Strain Hardening Ultra High Performance Research Articles

Strain-hardening ultra-high performance concrete (UHPC) with hybrid steel and ultra-high molecular weight polyethylene fibers

This study addresses the advancement of strain-hardening ultra-high performance concrete (UHPC) by investigating its tensile behavior with hybrid steel and ultra-high molecular weight polyethylene fibers. Through direct tensile testing of UHPC samples with varying volumes of steel and polyethylene fibers (totaling 2.0%), significant improvements in tensile strain capacity and energy absorption capacity (tensile toughness) were observed upon replacing steel fibers with polyethylene fibers. Despite marginal variations in first cracking strength and ultimate tensile strength, these properties could be finely tuned by tailing fiber length and hybrid ratio. Quantitative analysis established correlations between the hybrid fiber factor and key mechanical properties, advancing a deeper understanding of UHPC behavior. The findings underscore the benefits of multiscale fiber hybridization in UHPC, offering scientific insights into predicting and optimizing its performance for structural applications. This study helps close the gap in UHPC development, emphasizing the potential of multiscale fiber hybridization to achieve concurrently high tensile strength and strain capacity.

Effect of fiber content and fiber length on the dynamic compressive properties of strain-hardening ultra-high performance concrete

Strain-hardening ultra-high performance concrete (SH-UHPC) exhibits excellent mechanical properties at the quasi-static state and becomes a promising material for protective structures exposed to impact or blast threats. But its performance under loads with high magnitude and short duration remains unanswered. The focus of this study is to investigate the dynamic compressive properties of SH-UHPC under high strain rates, which are achieved by split Hopkinson pressure bar (SHPB) tests. SH-UHPC with different fiber contents of 1, 1.5, and 2 vol% and different fiber lengths of 6, 12, and 18 mm are tested. The considered strain rate ranges from ∼ 180 s−1 to ∼ 300 s−1. The results show that SH-UHPC exhibits good impact resistance in terms of integrity. Samples could remain good integrity at a strain rate of ∼ 240 s−1. The key parameters to measure the dynamic properties, i.e. energy absorption, dynamic compressive strength, and dynamic increase factor (DIF) of SH-UHPC show strain rate sensitivity and all of them increase with strain rate. Increasing fiber content leads to a slight improvement of the impact resistance of SH-UHPC while reducing fiber length impairs the impact resistance. Empirical DIF relations of SH-UHPC are firstly proposed in this study.

Experimental study and calculation method on the flexural resistance of reinforced concrete beam strengthened using high strain‐hardening ultra high performance concrete

AbstractIn this study, the efficiency of using high strain hardening ultra high performance concrete (HSH‐UHPC) on strengthening existing reinforced concrete beams was investigated by experimental and analytical means. First, the HSH‐UHPC constitutive relation was obtained using “Dog‐bone” specimen under uniaxial tensile test. Then three reinforced concrete beams strengthened with HSH‐UHPC layers and one control beam were tested under flexural loading. The midspan deflection, strain distribution, crack propagation and failure mode of the tested beams were described in detail. The tests verified that the middle section of UHPC‐NC composite beam basically conforms to the assumption of plane section. The flexural resistance of composite structure can be greatly improved by installing proper longitudinal reinforcement in UHPC layer. Meanwhile, numerical studies have been conducted on the tested beams using the experimentally obtained constitutive relations. The result shows that the finite element analysis results are consistent with the test results. Finally, a contribution factor for the HSH‐UHPC layer on the flexural resistance of the composite beam was proposed. At the same time, it is found that the contribution efficiency of the HSH‐UHPC to the flexural resistance increases with longitudinal reinforcement ratios in the HSH‐UHPC.

Effect of Fiber Content and Fiber Length on the Dynamic Compressive Properties of Strain-Hardening Ultra-High Performance Concrete

Effect of Fiber Content and Fiber Length on the Dynamic Compressive Properties of Strain-Hardening Ultra-High Performance Concrete

Cyclic tensile behavior of high strain hardening UHPC analyzed by acoustic emission techniques

