Visual-Based Evaluation Method for Optimizing the Dosage of PCE-Based Superplasticizer for SCC Paste and Concrete Mixtures

Abstract

Marsh cone and mini-slump flow tests are used widely to optimize the paste phase of self-compacting concrete (SCC). These tests are used to define the saturation point of a superplasticizer (SP) and to investigate their compatibility with cement and fine materials. However, neither tests provide a direct assessment regarding the segregation tendency of cement paste at high SP dosages. In this paper, a visual index was proposed to select the optimum dosage of polycarboxylate (PCE) superplasticizers for SCC paste and concrete mixtures. The method is based on the observation that the cementitious matrix undergoes an internal disruption once the optimum SP dosage is surpassed. This disruption is manifested with time in the form of bleeding and segregation, with a gradual formation of a white layer at the top surface. This phenomenon was captured photographically for mixtures with increasing SP dosage and presented as a visual deposit surface index (VDSI) to highlight the segregation tendencies of the mixtures at increased SP dosage. VDSI can be used to identify the saturation dosage of SP with results comparable to those obtained from the mini-slump flow test. The method is shown to be valid for mixtures incorporating cement alone or blended with limestone dust (LSD) and natural pozzolan (NP). It was successively applied to both cement paste and concrete mixtures.