Aveline Darquennes

Determination of time-zero and its effect on autogenous deformation evolution

ABSTRACT The aim of this study is the determination of time-zero, a reference time from which the autogenous deformation is expressed, and the effect of this choice on the evolution of autogeneous deformation of concretes with different slag proportions (0 and 71%). The determination of time-zero is based on the setting evolution measured by means of several experimental methods (mechanical, electrical and ultrasonic). Autogenous deformation is also measured with a new shrinkage testing set-up developed at the Université Libre de Bruxelles. Following the experimental results it appears that a time-zero equal to the final setting is the most adequate choice for classic concretes. For slag cement concretes, characterized by an expansion of their cement matrix at early age, time-zero should be taken equal to the time of the expansion peak.

Behaviour of slag cement concrete under restraint conditions

ABSTRACT Some constructions built with slag cement concrete exhibit cracking at early age due to restrained shrinkage. To determine the slag effect on cracking, this study focuses on the autogenous deformation evolution of concretes characterized by different percentages of slag (0 and 42% of the binder mass) under free and restraint conditions by means of the TSTM device (Temperature Stress Testing Machine). Despite the fast kinetics of its autogenous deformation, the cracking appears later for the slag cement concrete than for the Portland cement concrete. This behaviour is related to the swelling of its cementitious matrix at early age and its large capacity for relaxing the stresses.

Restrained shrinkage of massive reinforced concrete structures: results of the project CEOS.fr

Within the CEOS.fr national research project, several experiments on massive concrete structures were conducted to improve the knowledge on the cracking phenomenon. In this paper, experiments where deformations at early age are restrained are presented. Testing bodies are I-shaped and two largely dimensioned steel struts are placed laterally between the two transverse heads to prevent almost any shrinkage. Three testing bodies were realized: RG8, the reference one; RG9, with a reduced reinforcement and RG10, with an increased cover. A full set of measurement was used for auscultation of these beams during early age. Optical long base fibres gave information on the relative displacement of the central part of the beam. Local measurements of strains in concrete were given thanks to Vibrating Wire Extensometers. Gauges on rebars produced data of the strain on the first reinforcement layer, and the force in struts was monitored. With this, the force and stresses in concrete and rebars could be deduced. This huge amount of data allows verifying the phenomenology of the concrete. Various hypotheses were analysed to explain the strain measured and the corresponding forces in each component during specific period of early age. A first analysis of the cracking process shows that the cracks could appear for stresses below the tensile strength.

Shrinkage of slag cement concrete in free and restrained conditions

Slag cement concrete offers many advantages that lead to its intensive use in the Construction Industry in Belgium but it would present a higher sensitivity to cracking at early ages than Portland cement concrete. This preliminary study focuses on the evolution of autogenous and drying shrinkage and their effect on cracking sensitivity at early ages of slag cement concretes. Moreover, the influence of an extra period of the curing time on the total shrinkage is also underlined. In this way, various slag cement concretes were studied and compared to reference Portland cement concretes characterized by similar rheological and mechanical performances in free and restrained conditions. This experimental study highlights the role of the autogenous deformations in the evolution of the total shrinkage of slag cement concrete at early ages. Actually, our experimental data show that its evolution in terms of magnitude and kinetics is very different from that of Portland cement concrete. It was also observed that the slag cement concrete under restrained condition cracks earlier (between 15 and 60 days) than Portland cement concrete (after several months). This experimental research has also shown that the total shrinkage of slag cement concrete can be decreased partially by prolonging the curing time.