Ultra-high Performance Concrete Structures Research Articles

Predicting shear strength in UHPC beams through an innovative neural network with SHAP interpretation

Ultra-High performance concrete (UHPC) has garnered considerable attention in the construction industry due to its exceptional mechanical properties and durability. In the design of UHPC beams, accurately predicting shear strength is crucial. Inspired by the dense blocks in DenseNet, this paper proposes a novel neural network for predicting the shear strength of UHPC beams. A comprehensive database of UHPC beams was initially established through extensive literature data collection, amassing 619 experimental data samples. Preprocessing was subsequently conducted using mahalanobis distance (DM) to eliminate outliers. Subsequently, the database underwent detailed parameter analysis. In practical testing, the novel neural network model exhibited superior accuracy in predicting the shear strength of UHPC beams, surpassing traditional machine learning (ML) models and empirical formulas. Furthermore, it demonstrated excellent generalization capabilities, achieving an R2 value of 0.988 in the training set and 0.964 in the test set. The study introduced the shapley additive explanations (SHAP) method to conduct global and local interpretability analysis of the network model, demonstrating the contributions of each input parameter to the model predictions. Finally, based on this network model, a graphical user interface (GUI) was developed for researchers engaged in UHPC beam design, enabling users to easily input relevant parameters and obtain shear strength predictions. The findings of this study are expected to promote the application of ML models in predicting the complex nonlinear behavior of UHPC structures, providing strong support for engineering practice.

Advancing ultimate bond stress–slip model of UHPC structures through a novel hybrid machine learning approach

The significance of Ultimate Bond Stress-Slip (UBS-S) in reinforced Ultra-High Performance Concrete (UHPC) structures cannot be overstated, as it directly affects their load-carrying capacity, structural integrity, and long-term performance. A comprehensive analysis of the UHPC-Parallel Micro Element System (UHPC-PMES), including 144 specimens, evaluated the computational efficiency of the proposed UBS-S model. To this end, the most critical settings of steel bar diameter, concrete cover, bond length, and UHPC compressive strength (db,c,lb,fUHPC′) were directed to create this parametric research. Applying the hybrid approach of three different optimization techniques, namely Physics-Informed Neural Networks (PINN), Genetic Algorithms (GA), and Multiple Linear Regression (MLR), this study predicted the UBS-S at the interface of UHPC and steel bars. It formulated the hyper-parameters effect values (a,m,β,G). The presented research used these algorithms to solve an inverse problem in structural engineering. Comparing the results obtained from PINN, GA, and MLR demonstrated that machine learning techniques and the proposed PMES model could effectively and accurately investigate the ultimate bond stress-slip for reinforced UHPC structures.

Hybrid machine learning model and predictive equations for compressive stress-strain constitutive modelling of confined ultra-high-performance concrete (UHPC) with normal-strength steel and high-strength steel spirals

Stress-strain constitutive model for confined ultra-high-performance concrete (UHPC) plays a pivotal role in the design and modelling of UHPC structures. However, while extensive research exists on the stress-strain responses of unconfined UHPC, a notable dearth of studies focuses on the stress-strain constitutive model for confined UHPC. Accordingly, this study introduces a novel hybrid machine learning (ML)-based stress-strain constitutive model for UHPC confined with normal-strength steel (NSS) or high-strength steel (HSS) spirals. Given the limited experimental data available, a numerical model was developed and validated against experimental results of confined UHPC. Subsequently, it was employed to generate a comprehensive dataset of confined UHPC stress-strain responses, which were then used to train the hybrid ML model. Hyperparameters of the model are subsequently optimized using Optuna along with a cross-validation technique. The optimized hybrid ML model exhibited exceptional accuracy in predicting the stress-strain response of confined UHPC with NSS or HSS spirals, as demonstrated by high coefficients of determination (R2) values ranging from 0.905 to 0.999 and low error metrics across varying stress levels and datasets. Moreover, the capability of the model to simultaneously predict both stress and strain responses was further validated through canonical correlation analysis (CCA). The results of the analysis demonstrated robust predictive performance of the model, with high CCA scores that ranged between 93.3% and 99.4%. To facilitate the practical implementation of the model, an end-to-end interactive software tool is developed. In addition, this study proposes predictive equations for the peak and ultimate stress-strain responses of confined UHPC. The evaluation of the developed predictive equations demonstrates their efficacy in accurately predicting the stress-strain responses of confined UHPC, as evidenced by the high R2 values, which ranged from 83–91%.

