Analysis of concrete workability through its rheological properties

Abstract

Single point tests, which were classified by Tattersall as being Quantitive Empirical Tests (Tattersall, 1976), have been used to test the workability characteristics of fresh concrete for decades. However, these kinds of tests have their limitations. Tattersall wrote that "it remains common experience for two concretes giving identical slump values to behave quite differently" (Banfill, 2010). This led to the development of the first viscometer, making it possible to study concrete rheology in a more precise manner. In this research, various concrete mixes varying in water content, super plasticizer dosage and different cement replacement percentages by limestone filler and pulverized fuel ash, were assessed in their plastic form. This was done by means of both Quantitive Empirical tests (single point tests: slump test and flow table), as well as Quantitive Fundamental tests (two-point test: concrete viscometer). Results obtained from different mixes were then extracted and analyzed. The research also looked into possible correlation between single point test results and rheological properties obtained by means of the two point workability test. The validity and reliability of these results were then analyzed by comparing them to results published in literature. From the results obtained, it was confirmed that assessing concrete workability through single point tests is not adequate and can be misleading. Assessing concrete workability by means of a concrete viscometer also made it possible to investigate concrete thixotropy. Thixotropic indexes obtained from the concrete viscometer were also compared with results obtained by means of the Portable Vane test apparatus which was developed as part of this research. Portable Vane test results were also compared with other single point tests results and were found to correlate well with the yield value. A number of concrete samples were also cast and tested for their compressive and tensile splitting strength. This was done to assess the effect of the different variables assessed in their plastic form on the hardened properties of concrete.