Understanding the cyclic tensile behavior of high strain hardening ultra high performance concrete (UHPC) is important in structural design and for predicting its damage state. In this study, acoustic emission (AE) analysis was applied to monitor the damage evolution process of high strain hardening UHPC under direct tensile cyclic loading at different objective tensile strain levels. The test results show that when the objective strain is in the elastic state, the tensile cyclic stress–strain curves show marginal change, whereas when it is in the strain hardening state, the residual strain and the stiffness of UHPC are reduced by increasing the number of cycles and finally tends toward stability, the maximum stress remains stable. The AE analysis results can help to explain the evolution of the mechanical properties of high strain hardening UHPC under cyclic tensile loading and proves the existence of the Kaiser effect, which can be used to quantify the deformation and the degree of damage of UHPC. The stiffness of UHPC under cyclic tension is controlled by the nominal debonding length of the steel fibers. Finally, the relationship between the total tensile strain and the loading stiffness degradation ratio under cyclic tension was proposed and could be employed in evaluate of stiffness of the high strain hardening UHPC under cyclic tension in some extent.

Spalling resistance and mechanical properties of strain-hardening ultra-high performance concrete at elevated temperature

This study aimed to investigate fire resistance of strain hardening ultra-high performance concrete (SHUHPC). A series of mechanical tests, spalling tests, thermal analysis, and microscopic observation were conducted. Polypropylene (PP) fibers of different dosages were adopted to mitigate spalling of SHUHPC. The finding showed that SHUHPC showed severe spalling even with 1.5 vol% polyethylene (PE) fibers. PE fibers were ineffective in spalling prevention. Creation of empty channels by PE fibers after exposure to 200 °C did not lead to a good spalling resistance of SHUHPC. To prevent spalling of SHUHPC, a dosage of 0.3 vol% of PP fibers was required. High thermal expansion of PP fibers before melting allowed the PP fibers to create microcracks in concrete and to enhance permeability at 150 °C, resulting in good spalling resistance of SHUHPC. Further, partially replacing PE fibers by PP fibers negatively affected tensile properties of SHUHPC at ambient temperature. Besides, the compressive of SHUHC was not affected by different proportions of PE and PP fibers at ambient temperature and high temperature.

Biaxial flexural fatigue behavior of strain-hardening UHPFRC thin slab elements

The biaxial flexural fatigue behavior of thin slab elements made of strain-hardening Ultra High Performance Fiber Reinforced Cementitious Composite (UHPFRC) is investigated experimentally by means of the ring-on-ring test method. Fourteen flexural fatigue tests under constant amplitude fatigue cycles up to the Very High Cycle Fatigue domain (20 million cycles) are conducted with varying maximum fatigue stress level S ranging from 0.50 to 0.68. Digital Image Correlation (DIC) technology is applied to capture the 3D full-field strain contours on the tensile surface through the entire fatigue test. Test results presented in the S-N diagram reveal a fatigue endurance limit under biaxial flexural fatigue at S = 0.54. Fatigue tests exhibiting failure show four distinct phases of damage evolution, while only the first two phases are observed in the case of run-out tests. DIC analysis reveal formation and propagation of multiple fine fictitious cracks that dominate the stable fatigue propagation phase with slow rate, representing the longest part of fatigue life of the UHPFRC specimen. Finally, the secant modulus of deflection and fictitious crack opening with respect to fatigue cycles is found to characterize quantitatively fatigue damage evolution.

Modelling of the tensile behavior of SH-UHPFRC at low and high stress levels, under very low loading rates

The tensile behavior under low loading rates governs to a large extend the mechanical response of Strain Hardening Ultra High Performance Fiber Reinforced Concretes (SH-UHPFRC) at early age and long term, in applications of rehabilitation. A viscoelastic-viscohardening model was developed and applied to predict the tensile response of two types of SH-UHPFRC; Mix I with pure type I cement, silica fume and steel fibers and Mix II with 50% replacement of cement with limestone filler and a similar fibrous mix, and compare their time dependent performance. Different tensile loading conditions were investigated, including the behavior under restrained shrinkage deformations, the effect of very low monotonic strain rates, and linear and non-linear relaxation and creep tests. The predictions of the model were also compared with experimental results from literature. The interaction of ageing, hydration, early age volume changes, viscoelastic phenomena and damage and their influence on the overall tensile behavior of UHPFRC was discussed.

