Evaluation of ASTM C 494 procedures for polycarboxylate admixtures used in precast concrete elements

Stacey, Stephen Michael
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This thesis presents the investigation towards increasing the understanding of the effect of polycarboxylate (PCE) High-Range Water-Reducers (HRWRs) on the performance of concrete mixtures, with a special focus on concrete mixtures that will be used in precast concrete beam applications. Since the mid-2000’s, precast concrete elements at several major precast plants throughout Texas have displayed extensive micro-cracking along their exposed surfaces despite never being subjected to service load conditions. The precast industry’s transition from naphthalene based HRWRs to polycarboxylate based HRWRs was identified as a potential culprit for inducing the cracking phenomenon. The admixture qualification standard, ASTM C494, was closely investigated for its relevancy in passing and failing HRWR agents with respect to a mixture’s propensity towards the latent observed cracking effect. Key testing procedures of ASTM C494 were investigated by subjecting mixture designs that had performed both good and bad in the field with respect to micro-cracking. Testing proved that ASTM C494 testing protocols were insufficient and lacking towards predicting the micro-cracking behavior. Therefore, an additional parametric study through concrete and paste testing was created to investigate precast concrete mixture designs and their polycarboxylate admixture influences. The testing program focused on autogenous shrinkage through concrete restrained ring testing and paste analysis via the corrugated tube test. Tests were carried out by introducing different HRWR types and dosages, and quantifying their outlying observed strain effects with respect to one anther. Results have shown that HRWR type and dosage play a vital role in the delay of final set and extent of autogenous shrinkage observed. In addition to the parametric study, a field study was performed in order to correlate the laboratory testing back to the micro-cracking development on concrete elements exposed to natural conditions.