AbstractA novel model based on the three-parameter Weibull function is proposed to describe the three-stage fatigue deformation behavior of plain and fiber-reinforced concrete. The fatigue strain at ...a particular stress between zero and the maximum fatigue stress can be modeled using the proposed model, and all the model parameters have clear physical meanings. This model is validated via comparison of its results with previously reported results of compressive, tensile, and flexural fatigue tests. Cases of application of the model to plain concrete and fiber-reinforced concrete with high ductility are examined in order to investigate the variation of the model parameters. Additionally, a deformation-based method for prediction of the fatigue life of concrete is presented, and the prediction results demonstrate that the proposed model can be successfully applied to the estimation of the fatigue life of concrete materials.
A systematic study of the basic mechanical properties of ultra-high ductility cementitous composites (UHDCC), including the compressive, tensile and shear properties from normal strength to high ...strength were investigated in present research. The compressive strength of cylinder specimens with diameter of 100 mm and height of 200 mm were in the range of 43–115 MPa. The compressive strain corresponding to the peak stress, the Young’s modulus and the Poisson’s ratio were calculated. The tensile properties including the peak stress, the strain capacity and the strain energy were obtained as well as the relationships between the aforementioned parameters and the compressive strength. The peak tensile stress and the strain capacity of UHDCC were ranged from 6.2 MPa to 16.5 MPa, and 8.0% to 11.1%, respectively. Additionally, the shear stress-strain curves and the relationship between the shear strength and the corresponding compressive strength were also established. During the whole loading process of direct tensile and shear tests, the crack patterns were monitored by digital image correlation method.
This paper presents a development of reinforced ultra-high toughness cementitious composite (UHTCC) participating permanent formwork, which acts as a self-supporting formwork during construction and ...contributes to the load capacity and durability of the concrete member through the service life of the structure. The flexural behavior of the specimens made with the formwork is analyzed with the help of Digital Image Correlation (DIC) technology. Compared to reinforced concrete beam, the specimens made with UHTCC formwork own a larger deflection, when their maximum crack width reached 0.45mm. The cracks caused by the delamination of the formwork have little effect on the longitudinal bars, but they would reduce the load capacity in some circumstance. Based on the failure process of specimens, several design details, including the load capacity of various sections and the surface treatment methods, were given to improve the performance of the formwork.
Fiber-reinforced cementitious composites with high ductility exhibit significant pseudo-strain-hardening response and multi-cracking behavior. In this paper, a systematical investigation of a ...sprayable fiber-reinforced cementitious material with high ductility is presented from material design to practical application. This sprayable material is formed using the wet-mix spray process, and its compressive, tensile, and flexural strengths are higher than those of the cast ones with the same proportion. A series of reinforced concrete beams, including unloaded and pre-loaded ones, is strengthened with this material, and the four-point bending test is performed to evaluate the flexural performance of the composite elements. A theoretical analysis is conducted to predict the load capacity of the specimens. The application cases of this sprayable material in China are presented to show its potential application in the construction of durable concrete structures and restoration of aged structures.
•An ultra-high performance engineered cementitious composites was developed.•Flexural and shear behaviors of reinforced UHP-ECC beams show superior performance than RC beams.•Feasibility of utilizing ...UHP-ECC to largely reduce steel bar in structural member was proved.
The structural behaviors of steel reinforced ultra-high performance engineered cementitious composites (UHP-ECC) beams under bending were experimentally explored and compared to the ordinary reinforced concrete (RC) beams in present research. Two series of beams, reinforced or unreinforced with stirrups, were prepared to study the flexural and shear performance of steel reinforced UHP-ECC (RU) beams. The UHP-ECC was featured with the tensile strength of 16 MPa and the tensile strain capacity around 8%. The steel reinforcement ratios of longitudinal bar for both RU and RC beams were 0.69%, 1.86% and 2.94%, respectively. The crack propagation and the failure mode of RU and RC beams were monitored by digital image correlation (DIC) method. Additionally, the load-displacement relationships, the ductility index values and the component strains including steel bar and UHP-ECC of RU beams were obtained and compared with RC beams. It was found that the RU beams showed superior performance in both load capacity and ductility at the ultimate limit state; while the crack width and deflection at the serviceability limit state of RU beams were much smaller than those of RC beams. The loading capacity of non-steel reinforced UHP-ECC (NU) beams was approaching to the one of RC beams with reinforcement ratio of 1.86%, which indicated the feasibility of utilizing the UHP-ECC to substantially reduce or replace the steel bar in structural members.
•Static and fatigue behavior of RC-UHTCC beams is investigated using a four-point bending test.•With an increase in thickness of UHTCC, the fatigue life and mid-span deflection of RC-UHTCC beam ...increase.•A simplified method is introduced to model the fatigue performance of RC-UHTCC beam.•UHTCC layer can lower the tensile stress, strain localization and stress concentration of the longitudinal reinforcements.
The static and fatigue performance of reinforced concrete beams strengthened by strain-hardening fiber-reinforced cementitious composite is investigated. Two series of strengthened beam specimens are prepared with different thicknesses of the enhancement layer (40 mm and 50 mm), and three fatigue stress levels (0.9, 0.8, and 0.7) are tested. The fatigue life, mid-span deflection, and crack mode of the tested specimens are analyzed. Emphasis is placed on the fatigue response of the strain-hardening fiber-reinforced cementitious composite layer and longitudinal reinforcements. A simplified method is proposed to model the fatigue performance of the composite beam. The mechanism of the fatigue enhancement of the strengthened beam compared to a conventional reinforced concrete beam is as follows: (1) the enhancement layer physically contributes by taking part of the stress in the tension zone, and (2) the enhancement layer can lower the strain localization and stress concentration of the longitudinal reinforcements. Several methods for further improvement in the fatigue performance of reinforced concrete beams are suggested.
