Thermo mechanical effects in Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) at early age

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.

Eugen Brühwiler, Emmanuel Denarié, Aicha Kamen, Hamid Sadouki

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 strain hardening UHPFRC, with different binders, under various thermo mechanical conditions. Isotherm tests as well as tests 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. 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. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep at low stress levels. As expected, UHPFRC tensile creep behaviour was also sensitive to the stress level. Above 35 % of the tensile strength at the loading age of 3 days, the material exhibited a non linear behaviour.

2010

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.

EPFL2011

Analyse de l'état de contrainte et de l'endommagement d'assemblages sertis en matériau mixte métal-composite sollicités en traction ou en flexion

Alain Michel Prenleloup

The constant improvement of polymer composite materials has allowed their integration in many industrial fields where their mechanical and chemical properties are apparently attractive. Mixed metal-composite insulators are now increasingly used for high voltage networks instead of conventional porcelain structures because of their high strength and reduced maintenance requirements due to their strong resistance against chemical degradation and dynamic loading. The composite insulators investigated in this study are made of an epoxy rod reinforced with glass fibres, two steel fittings being strongly crimped to both ends of the rod to transfer loads between the suspended electrical components and the fixation points. Due to the plastic deformation of the steel induced by the crimping process, a purely mechanical link results from the residual radial stresses at the interface between the composite rod and the metallic fitting. The strength of the joints plays an important role during the tensile and bending loading of the assemblies (clamping of the high-voltage line on the electrical tower and guiding of the electrical transformation substation). Experimental and numerical studies on the crimped joints are performed simultaneously in order to determine the strength of the structures and to characterise the damage and fracture mechanisms. The traction and bending tests are achieved to highlight the failure modes appearing for very high loads. In parallel, the numerical analysis provides complementary information on the stress state and the sensitivity of some influent parameters. The proposed dual approach allows the identification of the critical loads corresponding to the failure initiation and the possible mechanisms associated. The results for the tensile tests performed on structures subjected to a crimping load between 290 and 460 kN exhibit a nonlinear increase from 110 to 160 kN of the maximum pull-out load, and a transition of the failure mode of the link from a sliding of the rod out of the end-fitting for weakly crimped insulators and a delamination of the composite for strongly crimped ones. For still higher crimping conditions, the maximum tensile load decreases due to the damage introduced before tensile loading. In parallel to the experimental study, the numerical model developed for the crimping and traction steps provides the stress state in the joint during the solicitations. The comparison between the numerical predictions and the experimental observations highlights the strength limit of the bond by delamination of the outer layers of the composite. The use in the numerical model of a tensor damage factor depending on the stress state in the composite provides additional information on the failure initiation resulting from the stress combination and allows the characterisation of the critical crimping and loading conditions. The analysis of the crimped joints under tensile loading is extended to the study of insulators subjected to bending loading. Experimental tests are performed for two different sizes of insulators showing different final failure modes. For 51mm-thick insulators, a collapse of the compressive part of the rod occurs while for the 63mm-thick assemblies a shear crack propagation on the mid-plane of the rod can be observed. In addition to the fracture behaviour, the tests show a progressive embrittlement of the joint due to the damage onset appearing at 60 and 50% of the maximum mechanical loading of 15.7 and 21 kN found for the 51mm- and 63mm-thick rods respectively. Moreover, the numerical simulations contribute to the understanding of the failure zones and allow the identification of the critical loads and the stress state associated with the damage observed. Owing to the numerical and experimental analysis, structural modifications are proposed for improving the current bearing capacities of the crimped structures in order to shift the damage initiation and to increase the maximal strength of these assemblies under industrial use. In the tensile loading case, an optimal contact area between the rod and the end-fitting ensures an increase of 50% of the maximum mechanical load for the 18.7mm-thick insulator. For the bending case, the damage initiation at loading rates which are much lower than the bearing capacity can be postponed by changing the conical form of the metal end-fitting. However, the maximum bending load cannot be improved since it is directly limited by the diameter of the rod.