Brice Delsaute

Decoupling Thermal and Autogenous Strain of Concretes with Different Water/Cement Ratios During the Hardening Process

At early age, the autogenous strain and the coefficient of thermal expansion (CTE) are two of the most important concrete properties that are responsible for volume changes of cement-based materials and therefore for possible cracking in concrete structures (especially massive structures). Autogenous strain and CTE are dependent on the water/cement (W/C) ratio. A new experimental approach was developed to define both properties as well as the initial and final setting time with a single test. Every two hours, thermal variations of 3 °C are applied on a concrete sample with the device so-called BTJADE (from the French acronym BeTon au Jeune Age, Deformation Endogene). The initial setting time is the moment when the value of the CTE no longer corresponds to a liquid. The final setting time corresponds to a zero value of the derivative of the autogenous strain before the swelling of the concrete. Results of the initial and final setting time are compared to the evolution of the transmission of the ultrasound P- and S-waves. Three different compositions are investigated to define the influence of the W/C ratio (0.4, 0.5, and 0.6), and a fourth composition, for which the binder is composed of only 25 % cement, is also used in order to validate all observations. A new model expressed in function of the equivalent time or the advancement degree of reaction (defined with isothermal calorimetry) is proposed for computational modelling of the autogenous strain and the CTE. Effect of the W/C ratio and the physical mechanisms are highlighted.

Applicability of ultrasonic measurement on the monitoring of the setting of cement pastes: effect of water content and mineral additions

Until now, standardized methods developed for the detection of initial and final setting time of cement paste were destructive and non-continuous. The well-established ultrasonic pulse transmission velocity (UPV) method can be correlated to the setting process of cement-based materials by the measurement of P-waves. However, S-waves seem to be better indicators of the setting of cement-based materials, because they are more sensitive to the solid matrix connectivity and less dependent to the air content. A new methodology, based on the combined monitoring of P-waves and S-waves, has already been developed to determine the setting times of mortar and concrete. The purpose of this study is to extend this methodology to cement pastes. Ordinary portland cement pastes with three water/cement ratio between 0.3 and 0.5 have been investigated. In addition, a portland cement, blast-furnace slag and limestone filler blended cement paste with a 0.4 water/cement ratio is studied in order to validate all observations. This study shows that the dynamic elastic properties can be linked to the initial and final setting times of cement paste. The early age evolution of the elastic properties depends on the air content and the binder nature. The combined monitoring of P-waves and S-waves provide accurate indicators of the setting process.

Mixture Proportioning for Crack Avoidance

In this chapter, the nature, the physical and chemical properties and the content of the concrete constituents will be discussed for the understanding of the way of making an optimized mixture proportioning of the concrete for massive applications where the temperature rise must be minimized.