Ultra High Toughness Cementitious Composites Research Articles

Quasi-static and dynamic compressive behavior of ultra-high toughness cementitious composites in dry and wet conditions

This paper investigated the quasi-static and dynamic compressive behavior of ultra high toughness cementitious composites (UHTCC) in dry and wet conditions. The presence of water decreased the quasi-static compressive strength and peak strain of UHTCC, whereas that led to the increase in the elasticity modulus and Poisson's ratio. Under dynamic loading, the reduction of compressive strength and the strain rate sensitivity in terms of strength in UHTCC were observed after water immersion, which was different to those in normal concrete. The underlying mechanism for such phenomenon may attribute to the bridging effect of polyvinyl alcohol (PVA) fibers in the matrix. In order to verify this assumption, the specimens reinforced with different volume fractions (0%, 0.75%, 1.5%, 2.0%) of PVA fiber were tested in this study. Test results showed that, in the case of the specimens without fiber, wet specimens exhibited higher dynamic compressive strength compared with dry specimens, which was consistent with the previous results for normal concrete. Moreover, the influence effect of free water on the strain rate sensitivity in terms of strength of composites was relative to the fiber content.

Coupled effects of high temperature and strain rate on compressive properties of hybrid fiber UHTCC

This study investigates the combined effect of the strain rate and temperature on the compressive properties of hybrid fiber ultra high toughness cementitious composites (UHTCCs) using a Split Hopkinson pressure bar. Specimens were first heated to different exposure temperatures, e.g. ambient temperature, 200, 400, 500, 600 and 800 °C, and subsequently, cooled to ambient temperature. Thereafter, the specimens were tested at four different strain rates. The test results show that the dynamic compressive strength of the UHTCC is enhanced at a temperature of 200 °C, and subsequently, decreases with the increase in exposure temperature. The strain rate sensitivity of UHTCC is largely enhanced with the increase in exposure temperature. The possible mechanism of this phenomenon was discussed based on the high-speed photography of the crack propagation process on the surface of the specimens and microscopic observation of fibers condition on their fracture surfaces. Moreover, an empirical relationship is established to express the dynamic strength enhancement of fire-damaged UHTCC as a function of strain rate.

Thermal and mechanical properties of ultrahigh toughness cementitious composite with hybrid PVA and steel fibers at elevated temperatures

For fire resistance calculations, the thermal and mechanical properties of an ultrahigh toughness cementitious composite (UHTCC) with hybrid polyvinyl alcohol (PVA) and steel fibers at elevated temperatures were investigated. The thermal conductivity and flexural strength were tested at temperatures from 25 to 900 °C. The density, mass loss, and residual compressive strength of the UHTCC were tested after exposure to elevated temperatures with the failure mode scanned by X-ray computed tomography and the microstructure was observed using field-emission scanning electron microscopy. A finite element (FE) model was established to predict the temperature distribution of a steel-reinforced UHTCC beam using the measured thermal property data. Experimental results show that the thermal conductivity of the UHTCC, which is approximately 0.5 W/(m ⋅K), remains stable from 25 to 900 °C. The flexural strength of the UHTCC decreases with increasing temperature, whereas the residual compressive strength increases by approximately 8% at 210 °C compared to its original strength at ambient temperature. Explosive spalling of the cement-based material is effectively prevented owing to the presence of the PVA fibers. This work can provide basic property data for UHTCC structural calculations in the event of a fire.

Flexural behaviour of corroded reinforced concrete beams repaired with ultra-high toughness cementitious composite

The repair of corroded reinforced concrete (RC) structures is necessary to improve their serviceability. Ultra-high toughness cementitious composite (UHTCC), characterised by excellent tensile behaviour, crack control capacity, and durability, shows great potential for the repair and strengthening of corroded members, especially in harsh marine environments. This paper presents an experimental study and a theoretical analysis of the flexural performance of corroded RC beams repaired with UHTCC. The experimental results indicated that macro cracks in concrete were transformed into multiple fine cracks and a slight nonlinear strain distribution was observed along the sectional depth. The replacement of damaged concrete with UHTCC can recover the initial load-carrying capacity at corrosion ratios within 11.2%. After yielding of beams, the development of UHTCC-concrete interface crack in the pure bending region was beneficial to ductility improvement. A theoretical model considering the effect of pitting corrosion was proposed for calculating flexural capacity of repaired beams.

