Effects of SP on flowability and cohesiveness of cement-sand mortar

The use of superplasticizer (SP) can effectively improve the flowability of concrete but adding an excessive dosage of SP, especially when the water content is high, may impair cohesiveness and thus cause segregation. This study was to evaluate the effects of SP on the rheology of mortar with different water contents. A number of cement-sand mortar samples were produced and tested for their flowability in terms of flow spread and flow rate, and for their cohesiveness using a microversion of the sieve segregation test. The results showed that the effects of SP on flowability and cohesiveness are dependent not only on the SP dosage but also on the water content. From these results, the “saturation dosage” for achieving highest flowability and the “segregation dosage” that would cause segregation were evaluated to provide design guidance. In-depth analysis revealed that the saturation dosage and segregation dosage are different and therefore should not be mixed up. Furthermore, for best overall flowability-cohesiveness performance, the highest permissible water content plus a relatively low SP dosage should be adopted.