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186 4.1 introduction This chapter provides essential information and steps to be considered in developing a bridge-deck system for a particular project in order to meet both strength and service life requirements. Section 4.2 describes various deck systems and their known advan- tages and disadvantages, as summarized in Table 4.1. Section 4.3 summarizes factors that affect the service life of bridge decks by using the fault tree format. Refer to Chapter 1 for a description of fault trees and how they are constructed. Section 4.4 provides strategies that can be used to mitigate most of the factors affecting the service life of bridge decks, as described in Section 4.3. Section 4.5 provides a framework for systematically addressing the service life design of bridge decks designed for strength, based on design provisions stated in the LRFD Bridge Design Specifications (LRFD specifications) (AASHTO 2012). 4.2 deScriPtion oF bridge-deck tyPeS The primary function of a bridge deck is to provide a safe riding surface for traffic, ensuring direct structural support of wheel loads. Two principal superstructure types are considered as bridge decks in this section: (1) bridge decks cast on top of beams or stringers, acting either compositely or noncompositely with superstructure supporting elements; and (2) superstructure systems in which the top of the superstructure ele- ment forms the top of the riding surface. By definition, numerous types of systems qualify as bridge decks, including con- crete deck systems, metal deck systems, timber deck systems, and fiber-reinforced poly- mer (FRP) deck systems. The factors affecting service life of concrete deck systems, 4 BRiDGE DECKS
187 Chapter 4. BRiDGE DECKS BRiDGE DECKS the main system used in the United States, are further described in this chapter. Metal, timber, and FRP bridge-deck systems are not addressed. Major bridge-deck systems are summarized in Table 4.1 and are described in this section. 4.2.1 Concrete Bridge-Deck Systems Concrete bridge-deck systems can consist of cast-in-place (CIP) systems and precast systems. The predominant bridge-deck system in the United States consists of CIP rein- forced concrete. CIP concrete systems are defined as concrete bridge decks that are cast in their final position. Typical CIP systems include â¢ Bridge decks on beams or stringers; â¢ Full-depth concrete slab superstructure; â¢ Multicell box girders; and â¢ CIP segmental construction. Precast concrete systems are defined as concrete bridge decks that are cast remotely and then brought to the bridge site for assembly into the final structure. Typical precast systems include â¢ Adjacent member; â¢ Deck panel over beams or stringers; and â¢ Precast segmental construction. tABLE 4.1. bridge-deck SyStemS Type Advantage Disadvantage Cast-in-place concrete deck systems Readily available material. Accommodates tolerances. Low cost. Susceptible to cracking and corrosion. Precast concrete deck systems Readily available material. Typically prestressed, reducing cracking. Requires construction joints between components. Higher initial cost. Metal deck systems Lightweight system. Prefabricated system. Requires protective coatings. Difficult tolerance adjustments. High cost. Timber deck systems Lightweight system. Constructible with unskilled labor. Low cost. Limited span range. Susceptible to wear without overlays. Susceptible to moisture degradation. FRP deck systems Lightweight system. Noncorrosive system. High cost. Limited history. Requires overlay for traction.
188 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 126.96.36.199 Cast-in-Place Concrete Systems on Beams or Stringers As shown in Figure 4.1, CIP bridge decks on beams or stringers are typically reinforced with mild steel reinforcement. They are generally 7.5 to 9 in. in thickness and are cast on forms that span between beams or stringers. These forms can either be removable or stay-in-place. 188.8.131.52 Full-Depth Cast-in-Place Concrete Systems Full-depth CIP concrete deck slab superstructures are a classification of bridge decks that span between pier supports without the aid of supporting beams and stringers. These deck slabs can be solid, can contain circular voids (as shown in Figure 4.2), or can contain more trapezoidal-shaped voids such as those used in CIP multicell box structures and CIP segmental structures. Voids are introduced to reduce the dead Source: Courtesy Atkins North America, Inc. Figure 4.2. Full-depth cast-in-place concrete slab posttensioned with voids. Figure 4.1. Cast-in-place concrete deck over longitudinal beams and stringers. Source: Courtesy Atkins North America, Inc. Figure 4.2. Full-depth cast-in-place concrete slab posttensioned with voids. Source: Courtesy Atkins North America, Inc.
189 Chapter 4. BRiDGE DECKS weight of the bridge. These bridge systems are usually conventionally reinforced with mild steel reinforcing, but they can be posttensioned longitudinally and transversely to achieve longer span lengths. 184.108.40.206 Precast Adjacent-Member Concrete Systems One of the most commonly used superstructure systems is the adjacent-member su- perstructure system that consists of prefabricated beam elements placed side-by-side in close proximity. This system has been used in various forms to expedite construction and minimize field forming and placing of concrete. These members are predominantly prestressed concrete beam elements in the form of prestressed solid and hollow-cored slab units (as shown in Figure 4.3), deck bulb-tees, double Ts, channels, and adjacent box beams. These deck systems are typically built as simple spans, but they can be constructed as continuous members. Typically, the members are tied together with a continuous longitudinal grout- or concrete-filled shear key that allows for the trans- verse distribution of applied vertical forces across the joint and prevents differential movement between adjacent members. The members may also be either transversely connected with conventional reinforcement or posttensioned together to develop the moments across the joint. 220.127.116.11 Precast Concrete Deck Panels As shown in Figure 4.4, precast concrete deck panel systems employ a series of precast concrete panels that are usually full-depth in thickness and have a length and width determined by specific bridge geometry. The length of the panel along the roadway is approximately 8 to 12 ft and is typically dictated by transportation limitations and crane capacity. Panels span the supporting girders and are designed with conventional reinforcement or as prestressed concrete. The general preference of precasters and contractors is to use prestressed concrete to eliminate possible cracking from handling and shipping. Figure 4.3. Example of adjacent-member slab unit superstructure system. Source: Courtesy Atkins North America, Inc.
190 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Precast concrete deck panel systems include both transverse and longitudinal slots for connections. The transverse slots are typically grout-filled keyways connected in a manner similar to the adjacent-member bridge systems. The longitudinal slots may consist of grouted (or concreted) pockets or block-outs to accommodate the shear connections to the girder. The system may also require temporary support and forms along the girder to retain the grout and some type of overlay to improve pavement ride quality. Longitudinal posttensioning is typically included in the system to tie the panels together; however, systems without posttensioning have been used. At the time of writing this guide (2012), new nonposttensioned connections were being developed. 18.104.22.168 Precast Segmental Concrete Superstructure Systems This structural system consists of numerous precast bridge elements that are post- tensioned together to form either simple-span units or, more commonly, continuous spans. Segmental construction has gained favor in locations where access is challeng- ing, such as in deep valleys, environmentally sensitive areas, across existing roadways, and where accelerated construction is warranted. The basic cross section of a segmen- tal bridge is usually a box shape with a top slab serving as the bridge-deck riding sur- face, as shown in Figure 4.5. The primary longitudinal reinforcement consists of either posttensioning tendons or bars that can be installed either internal to the web or ex- ternally inside the box section. The bridge deck is typically posttensioned transversely. 4.2.2 metal Deck Systems Metal deck systems are bridge-deck systems that rely on a metal such as steel or alumi- num to provide the structural resistance to vehicle wheel loads. Metal deck systems can consist of metal grid decks, orthotropic steel decks, or orthotropic aluminum decks. Figure 4.4. Full-depth precast panel system. Source: Courtesy University of Nebraska, Omaha.
191 Chapter 4. BRiDGE DECKS 22.214.171.124 Metal Grid Decks A metal grid deck system is a prefabricated module system consisting of main I- or T-shaped sections and secondary crossbars combined to form a rectangular or diago- nal pattern. These members can be either steel or aluminum, and the main elements span between beams, stringers, or other crossbeams. This system is typically used for movable bridges and for long-span structures in which a reduced bridge-deck weight is demonstrated to have an economic advantage. It has also been used in deck replace- ment projects. The system consists of open grid deck or can be combined with con- crete to form a partially or fully filled grid deck. The partially or fully filled concrete is typically cast flush with the grid service, or it can be cast above the unfilled deck. This system is known as the Exodermicâ¢ bridge-deck system. The addition of concrete in these systems reduces noise, improves fatigue performance, and improves the ability to channelize and collect storm water. 126.96.36.199 Steel Orthotropic Decks Bridge structures can utilize the orthotropic steel plate as one of the key structural sys- tems in the distribution of deck traffic loads and for stiffening the supporting slender plate elements in compression. Generally, the orthotropic system consists of a flat, thin steel plate stiffened by a series of closely spaced longitudinal ribs at right angles or or- thogonal to intermediate floor beams (see Figure 4.6). The orthotropic deck is typically made integral with the supporting bridge superstructure as a common top flange to the floor beams and girders. This arrangement results in cost savings in the design of these other components. The defining characteristic of the orthotropic steel bridge is that it results in a nearly all-steel superstructure. The orthotropic system has been used for many bridges worldwide, especially in Europe, Asia, the Far East, and South America. The United States has not yet fully embraced this technology and currently has fewer than 100 such bridges in inventory. Figure 4.5. Segmental superstructure. Source: Courtesy Atkins North America, Inc.
192 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The orthotropic deck has been most commonly used in the United States for long- span bridges in which the minimization of dead load is paramount and for redecking bridges on urban arterials. Orthotropic construction has tremendous potential for use in short- to medium-span girder bridges. The system has not been used more exten- sively for economic reasons; however, its light weight makes it beneficial for increasing a bridgeâs load rating during a deck replacement when replacement of the bridge may have been the only other alternative. 188.8.131.52 Aluminum Orthotropic Decks The aluminum orthotropic deck system configuration is similar to the steel orthotropic deck described in Section 184.108.40.206. The use of aluminum provides a corrosion resistance advantage that can result in lower maintenance costs, as it does not need periodic painting. Although aluminum is lighter than steel, its additional cost has often deterred its use in the United States. Other factors to carefully consider that make aluminum orthotropic decks different from the steel orthotropic deck system include differences in thermal expansion coefficients, reactions with dissimilar materials, lower modulus of elasticity, and lower fatigue strength of the material, particularly at weld locations. Figure 4.6. Orthotropic steel deck bridge. Source: Wolchuk 1963. Source: Wolc uk (1963). Figure 4.6. Orthotropic steel deck bridge.
