Ultra-high-performance Fiber-reinforced Concrete Research Articles

In-situ nonlinear ultrasonic technique for monitoring damage in ultra-high performance fibre reinforced concrete (UHPFRC) during direct tensile test

This paper aims to employ the nonlinear ultrasonic technique for the in-situ monitoring of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) structures subject to tensile loads. The UHFPRC, characterized by specific mechanical properties of high compressive and tensile strength, provides the basis for this investigation. Particularly, the nonlinear parameter β of the nonlinear ultrasonic technique is extracted by formulating a theoretical model that accounts for the interaction of contacting rough surfaces under tensile loads. The experimental findings substantiate that β provides enhanced sensitivity and feasibility for evaluating crack conditions compared to the measurements obtained through conventional Linear Variable Differential Transformers (LVDT). Furthermore, this study explores the potential application of β to estimate tensile load condition by: i) establishing an estimation equation grounded in the relationship between tensile load and displacement (kN-mm); ii) incorporating β into this estimation equation to calculate the anticipated tensile load; and iii) comparing real and estimated tensile loads. The analysis outcomes reveal that estimated tensile loads are in good agreement with the actual loads. Although the estimation equation is based on the findings of this study, it underscores the potential applicability of β for estimating the load conditions in various structures.

Experimental investigation of the shear response of large-scale fibre-reinforced concrete panels

Although fibre-reinforced concrete has been the subject of extensive research for decades, its mechanical behaviour in shear is still not well understood. Furthermore, hardly any large-scale shear tests on elements with moderate fibre contents have been conducted to date. The present paper addresses this knowledge gap by presenting and discussing the results of an experimental campaign on large-scale fibre-reinforced concrete shear panels. Six panels with dimensions of 2.00 m × 2.00 m × 0.27 m were tested in the Large Universal Shell Element Tester at ETH Zurich by applying homogeneous shear. In four of these experiments, the in-plane shear was combined with homogeneous uniaxial compression by simulating the longitudinal restraint provided by the chords in a girder. Five tests focused on concrete of normal strength with moderate fibre contents and one test studied the behaviour of ultra high-performance fibre-reinforced concrete. The combined application of digital image correlation instrumentation and distributed fibre optical sensing allowed gaining a deeper insight into the structural behaviour of the panels. Overall, the test results showed the high efficiency of the fibres as shear reinforcement in large-scale web elements. However, failure of the panels without any steel reinforcing bars was rather brittle, with only few cracks forming over the element. The provision of a moderate amount of distributed longitudinal bar reinforcement has been found to be highly beneficial to the crack behaviour and thus significantly increase the ductility.

Effect of hybrid steel, polypropylene, polyvinyl alcohol, and jute fibers on the properties of ultra‐high performance fiber reinforced concrete exposed to elevated temperature

AbstractEvaluation of the effects of different fiber types with different ratios on the mechanical properties and explosive spalling prevention of Ultra‐High Performance Fiber Reinforced Concrete (UHPFRC) at elevated temperatures is the primary objective of the experimental work. Several fiber types, including steel fiber, polypropylene fiber, polyvinyl alcohol fiber, and natural jute fiber were used. Eight UHPFRC mixes were cast comprising the different hybrid fiber types. The results show that replacing polypropylene, polyvinyl alcohol, and jute fibers with steel fiber decreases the fresh and hardened properties of unheated UHPFRC while increasing durability. The existence of hybrid fibers improves the fire resistance of UHPFRC and fully prevents explosive spalling because of a considerable improvement in permeability. According to the test results, the optimum replacement ratio for UHPFRC mix is to replace polyvinyl alcohol fiber with an equivalent volume fraction of steel fiber, as it has a slight influence on the mechanical properties and prevented explosive spalling.

