Ultra High Performance Fibre Reinforced Concrete Slabs Research Articles

Punching shear behavior of ultra-high-performance fiber-reinforced concrete and normal strength concrete composite flat slabs

This paper presents the results of tests on flat slabs with partial application of ultra-high-performance fiber-reinforced concrete (UHPFRC) mixtures to improve the punching shear capacity of slab-column connections. One normal strength concrete (NSC) slab and two UHPFRC slabs were tested as a reference for tests on 12 UHPFRC-NSC composite slabs with full depth UHPFRC in the center area of the NSC slab. The main test parameters were the steel fiber volume (0.8 % and 1.6 % volume fractions) and the dimensions of UHPFRC mixtures. The load-column displacement curves, crack patterns, slab rotations at failure, and failure loads are presented and discussed. The failure loads are compared with estimates obtained following the yield line analysis, design codes, and other theoretical models. Test results showed that the use of UHPFRC mixtures led to an increase in initial stiffness, punching shear strength, and deformation capacity, along with a change in the slab failure mode. The shear capacity of the slab improved proportionally to the increase in the dimension of the UHPFRC. These tests confirmed the effectiveness of the UHPFRC mixtures as an alternative to enhance the punching shear capacity for slab-column connections.

Punching shear and flexural performance of ultra-high performance fibre reinforced concrete (UHPFRC) slabs

Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) is a high-performance cementitious composite that is characterised by superior mechanical properties, high energy absorption capacity and enhanced durability. The mechanical characteristics of UHPFRC and more specifically the enhanced tensile strength and the significant energy absorption and crack resistance make it a very popular material, especially for extreme loading conditions or in cases of thin load bearing elements.UHPFRC has been recently introduced in applications such as bridge decks, and thin slabs and for the strengthening of existing elements. However, the punching shear performance and the effect of the fibres on the punching shear strength of UHPFRC have not been sufficiently studied. Also, there is a lack of provisions for the punching shear design of UHPFRC, which limits the extensive application of UHPFRC in thin elements especially when there are concentrated loads.In this study, extensive experimental work has been conducted on slabs with different geometries and various mixes and valuable results are presented for the flexural and the punching shear capacities of UHPFRC. Also, methodologies for the calculation of the punching shear and the flexural capacities of UHPFRC slabs have been examined and important conclusions have been drawn.

Experimental and multi-scale numerical investigation of ultra-high performance fiber reinforced concrete (UHPFRC) with different coarse aggregate content and fiber volume fraction

This study investigates the influence of the coarse aggregate content and steel fiber volume fraction on the mechanical properties of UHPFRC. On the meso-scale, according to Mori-Tanaka homogenization method and progressive damage theory, the equivalent damage constitutive model of UHPFRC composite is established. On the macro-scale, the mesoscopic equivalent properties of UHPFRC material are input into the finite element model corresponding to the test for numerical simulation, and the mesoscopic material properties are coupled with the macroscopic finite element model to study the influence of mesoscopic components of UHPFRC on its macroscopic mechanical properties. The compression, tension and four-point bending numerical simulation results of UHPFRC show that the predicted results of the multi-scale numerical model are in good agreement with the test results. It is also found that with the increase of fiber volume fraction, the compressive strength, tensile strength and four-point bending strength of UHPFRC increase first and then decrease. The improvement effect of coarse aggregate on the properties of UHPFRC is not as obvious as that of fiber, and the increase of fiber volume fraction has a significant enhancement effect on the ductility and post peak toughness of UHPFRC slab.

Experimental and Numerical Evaluation of UHPFRC Slabs Subjected to Contact and Close-In Explosion

This paper presents achievements in the field of the numerical simulation of the fibrere reinforced concrete (FRC) and ultra-high performance fibre reinforced concrete (UHPFRC). The numerical simulations were performed to verify results of two experimental programmes focused on the blast resistance of FRC and UHPFRC. The response of the FRC and UHPFRC slabs to the contact and near-field blast was studied in these two experiments. As the detail behaviour of specimens could not be observed because of the blast load, the numerical models were prepared. The accuracy of the numerical models was evaluated based on the comparison of numerical and experimental results. Different approaches for blast simulation were tested and compared. The results indicate that the various phenomena (e.g. overpressure propagation, stress cumulation, crack propagation and damage extend) can be successfully simulated. However, the comparison of the soffit velocity, measured with the PDV unit and numerical model showed shortcomings of the numerical model. These numerical model inaccuracies are discussed and their reasons presented.