193 Chapter 4. BRiDGE DECKS 4.2.3 timber Bridge Decks Timber bridge decks have been used for hundreds of years, but increases in vehicle loads have typically restricted their use to low-volume roadways. The materials used for these bridge decks can be rough sawn timbers or glue-laminated panels. Their per- formance can be enhanced through the use of protective coatings that can minimize water absorption, which can be detrimental to the service life of the timber. The timbers can be posttensioned together to form stress-laminated decks or nailed together to form spike-laminated decks. Overlays are typically provided on these bridges to improve skid resistance; however, the overlay requires extensive main- tenance due to the flexibility of the timbers and the numerous connections between members. 4.2.4 fiber-Reinforced Polymer Bridge Decks FRP bridge decks and superstructure systems are an emerging technology. FRP decks have been used for short-span bridges and for deck replacement on bridges. The prin- cipal advantages of FRP as a material are that it is lightweight and does not corrode under the same conditions as steel materials. It has shown promise for use in projects for which deck replacement is needed (as shown in Figure 4.7), particularly if total load capacity is relatively low. FRP bridge decks and superstructures have been constructed in many states. Com- parisons with traditional CIP concrete bridge-deck systems have shown that they exhibit lower dead loads, higher live-load fatigue ranges, and lower dynamic allow- ance (impact) (Albers et al. 2007). Because the surface of the FRP material has low skid resistance and the material itself is soft, overlay systems are required to provide a safe riding surface that has ade- quate surface friction and can withstand daily traffic wheel-load abrasion. Failure of Figure 4.7. FRP bridge-deck and superstructure applications. Source: Aboutaha 2001.
194 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE overlay adherence to the FRP material was noted in early applications of this technol- ogy. Connections for crashworthy barriers for FRP decks present additional challenges. Further research is recommended to study the long-term behavior of this new material to demonstrate its ability to provide a sufficiently long service life. 4.3 FActorS inFLuencing bridge-deck Service LiFe Bridge decks are one of the most costly maintenance items within a typical bridge system. The reduced service life of bridge decks can be attributed to two causes: (1) obsolescence, which is a functional planning issue and not a factor relating to durability issues; and (2) material service life performance deficiencies, which may be load induced; caused by human activity or natural hazards; or result from produc- tion defects in construction processes, design details, or operational procedures. These deficiencies are illustrated in the fault tree shown in Figure 4.8. Figure 4.8. Bridge-deck reduced service life fault tree. Reduced Service Life of Cast-in-Place Bridge Deck Caused by Deï¬ciency Caused by Obsolescence Natural or Man-Made HazardsLoad - Induced Production/ Operation Defects
195 Chapter 4. BRiDGE DECKS Many interrelated factors during the design, construction, and management phases of a bridge deckâs service life must be considered in developing long-lasting, cost-effective bridge decks. These factors vary depending on the bridge-deck system used, which can be arranged in four broad categories: â¢ Concrete bridge-deck systems, including â CIP concrete bridge-deck systems, and â Precast concrete bridge-deck systems; â¢ Metal deck systems; â¢ Timber deck systems; and â¢ FRP bridge-deck systems. The factors affecting the service life of CIP concrete and precast concrete bridge- deck systems are further described in this chapter. Metal, timber, and FRP bridge-deck systems are not addressed in the Guide. 4.3.1 Cast-in-Place Concrete Bridge Decks CIP bridge-deck systems are systems in which the concrete for the bridge deck is cast in the field as an integral part of the final superstructure. This bridge-deck system is one of the most common systems used in the United States today. These decks provide a major constructability advantage in that the casting process easily molds the bridge deck to meet geometric requirements (such as skews, lane tapering, and super elevation transitions) and to match existing locations of supporting elements that are not pre- cisely located in accordance with the plans. The main disadvantages of these decks include the quality of concrete produced as a result of workmanship and the curing processes. Inspections of bridge decks have revealed numerous performance issues with CIP concrete, including cracking, corrosion of reinforcement, spalling, delamination, and concrete deterioration evidenced by scaling, wear, and abrasion. Although concrete in compression is considered a very durable construction material, tension introduced through various loading and bridge restraint conditions can result in significant ten- sion that can exceed the materialâs tension strength limits, resulting in cracking. Crack- ing of bridge-deck concrete reduces the integrity of the passivated concrete layer that surrounds the reinforcing steel, significantly reducing the encased reinforcementâs resistance to corrosion. The following subsections discuss factors affecting the service life of CIP bridge decks. 220.127.116.11 Load-Induced Bridge Deck Considerations Load-induced bridge-deck deterioration can be attributed either to loads induced by the traffic or by characteristics dependent on the overall bridge system. These load- induced factors are shown in the fault tree in Figure 4.9.
196 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 18.104.22.168.1 Traffic-Induced Loads Traffic-induced loads include the effects of truck and other vehicular traffic on the rid- ing surface of the bridge. Bridge-deck loading has a degree of uncertainty that must be addressed during the design of the bridge, especially when achieving long service life is an objective. Typically the service life of bridge decks will be affected by fatigue, overload, and wear and abrasion. Fatigue. CIP concrete deck consists of two materials, steel and concrete, both of which can fail by fatigue. Design provisions for fatigue are addressed in the LRFD specifications. Overload. Despite weight limit regulations in most states that define load limits for permit and legal truck configurations, overloads exceeding these limits do occur. Overload is one of the main reasons for reduced service life of bridges. Overloads result in additional flexural stresses in bridge decks that can cause excessive cracking not accommodated by the original design. Heavier tire loads may also affect the wear and abrasion on the structure, and multiple applications of these loads can affect the fatigue behavior of the deck. Figure 4.9. Cast-in-place bridge-deck load-induced deficiency fault tree. Figure 4.9. Cast-in-place bridge-deck load-induced deficiency fault tree. Load-Induced System-Dependent Loads Traffic-Induced Loads Wear and Abrasion Fatigue Differential Shrinkage System- Framing Restraint Thermal Overload
197 Chapter 4. BRiDGE DECKS Wear and abrasion. Wear and abrasion is typically affected by high traffic volume, high tire loads, and the types of tires used on the facility. Tires in cold climates may have features to aid in traction, such as deep grooves, studs, and chains. These added tire features, while aiding traction, can abrade the surface of the bridge deck. Wear and abrasion can result in reduced thickness of the bridge deck, which in turn reduces the concrete cover protecting the reinforcement from corrosion; can reduce the load-resisting section, resulting in higher stresses and cracking; and can change the deck stiffness assumed for distribution of loads between superstructure elements. 22.214.171.124.2 System-Dependent Loads System-induced loads include the effects of the bridge system configuration on the be- havior of the bridge deck, such as restraint of integral abutment systems. Differential shrinkage. Differential shrinkage occurs when bridge-deck concrete is cast over previously cured concrete or over steel girders. The shrinkage of fresh concrete is restrained by the cured concrete or steel stringers, which results in a set of equal and opposite forces causing tension in the deck and compression in the girders. Heat of hydration also contributes to development of tensile forces in the freshly cast concrete deck. Thermal. Temperature changes can result in the development of axial forces in the bridge deck. These thermal forces are due to uniform internal temperature changes and temperature gradient. The level of these thermally induced axial forces is a func- tion of bridge system boundary conditions. System-framing restraint. Bridge decks can be subject to additional axial forces cre- ated by bridge boundary conditions. Bridge system boundary conditions are set at the design stage, during bridge system selection. For instance, in integral abutment joint- less bridge systems, the elimination of expansion devices and reliance on flexibility of piles to resist the bridge expansion and contraction add axial forces to the deck. Other examples of boundary conditions capable of creating axial forces in the bridge deck include choices for bearings and connections between superstructure and substructure made during design. These axial forces range from compression during system expan- sion to tensile forces during system contraction. Resistance to system contraction can create tensile forces in the bridge deck and cause cracking. Calculation of these addi- tional axial forces is important and can be achieved through conducting proper analysis methods that correctly model the bridge boundary conditions. Improper function, or seizing of the bearings, results in unintended movement restraint that can raise the force resisted by the substructure well above the intended design. This unintended restraint can cause unanticipated cracking with greater poten- tial for corrosion. Proper bearing function, which is addressed in Chapter 10, is essen- tial to substructure durability. Lack of maintenance may also result in bearings losing the movement capability intended by their design.
198 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 126.96.36.199 Natural or Man-Made Hazard Bridge-Deck Considerations The environment to which the bridge deck is subjected can have a significant influence on its service life. Environmental influences comprise hazards from both natural and sources and include effects from areas with adverse thermal climate, climates, and chemical climates, as well as from chemical properties of the materials and outside agents, such as fire. These natural and man-made hazards are listed in the fault tree in Figure 4.10. 188.8.131.52.1 Thermal Climate Thermal climate influences on bridge-deck service life performance are primarily due to cold weather. These influences are both man-made, from the application of deicing salts, and natural, in the case of freezeâthaw. Agencies in cold weather climates that deal with ice and snow on roadways and bridges have traditionally applied deicing salts to melt the ice and snow to facilitate tire traction. The application of these deicing salts is viewed as a safety enhancement for the traveling public; however, these chloride-laden compounds tend to ingress into the concrete deck either through porosity in the concrete or through open deck cracks. The chloride ingress into the bridge deck continues to reduce the effectiveness of the passivating layer around the reinforcing steel, eventually initiating reinforcement cor- rosion. The reinforcement corrosion process causes the bar to expand, resulting in deck cracking, spalling, and delamination. The cross slope built into bridge decks for drainage purposes causes the salt to wash down toward the bridge gutter adjacent Figure 4.10. Cast-in-place bridge deck natural or man-made hazards fault tree. [LANDSCAPE IN FINAL] Figure 4.10. Cast-in-place bridge deck natural or man-made hazards fault tree. Natural or Man- Made Hazards Thermal Climate Coastal Climate Chemical Climate Fire/Blast Vehicle/ Vessel Collision Reactive Materials Freezeâ Thaw Humidity Salt Water/ Spray Corrosion Corrosion- Inducing Chemicals Sulfate Attack ACR ASR Deicing Salts Corrosion Hydraulic Action Scour Flood/ Storm Surge Seismic Extreme Events
199 Chapter 4. BRiDGE DECKS to the traffic railing barriers bounding the bridge. Removal equipment that scrapes snow from the bridge deck also deposits residual snow laden with deicing salts at this location, resulting in a very high concentration of chlorides. Construction joints at this location are particularly susceptible to chloride intrusion, and in many cases this susceptibility has led to corrosion of the barrier reinforcement. Water absorbed into the concrete deck surface and contained in cracks can freeze in cold weather conditions. The frozen water tends to expand, causing stresses within the concrete. Cyclic freezing and thawing of the water absorbed in the deck surface can result in bridge-deck deterioration in the form of cracking, scaling, and spalling. Refer to Chapter 3 for additional information on freezeâthaw in concrete. 184.108.40.206.2 Coastal Climate Coastal climate influences on bridge-deck service life performance are primarily due to the introduction of chlorides through salt spray and from the effects of high humidity. Both of these influences occur naturally. Coastal regions are subjected to a chloride-laden saltwater environment and a combination of wind and wave action that causes these chlorides to become airborne as salt spray. The susceptibility of the bridge deck to these environmental influences depends on the height of the bridge deck above the water elevation and the distance to coastal areas. The action of waves hitting substructure units and seawalls or abut- ments under the bridge tends to cause the salt spray to explode upwards, wetting the bottoms of lower-level bridge decks. The salt spray can also be deposited on the bridge-deck surface, particularly on windy days. When the salt spray wets the surfaces, it leaves a chloride residual that can absorb into the concrete, resulting in reinforce- ment corrosion. High humidity in coastal regions also results in cyclical wetting and drying of con- crete surfaces. Concrete materials sensitive to repeated wetting, such as those where reactive aggregates are used, can have an adverse effect on the bridge-deck service life. 220.127.116.11.3 Chemical Climate Chemical climate influences on bridge-deck service life performance can be attributed to corrosion-inducing chemicals and sulfate attack. These influences can occur natu- rally or can be man-made. Corrosion-inducing chemicals can be introduced to the bridge deck from adjacent industries, where residuals from pollution can contribute to reduction in bridge-deck service life. For example, oil- and coal-burning facilities release sulfur dioxide and nitrogen oxide into the air, which causes acid rain consisting of sulfuric and nitric acids. These acids can dissolve cement compounds in the cement paste and calcareous aggregates and can leave crystallized salts on concrete surfaces that can lead to spalling and the corrosion of reinforcing bars. Exposure to sulfates can cause expansion of the concrete material and conse- quently result in spalling and cracking of the bridge deck. Refer to Chapter 3 for addi- tional information on sulfate attack in concrete.
