Increasing Fiber Content Research Articles

Flexural toughness and its evaluation method of ultra-high performance concrete cured at room temperature

This study investigates the effects of water-binder ratio and steel fiber content on the flexural response characteristics of UHPC cured at room temperature by four-point bending tests. These flexural properties include load-deflection curve, flexural strength, initial cracking strength and flexural toughness. Based on the review of existing evaluation methods about flexural toughness, a deflection offset method that can quantitatively determine the initial crack point of UHPC in bending tests is proposed, and the optimized evaluation indicators of UHPC flexural toughness are further established from the perspective of equivalent strength. The proposed method with clear physical meaning and strong applicability can be used to evaluate the flexural toughness of UHPC cured at room temperature during the whole loading process. The results indicate that: (1) The flexural strength and deflection decrease as the water-binder ratio increases from 0.14 to 0.20, in which the decrease of peak deflection is the most significant (56.95%). (2) Although the flexural strength increases linearly with the increase of steel fiber content, the peak deflection increases slowly when the fiber content exceeds 1.0%. (3) The strength and deflection corresponding to the initial cracking point are about 2/3 of the flexural strength and 1/3 of the peak deflection respectively. (4) The flexural toughness decreases with the increasing water-binder ratio, especially when the water-binder ratio exceeds 0.18. (5) The flexural toughness of UHPC increases and the toughening effect of fiber first increases and then decreases with the increase of steel fiber content, and UHPC mixed with 1.5% steel fiber achieves the most remarkable toughening effect. Therefore, the superior flexural toughness and load-bearing capacity of UHPC cured at room temperature are highlighted when the water-binder ratio is less than 0.18 and the steel fiber content exceeds 1.0%.

Study on fracture performance and failure mechanism of hydraulic basalt fiber asphalt concrete by considering temperature effect

An asphalt concrete anti-seepage core wall may be faced with the risk of cracking due to problems like uneven settlement of the foundation during the service period. Considering that hydraulic asphalt concrete (HAC) has a high asphalt content while basalt fibers have excellent tensile properties and good compatibility with asphalt, adding basalt fibers into HAC may potentially form a useful material for hydraulic engineering. Therefore, it is of great significance to comprehensively examine the fracture properties of this new composite material from both macroscopic and mesoscopic perspectives. To explore the effect of basalt fibers on the fracture performance of HAC, an experimental study was carried out on hydraulic basalt fiber asphalt concrete (HBFAC) under different temperatures by considering 3 fiber lengths and 4 fiber contents. The results showed that, when the temperature remained constant, the fracture load, fracture toughness and fracture energy of HAC all exhibited an upward trend with the increase of fiber content, and reached the maximum values at the fiber content of 4‰. Of different fiber lengths, the maximum fracture load, fracture toughness and fracture energy were observed at the fiber length of 6 mm. With the growth of temperature, the fracture load and fracture toughness exhibited a decreasing trend, while the fracture energy showed an increasing trend. At the same time, the increase in fracture load and fracture toughness due to the fiber reinforcing effect was gradually enlarged, but the increase in fracture energy was gradually reduced. Overall, the optimum fiber length and fiber content were determined to be 6 mm and 4‰, respectively. Subsequently, the fiber properties were further investigated with the discrete element method, and the results of internal mesoscopic mechanical behavior, crack development and particle displacement indicate that basalt fibers play a positive role in dispersing stress and inhibiting crack development so as to improve the toughness and cracking resistance of HAC. Finally, the failure mechanism of HBFAC at different temperatures was revealed from a mesoscopic level. The outcome of this paper provides a theoretical basis for the research and application of basalt fibers in HAC.

Performance of Pantaneira breed cows in precision grazing system.

