Modification of Excess Mortar Model for Paste Threshold Theory of Self-compacting Concrete

The performance of self-compacting concrete (SCC) could be predicted through the paste rheological threshold theory based on certain assumptions. As the basic assumption, the excess mortar model regards all the coarse aggregate particles ignored the particle size distribution. That neglects the arrangement changes caused by actual factors such as shape and particle size distribution. This paper focused on modification of the excess mortar model considering the influence of the coarse aggregate packing characteristics. To study the effects caused by packing state of coarse aggregate particle system on the excess mortar system, the size ratio sG and void content coefficient β were proposed for a better description of the packing status of coarse aggregate particles. Thus, the calculation methods for correcting the excess mortar volume and the related excess mortar film thickness were proposed, and a modified excess mortar model was established. According to the modified model, the calculation formulas for the paste rheological threshold were modified. The validation of the proposed model was conducted by paste and SCC performance tests. As visualization results of the paste threshold theory, the self-compacting zones could reveal the prediction accuracies of the calculated thresholds, by which the prediction accuracy comparison could be done. The results show that through the modification, the relative accuracy index Δε ≥ 0, and the modification index Δ|ε′ − 1| < 0, indicating that the modification increased the prediction accuracies. Both the overlap area and relative position of the self-compacting paste and concrete zones achieved by the original and new methods could be evaluated. Finally, the variation trends of paste thresholds under the effect of coarse aggregate were analyzed. The modified excess mortar model can achieve a more accurate prediction of SCC performance through simple paste performance test.