A comprehensive review of ultra-high performance concrete (UHPC) behaviour under blast loads

Blasts resulted from industrial/residential accidents and deliberate attacks can impose extremely hazardous loading conditions upon nearby structures. The repercussions of these blasts encompass not only direct effects such as blast overpressure and fragments but also indirect outcomes like progressive collapse of structures. In the pursuit of enhanced protection for concrete structures, substantial endeavors have been directed towards developing innovative construction materials with outstanding properties, among which the ultra-high performance concrete (UHPC) has emerged as a notable representative. Drawing from existing experimental, numerical and theoretical studies, this paper presents an inclusive overview of recent progress in UHPC structural members (slabs, beams and columns) and UHPC-based composite structures (mesh reinforced UHPC, UHPC-filled steel tube and UHPC strengthening of normal reinforced concrete (NRC) structures) in their ability to withstand blast loads. While discussing the exceptional material and structural dynamic performance of UHPC, recommendations are offered for the further research directions in utilising UHPC structures to resist blast loads, which include exploring eco-friendly UHPC, integrating 3D printing technology and employing machine learning analytical methods.

Moment Redistribution in UHPC Continuous Beams Reinforced with High-Strength Steel Bars: Numerical Investigation and Prediction Model

Considering moment redistribution in the design of ultra-high-performance concrete (UHPC) statically indeterminate structures can fully exploit the load-bearing potential of members, simplify reinforcement details, and save construction costs. Due to the excellent properties of ultra-high-performance concrete (UHPC) that distinguish it from conventional concrete, new characteristics of the moment redistribution manifest in UHPC structures. In this study, a finite element (FE) analytical model was developed to simulate and analyze the bending behavior and moment redistribution of UHPC continuous beams reinforced with high-strength steel bars. The simulation and test results exhibited excellent agreement with the experimental research. Based on the FE model, a fine analysis for nine simulated two-span UHPC continuous beams was conducted with a detailed discussion of the failure modes, load-displacement curves, variations of support reaction forces, tensile strains of steel bars, and the whole process of moment redistribution. Subsequently, the variation rules of moment redistribution in UHPC continuous beams were explored by an extensive parametric study of 108 simulated beams. The studied parameters included a neutral axis depth factor, concrete strength, yielding strength of reinforcement, beam depth, span–depth ratio, reinforcement ratio between the mid-span and intermediate support section, as well as load forms. According to the numerical results, new formulas for estimating the two-stage moment redistribution in UHPC continuous beams with high-strength reinforcement were established. Finally, a comparison of moment redistribution between normal concrete continuous beams and UHPC continuous beams was performed. It can be observed that the elastic moment distribution in UHPC continuous beams was comparatively smaller, while the plastic moment distribution was relatively larger than those of normal concrete continuous beams. Overall, the degree of the total moment distribution in UHPC structures was greater than that of normal concrete structures due to the high ductility of UHPC. The research in this study may provide a technical reference for the practical engineering of UHPC.

