Concrete elements at early age experience a temperature increase due to the exothermic nature of cement hydration. The initial heating and expansion are followed by cooling when the temperature equilibrates with the ambient. Since the surface zones are cooling down faster than the bulk, an element experiences temperature gradients and hence non-uniform thermal deformations.
This, together with adjacent restraining elements, in turn leads to buildup of self-restraint stresses and increases the risk of macro-cracking (Fig.1). In addition, the mismatch in thermal deformations at the mesoscopic scale, i.e. between cement paste and aggregates or reinforcement leads to buildup of internal stresses and possibly micro-cracking. Thermal cracks can seriously impair durability and serviceability of concrete structures. The risk of thermal cracking has been usually considered relevant only for massive concrete structures, but recently has become important also for relatively small concrete elements made of high-performance concrete, owing to their high cement contents (i.e. low w/c and low aggregates content) and therefore high heat of hydration.
Prediction of thermal deformations requires precise measurements of the coefficient of thermal expansion (CTE). Studying the CTE has met serious difficulties in the past. It is especially challenging at early ages (first days of hydration), due to the ongoing evolution of material properties (in particular mechanical properties, internal RH). In the case of low w/c cement pastes, the thermal deformations occur simultaneously with autogenous shrinkage, calling for special measures for separating the two types of deformations. A novel technique suitable for measuring the CTE from casting based on a volumetric principle (buoyancy method, see Fig.2) has been proposed in our lab:
Loser, R., Münch, B. and Lura, P., 2010. A volumetric technique for measuring the coefficient of thermal expansion of hardening cement paste and mortar. Cement and Concrete Research, 40(7), pp: 1138-1147.
One of the main activities of the lab in the field of early-age concrete is studying the fundamental mechanisms of thermal deformations, in particular the evolution of the CTE at early ages. It has been long recognized that the CTE is strongly dependent upon the moisture state of the concrete. This topic has been studied in the following publication, where it was proven, for the first time quantitatively, that the evolution (increase) of the CTE of concrete at early ages is exclusively due to the temperature-dependence of the internal RH (increase of RH upon heating):
Wyrzykowski, M. and Lura, P., 2013. Moisture dependence of thermal expansion in cement-based materials at early ages. Cement and Concrete Research, 53, pp: 25-35.
The microstructural mechanisms of the RH increase upon heating, also referred to as the temperature-dependence of sorption, have been a topic of a long-lasting debate. Our study of the water redistribution phenomena occurring in the microstructure of hardened cement paste upon heating/cooling has for the first time shown that heating causes redistribution of water from the smallest interlayer C-S-H spaces (below 1 nm) to the larger gel pores within the C-S-H (couple of nm in size). This effect is responsible for reducing the curvature of the pore fluid-vapor interfaces (menisci) and increasing the RH upon heating. The effect is virtually instantaneous and fully reversible upon cooling. Explicit measurements of water redistribution at the microscopic scale were possible thanks to the 1H Nuclear Magnetic Resonance technique, with temperature changes imposed on the cement paste samples during the actual measurements (Fig.3). This study was carried out in collaboration with EPF Lausanne (Switzerland) and University of Surrey (UK):
Wyrzykowski, M., McDonald, P., Scrivener, K. and Lura, P. 2017. Water Redistribution within the Microstructure of Cementitious Materials due to Temperature Changes Studied with 1H NMR. The Journal of Physical Chemistry, C 121(50), pp: 27950-27962.
A proper understanding of the mechanisms behind the moisture-dependence of the CTE, namely the fact that the CTE increases upon desaturation of the pores (caused by hydration or external drying) allowed us to propose a novel application of the superabsorbent polymers (SAP), conveniently used for reducing autogenous shrinkage, for controlling the CTE of high-performance concrete (Fig.4). Superabsorbent polymers provide internal curing and allow to maintain high RH in the pores. This, in turn, reduces the CTE increment occurring normally during the first days of hydration.
Wyrzykowski, M. and Lura, P., 2013. Controlling the coefficient of thermal expansion of cementitious materials - A new application for superabsorbent polymers. Cement and Concrete Composites, 35(1), pp: 49-58.
Other important publications: