Control of Concrete Cracking in Bridges (2017)

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44 Yield Strength of reinforcement It is generally accepted that crack widths in nonprestressed concrete members increase as the tensile stress in the reinforcement increases. Consequently, the use of higher strength reinforcement will lead to wider crack widths unless an upper limit is placed on the tensile stress in the reinforcement under service loads. Some agencies have chosen to do this. In doing so, the advantage of the higher strength reinforcement is reduced. Sharooz et al. (2011) collected extensive data on crack widths during flexural tests of concrete beams reinforced with ASTM A1035 Grade 100 reinforcement. They concluded that, at service load levels as great as a steel stress level of 72 ksi, average crack widths were less than the limits of 0.017 and 0.013 in. that are assumed for Class 1 and 2 exposure conditions, respectively, in Article 5.6.7 (formerly 5.7.3.4) of the AASHTO LRFD Specifications. When the AASHTO LRFD Specifications were revised to permit the use of reinforcement with a specified yield strength, fy, to 100 ksi, the calculated stress in nonprestressed reinforcement at the service limit state was limited to 0.60fy (AASHTO 2013). corroSion-reSiStant Steel reinforcement In the survey for this synthesis, agencies identified the types of corrosion-resistant steel reinforcement used in CIP concrete bridge decks. The results are shown in Table 6. Clearly, epoxy-coated reinforcement remains the predominant type of corrosion-resistant reinforce- ment used in bridge decks. All agencies that reported the use of epoxy-coated reinforcement in bridge decks reported using it in both layers of reinforcement. Agencies also reported the use of zinc coating, stainless steel coating, and solid stainless steel reinforcement and that their use did not affect deck cracking. The use of ASTM A1035 reinforcement is relatively new, and no information was reported about its effect on deck cracking. Tension tests of reinforced concrete prisms by Patnaik and Baah (2015) revealed that the concrete specimens with epoxy-coated bars developed a first crack at smaller loads and developed larger crack widths that did corresponding specimens with uncoated bars. Flexural tests of reinforced concrete slabs with epoxy-coated bars showed a first crack at smaller loads, wider cracks and a larger number of cracks, and failure at smaller loads than was seen in the corresponding test specimens with uncoated bars. To investigate a preventive measure, slab specimens with basalt MiniBar or polypropylene fibers were included in the test program. These specimens exhibited higher cracking loads, smaller crack widths, smaller midspan deflections, and higher failure loads than those seen with the slab specimens without fibers. The authors concluded that merely satisfying the reinforcement spacing requirements given in the AASHTO LRFD Specifications was not adequate to limit cracking below the maximum limits recommended by ACI 224R-01 (ACI Committee 224 2008), even though all the relevant design requirements were otherwise met. Hart and Lysogorski (2005) reported on 27 state projects that had used corrosion-resistant reinforce- ment under the FHWA Innovative Bridge Research and Construction Program. The different reinforce- ment types included solid stainless steel Types 316, 2201LDX, and 2205; stainless Type 316 clad bars; low-carbon chromium steel bars; and galvanized steel bars. The various state projects demonstrated chapter five effectS of reinforcement tYpe on crack control

45 that corrosion-resistant reinforcing bars can be used in bridge construction with relative ease. No information about the effect on cracking was reported. Salomon and Moen (2014) showed, by testing and analysis, that slab specimens reinforced with ASTM A1035 or UNS S32304 steel bars had similar deformability ratios and crack widths that com- plied with current AASHTO requirements, with as much as 36% less reinforcing steel compared with Grade 60 reinforcement. Bridge deck slabs employing high-strength reinforcement without a defined yield plateau provided ductility consistent with AASHTO ductility limits at a strength limit state. Sim (2014) tested twelve 8-in.