Influence of Admixtures and Temperature on the Performance of Shrinkage Compensating Concrete

摘要

The performance of shrinkage-compensating concrete, in terms of reducing the risk of cracking, mainly depends on correct synchronisation between two different processes inside the concrete: the evolution of its early age mechanical strength which is controlled by cement hydration, and the expansion rate which is controlled by expansive agent hydration. Recently the use of shrinkage reducing admixtures (SRA) has been recommended in shrinkagecompensating concrete due to the improvement in its performance in the absence of prolonged wet curing, thanks to the synergy achieved by combining SRA and CaO or MgO-based expansive agents. Some studies state that this positive combination is due to retardation in the hydration rate of the expansive agent which is able to induce a more efficient reinforcement tension (compressive stress in the concrete) due to improved steel-concrete bonding. In this paper, the influence of the type of superplasticizer and temperature on the hydration processes of the cement and the expansive agent (CaO) have been analysed. In order to evaluate the CaO hydration, several expansive cement paste samples with different polyacrylated ester-based superplasticizers, with and without SRA, were prepared at different temperatures and analysed using TG (thermogravimetry). Compressive strength, workability loss, setting time and restrained expansion tests were also carried out on concrete. The results indicate that the type of superplasticizer used and/or the temperature have different effects on the rate of cement hydration and on the calcium oxide, especially during the first 24 hours of hydration. Some indications of the correct way to design a shrinkage-compensating concrete in different situations are presented.