200 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 18.104.22.168.4 Reactive Ingredients Reactive ingredients within the mix used for bridge decks can affect service life perfor- mance as the reactive ingredients alter the volumetric stability of the concrete. These influences primarily occur naturally. Alkali-silica reactivity (ASR) results in swelling within concrete that can lead to spalling, cracking, and general concrete deterioration. Refer to Chapter 3 for addi- tional information on ASR in concrete. Alkali-carbonate reactivity (ACR) results in aggregate expansion within concrete that can lead to spalling, cracking, and general concrete deterioration. Refer to Chap- ter 3 for additional information on ACR in concrete. 22.214.171.124.5 Fire A key factor in the amount of damage caused to concrete by fire is the duration of the fire and the heat levels generated. Because of the low thermal conductivity of concrete, it takes considerable time for the interior of concrete to reach damaging temperatures. When concrete is exposed to the extreme heat of a fire, the chemical bonds between the water molecules in the concrete break, resulting in dehydration and the destruc- tion of the cement binder. The concrete loses its mechanical properties, exhibiting cracking and spalling, and exposes steel, leaving it unprotected (ACI 216, 1989). Once the reinforce ment has become exposed, it conducts heat and accelerates this action. Reinforcing steel in bridge decks subjected to temperatures above 550Â°C (1,022Â°F) exhibits a rapid reduction of strength, which can lead to collapse. In addition, spalling can result from the rapid quenching of hot fires by fire hoses. 126.96.36.199 Design, Construction (Production), and Operation Bridge-Deck Considerations Decisions made for the design and construction of bridge decks and the activities that will occur during their operation can significantly influence service life. These influ- ences, which are listed in the fault tree in Figure 4.11, include decisions made during the design and detailing of the bridge deck, the quality of construction, the level of inspection, and the testing performed during operations and maintenance. 188.8.131.52.1 Design and Detailing Bridge-Deck Considerations Decisions made during the design and detailing phase of a bridge project can signifi- cantly affect the service life of the bridge. It is incumbent on designers to understand the implications of these decisions in order to make rational choices that will improve the service life of bridge decks. These decisions are listed in the fault tree in Figure 4.12 and include choices in design philosophy, expansion joints, construction joints, con- crete mix design, and bridge-deck drainage. 184.108.40.206.1a Design Philosophy There are two principal methods presented in the LRFD specifications for the design of bridge decks: the traditional design method and the empirical design method.
201 Chapter 4. BRiDGE DECKS The traditional design method assumes flexural action to describe the behavior of bridge-deck spanning between supporting girders and ignores the axial forces created in the bridge deck as a result of arching action. Under this assumption, the amount of reinforcement needed in the bridge deck will generally far exceed the demand, usually by more than a factor of two. Providing additional reinforcement in the bridge deck creates additional means for corrosion and deterioration of the bridge deck. The empirical design method provides better estimation of bridge-deck resistance to applied traffic loads than the traditional design method. Test results (Fang 1985; Holowka et al. 1980) show that the principal mechanism for resisting the applied traffic loads in the bridge deck is the creation of axial compressive loads, commonly referred to as arching action. These axial compressive loads are resisted by supporting longitudinal beams. Consequently, the use of the empirical method is not applicable to cantilever portions of the deck. The axial compressive loads in the bridge deck Figure 4.11. Cast-in-place bri dge-deck design, construction (production), and operation defects fault tree. Production/ Operation Defects Design/Detailing Construction Placement Dissimilar Metals Field Bending Inspection Protection and Repair Visual Nondestructive Testing Maintenance Figure 4.11. Cast-in-place bridge-deck design, construction (production) and operation defects fault tree.
202 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE significantly reduce the need for reinforcement in the bridge deck, and reduction of reinforcement in the bridge deck significantly reduces the sources of corrosion. Research in Canada in the past 20 years (Newhook and Mufti 1996) has focused on eliminating bridge-deck reinforcement corrosion through the development of âsteel-freeâ bridge decks. Similar to the methodology employed by the empirical design method, this concept provides the tension tie required to resist the compressive forces created in the bridge deck by arching action. Under this concept, the tension ties are attached to top flanges of the supporting beams or stringers. These bridges have experienced some temperature and shrinkage cracking. To control this cracking to acceptable levels, recent recommendations suggest supplementing the steel tension ties below the deck with a mat of FRP reinforcing bars (Memon and Mufti 2004). Figure 4.12. Cast-in-place bridge-deck design and detailing deficiency fault tree. Design/ Detailing Design Philosophy Permeability Cracking Resistance Passivity Mix Design Workability Modular Construction Creep and Shrinkage Construction Joints Drainage Phasing Expansion Joints Empirical Traditional Target Design Life Composite Action Other Methods Figure 4.12. Cast-in-place bridge-deck design and detailing deficiency fault tree.
203 Chapter 4. BRiDGE DECKS 220.127.116.11.1b Expansion Joints Expansion joints are provided to relieve system-framing restraints that can cause a buildup of tension stresses in the superstructure and the bridge deck. Refer to Section 18.104.22.168.2 for additional information on system-framing restraints and to Chapter 9 for additional information on expansion joints. Dirt and other deleterious material, such as deicing salts, can collect on expan- sion devices within the bridge deck and produce an adverse effect on the service life of bridge decks. Impact from vehicles and from snow removal equipment can cause spalling, which reduces the protective concrete cover over reinforcement or exposes the reinforcement, which leads to corrosion. 22.214.171.124.1c Construction Joints Construction joints are surface discontinuities at which successive concrete placement regions meet; they are generally specified by the designers or construction contrac- tors. Typical construction methods for which construction joints are required include modular construction and phasing of construction. The modular construction used to accelerate bridge construction requires con- struction joints within adjacent-member superstructure systems. Typically these joints are designed to transfer shear and moment across the interface, and if not properly designed and detailed they may lead to cracking along the adjacent-member interface. Phasing of bridge construction is often a result of public pressure and demand for uninterrupted traffic flow during bridge construction. A phased approach is used both for widening of existing bridges and the construction of new bridges. In the case of new construction, a portion of the new bridge is constructed (Phase 1) while the exist- ing bridge carries the traffic. Traffic is then transferred to the new bridge (Phase 1), the existing bridge is demolished, and the new bridge is completed (Phase 2). Finally, the two phases are typically joined using a closure pour. The phases will experience differ- ent deflections at the time of placing the closure pour; this differential deflection can result in major construction problems. One of the characteristics of phased-constructed bridges is that transverse and longitudinal cracks form near the points at which Phases 1 and 2 are joined. Forma- tion of these cracks is a well-known feature of these bridge types. Further, closure-pour regions need to be water proofed to prevent deterioration of the deck in these regions as a result of chloride ingress and initiation of reinforcement corrosion. Placing addi- tional reinforcement in the deck will not prevent formation of these cracks; it will only make the crack width smaller. A well-constructed phase bridge can perform very satisfactorily (Azizinamini et al. 2003b). Nevertheless, several factors (described in this subsection) need to be taken into consideration. Composite sections experience long-term displacement because of creep and shrinkage. Figure 4.13 shows an exaggerated difference between elevations of Phase 1 and Phase 2 for newly constructed bridges. The same phenomenon also exists for widening projects.
204 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The reason for the observed differential displacement shown in Figure 4.13 is illustrated in Figure 4.14, which shows the displacement of the Phase 1 girders due to creep and shrinkage. At about 90 days after completion of Phase 1, the girders experi- ence maximum creep and shrinkage displacement. As shown along the horizontal axis, construction of Phase 2 starts after Phase 1 is completed. Phase 2 also experiences the creep and shrinkage displacements and, depending on the time of casting the closure- pour region, differential displacement will exist between Phase 1 and Phase 2 girders. In the case of steel bridges, this differential deflection between the two phases has resulted in major fit-up problems for the cross frame in the bay containing the closure pour. Once the closure-pour region is cast, the two systems are locked in, but Phase 2 continues to experience additional displacements that subject the deck in the closure- pour region to additional stresses. Figure 4.13. Exaggerated differential displacement between phases. Source: Azizinamini et al. 2003b. Time D eï¬ ec ti on Figure 4.14. Displacement of Phase 1 and Phase 2 portions of the bridge. Source: Azizinamini et al. 2003b.
205 Chapter 4. BRiDGE DECKS When the deck elevations of the two phases do not match, the contractor may attempt to force the two separate superstructure portions together. This practice sub- jects the deck in the closure-pour region to additional stresses, which are difficult to estimate and may also jeopardize the service life of the deck concrete. There is debate among bridge engineers on conditions under which the closure- pour region should be cast. Some contractors prefer to close traffic completely until the concrete in the closure-pour region is set, but others believe that vibration caused by traffic is helpful for better consolidation of the concrete in the closure-pour region. Generally, traffic closure is not an option because of public demand that traffic inter- ruptions be minimized. Another major condition that could facilitate the construction of phased bridges is the elimination of cross frames in the bay containing the closure pour, if possible. However, the pros and cons of such action need to be investigated. 126.96.36.199.1d Mix Design Chapter 3 provides detailed information concerning factors that affect the service life of concrete. The following paragraphs briefly describe mix design factors that affect the service life of bridge decks: permeability, passivity around reinforcement, crack resistance, workability, and creep and shrinkage. Permeability. The durability of concrete largely depends on its ability to resist the infiltration of water and aggressive solutions. Concretes with high permeability pro- vide less resistance to aggressive solutions or water penetrating the concrete and pos- sibly causing expansive forces due to physical (freezeâthaw) or chemical (corrosion, ASR, sulfate attack) factors. Passivity around reinforcement. The loss of passivity of the outer layer of the reinforcing steel initiates a corrosion process that deteriorates the steel. This corrosion process begins by the diffusion of chloride ions to the depth of the reinforcing steel and/or carbonation, which reduces the pH of the concrete to the passivating layer sur- rounding the concrete. Crack resistance. Mix design can affect the extent of cracking for all CIP concrete bridge decks and slab superstructures. Mixtures with high water and paste content are prone to shrinkage cracks that occur over time. The use of large aggregate sizes and well-graded aggregates reduces the water and paste content and minimizes shrinkage. In fresh concrete, when the rate of evaporation exceeds the rate of bleeding, plastic shrinkage occurs. Concrete with low-bleed water, stiff consistency, and a low water- cement ratio is prone to plastic shrinkage cracking. Prevention of plastic shrinkage cracking depends on prompt, effective curing. Workability. Concrete mix designs must include good-quality aggregates and appropriate admixtures to facilitate construction. Mix designs with poor workability can cause uncontrolled field adjustments to the mix through the addition of water in the field, resulting in higher water-cement ratios and overvibration that can cause aggregate segregation. Proper workability must be ensured for the integrity of the mix design to provide concrete with the intended properties.