Use of a grazing system that takes into account the ingestive behavior of animals and the physiological characteristics of plants maximizes land use and promotes positive effects on the environment. In this sense, the objective of this study was to evaluate the performance of Pantaneira breed cows, kept in rotated grazing in Mombasa grass (Megathyrsus maximum), with different grazing times. Fifty animals were divided into two treatments: Continuous T1: 24h and T2-Inverted: 12h. The experiment lasted 98days, being determined the production and nutritional quality of the forage, digestibility, intake, and performance of the animals. The design was randomized blocks at 5% probability, with the means compared by the F test. The design used was completely randomized at 5% probability by the T test. There was no significant difference for biomass production (P > 0.05). However, after grazing the Inverted group, the forage showed a lower percentage of leaves and an increase in neutral detergent fiber and acid contents and total carbohydrates, and a reduction in crude protein and ether extract values, as well as a higher digestibility (P < 0.05). The animals of the Continuous group intaked more forage (P < 0.05). The Inverted group had a higher average daily weight gain, in addition to a lower feed conversion (P < 0.001). The animals in the Control group spent more time eating (P < 0.0001) and sleeping (P < 0.0001), while the animals of the Inverted group spent more time in idleness (P < 0.0001) and interacting (P < 0.05). There was no difference in rumination time between treatments (P > 0.05). It was concluded that Inverted grazing improved the quality of Mombasa grass and the performance of cows.

Foundation of multilevel apploach to fracture modeling for materials with submicrostructure applicable for Arctic and Subarctic environment

A short review of research on multiscale modeling of the fracture processes of heterogeneous materials with submicrostructures applicable to Arctic and Subarctic environments is presented. The results of solving a number of model problems, both in terms of macroscopic strength and main crack growth, and describing the accumulation of microscopic defects and hierarchical fracture processes by the mechanisms of formation, growth, and fusion of secondary and microcracks, and microscopic pores are provided. Thus, for the fracture process of samples of porous concrete modified with oil palm fruit fibers to improve its consumer qualities, a simulation of crack growth was conducted, which showed a decrease in the crack resistance of the material with increasing fiber content. A three-point bend test was used to evaluate the bending strength and modulus of elasticity, and the compressive strength and modulus of elasticity were determined. An increase in the fiber content led to a decrease in the compressive strength and modulus of elasticity. The fracture surface analysis revealed the mechanism of crack propagation through the coalescence of micropores. The basis of the modeling was an experimentally substantiated criterion for concentrated fractures during crack formation based on percolation theory. Macroscopic finite-element and stochastic modeling of fractures during the bending of a beam made of wood have also been carried out. For wooden structures, the peculiarity is the anisotropic behavior of the material in the loading direction. The structure of the Bilinga tree timber at the mesoscopic and microscale levels was considered, and a hierarchy of the spatial and temporal scales of the fracture process was constructed. The crack growth rates at different scales were determined and compared with macroscopic FE simulations. Another task was to determine the dependence of the crack growth rate in the soil, such as regolith, under the influence of an ultrasonic impactor. Stochastic modeling of the crack growth in the array of micropores revealed the crack growth rate dependence on the ultrasonic transmitter beater parameters and pulse amplitude.

Pullout performance of headed anchor studs in Engineered Cementitious Composites (ECC) with Polyvinyl Alcohol fibers

To study the pull-out resistance of headed anchor studs in Engineered Cementitious Composites (ECC) for steel-ECC composite structures, pull-out tests were carried out on 9 scenarios of ECC specimens. The failure modes, load–displacement curves, peak load (Nu), initial stiffness (K0.3), and ultimate displacement (Du) of studs in Polyvinyl Alcohol fiber (PVA)-ECC under different stud sizes (d = 13 mm and 16 mm), stud embedment depths (hef = 40 mm, 60 mm, and 80 mm) and fiber content (Vf = 2.0% and 2.5%) were obtained. Failure modes include (ductile) concrete cone failure, cone-splitting combined failure, and steel failure. For the specimens with concrete cone, either increasing d or hef increases K0.3, Nu, and Du, and Nu shows the greatest improvement. Increasing fiber content (Vf) exerts a significant enhancing effect on Nu, but exerts little effect on K0.3 and Du. Through the comparisons to the pull-out resistance of studs in high-strength concrete (HSC), the studs in ECC show evident advantages in ductility, toughness, and Nu. Based on the experimental results, the applicability of the original CCD model and two modified CCD models from literature to studs in ECC is discussed. The modified CCD model including the effect of stud size and specimen size presents the best accuracy in predicting Nu of studs in ECC. Overall, ECC is a promising material for steel–concrete composite structures, considering it evidently improves the pull-out performance of studs, thus the anchorage between steel and ECC.

