Bursting and Spalling in Pretensioned U-Beams - Ferguson ...

3.3 CONCRETE PROPERTIES3.3.1 Concrete Mixture DesignEfficient precast concrete fabrication demands high early-strength concretes.Typically, large amounts of Type III cement (6 to 7 sacks, or 570 to 670 lb/cy) is mixedwith small amounts of water (water-cement ratio, w/c < 0.40) so that beams can reachtheir release strengths (4 to 7 ksi) within 18 to 24 hours. Beams can then be released andmoved from the pretensioning bed the day after casting.Precast concrete mixes are highly engineered: much high-range water-reducer isneeded for workable concrete with the low w/c used. The balance of coarse aggregate tofine aggregate may be lower than in conventional concrete (1 to 1.5:1, compared to 1.5 to2:1) to avoid segregation while the concrete is fresh. Lastly chemical retarders are oftenused to prevent flash setting, especially during hot Texas summers.Concrete made with Type III cement is not commercially available as ready-mixin the Austin area at present, likely due to insufficient demand and concerns over flashsetting. A local precast concrete fabricator, Coreslab Structures, was able to provide helpdesigning the concrete mixture proportions for application on this project, and donatedthe concrete, batched at their facility 15 miles north of the Ferguson Laboratory. Theconcrete mixture design used for both beams is shown in Table 3.1. This design is similarto high-early-strength mixtures used by Texas precast highway-beam fabricators.The design used is a “straight-cement” concrete mixture, containing no fly ashreplacement. Within the first year of this project, TxDOT mandated the use of Class F flyash in precast concrete mixes at 25% replacement of cementitious material (Marek,2008). The purpose of this new prescription was control of alkali-silica reaction (ASR), aslow-acting chemical reaction in which alkalis from cement and other sources interactwith reactive (glassy) silica in aggregate to form an expansive gel. The provision of fly104