Experimental evaluation of flexural behavior of UHPFRC slabs utilizing local constituents

New ultra-high performance fiber reinforced concretes (UHPFRCs) are being developed throughout the world using locally-sourced materials to reduce financial and logistical burdens. Understanding force protection capabilities of these new materials requires experimental data that are useful for validating constitutive models or fast-running computational tools. Flexural performance of two UHPFRCs from different regions of the world are compared via one-way slabs subjected to uniform quasi-static loads. Results are also compared to a normal-strength concrete and two high-strength concretes from different regions. Results showed increased unconfined compressive strength of concrete leads to increased flexural resistance and stiffness. Furthermore, fiber reinforcement greatly increases flexural capacity in slabs but can cause brittle failure when combined with high reinforcement ratios of conventional reinforcement. While the UHPFRCs had different mechanical properties, both increased the maximum flexural capacity and showed potential for significantly improved performance for force protection applications.

Effectiveness of flange plates on torsional behaviors of ultra-high-performance fiber-reinforced concrete hollow beams

This study investigates the effectiveness of flanges with various types of stirrup and cross-sectional dimensions on the torsional behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) hollow beams. A finite element model was developed and validated for simulating the torsional behavior. The impact of the essential parameters of flanges was investigated through experimental and numerical methodologies. The results showed that the inclusion of flanges substantially influenced the torsional strength of UHPFRC hollow beams, attributed to the section torsional plastic resistance moment and UHPFRC tensile capacity in flanges. The effectiveness of flange plates was considerably influenced by the web wall thickness. The stirrup type and the increase in flange reinforcement ratio, flange width, and thickness had a favorable effect on the torsional behaviors of thick-walled UHPFRC beams but minimal effect on thin-walled UHPFRC beams. The failure of thin-walled beams was determined by the web thickness and web reinforcements.

A novel damage evaluation method for ultra‐high‐performance fiber reinforced concrete under multiple impact loads

AbstractIn this study, repeated compression tests were conducted for ultra‐high‐performance fiber reinforced concrete (UHPFRC) under three different impact pressures. Results showed that as the impact number increased, the strain rate approximately showed a proportional increase, and the plateau region of the dynamic stress–strain curve became more prominent. Besides, when the repeated impacts increased, the maximum strain and dynamic toughness both exhibited upward trends, while the peak stress decreased. The average toughness was positively correlated with the strain rate. Additionally, under high repeated impact loads, a single crack propagated rapidly within the UHPFRC specimen after single impact. Conversely, cracks propagated from the center to the periphery on the surface of the specimen when repeated loads were low. Furthermore, a quantitative evaluation method for the damage of UHPFRC under repeated impact loads was proposed based on the characteristic parameters of the gray‐level co‐occurrence matrix (GLCM).

Shear behaviour of reinforced concrete beams strengthened with ultra-high performance fiber reinforced concrete (UHPFRC)

Ultra-high performance fiber reinforced concrete (UHPFRC) is an advanced cementitious composite with high compressive strength and low permeability. Due to its excellent mechanical properties and superior durability, UHPFRC is considered promising for strengthening of the existing concrete bridges. In order to examine its strengthening efficiency for shear capacity, an experimental study is carried out on shear-deficient beams without stirrups. Strengthening method comprising precast UHPFRC laminates being glued with epoxy resin on two lateral sides of the reinforced concrete beams, is examined. To investigate the robustness of the system under severe exposure conditions, some beams are subjected to freeze-thaw (FT) cycles. Beams are tested to failure under three-point bending configuration. Test results show that for epoxy resin bonding, UHPFRC shear strengthening is a promising method to increase the load and deformational capacity, and to limit the crack openings. The load capacity is doubled, and the deformational capacity is increased by around 60%. After exposure to 30 FT cycles, the strengthening efficiency and fracture behaviour of UHPFRC composite beams seem not to be affected. It seems that the interfacial bond strength is sufficient to prevent premature debonding between UHPFRC and NC, which under combined action of environmental exposure (e.g. FT) and mechanical loading might become a challenge. Finally, a finite element model is developed to predict and understand the shear behaviour of the reference and strengthened beams. In general numerical results show good agreement with the experimental results in terms of failure pattern and peak load prediction once the perfect bond model is used for the interface between UHPFRC and NC. In order to better understand the role of governing parameters on the shear capacity of the composite member, parametric studies are conducted focusing on the role of varying UHPFRC softening behaviour and UHPFRC-concrete interface properties.