An Adaptive Crack Width Prediction for Flexural Steel Reinforced UHPC Beams

The crack pattern of steel reinforced ultrahigh performance concrete (UHPC) beam is usually characterized by many densely distributed fine cracks (i.e., multiple microcracks) along with localized macrocrack, and the crack width development rate along the beam height is smaller than that of normal concrete since steel fibers and steel reinforcement bars are supposed to be effective in controlling crack width propagation of the UHPC beam. However, an effective crack width prediction formula is still underdeveloped for steel reinforced UHPC beam. The present study aims to formulate a crack width prediction equation based on the equations in Chinese code GB50010 where the parameters can be regressed and calibrated. Ten UHPC beams with different steel fiber volumes and reinforcing ratios are experimentally tested to collect crack width and spacing data for comparison and validation purposes. Nonuniformity distribution coefficient of rebar strain and average crack spacing are calibrated by the test data. Also, rebar stress is calculated with considering residual tensile strength of UHPC based on a sectional analysis. The modified crack width equation is validated with the test results, showing the best prediction accuracy of 0.97 and standard deviation of 0.11 for the test beams in this study compared to those predicted by JTG 3362, CECS 38, MC and AFGC. This study is emphasizing crack width prediction and control in designing UHPC structures.

Effects of thermal properties on temperature field of UHPC structures under fire conditions

Ultra-high-performance concrete (UHPC) is increasingly used for bold structures worldwide due to its exceptional mechanical properties, but its fire performance is not well known. Standards and literature provide limited information and data on the thermal-physical properties of some UHPC mixes exposed to high temperatures. This study aims to partially fill this gap. The thermal parameters of UHPC were determined experimentally and then compared with another UHPC mix proposed in the literature and also with those defined in EN 1992–1.2 for normal strength concrete (NSC). To understand the thermal diffusivity (i.e., the thermal field) of UHPC and NSC structures, a FEM -based numerical application was performed using Abaqus software, assuming the thermal-physical data of UHPC and NSC. Cross sections with the experimentally determined UHPC properties showed quite similar behavior to NSC. However, using the UHPC data proposed in the literature, the thermal diffusion in the cross-section appeared to be much more severe. The thermal-physical properties of UHPC can vary greatly depending on the mix design, with UHPC cross-sections exhibiting a maximum 110% greater thermal field than those simulated with NSC properties.

Chloride ion transport in unsaturated ultra-high performance concrete cracks under high hydraulic pressures

The high strength, durability, and impermeability of ultra-high performance concrete (UHPC) make it a vital material for the construction of deep underground facilities, such as tunnels, nuclear waste repositories, and research laboratories. However, the presence of cracks and high hydraulic pressure in deep underground greatly accelerates the transport of chloride ions, which can shorten the service life of UHPC. Unfortunately, the underlying mechanism for this is still unclear. Here, we investigated the influence of hydraulic pressure and capillary force on chloride ion transport in cracked UHPC using a self-designed testing module. It was found that the presence of trapped gas creates a gas–liquid two-phase state within the cracks. Theoretical models were established using the Laplace equation, which effectively describe the hindrance of gas within cracks on the transport of chloride ions. Contrary to current understanding, the primary driving force for chloride ion transport within the crack is the capillary pores along the crack walls, rather than the capillary forces associated with the crack itself. In addition, the unhydrated regions within the UHPC interior expedite the adsorption of water molecules compared to ordinary concrete. Hydraulic pressure significantly accelerates the depth of chloride ion penetration. Subsequently, the visualization of the solution within cracks was carried out through the application of X-ray computed tomography (X-CT) and capillary tests, thereby providing a means to validate the accuracy of the model. This study contributes to the advancement of our knowledge regarding the influence of capillary force and high hydraulic pressure on the transport of chloride ions. The findings have important implications for the analysis of the durability of UHPC structures situated in deep underground environments.

Bond properties between lapped steel rebars and UHPC under direct tension

Ultra-High Performance Concrete (UHPC) is a highly advanced cement-based composite that offers higher mechanical properties and exceptional durability over Normal-Strength Concrete (NSC). Bond properties between steel rebars and UHPC are the key to designing reinforced UHPC structures. The goal of this study was to characterize the bond properties between lapped steel rebars (uncoated) and UHPC. A total of 20 modified pullout tests having two-row lap splices were performed to examine the variation of diameters of 10, 14, 20, 25 and 32 mm. It was shown that the splitting of UHPC covers was a dominant failure mode, and samples with smaller bar diameters exhibited good ductility. Higher bond strengths were found in samples with smaller bar diameters. Specifically, bond strengths in samples having diameters of 20 mm, 25 mm, and 32 mm showed reductions of 14.8 %, 27.6 %, and 35.2 % in comparison to those featuring a diameter of 10 mm. The distributed curves of bond stress with two peaks were identified when maintaining a lapped length of four times the bar diameter. Furthermore, the mechanical model for lapped steel rebars in UHPC and the theoretical equation for calculating the bond-splitting strength and minimum lapped length were presented and validated by the available studies.

