The patch microstructure in concrete: The effect of superplasticizer

It has been previously shown that laboratory- and field-mixed concretes exhibit dense areas or patches of hardened cement paste (hcp) which are sharply delineated from adjacent, highly porous areas. Direct experiment with long-continued concrete mixing showed that this microstructural pattern is not due to inadequate mixing. An experiment was conducted to determine whether this distinctive microstructure was associated with the flocculation inherent in most fresh concretes. A conventional laboratory concrete was batched and after preliminary mixing a polycarboxylate-type superplasticizer was incorporated. The superplasticizer was added in two successive stages, a modest dose causing some increase in slump, and then a heavy additional dose sufficient to cause complete collapse of slump. SEM examination indicated that the patchy microstructure existed in the ‘base’ concrete, and was retained in the superplasticized concretes as well. Thus the patchy microstructural pattern is not generated as a consequence of the flocculated state that exists in most fresh concretes.