The failure mechanisms of fibre reinforced concrete (FRC) are complicated and governed by random phases at meso-scale, typically including fibres, mortar and fibre-mortar interfaces. This work ...develops an efficient numerical method using non-conforming meshes between the fibres and mortar. Their interfacial bond-slip behaviour is explicitly modelled by inserting zero-thickness cohesive interface elements between fibres and their replicas, i.e. the proposed “shadow fibres”. The shadow fibres are then constrained by the background mortar through a coupling technique proposed in the literature to ensure the displacement compatibility and force transmission. The damage and fracture of the mortar are captured by the concrete damage plasticity model, and the fibres are modelled by finite beam elements with elastoplastic constitutive relations. The proposed approach is validated by single fibre pullout tests, single fibre-bridged notched tension test-pieces, and direct tensile tests of ultra-high performance fibre reinforced concrete samples with random fibres. After comprehensive analyses of the key material and geometric parameters, the presented framework proves capable of capturing the major failure mechanisms at meso-scale, such as fibre pullout, bending and yielding, mortar cracking and spalling and fibre-mortar interfacial debonding, which can be used to enhance the understanding of FRC damage and fracture behaviour.
•Propose a rate-dependent phase field model considering both viscoelasticity and micro-viscosity;.•Develop VUEL and VUMAT subroutines to integrate the model through explicit numerical solution ...schemes;.•Successfully predict dynamic tensile behaviour of quasi-brittle concrete material;.•Elucidate complicated dynamic fracture mechanisms using CT image-based real meso‑structures.
To investigate dynamic fracture mechanisms of quasi-brittle materials, this work proposes a rate-dependent phase field model that integrates both macroscopic viscoelasticity and micro-viscosity to reflect the rate effects by free water and unhydrated inclusions. Based on the unified phase field theory, the model introduces a linear viscoelastic constitutive relation in effective stress space to consider the macro-viscosity of the bulk material. Additionally, the micro-force balance concept is utilized with the micro-viscosity to derive a parabolic phase field evolution law that accurately describes the dynamic micro-crack development. Explicit numerical solution schemes are established for the governing equations by developing VUEL and VUMAT subroutines in ABAQUS. This eliminates the convergence issue in implicit phase field modelling. Four typical benchmarks are investigated to validate the proposed model for macroscale and mesoscale heterogeneous problems. It is found that the proposed model can well capture the crack branching, delaying characteristic of micro-crack growth, and increase of macroscopic strength under higher strain rates. Using real meso‑structures from CT images, the complicated dynamic behaviour of concrete is investigated which yields deeper insight into stress wave propagation, crack evolution and load-carrying capacities.
In this work, a localizing gradient damage model (LGDM) based on the generalized micromorphic theory is adopted to investigate the quasi-brittle fracture behaviour of concrete at mesoscale for the ...first time. Micro Computed Tomography (CT) image-based realistic concrete models are generated including aggregates, mortar, pores, and aggregate-mortar interfacial transition zones (ITZs). In the LGDM, an additional micro-force equilibrium equation is formulated with a damage-dependent interaction function to prevent spurious energetic interactions during the softening stage. The damage driving forces are evaluated as history variables through the maximum nonlocal equivalent strains at the integration points. The stiffness matrices and residual vectors for displacement and nonlocal equivalent strain fields are formulated through the conventional shape function and strain matrices of finite elements, by using the user-defined element (UEL) subroutine implemented with the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm to improve numerical performance. Typical nonlinear fracture benchmarks are presented to investigate the variability of mesoscale fracture evolution, crack trajectories, and load–displacement curves as well as numerical performance. It is found that the LGDM significantly improves the computational efficiency over the conventional phase-field cohesive zone model (PFCZM) with 60% savings of CPU time, and holds great potential for multiscale fracture evaluation of quasi-brittle composite materials, through the high-fidelity representation of real materials’ microstructures, and flexible simulation of the complicated nonlinear fracture without the need for re-meshing or mesh enrichments.
•Simulate mesoscale fracture of concrete using a localizing gradient damage model (LGDM) for the first time.•Enable a robust and efficient monolithic framework by the BFGS quasi-Newton algorithm.•Successfully capture quasi-brittle fracture behaviour of mesoscale concrete using CT images with high fidelity.•Demonstrate considerable reduction (60%) in computational time over the phase-field cohesive zone model.•Widen the applicability and acceptability of the LGDM among researchers.
For fire resistance calculations, the thermal and mechanical properties of an ultrahigh toughness cementitious composite (UHTCC) with hybrid polyvinyl alcohol (PVA) and steel fibers at elevated ...temperatures were investigated. The thermal conductivity and flexural strength were tested at temperatures from 25 to 900 °C. The density, mass loss, and residual compressive strength of the UHTCC were tested after exposure to elevated temperatures with the failure mode scanned by X-ray computed tomography and the microstructure was observed using field-emission scanning electron microscopy. A finite element (FE) model was established to predict the temperature distribution of a steel-reinforced UHTCC beam using the measured thermal property data. Experimental results show that the thermal conductivity of the UHTCC, which is approximately 0.5 W/(m ⋅K), remains stable from 25 to 900 °C. The flexural strength of the UHTCC decreases with increasing temperature, whereas the residual compressive strength increases by approximately 8% at 210 °C compared to its original strength at ambient temperature. Explosive spalling of the cement-based material is effectively prevented owing to the presence of the PVA fibers. This work can provide basic property data for UHTCC structural calculations in the event of a fire.