Shear behaviour of reinforced concrete beams using treated recycled coarse aggregates and steel fibres

This paper has made an attempt to investigate the shear behaviour of reinforced concrete beams made of treated recycled coarse aggregate (RCA) experimentally after adding three different volume fractions of hook end steel fibres, i.e., 0.5, 1 and 1.5%. The treatment method adopted for strengthening recycled aggregate concrete (RAC) was pre-soaking recycled aggregates in cement and fly ash slurry. The percentage of replacement of treated RCA was kept constant at 60%. Simply supported RC beams made of treated RCA without stirrups after incorporating steel fibres were tested under an incremental four-point loading mechanism. The results from the experiment showed that the first crack load and ultimate load ascended as we increased the steel fibre content. Maximum deflection at mid-span deflection reduced with an increase in fibre content. The failure mode has shifted from shear to flexure by increasing the steel fibre content. Hence, steel fibres can make up for the lack of transverse reinforcement during shear.

An NMR study of the role of coir fibers in the hydration and drying of cement paste at early age

Natural fibers such as coir fibers have been increasingly explored as reinforcement in the cementitious composite to promote sustainable and economical construction. However, the in-situ application of this composite presents challenges due to the complex interplay of water behaviors, involving the hydrophilicity of natural fibers, the hydration of cement and the evaporation due to drying. Although these water-related processes play a crucial role in achieving the desired properties, they have been relatively underreported in the literature. This study aims to investigate the role of coir fibers in the water distribution and pore size distribution during the early age (initial 3 days) of cement paste hydration under drying conditions. Accordingly, Nuclear Magnetic Resonance (NMR) methods involving the moisture profiling and transverse relaxation are used to monitor the water distribution and pore size distribution in time but also in space. The samples are made by adding water-saturated coir fibers to the cement paste at different dosages (0, 1%, 2% and 4%). The effects of drying and coir fiber dosages on the samples are evaluated. The results show that drying leads to a moisture gradient and a larger transverse relaxation time T2, i.e., a coarser pore structure near the drying surface. By adding the saturated coir fibers, the moisture gradient caused by drying is decreased and spread out along the sample. As the coir fiber content increases, the water content along the sample and the total water content of the sample increase. As a result, the samples with coir fibers exhibit a lower T2, i.e., a denser pore structure near the drying surface. Coir fibers are shown to eliminate the adverse effect of drying on cement hydration through three effects, including the internal curing, improved water transport and crack control. The results from this study reveal the promising potential of coir fibers for in-situ construction applications.

The effect of natural bamboo fiber and basalt fiber on the properties of autoclaved aerated concrete

Autoclaved aerated concrete (AAC) has the advantages of low density and good thermal insulation performance, but its mechanical properties are not so good. In this experiment, natural bamboo fiber and basalt fiber were used to strengthen AAC, and the effect of fiber content and fiber length on the AAC properties was investigated. The experimental results showed that with the increase of fiber content, the slurry fluidity, foaming height of AAC mixture decreased, the area of hazardous pore, the density, mechanical strength and thermal conductivity of AAC increased. As the fiber length increased, similar results were observed in the mechanical strength and thermal conductivity of AAC but not found in the slurry fluidity, foaming curve and pore structure. By contrast, natural bamboo fiber had greater impact on the properties of AAC than basalt fiber. The scanning electron microscope (SEM) results showed natural bamboo did not react with AAC matrix and the fiber was pulled out from AAC matrix when the AAC cracked, while basalt fiber was corroded in alkaline matrix and broke with AAC matrix together under external load.

Effect of hybrid steel-polypropylene fiber on punching shear behavior of flat slab with an opening

This research aimed to gain a better understanding of how the addition of fiber influences the punching shear capacity of two-way slabs by conducting an experiment into the structural behavior of flat slabs with and without a square opening using different volume fractions of hybrid steel-polypropylene fiber (0%, 0.9%, 1.05% and 1.8%). Ten 700 × 700 × 70 mm slabs were divided into five pairs, with two samples used as control samples (with and without openings), and eight other samples with different volume fraction of fibers. Results showed that an increase in fiber content enhanced the shear strength of the slabs. For example, as the volume fraction of hybrid fiber increased from 0.0 to 1.8%, the ultimate load increased by 52% for slabs without an opening and up to 42% for slabs with an opening.