Effect of Post-Fire Curing on the Compressive Properties of Fire-Damaged Ultra-High Toughness Cementitious Composites

Abstract After exposure to fire or high temperatures, the post-fire curing methods have a significant effect on the mechanical properties of concrete material. In present research, the influence of post-fire curing methods on the mechanical properties of fire-damaged ultra-high toughness cementitious composites (UHTCC) was investigated. A heating temperature up to 800°C, lasting for 2 hours, was adopted to simulate the fire damage, after which the heated specimens were subjected to three types of cooling regimes (i.e., cooling in air, quenching in water for 5 minutes, and quenching in water for 30 minutes). The quenched specimens were then cured in air for 1 hour, 1 day, 7 days, and 28 days. It was found that quenching in water helped the recovery of the compressive properties of UHTCC, and the extent of recovery increased with the quenching time as well as the post-exposure curing age. The microstructural characterization of UHTCC was also examined before and after exposure using scanning electron microscopy. The pore size distribution was obtained by the mercury intrusion porosimetry analysis. Results from the microscopic observations well explained the mechanical properties of post-exposure UHTCC specimens.

Experimental study on the interfacial bonding behaviors between sprayed UHTCC and concrete substrate

Ultra high toughness cementitious composites suitable for spraying (sprayed UHTCCs) is characterized by strain-hardening property under direct tensile stress with tight crack-control ability. It has a great potential in the application of repairing and retrofitting cracked or corroded concrete structures. The utilization efficiency of sprayed UHTCC is significantly affected by the integrity of the repair or strengthening system, which depends on the interfacial bonding behavior between the sprayed UHTCC and concrete substrate. A Type L934 orthogonal experiment and scanning electron microscope (SEM) observation were performed to investigate the bond behavior between sprayed UHTCC and concrete substrate, where three factors, namely sprayed direction, surface roughness of concrete substrate, and application of primer, were taken into consideration with three test levels for each of the former two factors and two levels for the other factor provided. The results demonstrated that the spraying direction had a significant influence on the bond splitting tensile strength and a quite significant influence on the interfacial shear strength between sprayed UHTCC and concrete substrate, and the surface roughness of concrete substrate had significant influence on the interfacial shear strength but not much significant influence on the bond splitting tensile strength. It had little influence on the bond behavior between sprayed UHTCC and concrete substrate whether a thin layer of primer, which contained the same matrix of sprayed UHTCC without polyvinyl alcohol (PVA) fibers, was adopted or not.

Bond-slip behaviour of corroded reinforcement and ultra-high toughness cementitious composite in flexural members

Bonding between ultra-high toughness cementitious composite (UHTCC) and corroded reinforcement has a great effect on the mechanical behaviour of reinforced UHTCC members subjected to chloride attack and of corroded structural members repaired with UHTCC. This paper presents an experimental study on bonding between UHTCC and post- and pre-corroded rebar using bending tests. The test parameters included corrosion ratio and bond length. Failure and cracking pattern, bond-slip curves, bond strength, and bond toughness were compared. The bond strength decreased with increasing bond length, and increased initially and then decreased with increasing corrosion ratio. The critical corrosion ratio, at which the bond strength became lower than the initial strength, was approximately 12–13.5% for post-corrosion specimens and 13.5–18% for pre-corrosion specimens. The residual bond strength of pre-corrosion specimens was superior to that of post-corrosion specimens. Rebar corrosion obtained at these corrosion levels had hardly any negative effect on the bond toughness index.

Multiple Impact Resistance of Hybrid Fiber Ultrahigh Toughness Cementitious Composites with Different Degrees of Initial Damage

AbstractIn this research, the multiple impact resistance of hybrid fiber ultrahigh toughness cementitious composites (UHTCC) was studied. By varying the impact velocity, different degrees of initia...