-thick slabs reinforced with conventional uncoated bars and six dif- ferent corrosion-resistant bars to evaluate the influence of various materials on cracking. The bar types affected the spacing and width of primary cracks. For the control of crack widths, the author recom- mended that crack widths be calculated based on conventional bars and multiplied by modification factors. These factors could also be used to reduce the bar spacings calculated for conventional bars. fiber-reinforced polYmer reinforcement Fiber-reinforced polymer (FRP) reinforcement consists of a continuous fiber, such as glass, carbon, or aramid, embedded in a resin matrix, such as epoxy, polyester, vinylester, or phenolics (ACI Committee 440 2007). The advantages of this type of reinforcement are that it does not corrode as does uncoated steel reinforcement and is lighter to ship and install than is steel reinforcement. The first bridge built in the United States using FRP reinforcement in the concrete deck was in West Virginia in 1996. The bridge used glass FRP bars as deck reinforcement (Thippeswamy et al. 1998). Subsequently several other agencies used FRP in bridge decks on an experimental basis, including ones in Kentucky (Trejo et al. 2000), Michigan (Trejo et al. 2000), New Hampshire (Goodspeed et al. 2002), Ohio (Eitel and Huckelbridge 2002; Huckelbridge and Eitel 2003), Texas (Bradberry and Wallace 2003), Vermont (Benmokrane et al. 2006), Manitoba (Rizkalla et al. 1998), Québec (Tadros 2000; Benmokrane et al. 1999), and Calgary (Tadros 2000). As part of the survey for this synthesis, 17 of 42 agencies responding identified that they had used FRP reinforcement in CIP concrete bridge decks. Three agencies reported that its use was beneficial in reduc- ing cracking. Other agencies did not respond to the question or did not know if FRP had been used. The amount of FRP reinforcement often is based on control of crack widths, as discussed later in this chapter. Nawy and Neuwerth (1977) reported that beams reinforced with FRP reinforcement had more cracks than corresponding beams with steel reinforcement. The large number of well-distributed cracks in the FRP-reinforced beams indicated that good mechanical bond was developed between the FRP bar and surrounding concrete. Faza and GangaRao (1992) determined that concrete beams reinforced with spirally deformed FRP-reinforcing bars in 4.0-ksi compressive strength concrete exhibited crack formation that was Type of Reinforcement Respondents Number Percentage Epoxy-coated in top layer 33 87 Epoxy-coated in bottom layer 33 87 Epoxy-coated in both layers 33 87 Epoxy-coated reinforcement projecting into the deck from the beam 23 64 Zinc-coated 19 45 Stainless-steel–coated 13 13 Solid stainless steel 18 43 ASTM A1035 at 100 ksi 3 7 Other (type not listed) 3 3 TABLE 6 USE OF CORROSION-RESISTANT REINFORCEMENT

46 sudden and propagated toward the compression zone as soon as the concrete stress reached its ten- sile strength. Crack spacing was determined by the stirrup spacing. With higher strength concrete and sand-coated FRP reinforcing bars, the propagation of cracks and crack widths decreased. An expression for maximum crack spacing was developed. In 2000, the Texas DOT used glass fiber-reinforced polymer (GFRP) bars as the top mat of rein- forcement in the CIP topping on partial-depth, precast, prestressed concrete panels on the Sierrita de la Cruz Creek Bridge near Amarillo (Bradberry and Wallace 2003). The design was governed by serviceability considerations with the estimated crack width being the controlling parameter. The designer chose the maximum crack width of 0.02 in. recommended by the Canadian Standards Association (CSA 1996). The calculated maximum stress for this crack width for this slab was less than 15% of the guaranteed ultimate strength of the bar. This resulted in No. 6 GFRP bars with cen- ters spaced at 5½ in. With epoxy-coated reinforcement, No. 5 bars at 6-in. spacing would have been required. Thus, the use of GFRP bars required 42% more area of reinforcement and 7% closer bar spacing compared with the use of epoxy-coated reinforcement (Bradberry 2001). In 2015, nine concrete cores were extracted from different locations on the bridge for various analy- ses (Gooranorimi et al. 2016). Carbonation depth and pH of the concrete surrounding the GFRP bars were measured. Scanning electron microscopy imaging and energy dispersive X-ray spectroscopy were performed to monitor any microstructural degradation or change in the GFRP chemical compositions. Finally, GFRP interlaminar (horizontal) shear strength, glass transition