Effect of waste colemanite and PVA fibers on GBFS-Metakaolin based high early strength geopolymer composites (HESGC): Mechanical, microstructure and carbon footprint characteristics

This study investigated the effects of polyvinyl alcohol (PVA) fiber and colemanite on the physical, mechanical and transport characteristics of GBFS-metakaolin-based geopolymer composites with high early-age strength (HESGC). The microstructure was intensified using 100, 150 and 200 kg/m3 colemanite instead of quartz powder. In addition, 0.25% and 0.50% PVA fiber were added to the blend to prevent the permeability problem in the condensed microstructure. HESGCs were cured in 3 different environments: ambient, 60 and 80 °C. The binder dosage (GBFS + Metakaolin) in HESGCs is 750 kg/m3. The alkali solution ratio is approximately 75% (according to binder amount. While the apparent porosity exceeds 10% in HESGCs subject to ambient cure, the porosity % in heat-cured HESGCs) In addition, the porosity and water absorption decrease when colemanite is used in heat cured HESGCs. Adding 0.25% PVA fiber to HESGCs creates a micro-aggregate effect, reducing porosity and water absorption relatively. It varies between 2393 kg/m3. The compressive strength (fc) of HESGCs subjected to ambient cure varies between 27.5 and 43.8, and the fc of HESGCs heat cured at 80 °C varies between 77.4 and 95.6 MPa. The flexural strength (ff) of HESGCs varies between 3.6 and 13.3 MPa. As the PVA fiber content and curing temperature increase, the ff increases. If 0.5% PVA fiber is used, the ff of HESGCs is increased between 8.3% and 89.6%0.200 kg/m3 colemanite is used in heat-cured HESGCs fc can increase up to 20%. While 0.25% PVA fiber ratio is the most suitable ratio in fc, 0.5% PVA fiber ratio is more suitable in ff. The capillary water absorption of HESGCs varies between 0.27–2.67 kg/m2. In the case of heat curing of HESGCs at 80 °C instead of ambient curing, capillary water absorption can be reduced up to 88%. Using colemanite instead of fine aggregate in microstructure investigations intensified the microstructure, but using 0.5% PVA fiber increased the porosity relatively. As a result, it has been observed that heat curing must be used in colemanite-containing HESGCs. When a heat cure is applied to HESGCs containing 0.25% PVA fiber and 200 kg/m3 colemanite, a fc of more than 12 MPa can be obtained.

Performances of Heat-Insulating Concrete Doped with Straw Fibers for Use in Tunnels

Current research efforts aim to develop insulating building materials to cope with hot tunnels by using crop straw fibers. The amount of straw fiber incorporated included different percentages of the gelling material mass: 0% (reference specimen), 2.5%, 5%, 7.5%, and 10%. The study focused on the concrete’s compressive strength, permeability, and thermal coefficient. The thermal coefficient of dry concrete decreased with increasing fiber content. However, the porosity of concrete mixed with more straw fibers increased, thus decreasing compressive strength. Concrete with a fiber content of 7.5% had the best overall performance, and it had a low thermal conductivity (0.158 W/(m·k)). According to COMSOL simulation results, the tunnel wind flow temperature of concrete with 7.5% fiber content was lower than the tunnel wind flow temperature of plain concrete. Thus, the obtained straw fiber heat-insulating concrete is a promising candidate material for tunnel insulation.

