Residual Flexural Strength Research Articles

High-Temperature Fatigue Degradation Behaviors of a 3D Braided C/SiC with a Thin Interlayer in Different Dry Oxygen Atmospheres

In order to evaluate the increase in the flexural strength of a 3D braided C/SiC composite comprised with a thin pyrolytic carbon (PyC) interlayer (TI C/SiC) under a load of 60 MPa with an amplitude of ±20 MPa at an oxygen partial pressure of 8000 Pa, the effect of temperature, oxidation and stress value on the length change in the sample, fracture behavior, residual flexural strength and fracture morphology were studied up to 1500 °C. It was found that the gauge length change behaviors of the material are related to (i) the positive damage of the thin interlayer and (ii) to the negative damage of the C phase. The most serious damage of TI C/SiC under 60 ± 20 MPa occurs in an oxygen partial pressure of 17,000 Pa at 1300 °C. When the oxygen partial pressure and/or the temperature are reduced, the positive C phase damage is relieved. In the case that the oxygen partial pressure, temperature and stress increase, the negative C phase damage is facilitated. The oxidation mechanism of the C phase is controlled by the inward diffusion of oxygen from the sample surface to the center; however, a higher stress is considered to change the oxygen diffusion mechanism by increasing the reaction of the C phase, with oxygen causing a widening of microcracks.

Comparative analysis of flexural strength prediction in SFRC using frequentist, Bayesian, and Machine Learning approaches

Steel fiber reinforcement significantly enhances the flexural strength of concrete, which is vital for structural integrity. Annex L of the new Eurocode 2 classifies steel fiber-reinforced concrete by its flexural performance, aiding engineers in designing resilient structures. This study investigates the flexural behavior of steel fiber-reinforced concrete (SFRC) using three data-driven methodologies: Frequentist Inference (FI), Bayesian Inference (BI), and Machine Learning (ML). A comprehensive database was constructed from three-point bending tests on SFRC specimens, encompassing various compressive strengths, fiber quantities, and geometric parameters, to identify key factors influencing material properties.The findings indicate that all three methodologies yield comparable predictive capabilities for flexural responses in SFRC. Notably, FI models emphasize the importance of compressive strength and fiber volume fraction, along with fiber properties such as non-dimensional length and tensile strength. BI models enhance predictive stability by integrating prior knowledge and quantifying uncertainty, demonstrating their advantage, particularly in data-scarce situations. Additionally, ML analysis reveals that linear regression (LR) models can achieve accuracy similar to or greater than that of more complex models. This research provides novel insights into the application of BI and ML in concrete technology, emphasizing their potential to enhance predictive modeling. Additionally, it offers practical guidelines for optimizing SFRC design through a case study that compares residual flexural strengths obtained via Bayesian analysis, classifying the material in accordance with Annex L of the new Eurocode 2.

Effect of recycled aggregates with different parent strength and fly ash on flexural strength of recycled aggregate concrete prisms

Fast consumption of conventional ingredients of concrete and waste generated due to demolishing old buildings is a multi-fold problem in the concrete industry. Hence the sustainable alternative of it is the need of the day. In this research work effect of binary blending of demolished waste and fly ash on the flexural strength of plain concrete prism is presented. The properties of aggregates and cement are evaluated. The physical and chemical properties of fly ash were determined. Recycled aggregates from demolished waste with different mix ratios (1:2:4 and 1:1.5:3) were used. Optimization of recycled aggregates; by evaluating concrete cylinders in 12 batches; showed 35% as optimum. At the optimum dosage of recycled aggregates fly ash was used from 2.5% to 15% with an increment of 2.5% to optimize its dosage. Compressive strength test of the standard-size cylinders and their comparison with control concrete showed a 10% dosage of a fly as the optimum percentage to replace the cement in the concrete matrix. Using both optimized dosages of recycled aggregates and fly ash prism specimens of900mmx150mmx300mm were cast in four batches. The beam specimens were cured for 28 days followed by an evaluation of flexural strength and deflection under centric load in the universal testing machine. Test results of flexural strength (only 15% loss) showed good potential for both waste materials in the concrete matrix. Recycled aggregate with higher parent strength showed better performance than its other counterparts. With higher-strength recycled aggregates residual flexural strength was recorded as equal to 93%. For all specimens recorded deflection was within the allowable limits of ACI-318.

