National Academies Press: OpenBook

Control of Concrete Cracking in Bridges (2017)

Chapter: Chapter Three - Effects of Concrete Constituent Materials on Cracking

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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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Suggested Citation:"Chapter Three - Effects of Concrete Constituent Materials on Cracking ." National Academies of Sciences, Engineering, and Medicine. 2017. Control of Concrete Cracking in Bridges. Washington, DC: The National Academies Press. doi: 10.17226/24689.
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31 Concrete constituent materials have an important impact on cracking, particularly in bridge decks, because they affect the magnitude of shrinkage. ConCrete Constituent Materials for Bridge deCks The primary benefit achieved by careful selection of concrete constituents is the reduction in drying shrinkage and the associated reduction in cracking. This section summarizes practices and recom- mendations that have been made to reduce shrinkage and concrete cracking in bridge decks. general Forty-eight bridge decks containing 10 different concrete mixes were inspected in Ohio for deck crack- ing on the top and bottom surfaces (Lefchik 1994). The report noted that many decks showed varying degrees of cracking over the surface of the deck with some isolated areas having more or less cracking. In some cases, one end of a deck was relatively crack free, whereas the other end exhibited cracking. The report concluded that factors other than concrete mix had a significant effect on the amount of cracking. Krauss and Rogalla (1996) concluded that concrete material factors important in reducing cracking include low shrinkage, low modulus of elasticity, high creep, low heat of hydration, and selection of aggregates and concrete that provide a low cracking tendency. The material factors helpful in reducing cracking included lower cement content, higher water-cement ratio, using shrinkage-compensating concrete, and avoiding materials that produce high early compressive strengths and modulus of elasticity values. An evaluation of existing bridge decks in Kansas showed that cracking increased with increasing values of slump, percent volume of water and cement, water content, cement content, and compressive strength (Schmitt and Darwin 1995, 1999; Miller and Darwin 2000; Lindquist et al. 2005). Decreases in cracking were noted with increases in air content. A study by North Carolina State University noted a tendency for reduced cracking in the presence of higher slump and higher air concrete (Cheng and Johnston 1985). Based on a comprehensive literature search, Hadidi and Saadeghvaziri (2003) made the following recommendations as positive steps to reduce the potential for deck cracking: • Reduce cement content to 650 to 660 lb/yd3. • Consider using a low early-strength concrete when early opening of the deck is not required. • Limit the water-cement ratio to 0.40 to 0.45 or lower with the use of water reducers. • Use the largest maximum aggregate size and the maximum aggregate content. Menkulasi et al. (2015) recommended that a concrete mix for a CIP concrete deck should have low shrinkage and high creep properties to reduce the likelihood of excessive cracking in bridge decks. Low free shrinkage reduces the tensile stresses that develop because of restrained differential shrinkage. High creep helps relax any tensile stresses that may develop. In addition, the short-term properties that will help reduce the extent of cracking caused by differential shrinkage include a mix with high tensile strength and low modulus of elasticity. chapter three effeCts of ConCrete Constituent Materials on CraCking

32 Because it is difficult to find a concrete mix that embodies all of these long-term and short-term properties, priority should be given to a mix with the lowest shrinkage because it is the free shrinkage of the deck that serves as a catalyst for the creation of tensile stresses in the CIP topping (Menkulasi et al. 2015). Measurements on seven different concrete mixes showed that the mix with normal weight coarse aggregate and saturated lightweight fine aggregate exhibited the lowest shrinkage strain and the highest creep coefficient. Before 2013, the Illinois Tollway used a prescriptive approach for mix proportioning and had standard mixes for bridge decks (Gillen and Gancarz 2016). The mixes were made up of 605 to 705 lb/yd3 of cement, no SCMs or only small amounts of fly ash, and one coarse aggregate grada- tion. Early-age cracking was seen on many bridge decks. In 2013, the Illinois Tollway introduced a performance specification, which included requirements for linear shrinkage and restrained shrink- age. Since 2013, 77 bridges have been built under