temperature, and fiber content were determined and compared with the results of similar tests performed on control samples at the time of construction. Microscopic examination revealed no GFRP degradation. Fibers did not lose any cross- sectional area, the matrix was intact, and no damage was observed at the fiber-matrix interface. In addi- tion, the concrete-GFRP interface was maintained properly, and no interfacial bond loss was observed. Soroushian et al. (2001) caution that the substitution of FRP reinforcement for steel reinforcement on an equal area basis typically results in significantly higher deflections with wider crack widths. In addition, shear capacity is likely to be significantly reduced as a result of increased crack widths and reduced size of the compressive stress blocks. In Virginia, the deck of one end span of the Gills Creek Bridge was constructed with GFRP bars as the top mat and epoxy-coated steel bars as the bottom mat (Phillips et al. 2005). Live load tests were performed in 2003, shortly after completion of construction, and again in 2004. In addition, tests were performed on the deck of the opposite end span, which had all epoxy-coated steel reinforcement. There were no significant differences in the behavior of the deck after 1 year of service, and there was no visible cracking. The behavior of the two end spans was similar, and the mea- sured girder distribution factors were less than the AASHTO design recommendations. The impact factors were less-than-design values for the 2003 tests but higher-than-design values for the 2004 tests. Stresses in the GFRP-reinforcing bars were much less than the design allowable stress and did not change significantly after 1 year of service. In 2013, the Kansas DOT replaced the decks of the I-635 bridges over State Avenue in Kansas City with traditional epoxy-coated steel reinforcement in the northbound bridge and GFRP reinforcement in the southbound bridge (Koch and Karst 2015). The deck with the epoxy-coated steel reinforcement con- tained No. 5 bars at 6-in. centers, whereas No. 6 bars at 6-in. centers were used in the GFRP-reinforced deck. There was a small premium to use GFRP reinforcement over traditional steel reinforcement. This is expected to be offset by an increase in the service life of the deck. ACI Committee 440: Fiber-Reinforced Polymer Reinforcement recommends the use of the Canadian Standards Association’s limits for crack widths with FRP reinforcement (ACI Commit- tee 440 2006). The Canadian standard (CSA 2002) implicitly allows crack widths of 0.020 in. for exterior exposure. However, Committee 440 cautions that the limit may not be sufficiently restric- tive for structures exposed to aggressive environments. Committee 440 also proposes that a modified version of the Frosch equation (Frosch 1999) be used to calculate maximum probable crack width:

47 ( )( )= β +2 2 (3)2 2w fE k d scu ff s b c where wcu = maximum probable crack width for FRP reinforcement (in.); ff = stress in FRP reinforcement (ksi); Ef = modulus of elasticity of FRP reinforcing bars (ksi); βs = ratio of distance between the neutral axis and tension face to the distance between the neutral axis and the centroid of the reinforcement; kb = bond quality coefficient; dc = thickness of concrete cover measured from extreme tension fiber to center of the flexural reinforcement located closest thereto (in.); and s = spacing of nonprestressed reinforcement in the layer closest to the tension face (in.). Analysis of available data indicated that kb could vary from 0.60 to 1.72, depending on the surface char- acteristics of the bar. A value of 1.4 was recommended in instances in which the actual value is unknown. A design example for a beam illustrates that crack width criteria control the amount of FRP reinforce- ment (ACI Committee 440 2006). The use of the Frosch equation with steel reinforcement is discussed in the next chapter. Although the highly alkaline environment of concrete is beneficial in preventing corrosion of conventional uncoated steel reinforcement, its effect on FRP reinforcement may be detrimental (ACI Committee 440 2007). Tests have shown that FRP bars placed in a highly alkaline solution can lose tensile strength (Mehus 1995; ElSafty et al. 2014). Thus, it is important that FRP bars be evaluated for alkali resistance. SpecificationS for crack control This section of the synthesis contains a summary of the articles in the AASHTO LRFD