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

Correspondence between the values of prestressing force measured by thedifferent methods was good (3.7% mean discrepancy between methods) and less than the5% code maximum (ACI 318, 2008, Provision 18.20.1). Precision improved during thesecond beam cast, with the error halved (1.4% discrepancy). The reported prestressingforce applied for each beam represents the mean prestressing force associated withmeasurements of ram pressure and strand elongation.3.6.4 Beam CastingAfter the strands were fully tensioned, the beams could be cast. A similarprocedure to that observed at the precast-beam fabricator (Section 3.6.1) was used; a fewparticulars of the casting process used at the Ferguson Laboratory are noted.Concrete for the U-beams was provided by Coreslab Structures, as previouslydiscussed in Section 3.3.1. After batching, this concrete was loaded into mixing trucksrented from a local ready-mix concrete provider and transported to the laboratory(approximately 15 miles). Because of the journey, and concerns about concrete flashsettingin the trucks, chemical retarder (5 oz/Cwt) was incorporated in the concretemixture design (Table 3.1).When the concrete trucks arrived at the laboratory, the concrete slump wasmeasured as shown in Figure 3.43. High-slump concrete (9 in. goal) was desired for easyplacement of the concrete, particularly in the bottom flange, congested by the largenumber of prestressing strands used. If trucks arrived with low-slump concrete, waterand/or superplasticizer could be mixed in before placement.A large number of cylinders were cast alongside the beams (Figure 3.44). A totalof 48 cylinders were match-cured to locations in the beam, with the majoritycorresponding to the “release point” and “hot spot” locations (described previously in140