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.

Autogenous Shrinkage and Hydration Kinetics of SH-UHPFRC under Moderate to Low Temperature Curing Conditions

Mohammadhadi Kazemi Kamyab

Strain Hardening Ultra High Performance Fiber Reinforced Concrete (SH-UHPFRC) were used successfully over the last 8 years in numerous cast in place rehabilitation applications as long lasting waterproofing layers (20 to 30 mm thickness) and, combined with rebar, to increase the structural load bearing capacity of existing bridge decks or building slabs (50 to 70 mm thickness). In composite applications on reinforced concrete substrates, SH-UHPFRC provide a deformation capability (strain hardening) larger than their free shrinkage which makes localized macro cracking at service state very unlikely. On the other hand some specific conditions of microclimatic state of the substrate (moisture and thermal conditions) and climatological (temperature and humidity) of the site such as casting at low temperatures (winter conditions) might increase the autogenous shrinkage and thus eigenstresses of the freshly cast SH-UHPFRC and hinder the development of its tensile strength or/and deformability. Combination of these specific circumstances could compromise the SH-UHPFRC protective functions due to the occurrence of localized macro cracks, making the objective of using SH-UHPFRC as a onetime intervention strategy in rehabilitation application obsolete. Advancement in hydration at early age leads to formation of partially emptied capillary spaces contributing to self-desiccation, decrease of the relative humidity and increase of the capillary depression. Since water is under depression, the solid porous material is under compression forcing the material to shrink. From the aforementioned, it can be deduced that the autogenous shrinkage is a force driven phenomenon: a force is applied on the ageing viscoelastic porous skeleton and the apparent structural response of the bulk cementitious material is the autogenous shrinkage strain. Based on the aforementioned two levels of investigation were considered in this thesis: I. Material level: ageing viscoelastic porous skeleton of hydrating cementitious material II. Structural level: autogenous shrinkage development under the action of its driving forces The objective of this thesis was to study the development of the autogenous shrinkage and its relation with the development of microstructure and mechanical properties in SH-UHPFRC cured at moderate and low temperature conditions (1 to 20°C). Focus was put on: (1) kinetics and magnitude of hydration of silica fume and cement, (2) activation energy of the processes, (3) stiffness (E modulus and Poisson ratio) development, and (4) states of water and pore development. Finally a cross link analysis of information obtained from material and structural level was carried out to better highlight the mechanisms and driving forces of autogenous shrinkage. CM22-TKK, a SH-UHPFRC developed at MCS for rehabilitation applications, on the basis of CEMTECmultiscale® fibrous mixes, was used. Silica fume is one of the major constituents of CM22-TKK (26% by mass of cement). Extent of the silica fume pozzolanic reaction significantly affects the pore size distribution of the UHPFRC matrix which then influences the development of the relative humidity, self-desiccation and finally autogenous deformation. Detailed experimental investigation performed showed that only maximum 8% by mass of cement out of 26% silica fume used in the recipe reacts in moderate and low temperature curing conditions. Finally, a model of silica-fume degree of reaction as a function of curing temperature (5 to 250 °C) was proposed on the basis of a detailed literature survey of existing works with low W/B mixes. The activation energy of the thermo-mechanical properties was evaluated for thermo (cumulative heat of hydration) and mechanical properties development (such as dynamic Elastic modulus, dynamic Poisson’s ratio and compressive strength). A single activation energy Ea of 27.4 KJ/mol could successfully model the activation of all properties. Since maturity was not a proper reference to compare the autogenous deformations at various temperatures, a more fundamental reference such as degree of hydration was used. Experimental investigation was carried out using different techniques such as isothermal calorimetry and 29Si solid NMR to evaluate the development of the degree of hydration of cement and silica fume in UHPFRC at moderate and low temperature curing conditions. The ultimate degree of hydration of cement in CM22-TKK for a closed system was 0.34. There was a close correspondence between the experimental results and the predictions of the volumetric phase distribution models of Waller (0.28) and Jensen (0.32). Finally, the De Schutter et al. 1996 model was used to describe the mechanical properties (Emodulus and Poisson ratio) as function of degree of hydration. Nondestructive and non-invasive 1H-NMR was used to evaluate volumetric phase distribution development at 20°C, especially the classification of the water types (capillary, gel and C-S-H interlayer water) which brought fully new and valuable information about the hydration kinetics and pore size development. The results were also compared with Jensen volumetric phase distribution model to check the advantages and limitation of both approaches. Finally experimental investigation was carried out to follow the development of autogenous shrinkage and eigenstresses (in case of restrained shrinkage) of CM22-TKK cured in isotherm conditions (1 to 20°C). Eigenstresses were obtained and used to highlight the effect of the viscous response of the material at the different temperatures investigated. The previously mentioned results on the kinetics of the silica fume reaction, cement hydration and evolution of states of water and development of stiffness were then put into perspective with the trends observed on the autogenous shrinkage and eigenstresses development.

