Inverse Analysis of R-UHPFRC Beams to Determine the Flexural Response under Service Loading and at Ultimate Resistance

Ultra high performance fiber reinforced cementitious composites (UHPFRCs) have demonstrated their outstanding efficiency as structural materials. Determination of the bending resistance of members combining UHPFRC with steel reinforcement bars (R-UHPFRC) using material properties obtained by material testing gives satisfactory results. However, the synergetic interaction between rebar and UHPFRC in the tensile action due to bending, especially under loading–unloading of the structure is not yet well understood. This paper compared methods of analytical inverse analyses, as well as finite-element modeling and nondestructive testing (NDT), for determining UHPFRC material properties based on plates and R-UHPFRC members’ test results. It was demonstrated that UHPFRC in the tensile zone of R-UHPFRC members can enter into compression during unloading, influencing the structural response under service loading.

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

Static and Fatigue Biaxial Flexural Behavior of Strain-Hardening UHPFRC Thin Slab Elements

Xiujiang Shen

Cast-in-place thin layers of Ultra-High Performance Fiber Reinforced Cementitious Composites (UHPFRC) on the specific zones of existing reinforced concrete (RC) bridge decks has been demonstrated to be a technically efficient and economic rehabilitation and strengthening method. In these applications, the thin UHPFRC layer, serving as tensile reinforcement, increases the bending and shear resistance of UHPFRC-RC composite members through its in-plane tensile resistance and deformability. Bridge deck slabs are the most fatigue loaded structural elements in bridges, and the actual stress state caused by wheel loading is nearly equi-biaxial and far from uniaxial. Bridge decks are expected to be subjected to a high number of fatigue stress cycles, which may exceed several hundred million. Accordingly, this thesis is devoted to study experimentally and analytically the static and fatigue biaxial flexural response of strain-hardening UHPFRC thin slab elements. For a given UHPFRC mix, there are no intrinsic tensile properties. The representative behavior, especially the strain-hardening response, is dependent on the fiber distribution. The present thesis introduces firstly the uniformity factor for considering the local fiber distribution within an UHPFRC element. Accompanied with the fiber orientation factor and efficiency factor, the influence of uniformity on the strain-hardening response of UHPFRC under uniaxial tension is investigated quantitatively by means of experimental campaign and mechanical analysis. The direct tensile test (DTT) was carried out on dumbbell specimens, extracted from a UHPFRC slab element, to characterize the tensile behavior of UHPFRC. Before tensile testing, actual fiber distribution of each specimen was measured by the non-destructive test (NDT) method using a magnetic probe. Finally, the most likely ÎŒ_2 values are determined for UHPFRC layers with different thickness. In a next step, the biaxial flexural response of UHPFRC thin slab elements using ring-on-ring test method is investigated experimentally and analytically. Based on the testing results, a quasi-elastic limit is introduced to characterize the flexural response of UHPFRC under biaxial stress condition. In addition, an in-depth comparison between ring-on-ring test and 4-Point-Bending-Test (4PBT) results, with special emphasis on flexural strength and matrix discontinuity development, is conducted. An original analytical inverse analysis method for determining the biaxial tensile properties of UHPFRC from ring-on-ring test is developed based on yield line and elastic slab bending theories. The inverse analysis results are validated against the experimental evidence, particularly based on DIC analysis. It is concluded that the elastic limit is 18% lower, and almost equivalent tensile strength of UHPFRC subjected to biaxial stresses under biaxial stress condition, compared with those under uniaxial condition. And a relatively small biaxial hardening strain is highlighted. Finally, four series of flexural fatigue tests under constant amplitude fatigue cycles up to Very High Cycle Fatigue (20 million cycles) are conducted using circular slab elements. The fatigue stress level S is ranging from 0.50 to 0.68, targeting the fatigue endurance limit of UHPFRC material and fatigue strength under biaxial stress condition. The fatigue damage evolution is analyzed in terms of central deflection as well as development of fictitious crack propagation and opening.

EPFL2020

Structural behaviour of elements combining ultra-high performance fibre reinforced concretes (UHPFRC) and reinforced concrete

Katrin Habel

Ultra-High Performance Fibre-Reinforced Concretes (UHPFRC) have high mechanical strengths (fU,c > 150 MPa, fU,t > 6 MPa) and exhibit quasi-strain hardening in tension. Their very low permeability prevents the ingress of detrimental substances. In composite structural elements formed of normal strength reinforced concrete and Advanced Cementitious Materials (ACM), UHPFRC offer a high potential in view of the load carrying and protection function of the ACM layer. The objectives of the study described in this thesis are to determine the performance and structural behaviour of composite "UHPFRC-concrete" elements in bending. Towards this end, the current knowledge of UHPFRC properties is to be extended and modelling tools are to be developed in order to predict the structural behaviour of such composite elements and to make recommendations for their design. The experimental program is performed in order to characterize the UHPFRC and determine the structural behaviour of composite "UHPFRC-concrete" elements through 15 full-scale beam bending tests. The material tests focus on UHPFRC early age behaviour and on the determination of its outstanding tensile properties with an original uniaxial tensile test. They show that the UHPFRC properties become virtually constant after 90 days. The time-dependent behaviour of the composite beams is investigated during 11 weeks starting from the casting of the UHPFRC layer. After these long-term tests, the beams are subjected to bending until failure with the UHPFRC layer in tension. The parameters are the thickness of the UHPFRC layer, the presence of rebar in the UHPFRC and the static system. The time-dependent behaviour of composite "UHPFRC-concrete" members is investigated focusing on early age by means of test results and an existing numerical model. Using the results of the material tests as input, the numerical model is able to predict the behaviour of composite "UHPFRC-concrete" elements with the exception of self-desiccation of the UHPFRC. However, the deformations of composite "UHPFRC-concrete" members are correctly modelled by introducing autogenous shrinkage directly as volumetric deformation in the model. The structural response of the composite members under bending with the UHPFRC layer in tension is investigated with an original analytical model, which is an extension of the classical bending model for reinforced concrete. The influences of cross-section geometry, the UHPFRC tensile properties, the compressive strength of the substrate and the type of reinforcement are studied. This study demonstrates that the use of UHPFRC enhances the performance of composite "UHPFRC- concrete" elements in terms of resistance and stiffness. Furthermore, durability is extended due to the low permeability and tensile strain hardening properties of UHPFRC. The incorporation of rebar in the UHPFRC layer leads to a further increase in resistance and stiffness of the composite element and to a higher apparent magnitude of hardening in the UHPFRC. The investigated composite elements show monolithic behaviour under service conditions. The time-dependent behaviour is mainly controlled by autogenous shrinkage of the UHPFRC, which may induce a few evenly distributed small-width macrocracks in the case of statically indeterminate systems, thin UHPFRC layers (≤ 1 cm) or high magnitudes of autogenous shrinkage (≥ 750 µm/m at 28 days). The results show that the magnitude of autogenous shrinkage should not exceed 1000 µm/m (at 28 days) in order to avoid debonding and extensive formation of distributed macrocracks. Finally, three basic composite "UHPFRC-concrete" element configurations are proposed: Configuration P is designed for the protection function and consists of a thin UHPFRC layer. Configuration PR is proposed for existing elements with strongly deteriorated rebar and for new construction. It consists of an UHPFRC layer with reinforcement, and assumes no reinforcement in the concrete layer near the interface zone. Configuration R is proposed for existing structures requiring an enhancement of the structural behaviour. It is made of an UHPFRC layer with reinforcement, and assumes that there is reinforcement in the concrete layer near the interface zone.

EPFL2004