High Performance Concrete Specifications and Practices for Bridges (2013)

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18 chapter three cURRENT SPEcIFIcATIONS AND PRAcTIcES FOR cAST-IN-PLAcE cONcRETE a w/cm ratio, at least two states specify an upper limit for the total water content. The total water content includes any water in the SCMs and admixtures and any water beyond the saturated surface dry condition of the fine and coarse aggre- gates. At least 44 states specify a minimum cementitious materials content. Specified slump varies from a low of zero to a high of 8 in. The tolerances of most slump values are either ±1 in. or ±1½ in. Most slumps are specified to be applicable before the addition of any water-reducing admixtures. Some states provide a wide range for slump and require the contractor to select a target value to which a tolerance is then applied. Specified air contents also vary considerably from a low of zero to a high of 8%. The lower values are specified by states with less likelihood of freezing. Most specified air contents have a tolerance of ± 1½%. Agencies were asked which characteristics were speci- fied in their performance specifications for CIP concrete and which characteristics were considered in developing their prescriptive specifications. The results are shown in Figure 6. The four dominant characteristics in both types of specifications were compressive strength, permeability, workability, and ASR resistance. The selection of compressive strength and permeability by many agencies is consistent with the high usage in the demonstration projects. At that time, workability and ASR resistance were not included as characteristics. Other characteristics that agencies listed were restrained shrinkage cracking, surface resistivity, air content, w/c ratio, strength gain, corrosion resistance, and reduced maximum cementitious materials content. Some of these properties are not related to HPC characteristics. The review of the state standard specifications revealed that several states include a high performance class of con- crete or require an HPC performance characteristic for some of their other classes of concrete. The specifications for those concretes require the use of one or more SCMs, with at least 16 states specifying a maximum permeability. The specified permeabilities in terms of charge passed per AASHTO T 277 range from 750 to 4000 coulombs, with most values in the 1000 to 2000 coulomb range. This chapter addresses the specifications and practices for the use of CIP HPC primarily in bridge decks. The information was obtained from a review of the state specifications and information obtained from the survey. A list of the websites for the state standard specifications is provided in Appendix C. The standard specifications of nearly every state include a table that provides the basic requirements for several different classes or grades of concrete intended for different applications. The table generally includes information on cementitious materials content, w/cm ratio or water content, air content, slump, and compressive strength. Some tables include com- plete mix proportions. Numerous footnotes or related text provide further information about the type and quantities of materials, substitutions, and exceptions. This chapter and chapter four include information from these tables and the related text. As part of the survey for this synthesis, agencies were asked if they had used HPC for CIP bridge decks. Thirty-one agencies responded that they had and eight agencies responded that they had not. Reasons for not using HPC included: • Agency used HPC for deck overlays only. • Agency had tried it in comparison with conventional concrete on one project and observed no noticeable difference. • The improved curing process improved the durability of decks, but the additional expense of HPC was not justified. • Standard concrete with epoxy-coated reinforcement was performing well. • Cost-benefit ratio was not favorable. • Agency does not call it HPC but uses a prescriptive approach. • HPC was not needed. SPEcIFIED PROPERTIES The standard specifications of all states have some prescriptive requirements for the concrete. These include values for the amounts of cementitious materials, w/cm ratio, slump, and air content. The amounts of cementitious materials are discussed in the next section. At least 45 states specify an upper limit or an exact value for the w/cm ratio. About 75% of the states specify a maximum w/cm ratio between 0.40 and 0.45 for concrete to be used in bridge decks. Rather than specifying

