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35 chapter six CONCLUSIONS use of SCMs reduces the permeability of concrete, which slows the rate of penetration of water and chlorides into the concrete. Many states report that the use of HPC has improved concrete performance, but they still express concern about deck cracking. The following practices were identified as factors in reduc- ing cracking of CIP concrete bridge decks: ⢠Decreasing the amount of water and cementitious materials in the concrete mix while achieving the spec- ified properties, ⢠Using the largest practical maximum size aggregate to reduce water content, ⢠Avoiding concrete compressive strengths higher than normally required, ⢠Applying wet curing immediately after finishing the surface and curing for at least seven days, preferably longer, and ⢠Applying a curing compound after the wet curing period to slow down the drying shrinkage and enhance the other concrete properties. The following practices were identified as less effective in minimizing deck cracking: ⢠Using lower w/cm ratios, ⢠Having high cement contents, ⢠Using high compressive strength concrete, ⢠Specifying maximum slump, ⢠Using prescriptive mix designs, ⢠Specifying maximum and minimum concrete tempera- tures, ⢠Using curing membranes and evaporation retardants, and ⢠Not requiring a test slab before casting the deck. Specifications for precast concrete are more performance- oriented than those for CIP concrete, with compressive strength being the property specified most frequently. Speci- fications for precast concrete place more reliance on the capability of the producer to develop mix proportions that will meet the specifications instead of providing detailed pre- scriptive requirements. Precast, prestressed concrete beams appear to be performing satisfactorily using the existing specifications with only a few performance issues reported in the survey. CONCLUSIONS In 1993, the FHWA initiated a national program to implement the use of high performance concrete (HPC) in bridges by encouraging the use of performance or performance-based specifications. States adopted this concept to varying degrees. Some states modified their specifications to include aspects of HPC without specifically calling the concrete HPC. At the same time, the state agencies have developed a wide range of specifications for HPC. Consequently, the use of HPC has increased but the results have been variable. All state specifications for concrete to be used in bridge structures follow the traditional approach of being prescriptive in nature. The use of performance specifications as proposed by the FHWA implementation program is largely limited to special provisions for specific projects. In general, the perfor- mance specifications only address strength and permeability. Nevertheless, numerous changes have been made to the speci- fications to enhance the performance of concrete particularly when used in bridge decks even though the concrete may not be called HPC. All state specifications now specifically permit the use of one or more secondary cementitious materials (SCMs)â fly ash, silica fume, and slag cementâalong with the use of hydraulic cements. The specifications, however, limit the quantities of these materials that may be used for both cast- in-place (CIP) and precast concrete. The following limitations were identified during a review of state specifications. ⢠Minimum cementitious materials content of 560 to 750 lb/yd3, ⢠Maximum cementitious materials content of 700 to 800 lb/yd3 for cast-in-place (CIP) concrete and 750 to 925 lb/yd3 for precast concrete, ⢠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, if used, and ⢠Maximum slag cement content of 30% to 50% of the total cementitious materials content. Specified maximum w/cm ratios range from 0.40 to 0.50 for CIP concrete and 0.38 to 0.44 for precast concrete. The
36 From the survey of state agencies, numerous changes in specifications and practices were identified that have improved performance. These include the following: ⢠Developing special provisions for specific projects, ⢠Using a performance-based specification for bridge decks, ⢠Implementing a specification addressing alkali-silica reaction (ASR), ⢠Using high-strength concrete in precast girders to improve durability, ⢠Providing multiple options for concrete constituent materials, ⢠Specifying the amount of drying shrinkage, ⢠Specifying permeability limits, ⢠Starting wet curing immediately after concrete placement, ⢠Specifying and ensuring a longer wet curing period than used previously, ⢠Testing for more concrete properties than previously, ⢠Specifying limits on rate of strength gain, ⢠Using lower cement contents, ⢠Using SCMs (fly ash, silica fume, and slag cement) in more applications, ⢠Optimizing aggregate gradation, ⢠Using a corrosion inhibitor, ⢠Using self-consolidating concrete (SCC), ⢠Placing concrete at night, ⢠Controlling evaporation rates, and ⢠Strictly enforcing air content requirements. The following practices were identified as contributing to adverse performance: ⢠High early-strength mixes used to allow early opening to traffic were more prone to cracking. ⢠High concrete strengths have led to cracking in bridge decks. ⢠The use of silica fume reduced workability and led to cracking in new decks. ⢠The use of shrinkage reducing admixtures meant that specified air content could not be maintained in the field. ⢠The use of shrinkage reducing admixtures has helped reduce cracking but not to a satisfactory degree. ⢠Increasing the cement content to obtain lower perme- ability resulted in more cracks in the deck. ⢠Use of an evaporation retardant resulted in excessive cracking in decks. ⢠The use of fly ash in bridge decks has resulted in increased cracking. ⢠The use of Class F fly ash content greater than 30% only produced limited success. ⢠Concrete with a rapid strength gain resulted in more deck cracking. ⢠The use of 14-day wet curing for decks did not eliminate deck cracking. These two lists indicate that different states have had conflicting experiences with some of the same practices. Overall, the information collected for this synthesis indicates that there is no single practice that can be used to enhance concrete bridge deck performance. SUGGESTIONS FOR FUTURE RESEARCH Responses to the survey for this synthesis provided the follow- ing suggestions for future research and development programs: ⢠Identify causes of cracks in concrete bridge decks. ⢠Study several types of structures to see if they can be improved to reduce deck cracking. ⢠Define how to achieve a low permeability concrete deck without shrinkage cracks and determine if expansive additives or polypropylene fibers would be effective. ⢠Develop effective means, using non-destructive or other tests, to ensure that concrete meets the required perfor- mance criteria for the intended environment. Tests that can be performed on fresh concrete would be particularly useful. ⢠Identify the most cost-effective methods of sealing cracks in decks to reduce future maintenance. ⢠Evaluate the effect of concrete compressive strength, modulus of elasticity, drying shrinkage, and creep on cracking. ⢠Investigate the use of internal curing to reduce deck cracking.