Comparative study on the effect of steel and plastic synthetic fibers on the dynamic compression properties and microstructure of ultra-high-performance concrete (UHPC)

Ultra-high-performance concrete (UHPC) shows great potential in constructing concrete structures subjected to impact and explosive forces. In order to make UHPC structures more ductile and cost-effective, this study used four-volume fractions of plastic synthetic fiber (PSF) instead of conventional steel fibers (SF) to prepare ultra-high-performance hybrid fiber-reinforced concrete (UHP–HFRC) with a total fiber weight of 2%. The dynamic compression properties at four strain rates were also investigated using a split Hopkinson pressure bar (SHPB) test. The results show that the addition of PSF reduces the flowability and compressive strength of UHPC but contributes significantly to the flexural strength of UHPC. UHP-HFRC demonstrates higher integrity under impact loading and significant sensitivity to strain rates. As the strain rate increases, both the dynamic compressive strength and dynamic growth factor of UHP-HFRC increase, with the optimal mixing ratio of PSF and SF being 1:3.Combined with the SEM and nanoindentation test results, it can be found that the bonding ability of PSF to the substrate is stronger, and the interface zone between PSF and the substrate is 10um smaller than that between sf and the substrate. It can be concluded that it is difficult for PSF to fully replace steel fiber in referencing UHPC, while hybrid fibers should be a good choice to produce excellent UHPC composite.

Degradation Mechanism of Ultra High-Performance Concrete under Coupling Erosion of Salt and Frost

Abstract With the use of ultra high-performance concrete (UHPC) in frozen areas, UHPC structures suffer from severe erosion under the interaction of salt ions and frost, resulting in significant performance degradation of UHPC structures. However, the performance degradation mechanism of different compositions and structures of UHPC under the coupling erosion of salt and frost is still unclear at present. In order to study the degraded behavior and mechanism of UHPC under the coupling erosion of salt and frost, various UHPCs were prepared and exposed to freeze–thaw (F-T) cycles and salt–freeze–thaw (S-F-T) cycles. The mass and compressive strength of the UHPCs before and after the F-T and S-F-T tests were compared to evaluate the performance degradation of UHPCs. The SPSS software correlation method, scanning electron microscope, and mercury intrusion porosimetry tests were used to analyze the mechanism of the coupling erosion of salt and frost on the UHPCs. Results indicate that UHPC with a lower water–binder ratio (WBR) and higher superplasticizer content exhibits better resistance to the coupling erosion of salt and frost. The resistance of UHPC to the coupling erosion of salt and frost will decline with the rise of WBR, and the WBR is the most significant factor affecting the coupling erosion of salt and frost for UHPC. The coupling of salt and frost decreases the compressive strength and leads to cracking of UHPC, which can be ascribed to the larger average pore size and higher cumulative pore volume caused by the salt ions. The hygroscopicity of salt ions increases the moisture in pores, and salt crystallization generates pressure in the pores, further aggravating the damage of UHPC.