Challenges of a Circular Economy: The Example of Raw Recycled Tyre Steel Fibres Added to Concrete.

This research was conducted to analyse the possibility of using raw, untreated recycled tyre fibres as an effective concrete reinforcement according to circular economy principles. The aim of the article was also to develop a method for dispensing tire fibres on a real scale. Additional treatment and homogenisation of recycled steel fibres entail higher energy consumption, emissions of greenhouse gases, and increased costs. However, obtaining durable and safe concrete effectively reinforced with steel fibres is critical. Finding a balance between environmental friendliness and product durability is a circular economic challenge. Reference concrete with commercial steel fibres (15 kg/m3) and two concretes containing various quantities of non-treated, raw tyre recycled fibres (25 kg/m3 and 45 kg/m3) were industrially produced. Tests were carried out on the properties of the concrete mixture and hardened concrete, such as compressive strength, flexural strength, splitting strength, modulus of elasticity, residual flexural tensile strength, and fibre distribution in concrete. Tests revealed that increasing the amount of raw tyre fibres disturbs the structure and causes air entrainment and the formation of fibre clusters. Smaller quantities of raw tyre fibres turn out an effective concrete reinforcement. The use of non-treated tyre fibres as concrete reinforcement is possible but requires more stringent control of the concrete parameters. Implementation tests on an industrial scale are a novelty in this study, presenting an analysis of the possible dispensing of tyre fibres in a ready-mixed concrete production plant and testing the characteristics of manufactured concrete.

Assessment of ply thickness and aluminum foils interleaving on the impact response of CFRP composites designed for cryogenic pressure vessels

Carbon fiber reinforced polymer (CFRP) proved to be the best choice to produce cryogenic pressure vessels coupling a reduced weight with superior mechanical performance. Despite this, CFRPs are prone to fuel leakage due to interconnected matrix cracks resulting from mechanical and thermal stresses. Two strategies were combined in this work to reduce fuel permeability, i.e., increase of plies number and metallic film interleaving, evaluating the cryogenic impact response of these new configurations. At first, the only ply thickness effect was assessed and the hybrid sandwich-like (SL) configuration with traditional CFRP (RC) skins and a thin-ply (TP) core displayed impact and residual flexural properties close to RC ones. Then, the effect of aluminum foils interleaving was investigated, and the interleaved SL configuration displayed higher residual flexural properties than interleaved RC for all temperatures, e.g., the residual flexural strength was 30.3 % higher at room temperature and 14.6 % higher at −70 °C, respectively.

Experimental investigation on the residual strength of concrete under the multi-factor coupling action of chloride penetration, freeze-thaw, and fatigue effect

With the ongoing process of globalization and the rapid expansion of the global economy, the proliferation of coastal constructions and infrastructures has witnessed a notable surge. These structures often encounter a confluence of environmental stressors in practical engineering, including cyclic fatigue loading, chloride salt erosion, and freeze-thaw cycles. These synergistic factors accelerate concrete’s failure, resulting in accelerated deterioration and significantly reduced durability. A comprehensive understanding of the damage characteristics of concrete materials in offshore environments, under the multi-factor coupled action of freeze-thaw phenomena, along with an assessment of their residual strength, is crucial for the development of relevant structural analysis and lifetime prediction models. To address this, twenty-seven dedicated specimens were designed and fabricated for this investigation. They were employed to scrutinize the residual strength of concrete under the combined influences of fatigue, freeze-thaw, and chloride penetration. Nine distinct multi-factor coupling actions were meticulously devised and characterized by different parameters. Following the multi-factor coupling action, tests were conducted to assess the residual flexural and compressive strengths, as well as the chloride ion content near the specimen edges. The findings revealed a substantial negative correlation between the residual flexural and compressive strengths with both the number of freeze-thaw cycles and fatigue loading iterations. In contrast, chloride ion concentration and the stress level of fatigue loading (stress ratio: 0.1–0.3) exhibited a comparatively marginal influence. Despite the relatively short test duration, it was evident that both freeze-thaw cycles and fatigue loading exerted a discernible promotional effect on chloride ion ingress in the concrete of the specimens.