Tensile response of UHPFRC under very low strain rates and low temperatures

This paper addresses the uniaxial tensile response of Strain Hardening Ultra High-Performance Fiber Reinforced Concretes (SH-UHPFRC) subjected to very low strain rates and low temperatures. The influence of four different strain rates; 1 × 10−5, 1 × 10−7, 1 × 10−8 and 5 × 10−9 1/s on the tensile properties like elastic limit, elastic modulus, tensile strength and the strain at tensile strength, was studied for two types of SH-UHPFRC mixes; Mix I with type I cement and silica fume, and Mix II with silica fume and 50% mass replacement of type I cement with limestone filler, at three curing temperatures; 20 °C, 10 °C and 5 °C. The tests at strain rates lesser than 1 × 10−6 1/s are the first of their kind for UHPFRC materials and the results show a considerable impact on the elastic limit of the mixes. Acoustic Emission tests were also carried out for validation of test results of the elastic limit.

Influence of Fiber Volume Fraction and Fiber Orientation on the Uniaxial Tensile Behavior of Rebar-Reinforced Ultra-High Performance Concrete

This paper studied the influence of fiber volume fraction ( V f ), fiber orientation, and type of reinforcement bar (rebar) on the uniaxial tensile behavior of rebar-reinforced strain-hardening ultra-high performance concrete (UHPC). It was observed that the tensile strength increased with the increase in V f . When V f was kept constant at 1%, rebar-reinforced UHPC with fibers aligned with the load direction registered the highest strength and that with fibers oriented perpendicular to the load direction recorded the lowest strength. The strength of the composite with random fibers laid in between. Moreover, the strength, as well as the ductility, increased when the normal strength grade 60 rebars embedded in UHPC were replaced with high strength grade 100 rebars with all other conditions remaining unchanged. In addition, this paper discusses the potential of sudden failure of rebar-reinforced strain hardening UHPC and it is suggested that the composite attains a minimum strain of 1% at the peak stress to enable the members to have sufficient ductility.

Tensile response of low clinker UHPFRC subjected to fully restrained shrinkage

This paper addresses the tensile response of Strain Hardening Ultra High Performance Fiber Reinforced Concretes (SH-UHPFRC) subjected to restrained autogenous shrinkage deformations under full and partial restraint conditions, right after casting and until one month. The development of autogenous shrinkage and corresponding eigenstresses under various degrees of restraint were studied for two types of mixes; Mix I with type I cement and silica fume, and Mix II with silica fume and 50% mass replacement of type I cement with limestone filler. The tests under 100% restraint conditions are the first of their kind on SH-UHPFRC and the results show that under these conditions, the material enters into the strain-hardening domain of the tensile response. The development of eigenstresses was much slower in the Mix II when compared to that of Mix I. The development of the dynamic elastic modulus and heat of hydration were also studied and put into perspective.

Properties of strain hardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading

Enhanced matrix packing density and tailored fiber-to-matrix interface bond properties have led to the recent development of ultra-high performance fiber reinforced concrete (UHP-FRC) with improved material tensile performance in terms of strength, ductility and energy absorption capacity. The objective of this research is to experimentally investigate and analyze the uniaxial tensile behavior of the new material. The paper reviews and categorizes a variety of tensile test setups used by other researchers and presents a revised tensile set up tailored to obtain reliable results with minimal preparation effort. The experimental investigation considers three types of steel fibers, each in three different volume fractions. Elastic, strain hardening and softening tensile parameters, such as first cracking stress and strain, elastic and strain hardening modulus, composite strength and energy dissipation capacity, of the UHP-FRCs are characterized, analyzed and linked to the crack pattern observed by microscopic analysis. Models are proposed for representing the tensile stress–strain response of the material.

Strain-hardening Ultra-high Performance Fibre Reinforced Concrete: Deformability versus Strength Optimization

High Performance Cement-based composites such as some UHPFRC or SHCC exhibit a tensile strain hardening response associated to the progressive formation of finely distributed microcracks. Strain Hardening (SH) - UHPFRC with suitable fibrous mixes have a dense matrix, a very low capillary absorption of liquids, significant viscoelastic potential and limited shrinkage. They exhibit a tensile strain hardening response up to 0.3 ‰. Strain hardening materials are candidates for improving existing or new structures to increase their durability and mechanical performance. With this aim in view, the impact of macro and microcracking on the protective function and corrosion mechanisms in cementitious materials has to be critically weighed to choose adequate materials in terms of strength and deformability, to fulfil durability requirements. From this basis, the mechanical response (strength, deformability and associated crack formation) can be tailored according to the applications. Finally, future steps for the development and applications of these advanced contemporary materials can be defined.