206 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Creep and shrinkage. The creep and shrinkage properties of concrete mixes can affect the service life performance of bridge decks. The adverse effects of concreteâs shrinkage characteristics are discussed in conjunction with system-dependent loads in Section 188.8.131.52.2. In contrast to the effects of shrinkage, high levels of creep can be either beneficial or detrimental depending on the application. High creep levels are beneficial when differential shrinkage occurs, particularly within bridge decks. In instances of higher creep, the restraining force between the bridge deck and the sup- porting superstructure will reduce significantly, consequently reducing the potential for cracking. However, high creep levels are detrimental when the creep results in unrestrained volumetric changes that cause significant, unintended movements of the structure, such as in posttensioned structures. 184.108.40.206.1e Drainage In design, poor drainage details can result in ponding and prolonged exposure of bridge components to moisture and aggressive solutions, causing corrosion and other environmental distress. When concrete gains moisture, it expands slightly or swells. When concrete loses moisture, the concrete contracts or shrinks. As drying occurs, the portion of concrete near the surface dries and shrinks faster than the inner portion of the concrete. This process results in a differential moisture condition in which tensile stresses that can cause cracks may occur on the surface. The degree of frost damage to concrete is also highly dependent on the degree of saturation. Ponding of water on bridge decks can cause critical saturation that results in bridge damage. High moisture is also detrimental to concrete susceptible to ASR expansion, which can cause spalling and cracking. 220.127.116.11.2 Construction Attention to good practices during construction is crucial to the long-term durability of reinforced concrete. A well-qualified and well-trained work force and work that is well executed increase productivity, reduce material waste, and provide expected service life. The proper use of appropriate equipment provides better workability by increasing efficiency, and well-planned construction schedules reduce overall costs by providing set times for equipment rental and reducing downtime. The correct imple- mentation of test methods ensures quality concrete. 18.104.22.168.2a Placement and Curing of Concrete Good construction practices, which ensure the proper location of reinforcing steel for proper cover depth, consolidation, and curing, are essential for longevity. Proper con- solidation minimizes entrapped air voids, which can reduce strength and durability. Proper curing is necessary for formation of the binder and control of volumetric changes and includes both moisture and temperature control. In bridge structures, the deck surfaces require special attention because of their large surface areas, where loss of moisture is a concern.
207 Chapter 4. BRiDGE DECKS Handling of concrete affects the final product. Delay in placement, particularly on hot days, should be avoided as it can lead to stiffening of the concrete that can cause tearing of the deck surface during finishing, resulting in a poor surface finish and reduced durability. 22.214.171.124.2b Formwork The type of concrete formwork can affect the surface finish of the concrete. Imperme- able forms can allow surface voids to occur, resulting in increased surface permeability, reduced strength, and an overall decrease in durability. CIP bridge decks on beams or stringers are cast on forms that span the beams or stringers. These forms can be either removable or stay-in-place. Stay-in-place forms are typically made of galvanized steel (as shown in Figure 4.1), precast concrete panels (either conventionally reinforced or prestressed), and FRP. Many owners do not allow the use of stay-in-place forms, citing the inability to inspect the bottom surface of the concrete deck and the potential for the collection of water intruding through the cracks. The use of precast concrete stay-in-place panels designed to be composite or non- composite with the deck pour above has resulted in reflective cracking over the panel joints in past applications and has raised questions about their long-term durability. Research is needed to develop acceptable details for control or elimination of reflective cracks. 126.96.36.199.2c System Vibration During Construction Excessive traffic-induced vibrations during construction can occur during the widening of a new bridge deck that is being cast against an existing bridge deck subjected to ac- tive traffic. The effect of these vibrations on the quality of the finished deck, especially in the closure-pour regions, is not well understood and requires further investigation. 188.8.131.52.2d Casting Schedule The casting schedule for bridge decks should carefully consider weather conditions, particularly hot days. The rise of heat from hydration in concrete can be exacerbated by a concurrent rise in the ambient temperature, resulting in a greater cooling differ- ential that can cause bridge-deck cracks. It takes approximately 18 hours for heat of hydration to reach its peak value. After casting, the concrete at the surface is always at ambient temperature, while within the deck, the temperature varies due to the develop- ment of heat of hydration. The center of the deck cools last. As a result, the maximum temperature differential takes place between the center of the deck and the deck sur- face. When the maximum temperature differential exceeds a certain limit, which most departments of transportation limit to about 30Â°F, the deck can crack. Therefore, this maximum temperature differential needs to be controlled. When casting is performed at night, the peak ambient temperature generally occurs 12 to 18 hours later, when the center of the deck is at its highest temperature. Timing the casting in this way results in a minimum temperature differential between the center and outside surfaces of deck
208 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE concrete. In contrast, when the deck is cast in the morning, the temperature differential between the center of the deck and the surface of deck is much higher. In conclusion, although casting a deck at night is ideal, the common practice is to cast the concrete deck in the morning, the most undesirable time. 184.108.40.206.2e Casting Sequence Construction joints within bridge decks control the sequence of casting bridge decks, either as a result of concrete volume placement constraints, or, in the case of continu- ous steel girders, in order to minimize bridge-deck tension in the negative moment area over substructure elements. These locations form a discontinuity that can open as cracks within the bridge deck and allow ingress of moisture. 220.127.116.11.3 Visual Inspection Although inspections are valuable tools for identifying deficiencies in bridge decks, they are typically visual, making them subject to the ability, training, and disposition of the individual inspector. Often a deficiency is not easily detectable and may show only subtle signs that can easily be missed by cursory inspections or by inexperienced inspectors. Deficiencies can also be located below the undamaged surface or in inac- cessible areas. The inability to see the deficiency leads to inadequate identification of repair methods, scope, and material selection and could cause failure of the structure without visible signs, because surficial repairs may cover the damaged area. 18.104.22.168.4 Maintenance Lack of preventive maintenance reduces the service life of bridge decks. Sometimes simple maintenance tasks are delayed until a problem becomes a safety issue, at which time the required repairs may be either significantly more extensive or ultimately irreparable. 4.3.2 Precast Concrete Bridge-Deck Systems Precast concrete bridge-deck systems are systems in which the concrete components for the bridge deck are produced in a controlled environment, minimizing variability in concrete uniformity of both material behavior and construction personnel per- formance. Use of these systems can minimize traffic disruption caused by prolonged concrete casting operations over active roadway facilities; such systems are a key con- sideration for accelerated bridge construction. Inspections of precast concrete bridge decks have revealed many of the same per- formance issues experienced with CIP concrete as described in Section 4.3.1, including cracking, corrosion of reinforcement, spalling, delamination, and concrete deterio- ration evidenced by scaling, wear, and abrasion. Precast concrete bridge-deck com- ponents introduce numerous joints in the superstructure that are usually the source of many bridge-deck service life issues, particularly when the material provided to seal these construction joints breaks down, causing cracking, leakage, and eventually reinforce ment corrosion.
209 Chapter 4. BRiDGE DECKS Rather than repeat the discussion of the many performance-related service life issues inherent with concrete systems described in Section 4.3.1, this section describes the factors affecting service life specific only to precast concrete bridge-deck systems. The user of this Guide should also become familiar with the other factors described in Section 4.3.1 when considering precast concrete bridge-deck systems. Load-induced (Section 22.214.171.124) and natural or man-made hazard (Section 126.96.36.199) bridge-deck considerations for precast bridge-deck systems are the same as those for CIP bridge-deck systems. The following subsections discuss the factors affecting pre- cast concrete bridge-deck service life in production and operation. 188.8.131.52 Design, Construction (Production), and Operation Bridge-Deck Considerations Decisions regarding the production of a bridge deck and the activities that will occur during its operation can have a significant influence on service life. These produc- tion and operation influences are listed in the fault tree in Figure 4.15. They include Figure 4.15. Precast concrete bridge-deck design, construction (production), and operation defects fault tree. Production/ Operation Defects Design/Detailing Construction Placement Curing Vibration During Construction Casting Schedule Connection IntegrityFormwork MaintenanceVisual Inspection Fabrication/ Manufacturing Tolerances/ Field Fit - up Lifting Embedments Figure 4.15. Precast concrete bridge-deck design, construction (production) and operation defects fault tree.
210 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE decisions made during the design and detailing of the bridge deck, fabrication and manufacturing requirement, quality of construction, and decisions concerning the level of inspection and testing performed during future operation and maintenance. 184.108.40.206.1 Design and Detailing Bridge-Deck Considerations Decisions made during the design and detailing phase of a bridge project can signifi- cantly influence the service life of the precast bridge deck. It is incumbent on design- ers to understand the implications of these decisions in order to help make rational choices that will improve service life. These influences, which are listed in the fault tree in Figure 4.16, include choices regarding design philosophy, expansion joints, construction joints, concrete mix design, and bridge-deck drainage. Again, many of these influences are similar to those for a CIP concrete deck, except for the addition of composite action considerations for precast decks. Design/Detailing Design Philosophy Permeability Cracking Resistance Passivity Mix Design Workability Modular Construction Creep and Shrinkage Construction Joints Drainage Phasing Expansion Joints Empirical Traditional Target Design Life Composite Action Other Methods Figure 4.16. Precast concrete bridge-deck design and detailing deficiency fault tree.