Stray current induced chloride ion transport and corrosion characteristics of cracked ultra-high performance concrete

Ultra-high-performance concrete (UHPC) is a strong, durable, and corrosion-resistant material that is poised to revolutionize construction in complex environments, such as the deep sea and underground. However, the formation of cracks of UHPC during service and the occurrence of stray currents in underground stations and tunnels greatly accelerate the transport of chloride ions, posing a threat to the safety of underground construction. Here, we used the rapid chloride migration (RCM) method to study the effect of stray current in cracked UHPC. Essentially, electric field accelerates chloride ion transport in cracks by increasing ion transport in the uncracked zone. The wider the crack, the more severe the corrosion of steel fibers, which affects the pore structure of UHPC and leads to an increase in the diffusion coefficient in the uncracked area. In addition, area normalization was employed to precisely monitor the chloride ion concentration in the cracks, which was validated by numerical simulations. Finally, microscopy and atomic force microscopy (AFM) showed that larger crack widths led to deeper corrosion and more severe steel fiber corrosion in UHPC. These results have important implications for the durability design of UHPC structures in stray current environments.

Behavior and constitutive model of ultra-high-performance concrete under monotonic and cyclic tensile loading

Ultra-high-performance concrete (UHPC) holds great promise for constructing lightweight and high-performance concrete structures. Hence, the proportion of live load increases, in turn, leads to a higher live-to-dead load ratio. UHPC structures suffer from cyclic stress amplitude in higher possibility. Therefore, the behavior and constitutive model of UHPC under cyclic tensile loading is of significance for the understanding of bearing capacity and deformation property for structures subjected to cyclic loading, which, nevertheless, is seldomly reported. In this regard, the present study conducted direct tensile tests on 35 UHPC dog-bones with four steel fiber volume fractions (i.e., 0%, 1%, 2%, and 3%) and two fiber shapes (i.e., straight and hooked-end). A constitutive model for UHPC under tensile loading was proposed, implemented into OpenSees, and employed in numerical simulations of UHPC beams subjected to bending in cyclic loading. The numerical results were compared with test results on UHPC beams for model verification. The experimental and numerical results show that the UHPC with straight fiber volume fractions less than 1% demonstrated a brittle failure, whereas the UHPC with straight fiber volume fractions higher than 1% or with a hooked-end fiber volume fraction of 2% exhibited a ductile failure. The increase in fiber volume fraction resulted in great enhancements of the initial crack and peak stresses, and the peak and ultimate strains, and the energy absorption capacity. Besides, the use of hooded-end fibers also led to a slight improvement in tensile strain behavior of UHPC but negatively affected the initial crack stress and peak stress. It also provided sufficient bonding between fiber and matrix. The envelope curves and failure patterns for UHPC under monotonic and cyclic loading are close. The proposed constitutive model for UHPC under tensile loading is composed of an envelope curve that is assumed bi-linear for strain softening UHPC or tri-linear for strain hardening UHPC, an unloading curve and a reloading curve described by an exponential function, and a damage index defined as the reduction of unloading modulus. The proposed constitutive model for UHPC under tensile loading can provide acceptable results for the prediction of the bearing capacity and deformation property of UHPC beams without reinforcements subjected to bending in cyclic loading.

Experimental study of the bonding behavior between coarse aggregate ultra-high performance concrete and steel rebar

The shrinkage and cost of ultra-high performance concrete (UHPC) could be effectively controlled by adding coarse aggregate (CA). The bond performance between UHPC and steel rebar was critical to UHPC structures. This paper conducted a pull-out test to investigate the effects of CA strength, size, content (volume fraction), fiber content, cover thickness, and bond length on the bonding behavior of UHPC and steel rebar. The results showed that CA significantly affected the bond strength because it could delay and prevent the expansion of microcracks. Improving the CA strength (106.3 MPa-202.4 MPa) could substantially increase the bond strength. As the CA content increased from 0 to 600 kg/m3, the bond strength increased by 14.49%. The CA size of 5–8 mm improved bond strength better than other sizes. Further, a formula for calculating bond strength and the bond-slip constitutive model was proposed, which agreed with the experimental results.