19 cONcRETE cONSTITUENT MATERIALS Most state bridge standards define that concrete shall con- sist of hydraulic cement or portland cement, SCMs, fine aggregate, coarse aggregate, water, chemical admixtures, and air-entraining admixtures. Other state specifications do not include a precise definition, although all the concrete constituent materials are included in the specifications. cementitious Materials At least 39 state specifications limit the types of cement that may be used in bridges or define specific chemical require- ments for the cement. Some specifications restrict the use of some types of cement to specific components, such as the use of Type III cement in only precast concrete members. At least 44 states specify a minimum cementitious materials content, which generally ranges from 560 to 750 lb/yd3. Some specifications also include an upper limit, which is generally in the 700 to 800 lb/yd3 range. All states permit the use of fly ash. Two states specify only Class C fly ash, 10 states specify only Class F fly ash, and 38 states specify both classes. At least 12 states specifically state that Class N pozzolan may be used. At least 47 state specifications have an upper limit on the amount of fly ash that may be included. The upper limit is usually in the range of 15% to 30% of the total cementitious materials content with some as low as 10% or as high as 35%. Some states require the use of a higher percentage of pozzolans such as fly ash to control ASR. In contrast to the optional use of SCMs by most states, the California specifications require the use of minimum amounts of SCMs. The minimum quantity of SCMs varies depending on the exposure condition and aggregate reactivity. Silica fume is specifically permitted by at least 36 states, while others do not address its use. Where permitted, its use is generally restricted to an upper limit that ranges between 7% and 10% of the total cementitious materials content. Some specifications also include a lower limit of 5% or 7% when silica fume is used. The use of slag cement is permitted by at least 39 state stan- dard specifications. The upper limit for the maximum amount is usually in the range of 30% to 50% of the total cementitious materials content. To obtain information about the actual practices, agencies were asked to identify the percentage of bridges that use different SCMs in bridge deck concrete. The responses are shown in Table 6. From these data, it appears that Class F fly ash is the most frequently used SCM in HPC bridge decks, followed by silica fume and slag cement, which are used in about equal amounts. The least used SCM is the Class N pozzolan. 15 8 3 2 11 9 3 23 2 1 6 8 24 16 4 4 19 18 7 29 4 2 9 6 0 5 10 15 20 25 30 No. of Agencies Characteristic Performance Specifications Prescriptive Specifications FIGURE 6 Characteristics included in performance specifications and considered in prescriptive specifications for CIP concrete decks.

20 Aggregates Aggregates for concrete used in bridge decks may be normal weight aggregates conforming to AASHTO Specifications M 6 and M 80, lightweight aggregates conforming to AASHTO M 195, or a combination of them. The coarse aggregate size is generally selected to be the largest size practical under job conditions (Kosmatka and Wilson 2011). AASHTO Specifi- cations M 6 and M 43 contain grading requirements for the fine and coarse normal weight aggregates. In addition, a few states specify a combined grading. A standard specification for combined aggregates for hydraulic cement concrete is included in Section 8 of the AASHTO LRFD Bridge Construction Specifications (2010a). A combined grading can be used to improve the workability of concrete at given water and paste contents, minimize water and paste contents for a given work- ability, or improve workability and hardened properties of the concrete (Russell et al. 2006a). The possibility of includ- ing a small quantity of lightweight fine aggregate in concrete has received attention recently as a means to provide internal curing (ESCSI 2012). However, this approach was not iden- tified in any standard specifications. Admixtures Chemical admixtures are generally required to conform to AASHTO M 194 or ASTM C494. This specification lists seven types of admixtures (A through G) although not all seven are permitted by every state. Other admixtures included in state specifications are air entraining and corrosion inhibiting admixtures. Agencies were also asked about the percentage of total bridge decks using chemical admixtures conforming to AASHTO M 194, corrosion inhibitors, shrinkage reduc- ing admixtures, and expansive components. The number of respondents for each percentage range is shown in Table 7. Supplementary Cementitious Material Extent of Use as a Percentage of All Bridge Decks None 1 to 33 34 to 67 68 to 100 Fly Ash Class C 17 8 3 5 Fly Ash Class F 2 17 5 11 Pozzolan Class N 27 3 0 1 Silica Fume 11 16 1 7 Slag Cement 10 12 6 7 TABLE 6 NUMBER OF AGENCIES REPORTING THE OF USE OF SCMS IN HPC BRIDGE DECKS TABLE 7 NUMBER OF AGENCIES REPORTING THE USE OF ADMIXTURES IN HPC BRIDGE DECKS Admixture Extent of Use as a Percentage of All Bridge Decks None 1 to 33 34 to 67 68 to 100 AASHTO M 194 Type A—Water-reducing admixtures 4 4 5 22 AASHTO M 194 Type B—Retarding admixtures 9 11 7 7 AASHTO M 194 Type C—Accelerating admixtures 23 8 2 2 AASHTO M 194 Type D—Water-reducing and retarding admixtures 11 11 5 6 AASHTO M 194 Type E—Water-reducing and accelerating admixtures 23 8 1 0 AASHTO M 194 Type F—High range water-reducing admixtures 10 8 7 10 AASHTO M 194 Type G—High range water-reducing and retarding admixtures 16 11 2 3 Corrosion Inhibitors 20 9 0 4 Shrinkage Reducing Admixtures 25 5 0 1 Expansive Components 27 4 0 0