Analysis of synergistic effect of bending energy absorption capacity of fiber reinforced concrete based on fiber distribution characteristic

Macro fibers could contribute to absorb energy through fiber sliding, which significantly enhance the energy absorption capacity of concrete beams. In this research, the synergistic effect of macro steel fibers and polypropylene (PP) fibers on the energy absorption capacity of concrete under bending is under investigated. The three-point bending test is adopted to compared the residual flexural strength and energy absorption capacity of fiber reinforced concrete (FRC) beams. Besides traditional Load-deflection curves, the energy absorption-crack mouth opening displacement (CMOD) curves are adopted to reflect the toughness of FRC beams. The synergistic effect of steel fibers and PP fibers on the residual flexural strength and energy absorption capacity is evaluated. A linear function is proposed to describe the relationship between energy absorption and CMOD for FRC beams. In addition, fiber distribution on cracking surface of FRC beams is analyzed through fiber distribution density and fiber spacing to explain the synergistic effect. Experimental results indicate that the behaviors of FRC beams, that is, residual flexural strength and energy absorption, are improved by adding macro fibers, in particular macro steel fibers. Besides, the relationship between energy absorption and CMOD of FRC beams can be fitted very well by the linear function. The slope of the energy absorption and CMOD relationship is adopted to analyze the contribution of macro fibers to energy absorption. The uniformity of fiber distribution is improved by increasing of fiber dosage for SFRC and PFRC beams. The hybrid use of macro steel fibers and PP fibers demonstrates a positive synergistic effect on the energy absorption capacity and flexural strength of concrete. The improved fiber spacing and fiber distribution density of hybrid fiber on cracking surface contribute to the positive synergistic effect observed in hybrid fiber reinforced concrete (HyFRC) beams.

Porous concrete modification with silica fume and ground granulated blast furnace slag: Hydraulic and mechanical properties before and after freeze-thaw exposure

This study investigates the effects of incorporating silica fume (SF) and ground granulated blast furnace slag (GBFS), both individually and in hybrid combinations, on various properties of porous concrete (PC) were evaluated under room conditions and after freeze-thaw (F-T) cycles. The hardened density (HD), water permeability coefficient (k), apparent porosity (AP), as well as compressive strength (CS), splitting tensile strength (STS), and flexural strength (FS) at 7, 28, and 120 days. Additionally, the impact of SF and GBFS on the mass loss (ML), residual compressive strength (RCS), residual splitting tensile strength (RSTS), and residual flexural strength (RFS) of PC samples subjected to 30, 60, 120, and 180 freeze-thaw (F-T) cycles was analyzed. The findings revealed that the addition of SF and GBFS decreased the AP and k values of PC samples under room conditions. Furthermore, these materials significantly improved the CS, STS, and FS properties of PC samples, especially at the 28-day and 120-day marks, owing to their enhanced pozzolanic activity over time. SF and GBFS also contributed to higher RCS, RSTS, and RFS values in PC samples after F-T cycles compared to the control samples. This was due to their effective gap-filling properties and the formation of additional calcium-silicate-hydrate (C-S-H) gels within the matrix. Additionally, SF and GBFS led to slightly higher RDME values in the C1-C6 samples than in the control samples following F-T cycles. However, SF and GBFS did not have a significant impact on reducing the ML values of PC samples after F-T cycles.