Enhancing the initial cracking fracture toughness of steel-polyvinyl alcohol hybrid fibers ultra high toughness cementitious composites by incorporating multi-walled carbon nanotubes

In this study, the use of dispersed multi-walled carbon nanotubes (MWCNTs) to enhance the mechanical properties, especially the initial cracking fracture toughness, of the steel-polyvinyl alcohol hybrid fibers ultra high toughness cementitious composite (UHTCC) was reported. The influences of the MWCNTs contents (0.00%, 0.05%, 0.10%, 0.20% and 0.40% by the cement weight) were investigated by the fluidity test, compression test and uniaxial tensile test. An optimal concentration of 0.10 wt% was obtained, at which the compressive strength (to 60.8 MPa), tensile strength (to 6.8 MPa), elastic modulus (to 15.2 GPa), ultimate tensile strain (to 0.960%) were enhanced by about 18%, 42%, 12% and 16%, respectively. The pre-notched three-point bending beam tests were then conducted to study the improvements of the fracture properties at the optimal MWCNTs content, based on the double-K fracture model and the standard toughness index method. The initial cracking fracture toughness KIcini (to 1.8 MPa·m1/2) was improved by about 21%. The defined effective fracture toughness KIcun− (to 8.1 MPa·m1/2) and KIcun+ (to 20.5 MPa·m1/2) were improved by about 14% and 58%, respectively. The residual tensile strengths fR,1-CMOD (to 23.2 MPa), fR,2−CMOD (to 26.1 MPa), fR,3−CMOD (to 24.6 MPa) and fR,4−CMOD (to 20.7 MPa) were increased by about 24%, 27%, 47% and 59%, respectively. The toughening effect at the peak load state may imply that the bonding strengths between the hybrid fibers and the cement matrix were enhanced by MWCNTs addition. The scanning electron micrograph was used to observe the microstructures of the MWCNTs-enhanced hybrid fibers UHTCCs. Observations showed that the MWCNTs can act as nucleation spots to accelerate the hydration reaction, be embedded in the hydrates to form a more compact 3D cross network, and arrest the nano-cracks due to their bridging and load-transfer abilities.

Fatigue deformation behavior and fiber failure mechanism of ultra-high toughness cementitious composites in compression

Ultra-high toughness cementitious composites (UHTCCs) belong to a family of fiber-reinforced cementitious composites with strain-hardening behavior under tension. Such composites have potential application in structures sustaining fatigue loads. In this paper, the compressive fatigue deformation behavior of UHTCC under various stress levels S (S = 0.90, 0.85, 0.80, 0.75, 0.70 and 0.65) was investigated. It is found that the stress level has little influence on the cyclic creep curve of UHTCC, while the fatigue failure strain increases as the stress level decreases. The failure strains at different stress levels obey the two-parameter Weibull distribution, and a probabilistic model is proposed to consider the effect of stress level on the fatigue failure strain. Using X-ray computed tomography and a scanning electron microscope, three fatigue-induced failure modes of polyvinyl alcohol fibers in UHTCC are found and the possibility for further improvement of the compressive fatigue performance is discussed.

Theoretical Analysis of Reinforced Beams of UHTCC

UHTCC (Ultra High Toughness Cementitious Composite) is a new type of material which is widely used in this study. It is a kind of cement-based material with very good toughness. It is effective to improve the performance of damaged reinforced concrete and improve its durability. UHTCC is widely used in the reinforcement of concrete structures. There is no clear and effective method for calculating it. There is an approximate formula for the crack width of ordinary concrete. The concept of an average crack spacing is used in the derivation of the formula. The limit of UHTCC for cracks can be measured by the concept of average crack spacing. According to the obtained crack width limit, the reinforcement effect of UHTCC can be shown.

Shear bond assessment of UHTCC repair using push-out test

This investigation is aimed at assessing the shear bond strength between existing concrete and an ultra-high toughness cementitious composite (UHTCC). The main issues relevant to a push-out test are the type of interface treatment agent, the strength of the existing concrete, the substrate surface treatment, and the surface treatment of the precast UHTCC. An analysis of variance (ANOVA) is presented to evaluate the significant factors. In addition, mathematical models are built to estimate the shear bond strength. The results indicate that the primers and bonding agents can improve the interface bond, and the highest shear bond strength is achieved using a polymer-modified material. The strength of the existing concrete has a certain influence on the bond. A substrate surface treatment is also a vital factor, and has a strong effect on the shear bond strength. However, excessive mechanical action leads to interface damage. There are two bond planes for precast repair specimens, and it is imperative to maintain a good quality and consistent speed failure for both. ANOVA is an acceptable and effective way to determine the significance. The proposed models have high accuracy and satisfactory prediction capability.