21 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 the more frequently used types and Type E— water reducing and accelerating the least common. Corrosion inhibitors, shrinkage reducing admixtures, and expansive components are used by a few agencies. The review of the state specifications indicated that at least 16 states permit the use of a corrosion inhibitor, usually calcium nitrite. CONSTRUCTION PRACTICES All state standard specifications address construction prac- tices, although the amount of detail and the requirements vary considerably. Topics addressed in the specifications include concrete production, transportation, placement, finish- ing, curing, and quality control. For concrete bridge decks, curing practice is an important topic. With HPCs, the application of wet curing immediately after concrete finishing is extremely important because these concretes have less bleed water and greater likelihood of plastic shrinkage cracking (Khaleghi and Weigel 2001; Praul 2001; Schell and Konecny 2001). Whiting and Detwiler (1998) emphasized the importance of curing silica fume concrete. The lack of bleed water means that water lost from the surface as a result of evaporation cannot be readily replaced. States now specify that wet curing begin within a certain distance or a short time after final finishing. For example, the Kansas LC-HPC specifications require that wet burlap be applied within 10 minutes after strike-off, as shown in Figure 7. In a 2003 survey of agencies in the United States and Canada, 82% of the respondents reported that they specify that cur- ing must begin immediately after finishing any portion of the deck (Russell 2004). At least two states require that a curing compound be applied at the end of the wet curing period. This allows the concrete to dry out more slowly and leads to slower development of tensile stresses from drying shrinkage. Based on the results from the survey for this synthesis, all responding agencies wet-cure concrete bridge decks, as illustrated in Figure 8. The duration of wet curing, however, ranges from three to 14 days, as shown in Figure 9. In a survey for NCHRP Synthesis 333 published in 2004, agencies in the United States and Canada indicated a range of FIGURE 7 Application of wet burlap within 10 minutes after strike off [Photo courtesy of the University of Kansas, Transportation Research Institute]. FIGURE 8 Wet curing of a concrete bridge deck under polyethylene sheeting [Photo courtesy of Oregon Department of Transportation]. 1 1 23 1 2 9 0 5 10 15 20 25 30 0 3 5 7 8 10 14 No. of Agencies Wet Curing Duration, days FIGURE 9 Duration of wet curing for concrete bridge decks.

22 curing periods from three to 14 days, with the most frequent value being seven days (Russell 2004). However, between 2004 and the current survey, the percentage of agencies spec- ifying seven days or fewer has decreased from 87% to 67%, while the percentage specifying 14 days has increased from 11% to 24%. In the current survey, only two states reported fewer than seven days wet-curing. Some states require a trial deck placement before con- struction of the actual bridge deck. One state reported that eliminating the requirement for a trial placement was not beneficial. TESTING AND AccEPTANcE PRAcTIcES Responses to the survey for this synthesis revealed that all agencies have practices or tests for the acceptance of new HPC mixtures used for concrete bridge decks. The amount of testing, however, varies considerably. All responding states require a trial batch or batches with the measurement of one or more of the following properties: • Abrasion resistance • Air content • ASR on the aggregate • Compressive strength • Creep • Freeze-thaw resistance • Heat of hydration • Hydraulic cement content • Modulus of elasticity • Mortar bar expansion • Permeability • Rate of strength gain • Scaling • Drying shrinkage • Concrete temperature • Unit weight • Workability (slump). Two states mentioned that they require a trial placement similar to that required for the LC-HPC concrete in Kansas. Two states mentioned that they require trial batches from each plant that supplies concrete to the project. From the responses, it appears that most states are willing to accept a new HPC mix for bridge decks based on laboratory trial mixes only. PERFORMANcE OF IN-SERVIcE STRUcTURES In the responses to the survey for this synthesis, five agencies stated that they routinely conduct tests of the hardened CIP concrete to check end product performance. The listed tests were permeability, surface resistivity, and chloride penetration resistance. Twelve agencies responded that they sometimes do tests for permeability, chloride ion content, in-place strengths, in-place air content, surface resistivity, petrographic analysis, and chloride penetration resistance. Twenty agencies responded that they never do tests of the hardened CIP concrete to check in-service performance. Most tests of the in-place concrete are only performed when sub-standard concrete is suspected. Regarding the current practices to evaluate short- and long-term performance of HPC in bridge decks, all the responding states rely on the quality control tests during construction and the biannual bridge inspections. One state responded that the only formal evaluation occurs if the deck is part of a research project. In summary, very little is done to determine the properties of the in-place concrete. BRIDGE DEcK cRAcKING As part of the survey for this synthesis, agencies were asked to identify the strategies that they are currently using to minimize cracking in CIP concrete bridge decks. Their responses are provided in Table 8. Strategies that were mentioned included immediate wet curing, only allowing Type A or Type A/F admixtures, requiring 20% pozzolans, requiring 55% coarse aggregate as a percentage of the total aggregate, specifying a minimum w/cm ratio, permitting slump adjustments only with the addition of admixtures, requiring the contractor to have a weather station on site, nighttime concrete placements, use of internal curing, use of polypropylene fibers, and limiting hand finishing. The use of wet curing applied in a timely manner and maintained for at least seven days was listed most often as the most effective strategy in minimizing deck cracking. Other strategies mentioned by more than one respondent were reduction in cement or paste content, limit on maximum con- crete temperature, control of water content, and evaporation rate control. The use of fogging equipment to control evapo- ration rate is illustrated in Figure 10. A number of less effective strategies to minimize deck cracking were identified by the responding agencies. These strategies included specifying a maximum slump; using pre- scriptive mix designs; specifying maximum and minimum concrete temperatures; using curing membranes and evapora- tion retardants; having high cement contents, high compressive strength requirements, and low w/cm ratios; and not requiring a trial slab placement before casting the deck. Cracking in concrete bridge decks was discussed in NCHRP Synthesis Report 333 (Russell 2004) because of the increased amount of cracking that had been observed in concrete bridge decks. At that time (2003), agencies were also asked about strategies used to minimize cracking in bridge decks; percent-