Bridge Design Specifications (AASHTO 2017) that relate to the use of reinforcement to control cracking when it occurs. Some background to the articles and information about relevant research is also provided. Some of these articles originally were developed for Grade 60 steel reinforcement, and their appropriateness for use in crack control with higher strength reinforcement, corrosion-resistant reinforcement, and FRP reinforcement may not have been verified. article 5.6.3.3: minimum reinforcement Article 5.6.3.3 (formerly 5.7.3.3.2) contains provisions for minimum reinforcement intended to reduce the probability of brittle failure by providing a flexural capacity greater than the cracking moment of the member. Unless otherwise specified, at any section of a noncompression-controlled flexural component, the amount of prestressed and nonprestressed tensile reinforcement shall be adequate to develop a factored flexural resistance, Mcr, greater than or equal to the lesser of the following: • 1.33 times the factored moment required by the applicable strength load combination specified in Table 3.4.1-1; • ( )( ) ( )= g g + g − − M f f S M SScr r cpe c dnc cnc 1 5.6.3.3-1 (4)3 1 2 where Mcr = cracking moment (kip-in.); fr = modulus of rupture of concrete specified in Article 5.4.2.6 (ksi);

48 fcpe = compressive stress in concrete caused by effective prestress forces only (after allowance for all prestress losses) at extreme fiber of section where tensile stress is caused by exter- nally applied loads (ksi); Mdnc = total unfactored dead load moment acting on the monolithic or noncomposite section (kip-in.); Sc = section modulus for the extreme fiber of the composite section where tensile stress is caused by externally applied loads (in.3); Snc = section modulus for the extreme fiber of the monolithic or noncomposite section where tensile stress is caused by externally applied loads (in.3); g1 = flexural cracking variability factor; g2 = prestress variability factor; and g3 = ratio of specified minimum yield strength to ultimate tensile strength of the nonpre- stressed reinforcement. Equation 5.6.3.3-1 was developed by Holombo and Tadros (2009) to provide a rational design pro- cedure for minimum reinforcement to prevent brittle failure of concrete sections. This was achieved through the use of the gamma factors. A subsequent NCHRP Project 12-94 is investigating minimum flexural reinforcement requirements. article 5.6.7 (formerly 5.7.3.4): control of cracking by distribution of reinforcement Article 5.6.7 addresses the distribution of tension reinforcement to control flexural cracking for all concrete components, except deck slabs designed in accordance with Article 9.7.2: Empirical Design. The article requires that the spacing, s, of nonprestressed reinforcement in the layer closest to the tension face shall satisfy the following equation: ( )≤ gβ − 700 2 5.6.7-1 (5)s f d e s ss c in which ( ) ( )β = + −1 0.7 5.6.7-2 (6) d h ds c c where βs = ratio of flexural strain at the extreme tension face to the strain at the centroid of the reinforce- ment layer nearest the tension face; ge = exposure factor; = 1.00 for Class 1 exposure condition; = 0.75 for Class 2 exposure condition; dc = thickness of concrete cover measured from extreme tension fiber to center of the flexural reinforcement located closest thereto (in.); fss = calculated tensile stress in steel reinforcement at the service limit state not to exceed 0.60 fy (ksi); and h = overall thickness or depth of the component (in.). Class 1 exposure condition relates to an estimated maximum crack width of 0.017 in., and Class 2 relates to an estimated maximum crack width of 0.013 in. Class 2 typically is used for situa- tions in which the concrete is subjected to severe corrosion conditions, such as bridge decks exposed to deicing salts and substructures exposed to water. Class 1 is used for less corrosive conditions and could be thought of as an upper bound in regard to crack width for appearance and corrosion (SHRP 2 2015). The different classes of exposure conditions have been so defined in the design specifications to provide flexibility in the application of these provisions to meet the needs of the bridge owner. A calibration study of Equation 5.6.7-1 determined that the reliability indices for a 1-year return period and 5,000 average daily truck traffic were 1.6 and 1.0 for Class 1 and Class 2 exposure conditions, respectively (SHRP 2 2015).

