High Performance Concrete Specifications and Practices for Bridges (2013)

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25 chapter four CURRENT SPECIFICATIONS AND PRACTICES FOR PRECAST, PRESTRESSED CONCRETE GIRDERS AND DECK PANELS stated. Other specified properties such as slump and air content are similar to corresponding values for CIP deck concrete. Some states, however, do not consistently specify an air content for the higher strength concretes used in precast concrete beams. Agencies were asked which characteristics were specified in their performance specifications and which were considered in developing their prescriptive specifications. The results are shown in Figures 11 and 12 for beams and deck panels, respectively. For precast, prestressed concrete beams, the characteristic most frequently specified for the performance specifications and considered for prescriptive specifications was concrete compressive strength. Workability, permeability, freeze-thaw resistance, and ASR resistance were the next most frequently reported characteristics, but more commonly listed in the development of prescriptive specifications than in performance specifications. For precast, prestressed concrete deck panels, it was difficult to identify a clear pattern from the survey results for both the performance and prescriptive specifications. This may be the result of fewer states using precast concrete deck panels and a lack of experience in their use. The use of precast deck panels is shown in Figure 13. CONCRETE CONSTITUENT MATERIALS The concrete constituent materials specified for use in precast, prestressed concrete members are similar to those specified for CIP concrete and described in chapter three. Most specifi- cations explicitly allow the use of Type III cement and silica fume. Both of these materials facilitate the development of high early strengths required for prestress transfer. Silica fume is also beneficial in reducing concrete permeability. Concrete for use in precast, prestressed concrete members is generally specified to have a minimum cementitious materials content that ranges from 560 to 750 lb/yd3 of concrete. The specified maximum cementitious materials content ranges from 750 to 925 lb/yd3. However, exceptions may be made to achieve higher strength concretes. The specified percentage limits for cementitious materials contents for precast concrete are similar to those for CIP concrete. This chapter addresses the specifications and practices for the use of HPC in precast, prestressed concrete for bridge girders and deck panels. The information was obtained from the synthesis survey and from a review of the state specifications, some of which include the information in the same sections that deal with CIP concrete and others of which address precast, prestressed concrete in a separate section. The state standard specifications for concrete to be used in precast, prestressed concrete members are less prescriptive than for CIP concrete. More reliance appears to be placed on the capability of the precaster to develop the concrete mix proportions and to produce an acceptable finished product, allowing the specifications for precast products to be more performance-based. As part of the survey, agencies were asked if they had imple- mented HPC in precast, prestressed concrete components. Twenty-one agencies responded that they had and 16 agencies responded that they had not. Several agencies stated that they had not seen any need for or advantage to using HPC because precast, prestressed girders have performed adequately. SPECIFIED PROPERTIES The primary performance criteria for precast, prestressed con- crete beams and deck panels are concrete compressive strength at transfer of the prestressing force and at a later age. The age at which the prestressing transfer occurs is determined by the precaster’s production schedule and can be as short as 12 hours for a daily production cycle or as long as three days over a weekend. The later age for the design strength is usually specified at 28 days, although 56 days is sometimes used for higher strength concretes. In addition to specifying compressive strength, some states have developed permeability specifications. The factors that contribute to high-strength concrete also help produce a low permeability concrete. It is, therefore, much easier to achieve lower permeabilities with precast, prestressed con- crete because of the higher quantity of cementitious materials used in the product, the greater use of SCMs, the lower w/cm ratio, and the use of heat curing (PCI 2011). The specified maximum w/cm ratios for precast, pre- stressed concrete vary from 0.38 to 0.44. No minimum is

26 7 8 2 2 6 8 0 17 5 2 2 2 11 11 2 1 11 10 4 19 3 2 4 0 0 5 10 15 20 25 30 No. of Agencies Characteristic Performance Specifications Prescriptive Specifications FIGURE 11 Characteristics included in performance specifications and considered in prescriptive specifications for precast, prestressed concrete beams. 5 8 3 4 3 9 2 8 5 3 3 0 3 8 2 1 2 6 4 4 2 2 3 1 0 5 10 15 20 25 30 No. of Agencies Characteristic Performance Specifications Prescriptive Specifications FIGURE 12 Characteristics included in performance specifications and considered in prescriptive specifications for precast, prestressed concrete panels.