Mechanical behavior of ultra-high toughness cementitious composite strengthened with Fiber Reinforced Polymer grid

A new strengthening composite system, namely Basalt Fiber Reinforced Polymer (BFRP) grid – Ultra-High Toughness Cementitious Composite (UHTCC) for Reinforced Concrete (RC) structures is explored in this paper. Thirty UHTCC specimens internally strengthened with BFRP grid and six similar reference specimens without strengthening were tested to investigate the tensile mechanical behavior. The reinforcement ratio of the BFRP grid (0.17%, 0.68%, and 1.16%) and the mix proportion of the UHTCC were the two main test parameters. The experimental results highlighted two failure modes: 1) rupture or slip off failure of chopped PolyVinyl Alcohol (PVA) fibers at the critical crack sections in the reference specimens, and 2) partial rupture failure of BFRP grid within the UHTCC in all strengthened specimens. Moreover, the relative slip at the interface between the BFRP grid and the UHTCC substrate was not observed during testing. The tensile force capacity of the strengthened BFRP–UHTCC specimens increased by 42% to 172% compared to the reference specimens depending on the reinforcement ratio of the BFRP grid. On the other hand, the tensile force capacity of BFRP–UHTCC specimens slightly decreased by 1% to 14% with the increase of the water-to-cement material ratio of the UHTCC layer from 24% to 38%. Additionally, a stress–strain relationship and strength models of the strengthened specimens are proposed and verified with the test results to predict the tensile mechanical behavior.

Waterproof ultra-high toughness cementitious composites containing nano reservoir silts

The waterproof ability of cementitious composites is essentially important for the uses in the aspects of construction, decoration and repair. Here we reported a waterproof ultra-high toughness cementitious composite (WUHTCC) that has the function of seepage resistance. The waterproof effect was improved by incorporating nano reservoir silts (NRSs) into the binders of WUHTCC. The mechanical properties, microstructures and water resistances of the WUHTCCs with different loadings of NRSs were tested to assess the affecting factors. The results show that increasing the loadings of NRSs maintains and/or enhances the toughness of WUHTCCs, but decreases the compressive strength. The water absorption is decreased, and the penetration pressure and time are increased significantly with increasing the NRS content. While the incorporation of NRS particles into cementitious matrix leads to a slight increase of porosity, it shows no significant changes of micro morphology. The variations of the mechanical and waterproof properties of WUHTCCs are induced by the hydrophobic and sheet-like NRS particles that alter the surface properties and microstructures of the material substrate. The WUHTCCs with good material properties developed in this study for seepage control facilitate their applications in the fields of construction, decoration and repair.

Development of reinforced ultra-high toughness cementitious composite permanent formwork: Experimental study and Digital Image Correlation analysis

This paper presents a development of reinforced ultra-high toughness cementitious composite (UHTCC) participating permanent formwork, which acts as a self-supporting formwork during construction and contributes to the load capacity and durability of the concrete member through the service life of the structure. The flexural behavior of the specimens made with the formwork is analyzed with the help of Digital Image Correlation (DIC) technology. Compared to reinforced concrete beam, the specimens made with UHTCC formwork own a larger deflection, when their maximum crack width reached 0.45mm. The cracks caused by the delamination of the formwork have little effect on the longitudinal bars, but they would reduce the load capacity in some circumstance. Based on the failure process of specimens, several design details, including the load capacity of various sections and the surface treatment methods, were given to improve the performance of the formwork.

Frequency Effect on the Compressive Fatigue Behavior of Ultrahigh Toughness Cementitious Composites: Experimental Study and Probabilistic Analysis

AbstractUltrahigh toughness cementitious composite is a cementitious material that exhibits the characteristic of pseudo strain hardening under uniaxial tension, and has potential use in structures...

