Developments of the two-point workability test for high-performance concrete

An improved version of the Tattersall two-point workability apparatus has been developed for the study of the rheology of high-performance concrete mixes. The improvements include a more compact arrangement of motor, impeller and bowl and the computer logging of the speed/torque data so that the flow curve can be obtained in a continuous sweep of about 30 s. Two impeller systems were used—an axial interrupted helix and an H-shaped impeller driven in a planetary motion with an offset gear, that is corresponding to the MH and LM systems on the previous apparatus. Both impeller systems were calibrated to enable the Bingham parameters of yield stress and plastic viscosity to be expressed in fundamental units. The performance of the apparatus was assessed by tests on a range of moderately high-performance mixes incorporating superplasticizers and cement replacement materials. The offset H impeller system was found to be suitable for testing concrete with slumps ranging from about 50 mm up to collapse, and for assessing the development of rheology with time after mixing, and is therefore preferred for high-performance concrete.