the new provisions, and shrinkage cracking has been reduced. Cement In the survey for this synthesis, agencies were asked to identify the frequency of use of different cements in concrete bridge decks. Their responses are summarized in Table 3. Clearly, the most frequently used cements are Types I and II. The source of cement may have a large effect on drying shrinkage. Cements with high alkali con- tent, high C3S and C3A contents, low C4AF, and high fineness have high strength gain and are found to have higher cracking tendencies (TRB 2006). Thus, Type III cements should be used with caution for deck applications. Some agencies restrict the use of Type III cements to precast concrete mem- bers (Russell 2013). In an effort to control temperatures, Type II or Type IV cements, with their lower heat of hydration, often are used in lieu of Type I cement, especially when warmer ambient condi- tions exist. Slower-setting cements can be expected to have reduced drying shrinkage and cracking. Decks constructed with Type II cement cracked less than did those constructed with Type I cement (TRB 2006). The use of Type II cement was recommended by Saadeghvaziri and Hadidi (2002). Krauss and Rogalla (1996) pointed out that the general chemistry of cements has changed over time. In addition, today’s cements are finer than previous ones. Consequently, concrete made with today’s cements gain strength more rapidly than did concrete made with previous cements. As a result, modern concretes with a high early compressive strength and modulus of elasticity provide an increased risk of cracking because of the high stresses that develop as a result of early shrinkage and thermal strains. One material that has been used in concrete bridge decks to reduce cracking is a shrinkage- compensating concrete. A shrinkage-compensating concrete incorporates an expansive cement or Cement Never Sometimes Often Always AASHTO M 85 Types I and IA 7 4 13 9 AASHTO M 85 Types II and IIA 3 6 13 11 AASHTO M 85 Types II(MH) and II(MH)A 19 3 6 1 AASHTO M 85 Types III and IA 23 7 1 1 AASHTO M 85 Type IV 32 0 0 0 AASHTO M 85 Type V 29 2 1 0 AASHTO M 240 22 5 3 1 ASTM C1157 23 3 2 1 TABLE 3 FREqUENCy OF USE OF CEMENTS

33 expansive component that causes the concrete to expand during the first few days. Thereafter, the concrete shrinks in a manner similar to that of conventional concrete (ACI Committee 223 2010). McLean et al. (2016) reported that approximately 600 bridge decks in the United States have been built using shrinkage-compensating concrete. The Ohio Turnpike is reported to have built more than 500 bridge decks using shrinkage- compensating concrete, and cracking was reduced (Ramey et al. 1999). Gruner and Plain (1993) also reported that the use of an expansive cement in place of a Type I portland cement on Ohio bridges resulted in excellent performance. The New york Thruway Authority placed 47 bridge decks from 1991 to 1994 using shrinkage- compensating concrete. Most of the decks developed scaling problems to such an extent that the authority discontinued the use of shrinkage-compensating concrete. An investigation of the causes of the scaling determined that it had nothing to do with the use of shrinkage-compensating concrete (Ramey et al. 1999). Cope and Ramey (2001) investigated the use of shrinkage-compensating concrete with and without silica fume and the use of an SRA to reduce early-age deck cracking. They concluded that the shrinkage- compensating concrete without silica fume was a viable candidate for mitigating drying shrinkage crack- ing. The Illinois DOT conducted laboratory tests of shrinkage-compensating concrete containing Type K and Type G expansive components with and without SCMs (Chaunsali et al. 2013). Saadeghvaziri and Hadidi (2002) also suggested the use of shrinkage-compensating concrete when available. The Virginia DOT investigated the effectiveness of shrinkage-compensating concrete in reducing cracks in bridge decks (Nair et al. 2016a). The bridge deck on the Route 613 Bridge over the South Fork Shenandoah River in Warren County was selected for the study. The results showed that the bridge deck with preblended Type K cement concrete had fewer transverse cracks than typically found in decks constructed with Type I/II cement. There were several longitudinal cracks caused by the differential movement at the keyway of the adjacent prestressed concrete box beams that could not be prevented by the use of shrinkage-compensating concrete. Slump loss under hot weather conditions was a more serious problem in shrinkage-compensating than in normal portland cement concrete. supplementary Cementitious Materials Supplementary cementitious materials, such as fly ash, slag, and silica fume, are used frequently in mixtures to enhance early and long-term performance characteristics (TRB 