The effect of in situ-synthesized Y2SiO5 on the thermal shock resistance of Y2O3 ceramic materials

Y2O3 ceramics are increasingly recognized for their excellent vacuum stability and high-temperature chemical stability in the field of functional materials research. However, their poor thermal shock resistance renders them susceptible to spalling and damage during use, thus restricting further applications. To address this issue, the low thermal expansion coefficients of Y2SiO5 materials were leveraged. Through an in situ reaction between SiO2 and Y2O3, a Y2SiO5 phase was generated that induced a thermal expansion mismatch with Y2O3, thereby enhancing the thermal shock resistance of the ceramic. Using Y2O3 fine powder as the primary material, silica sol and nano-SiO2 were selected as reactive silicon sources to prepare Y2O3–Y2SiO5 composite ceramics at 1750 °C. The impact of the different silicon sources and Y2SiO5 phase content on the microstructure, mechanical, and thermal properties of the composite ceramics was studied. The results demonstrated that the in situ formation of Y2SiO5 significantly improved the thermal shock resistance of the composite ceramic. Specifically, increasing the SiO2 content led to a gradual rise in the volume fraction of Y2SiO5 formed between Y2O3 grains, thereby increasing the residual flexural strength and enhancing the hardness and elastic modulus. The low thermal expansion coefficient of Y2SiO5 reduced the overall thermal expansion coefficient of the composite material, thereby improving the thermal shock resistance. Moreover, the thermal expansion coefficient mismatch between the Y2O3 and Y2SiO5 phases induced numerous microcracks within the material, providing thermal stress relief within the microcracks and markedly improving the resistance of the ceramic to thermal shock.

Research on improving the flexural performance of alkali-activated geopolymer cemented coal gangue through layered addition of fibers

The brittle characteristics of geopolymers during failure seriously hinder their practical engineering applications. To improve the toughness of low-energy alkali-activated geopolymer cementing coal gangue (AGCCG), the flexural performance of alkali-activated geopolymer cementing coal gangue (AGCCG) was enhanced by layering polypropylene fibers. Based on DIC digital speckle technology, the four-point flexure test of AGCCG was carried out, and the strength formation mechanism of the material was analyzed in depth by combining SEM-EDS and XRD. 3D visualization and analysis of the inside of the specimen with the help of CT scanning technology. The internal crack evolution law of the sample was illustrated by PFC numerical simulation. The results showed that the crack formation threshold, peak flexural strength and residual flexural strength of the specimens reached a local maximum at 3 % NaOH doping. With the increase of NaOH dosage, the cementitious product becomes more and more dense and homogeneous, and the area of void defects in the hydration product within the field of view becomes smaller and smaller. The peak bending strength of specimen B4-#4 is 79.43 % of that of specimen B5-#1–4, in which the cost of specimen B5-#1–4 is four times of that of specimen B4-#4, and the way of fiber assignment in specimen B4-#4 is more reasonable when considered together. The layered addition of fibers has a directional strengthening effect on the mechanical properties of geopolymer, and the overall addition of fibers can improve the overall strength and toughness of geopolymer. The numerical simulation results basically coincide with the macroscopic test results, and are consistent with the progressive damage law of the specimens obtained from the numerical scattering test. The results of the study can provide a theoretical basis for improving the bearing capacity of geopolymer and optimizing its structural performance. As well as reducing the impact of solid waste on the environment and improving the efficiency of resource use.

From Mg(OH)2 to lightweight closed-pore MgO refractory aggregates: The role of NiO additive

MgO refractory aggregates with high closed porosity were prepared using light-burned MgO and Mg(OH)2 with adding NiO additive. The evolutions of microstructure and pores and their effects on the properties were investigated. The introduction of NiO effectively enhanced growth of MgO grains and transformation of large open pores into small closed ones, attributing to the lattice distortion caused by MgxNi1-xO solid solution formation. Due to the occurrence of more closed pores and lattice distortion, thermal insulation performance was improved significantly. Moreover, the formed circular closed pores and MgxNi1-xO could absorb thermal stress, lower thermal expansion coefficient and induce crack deflection, thus effectively enhancing thermal shock resistance. The specimen with 4.0 wt% NiO showed the best performance, with a median pore diameter of 5.69 μm, closed/apparent porosity of 9.4 %/3.6 %, thermal conductivity of 9.9 W/(m·K) (500 °C), flexural strength of 68.5 MPa, and residual flexural strength of 10.8 MPa after thermal shock.