211 Chapter 4. BRiDGE DECKS 220.127.116.11.2 Composite Action For precast systems such as full-depth deck panels, the design philosophy addresses either a composite or a noncomposite connection of the deck panels to the supporting superstructure element. Composite bridge-deck systems add to the stiffness of the overall bridge system, reducing deflection and vibration and improving bridge-deck performance. The con- nection requirements for composite systems are developed through field casting of concrete or grout around shear connectors or studs accessed through continuous full-depth open pockets (as shown in Figure 4.4); through localized, full-depth open pockets; or through continuous or localized embedded channels in the panels under the deck surface. Open-pocket systems introduce construction joints, forming a dis- continuity that can open as cracks. Embedded channel systems require pressure grout- ing to fill the void. Improper grout installation can lead to entrapped air voids that can fill with bleed water and water intruding through deck cracks, leading to freezeâthaw issues and increased potential for reinforcement corrosion. In noncomposite systems, excessive flexibility and inconsistent friction between the deck panels and the superstructure along the length of the supporting stringer could result in localized stress that can cause cracking and delamination in the con- crete. Excessive vibration can also lead to fatigue issues. 18.104.22.168.3 Modular Joint Construction In precast systems consisting of adjacent members or segmental construction, the de- sign and detailing of the joint is essential to its proper performance. These joints can open as cracks within the bridge deck if not properly considered, leading to leakage, spalling, and reinforcement corrosion. 22.214.171.124.4 Precast Component Fabrication and Manufacturing Considerations Decisions relating to the fabrication and manufacturing of precast components for bridge decks can significantly affect the service life of the bridge. It is incumbent on the fabricators to understand the implications of these decisions in order to help make rational choices for improving the service life of bridge decks. These decisions are listed in the fault tree in Figure 4.15; they include choices in field fit-up and casting tolerances, as well as methods for lifting the precast elements in the precast yard and for erection in the field. 126.96.36.199.4a Tolerances and Field Fit-Up Careful planning is needed to incorporate precast components as bridge decks on bridges. These components must be aligned in the field fairly accurately to ensure a smooth, safe ride for the traveling public. Often the construction sequence and schedule must be assessed to establish casting dimensions for the precast compo- nents. Liberal tolerances and insufficient control in the precasting facility can result
212 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE in ill-fitting pieces that require unintended adjustments in the field, leading to spalling and other structural failures caused by localized, nonuniform contact surfaces that were not anticipated during design. 188.8.131.52.4b Transportation and Lifting Methods Precast components must be moved from the casting facility to their final position in the bridge. This transport may consist of multiple lifts, depending on storage require- ments and transportation schedules. The components must also be transported be- tween the casting bed, storage facilities, and the project site. Improper consideration of the stresses imposed on the precast components during these critical events can result in cracking, spalling, and sometimes failure. Lifting precast components also requires devices for attaching slings or cables (or both), usually consisting of steel embedded in the finished surface of the concrete. Improper removal and treatment of these embedments once the precast component is set in the field can result in localized spalls due to steel corrosion. 184.108.40.206.5 Construction Attention to good practices during construction is crucial to the long-term service life of reinforced concrete. Well-qualified and well-trained workers and well-executed workmanship increase productivity, reduce material waste, and provide expected ser- vice life. Proper use of adequate equipment provides better workability by increasing efficiency, and a well-planned construction schedule reduces the overall cost of the project by providing set times for equipment rental and reducing downtime. Proper use of test methods is needed to ensure that quality concrete is achieved. These prac- tices, which are listed in the fault tree in Figure 4.15, include workmanship related to the connectivity of the precast components. Connection of precast components is performed in several ways. Match-cast com- ponents are typically joined with epoxy and prestressed across the joint. Improper epoxy material for the temperature of application, inadequate epoxy set time, and inconsistent, nonuniform application can cause spalling of the joint as the prestressing compresses the joint across a nonuniform contact surface. Components that are not match cast are typically detailed with open joints to be filled with concrete or grout. Improper surface preparation and incomplete filling of these joints can result in early breakdown of the joint filler material, resulting in cracks, leakage, reinforcement corrosion, and a reduction in load distribution charac- teristics that can be detrimental to the carrying capacity of the bridge. 4.4 individuAL StrAtegieS to mitigAte FActorS AFFecting Service LiFe Section 4.3 defines numerous factors affecting the service life of bridge decks. This sec- tion provides individual strategies to mitigate those factors. Table 4.2 summarizes the areas in which strategies are provided.
213 Chapter 4. BRiDGE DECKS 4.4.1 Strategies to mitigate Load-induced Effects This section addresses concrete bridge decks. Load-induced effects are created from the traffic using the bridge and from system-dependent framing restraints. Strategies for mitigating deterioration from these effects are provided in this section. 220.127.116.11 Strategies to Mitigate Traffic-Induced Loads A complete understanding of the characteristics of the traffic on the structure is re- quired to define the strategies required for enhancing the service life of a bridge deck. These characteristics include vehicle configuration, such as axle and wheel spacing and individual wheel weights; type of wheel or tire; potential for overloads; type of suspension system; traffic volumes and frequency of truck and overload application; and vehicle location on the deck. In order to establish criteria to adequately address fatigue response, overload, wear, and abrasion, these characteristics must be understood. Table 4.3 identifies the strategies for these service life issues. Bridge-deck systems can be adequately designed for fatigue by considering indi- vidual wheel loads, dynamic impact effects, and the frequency of load application developed from the volume of truck traffic to which the bridge deck will be subjected. The fatigue design of reinforcing steel within the concrete deck is adequately addressed by the threshold design methods provided in the LRFD specifications. tABLE 4.2. mitigAtion cAtegorieS Section Mitigation Category 4.4.1 Strategies to mitigate load-induced effects 4.4.2 Strategies to mitigate system-dependent loads 4.4.3 Strategies to mitigate natural or man-made environment deterioration 4.4.4 Strategies to improve production and operations tABLE 4.3. mitigAting StrAtegieS For trAFFic-induced LoAdS Service Life Issue Mitigating Strategy Advantage Disadvantage Fatigue Design per LRFD specifications Minimizes the possibility of reinforcement failure May increase the area of steel Overload Increase deck thickness Minimizes cracking Adds weight to bridge structure, increases cost Wear and abrasion Implement concrete mix design strategies See Chapter 3, Materials See Chapter 3, Materials Implement membranes and overlays Protects surface from direct contact with tires Requires rehabilitation every 10 to 20 years
214 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Bridge decks can also be designed for overload conditions with adequate determi- nation of the potential for overload and the frequency of its application. Additional strength requirements for overloads can be addressed by increasing the thickness of the deck. Additional information on wear and abrasion can be found in Chapter 3. Mem- branes and overlays can be used to separate the wheel contact surface from the deck surface. 4.4.2 Strategies to mitigate System-Dependent Loads Bridge-deck performance can be enhanced by the proper selection of a system to ac- commodate bridge movements, whether they are caused by differential shrinkage or from system-framing restraints of movements from thermal expansion and contrac- tion or creep and shrinkage. Table 4.4 identifies the strategies for these service life issues; these strategies are expanded below. 18.104.22.168 Differential Shrinkage Several enhancements are viable for addressing the restraint forces at the interface of the bridge deck and supporting beam or stringer superstructure elements. Potential enhancements include the following: tABLE 4.4. mitigAting StrAtegieS For SyStem-dePendent LoAdS Service Life Issue Mitigating Strategy Advantage Disadvantage Differential shrinkage Use low-modulus concrete mix designed for composite decks Allows additional strain to be accommodated up to cracking stress Typically lower in strength and may be subject to wear and abrasion Use high-creep concrete mix designed for composite decks Reduces locked-in stresses Uncommon mix design. Difficult to assess stress relief. Develop composite action after concrete has hardened Allows slippage between deck and supporting members, minimizing locked-in stresses Very limited knowledge on available systems capable of developing composite action after hardening of bridge deck Use precast deck panels Allows slippage between deck and supporting members, minimizing locked-in stresses Introduces numerous construction joints Thermal restraint Develop an accurate system model for analysis purposes Identifies design criteria for establishing stresses Analysis is time consuming System-framing restraint Develop an accurate system model for analysis purposes Identifies design criteria for establishing stresses Analysis is time consuming
215 Chapter 4. BRiDGE DECKS â¢ Using low-modulus concrete mix design to allow the deck to accommodate the shrinkage strain with less tension force, which can reduce cracking. Refer to Chap- ter 3 for additional discussion of low-modulus concrete mix designs. â¢ Using a high-creep concrete mix in the supporting superstructure element, which continues to reduce the locked-in tension force in the bridge. Refer to Chapter 3 for additional discussion of high-creep concrete mix designs. â¢ Using delayed composite action systems in which the interface of the bridge deck and the supporting beam or stringer superstructure elements is not made compos- ite until a majority of the deck shrinkage has occurred. 22.214.171.124 System-Framing Restraint Superstructure and substructure systems must be designed to provide either movement or restraint of the structure, with proper consideration of internally induced forces. For additional information on the proper system selection, see Chapter 2. 126.96.36.199.1 Fully Integral Deck Systems Eliminating expansion joints at abutments and over piers can enhance bridge-deck performance. This bridge system, commonly referred to as a jointless bridge, is ad- dressed in Chapter 8. 188.8.131.52.2 Semi-Integral Deck Systems A significant number of states use a semi-integral approach to bridge-deck systems. This system provides expansion joints at the beginning and end bridge abutments and no joints (or limited joints) in the remainder of the bridge. Bridge-deck performance is improved by eliminating joints. Separating the bridge deck from the substructure at the abutment locations reduces the tensile forces that could otherwise be generated in the bridge deck during deck contraction as a result of traffic and thermal loads. 4.4.3 Strategies to mitigate natural or man-made Environment Deterioration Proper studies for identifying environmental exposures detrimental to bridge-deck per- formance should be performed. Understanding the causes of deterioration leads to proper consideration during design. Tables 4.5, 4.6, and 4.7 describe the strategies developed for natural and man- made environment deterioration. Tables 4.4 through 4.7 present strategies for addressing the various factors affect- ing service life presented in Section 4.3. Proper incorporation of design features and materials is important for enhancing the service life of CIP and precast bridge decks. Likewise, there are numerous protection strategies for enhancing the service life of concrete bridge decks. These include providing adequate concrete cover, proper con- crete mix design, proper reinforcement selection and protection, proper drainage,
216 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE application of prestressing, and the use of external protection systems. These enhance- ment strategies are described in the following sections. 184.108.40.206 Concrete Cover Concrete cover is recognized as an effective method for protecting steel from corro- sion. A minimum top concrete cover of 2 in. is required by the LRFD specifications. Generally, 2.5 in. of cover is used to allow for 0.5 in. of wear over the life of the deck. Cracking of the deck, however, can allow chlorides to quickly penetrate to the level of the reinforcement, initiating the corrosion process. tABLE 4.5. mitigAting StrAtegieS For thermAL cLimAte environment deteriorAtion Service Life Issue Mitigating Strategy Advantage Disadvantage Thermal deicing salts Use impermeable concrete. Increases passivity around reinforcement. Refer to Chapter 6, Corrosion Prevention of Steel Bridges. High initial shrinkage, which can result in cracking. Use corrosion-resistant reinforcement. Eliminates deck spalls, delaminations, and cracking from reinforcement corrosion. High cost. Limited availability. Some performance issues as noted in Chapter 3, Materials. Use waterproof membranes or overlays. Minimizes intrusion of dissolved chlorides into deck. Easily rehabilitated. Requires rehabilitation to replace riding surface every 5 to 20 years. Use external protection methods, such as cathodic protection. Reduces corrosion. Refer to Chapter 5, Corrosion of Steel in Reinforced Concrete Bridges. High cost. Requires extensive maintenance and anode and battery replacement. Could have limited effectiveness. Use effective drainage to keep surfaces dry and minimize ponding. Minimizes intrusion of dissolved chlorides into deck. Requires maintenance of drainage. Use periodic pressure washing to remove contaminants. Minimizes intrusion of dissolved chlorides into the deck. Low cost. Requires dedicated maintenance staff and appropriate budget. Use nonchloride-based deicing solution. Eliminates corrosion from chlorides. High cost. Freezeâthaw Refer to Chapter 3, Materials, for strategies relating to freezeâthaw deterioration.