Bond between Ultra High‐Performance Fiber Reinforced Concrete and existing concrete: a review

AbstractThe application of Ultra High‐Performance Fiber Reinforced Concrete (UHPFRC) is increasing worldwide to repair and strengthen existing structures. One of the main applications is in bridge components, e.g., to repair bridge decks and piers, but also to strengthen existing floors in residential or industrial buildings and to realize construction joints in prefabricated members. To properly design a strengthening intervention with UHPFRC, bond between the existing structure and the new layer of reinforcing material should be properly developed. In this work, a review about the interface behavior between UHPFRC and concrete is carried out, analyzing in detail the available test methods, and the variables influencing the bond behavior between the two materials. Among them, the role of moisture exchanges, the material properties of the concrete, the surface roughness, and the effect of bonding agents are discussed in detail. This work shows only proper surface treatments can enhance bond strength, otherwise they could be detrimental. In most previous experiences, UHPFRC bond efficiently with the substrate, resulting in composites that behaves monolithically.

Modeling the ductility characteristics of ultrahigh-performance fiber-reinforced concrete using a precise, computation-efficient, and comprehensive data-driven random forests-based approach

The most relevant features of ultra-high-performance fiber-reinforced concrete (UHPFRC) are its ductility, toughness, and pos-cracking (residual or strain-hardening) parameters under direct tensile stress. Nonetheless, measuring these parameters requires complicated and delicate tests in which minor variations in test performance might render the findings invalid. This research is aimed at creating four different machine-learning algorithmic regressions for estimating the uniaxial tensile performance of UHPFRC. Hence, the first crack stress σcc, the maximum post-cracking strength σpc, and its associated strains, εcc, and εpc, were modeled through the Random Forest algorithm. The developed approaches showed a high goodness-of-fit for regression measured in the test subset, presenting R2 values of 0.894, 0.909, 0.930, and 0.911 in guesstimating the behavior of UHPFRC under direct tensile stress (viz., σcc, σpc, εcc, and εpc). The proposed models are a tool for assessing UHPFRC under ductility requirements and reducing the experimental campaign's time and costs by pre-selecting the available constituents with the proper performance on the algorithmic approaches.

Tensile behavior of UHPFRC made by a fiber orientation device with adjustable vibrational frequency

Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) not only has high compressive and tensile strength, but also exhibits strain-hardening characteristics in tension due to the addition of steel fibers. In this study, a new-style fiber orientation device equipped with a vibration motor of adjustable vibrational frequency was used to prepare UHPFRC. The wall effect and fiber length were considered in the manufacturing process of the device. Uniaxial tensile tests were conducted on fiber-oriented UHPFRC to explore the effects of vibrational frequency and corresponding vibrational force on the tensile behaviors. The test results showed that the elastic ultimate tensile strength, elastic ultimate tensile strain, tensile strength, peak tensile strain, and tensile elastic modulus of UHPFRC specimens cast by fiber orientation process at various vibrational frequencies were different. The fiber orientation parameters were quantitatively evaluated by the image analysis technique to explain the differences in tensile properties of the specimens. The effect of different vibrational frequencies on the tensile strength of UHPFRC matrix and its porosity was also considered. Therefore, this study provides a reference for how to adjust the vibrational frequency of the vibrating platform under the directional channel when pouring relevant components.

Design and production of an experimental UHPFRC bending-active footbridge

This paper describes the design, production and testing of an experimental footbridge made of ultra-high-performance fibre-reinforced concrete (UHPFRC). The prototype has a span length of 5.4 m and is made of a very slender pre-stressed UHPFRC deck (6 cm deep, 1.2 m wide), a 0.55 m high central deviator of the same material, and an external lower tensioning stainless-steel cable system. The service configuration of the structure involves a significant permanent bending of the deck, because of its slenderness; this initial bending is introduced in a controlled manner during the production process and prior to the installation, and results in an efficient, lightweight, and elegant structure with a high stiffness-to-self-weight ratio. The structural concept belongs to the category named active bending, which has been recent subject of research. Specifically, the structure is a bow-string system in which the bending-active member is made of UHPFRC, which constitutes the main novelty of this research. The suitability of UHPFRC as a material for bending active applications has been investigated, and a method to determine the limiting pre-stress of UHPFRC bending-active cross sections has been developed. The activation process and relevant limit states have been checked by means of a 3D frame finite-element model. The paper concludes with the description of the production process and the installation of the footbridge in the final location, as well as the successful results of the load tests and the geometric control of the novel footbridge prototype.