49 The intent of the article is to control flexural cracking in which the crack width is assumed to be pro- portional to its distance from the neutral axis as represented by βs. However, most cracks in the bridge decks are caused by restrained shrinkage, differential temperatures, or discontinuities in the supporting beams or slabs. The cracks are usually full depth and have a width somewhat constant. Therefore, the use of the article to control some types of cracking in bridge decks may not be appropriate. As part of the survey for this synthesis, the states were asked whether they used Article 5.6.7 of the AASHTO LRFD Specifications to determine maximum spacing of reinforcement in bridge decks. Responses were as follows: • Yes, with no modifications: 26 U.S. agencies; • Yes, with modifications: eight U.S. agencies; and • No: four U.S. agencies. Most agencies reported using an exposure factor of 1.0, with 12 agencies reporting a value of 0.75 for some or all applications. The agencies also listed the following modifications related to Article 5.6.7: • Decks made continuous (link slabs) have supplemental reinforcement. • AASHTO LRFD Specifications were used before the 2005 Interim revisions. • Maximum value for cover used to calculate dc was 2 in., whereas actual cover was 3 in. • Quantity of reinforcement was checked against the Frosch et al. (2002) equation. • Standardized deck designs were used. Article 5.6.7 was rarely used. • Concrete compressive stress was limited at the service limit state because of the positive bend- ing moment between girders to 0.4 f ′c, bar spacing to 8 in. maximum, and reinforcement bar size in decks to No. 6 maximum. • The definition of dc was modified and defined where Class 1 and Class 2 apply. • Redistribution percentage was modified in terms of c/de ratio. article 5.7.2.5 (formerly 5.8.2.5): minimum transverse reinforcement Where transverse reinforcement is required and nonprestressed reinforcement is used to satisfy that requirement, the area of steel shall satisfy: ( )≥ l ′  0.0316 5.7.2.5-1 (7)A f b sfv c vy where Av = area of transverse reinforcement within distance s (in.2); bv = width of web adjusted for the presence of ducts as specified in Article 5.7.2.8 (in.); s = spacing of transverse reinforcement (in.); fy = yield strength of transverse reinforcement (ksi) ≤ 100 ksi; and l = concrete density modification factor, as specified in Article 5.4.2.8. A minimum amount of transverse reinforcement is required to restrain the growth of diagonal shear cracks and ensure that the member has adequate ductility. article 5.7.2.6 (formerly 5.8.2.7): maximum Spacing of transverse reinforcement Article 5.7.2.6 specifies a maximum spacing of transverse reinforcement to ensure that any diagonal crack is intersected by a reinforcing bar. The spacing of the transverse reinforcement shall not exceed the maximum permitted spacing, smax, determined as: • If vu < 0.125 f ′c, then: s dv )(= ≤0.8 24.0 in. 5.7.2.6-1 (8)max

50 • If vu ≥ 0.125 f ′c, then: s dv )(= ≤0.4 12.0 in. 5.7.2.6-2 (9)max where vu = shear stress calculated in accordance with Article 5.7.2.8 (ksi); and dv = effective shear depth as defined in Article 5.7.2.8 (in.). article 5.8.2.6 (formerly 5.6.3.6): crack control reinforcement For members designed using the strut-and-tie method, Article 5.8.2.6 requires the use of orthogonal grids of bonded reinforcement in structures and components or regions thereof, except for slabs and footings, to control the width of cracks and ensure a minimum ductility for the member so that, if required, significant redistribution of internal stresses is possible. The spacing of the bars in these grids shall not exceed the smaller of d/4 and 12.0 in. The rein- forcement in the vertical direction shall satisfy the following: ( )≥ 0.003 5.8.2.6-1 (10)Ab s v w v and the reinforcement in the horizontal direction shall satisfy the following: ( )≥ 0.003 5.8.2.6-2 (11)Ab s h w h where Ah = total area of horizontal crack control reinforcement within spacing sh (in.2); Av = total area of vertical crack control reinforcement within spacing sv (in.2); bw = width of member’s web (in.); and sv, sh = spacing of vertical and horizontal crack control reinforcement, respectively (in.). Where provided, crack control reinforcement shall be distributed evenly near the side faces of the strut. The required minimum reinforcement is based on the work of Birrcher et al. (2009) and Larson et al. (2013). article 5.9.4.4.1 (formerly 5.10.10.1): Splitting resistance Article 5.9.4.4.1 requires a minimum amount of splitting resistance in the webs of I-beams or webs and bottom flanges of boxes and U-girders to control end region splitting cracks that may develop parallel to the strands. The minimum amount of reinforcement is calculated as: = (12)A Pfs r s where As = total area of reinforcement located within the distance h/4 from the end of the beam (in.2); Pr = splitting resistance taken as not less than 4% of the total prestressing force before transfer (kip); fs = stress in steel not to exceed 20.0 ksi (ksi); and h = overall dimension of precast member in the direction in which splitting resistance is being evaluated (in.).

