TENSILE CREEP OF UHPFRC UNDER LOW AND HIGH STRESSES

The purpose of this paper is to investigate experimentally the UHPFRC time-dependent behavior under sustained tensile loading and to determine the effect of the stress level on the creep response. Uniaxial tensile creep tests were performed at various stress levels by means of an optimized testing set-up. An Acoustic Emissions equipment was used in parallel to correlate the creep deformations with damage. From 29 % to 85 % stress level, the material exhibited a linear viscoelastic behavior. The UHPFRC response under higher stress levels close to strain hardening remains an open question.

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

Etude expérimentale des couplages viscoélasticité-croissance des fissures dans les bétons de ciment

Emmanuel Denarié

Viscoelasticity and crack growth govern the long-term deformability of concrete and thus its service behaviour and its durability. For low load levels, viscoelasticity behaves quasilinearly and crack growth is inactive. On the other hand, for high load levels, cracks grow and interact with viscoelasticity. Numerous authors have long demonstrated the influence of microcracking on creep qualitatively. Moreover, rate effects on the fracture behaviour of concrete are clear for low and high loading rates. Nevertheless, the mechanisms involved in these effects are not yet clearly determined. Cohesive crack approaches extended to time dependent parameters, give some elements of explanation, although without providing a general tool capable of explaining all the phenomena. The dissipative nature of crack growth and viscoelasticity naturally encourages one to model their couplings by means of an energybased approach to fracture. This is the meaning of the continuum thermodynamics of dissipative multicracked granular bodies developed by Huet (1997). This theoretical formalism puts the emphasis on the effect of viscoelasticity on the driving (reactive) part of the propagation criteria (energy release rate). The aim of this experimental research was to investigate coupling effects between viscoelasticity and crack growth in concrete. With this aim in view, 4 different types of fracture test have been performed on the same material (concrete with a maximum aggregate size of 8 mm), with a constant specimen geometry (rectangular wedge splitting specimens 20 by 20 by 10 cm). The first type of test consisted of performing a series of successive relaxations, at various increasing load levels, before, on and after the peak force, following the envelope of failure. Special attention was devoted to the influence of the control parameter ("active": with respect to displacements measured on the specimen, or "passive": crosshead displacement) and loading history. The results showed a progressive deviation from the linear viscoelastic behaviour starting from a load level of approximately 50 % of the peak force, before the peak. This deviation was significantly higher with crosshead control ("passive"). It depended on the loading history. After the peak, in "active" displacement control, relative relaxations tended to be similar whatever the load level. The displacement parameters (other than control) measured on the specimen or crosshead during relaxation, showed an evolution dependent on the control parameter. This effect was significant at the beginning of the relaxation and tended to disappear later. Furthermore, acoustic emissions could be detected at the beginning and during some relaxations, for high load levels (nearby peak force and after). The second type of test consisted of one or more successive creep levels up to eventual failure under sustained force. In certain cases, during creep levels leading to fracture, alternating secondary and tertiary creep with multiple concavity changes correlated with measured acoustic emissions could be observed. The tertiary creep leading to fracture developed over several minutes. Unstable sudden crack propagation was only seen at the end of the tertiary creep levels, accompanied by a sudden and very fast increase of the number of acoustic emissions. Moreover, during tertiary creep, an increase of the crack length measured on the specimen's surface by means of a conductive crack graphite gauge could be observed, correlated with the evolution of the creep displacements. The third type of test was performed under constant displacement speed (displacement measured on the specimen in the axis of the splitting force) with a range of speeds from 5 x 10-4 to 5 x 10-1 mm/minute. The results show a clear influence of the displacement speed on the response in terms of force-displacement curve, in the vicinity of the peak. The maximum force decreases with the displacement speed. On the other hand, the displacement at peak force was not significantly influenced by the displacement speed. Surface crack length measurements by means of a conductive graphite gauge enabled the determination of the surface crack speed as a function of the imposed displacement. The obtained surface crack speed depended quasi linearly on the imposed displacement speed. The fourth type of test, complementary to the former ones, was especially for the study of the evolution of internal micromechanical parameters during crack growth. Two wedge splitting specimens were equipped internally, near to the potential crack path with optical strain gauges (Bragg gratings). The internal strain measurements, revealed two zones of very different behaviour, with similar tends for the two specimens. Optical strain gauges close to the fracture plane (less than 5 mm from gauge axis) measured high strains (around 1000 με) significantly greater than the ultimate tensile strain of normal concrete (0.1‰ or 100 με) and mostly irreversible. At the contrary, optical strain gauges farther from the fracture plane (over 10 to 15 mm) measured strains that were always smaller than 100 με , and mostly reversible. Finally, two types of computer simulation (homogeneous materials) were performed in order to complement the interpretation of experimental results. A linear viscoelastic calculation, for a constant crack pattern, was used to illustrate quantitatively the progressive deviation from linearity observed in the experimental successive relaxations. A non-linear calculation of the propagation of a discrete crack with softening behaviour, in a linear elastic material, accurately predicted some of the strains measured with optical strain gauges during crack propagation. The experimental results demonstrated many indications of crack growth activity during creep as well as relaxation levels. These measurements confirm the significant contribution of propagation and coalescence of microcracks to the non-linear viscoelastic response of concrete. Indications of microcrack growth during relaxation can be explained by unstable propagation phenomena triggered by the small size of the microcracks. These phenomena could be amplified by viscoelastic effects of the redistribution of internal stresses in the heterogeneous structure of concrete. All the work performed clearly shows the need to complement macroscopic measurements such as the reaction force of a specimen or external displacements, with the measurement of micromechanical parameters in order to distinguish the contributions of the different active phenomena and of their coupling.

EPFL2000

Comportement mécanique des milieux granulaires sous sollicitations cycliques

Frédéric Mayoraz

The following work deals with the characterisation of the behaviour of road bases granular material, made of tunnel excavation material and loaded by passing vehicle axles. This granular media, don't correspond generally to the standards demanded for the construction of road pavements. The thesis is a part of the project COST 337 ("Unbound granular materials for road pavements") whose aim is to develop a guideline for the design of road pavements in Europe (material production, laboratory tests, modelling of mechanical behaviour). At the beginning, several loadings of road pavements are described (mechanical, thermal, hydric), concentrating on mechanical loading. Full-scale tests and finite element simulations led to the knowledge of the order of magnitude of stresses and deformations as well as to the understanding of the rotation of the principal stresses, due to the passing of a vehicle axle. This theoretical part is followed by laboratory cyclic tests with sand to estimate the influence of the principal stress rotation on the deformability of granular media and the importance to take this phenomenon into account for the modelling. Further laboratory tests were carried out on high diameter samples (160 and 250 mm) made of tunnel excavation material. Those tests were made by performing a large number of cycles (80'000 to 250'000 cycles) and various stress paths. The aim of the tests was to measure the evolution of elastic (stiffening of material) and plastic (accumulation of permanent deformations) material characteristics during the cycles. Tools have been developed in the framework of image analysis to assess the eventual change of morphology of this non-standard granular media. Finally the modelling of the accumulation of plastic deformations is discussed in accordance to the theory of elasto-plasticity and visco-plasticity. To remove the limits of elasto-plasticity to simulate a large number of cycles, a visco-plastic approach is developed. The elegant approach, replacing the time by the number of cycles, allows to approximate the permanent deformations of a road pavement in dependence on the stress path. It provides a tool to estimate the rutting in road pavements.

EPFL2002