Fabrication and Characterization of Partial Bio-nano-silica Inclusion in Fibre-Reinforced Concrete for High-performance Applications

Ultra-high-performance fibre-reinforced concrete (UHPFRC) is a specialized type of concrete (to create a very dense matrix) that is used for both new construction and renovation projects in order to improve the lifespan of structures. Researchers analyse and evaluate only the microstructure, porosity, and fresh and hardened concrete properties of UHPFRC but limited their exploration on the reduction of the mechanical properties of UHPFRC due to the presence of metallic particles and micro-fractures that occur during the generation of hydrogen. Hence, the present study aims to eliminate the existing problem by hybridization approach (mixing of bio-nano-silica (nS) and polypropylene) with different percentages to further improve the strength properties of UHPFRC. The result showed that the compressive strength is increased by 15.5% compared to traditional concrete due to the filling ratio of nS in the pores of the concrete; in addition, the fibre’s surface and roughness also contributed to the strength enhancement.

Strengthening of Reinforced Concrete Columns Using Ultra-High Performance Fiber-Reinforced Concrete Jacket

The use of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) increased in the field of civil engineering throughout the last few decades. UHPFRC is being used considerably on a large scale in megastructure applications. High compressive and tensile strength permits reconstruction and optimization of the structural members. At the same time, its improved durability properties make it easier to extend the life of the design and can be used as thin layers, cladding, repairs, and column coverings. Although UHPC has an extremely high compressive strength, it exhibits very brittle fracture behavior compared to normal strength concrete (NSC). Since the ductility and fracture toughness of UHPC can be enhanced by adding fibers, the addition of fibers to production adds innovative features to UHPFRC structures and opens up new application areas for UHPFRC. The aim of this study is to investigate the axial behavior of square reinforced concrete (RC) columns strengthened with UHPFRC jackets. Nineteen specimens were cast (1000 mm in height and a cross-sectional area of 150 × 150 mm, whose interface treatment methods were prepared through vertical grooving (VG), horizontal grooving (HG), and without grooving (NG), with the jacket thickness (20 mm and 40 mm) and the number of strengthened sides of the column (two, three, and four sides). The results show a brittle failure for all strengthened specimens. The UHPFRC-reinforced RC columns with vertical grooving (VG) showed a higher ultimate load capacity compared to the columns with horizontal grooving (HG) and the columns without grooving (NG). The horizontal grooving (HG) gives a better result than the jacket without grooving (NG) and increases the cohesion area between the jacket and the column for two and three of the RC columns’ strengthened sides. But, in the case of strengthening the columns on four sides, the effect of confining the jacket to the column appears, and the grooving causes weakness in the body of the original column so that the jacket without grooving (NG) gives a better result than the jacket with horizontal grooving (HG).

Flexural properties of ultra-high performance fiber reinforced concrete based on three-dimensional random distribution of steel fibers

The distribution of fibers and the fiber bridging affect the flexural strength and energy dissipation of ultra-high-performance fiber reinforced concrete (UHFRC) members. In this study, an analytical model to predict the flexural behavior of UHPFRC was developed based on the three-dimensional random distribution of fibers. The probability distribution model of the fiber orientation was developed as a sine function, which agreed to the measured probability data from the randomly distributed fiber specimens. The number of fibers at the fracture surface was determined addressing the effect of fiber orientation. Based on the proposed model, the method to predict the load–deflection curve of UHPFRC beam was suggested addressing the fiber bridging stress at fracture surface and tension softening effect. For verification, the four points flexural tests were conducted for six UHPFRC beam specimens. The test results showed that the peak flexural strength and energy dissipation can be predicted by the proposed model. Further, the proposed model was applicable to predict the strength and deformation capacity of existing test specimens. Based on the proposed model, the aspect ratio of fibers to prevent the brittle failure due to fiber rupture was suggested addressing the fiber tensile strength and bond strength.

