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68 ConClusions Concrete cracking in bridges is complex and unlikely to be caused by a single effect. In some situ- ations, cracking cannot be avoided. Nevertheless, crack quantities and widths can be minimized by the careful selection of materials, proper reinforcement design details, and appropriate construction practices. Full-Depth, Cast-in-Place Concrete Bridge Decks It is the consensus of bridge industry personnel that the primary causes of bridge deck cracking are drying shrinkage of the deck concrete and temperature differences between the deck concrete and the supporting beams. However, the industry does not have a good record for consistently, reliably, and predictably producing concrete structures with minimal or no concrete cracking. The practices described here have been identified previously as ways to reduce and control cracking, yet cracking in bridge decks continues to be a major concern. Perhaps the real need is proper implementation of these practices. Practices recommended for reducing shrinkage and associated shrinkage cracking in cast-in- place (CIP) concrete bridge decks are as follows: ⢠Using a w/cm ratio in the range of 0.40 to 0.45. ⢠Using a cement content not greater than 650 lb/yd3. ⢠Using the lowest quantity of water and cement paste in the concrete consistent with achieving other required properties. ⢠Using the largest practical maximum size coarse aggregate and maximum coarse aggregate content to reduce the water demand. ⢠Using an optimum combined aggregate gradation. ⢠Avoiding actual concrete compressive strengths greater than 6.0 ksi. Practices recommended for reducing cracking from temperature differences include the following: ⢠Minimizing the temperature difference between the CIP concrete deck and the supporting steel or concrete beams. ⢠Specifying and ensuring minimum and maximum concrete temperatures at time of placement of 55°F and 75°F, respectively. ⢠Minimizing cement content. ⢠Using a Type II cement. ⢠Using aggregates with low modulus of elasticity, low coefficient of thermal expansion, and high thermal conductivity. Recommended design practices that can reduce the likelihood of cracking and control crack widths include the following: ⢠Specifying the lowest acceptable concrete compressive strength. ⢠Specifying a minimum shrinkage of 300 to 350 millionths after 28 days of drying when tested in accordance with AASHTO T 160. chapter eight ConClusions anD suggestions For Future researCh
69 ⢠Specifying a placement sequence that will minimize tensile stresses in previously placed concrete. ⢠Using the minimum bar sizes and spacings to control crack widths. Recommended construction practices that can reduce the likelihood of deck cracking include the following: ⢠Where practical, using a placement sequence that will minimize tensile stresses in previously placed fresh concrete. ⢠Using windbreaks and fogging equipment, when necessary, to minimize surface evaporation from fresh concrete. ⢠In hot or low humidity conditions, placing concrete at night. ⢠Applying wet curing procedures immediately after concrete finishing and maintaining the surface wet for at least 7 days. ⢠Applying a curing compound after the wet curing period to slow shrinkage and enhance the concrete properties. Other practices that some state agencies reported to have been effective include the following: ⢠Using internal curing. ⢠Using an SRA. ⢠Including SCMs in the concrete. ⢠Using a shrinkage-compensating concrete. Overall, no single practice can be used to enhance concrete bridge deck performance. When the structural system of the bridge includes skewed supports, diagonal cracks are likely to occur near the supports. When the structural system of the bridge includes continuity over the sup- ports, negative moment transverse cracks are likely to occur. When construction joints are present, cracks are likely to occur at the joints. To control crack widths and spacing in these situations, using the minimum bar sizes and minimum bar spacings that are practical is recommended. Partial-Depth, Precast Concrete Panels with a Cast-in-Place topping The main concern with the use of partial-depth, precast concrete panels with a CIP topping is the reflective cracking that occurs in the topping above the edges of the panels. This type of cracking may be reduced by saturating the surface of the panel before casting the topping, special joint detailing, and delaying erection of the panels until most of the creep and shrinkage have occurred. Crack widths can be controlled by the reinforcement in the topping. Nevertheless, research is needed to clearly determine the primary factors causing reflective cracking and identify ways to reduce or eliminate it. Full-Depth, Precast Concrete Panels Full-depth panels generally are pretensioned in the transverse direction of the bridge and may be posttensioned in the longitudinal direction. Although the use of two-directional prestressing offers the potential to produce a crack-free bridge deck, the results from the survey indicate that this is not always accomplished. Additional research is needed on this topic to provide a solution for crack control of prestressed and nonprestressed panels. adjacent Precast Concrete Box Beam Bridges and slab Beam Bridges Lateral ties are used in adjacent precast concrete box beam bridges to tie adjacent beams together. As such, the ties function to control the crack widths. The degree of restraint varies from requiring a minimum compressive stress across the longitudinal joint to providing a passive tie. NCHRP Proj- ect 12-95 is ongoing, with the objective of developing guidelines for the design and construction of