51 For pretensioned I-girders or bulb tees, h is the overall height of the member. For pretensioned solid or voided slabs, h is the overall width of the member. For pretensioned box or tub girders, h is the lesser of the overall width or height of the member. As part of the survey for this synthesis, the U.S. state agencies were asked whether they used Arti- cle 5.9.4.4.1 to design splitting reinforcement at the ends of prestressed concrete beams. Responses were as follows: • Yes, with no modifications: 23 U.S. agencies; • Yes, with modifications: five U.S. agencies; and • No: seven U.S. agencies Modifications included the following: • Use a minimum spacing of 10 in. for a distance of h/4. • Provide the reinforcement over a distance greater than h/4 to facilitate concrete placement. • Provide additional reinforcement to keep crack widths less than 0.012 in. • Use a maximum spacing of 3 in. • Consider end blocks when prestressing forces exceed 1,800 kip. • Anchor closely spaced grids for members with prestressing forces in excess of 1,800 kip. • Spread bars beyond h/4. Based on NCHRP Project 18-14, Tadros et al. (2010) recommended that at least 50% of the split- ting reinforcement be placed in the end h/8 of the member. The balance of the splitting reinforcement was recommended to be placed in the distance between h/8 and h/2 from the member end. The reason for this distribution was to concentrate the reinforcement at the location where the highest bursting stresses are expected to exist. article 5.9.4.4.2 (formerly 5.10.10.2): confinement reinforcement Article 5.9.4.4.2 requires that reinforcement be placed to confine the prestressing steel in the bottom flange of beams other than box beams for a distance of 1.5d from the end of the beam. The reinforce- ment shall not be less than No. 3 deformed bars, with a spacing not exceeding 6 in. and shaped to enclose the strands. For box beams, the transverse reinforcement must be provided and anchored by extending the legs of stirrups into the webs of the beam. As part of the survey for this synthesis, the states were asked whether they used Article 5.9.4.4.2 to design confinement reinforcement at the ends of prestressed concrete beams. Responses were as follows: • Yes, with no modifications: 29 U.S. agencies; • Yes, with modifications: three U.S. agencies; and • No: three U.S. agencies. Modifications included the following: • Use closer spacing at beam ends but do not always extend the confinement reinforcement to the full 1.5d. • Use closer spacing at beam ends and extend the confinement reinforcement for the full length of the beam using a wider spacing. • Extend the confinement to 1⁄3 of the span with No. 4 bars at a spacing to match the vertical stirrup spacing with a maximum spacing of 21 in. article 5.10.3.2 (formerly 5.10.3.2): maximum Spacing of reinforcing bars Article 5.10.3.2 limits the maximum spacing of reinforcement in walls and slabs to 1.5 times the thickness of the member or 18.0 in., whichever is the lesser.