217 Chapter 4. BRiDGE DECKS 220.127.116.11 Concrete Mix Design The impermeability of concrete enhances the protection of bridge-deck reinforcement. Concrete mix design is addressed earlier in this section and is further discussed in Chapter 3. Enhanced service life of bridge decks can be achieved by implementing a mix design to obtain desirable properties for mitigating the potential for deficiencies. The desirable properties for enhanced performance include crack resistance through improved tension capacity, low permeability to delay chloride intrusion, low modulus tABLE 4.6. mitigAting StrAtegieS For coAStAL cLimAte environment deteriorAtion: nAturAL or humAn-cAuSed Service Life Issue Mitigating Strategy Advantage Disadvantage Salt spray Use impermeable concrete. Increases passivity around reinforcement. Refer to Chapter 3, Materials. Higher cost. Not effective at transverse cracking locations. Use corrosion-resistant reinforcement. Eliminates deck spalls, delaminations, and cracking from reinforcement corrosion. Refer to Chapter 3, Materials. High cost. Limited availability. Some performance issues as noted in Chapter 3, Materials. Use waterproof membranes or overlays on travel surfaces of bridge deck. Minimizes intrusion of dissolved chlorides into the deck. Requires rehabilitation every 5 to 20 years. Use external protection methods, such as cathodic protection. Reduces corrosion. Refer to Chapter 5, Corrosion of Steel in Reinforced Concrete Bridges. High cost. Requires extensive maintenance and anode and battery replacement. Use sealers on nontravel surfaces of bridge deck. Minimizes intrusion of dissolved chlorides into deck. Requires rehabilitation every 5 to 10 years. Use corrosion-resistant stay-in- place forms on bottom of bridge deck. Minimizes intrusion of dissolved chlorides into deck. Difficult to inspect. Use effective drainage to keep surface dry. Minimizes intrusion of dissolved chlorides into deck. Requires maintenance of drainage and periodic cleaning. Use periodic pressure washing to remove contaminants. Minimizes intrusion of dissolved chlorides into deck. Requires dedicated maintenance staff and appropriate budget. Humidity Use materials that are not sensitive to moisture content. Refer to Chapter 3, Materials.
218 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE of elasticity to allow deck strain with lower tension force, and high creep to allow reduced locked-in stresses over time. Bridge-deck concrete can also be enhanced by incorporating proper materials and admixtures: â¢ Proper cement selection. In areas where sulfate attack may be a concern, Type II or Type V cements may be used to provide added resistance to its detrimental effects. Heat of hydration, which adds to the differential shrinkage strain, may be reduced by using a Type IV cement. â¢ Proper aggregate selection. Some readily available aggregates may be reactive to the internal concrete chemistry and be more susceptible to ASR and ACR. Ad- mixtures and/or proper blending with nonreactive aggregate can minimize these effects. Using high-quality aggregates also enhances abrasion resistance. â¢ Proper air entrainment agent. Air entrainment increases workability in the field and also enhances concrete performance when concrete is subjected to freezeâ thaw cycles. â¢ Proper admixture selection. Numerous admixtures are available to enhance the properties of concrete and improve concrete durability substantially. In particular, admixtures for concrete decks can inhibit corrosion, improve workability (pro- viding a proper durable concrete finish), delay initial set to provide time for con- crete placement and finishing, and reduce water requirements to improve concrete strength and density. Some of these enhancements may conflict, and therefore a mix design must incor- porate desired features with the understanding that all enhancement strategies cannot be achieved. For more information on mix designs, refer to Chapter 3. tABLE 4.7. mitigAting StrAtegieS For environmentAL deteriorAtion by chemicAL cLimAte, reActive ingredient, And Fire: nAturAL or humAn-cAuSed Service Life Issue Mitigating Strategy Advantage Disadvantage Corrosion-inducing chemicals and sulfate Use materials and mix designs that are not sensitive to chemical attack. Refer to Chapter 3, Materials. Reactive Ingredients: ASR and ACR Use materials and mix designs that are not sensitive to aggregate reactivity. Refer to Chapter 3, Materials. Extreme events: fire Incorporate fire rating. The height of the concrete structure and concrete cover can protect reinforcement from softening significantly, lessening collapse risk. Can result in high cost. Increases weight with increased fire rating. Provide fire-protective coatings. Increases fire rating, reduces collapse risk. Aesthetically unappealing. Subject to deterioration in an exposed environment.
219 Chapter 4. BRiDGE DECKS 18.104.22.168 Reinforcement Selection The selection of bridge-deck reinforcement can enhance the service life of the bridge deck and increase resistance from corrosion and section loss, particularly in marine en- vironments or in areas where deicing salts are used. Enhanced reinforcing steel includes â¢ Corrosion-resistant reinforcing, such as FRP, stainless steel, and titanium bars; â¢ Reinforcement protection systems, such as epoxy coating and galvanizing; and â¢ Multiple posttensioning protection strategies, such as those defined by FHWA in the Post-Tensioning Tendon Installation and Grouting Manual (Corven and Moreton 2004). Refer to Chapter 3 for more information on reinforcing materials. 22.214.171.124 Bridge-Deck Drainage Eliminating prolonged exposure to moisture and allowing the bridge deck to be main- tained in a dry condition can enhance the performance of bridge decks. Proper deck slopes, both transversely and longitudinally, should be provided to channel water to appropriate collection points. Construction joints at these collection points, as shown on the left in Figure 4.17, should be eliminated or moved away from the collection point, as shown on the right in Figure 4.17, to minimize contaminant intrusion, which can lead to the deterioration of reinforcement. Bridge drains, drain grates, and piping should be sized appropriately to be self-flushing, minimizing maintenance requirements. 126.96.36.199 Application of Compression to Relieve Tension The elimination of tension in a CIP bridge deck enhances the performance of the bridge deck by eliminating or significantly reducing deck cracking. Compression is typically introduced by posttensioning the concrete; however, the durability of the Figure 4.17. Construction joint at barrierâbridge deck interface.
220 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE posttensioning is contingent on the proper incorporation of durability enhancements, as well. Refer to Chapter 3 for additional information on the durability concerns of posttensioned systems. Compression can also be introduced into the bridge deck of continuous girder bridge systems through the use of a self-stressing bridge-deck system developed by SHRP 2 Project R19A (da Silva 2011). The compression in this system, typically appli- cable to a two-span steel girder bridge unit, is introduced by casting the bridge deck with the intermediate support higher than required. After the deck has cured, the inter- mediate support is lowered to its final position, thereby introducing compression into the area of the deck usually subjected to tension forces from negative moments. This system requires additional research to establish a history of satisfactory performance. Refer to Appendix A for more information on this system. 188.8.131.52 Membranes Membranes are placed on top of the concrete and are protected by an asphalt layer that also functions as a riding surface. Effective waterproofing enhances the service life of the membrane system, and in turn, the bridge deck. 184.108.40.206 Overlays The purpose of concrete overlays is to create a low-permeability protective layer over the conventional concrete on bridge decks. An overlay serves as a barrier to chloride ions and thus increases the time required for the concentration of the ions at the level of the reinforcement to reach the threshold for corrosion. Low-permeability overlays also decrease water penetration into a structure, allowing it to dry out, which reduces chlo- ride ion mobility. Overlays can be applied to new decks or as a rehabilitation method to existing decks. However, overlays are not as effective when applied to existing decks because if chloride ions are already present in the deck when the overlay is placed, then the only protection that the overlay can offer is a decrease in moisture infiltration. The most common type of overlay has been a low-slump dense concrete overlay, which has been effective in extending the service life of damaged bridge decks in some states. Special equipment is required to handle the very stiff concrete; special atten- tion to placement and consolidation are needed; and the overlays are prone to rapid loss of moisture, necessitating extra care in curing. Recently, silicaâfume concrete and latex-modified concrete overlays have been successfully used in extending the service life of contaminated structures; these concretes have improved workability compared with low-slump concrete overlays. Polymer concrete overlays are also available, gen- erally as a temporary repair method on damaged bridge decks. Refer to Chapter 3, Materials, for more information on overlays. 220.127.116.11 Sealers Sealers are expected to minimize the intrusion of aggressive solutions into concrete. The primary purpose of sealers is to prevent water and chloride ions from penetrating the con- crete and thereby reduce the corrosion of reinforcement or the deterioration of concrete.
221 Chapter 4. BRiDGE DECKS An important property of a sealer is its vapor transmission characteristics. Mois- ture within the concrete needs to pass through the sealer and escape in order to prevent high vapor pressures from building up in the concrete during drying periods, which could cause the sealer to blister and peel. Sealers can be either pore blockers or water repellents. Pore-blocker sealers work by forming a microscopically thin (up to 2 mm) impermeable layer on the concrete surface. Most pore-blocker sealers are not appropriate for use on bridge decks because they do not offer good skid resistance and do not hold up under traffic wear. Water- repellent sealers, in contrast, work by penetrating slightly into the concrete and act- ing as hydrophobic agents. Hydrophobic sealers for bridge decks include silanes and siloxanes. Sealers can protect all of the exposed concrete surfaces of the structure, including bridge decks, superstructure members, substructure members, and deck undersides. Proper surface preparation and consideration of application rates are key factors to be considered during installation of the sealer. Abrasion, sunlight, and the environ- ment can affect the effective life of sealers, and resealing of the bridge deck could be expected every 2 to 5 years. 18.104.22.168 External Protection Systems Several external protection systems are available to enhance the service life of CIP bridge decks, including electrochemical chloride extraction and cathodic protection systems. â¢ Electrochemical chloride extraction involves the application of a direct current to an existing bridge deck for a 4- to 8-week period. Electrochemical chloride extrac- tion extracts chlorides from concrete and enhances the passivated zone around the reinforcement. An anode is provided by a titanium mesh or steel anode, which is temporarily placed on the concrete cover. This removes an average of 40% to 90% of the initial free chlorides. The extraction depends on the depth and location of the reinforcement. The pH of the concrete is increased, and the remaining chloride contents are typically below threshold levels near the reinforcement and increase with distance from the rebar. Prior application of this technology has resulted in more than 20 years of a passive noncorroding condition. Typically applied to ex- isting bridge decks, this system also has the potential to pretreat a new concrete bridge deck, enhancing the passivated zone around the reinforcement. â¢ Cathodic protection is used to prevent corrosion from initiating, thereby reducing the concentration of chloride ions. This method is mainly used to prevent further corrosion after repair of damaged structures, and it has recently been used to prevent corrosion from initiating in new structures. The most common impressed- current anode uses a titanium mesh anode in conjunction with a concrete overlay or titanium ribbon. The current must be uniformly distributed, and the system must be regularly monitored and inspected to ensure that polarization is in the desired range.