Modeling the fracture behavior of Ultra - High Performance Fiber Reinforced Concrete slabs under contact Blast Loading

This paper presents a fracture behavior modeling of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) slabs under contact blast loading using the finite element method. The UHPFRC is supposed to be brittle material and follow the Johnson–Holmquist-II(JH-2) model. The steel rebar is modeled using the elasto-plastic model. The Emulsion Explosive has been used and modeled by the SPH method. UHPFRC slabs with the dimensions 1000mm of length, 800mm of width and 120mm of thickness is considered. The steel fibers with a volume fraction of 2% is used. The UHPFRC material is fabricated in laboratory using the material available in Vietnam. The concrete crater and spall damage of UHPFRC slab under contact blast loading are considered. The numerical results are compared with experiment. These results allow to evaluate the resistance against blast load of UHPFRC fabricated in lab.

Influence of key design parameters of ultra-high performance fibre reinforced concrete on in-service bullet resistance

This study investigates the influence of key parameters on in-service bullet impact resistance of ultra-high performance fibre reinforced concrete (UHPFRC), with the aim to provide design guidance for the engineering applications. The effects of steel fibre type and dosage, matrix strength, coarse basalt aggregates, and target thickness are researched by subjecting the UHPFRC to a 7.62 mm bullet shooting with velocities of 843–926 m/s. The results show that the UHPFRC, designed by using a particle packing model with compressive strength around 150 MPa, is appropriate to develop protective elements considering both anti-penetration performance and cost-efficiency. The 13 mm short straight steel fibres show better anti-penetration than the 30 mm hook-ended ones, and the optimum volume dosage is approximately 2% by considering both the penetration and crack inhibition. Introducing coarse basalt aggregates with the particle size up to 25 mm into UHPFRC reduces the powder consumption from 900 kg/m3 to 700 kg/m3, and results in slightly higher mechanical strength and significantly enhanced bullet impact resistance with 14.5% reduction of penetration depth. The safe thicknesses (perforation limit) of the designed UHPFRC slabs are approximately 85 mm and 95 mm to withstand the 7.62 × 51 mm NATO armor-piercing bullet impact under velocity 843 mm/s and 926 mm/s, respectively.

Behaviour and analysis of ultra high performance fibre reinforced concrete (UHPFRC) skew slabs

This study presents an experimental and theoretical investigation of the performance of ultra-high performance fibre reinforced concrete (UHPFRC) skew bridge decks. Five simply-supported UHPFRC slabs with different skew angles were tested under monotonically increasing concentrated load. It was observed that an increase in the skewness of a slab led to a more unsymmetrical vertical deflection profile, and also resulted in a decrease in the magnitude of reaction forces at the acute corners and a corresponding increase in the reaction forces at the obtuse corners. The test results also show that, although some reduction in ductility was seen with increasing skewness of a slab, the specimen with over 25° skew angle still maintained a desirable load carrying capacity and ductility compared to those of a conventional RC straight slab. In addition to the experimental investigations, a fundamental mechanics based closed-form model was developed based on strain energy theorem to predict the performance of a UHPFRC skew slab under linear elastic material condition. At its ultimate limit state, a novel yield-line analysis using a numerically-generated mechanics based moment-rotation relationship was adopted to predict the ultimate load carrying capacity of a skew UHPFRC slab. Both the generic analytical procedures were compared and against test results obtained from this study, as well as those reported in literature, and the results show that the models developed can be applied to UHPFRC skew slabs.

New UHPFRC bridges in the Czech Republic

This paper deals with new UHPFRC (Ultra High Performance Fibre Reinforced Concrete) bridges produced or constructed in 2018 in the Czech Republic. The paper includes a short description of 3 unique pedestrian bridges with load bearing structure made of UHPFRC and 2 steel bridges over the railway track with lost formwork for bridge decks and cornices made of thin UHPFRC slabs. Focus is given on the testing of the material, comparison of test specimens and summary of results from UHPFRC production in real scale. The size effect of control specimen and the results analysis is given in the last chapter. The 3 mentioned unique pedestrian bridges are Footbridge over the Lubina river in city Příbor, Pedestrian and cycle bridge in the vicinity of Black Bridges in city Tábor and Footbridge over Dřetovice stream in Vrapice - a district of Kladno city. Thin panels as permanent lost formwork for two bridges over the railway tracks are in city Přerov. All new UHPFRC structures should be worthy of attention of bridge experts and community of specialists in the field of concrete engineering.

