Tensile response of Strain-Hardening Ultra High Performance Fiber Reinforced Concretes under low loading rates and low temperatures

Strain Hardening Ultra High Performance Fiber Reinforced Concretes (SH-UHPFRC) are cementitious materials with exceptional mechanical properties and outstanding durability making them ideal materials for rehabilitation, for the improvement of load carrying capacity and protective functions of existing structures. However, cast in-situ UHPFRC undergoes restrained autogenous deformations leading to the development of tensile eigenstresses with very low strain rates. Even though, in majority of the cases, the strain hardening capability and tensile viscoelastic potential of the material help in mitigating the effect of these eigenstresses, in specific cases, a combination of detrimental fiber orientation, matrices favoring higher shrinkage, and adverse climatic conditions (especially low temperature winter casting) may lead to the exhaustion of the strain hardening potential. This results in the development of localized macrocracks that hinder the protective function of UHPFRC, questioning the relevance of their use as protective layers in a onetime intervention strategy in rehabilitation application. The main objective of this thesis was to investigate the influence of very low strain rates (in the range of those observed for shrinkage deformations) and winter temperatures on the tensile response of two types of SH-UHPFRC mixes; Mix I with pure type I cement, silica fume and steel fibers and Mix II with 50% mass replacement of cement with two complementary limestone fillers and a similar fibrous mix. An extensive experimental campaign was carried out to investigate the tensile response of the mixes at different temperatures (5°C, 10°C and 20°C). At the material level, Isothermal Calorimetry, 29Si MAS NMR and Vibration Resonance Frequency tests were carried out to study the development of the degree of hydration and reaction of cement and silica fume as well as the development of the elastic modulus. At a structural level, a Temperature Stress Testing Machine (TSTM) was used to determine the autogenous deformations and associated eigenstresses, with, for the first time, full restraint conditions. An electromechanical test setup was used to investigate the influence of very low strain rates (down to 5x10-9 1/s) on the tensile response under monotonic loading under quasi-isotherm conditions. The results indicated that the elastic limit decreased significantly with the strain rate, with a reduction of more than 20% at a strain rate of 5x10-9 1/s when compared to that at a quasi-static strain rate. The full restraint tests in the TSTM revealed that, except for Mix I cured at 20°C, the eigenstresses in none of the other tests reached the respective elastic limit, after one month, even under low temperatures. Moreover, because of its higher relaxation potential, the eigenstresses in Mix II at any age were considerably lower than those observed for Mix I. The results also revealed that Mix II exhibited a slightly slower development of hydration when compared to Mix I. A viscoelastic-viscohardening model was developed to discuss the interaction of ageing, hydration, early age volume changes, viscoelastic phenomena and damage and their influence on the overall tensile behavior of UHPFRC. This study highlights for the first time the excellent tensile performances of SH-UHPFRC mixes with massive replacement of clinker by limestone filler, as well as the limited impact of autogenous shrinkage on the development of eigenstresses for those mixes.