Mechanical Behavior of Low-Strength Hydraulic Lime Concrete Reinforced with Flexible Fibers under Quasi-Static and Dynamic Conditions

We investigate the effect of flexible fiber reinforcement on low-strength hydraulic lime concrete. This type of concrete is occasionally necessary to ensure compatibility with the substrate, particularly in the conservation and rehabilitation of historical heritage. For this purpose, we designed a matrix of hydraulic lime concrete based on a mix design method we proposed previously and added different amounts of polyvinyl alcohol fiber (volumetric contents of 0.3%, 0.6%, 0.9%, and 1.2%). We then conducted three-point bending tests on prismatic specimens with a central notch under quasi-static (displacement rate of 4 × 10−4 mm/s) and dynamic (4 mm/s) conditions, using a servo-hydraulic machine. The results indicate that, in both quasi-static and dynamic regimes, the flexural strength, the residual flexural strengths for different crack openings, and the work of fracture increase as the fiber content increases. Furthermore, transitioning from one regime to another (by increasing the strain rate or velocity) leads to a significant increase in these mechanical parameters.

Yield line-based design of fibre reinforced concrete: assessment of suitability of material models

The present research is intended to address the adequacy of flexural toughness-based material models such as equivalent and residual flexural strength, and the deflection limits at which these parameters are estimated for design of fibre reinforced concrete (FRC), for various applications such as slabs-on-grade/pavements. The work is an experimental study using FRC concrete mixes with individual and hybrid combinations of hooked-end steel (SF) and macro polypropylene fibres (PF). The experimental investigation consisted of toughness testing unnotched and notched prisms and square slabs, prepared with a typical grade of concrete used for FRC pavement applications. To validate the material models, a comparison was made between the theoretical moment/load calculated based on material flexural toughness parameters and the actual moment obtained from slabs using actual yield line formed during testing. From the study it is observed that overall, both equivalent (from unnotched prism test) and residual (from notched prism test) flexural strength, are suitable for designing FRC slabs, however the equivalent strength parameter shows better correlation to the slab response. The results also indicate that the hybrid mixes of PF and SF, with a higher volume fraction of 0.6% Vf, shows a marginal increase in flexural performance, in comparison to the mixes with lower volume fraction but no or minimal synergy due to hybridisation.

Development of an enhanced damage law for typical steel fiber reinforced concrete based on uniaxial compression and tension tests

Understanding the stiffness of a concrete structure is crucial to analyze it, particularly for statically indeterminate structures. Stiffness degradation – commonly referred to as damage – occurs with the onset of cracking or large compressive strains. For most conventional and specialized types of concrete, damage studies and models for predicting damage development are available. However, more information is needed about the damage behavior for the most common steel fiber reinforced concrete in Europe with strength class C30/37 and modern end-anchored high-strength fibers in dosages of 20–40 kg/m3. Therefore, in this study, these common steel fiber concretes were subjected to multiple load cycles in (1) uniaxial compression tests on cylinders and (2) direct tensile tests on bone specimens to investigate their damage behavior. The resulting damage was then compared to known damage laws, but none of the models predicted accurate damage results. Finally, an existing damage law for plain concrete was modified as a function of the residual flexural tensile strength—the relevant parameter for describing the performance of the steel fiber reinforced concrete. Hereby, we were able to decisively improve the agreement between experimental results and the theoretical prognosis by utilizing our modified damage law.

An Experimental Study on the Influence of Different Cooling Methods on the Mechanical Properties of PVA Fiber-Reinforced High-Strength Concrete after High-Temperature Action.

