The influence of thermal curing on the evolution of the material properties and the UHPFRC behaviour was investigated. Tests results showed a beneficial effect of a high temperature curing on the early age material properties due to the thermo-activation effect on the hydration process. However, an inverse effect was observed at long-term. In our study, activation energy of UHPFRC was evaluated from experimental data by means of empirical models. The traditional maturity-function based on Arrhenius law, generally used to describe thermally activated physical or chemical processes, was used to predict the evolution of the UHPFRC autogenous shrinkage and to validate the applicability of this concept for such cement-based materials. Results showed that the concept based on Arrhenius law could describe correctly temperature effects on UHPFRC for temperature lower than 30°C.

Ultra high performance fibre reinforced concrete (UHPFRC) early age viscoelastic behaviour under tension was investigated. The tests results showed a high creep potential due to the high volume paste (88%). This result is of major importance because the viscoelastic properties contribute to mitigating the high early age stresses generated under restrained shrinkage. This beneficial effect was reflected by the increased linear-relationship between tensile creep and shrinkage. As expected, UHPFRC tensile creep behaviour was also sensitive to the loading level. Above 35% of the tensile strength at the loading age, the material exhibited viscoplastic behaviour. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep and confirms the induced non-linear response.

This paper presents test results of mechanical properties of sprayed sand concrete [BS] by wet-mix process on 3D panel, as a substitution to ordinary dry-mix shotcrete [BO (3/8)] made of aggregates up to 8 mm.

The thermal effects on the Ultra High Performance Fiber Reinforced Concrete (UHPFRC) behavior are still not completely known. Under high temperatures, the processes that interact at early age are thermo-activated. All the experimental results obtained within this research confirmed it. Besides, certain authors observed a non-monotonous effect of temperature on the autogenous shrinkage and the creep of normal and high performance concretes. To clarify the thermal effect on the early age UHPFRC behavior an extensive experimental study was carried out. Tests results showed a non-monotonous effect of temperature on the autogenous shrinkage. In addition, finite element numerical simulations were performed to predict the experimental results and to validate existing models for different curing conditions. These models are necessary to predict accurately the deformations, the stresses and the damage (cracking) in the case of structural elements subjected to in-situ temperature variations.

ABSTRACT: Thin ultra high performance fibre reinforced concretes overlays are suitable for rehabilitation of structures thanks to their enhanced properties. However, in such composite structures, restrained differential shrinkage cracking is an important concern. To investigate this problem a test set-up, inspired by previous research, was developed. The set-up simulates real restraint and thermal conditions commonly encountered in composite members. An analysis was first performed to identify the adapted configuration of the set-up from the point of view of geometry and cooling system, using a finite element program for time dependent behaviour of multilayer systems developed by (Femmasse 2007). The predicted temperatures and displacements are in reasonable concordance with measurements. The test results showed that the proposed set- up and the testing procedure are a practical and efficient approach for investigating the early age UHPFRC behaviour especially the evolution of the internal stresses, the autogenous shrinkage and the thermal effects.

Les Betons a Ultra Hautes Performances (BFUP) constituent des materiaux prometteurs dans le domaine de la habilitation des ouvrages en particulier ceux soumis a un environnement agressif et fortement sollicites grâce a leurs proprietes specifiques a savoir leur tres faible permeabilite et performances mecaniques elevees. Cependant, de telles structures composees peuvent etre sujettes au risque de fissuration au jeune âge sous conditions ’entrave. Pour etudier ce probleme un appareil d’essai a ete developpe en s’inspirant des recherches precedentes. L’appareil permet de simuler les conditions d’entrave et thermiques reelles susceptibles de se produire dans des elements mixtes. Une analyse numerique a ete effectuee pour identifier la configuration adaptee de l’appareil du point de vue de la geometrie et du systeme de conditionnement, par le biais d’un programme d’elements finis. Ce programme permet d’etudier le comportement dans le temps de systemes composes et est developpe par (Femmasse, 2007). Les temperatures et les deplacements predits sont coherents avec nos mesures experimentales. Les resultats d’essais ont montres que le systeme propose et la procedure d’essai constituent une approche fiable pour caracteriser le comportement au jeune âge du BFUP en particulier l’evolution des contraintes internes, le retrait endogene et les effets thermiques

Keywords: BFUP ; jeune âge Note: pp. 77-86 Reference MCS-PRESENTATION-2007-016 Record created on 2007-07-26, modified on 2016-08-08

Material characterization tests of an ultra-high-performance fiber-reinforced concrete (UHPFRC) were performed at various ages. A linear relationship was obtained between the mechanical properties and the degree of hydration. In parallel. the influence of curing conditions on the physico-mechanical properties and the time dependent behavior of this UHPFRC was investigated A temperature increase accelerated the hydration process at early age and therefore improved the material's compressive strength and the carrying capacity in four-point bending tests; but at a long term, a higher temperature had adverse effects on the mechanical properties.Moreover the UHPFRC exhibited. autogenous shrinkage at long term comparable with normal concrete. An increase of curing temperature increased the autogenous shrinkage. This effect may be due to the hydration and the self-desiccation processes that are accelerated at high temperatures.