EPFL2013

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

Precipitation in the high strength AA7449 aluminium alloy

Patrick Schloth

Al-Zn-Mg(-Cu) (AA7xxx) alloys are widely used structural materials owing to their excellent mechanical properties and low corrosion susceptibility. The high strength of these materials is obtained by the formation of secondary phases, e.g. precipitation. This present thesis aims at contributing to the understanding of the influence of precipitation in the high strength AA7449 alloy on the internal stress formation at different length scales. Two different cases are studied in this thesis. In the first case, the influence of precipitation during quench on macroscopic residual stresses (RS) is studied in an industrial 75 mm AA7449 thick plate. For some investigations the behaviour of the AA7449 alloy is compared with the medium strength AA7040 alloy. In the second case, the influence of different precipitation states on the internal strain formation is investigated at the microstructural length scale. For the first case, in situ small angle X-ray scattering evidenced that during quenching the heterogeneous eta phase forms at high temperatures in contrast to homogeneous GP(I) zones, which form at low temperatures. The eta formation is influenced by the cooling conditions but also by macrosegregation in the thick plate. Yet, the volume fraction of eta is very low and has a negligible effect on the macroscopic RS in thick plates. The homogeneous GP(I) zones form during quench with radii in the subnanometre range. Their formation is strongly influenced by excess vacancies. Fast cooling rates can lead to higher radius and volume fraction of GP(I) zones compared to slower cooling. The GP(I) zone formation increases the yield strength during quench and is responsible for the high RS in industrial thick plates. Therefore, the GP(I) zone formation during quench needs to be taken into account in macroscopic finite element (FE) residual stress simulations by using precipitation modelling. The GP(I) zone formation during quench is simulated by using a thermodynamic based Eulerian multi-class model. Therefore, a thermodynamic description for GP(I) zones is derived from reversion heat treatments by using the solubility product. The influence of excess vacancies is taken into account by adapted effective diffusion coefficients. This simplified approach allows reproducing reasonably well the measured GP(I) zone formation during rapid coolings. The coupling of the precipitation model with FE residual stress calculations (performed by the other PhD studentt) allows predicting very well the RS in two AA7449 thick plates. For the second case, in situ mechanical testing during tensile deformation using X-rays and neutrons evidence that the presence of different precipitate types change the internal strain formation in the aluminium matrix at the microstructural length scale. The presence of small GP(I) zones leads to an internal strain formation that is dominated by the intergranular strains of the aluminium matrix. When large eta precipitates are embedded in the aluminium matrix the interphase strains clearly dominate the internal strain formation. Further, it is shown that the eta phase has a higher elastic modulus than the aluminium matrix and shows an anisotropic behaviour during elastic and plastic deformation. In addition, it is pointed out that the internal strain formation at the microstructural length scale can have a significant effect on macroscopic RS measurements performed by XRD when the plastically deformed sample contains eta' and eta.

EPFL2015