2006). In the survey for this synthesis, agencies were asked to identify the frequency of use of different SCMs in concrete bridge decks. Their responses are summarized in Table 4. From these data, it appears that Class F fly ash is the SCM most frequently used in bridge decks, and Class N pozzolan is used the least. This is similar to results obtained in a 2012 survey (Russell 2013). Fly ash and slag typically reduce the rate of strength gain, lower the heat of hydration, reduce the rate of stiffness development, and thus typically reduce the potential for cracking (TRB 2006). Material Never Sometimes Often Always Fly ash Class C 11 11 13 3 Fly ash Class F 5 11 17 6 Pozzolan Class N 26 8 2 0 Silica fume 11 16 9 5 Ground-granulated blast-furnace slag 12 10 15 1 Other 17 1 0 1 TABLE 4 FREqUENCy OF USE OF SUPPLEMENTARy CEMENTITIOUS MATERIALS

34 yuan et al. (2015) reported that the use of slag cement as part of the cementitious materials reduced the free shrinkage compared with mixtures containing only portland cement as the cementi- tious material. When slag cement was used in combination with a porous limestone coarse aggregate, an even greater reduction in free shrinkage was observed. Silica fume can increase the rate of strength development, increase the heat of hydration, reduce bleeding, and create conditions that are favorable for cracking. Some strongly discourage the use of silica fume in bridge deck applications (Hopper et al. 2015); however, others have reported that silica fume is not a cause of early cracking. According to Ozyildirim (1992), silica fume concrete is suscep- tible to plastic shrinkage cracking because of its lack of bleeding. Therefore, immediate application of sprays or misting after placement is essential to avoid the formation of plastic shrinkage cracks in silica fume concrete. According to laboratory tests by Whiting and Detwiler (1998), the cracking tendency of con- crete was influenced by the addition of silica fume only when the concrete was improperly cured. They recommended that specifications for silica fume concrete in bridge deck construction include a provision for a 7-day continuous moist curing of exposed surfaces. Concretes containing silica fume had somewhat higher shrinkages at early ages than did their counterparts not containing silica fume. However, the long-term shrinkage of the silica fume concretes was not greater than identical concretes without silica fume. aggregates Most recommendations specify a maximum size aggregate of 1½-in. or the smaller of one-third the deck thickness and three-fourths the minimum clear spacing between reinforcing bars (Krauss and Rogalla 1996). A 1961 PCA study (PCA 1970) recommended the use of the largest practical maximum size of coarse aggregate to minimize the water content to reduce cracking. Other recom- mendations included use of the lowest reasonable slump and keeping the maximum slump within the range of 2 to 3 in. Saadeghvaziri and Hadidi (2002) also recommended the use of the largest coarse aggregate size and maximum aggregate content. The use of soft aggregates, such as sandstone, results in increased drying shrinkage, whereas the use of hard aggregates, such as quartz, dolomite, and limestone, results in decreased shrinkage (TRB 2006). The use of optimized combined aggregate gradations can result in the use of less water, less cementi- tious materials, and thus less paste content (AASHTO 2010), which leads to less shrinkage. Optimized combined aggregate gradations may be determined using the combined fineness modulus, coarse factor chart, power chart, or percent retained on each sieve, as explained in the Appendix A8 of the AASHTO LRFD Bridge Construction Specifications (AASHTO 2010). The Virginia DOT investigated the performance of seven bridges with lightweight concrete decks that were built between 2012 and 2014 (Nair et al. 2016c). The lightweight concrete contained lightweight coarse aggregate and normal-weight fine aggregate. The concrete compressive strengths ranged from 4.5 to 6.0 ksi at 28 days. The authors reported that some bridge decks had no cracks, whereas others had fewer cracks than was typical of decks constructed with normal weight aggregate over the past 20 years. Based on a field inspection, Ozyildirim and Moruza (2014) concluded that lightweight, HPC bridge decks can be produced that have no visible cracking after 2 years in service. Chemical admixtures Chemical admixtures can have a positive, negative, or no effect on concrete shrinkage and crack- ing (TRB 2006). In the survey for this synthesis, agencies were asked about the frequency of use of chemical admixtures, corrosion inhibitors, SRAs, and expansive cement or expansive components for CIP concrete bridge decks. The respondents’ reported frequency of use of the admixtures is shown in Table 5.