Geometrical and boundary condition effects on restrained shrinkage behavior of UHPFRC slabs

Six large Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) slabs were fabricated and tested to investigate the restrained shrinkage and cracking behaviors. The use of expanded polystyrene and Teflon sheets with two different slab thicknesses was considered to improve the shrinkage crack resistance. Free shrinkage was simultaneously measured to evaluate the degree of restraint according to the above test parameters. The test results showed that free shrinkage strains of –689 με to –723 με were obtained after 9 days, and prismatic specimens with a higher exposed surface area-to-volume ratio (S/V) had slightly higher free shrinkage strains than those with a lower S/V. Increasing the concrete slab thickness and using expanded polystyrene and Teflon sheets were effective at reducing the degree of restraint and improving the shrinkage crack resistance of the UHPFRC slabs. Among the various specimens, the slabs with the expanded polystyrene exhibited the lowest degree of restraint by 0.45 after 9 days.

Ultra-high performance fibre-reinforced concrete under impact: experimental analysis of the mechanical response in extreme conditions and modelling using the Pontiroli, Rouquand and Mazars model.

To evaluate the vulnerability of ultra-high performance fibre-reinforced concrete (UHPFRC) infrastructure to rigid projectile penetration, over the last few years CEA-Gramat has led an experimental and numerical research programme in collaboration with French universities. During the penetration process, concrete is subjected to extreme conditions of pressure and strain rate. Plasticity mechanisms as well as dynamic tensile and/or shear damage are activated during the tunnelling phase and the cratering of the concrete target. Each mechanism has been investigated independently at the laboratory scale and the role of steel fibres especially has been analysed to understand their influence on the macroscopic behaviour. To extend the experimental results to the structural scale, penetration tests on UHPFRC slabs have been conducted by CEA-Gramat. The analysis of this dataset combined with material characterization experiments allows the role of steel fibres to be identified in the different plasticity and damage mechanisms occurring during penetration. In parallel, some improvements have been introduced into the concrete model developed by Pontiroli, Rouquand and Mazars (PRM model), especially to take into account the contribution made by the fibres in the tensile fracture process. After a primary phase of validation, the capabilities of the PRM model are illustrated by performing numerical simulations of projectile penetration into UHPFRC concrete structures.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

Response of Thin UHPFRC Targets with Various Fibre Volume Fractions to Deformable Projectile Impact

The main purpose of this study is to analyse the projectile impact resistance on slabs constructed of ultra-high-performance fibre-reinforced concrete (UHPFRC). The fibre volume fraction of the UHPFRC was set as the main test variable in the framework of this study. The fibre volume fraction was ranging from 0.125% to 2% and accordingly the compressive strength of the UHPFRC was ranging from 118MPa to 144MPa. The primary importance was given to the shape of the crater that was produced on the UHPFRC slabs as a result of the projectile impact. The resistance of the slab was therefore evaluated on the basis of the crater depth, crater area and crater volume at the front and rear side of the slab.The resistance to impact loading was tested on rectangular slabs with the measurements of 300mm × 400mm × 50mm (length × width × thickness). It was verified experimentally that fibres are important inhibitor of the damage when dealing with projectile impact. The damage of the slab was reduced significantly when the fibre volume fraction was increased. Also it was shown that the best performance was achieved with the highest fibre volume fraction. It is very important to note that using 2% of steel fibres in the thin UHPFRC targets resulted in no scabbing from the back face of the slab with only a front crater forming. This is very important for facade panels and walls that are used for the purpose of protecting people where no perforation and scabbing of the concrete are allowed to occur.

A new test method for investigating punching shear strength in Ultra High Performance Fibre Reinforced Concrete (UHPFRC) slabs

Punching shear capacity of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) slabs has been the subject of a number of studies. There is, however, only limited information available on this parameter of UHPFRC without conventional reinforcement. This is due to the complexity of punching shear behaviour within concrete and is also limited by the lack of suitable test methods currently available. Therefore, in this study, attempts to design a novel testing method to measure the punching shear capacity of the concrete was carried out. The designed test arrangement was employed to carry out an extensive experimental study on UHPFRC slabs subjected to punching shear failure. From the results obtained, the relationship between the punching shear load and the angle of the shear plane, the critical value of the basic control perimeter and failure mode were studied. The experimental study undertaken here provides significant insight into the punching shear capacity of UHPFRC slabs. The results illustrate and highlight many of the advantages of using UHPFRC compared to normal concrete in structural designs. The novel punching shear test presented here has established itself as a suitable procedure for testing UHPFRC and potentially, other fibre reinforced composites.