222 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 4.4.4 Strategies to improve Production and operations Improving the performance of bridge decks relies on following proper methods and procedures during construction. The strategies to improve production and operations are included in Table 4.8. The methods and procedures used to mitigate production and operation defects include proper construction joint selection, proper choice of formwork and bar supports, specification of enhanced placement procedures, and other maintenance considerations. 22.214.171.124 Construction Joint Selection 126.96.36.199.1 Sequence of Deck Casting Bridge-deck performance can be enhanced through proper selection of the deck- casting sequence. Concrete mix designs that delay initial set until the weight and vibration of casting and finishing machinery have passed reduce concrete cracking potential from movement of the supporting structure below. tABLE 4.8. StrAtegieS For mitigAting Production And oPerAtion deFectS Service Life Issue Mitigating Strategy Advantage Disadvantage Design philosophy Use empirical design. Uses less reinforcement. Limited application for future bridge widening. Expansion joints Eliminate expansion joints. See Chapter 8, Jointless Bridges. Construction joints Minimize construction joints. Reduces corrosion potential. Minimal cost. Requires larger casting volumes. Locate joint away from areas of ponding. Minimizes saturation at joint location. Reduces corrosion potential. Minimal cost. Could require additional formwork. Phased construction Use minimum-width closure pour with UHPC. Minimizes cracking. High cost. Difficult to finish. Tight tolerances. Use wider closure pour, conventional concrete with waterproofing, and overlay. Minimizes water intrusion and closure pour cracks. Shortens construction time. Accommodates residual differential deflection. Slower construction, more cracking from differential shrinkage and restraint. Allow sufficient time for creep deformation to stabilize before casting closure pour. Minimizes differential elevation between adjacent bridge sections. More construction time and higher cost. Note: UHPC = ultrahigh-performance concrete.
223 Chapter 4. BRiDGE DECKS If the bridge deck cannot be cast in one operation, the appropriate location of con- struction joints and the proper sequencing of the deck pour can improve performance. Performance is enhanced by delaying the casting of those sections of the bridge suscep- tible to tension from adjacent casting operations. This is typical of casting sequences for continuous steel girders in which the positive moment areas are cast first, followed by the negative moment areas. The casting sequence should be specified by the designer and noted on the design plans. 188.8.131.52.2 Adjacent Members Proper sealing (making the construction joint waterproof and preventing water intru- sion between the bridge-deck elements) enhances construction joints between adja- cent CIP members, such as transverse construction joints in CIP segmental structures. Water proofing strategies are addressed in the FHWA Post-Tensioning Tendon Installa- tion and Grouting Manual (Corven and Moreton 2004) and in Chapter 3, Materials. Numerous details have been used by many states for these types of structures. Many details transfer only the vertical shear across the construction joint, causing the adjacent members to act together vertically, but allowing the bridge system to flex at these joints. This design at times has resulted in a breakdown of the material in the field construction joint. In general, details in which the connection between these adjacent members is designed to transmit the applied vertical shears and transverse moments have performed significantly better over time. Enhanced service life of filled construction joints between adjacent CIP mem- bers can be achieved through the use of ultrahigh-performance concrete. UHPC, which is described in Chapter 3, provides high strength and stiffness with negligible permeability and improved durability. It is expected to reduce maintenance require- ments and extend service life. When UHPC is used in bridge-deck construction joints, consideration should be given to two issues: grinding the surface due to the higher strength of UHPC and dissimilarities in color. 184.108.40.206.3 Staged or Phased Construction Construction joints at the interface between adjacent phases of construction, such as in bridge widening, can be enhanced by a combination of the following: â¢ Properly locating the construction joint away from areas in which water and water borne contaminants can collect, such as at the construction joint between traffic railing barriers and the bridge deck, as shown in Figure 4.17; â¢ Ensuring proper reinforcement through the construction joint to control cracking; â¢ Applying epoxy to bond the surfaces together to prevent water intrusion, such as at the construction joint between traffic railing barriers and the bridge deck, as shown in Figure 4.17; â¢ Limiting live-load influence near the joint to prevent vibration and joint flexing until concrete has attained the appropriate resistance to tension;
224 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE â¢ Using admixtures in the concrete design to increase the time to initial set until all construction activities affecting the deflection of adjacent supporting members have been completed; â¢ Delaying the casting of the deck between adjacent phases of construction by add- ing a closure pour to be completed after the casting of the deck on the supporting members for both phases; and â¢ Addressing differential shrinkage between the phase-constructed closure pours and the adjacent completed bridge phases using procedures identified in Section 220.127.116.11. 18.104.22.168 Formwork Formwork for bridge decks can be either removable or stay-in-place. Removable forms can be made of various materials, but usually consist of some type of plywood. Stay- in-place forms can consist of steel panels supporting a full-depth CIP deck or precast panels that can be either composite or noncomposite with the CIP deck above. The use of improved formwork technologies can improve the quality of the con- crete surface, increasing its impermeability. Controlled permeability formwork is a special class of lined formwork that increases the strength and durability of the con- crete surface (Malone 1999). The formwork liner allows trapped air and excess water to pass through during concrete placement and consolidation. The result is a surface free of voids (bug holes), which increases the strength and durability of the surface. CIP concrete with metal stay-in-place forms has gained popularity nationwide. However, several states are reluctant to adopt it because the underside cannot be easily inspected. The steel forms are susceptible to corrosion from salt spray and should be limited to areas where this type of corrosion is not an issue. 22.214.171.124 Bar Supports Bar supports typically rest on concrete surfaces that are exposed to natural and man- made environmental hazards. Enhanced service life can be achieved through the use of noncorroding materials or noncorroding coatings on chair legs. 126.96.36.199 Bridge-Deck Construction Procedures Concrete placement procedures are fairly well established. Improvements in concrete mix design, placement, and curing specifications appear to have adequately addressed many of these service life issues, except for cracking and corrosion of reinforcement. Service life is enhanced through proper planning of the bridge-deckâcasting pro- cess. Concrete placement is enhanced by ensuring that sufficient vibration equipment is used; that vibration is effective at areas of congestion, such as at expansion joints; and that concrete is not dropped from excessive heights. Curing is among the most important factors in developing durable deck concrete (Darwin et al. 2010) and is essential for the continuation of hydration reactions and the control of cracking due to volumetric changes. Curing of concrete is enhanced by ensuring that moisture is not lost, which can be accomplished by using curing
225 Chapter 4. BRiDGE DECKS compounds and maintaining a wet curing environment, such as under a moist burlap covering, for 7 to 10 days. Performance enhancements are maximized with longer wet cure periods. The wet curing process also helps maintain thermal control of the bridge deck in its critical early stages of hydration. Refer to Chapter 3 on materials for more information on concrete curing. 188.8.131.52 Maintenance Considerations for Existing Bridge Decks To extend concrete bridge service life in existing structures, preventive maintenance should be emphasized in a maintenance plan, and proper repairs should be performed before extensive damage occurs and costly rehabilitation is required. The scope of re- pair or rehabilitation work can vary significantly, from sealing cracks to applying over- lays to replacing large components such as bridge decks. Preventive maintenance may also include tasks as simple as washing the structure to eliminate chloride buildup. 4.5 overALL StrAtegieS For enhAnced bridge-deck Service LiFe This section provides tools for selecting the most appropriate individual strategy to achieve the desired bridge-deck service life. It provides a template to the designer for selecting the optimum solution available and quantitatively predicting the service life, when applicable. The flowchart in Figure 4.18 shows the bridge-deck system component selection process. The flowchart in Figure 4.19 shows the service life factor mitigation process. An identifying number in each step designates each activity within the flowchart. These identifying numbers are used in the following discussion of the various elements of the flowcharts. Steps 1a and 1b. Development of Design Criteria The activities in Steps 1a and 1b of Figure 4.18 identify the projectâs local operational site requirements and the local factors affecting service life. They are crucial to the development of design criteria used to identify and evaluate bridge-deck alternatives. Examples of the information to be gathered during these activities are provided in Table 4.9. Step 2. identification of feasible Bridge-Deck Systems The next step in the process is the selection of various bridge-deckâtype alternatives that can meet the project requirements and the design provisions stated in the LRFD specifications. For example, CIP and precast deck systems could be identified as poten- tial alternatives. Table 4.1 in Section 4.2 provides the advantages and disadvantages of these various decks. The selection of potential deck alternatives should consider the requirements of other bridge subsystems, components, and elements and their interac- tion. Refer to Chapter 2 for more information on overall bridge system requirements. The most prominently used bridge-deck system is the CIP or precast concrete bridge deck. This bridge deck can either be self-supporting as part of the overall super- structure system, (e.g., voided slabs and segmental structures) or supported on beam or stringer superstructure elements. All these concrete deck systems are subject to cracking, which typically results in reduced life from corrosion.
226 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 1b. Identify local factors aï¬ecting service life. 2. Identify feasible deck alternatives satisfying design provisions of LRFD speciï¬cations, operational, site, and bridge system requirements. 1a. Identify local operational and site requirements. 3. For each alternative, identify factors aï¬ecting service life by following fault tree. Go to A Bridge-Deck System Component Selection Process Yes No 6. Identify maintenance requirements. 5a. Identify rehabilitation or replacement requirements. Yes 7. Develop life-cycle costs. No 5. Is deck service life greater than or equal to the system TDSL? 8. Additional deck alternative? 9. Compare alternatives and select deck system. B 8a. Go to the next alternative. 4. Figure 4.18. Bridge-deck system component selection process. For steps from A to B, see Figure 4.19.(TDSL means target design service life.)
227 Chapter 4. BRiDGE DECKS Selection of the overall concrete bridge-deck system may be affected by the follow- ing factors: â¢ Need for accelerated construction to shorten overall user impacts; â¢ Maintenance of traffic requirements that may dictate construction staging; â¢ Commitments made during the NEPA process, such as acceptable noise levels, access limitations, or environmental and biological limitations that may dictate a precast system; â¢ Availability of special mix designs to provide a more durable concrete; â¢ Availability and construction expertise to incorporate prestressing to compress the concrete, minimizing or eliminating tension in the concrete; Figure 1.18. Flowchart to identify factors affecting service life (Figure 4.19). 2A.b. Modify bridge deck conï¬guration. A 2A.a. Identify consequences and determine appropriate strategies for avoidance or mitigation. 1A. Identify individual factors aï¬ecting service life considering each branch of fault tree. 3A.a. Go to the next factor. 4A.Modiï¬ed bridge deck conï¬guration for deck alternative under consideration. Go To B Yes No 2A. Does factor apply? Yes No 3A. Are all factors considered? 4. Figure 4.19. Mitigation of factors affecting service life process.