35 The data indicate that agencies use a variety of the chemical admixtures specified in AASHTO M 194, with Type A–water-reducing and Type F–high-range, water-reducing being used most fre- quently and Type E–water-reducing and accelerating used the least. Corrosion inhibitors, SRAs, and expansive cements or components are used by only a few agencies. The results shown in Table 5 are similar to those obtained in a 2012 survey (Russell 2013). An SRA has a positive effect by reducing the surface tension of the pore water and thus lowering plastic and long-term shrinkage. Schemmel et al. (1999) reported that free shrinkage was reduced on the order of 50% with the use of an SRA. However, the use of SRAs can affect air content, set time, and bleed time (Kosmatka and Wilson 2016). California Department of Transportation (Caltrans) found that the use of an SRA in the concrete resulted in less shrinkage cracking in bridge decks (Maggenti et al. 2013). Beginning in the late summer of 2002, Caltrans began using SRAs in their concrete deck mixes. As a result, there was a dramatic reduction in cracking, and the decks remained free of visible cracking until at least 2013. Given the agency’s positive experience with these and subsequent bridges, Caltrans has selected SRAs as a method of crack control for CIP concrete decks on precast concrete girders. More details about the Caltrans approach are provided in chapter seven. Virginia DOT also investigated the effectiveness of SRAs in reducing drying shrinkage in concrete mixtures and thus reducing cracks in bridge decks (Nair et al. 2016b). Nine bridges were selected for study. Three different SRA products were used. With the exception of one mixture, the maximum cementitious materials content was limited to 600 lb/yd3. The results showed that concrete with low cementitious materials content and an SRA was effective in minimizing bridge deck cracking. The study showed that bridges with fewer and narrower cracks or no cracks can be constructed and recommended the use of an SRA and a cementitious materials content less than 600 lb/yd3 in Virginia DOT bridge deck concrete mixtures. A special provision was developed for the future use of SRAs in concrete mixtures. To provide the most effective resistance to drying shrinkage cracking in concrete bridge decks, Brown et al. (2007) recommended that mixtures containing an SRA, polypropylene fibers, expansive cement, or high volume fly ash be used. Cementitious Materials Content and Water-Cementitious Materials ratio The most important factor affecting drying shrinkage is the amount of water per unit volume of concrete (Kosmatka and Wilson 2016). Only about half of the water is used in the hydration process. Admixture Never Sometimes Often Always AASHTO M 194 Type A—Water-reducing admixtures 2 7 15 14 AASHTO M 194 Type B—Retarding admixtures 4 20 10 3 AASHTO M 194 Type C—Accelerating admixtures 19 15 2 1 AASHTO M 194 Type D—Water-reducing and retarding admixtures 5 18 11 3 AASHTO M 194 Type E—Water-reducing and accelerating admixtures 21 14 2 1 AASHTO M 194 Type F—High range water-reducing admixtures 8 12 13 5 AASHTO M 194 Type G—High range water-reducing and retarding admixtures 16 15 5 2 Corrosion inhibitors 25 11 2 1 Shrinkage-reducing admixtures 21 13 2 2 Expansive cement or components 30 8 0 0 TABLE 5 FREqUENCy OF USE OF ADMIxTURES IN CAST-IN-PLACE CONCRETE BRIDGE DECKS

36 The rest is there to provide workability and finishability. The excess free water that remains in the hardened concrete contributes to the drying shrinkage. Thus, shrinkage can be minimized by keep- ing the water content as low as possible. For the same w/cm ratio, this also reduces the cementitious materials content. Collectively, the paste content is reduced and the aggregate content is increased. A study of premature cracking in concrete bridge decks for the Wisconsin DOT resulted in recom- mendations to limit the water-cement ratio to a maximum of 0.40 and to use coarse aggregate with a maximum size greater than 0.75 in. (Kochanski et al. 1990). Saadeghvaziri and Hadidi (2002) recom- mended the use of a water-cement ratio between 0.40 and 0.45 and a cement content of 650 to 660 lb/yd3. Responses to a Michigan DOT 2002 survey indicated that 52% of the 31 responding states use a cement content of 658 lb/yd3 and 32% use 564 lb/yd3 in their bridge deck concretes (Aktan et al. 2003). When the South Carolina DOT introduced a Class E concrete, they found that most of the bridge decks experienced cracking occurring both before the bridge was opened to traffic and immediately thereafter (Hussein 2006). After an investigation, the South Carolina DOT introduced a concrete mix with less cementitious material, required that all bridge decks be wet cured for 7 full days, and wind barricades and foggers be used during placement of all bridge deck concrete. The new mix resulted in a concrete compressive strength of 4.0 ksi at 28 days. After implementation of the new requirements, a few projects with HPC bridge decks were constructed without cracking. According to the authors, it