Response of small scale ultra high performance fibre reinforced concrete slabs to blast loading

This paper provides a systematic study of the performance of ultra high performance fibre reinforced concrete (UHPFRC) slabs under close-in explosions. In the study, the UHPFRC slabs were manufactured with varying amounts and types of steel fibres. In field tests, various PE4 charge sizes were employed to generate blast load until a sudden increase in maximum deflection was observed. Peak 1/4 span deflections and development of slab cracks were recorded. With the results, the effect of steel fibres on performance of UHPFRC slabs can be better clarified. Static tests were also used to study the effect of fibres on the behaviour of UHPFRC specimens, results indicate that fibre distribution and orientation can clearly affect UHPFRC slab behaviour. Finally, finite element modelling was employed to simulate the performance of UHPFRC slabs under blast loading and these modelling results were compared to test data. Based on the comparison results, potential further enhancement of numerical models is discussed.

Hybrid FRP–UHPFRC composite girders: Part 2 – Analytical approach

A companion paper shows the experimental testing and numerical analysis of composite girders consisting of hybrid carbon/glass fiber-reinforced polymer (HFRP) I-girders topped with precast ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs. This paper presents derivations of analytical equations to examine the composite behavior of the HFRP–UHPFRC girders with varying levels of interaction at the slab/girder interface. The derived equations to predict interfacial slip strain, curvature, and deflection of the composite girders were validated by the experimental data. An equation to determine the effective moment of inertia of the HFRP–UHPFRC composite girders with partial interaction is proposed and compared against the experiment. A fairly good agreement between the experimental and analytical results was found. The derived/proposed equations can simply and reliably evaluate the composite behavior between the HFRP I-girder and the UHPFRC slab.

Hybrid FRP-UHPFRC composite girders: Part 1 – Experimental and numerical approach

This paper presents experimental testing and numerical analysis of durable composite girder systems consisting of hybrid carbon/glass fiber-reinforced polymer (HFRP) I-girders and precast ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs. Epoxy adhesive and/or bolt shear connectors (SCs) were used to transfer horizontal shear forces from the UHPFRC slabs to the HFRP I-girders. Push-out tests were performed to evaluate the shear interaction at the slab/girder interface. The results of the push-out tests were used to design five large-scale composite girders. The experimental variables included two types of bolt SCs (straight and inclined bolt SCs) and different slab geometries. The composite behavior between the HFRP I-girder and the UHPFRC slab was examined. The test results showed that the combined use of the bolt SCs and the epoxy adhesive was essential to achieve a full interaction between the HFRP I-girder and the UHPFRC slab. The use of the epoxy adhesive resulted in improving the strength and stiffness of the composite girders, reducing the stress concentration around the bolt-holes, and increasing the shear strength of the bolt SCs. The girders with the inclined bolt SCs showed a more ductile behavior than those with the straight headed bolt SCs. A three-dimensional finite element (FE) analysis of the HFRP-UHPFRC composite girder was carried out that considered slip effects at the slab/girder interface and the nonlinear behavior of the bolt SCs. Comparisons between the experimental results and the FE analysis were performed and a fairly good agreement was found. A companion paper shows derivations of analytical equations to predict composite behaviors of the HFRP-UHPFRC girders with varying levels of interaction at the slab/girder interface.

Shrinkage and cracking of restrained ultra-high-performance fiber-reinforced concrete slabs at early age

In this study, the combined effect of shrinkage-reducing admixture (SRA) and expansive admixture (EA) on the shrinkage and cracking behaviors of restrained ultra-high-performance fiber-reinforced concrete (UHPFRC) slabs was investigated. For this investigation, six full-scale UHPFRC slabs with three different thicknesses (h=40, 60, and 80mm) were fabricated using two different mixtures. Test results indicated that the combined use of 1% SRA and 7.5% EA is beneficial to improve the mechanical strengths and to reduce the free shrinkage strain of approximately 36–42% at 7days. Regardless of SRA and EA contents, the slabs with the lowest thickness of 40mm showed shrinkage cracking at a very early age, while the slabs with higher thicknesses of 60 and 80mm showed no cracking during testing. However, the UHPFRC slab including 1% SRA and 7.5% EA exhibited a shallow crack with a very small maximum crack width of below 0.04mm, while the slab without SRA and EA showed through cracks with a large maximum crack width of 0.2mm.