70 connection details for adjacent precast concrete box beam bridges to eliminate cracking and leakage in the longitudinal joints between adjacent boxes. Pretensioned Concrete Beams End zone cracking in prestressed concrete beams is an uncommon occurrence but can be prevented by modifying the detensioning sequence. Cracks that occur can be controlled through the use of split- ting and confinement reinforcement in the end zone region. Vertical cracking that occurs in the webs before transfer can be reduced by providing longer lengths of free strand in the bed. nonprestressed Concrete Beams Cracking in nonprestressed concrete beams is almost inevitable but is controlled by providing minimum amounts of reinforcement to control crack widths. substructures In general, cracking in substructures is an uncommon occurrence. Limiting the maximum stress in the reinforcement under service loads provides a means of crack width control. Design of deep compo- nents using the strut-and-tie method rather than the sectional design method along with the required reinforcement should provide improved crack control. It is recommended that a thermal control plan be developed to control thermal cracking in large members. effect of reinforcement type Epoxy-coated reinforcement continues to be the primary type of reinforcement used for corrosion protection, although agencies have used zinc-coated, stainless-steelâcoated, solid stainless steel, low-carbon chromium, and FRP reinforcement. The use of high-strength reinforcement leads to wider cracks unless an upper limit is placed on the allowable tensile stress in the reinforcement under service loads. Narrower crack widths result from using smaller diameter bars at a closer spacing. The AASHTO LRFD Specifications address the need to provide minimum amounts of reinforcement to ensure sectional strength. However, the LRFD articles do not encompass the full range of reinforce- ment types that are available today. effect of Cracking on long-term Performance Research has established that corrosion of conventional uncoated reinforcement will begin earlier in cracked concrete than in uncracked concrete. However, most analytical models assume uncracked concrete. The use of corrosion-resistant reinforcement prolongs the time to initiation of corrosion and thus service life. Stainless steel provides the longest life, although coated bars, such as epoxy-coated ones, can provide sufficient protection for many applications. Because the effect of cracking on service life is not well-defined, additional work is needed to be able to predict service life with cracked concrete and the range of reinforcement types available. Currently, no U.S. standards exist to predict long-term performance and service life. suggestions For Future researCh This synthesis has identified that many of the existing articles in the AASHTO LRFD Bridge Design Specifications for crack control were developed before the availability and use of certain types of reinforcement that exist today. These include steel reinforcement with a specified yield strength between 60 and 100 ksi, some types of corrosion-resistant reinforcement, and FRP reinforcement.
71 The application of the existing articles to control flexural cracking in structures with these new types of reinforcement may not be appropriate and in some cases can result in unnecessary increased costs or impractical designs. Consequently, a fresh look is needed to ensure that design provisions for flexural crack control are provided that address all types of reinforcement. Because most transverse cracks in full-depth concrete decks are caused by restrained shrinkage, there is a need to develop specifications to control shrinkage cracking. These may or may not be similar to the specifications for control of flexural cracking. Service life predictions generally are based on uncracked concrete with uncoated steel reinforce- ment. Additional work is needed to be able to predict service life with cracked concrete and the different types of reinforcement available. This synthesis has identified the need to determine the primary factors causing the reflective crack- ing in partial-depth, precast concrete panels and ways to reduce or eliminate it such cracking. Research is needed to provide a solution for crack control of prestressed and nonprestressed full-depth panels. Responses to the survey for this synthesis provided the following suggestions for future research and development programs related to concrete cracking: ⢠Coordination between designers and materials experts in identifying causes of cracking. ⢠Determination of permissible crack widths associated with different size reinforcing bars. ⢠Comparative study of deck cracking with partial-depth, precast concrete deck panels compared with conventional full-depth CIP decks and methods for reducing reflective cracking when partial-depth, precast concrete deck panels are used. ⢠Role of reinforcement layout on crack control for high-strength concrete. ⢠Use of additional reinforcement or control joints at the center of floor beams to mitigate cracking. ⢠Investigation of methods for reducing end-region cracking in pretensioned girders. ⢠Investigation of how to reduce and control vertical cracks in CIP traffic barriers on top of a concrete deck slab. ⢠Research to address shrinkage and cracking potential of concrete mixes, particularly HPC and SCC. Research to include multiple admixtures, expansive components, SRAs, SCMs, and internal curing. A research problem statement to address the control of flexural and shrinkage cracking by distribu- tion of reinforcement is provided in Appendix D. The statement incorporates several of the suggestions. Respondents also identified ongoing research at their agency. The responses are tabulated in Question 25 of Appendix B.
