Using early age concrete properties to determine setting time

Abstract

When fresh concrete sets, it changes from a Bingham fluid to a solid, with strength and stiff-ness. Changes in concrete volume however starts taking place from the moment when water comes into contact with the cement, starting the hydration process. The chemical reaction be-tween the water and the cement cause shrinkage because the resulting product is smaller than the two reactants. The reaction is very slow in the beginning, but when it speeds up it causes an increased temperature that can result in an expansion of the mix. These volumetric changes increase in high strength concrete due to the reduced water/cement ratio, the increased cement content and the use of High Range Water Reducing Agents (HRWRA). It is important to know the exact setting time of concrete as volume changes that takes place after setting, will cause internal stresses, which could exceed the early age strength, resulting in the formation of cracks. Existing setting time test methods do not take into account the effect of changes in water/ce-ment ratio and concrete composition on the setting time. The aim of this study was to determine a setting time for concrete based on the point in time when the concrete starts behaving as a solid. A variety of test methods were used to determine and compare early age properties of concrete. The point in time when a rapid change in behaviour was observed was used as point from where early age shrinkage would have an effect on the stresses or strains that develops in the concrete. The measured shrinkage results were recorded from as soon as possible after casting but the influence of the shrinkage should be considered from this time onwards. The results of this investigation confirm that neither the initial nor final setting time typically recorded are representative of the time when the concrete starts behaving as a solid. The time that it takes concrete to change from a viscous liquid to a solid is not a constant, but depends on w/c ratio, specimen size, temperature, inclusion of admixtures and the type of test used. The use of HRWRA retards the temperature increase caused by heat of hydration as well as the initial strength and stiffness development, but this trend is reversed within 24 hours of casting. A comparison between the setting times calculated from load application (such as penetration testing) and other setting time methods (such as derivatives of heat of hydration measurements) show that HRWRA have an influence on the setting times. The mixes without admixtures gave similar setting times, when the average for the different tests were used whether calculated from load test data or from other setting time test methods. The use of HRWRA resulted in differences of almost 2 hours for w/c ratios below 0.45 between the different types of setting time measurements. This confirms that when load tests are used in mixes containing admix-tures, the time when concrete change from a Bingham fluid to a solid can be wrongly estimated. This incorrect assumption of setting time could cause an over estimation of the early age shrink-age that can cause stresses or cracking in high strength concrete.