Experimental and theoretical analysis of cracking and tension stiffening in UHPFRC under high‐cycle fatigue

AbstractTension‐stiffening controls the serviceability behavior of concrete structures as it is responsible for crack formation and, consequently, the deflection of beams. In fiber reinforced concrete, such as ultra‐high performance fiber reinforced concrete (UHPFRC), fibers bridge cracks and thereby transfer tensile stresses across the cracked region, allowing for tensile stresses to be carried by the concrete within the cracked region. Due to structures being designed for longer design lives, the consideration of long‐term effects such as fatigue is required. Much research has examined tension‐stiffening under fatigue when subjected to low cyclic loading, but very little has considered the effects of high‐cycle fatigue, especially for UHPFRC. This paper presents the results of nine UHPFRC tension‐stiffening tests under high‐cycle fatigue in which the crack formation and development under varying cyclic ranges were studied. Specimens were subjected to as many as 5.7 million cycles, and crack readings were taken during each test. The experimental results demonstrate the random nature of cracking on UHPFRC as well as the increase in the crack width under cyclic loads. Finally, this research described the extension of an existing partial‐interaction mechanics model to allow for the stress in the fibers and the increase in crack width due to high cycle fatigue.

Corrosion risk and corrosion-induced deterioration of ultra-high performance fiber-reinforced concrete containing initial micro-defects

Micro-defects in UHPFRC, inevitably generated from the manufacturing to engineering service stage, impact its durability under extreme service environments. However, relevant understanding is still insufficient. This work assesses the corrosion risk and corrosion-induced deterioration in UHPFRC containing initial micro-defects, simulated by a combination of mechanical pre-loading and thermal treatment. Analytical analyses include electrochemical tests (OCP, Tafel, EIS), SEM, MIP, compressive strength measurements, etc. Results show that initial defect degree and steel fiber contents have significant effects on the corrosion resistance and mechanical performance of UHPFRC. Micro-cracks and pores are the major channels to deepen fiber corrosion risk, degrading mechanical performance up to 52%-56% in the most severely damaged UHPFRC. The porosity is increased by the corrosion/increased defects and fiber contents up to a growth rate of 35%, 56% and 78%, respectively, as corrosion triggers the occurrence of new defects (e.g., fiber splitting, newborn micro-cracks, pores). The present results provide a reference for predicting the corrosion potential of the defective UHPFRC.

Chloride induced mechanical degradation of ultra-high performance fiber-reinforced concrete: Insights from corrosion evolution paths

Chloride-induced corrosion of ultra-high-performance fiber-reinforced concrete (UHPFRC) inevitably affects structural durability. However, the process of multi-fiber corrosion and mechanical deterioration still lacks sufficient understanding. This work aims to reveal the fiber corrosion degradation mechanism from a microscopic to macroscopic view, applying multiple analytical analyses of atomic absorption spectrometry, SEM-EDS, nano-indentation, polarization, and macroscopic mechanical testing. Results show that the flexural strength of specimens decreases significantly with the increase of corrosion degree, and a clear reduction of up to 47% is found at a high corrosion degree. Elastic modulus and nano-hardness of corroded samples vary in a wide range of 30–189 GPa and 0.16–6.41 GPa. With the increase in fiber content, two distinctive corrosion mechanisms are proposed. The corrosion path deteriorates from fiber edge to inner by the invasion of erosive solution through the matrix at low contents (<2 vol%). Considering impurities, greater interfacial defects and macro-cell potential differences at high contents (≥2 vol%), another corrosion path originates from the fiber inner outward to the matrix. Fiber corrosion damages the fiber’s structural integrity and induces matrix deterioration, the micromechanics of the matrix along the fiber edge 20 μm decreases at least 10% more than the concrete matrix. This work firstly sheds light on the mechanical deterioration of UHPFRC from the perspective of fiber corrosion paths considering different initiation scenarios.