27 To obtain information about the actual practices, agencies were asked to identify the percentage usage of SCMs in precast, prestressed concrete beams and deck panels. The number of responses for each percentage range is summarized in Tables 9 and 10 for beams and deck panels, respectively. Both tables show similar trends, with Class F fly ash and silica fume being used most frequently in beams and panels. The frequent use of Class F fly ash is similar to that for CIP concrete decks. Agencies were also asked about the percentage usage of chemical admixtures conforming to AASHTO M 194, corrosion inhibitors, shrinkage reducing admixtures, and expansive components in precast, prestressed concrete beams and deck panels. The number of respondents for each percent- age range is shown in Tables 11 and 12 for beams and deck panels, respectively. The survey results for both beams and deck panels show similar patterns. Agencies use a variety of chemical admixtures specified in AASHTO M 194, with Types A and F being used the most. This is similar to those used for CIP bridge decks. Corrosion inhibitors, shrinkage reducing admixtures, and expansive components are used in a relatively small number of applications. CONSTRUCTION PRACTICES Agencies responding to the survey for this synthesis indicated that precast, prestressed concrete components may be heat cured with either steam or radiant heat until the specified strength for release of strands is achieved; or wet cured for a minimum period of three, four, seven, or 14 days. Some FIGURE 13 Precast concrete deck panels [Photo courtesy of NYSDOT]. Supplementary Cementitious Material Extent of Use as a Percentage of All Bridges None 1 to 33 34 to 67 68 to 100 Fly Ash Class C 24 2 1 3 Fly Ash Class F 10 11 2 6 Pozzolan Class N 23 4 0 1 Silica Fume 14 10 2 2 Slag Cement 16 5 3 3 TABLE 9 NUMBER OF AGENCIES REPORTING THE USE OF SCMS IN PRECAST, PRESTRESSED CONCRETE BEAMS Supplementary Cementitious Material Extent of Use as a Percentage of All Bridges None 1 to 33 34 to 67 68 to 100 Fly Ash Class C 22 2 1 1 Fly Ash Class F 12 6 3 4 Pozzolan Class N 22 3 0 0 Silica Fume 16 6 1 1 Slag Cement 18 3 3 1 TABLE 10 NUMBER OF AGENCIES REPORTING THE USE OF SCMS IN PRECAST, PRESTRESSED CONCRETE DECK PANELS

28 specifications limit the maximum temperature to 160° F for either the enclosure or the concrete during heat curing. TESTING AND ACCEPTANCE PRACTICES Many agencies reported that the only requirement for accep- tance of new HPC mixtures for precast, prestressed concrete components is compressive strength. Other agencies reported requiring test results for slump, air content, temperature, per- meability, or ASR of the aggregates. Some agencies require the same information for precast, prestressed concrete as for CIP concrete. Some states require or permit the use of match curing of cylinders for the measurement of compressive strength at the time of prestress transfer. PERFORMANCE OF IN-SERVICE STRUCTURES Four agencies reported that they routinely conduct tests of the hardened precast, prestressed concrete to check end product performance. The listed tests were surface resistivity, modulus of elasticity, and permeability. Seven agencies reported that they sometimes perform tests for in-place strength, surface resistivity, and permeability. Sixteen agencies reported that they never do in-place tests of the hardened concrete to check end product performance. It appears that most tests of the hardened precast, prestressed concrete are only performed when sub-standard concrete is suspected. Instead, agencies evaluate short- and long-term per- formance of HPC in precast, prestressed components based TABLE 11 NUMBER OF AGENCIES REPORTING THE USE OF ADMIXTURES IN PRECAST, PRESTRESSED CONCRETE BEAMS Admixture Extent of Use as a Percentage of All Bridges None 1 to 33 34 to 67 68 to 100 AASHTO M 194 Type A—Water-reducing admixtures 8 6 5 8 AASHTO M 194 Type B—Retarding admixtures 14 5 5 1 AASHTO M 194 Type C—Accelerating admixtures 18 3 4 0 AASHTO M 194 Type D—Water-reducing and retarding admixtures 12 7 4 2 AASHTO M 194 Type E—Water-reducing and accelerating admixtures 18 4 2 1 AASHTO M 194 Type F—High range water-reducing admixtures 3 3 7 15 AASHTO M 194 Type G—High range water-reducing and retarding admixtures 14 5 3 4 Corrosion Inhibitors 14 7 2 4 Shrinkage Reducing Admixtures 20 4 0 1 Expansive Components 24 0 0 0 TABLE 12 NUMBER OF AGENCIES REPORTING THE USE OF ADMIXTURES IN PRECAST, PRESTRESSED CONCRETE DECK PANELS Admixture Extent of Use as a Percentage of All Bridges None 1 to 33 34 to 67 68 to 100 AASHTO M 194 Type A—Water-reducing admixtures 9 5 4 6 AASHTO M 194 Type B—Retarding admixtures 14 5 4 0 AASHTO M 194 Type C—Accelerating admixtures 15 3 4 0 AASHTO M 194 Type D—Water-reducing and retarding admixtures 13 6 4 0 AASHTO M 194 Type E—Water-reducing and accelerating admixtures 15 4 2 1 AASHTO M 194 Type F—High range water-reducing admixtures 9 2 4 8 AASHTO M 194 Type G—High range water-reducing and retarding admixtures 16 4 1 2 Corrosion Inhibitors 1 4 4 2 3 Shrinkage Reducing Admixtures 19 3 0 0 Expansive Components 20 0 0 0

29 on information from the biannual bridge inspections. One state responded that the only formal evaluation occurs in connection with research projects. SUMMARY OF CURRENT SPECIFICATIONS AND PRACTICES FOR PRECAST, PRESTRESSED CONCRETE GIRDERS AND DECK PANELS All state specifications permit the use of SCMs, with Class F fly ash and silica fume being the most frequently used materials. Slag cement is used slightly more frequently than Class C fly ash and Class N pozzolan. All states permit the use of chemi- cal admixtures with AASHTO M 194 Type A water-reducing admixture and Type F—high range water-reducing admixture being the most frequently used. The quantities of SCMs are similar to those listed for CIP concrete at the end of chapter three. Specified maximum w/cm ratios generally range from 0.38 to 0.44, which are slightly lower than for CIP concrete. Mix proportions for precast, prestressed concrete are selected to provide a high early strength for release of the prestressing strand as well as a minimum 28- or 56-day com- pressive strength. More reliance appears to be placed on the capabilities of the precaster to develop the mix proportions rather than prescribing detailed requirements.