Systematic studies on behaviors of ultra-high performance concrete subject to freezing and thawing cycles combining DIC technology

Freezing and thawing alternation is one harsh weather condition for the concrete structures serving in seasonal frozen areas. As an emerging construction material, the frost-resistance of ultra-high performance concrete (UHPC) is highly necessary to be understood for propelling its practical application. In this study, the frost-resistance of UHPC was systematically evaluated based on the mechanical properties and the cracking behaviors. The fast freezing and thawing (FT) cycles were chosen as the experiment condition, along with the water-immersion condition for comparison. A combination of destructive mechanical and non-destructive resonance frequency test was used to characterize the behaviors. The results showed that, after undergoing 300 FT cycles, the tensile initial cracking strength, the tensile strength, the flexural strength, and the compressive strength exhibited a certain of degradations. The corresponding effects of the water-immersion action were more positive. The tensile strain capacity under the FT condition almost kept constant, whereas that under the water-immersion condition decreased. The two indices (including strain and strength) could reasonably reflect the magnitude of tensile strain. The digital image correlation (DIC) analysis unveiled that there may exist an additional plastic deformation in the tensile full-field deformation, apart from the elastic deformation and crack opening. Compared with the water-immersion action, the FT action resulted in a less zigzag cracking path, shorter cracking time, and larger crack opening under flexural loading. In addition, the variations of dynamic modulus of elasticity and re-hydration reaction degree under the FT condition were not coincident with those of mechanical properties. The findings could provide some valuable references for estimating the bearing capacity of the UHPC structures served in practical engineering.

FRP bar-reinforced ultra-high-performance concrete plates with a grouting sleeve connection: Development and flexural behavior

Fiber reinforced polymer (FRP) bars have been proposed for use in ultra-high performance concrete (UHPC) as the reinforcement to mitigate/avoid the post-peak strain softening of UHPC under tension and to reduce the fiber dosage used in UHPC. The so-formed FRP-reinforced UHPC plates could be adopted for both structural strengthening and constructing novel structural elements. In this paper, a novel connection based on FRP bars and steel grouting sleeves has been developed for prefabrication construction of FRP bar-reinforced UHPC structures. Flexural tests were conducted to gain understandings of the effects of the connection mode (the staggered connection and the horse tooth connection) and reinforcing fiber type (polyethylene (PE) fibers and steel fibers) in the UHPC on the flexural behavior of connected FRP bar-reinforced-UHPC composite plates. The test results revealed that the proposed connection system is reliable since the composite plates always failed outside the connection zone, and most of the crack opening displacements at the interface of the connection are smaller than 0.2 mm at ultimate. The connection mode and the fiber type had little influence on the first cracking load and the initial bending stiffness of the FRP bar-reinforced UHPC plates, while the horse tooth connection led to a better ductility of the composites plates than the staggered connection mode. At given tolerable deflection of 1/200 span (about 5 mm), the longitudinal strains developed in the CFRP bars were about 2000 με for all specimens.

Mechanical Strength and Microstructure of Ultrahigh-Performance Concrete under Long-Term Autoclaving

Solar thermal energy is a technology that collects radiant energy from the sun to generate heat or electricity. Thermal energy storage is required to compensate for the solar thermal energy variations across time scales, and a popular strategy is storing thermal energy in hot water tanks. Due to superior strength and durability, ultrahigh-performance concrete (UHPC) is exploited to construct such water tanks for applications at 200°C. Therefore, the mechanical strength and microstructure of UHPC after long-term temperature-pressure load (autoclaving) is studied. The compressive strength of UHPC can stay robust due to the accelerated formation of hydrates, while the flexural strength is vulnerable to long-term autoclaving due to the transformation of amorphous calcium silicate hydrate (C-S-H) to more ordered phases. The main hydrates in autoclaved samples are poorly crystallized C-S-H, hydroxylellestadite, and hydrogarnet. The porosity of autoclaved samples is not strictly related to the mechanical strength, and the influence of hydrate assemblage outweighs that of porosity on mechanical strength after long-term autoclaving. The partial replacement of cement by limestone powder can decrease crystalline hydrates and increase poorly crystallized C-S-H, which densifies the microstructure and enhances the mechanical strength. However, excessive poorly crystallized C-S-H aggravates the thermal mismatch between the matrix and quartz aggregates after autoclaved samples cool to room temperature, leading to interstice in matrix-quartz interfaces and, thus, reduced mechanical strength. Therefore, an appropriate addition of limestone powder is required to induce the hydrate assemblage with appropriate poorly crystallized C-S-H, ensuring durable UHPC structures under autoclaving.