Effect of retrofit type on the blast performance and failure mode of HSC beams retrofitted with UHPFRC

This paper presents a strategy to prevent shear failure and improve the structural performance of existing high-strength concrete (HSC) beams under blast loading. The strategy involves retrofitting with ultra-high performance fiber reinforced concrete (UHPFRC), a novel material which is well-adapted for the blast hardening of concrete structures. Shock-tube air-blast tests are conducted on seven HSC beams which are retrofitted with UHPFRC. The objective was to characterize the effectiveness of various retrofit types on the blast behaviour of both shear-deficient and flexural-dominant HSC members. Three retrofit types were considered: U-jacketing (UJ), full-jacketing (FJ) and a tension-sided (T) overlay. Other parameters included the effects of the concrete substrate roughening method, as well as the tension steel ratio (1.6% or 2.4%) and presence of stirrups in the beams. Both the UJ and FJ retrofits were found to be effective in preventing shear failure in shear-deficient HSC beams having varying shear demands. In addition, both retrofits increased member strength and stiffness, thereby reducing blast-induced displacements and increasing overall blast resistance. Comparing the two retrofits, optimal performance was obtained with full-jacketing due its ability to delay concrete crushing. In HSC beams with sufficient stirrups, both the T-sided and FJ retrofits reduced blast-induced displacements; however, the use of full-jacketing was better able to control damage and increase blast capacity. The roughening method had no substantial effect on the FJ retrofit performance or failure mode, with no debonding observed in the tests.

Fire Performance of Fiber-reinforced Ultra-High-Performance Concrete: A state-of-the-art review

Explosive spalling is a major problem for structures made of ultra-high-performance concrete (UHPC) after being exposed to fire, which hinders their practical applications. To overcome this obstacle, fibers have become an essential constituent of UHPC in past decades. This paper summarizes a state-of-the-art review related to the fire performance of UHPC using single and hybrid fibers. The spalling behavior, spalling mechanism, mechanical properties, and microstructure characteristics of fiber-reinforced UHPC are summarized, and structural performance of fiber-reinforced UHPC members at elevated temperatures is discussed. This review reveals that fire-induced spalling is a significant concern in UHPC, and it can be caused by various mechanisms. The addition of fibers can play a crucial role in preventing spalling, and different mechanisms for different types of fibers have been reported in the literature for UHPC with both single and hybrid fibers. Therefore, fiber hybridization is recommended to enhance the spalling resistance of UHPC. The future directions and challenges for UHPC at both the material and structural levels are discussed.

Performance of corroded RC beams strengthened in flexure using UHPC: Effect of configuration and thickness of the UHPC layers

In the present paper, an experimental investigation was carried out to examine the effect of configurations and thicknesses of ultra high performance fiber reinforced concrete (UHPC) layers on the flexural performance of reinforced concrete (RC) beams strengthened using UHPC layers after undergoing different degrees of accelerated reinforcement corrosion. The beams were tested under flexure to evaluate the effect of UHPC strengthening (with combinations of three UHPC layer thicknesses and two configurations) on failure modes, flexural strengths, and stiffnesses. Statistical analysis of the flexural test results using analysis of variance (ANOVA) showed the significant effects of both configuration and thickness of the UHPC layers on the performance of corrosion-damaged RC beams in flexure, irrespective of the degree of reinforcement corrosion. The RC beams strengthened with UHPC layers applied on three sides exhibited the most significant increase in the ultimate flexural strength in the range of 6 to 63% and an increase in the stiffness in the range of 28 to 112% as compared to that of the uncorroded control beam. Furthermore, analytical models were developed to predict the flexural strength of the corroded beams strengthened using different configurations of UHPC layers. The close agreement of the ultimate flexural strength between the experimentally obtained results and analytically predicted values for the strengthened RC beams indicates the accuracy of the developed analytical models, which can be used to select an optimum strategy for strengthening the corrosion-damaged RC beams.