Effect of corrosion on bond behaviors of rebar embedded in ultra-high toughness cementitious composite

The bond between corroded rebar and ultra-high toughness cementitious composite (UHTCC) is a key factor in the mechanical properties of corroded reinforced concrete members repaired using UHTCC. This paper presents an experimental study on bond behaviors of UHTCC and corroded reinforcement obtained in a concrete environment through central pull-out tests. The experimental results revealed that all the UHTCC specimens failed in pull-out and that almost all showed an apparent average slip plateau. The post-peak bond behavior was improved with an increase in corrosion ratio. The change in bond strength with corrosion ratio is related to bond length. Bond toughness first rose and then decreased with corrosion ratio, but still remained close to that of non-corroded samples at about 15% of corrosion ratio. An empirical model was proposed to predict the bond strength between corroded rebar and UHTCC, and a good agreement was obtained.

Influence of steel fiber on dynamic compressive behavior of hybrid fiber ultra high toughness cementitious composites at different strain rates

This paper describes an investigation of the influence of steel fiber (abbreviated as SF) on the dynamic compressive behavior of hybrid fiber ultra high toughness cementitious composites (UHTCC) at different strain rates. The volume content of SF was varied from 0.0% to 1.5%, whereas the volume content of polyvinyl alcohol (abbreviated as PVA) fiber remained constant at 2.0%. The cylindrical specimens were prepared and tested at strain rates ranging from 113.8 to 192.1s−1 by using an 80mm diameter split Hopkinson pressure bar (SHPB). The dynamic behavior of hybrid fiber UHTCC was evaluated in terms of failure patterns, dynamic compressive strength, dynamic increase factor (DIF), and energy absorption ability. Moreover, the visco-elastic model was employed to describe the viscoelastic behavior of hybrid fiber UHTCC. The experimental results show that the addition of SF improves the dynamic behavior of UHTCC significantly. In contrast to the previous studies, the rate sensitivity of dynamic strength in hybrid fiber UHTCC depends upon the fiber volume fraction. It was found that specimens with higher SF volume fractions exhibit lower rate sensitivity in dynamic strength than specimens with lower fiber volume fractions. In contrast, decreased strength sensitivity to SF content was observed with an increase of strain rate.

Rate dependence of ultra high toughness cementitious composite under direct tension

Ultra high toughness cementitious composite (UHTCC) usually shows strain hardening and multiple cracking under static tension loads. In practice, structures could be exposed to high strain rates during an earthquake. Whether UHTCC can maintain its unique properties and provide high structural performance under seismic loading rates largely determines whether it can successfully fulfil its intended function. To determine the rate dependence of UHTCC, uniaxial tensile tests with strain rates ranging from 4×10−6 s−1 to 1×10−1 s−1 were conducted with thin plates. The experimental results showed that UHTCC had significant strain hardening and excellent multiple cracking properties under all the rates tested. The ultimate tensile strain lay in the range of 3.7% to 4.1% and was almost immune to the change in strain rates. The rate of 1×10−3 s−1 seemed to be a threshold for dynamic increase effects of the first crack tensile strength, elastic modulus, ultimate tensile strength, and energy absorption capability. When the strain rate was higher than the threshold, the dynamic increase effects became more pronounced. The energy absorption capability was much higher than that of concrete, and the average ultimate crack widths were controlled below 0.1 mm under all rates. Several fitting formulas were obtained based on the experimental results.

Development of assembled permanent formwork using ultra high toughness cementitious composites

The present investigation shows new development on assembled permanent formwork using ultra high toughness cementitious composites with low permeability and high crack resistance. A bidirectional keel formwork is designed to act as surface cover to prevent corrosion of steel reinforcements for structures under aggressive environment. Main attention is focused on mechanical aspects of a reinforced concrete specimen made with permanent formwork in this article. The flexural behavior of specimen is measured with four-point bending test and experimental results are then presented. A method is proposed to predict the load capacity and displacement of specimen and theoretical prediction shows a coincidence with test results. Furthermore, a series of design details and suggestions are discussed. The results of this investigation show that specimen made by ultra high toughness cementitious composite assembled permanent formwork presents higher load-carrying capacity and more ductile behavior compared with reinforced concrete specimen.