UHPFRC viscoelastic behavior under compressive stresses has been investigated both in sealed and drying conditions. The tests results showed a high potential of creep of our UHPFRC when compared with another reactive powder concrete and a high performance concrete. This effect may be due to the high volume paste in the UHPFRC (88%) which increases the creep rate as it is the paste which deforms. According to our results, non-linear creep is reached beyond 43% of compressive stress. In the non-linear domain, creep under sealed conditions is close to that obtained under drying conditions. Existing models were applied to predict the UHPFRC creep and to validate their application for such materials. In parallel, shrinkage has been studied both in sealed and drying conditions. The test results showed that the investigated UHPFRC, despite its high paste volume, exhibits moderate autogenous shrinkage when compared to cement paste with similar water cement ratio. Autogenous shrinkage tends to stabilize. beyond 6 months.

Material characterization tests of a UHPFRC were performed at various ages. A linear relationship was obtained between the compressive strength and the degree of hydration. In parallel, the time dependent behaviour of this UHPFRC for different curing conditions was investigated. At a 20 °C temperature cure, the UHPFRC exhibited moderate autogenous shrinkage at long term with respect to normal concrete. An increase of curing temperature increased the autogenous shrinkage. This effect may be due to the hydration process and the self-dessiccation which are accelerated at high temperatures. The effect of fibres on the autogenous shrinkage is also reported in this paper. The test results demonstrate that the presence of fibres decreases the autogenous shrinkage by 35% in comparison to the UHPFRC matrix without fibres.

The effect of different curing temperatures (from 20 to 40 °C) on the compressive strength and hydration of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) have been investigated between 3 and 28 days, using the loss on ignition test (LOI) and thermogravimetry. The results have shown that the ultimate degree of hydration is affected by the curing temperature. An reversed correlation with temperature was observed beyond 7 days. Increased temperature initially accelerated the hydration due to the evolution of the material microstructure and thus the mechanical performance, especially compressive strength, increased. In long term, however, a decreased rate of hydration and compressive strength were observed. Finally the degree of hydration was calculated on the basis of the thermogravimetric and loss on ignition.

At early age, ultra high performance fibre reinforced concretes (UHPFRC)undergo significant volumes changes (thermal deformation and autogenous shrinkage)related to their composition. These deformations can be restrained by different elements respective to the material (rigid constituents) and the structure (formwork, underlying concrete and reinforcement) during construction. The consequences at material level are the development of tensile stresses that can initiate cracking and increase permeability. Therefore, their initial protective performance can be reduced. This paper presents the evolution of the temperature in various conditions, the degree of hydration, and the response at early age in free and restrained conditions determined by means of an original testing set-up developed at MCS-EPFL (autogenous shrinkage and stress generation) for a UHPFRC.

Note: edited by Swedish Cement and concrete Research Institute (CBI) RILEM PRO 43 : International RILEM TC 193-RLS Workshop on Bonded Concrete Overlays Reference MCS-PRESENTATION-2007-014 URL: http://www.rilem.net/ Record created on 2007-05-23, modified on 2016-08-08

The extremely low permeability of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC)associated to their outstanding mechanical properties make them especially suitable to locally "harden" reinforced concrete structures in critical zones subjected to an aggressive environment and to significant mechanical stresses. Composite UHPFRC-concrete structures promise a long-term durability which helps avoid multiple interventions on structures during their service life. Temperature Stress Testing Machines (TSTM) are very well suited to "experimentally simulate" the conditions encountered in composite structures in a new layer, at early age, subjected to restrained shrinkage under complex environmental conditions. They can also be used to determine intrinsic properties such as free shrinkage and creep response under well defined conditions of restraint and temperature. A new TSTM testing setup has been used to characterize the response of two types of strain hardening UHPFRC, with different binders (CEM I and CEM III), under various thermo mechanical conditions. Isotherm tests as well as test under realistic temperature conditions have been performed. Current numerical models have been applied to simulate the tests and the range of their applicability has been determined for temperatures between 10 and 30 °C. Moreover, the significant influence of non linearity in the creep response of UHPFRC, at early age, under moderate to high tensile load levels has been demonstrated. The test results showed a significant creep potential due to the high volume of paste. This beneficial effect was reflected by the linearly increasing -relationship between tensile creep and shrinkage. As expected, UHPFRC tensile creep behaviour was also sensitive to the loading level. Above 35 % of the tensile strength at the loading age, the material exhibited a non linear behaviour. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep and confirmed the observed non-linear response.