228 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE â¢ Availability of and the construction expertise to incorporate internal and/or ancil- lary protective systems for the bridge deck; and â¢ Ability to provide an alignment and/or a bridge drainage system to prevent pond- ing of water and soluble pollutants on the bridge deck. Step 3. identification of factors Affecting Service Life After feasible bridge-deck alternatives are selected and designed based on applicable provisions in the LRFD specifications (Step 2 in Figure 4.18), the next step is to use fault tree analysis to identify the factors affecting service life for each feasible deck alternative (Step 3 in Figure 4.18). The fault tree is described in Section 4.3. tABLE 4.9. identiFy bridge-deck demAndS: LocAL oPerAtionAL, Site, And Service LiFe reQuirementS Demand Examples Identify owner requirements Legal and permit loads Commitments made during the NEPA process Noise Access limitations Environmental and biological Other design directives Acceptable risk Bridge-deck target design service life Contingency planning for future expansion Identify traffic load demands Potential for overloads Construction loads Impact considerations from suspension systems Frequency of load application Tire type for wear and abrasion Identify system-dependent demands Coordinate with bridge system selection Differential shrinkage effects Boundary conditions for system-framing restraint Thermal exposure Identify natural and human-made hazard demands Deicing requirements Freezeâthaw potential Local aggregate reactivity Susceptibility to fire Susceptibility to collision Chloride concentrations (natural) Chloride concentrations (applied) Sulfate concentrations Humidity levels Identify other general demands Traffic maintenance requirements Construction phasing requirements Need for accelerated construction Drainage and storm water requirements Identify local construction practice expertise Note: NEPA = National Environmental Policy Act.
229 Chapter 4. BRiDGE DECKS After each feasible bridge-deck alternative has undergone fault tree analysis to identify all possible factors that may affect service life, the procedure continues with Process A, which is developed in Figure 4.19 and described next. Process A. Refinement of Alternatives: mitigation of factors Affecting Service Life Process A (Figure 4.19) develops mitigating design features for each factor affecting service life identified under Step 3. An alternative selection process would be to evalu- ate these strategies by examining material selection and protection strategies, construc- tion practice specification requirements, and maintenance requirements. A summary example of this alternative process is provided in Table 4.10. Further explanation of some for these elements is provided tABLE 4.10. ALternAtive bridge-deck SyStem deveLoPment ProceSS Demand Examples Determine function Strength design Accommodate span requirements and constraints Accommodate curvature and skew requirements Accommodate proposed deck joint layout and its effect on system restraint Identify deck deterioration modes Concrete cracking Reinforcement corrosion Wear and abrasion Aggregate reactivity Freezeâthaw Assess risk and deterioration consequences Loss of required strength Safety concerns from pot holing (large concrete spalls) Loss of skid resistance Develop mitigation strategies (Section 4.4) Traffic-induced loads System-dependent loads Natural, man-made, or environmental deterioration Develop design requirements Concrete cover Concrete mix design Reinforcement selection Bridge-deck drainage Introduction of compression to relieve tension Application of membrane, overlay, and sealers External protection systems Identify strategies to improve construction Sequence of deck casting Construction joint location and detailing Construction phasing and staging details Formwork selection Bar support selection Specification of construction procedures Quantification of maintenance requirements
230 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Process A includes the following steps: Step 1A Identify the individual factors affecting service life by considering each branch of the fault tree defined in Section 4.3. Step 2A For each identified factor, use the design criteria to evaluate whether the factor has an effect on the service life of the bridge deck. If the factor identified in Step 2A has an effect on the service life of the bridge deck, proceed to Steps 2A.a and 2A.b. Step 2A.a Identify the consequences of the factor and determine appropriate strategies to miti- gate or avoid the effects of deterioration. Refer to Section 4.4 for mitigation and avoid- ance strategies. There may be more than one strategy alternative to consider for each factor. For example, bridge decks subjected to wear and abrasion can be mitigated through concrete mix design alternatives and/or the use of waterproofing membranes and overlays. Note that the applicable factors identified in the circle symbols are basic factors that demand development of strategies to mitigate them. Step 2A.b Modify the bridge-deck alternative under consideration as needed to address the incor- poration of the chosen strategy or strategies. Steps 3A and 3A.a Continue with the evaluation process until every factor has been considered that may affect the service life for each feasible bridge alternative. This iterative process contin- ues (Step 3A.a) to ensure that all factors are considered. Step 4A Step 4A, the last step in Process A, is to finalize the modifications to the bridge-deck configuration for the bridge-deck alternative under consideration. Verify that strength and service performance have not been affected. A sample series of strategies for a bridge with a CIP concrete deck supported on stringers is shown in the next three tables. Table 4.11 provides strategies for CIP bridge-deck systems, and Table 4.12 provides strategies for precast bridge-deck sys- tems. The identified systems can achieve long service life with the proper inspection and maintenance as indicated. Table 4.13 provides strategies for rehabilitation of existing bridge decks, which may be used in the case of a bridge widening. Note that deteriora- tion modes addressed through proper design, such as fatigue and overload potential, are not addressed with a strategy in these tables. Other material deterioration modes, such as freezeâthaw and sulfate attack, are addressed in Chapter 3. The selection of appropriate material and protection strategies is fairly consistent for all of the concrete bridge-deck systems included in this chapter, with only minor differences in the durability performance selection process between CIP and precast deck systems. The appropriate strategies for each potential deterioration mode must be compared for conflicts in order to establish the overall strategy to be deployed for a specific project. For example, a bridge deck with the potential for deterioration from
231 Chapter 4. BRiDGE DECKS wear and abrasion and differential shrinkage cannot easily use concrete with both high strength and a low modulus. In this case, the wear and abrasion strategy using an overlay or membrane is more appropriate. Process B. identification of Rehabilitation, maintenance, and Life- Cycle Costs for System Alternatives and final System Selection Step 5. Check Service Life At this step in the process, the service life for the bridge-deck alternative is determined either through the use of deterioration models or through empirical evidence based on past performance. Refer to Chapter 1 for methods available for predicting service life. If the bridge-deck service life does not equal or exceed the bridge system design service life, then rehabilitation and/or replacement requirements (Step 5a) should be added to increase the longevity of the bridge. Step 6. Identify Maintenance Requirements The next step is to identify maintenance requirements for the proposed alternative and their associated costs. Table 4.14 identifies example maintenance issues. The cost for developing a maintenance plan should also be included in the bridge Ownerâs Manual. Refer to Chapter 1 for a detailed description of the bridge Ownerâs Manual. tABLE 4.11. bridge-deck SeLection StrAtegieS: ciP SyStemS Potential Deterioration Mode Material Selection and Protective Measures Selection Maintenance Mode Life-Cycle Costs Initial Long-Term Differential shrinkage and thermal restraint Low-modulus concrete Proper bearing design None Low Low Wear and abrasion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium Medium Wear and abrasion High concrete strength Hard aggregates None Medium Low Wear and abrasion Sacrificial thickness and overlay Continual overlay replacement 5 to 20 years Low Medium Reinforcement corrosion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium Medium Reinforcement corrosion Corrosion-resistant rebar None High Low Reinforcement corrosion External protection systems Continual inspection and system maintenance High High ASR Refer to Chapter 3, Materials na na na ASR Blended aggregates, proper drainage None Medium Low ASR Blended aggregates, waterproof membrane, proper drainage Continual overlay replacement 5 to 20 years Medium Medium ACR Refer to Chapter 3, Materials na na na Note: na = not applicable.
232 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 4.12. bridge-deck SeLection StrAtegieS: PrecASt SyStemS Potential Deterioration Mode Material Selection and Protective Measures Selection Maintenance Mode Life-Cycle Costs Initial Long-Term Differential shrinkage and thermal restraint Low-modulus concrete, proper bearing design None Low Low Wear and abrasion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium Medium Wear and abrasion High concrete strength, hard aggregates None Low Low Wear and abrasion Sacrificial thickness and overlay Continual overlay replacement 5 to 20 years Low Medium Reinforcement corrosion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium High Reinforcement corrosion Corrosion-resistant rebar None High Low Reinforcement corrosion External protection systems Continual inspection and system maintenance High High Reinforcement corrosion Application of compression by design to eliminate tension None Medium Low Reinforcement corrosion Application of compression through posttensioning Continual inspection, supplemental posttensioning Medium Medium ASR Refer to Chapter 3 for material componentâbased solutions na na na ASR Blended aggregates, proper drainage None Medium Low ASR Blended aggregates, waterproof membrane, proper drainage Continual overlay replacement 5 to 20 years Medium Medium ACR Refer to Chapter 3 for material componentâbased solutions na na na Note: na = not applicable. tABLE 4.13. bridge-deck SeLection StrAtegieS: exiSting bridge deckS Potential Deterioration Mode Material Selection and Protective Measures Selection Maintenance Mode Life-Cycle Costs Rehabilitation Long-Term Wear and abrasion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium Medium Reinforcement corrosion Overlay and/or membrane Continual overlay replacement 5 to 20 years Medium High Reinforcement corrosion External protection systems Continual inspection and system maintenance High High Reinforcement corrosion Epoxy injection of deck cracks Continual inspection and system maintenance Medium Medium ASR Waterproof membrane, proper drainage Continual overlay replacement 5 to 20 years Medium Medium
233 Chapter 4. BRiDGE DECKS tABLE 4.14. mAintenAnce reQuirementS Demand Examples Maintenance issues Inspection requirements and intervals Drainage system maintenance Membrane, overlay, and sealer maintenance Expansion joint maintenance Health monitoring Scheduled maintenance Examples of items to be included in bridge Ownerâs Manual Describe how bridge was designed, constructed, and intended to function from an operational perspective. Include the following: â¢ Design loads â¢ Expected movements at expansion joints â¢ Relevant as-built data, including, but not limited to, the following: â Concrete mix design â Slump test results â Chemical content of materials â Curing methods used â Compression cylinder test results â Reinforcing-steel material certifications â Coating tests on reinforcement â Formwork materials â Actual construction procedures â Temperature of concrete â Ambient temperature and time of casting â Timing of casting sequence and concrete delivery â Concrete cover measurements For each component, describe what is needed to achieve design service life for specific elements. Include the following: â¢ Required maintenance â¢ Expected rehabilitation and/or replacement of bridge elements with service life less than overall bridge system design service life â¢ Areas for inspection and types of adverse behavior to watch for Step 7. Life-Cycle Cost Analysis In Step 7 the bridge-deck alternative is evaluated to establish its life-cycle cost. Life- cycle cost analysis assesses the overall long-term cost of a strategy throughout the target service life of the structure. See Chapter 11, Life-Cycle Cost Analysis. Step 8. Consideration for Additional Bridge-Deck Alternatives Once the life-cycle cost is established, the bridge deck alternative under investigation is complete and the process resumes at Step 3 until all feasible bridge-deck alternatives are addressed.
234 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Step 9. Alternatives Comparison and Deck System Selection The final step in the process is the comparison of bridge-deck system alternatives. The life-cycle cost of these alternatives is combined with the analysis performed for each bridge subsystem, component, and element for a specific bridge system. Refer to Chapter 2 for the overall bridge system selection process. In general, the most cost- effective bridge-deck system should be chosen; however, the cost-effectiveness has to be weighed against the requirements of the overall bridge system. The selection should also be presented to the bridge owner for acceptance.