appears that the rich concrete mix was a major factor in causing the early-age cracking. Based on their study of cracking in 19 concrete bridge decks, Mokarem et al. (2009) concluded that a concrete mixture with a w/cm ratio between 0.35 and 0.40, a cementitious materials content between 600 and 700 lb/yd3, and appropriate construction practices, such as a 7-day wet curing, should be expected to result in a lower crack density. In their research to develop a low-cracking HPC, Darwin et al. (2012) recommended a maximum cementitious materials content of 540 lb/yd3 and a w/cm ratio of 0.44 to 0.45. fibers In the survey for this synthesis, 10 of 40 agencies reported that they had specified fibers as a strategy to minimize cracking in concrete bridge decks. One agency listed the use of fibers as a strategy that was most effective, whereas two agencies identified that it was the least effective. ASTM C1116—Standard Specification for Fiber-Reinforced Concrete classifies fiber-reinforced concrete into four different categories based on the material type of fiber: steel, glass, synthetic, and natural. Fibers are beneficial in reducing bleeding and plastic settlement cracking in fresh concrete and increasing energy absorption and load-carrying capacity after cracking (Kosmatka and Wilson 2016). Additional information about fibers is provided in ACI 544.1 (2009). Several states now require the use of fibers in bridge decks. For example, Caltrans specifies that each cubic yard of deck concrete must contain at least 1 lb of polymer microfibers and at least 3 lb of polymer macrofibers. Microfibers must have a length from 0.5 to 2 in. Macrofibers must have a length from 1.0 to 2.5 in. Oregon DOT specifies the use of synthetic fibers from the qualified prod- uct list in all bridge deck concrete. The specified quantity differs according to the manufacturer’s recommendations. ConCrete Constituent Materials for other CoMponents Cracking in components other than bridge decks is generally less of a concern for owners because the other components are exposed to a less severe environment than is the deck. In addition, the other components are thicker than the deck, which reduces the magnitude and rate of shrinkage. The use of continuous decks or watertight expansion joints over the piers and abutments in combination with proper drainage serve to protect the piers, columns, and abutments from rain, snow, ice, and deicing salts.

37 In all structures, a reduction in concrete shrinkage will reduce the tendency for shrinkage crack- ing (ACI Committee 224 2008). This can be achieved by using less water in the mix and the largest practical maximum aggregate size. A lower water content can be achieved by using a well-graded aggregate, stiffer consistency, and lower initial temperature. ACI Committee 224 (2008) lists the following items to be included in specifications to minimize drying shrinkage: • Cement should be AASHTO M 85 Types I, II, V; AASHTO M 240 Types IS and IP; or ASTM C845. • Aggregates should be well-graded and well-shaped with minimum amounts of clay, dirt, and excessive fines. Rock types that result in low shrinkage concrete should be used. • The largest practical maximum coarse aggregate size and the lowest practical fine aggregate size should be used. • The lowest practical slump and concrete temperature should be used. internal Curing Internal curing is defined by the ACI as “a process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water” (ACI 2013). Internally cured concrete uses absorptive materials in the mixture that supplement standard curing practices by supplying moisture to the interior of the concrete. This process adds moisture without affecting the w/cm ratio. The water addition can be achieved using a variety of materials (Kovler and Jensen 2007), but for bridges, the most likely material is prewetted, lightweight aggregate. More information about the use of prewetted, lightweight aggregate is provided in ACI Report (308-213)R-13 (ACI Committee 308 2013). The use of prewetted, lightweight aggregate (LWA) can minimize the development of autogenous shrinkage and help to avoid early-age cracking (Wei and Hansen 2008; Bentz and Weiss 2011). At the same time, the use of prewetted LWA has been shown to increase hydration, decrease shrinkage and permeability, and increase compressive strength. The use of LWA for internal curing is described in ASTM C1761/C1761M (“Standard Specification for Lightweight Aggregate for Internal Curing of Concrete”). An appendix provides guidance on cal- culating the quantity of LWA for internal curing. There is also a guide on the website of the Expanded Shale, Clay and Slate Institute (www.escsi.org). A procedure for mix proportioning has been reported by Bentz et al. (2005). Internal curing has been used in bridges in Illinois (Tollway), Indiana, Iowa, New york, Ohio, Texas, Utah, and Virginia. In 2010, a pair of bridge decks was cast in Monroe County, Indiana (Weiss et al. 2013). Both decks were cast using ready-mixed concrete. The first deck was cast using a conventional bridge deck