High-performance concrete (HPC) is a complex mixture often containing five to ten different materials. Interaction between the various materials can cause wide variations in workability, which also depends on the specific materials and proportions used. Determining workability properties by testing concrete is not always practical. Extensive concrete testing requires a large amount of materials and labor, which is expensive. There is therefore a need to predict the workability of concrete through a simpler, cheaper laboratory approach. This paper describes an attempt to utilize rheological measurements in cement paste as a reasonable indicator of concrete workability. Concrete workability is defined according to American Concrete Institute (ACI) as the ease of placement of concrete and is usually quantified by the result of the slump cone test.
Rheological tests on cement paste were used to select the type and dosage of mineral admixtures that improved concrete workability. The conclusions reached based on cement paste tests were validated by concrete slump tests. It was also deemed interesting to compare the fundamental parameters (yield stress and viscosity) measured with a fluid rheometer with the results from two commonly-used empirical tests, the mini-slump and the Marsh cone tests. If a relationship could be established, the empirical tests could be used to design materials for a given yield stress and viscosity or, at the very least, rank different materials on the basis of yield stress or viscosity.
The decision to study the influence of mineral admixtures was dictated by the recent increase in use of mineral admixtures for improved concrete durability. Economics (lower cement requirement) and environmental considerations have also had a role in the growth of mineral admixture usage. The lower cement requirement also leads to a reduction in the amount of carbon dioxide generated by the production of cement, while the use of a mineral admixture utilizes a product that would ordinarily be bound for the land fill. Thus there is a double environmental benefit from using mineral admixtures.