ABNORMAL CONCRETE PERFORMANCE IN THE PRESENCE OF ADMIXTURES

Review of literature on abnormal performance of concretes containing admixtures revealed (1) the importance of control of early hydration reactions by calcium sulfates for control of early stiffening, and (2) cases of rapid slump loss or cement/admixture incompatibility for which the causes were uncertain. Results of ordinary test methods for early stiffening of mortars or cement pastes do not appear to represent behavior of cements in concretes mainly because mixing shear rates are lower than in concretes. Concrete slumps are measures of yield stress, and are functions of the volume fraction and yield stress of the cement paste. Paste stiffness increases with power input during high-shear rate mixing. Mixing procedures maintaining good temperature control at higher-shear rates (500 to 4000 s to the -1) are described. Performance tests were developed for early stiffening and "slump loss" of cement pastes using a Haake Rotovisco RV 12 viscometer to determine shear stress at yield, and at higher shear rates to determine plastic viscosity. Paste "slump loss" tests were performed to match the slump loss test results for eight different field and laboratory mixed concretes. Good agreement between the concrete and paste results was obtained if the mixing power input into the Waring Blendor was adjusted properly for the batch size and water-cement ration (w/c). Required power input decreased with increasing w/c but was nearly independent of fly ash content. Viscometer test results were also correlated with minislump tests of 16 pastes of different compositions after being mixed by the new procedures. These test methods should provide results that relate reliably to the early stiffening and slump loss behavior of concretes when pastes of the same compositions as in concretes are tested at the same temperature. Pretesting cement/admixture combinations for compatibility should be an economical means to prevent early stiffening problems.