High-strength concrete (HSC) has a high compressive strength, high density, excellent durability, and seepage resistance, but its deformation ability is weak. Adding fibers can improve the physical and mechanical properties of HSC. Additionally, the HSC structure may face the threat of fire. In the process of fire extinguishing, the damage mechanism of high-temperature-resistant concrete is complicated due to the different contact conditions with water at different locations. Hence, it is essential to conduct pertinent research on the behavior of fiber-reinforced HSC with different cooling methods after high-temperature action. In this paper, polyvinyl alcohol fiber (PVA fiber) was selected to be added into the HSC to carry out high-temperature experimental research, so as to explore the apparent changes, failure pattern, and mass loss rate of the fiber-reinforced HSC using different cooling methods and analyze the influence of its residual compressive strength and flexural strength. The test results suggest that, with the increase in heating temperature, the color of the specimen's surface transitions from dark blue-gray to white, and the quantity of surface cracks on the specimen gradually rises. The mechanical strength gradually decreases as the heating temperature increases. At a consistent heating temperature, the mechanical strength initially rises, and then falls with an increase in fiber content. The maximum compressive strength and flexural strength were achieved at PVA fiber contents of 0.2% and 0.3%, respectively. For different temperatures and fiber contents, the mechanical strength after natural cooling is generally higher than that after immersion cooling. In addition, X-ray polycrystalline diffractometry (XRD) and scanning electron microscopy (SEM) tests were conducted to analyze the compositional alterations and microstructure of the fiber-reinforced HSC following high-temperature exposure, accompanied by an explanation of the factors influencing the alterations in the physical and mechanical properties. Therefore, the findings of this study can serve as a valuable reference for the utilization of HSC in engineering structures and contribute to the advancement of HSC technology.

Incorporating multi-walled carbon nanotubes in rubberized concrete: impact on physical, mechanical, and fire resistance properties

In the current study, multi-walled carbon nanotubes (MWCNTs) were added to the crumb rubber concrete (CRC) to enhance its physical, mechanical, and fire-resistance properties. A comprehensive investigation into thirteen distinct concrete mixes incorporating rubber aggregate (RA) at levels of 5, 10, and 15%, combined with varying percentages (0, 0.1, 0.2, and 0.3%) of MWCNTs particles, evaluated workability, density, compressive strength, split tensile strength, and flexural strength of MWCNT-mixed CRC at ambient and elevated temperatures (200, 400, and 600 °C). The study demonstrated that MWCNTs, used as a partial substitute for cement in CRC, significantly improved concrete properties, enhancing density, flexural strength, split tensile strength, and compressive strength. Notably, adding 0.2% MWCNTs to CRC resulted in remarkable increases: 0.96% in density, 21.91% in 28-day compressive strength, 10.19% in split tensile strength, and 15% in flexural strength. The workability of CRC decreased with the inclusion of MWCNTs. Furthermore, results indicated that MWCNTs bolstered the residual compressive, tensile, and flexural strengths of rubberized concrete at elevated temperatures, with substantial improvements particularly evident at 200 °C. The present work was focused on the analysis of feasibility of MWCNT mixed CRC toward sustainable construction practices by comparing its physical, mechanical, and fire resistance properties with conventional concrete.

Densification mechanism and thermal shock resistance improvement of MgF2‐doped Y2O3 ceramics

AbstractThis paper aims to improve the density and thermal shock resistance of Y2O3 ceramics for the preparation of ultra‐pure high‐temperature alloy crucible materials. The doping effect of MgF2 content on the densification behavior, physical properties, and thermal shock resistance of Y2O3 ceramics was systematically investigated in this paper. The results suggested that the presence of MgF2 greatly promoted the growth of Y2O3 grains and the transformation of the pore structure by liquid‐phase sintering. And the mechanical properties of the MgF2‐doped Y2O3 ceramics were significantly improved. Besides, the marked improvement in the thermal shock resistance of MgF2‐doped Y2O3 ceramics was attributed to the synergistic action resulting from the growth of grain size and the enhancement of the crack deflection effect. In particular, the relative density of Y2O3 ceramics doped with 1.5 wt% MgF2 reached 96.4% and the residual flexural strength ratio after thermal shock achieved 45.0%, showing an excellent application prospect.

Experimental Study on the Flexural Performance of Steel-Polyvinyl Alcohol Hybrid Fiber-Reinforced Concrete.

