Pierre Mounanga

Improved measurement methods for autogenous shrinkage of cement mortars at very early age

This paper presents improved test methods for the measurement of autogenous shrinkage of cement-based materials at very early age (0 to 48h). These five test methods enable the measurement of chemical shrinkage, volumetric autogenous shrinkage, linear autogenous shrinkage in horizontal and vertical directions and restrained autogenous deformations leading to early-age cracking of the material. The first experimental results are discussed and analyzed. It was shown that the presence of granular inclusions slightly accelerates the evolution of mortar chemical shrinkage. The rotary test device for the measurement of volumetric autogenous shrinkage makes it possible to eliminate bleeding and segregation problems related to medium or high water-to-cement ratio (W/C). The use of non-contact and waterproof eddy current sensors permits to quantify uniaxial autogenous deformations of the cementitious matrices since the casting of the material, in quasi-isothermal conditions. Finally, it is shown from ring test results that the age of cracking decreases when the temperature increases, but the cracking appears at nearly the same hydration degree.

From Chemical and Microstructural Evolution of Cement Pastes to the Development of Autogenous Deformations

This paper seeks to contribute to the understanding of physical and chemical mechanisms that are at the origin of the autogenous volume changes and cracking of cementitious materials, focusing on the first 24 hours. A micro-macro experimental study has been performed, from the end of mixing up to several months, on a set of plain cement pastes prepared with the same type I ordinary Portland cement and various water-to-cement ratios (W/C), and cured at various constant temperatures. Chemical shrinkage, volumetric and one-dimensional autogenous deformations have been measured and analyzed in relation to the hydration process and to the microstructural characteristics of the material. The effect of the curing temperature at early age (<24 hours) in the range 10-50 deg C, and of W/C in the range 0.25-0.60, have been investigated. The temperature-induced changes recorded on both the magnitude and the kinetics of volumetric autogenous shrinkage clearly show the irrelevance of using the usual maturity concept to describe such phenomena within the whole early-age period. In addition, a threshold is pointed out at about a degree of hydration of the cement = 7%, both defining the range where autogenous shrinkage is linearly related to degree of hydration of the cement and corresponding to the precipitation of calcium hydroxide. A W/C threshold also is pointed out both at the macro-level (autogenous deformations, etc.) and at the micro-level (characteristics of the hydration products, pore size distribution, etc.). The findings indicate the critical two-fold effect of calcium hydroxide: a chemical effect, in which the carbon hydroxide content of the cement past plays the role of a chemical indicator of the hydration process, and a microstructural effect, generated by large-sized carbon hydroxide crystals.

Autogenous and Isothermal Restrained Shrinkage of Hydrating Cement Pastes - Experimental Study

Numerous studies have been carried out to characterize the autogenous shrinkage of cement-based materials in free conditions at early and very early age. But the results obtained are not sufficient to understand the autogenous volume variations of concrete in realistic structural conditions. Indeed, the analysis of such deformations requires investigating the cementitious system’s behaviour in restrained conditions, when the internal stresses generated can lead to the premature cracking of the material.

INFLUENCE OF SUPERPLASTICIZER ON ACTIVATION ENERGY AND AUTOGENOUS SHRINKAGE OF CEMENT PASTE AT EARLY AGE

Three superplasticizers (SP) have been studied in this research: the first is based on modified polycarboxylic ether and is used to improve the workability of concrete and to obtain high mechanical characteristics at early age; the second, which contains naphthalene sulphonate, is used to reduce drastically mixing water in concrete and improve mechanical strength at early age; the third SP investigated is melamine-based and is used to improve the workability of concrete creating electrostatic repulsion between cement grains. The intention of the present investigation was to provide more information about the role of these SPs in concrete at early age. The apparent activation, initial and final set times by Vicat needle, chemical and autogenous shrinkage were measured for cement pates having a water/cement ratio of 0.25. The apparent activation energy has been determined by the setting times method at different temperatures: 10, 20, 30 and 40 degrees. The volumetric autogenous shrinkage was measured at the same temperatures immediately after setting. The experimental results show that the apparent energy activation is slightly modified by the presence of SP. Also, the evolution of chemical shrinkage shows clearly that the SP acts on the hydration kinetic of cement. The effect of a particular SP on autogenous shrinkage at different temperatures can be correctly predicted by means of the maturity concept.