23 Strategy to Minimize Bridge Deck Cracking 2012 Survey Responses 2003 Survey Responses No. %1 % Specify maximum w/cm ratio 34 94 — Specify minimum concrete compressive strength 33 94 — Specify maximum concrete temperature at placement 32 94 80 Specify maximum slump 30 86 98 Specify minimum concrete temperature at placement 30 83 — Required fogging when evaporation rates are high 27 77 67 Specify minimum cementitious materials content 26 76 — Specify maximum cementitious materials content 19 54 33 Require use of the ACI surface evaporation nomograph 18 55 — Specify maximum water content 14 42 — Require windbreaks during concrete placement 13 38 22 Specify maximum concrete temperature during curing 9 30 — Require evaporation retardants 9 28 29 Specify a ratio between 7- and 28-day compressive strengths 5 16 — Specify a maximum concrete compressive strength 4 13 4 1 Percentages appear inconsistent because not every respondent answered every option. — = not included in the survey. TABLE 8 STRATEGIES USED TO MINIMIZE CRACKING IN CIP BRIDGE DECKS age responses are shown in Table 8. In comparison with the current survey, the biggest differences are that agencies cur- rently specify a maximum cementitious materials content and fewer agencies are specifying wind breaks during concrete placement. Otherwise, the strategies appear to be similar in percentage usage. SUMMARY OF CURRENT SPECIFICATIONS AND PRACTICES FOR CAST-IN-PLACE CONCRETE BRIDGE DECKS State specifications, in general, are prescriptive except for the specification of concrete compressive strength. Other primary characteristics that are considered in the development of pre- scriptive and performance specifications are permeability, workability, and ASR resistance. Most state specifications permit the use of SCMs with Class F fly ash being the most frequently used material. Silica fume and slag cement are used to a lesser extent and Class C fly ash and Class N pozzolan are the least used. All states permit the use of chemical admixtures with AASHTO M 194 Type A water-reducing admixture being the most frequently used. The quantities of cementitious materials are generally in the following ranges: • Minimum cementitious materials content of 560 to 750 lb/yd3, • Maximum cementitious materials content of 700 to 800 lb/yd3, FIGURE 10 Fogging equipment is used to control evaporation rates [Photo courtesy of the University of Kansas Transportation Research Institute].

24 • Maximum fly ash content of 15% to 30% of the total cementitious materials content, • Maximum silica fume content of 7% to 10% of the total cementitious materials content, • Minimum silica fume content of 5% to 7% of the total cementitious materials content, when used, and • Maximum slag cement content of 30% to 50% of the total cementitious materials content. Specified maximum w/cm ratios generally range from 0.40 to 0.50 for bridge decks. Aggregate specifications are generally the same for HPC and conventional concrete. Some state specifications include a combined grading for coarse and fine aggregates, which can improve the properties of HPC. All state specifications address construction practices. Most states now require seven or 14 days wet curing of CIP concrete bridge decks. All states require trial batches of concrete prior to acceptance of a new HPC mix for bridge decks. The type of test data to be supplied with the trial batch information varies. Most states accept a new HPC mix for bridge decks based on laboratory trial mixes only. Five agencies reported that they routinely conduct tests of the hardened concrete to check the end product performance, measuring permeability, surface resistivity, and chloride penetration resistance. The other agencies only do tests when substandard concrete is suspected. Many states have implemented strategies to reduce bridge deck cracking. The primary ones are specifying a maximum w/cm ratio, minimum concrete strength, maximum concrete temperature at placement, maximum slump, minimum con- crete temperature at placement, fogging when evaporation rates are high, and minimum cementitious materials content. The most effective strategy was the use of wet curing applied in a timely manner and maintained for at least seven days. Other effective strategies included reducing cement or paste content, limiting maximum concrete temperature, controlling water content, and controlling evaporation rate.