concrete, and the second was cast using an internally cured concrete made using the same raw materials. Although the 1-day strength of the internally cured concrete was approximately 10% less than that of the conventional concrete, the conventional and internally cured concrete had equivalent strengths at approximately 10 days. After 3 months, the internally cured concrete was 20% stronger than the conventional concrete. Tests per AASHTO T 277 showed that the internally cured concrete had a 10% lower charge passed through at 28 days and nearly 40% lower charge passed through after 3 months. The internally cured mixture also had a lower shrinkage. Cracks developed in the conven- tional concrete deck after the first few months of service, whereas the internally cured concrete had no visible cracking at 1 year after placement. New york State DOT has used internal curing on at least 17 bridges located throughout the state (Streeter et al. 2012). The New york State DOT uses a special mixture design that is similar to their conventional deck concrete mix design (nearly 650 lb/yd3 of cementitious material with silica fume) but includes 200 lb/yd3 of fine LWA. According to Streeter et al. (2012), internal curing has been shown to provide improvements by reducing the cracking associated with concrete shrinkage but has not eliminated all deck cracking. It presented no problems to concrete suppliers when batching concrete or to contractors placing and finishing concrete on the bridges.

38 According to Bentz and Weiss (2011), there were no negatives associated with using internal curing on the New york State bridges; however, the potential benefits need to be quantified through com- parisons with conventional concrete bridge deck materials. Three internally cured decks were walked to assess their performance after 1 to 3 years, and only one small crack was observed in the negative moment region of one internally cured bridge deck. Conversely, the parapet walls and sidewalks pro- duced with concrete without internal curing showed several large cracks. The decks appeared to be wearing as expected. The New york State DOT permits these concretes to be pumped, and no problems have been reported (Bentz and Weiss 2011). In Ohio, the DOT used a modified HPC No. 4 mixture that contained 595 lb/yd3 of cementitious mate- rials and silica fume. Approximately 200 lb/yd3 of LWA was used for internal curing. The mixture was pumped to the deck without incident and maintained sufficient entrained air. The mixture was reported to have strengths that were similar or superior to the conventional mixture without internal curing. In May 2007, two bridge decks were cast in Euclid, Ohio; one deck used a standard Ohio DOT deck mixture, and one incorporated internal curing (Delatte and Crowl 2012). About 5 weeks after the decks were cast, two small cracks were observed on the underside of the bridge below the side- walk at points where two pieces of formwork came together. A second inspection 4½ years after casting revealed no additional cracks on both decks. The city of Cleveland has a major bridge across the Cuyahoga River, known as the Main Avenue Bridge, with a deck constructed of LWA concrete with expansive cement. This deck was placed dur- ing renovations in 1992. In 2011, the deck was still showing excellent performance with no cracks visible on the top of the deck. The underside could not be inspected because stay-in-place metal forms were used (Delatte and Crowl 2012). Four CIP concrete bridge decks over precast concrete deck panels were built in Northern Utah: two constructed with conventional concrete and two containing prewetted lightweight fine aggregate (Guthrie and yaede 2013; Bitnoff 2014). Data from sensors embedded in the CIP concrete decks indicated that the moisture content of the internally cured concrete was 2% to 4% higher than the moisture content of the conventional concrete for the first year after deck construction but less than 2% at 2 years. At 28 days, the average compressive strength of the internally cured concrete was 1% higher than that of the conventional concrete, but at 1 year the conventional concrete was 13% stron- ger. In rapid chloride permeability testing, the internally cured concrete consistently passed between 13% and 18% less current than did the conventional concrete. In the field, rebound hammer testing showed similar concrete strengths for both deck types at 1 year but showed that the internally cured concrete was weaker than the conventional concrete at 2 years. On average, the internally cured concrete exhibited between 2% and 16% and between 12% and 46% greater chloride concentration, depending on the depth interval, than did a conventional concrete at 1 and 2 years, respectively. After 2 months, three to five cracks 0.008 to 0.12 in. wide were found on each of the conventional concrete bridge decks, but no visible signs of cracking were found in the bridge decks with internal curing. At 5, 8, 12, and 24 months, the conventional concrete bridge decks had 4.8, 6.6, 2.5, and 1.3 times more