This paper explores the impact of steel-PVA hybrid fibers (S-PVA HF) on the flexural performance of panel concrete via three-point bending tests. Crack development in the concrete is analyzed through Digital Image Correlation (DIC) and Scanning Electron Microscope (SEM) experiments, unveiling the underlying mechanisms. The evolution of cracks in concrete is quantitatively analyzed based on fractal theory, and a predictive model for flexural strength (PMFS) is established. The results show that the S-PVA HF exhibits a synergistic effect in enhancing and toughening the concrete at multi-scale. The crack area of steel-PVA hybrid fiber concrete (S-PVA HFRC) is linearly correlated with deflection (δ), and it further reduces the crack development rate and crack area compared to steel fiber-reinforced concrete (SFRC). The S-PVA HF improves the proportional ultimate strength (fL) and residual flexural strength (fR,j) of concrete, and the optimal flexural performance of concrete is achieved when the steel fiber dosage is 1.0% and the PVA fiber dosage is 0.2%. The established PMFS of hybrid fiber-reinforced concrete (HFRC) can effectively predict the flexural strength of concrete.

Mechanical properties of polymer fibre reinforced concrete in the light of various standards

The article presents the results of a detailed experimental campaign including a compressive strength test, three- and four-point bending test (3PBT and 4PBT, respectively) of polymer fiber reinforced concrete with the addition of metakaolin. The comprehensive analysis included three Types of concrete mixture differing in amount and used polymer fibers. It was concluded that polymer fibers did not influence the maximum compressive and flexural tensile strength of concrete. On the other hand, they had a beneficial effect on the ductility, residual and equivalent flexural tensile strengths, and fracture energy of samples. The mixtures of Type 1 and 2 were characterized by softening behaviour but the mixture of Type 3 by soft-hardening behaviour. In the 3PBT, the residual flexural tensile strengths obtained according to EN 14651 did not correspond clearly with equivalent flexural tensile strengths calculated in compliance with RILEM TC 162-TDF. It is noteworthy that the effectiveness and correctness of equations presented in other work of the authors referring to dependencies between deflection, crack and tip mouth opening displacements for the 3PBT were confirmed on samples with different composition and fibers. Based on the 4PBT, the equivalent flexural tensile strengths according to JCI-SF4 standard were calculated and the correlations with the results from 3PBT were defined.

Degradation of flexural bond of CFRP bar in UHPFRC after exposure to elevated temperature

This paper investigated degradation of flexural bond of a ribbed carbon fiber reinforced polymer (CFRP) bar in ultra-high-performance fiber reinforced concrete (UHPFRC) after exposure to elevated temperatures. For this purpose, the mechanical properties of UHPFRC and thermal properties of CFRP bars after exposure to elevated temperatures were investigated, and a total of 16 groups of hinged beams were fabricated with variables of embedment length (4db, 10db, 20db, and 40db), bar type and exposed to elevated temperatures (200 °C, 300 °C, 400 °C, 600 °C). The mechanical properties of UHPFRC increased until elevated temperatures reached 400 °C and it was shown that the reduction of the ultimate tensile strength of CFRP bars were not notable after exposure to elevated temperatures of up to 300 °C. The ultimate moment of ribbed CFRP bars after exposure to elevated temperatures at 200 and 300 °C was similar. After exposure to an elevated temperature of 400 °C, it decreased by about 29–63 % compared to non-heated specimens, with bar rupture. Although similar decomposition temperatures of ribbed and sand coated CFRP bars occurred, the residual flexural bond strength of the ribbed CFRP bar was larger than that of the sand coated CFRP bar after exposure to elevated temperature at 400 °C. In addition, a relatively lower reduction of bond strength occurred in RC-40ED-400 compared to RC-10ED-400 and RC-20ED-400. Based on the test results, development length with consideration of degradation of developed tensile and normalized bond strength of CFRP bars were proposed, and it was found that the required development length of ribbed CFRP bars after exposure to elevated temperatures at 400 °C increased by 294 % compared to ambient temperature.