Resistance of slim UHPFRC targets to projectile impact using in-service bullets

Significant buildings, transportation hubs, protective or defence structures are at exposure risk to extreme load event such as blast loading or direct armed attack. This kind of loading is typical by its rapid increase in release of energy in a very short time. It is believed that emerging cementitious materials such as Engineered Cementitious Composite (ECC) or Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) provide better passive protection to significant buildings compared to High-Strength Concrete (HSC) or conventional Fibre Reinforced Concrete (FRC). In this study, response of several UHPFRC slabs to projectile impact was examined in order to simulate the effects of possible damage by small fire arms or by fragments generated by an explosion. Real ammunition was used with impact velocity in the range of 691–720 m/s and fibre content of the UHPFRC was selected as the main test variable. The optimal fibre content in the UHPFRC mixture was determined using damage parameters such as crater diameter, penetration depth, debris fragment mass and residual penetration potential of the bullet. It was verified experimentally that the optimal fibre content in the UHPFRC mixture is 2% by volume. Using less than 2% of fibre volume fraction might be unsafe in the slim UHPFRC targets due to increased volume of secondary fragments generated from the back side of the slab and also due to the higher residual penetration potential of the bullet exiting the back side of the slab. Using more that 2% of fibre volume fraction could be inefficient. No improvement in all damage parameters was observed when the fibre volume fraction was changed from 2% to 2.5% or 3%.

Push-out tests for shear connections between UHPFRC slabs and FRP girder

This paper presents an experimental investigation on the shear connections between carbon/glass Fiber-Reinforced Polymer I-girder and Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) slabs. Effects of straight/inclined bolt shear connectors and effective embedment depth-to-bolt diameter (hef/d) ratios were investigated. Fourteen push-out tests were conducted to evaluate the load–slip behavior and the ultimate resistance of the bolt shear connectors. With the use of the UHPFRC, the hef/d ratio needed for obtaining a shear connector failure mode was 2.7 times lower than that obtained from 43 push-out tests in the literature for normal weight and lightweight concrete slabs. The inclined bolt shear connectors showed a more ductile behavior than the straight bolt shear connectors. The experimentally obtained ultimate resistance of the bolt shear connectors was compared against the equations provided by ACI 318-11, AISC 2011, AASHTO LRFD 2010, PCI 2004, and EC-4 2004. An empirical equation to predict the ultimate shear connector resistance was proposed and validated by the experimental data. Idealized load–slip models and equations to predict the load versus slip relationship for all push-out tested specimens were proposed. Curve fitting was performed to find fitting parameters for all tested specimens and the results showed a very good correlation with the experiments.

Hybrid Fiber-Reinforced Polymer Girders Topped with Segmental Precast Concrete Slabs for Accelerated Bridge Construction

The behavior of composite girders made of hybrid fiber-reinforced polymer (HFRP) I-girders, topped with precast ultra-high-performance, fiber-reinforced concrete (UHPFRC) slabs is presented in this paper. HFRP I-girders were manufactured under the pultrusion process in which unidirectional carbon fibers and bidirectional fiberglass fabric or continuous strand mat were used. Four large-scale composite girders were tested under four-point flexural loading. In the first composite girder, the HFRP I-girder was topped with a full-length precast UHPFRC slab. Twelve precast UHPFRC segments were used in each slab of the other three composite girders. Either epoxy or mortar connections were used to connect the precast UHPFRC segments. The test results showed that the flexural stiffness of the composite girder with the epoxy-connected segmental precast slabs was similar to that of the full-length precast composite girder. The mortar-connected girder exhibited more ductile behavior than the epoxy-connected girder. All the composite girders exhibited significant improvements in strength and stiffness compared with the HFRP I-girder without the UHPFRC slab. The HFRP–UHPFRC composite girders were shown to provide a promising and sustainable solution for accelerated bridge construction.

Analysis of the behaviour of a bonded joint between laminated wood and ultra high performance fibre reinforced concrete using push-out test

Bonded composite constructions using timber and ultra-high performance fibre reinforced concrete (UHPFRC) are investigated as highly innovative structural elements technically and economically efficient, and having better environmental performances. Such technique could indeed offer new structures typologies for bridges for instance associating timber for main beams with UHPFRC slabs. In timber–concrete-composite structures, connection is traditionally achieved with mechanical means. The research presented herein describes the timber–concrete assembly by adhesive bonding and specially the behaviour of bonded joint between Laminated Wood (LW) and UHPFRC. This type of assembly has to be investigated concerning mainly the durability of the bonded assembly and its ability to transmit the forces concentrated on edges. The present study describes the development of a test called “Push-Out” used to characterize the behaviour of the timber–concrete bonded after or prior to environmental ageing. Following the development of the used test protocol, investigations are realized on four different commercial resins to study the influence of the adhesive stiffness on the force transfer focusing in particular on the failure mode, on the ultimate capacities and some local deformations. The mechanical behaviour of the bonded joint is investigated in particular from a theoretical point of view and from the determination of deformation field obtained by Digital Image Correlation (DIC).