TDOF model for UHPC members under lateral low-velocity impact

The dynamic behaviors of ultra-high performance concrete (UHPC) members under lateral low-velocity impact loading have been intensively studied by experiment and numerical simulation, while the corresponding structural design approach is still limited and mainly concerned in this study. Firstly, through comprehensively considering the compression arch action, strain rate effect, and unloading stiffness, the dynamic resistance function of UHPC members, e.g., reinforced UHPC (R-UHPC) and UHPC-filled steel tube (UHPC-FST) beams/columns, are built and verified. Then, a two degrees of freedom (TDOF) model of the drop hammer test on UHPC members is improved by considering the P-δ effect and stress wave propagation effect. Furthermore, the impact force- and mid-span deflection-time histories of the existing five series of drop hammer tests on UHPC members are applied to verify the validity and applicability of the established TDOF model. The influence of considering the stress wave propagation effect, strain rate effect, and steel fibers in UHPC for predictions of the TDOF model are discussed by parametric analyses. Finally, based on the energy balance, the explicit formulae are proposed to predict the maximum and residual mid-span deflections of the flexural critical UHPC members under the lateral low-velocity impact, which can be helpful for the design of UHPC structures under lateral impact loadings.

Experimental and Hybrid FEM/Peridynamic Study on the Fracture of Ultra-High-Performance Concretes Reinforced by Different Volume Fractions of Polyvinyl Alcohol Fibers.

In this study, a series of three-point bending tests were carried out with notched beam structures made of polyvinyl alcohol (PVA) fiber-reinforced ultra-high-performance concrete (UHPC) to study the effect of volume fractions of PVA fibers on the fracture characteristics of the UHPC-PVAs. Furthermore, in order to meet the increasing demand for time- and cost-saving design methods related to research and design experimentation for the UHPC structures, a relevant hybrid finite element and extended bond-based peridynamic numerical modeling approach is proposed to numerically analyze the fracture behaviors of the UHPC-PVA structures in 3D. In the proposed method, the random distribution of the fibers is considered according to their corresponding volume fractions. The predicted peak values of the applied force agree well with the experimental results, which validates the effectiveness and accuracy of the present method. Both the experimental and numerical results indicate that, increasing the PVA fiber volume fraction, the strength of the produced UHPC-PVAs will increase approximately linearly.

One-dimensional fiber beam-column model and two-dimensional fixed crack model for numerical simulation of UHPC structures

Ultra-high performance concrete (UHPC), with much higher compressive strength, tensile strength and ultimate tensile strain than normal concrete, has been experimentally investigated and applied in engineering structures more and more widely. However, there are few numerical models appropriate for the accurate and efficient simulation of UHPC or reinforced UHPC (R/UHPC) flexural members including beams and columns as well as shear-critical members such as shear walls till now, which brings obstacles to the analysis and design of UHPC structures. In this study, the uniaxial skeleton and hysteretic rules for UHPC under tensile and compressive loads are first proposed and verified by material tests. The formulas to calculate all the basic parameters in the model are put forward for convenient use. Then the fiber beam-column model for flexure-dominated UHPC members is established in ABAQUS 2017 software and six R/UHPC column specimens subjected to cyclic load are simulated. Based on simulation results, the equations of transition curves in the uniaxial rules are determined, which have significant influence on the predicted hysteresis curves. On the other hand, a new two-dimensional fixed crack based constitutive model for shear-critical UHPC members is further proposed and implemented in ABAQUS 2017 software by considering the compressive softening due to principal tensile strain and shear stress–strain relationship along cracks of UHPC. Finally, ten R/UHPC shear wall specimens under cyclic loads as well as four R/UHPC deep beams under monotonic loads are simulated and the accuracy of the proposed 2D model for UHPC is validated.