Desarrollo de un dispositivo de análisis de la reología del hormigón

Abstract

The aim of this study is the development of one device of fresh concrete rheological characterization (Horreómetro), valid for concrete with slump values ≥ 80 mm, and applicable to self-compacting concrete. The selection of the most appropriate impeller, between the two studied vane impellers, Pala Coruña and Pala Demo, and the calibration of the device, through the results of two tests, the Stress Growth Test ( SGT ) and Flow Curve Test (FCT ), are performed by comparison with a commercial rheometer. SGT consists on the rotation of a vane in a concrete mix at a constant low speed while recording the generated torque. The maximum torque, converted into stress, corresponds to the static yield stress. This test only requires the definition of the rotation speed. In order to adjuste the SGT each impeller was rotated at three constant speeds, 0.56rpm, 1.5 rpm and 3 rpm. The FCT measures shear stress at different shear rates (flow curve) after a pre-shear period at a constant speed that produce the structural breakdown and minimizes the effects of thixotropy. Torque and vane rotation speed datas on the decrease curve, converted to fundamental units of shear stress and shear rate, enable to represent the material flow curve. This test requires the definition of the rotational speed during the pre-shear period (breakdown time) and the definition of the elapsed time of this preriod. In order to adjuste the FCT, each impeller was rotated at two speeds, 10rpm and 30rpm, both combinated with tow different breakdown times of 5 and 90s. In the experimental work (both in FCT and in SWT), a concrete with a w/c ratio of 0.4 was used. Moreover, these two test were repited twice at different times (10, 15, 20 ,30 min). The tests were performed in the prototype rheometer and in the commercial rheometer at the same time, measuring before and after each test, the slump value. The results have allowed the selection of the proper vane and of the proper test parameters (speeds and breakdown time) accorging to the geometry and the engine conditions of the protoype rheometer.