Ahmed Loukili

Evaluation of plastic shrinkage cracking of self-consolidating concrete

Self-consolidating concrete (SCC) is a fluid concrete, cast without vibration, that has a self-leveling ability that makes it appropriate for flat structures such as slabs or industrial floors. This article reports on a study that evaluated plastic shrinkage cracking of SCC. The authors compared five SCC mixtures with compressive strengths ranging from 30 to 50 MPa to five ordinary concrete (OC) mixtures. Free and restrained plastic shrinkage tests were performed in different drying conditions. When the evaporation rate is moderate (drying at 20 deg C and a relative humidity of 50%), plastic shrinkage occurs before and during setting. When the evaporation rate is high (windy condition), plastic shrinkage occurs only in the plastic state, that is, before setting. In the presence of wind, SCC and OC mixtures have almost the same plastic shrinkage and restrained shrinkage tests reveal that cracks of SCC tend to be less wide than cracks of OC. However, when the evaporation rate is low, SCC mixtures exhibit a higher plastic shrinkage than OC mixtures, due to their lack of bleeding. The authors conclude by recommending curing in order to protect SCC against evaporation at the fresh state.

Transition from energy dissipation to crack openings during continuum–discontinuum fracture of concrete

Fracture in quasi-brittle materials like concrete is mainly governed by two processes: energy dissipation and discontinuous displacement fields or discrete cracks. In this study we discuss the simultaneous development of both types of fracture. Energy dissipation and crack openings are analyzed during the fracture development by acoustic emission and digital image correlation (DIC) respectively. DIC results are processed using nonlinear damage modelling and lattice element modelling to reveal the microcracking zone. It is observed that overall failure mechanism is governed by three processes (1) high energy dissipation phase during which energy release rate increases (2) continuum–discontinuum transitional phase where energy dissipation rate attains its maximum value, and (3) discontinuum phase where energy dissipation rate drops and crack openings start increasing.

Influence of the initial water saturation of aggregates on concrete shrinkage

The study presented in this paper deals with the influence of the water saturation of natural aggregates on shrinkage and cracking sensitivity of concrete at early age. Limestone crushed gravels were specially chosen for this study due to their high porosity. The degree of water saturation of gravels varied, from a given concrete mixture, keeping the total water content constant. Significant variations of the plastic shrinkage and early age autogenous deformations were observed. Due to the kinetics of absorption of gravels, the water content remaining in cement paste was different from the effective water content. This was confirmed by the values of mechanical properties and porosimetry. The evolutions of elastic modulus and tensile strength were experimentally assessed and used to compare the cracking sensitivity of the three studied concretes by evaluating the self generated stresses.