cracking, respectively, than did the internally cured con- crete decks. During surveys at 1 and 2 years, distinctive reflective cracks from the joints between the underlying precast, partial-depth deck panels were observed on all of the decks. A laboratory testing program was conducted for the Florida DOT to evaluate the properties of three standard concrete mixes and three corresponding internally cured concrete mixes with the same w/cm ratios and the same cementitious materials contents (Tia et al. 2015). The average compressive strength, flexural strength, modulus of elasticity, and splitting tensile strength of the internally cured mixes were lower than were those of the corresponding standard mixes. The differences ranged from 10% to 18%. The average cracking age of all internally cured mixes, as measured by the restrained shrinkage ring test, was 2.7 times that of the standard mixes. Jones et al. (2014) investigated autogenous shrinkage, restrained shrinkage cracking, and free shrinkage in a 50% relative humidity drying condition of internally cured concrete for use in Colo-

39 rado. Internally cured mixtures minimized autogenous shrinkage and caused initial expansion (or swelling) in a sealed system. Restrained shrinkage showed that internally cured concrete reduced the residual stress buildup in the material. Internally cured mixtures had less drying shrinkage because water present in the matrix allowed continuous hydration as the surface dried. A bibliography on internal curing is available at http://concrete.nist.gov/~bentz/phpct/database/ ic.html. self-Consolidating ConCrete Self-consolidating concrete (SCC) is defined as a highly flowable nonsegregating concrete that can spread into place, fill the formwork, and encapsulate the reinforcement without any mechanical consolidation (ACI Committee 237 2007). Its use is beneficial for casting complex shapes or mem- bers with congested reinforcement. SCC is made with conventional concrete materials except that a viscosity-modifying admixture may be included. SCC has been used in the construction of many bridges including its use in drilled shafts, pile caps, columns, precast bridge beams, precast deck panels, and connections between precast com- ponents. It has also been used in the rehabilitation of existing bridges. Its use in CIP decks has been limited because the concrete tends to flow downhill, making it difficult to cast sloping elements. Some states restrict its use to certain bridges components. SCC can be prone to plastic shrinkage cracking because the mixtures exhibit little or no surface bleeding (ACI Committee 237 2007). Thus, it is important to protect the concrete from early mois- ture loss (ACI Committee 237 2007). The increased paste volume in SCC creates a potential for increased drying shrinkage (PCI 2015). However, drying shrinkage of SCC can be similar to or less than that of conventional concrete. NCHRP Report 628: Self-Consolidating Concrete for Precast, Prestressed Concrete Bridge Elements provides guidelines for the use of SCC in precast, prestressed concrete bridge elements (Khayat and Mitchell 2009). Tests on four full-scale AASHTO Type II girders indicated that the greater shrinkage of SCC compared with that of HPC can lead to larger prestressing losses and smaller camber. SCC and HPC girders of similar compressive strengths exhibit similar flexural cracking moments and cracking shear strengths. Kim et al. (2015) reported that the AASHTO expressions for estimating the cracking moment of precast, prestressed concrete bridge girders were appropriate for a girder made with a river gravel SCC but slightly overestimated the cracking moment for a girder made with a limestone SCC. suMMary of the effeCts of ConCrete Constituent Materials The following materials and criteria have been identified as being beneficial in the reduction of cracking in concrete bridge decks: • Using a cement content not greater than 650 lb/yd3. • Using a Type II cement. • Using SCMs. • Using the largest maximum aggregate size and maximum aggregate content that can be prop- erly placed. • Using aggregates with an optimized combined aggregate gradation. • Using aggregates with low modulus of elasticity, low coefficient of thermal expansion, and high thermal conductivity. • Using an SRA. • Using internal curing. • Using a w/cm ratio in the range of 0.40 to 0.45.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 500: Control of Concrete Cracking in Bridges provides information on methods used to control concrete cracking in bridge superstructures and substructures, and on the influence of cracking on long-term durability. Cracking of concrete in bridges continues to be a concern for bridge owners, particularly with bridge decks exposed to severe environments. The control of cracking for aesthetic, durability, and structural reasons becomes increasingly important as service-life goals are extended and higher-strength concrete, higher-strength reinforcement, and different types of reinforcement are used in bridge construction.

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