52 article 5.10.6 (formerly 5.10.8): Shrinkage and temperature reinforcement Article 5.10.6 contains requirements for minimum amounts of reinforcement at each face to control shrinkage and temperature stresses in members exposed to daily temperature changes and in struc- tural mass concrete. For bars or welded wire reinforcement, the area of reinforcement per foot on each face and in each direction shall satisfy: ( ) ( )≥ + 1.30 5.10.6-1 (13)A bhb h fs y Except that As0.11 0.60 5.10.6-2 (14)( )≤ ≤ where As = area of reinforcement in each direction and each face (in.2/ft); b = least width of component section (in.); h = least thickness of component section (in.); and fy = specified yield strength of reinforcing bars ≤ 75 ksi. The coefficient of 1.3 in Equation 5.10.6-1 is the product of 0.0018, 60 ksi, and 12 in./ft. The equa- tion is written to show that the total required reinforcement is distributed uniformly around the perimeter of the component. article 5.12.2.3.3d (formerly 5.14.4.3.3d): longitudinal construction Joints Article 5.12.2.3.3d addresses longitudinal construction joints between precast concrete flexural components. The joint shall consist of a keyway filled with a nonshrinkage mortar attaining a compressive strength of 5.0 ksi within 24 hours. The depth of the keyway should not be less than 5.0 in. If the components are posttensioned together transversely, the amount of transverse pre- stress, after all losses, shall not be less than 0.25 ksi through the keyway. In the last 3.0 ft at a free end, the required transverse prestress shall be doubled. A similar article does not exist for concrete decks on concrete beams. article 6.10.1.7: minimum negative flexure concrete deck reinforcement Article 6.10.1.7 is applicable to CIP concrete decks on steel beams. It requires that an area of longi- tudinal reinforcement not less than 1% of the total cross-sectional area of the concrete deck be pro- vided where the longitudinal tensile stress in concrete exceeds the factored modulus of rupture of the concrete. Article 6.10.3.2.4 requires that this check be made for various loading conditions, including critical stages of construction. This article is intended to control cracking. A similar provision is not provided for CIP decks on concrete beams. article 9.7.2: empirical design and article 9.7.3: traditional design Two methods for concrete bridge deck design are provided in Section 9 of the AASHTO LRFD Bridge Design Specifications. The empirical method of Article 9.7.2 is based on the concept that the primary structural action is by internal arching. It is applicable when certain design conditions apply and requires that four layers of reinforcement be provided. The minimum amount of reinforcement is 0.27 in.2/ft for each bottom layer and 0.18 in.2/ft for each top layer. Checking of bar spacing to control flexural crack widths per Article 5.6.7 (formerly 5.7.3.4) is not required. Some states use the concept of the empirical method but not exactly as stated in the AASHTO Specifications. The traditional method of Article 9.7.3 is based on the assumption that the primary action is flexural. Four layers of reinforcement are required, with distribution reinforcement provided in the

53 direction perpendicular to the primary reinforcement at a percentage of the amount of primary reinforcement. Checking of bar spacing to control flexural crack widths per Article 5.6.7 (formerly 5.7.3.4) is required. concluSionS about the effectS of reinforcement tYpe on crack control Various types of corrosion-resistant steel reinforcement have been used in bridges. The predominant type continues to be epoxy-coated reinforcement. Zinc-coated, stainless-steel–coated, and solid stain- less steel reinforcement have been used. Agencies reported that the use of these materials did not affect deck cracking, although limited research shows opposite findings. Fiber-reinforced polymer reinforce- ment has been used as nonprestressed reinforcement in CIP concrete bridge decks. In this application, the amount of FRP reinforcement often is based on control of crack widths. The AASHTO LRFD Bridge Design Specifications contain numerous articles about providing mini- mum reinforcement to ensure minimum sectional strength if cracks occur. These articles also ensure that crack widths will be controlled after a crack occurs. Narrower crack widths result from using smaller diameter bars at a closer spacing. However, some agencies modify their design practices to supplement some of the AASHTO LRFD Specifications. The LRFD articles do not encompass the full range of reinforcement types available today.