Mechanical Analysis of Palm-Fiber-Reinforced Sand through Triaxial Tests

Triaxial tests were employed to investigate palm-fiber-reinforced sand under consolidated drained conditions in this study. Sixteen series of triaxial tests were carried out to investigate the properties of palm-fiber-reinforced sand. One series of pure sand was also employed for comparison. The deviator stress, stress path, shear strength, volume change, void ratio, and enhanced coefficient of fiber-reinforced sand were studied with different fiber lengths varying from 8 mm to 20 mm and fiber contents varying in mass from 0.3% to 0.9%. The test results indicate that palm fibers were beneficial for enhancing the shear strength of the sand. Compared to the peak shear strength increase of about 10% to 20%, the critical shear strength increased much more, by a little over 100%. Therefore, the fibers played a key role in enhancing the critical shear strength of the sand but not the peak shear strength of the sand. The addition of fiber to sand resulted in prolongation of the axial strain required to reach the critical void ratio and improved the sand’s ability to resist larger deformations, enhancing its toughness. Furthermore, the critical shear strength of the sand was positively correlated with both fiber content and fiber length, and the axial strain required to reach the critical shear strength increased with increasing fiber content and length. This study provides valuable experimental data and serves as a reference for temporary reinforcement in geotechnical engineering.

A comprehensive quantitative bottom-up analysis of fiber-reinforced recycled-aggregate concrete behavior

This study provides a more profound understanding of the influence of the phases of fiber-reinforced recycled-aggregate concrete (FRRAC), on its elastic properties, in particular Young’s modulus and Poisson’s ratio. Multi-scale modeling analyses of mortar and FRRAC were conducted to assess the effect of variations in the fiber content, fiber elastic modulus, RA content, and water-to-cement ratio (w/c) on the elastic properties at each scale. Thus, the analytic Mori–Tanaka (MT) homogenization algorithm developed in Python programming language and the three-dimensional finite element (FE) homogenization scheme were applied to evaluate the elastic properties of FRRAC. As such, different fiber types including steel, basalt, glass, and propylene, at a volume fraction range of 0–2%, along with the variations in fiber elastic modulus, and different RA replacement levels ranging from 0 to 100% were incorporated in the modeling process at different w/c ratio. Based on the results, the Poisson’s ratio of steel FRRAC in the MT approach surges with increasing fiber content. Furthermore, the elastic modulus of FRRAC is highly susceptible to an increase in Young’s modulus of polypropylene fiber, among other fiber types. The elastic modulus of concrete experiences a sharp decrease with increasing w/c for all fiber types in both FE and MT approaches.

Study on Dynamic Impact Mechanical Properties of UHPC with High-Content and Directional Reinforced Steel Fiber

Ultra-high-performance concrete (UHPC) is a kind of building material with ultra-high strength, toughness, and durability. However, under the conditions of ordinary molding technology, most of the fibers cannot play a bridging role in the direction of force. In this study, UHPC specimens with different steel fiber contents (0%, 2%, 4%, and 6% by volume) and directional reinforced fiber were prepared. Based on the split-Hopkinson pressure bar (SHPB), the influence of directional distributed steel fiber on the dynamic impact mechanical properties of the UHPC specimen were systematically investigated. The stress–strain curves, stress peaks, dynamic increase factor (DIF), and ductile energy absorption properties of the specimens at different strain rates were obtained. The results showed that oriented steel fiber significantly increases the dynamic property of UHPC. The dynamic impact peak strain, peak stress, and DIF of the UHPC specimen with 2% oriented steel fiber were 35.78%, 8.8%, and 12.6% higher than that prepared by normal molding technology, respectively. Moreover, with the increase of fiber content, the peak stress, energy absorption, and multiple-impact compression resistance of the specimen were greatly improved. When the fiber content was 6%, the dynamic impact peak strain, dynamic impact compressive strength ratio, and energy absorption capacity of the specimen were 3.09, 1.45, and 4.1 times the reference group, respectively.

Performance Study of Lightweight Insulating Mortar Reinforced with Straw Fiber.

The current research aimed to develop lightweight, environmentally friendly mortar materials using crop straw fibers with better insulation properties. The lightweight mortar samples were tested for moisture content, thermal conductivity and compressive strength on days 3, 7 and 28, respectively. Scanning electron tomography (SEM) was performed on the fiber-matrix bonding interface and internal fiber structure. The permeability rating was also measured to check the impermeability of the lightweight fiber mortar. Due to the high hygroscopicity of plant fibers, the thermal conductivity of the mortar was high at the initial molding stage; the thermal conductivity measured at day 28 decreased with increasing fiber content, while the mechanical properties gradually decreased. The impermeability test showed that the straw fiber mortar had better impermeability than the standard mortar. However, with the addition of 2% of 10 mm long fibers, we increased the compressive strength and thermal insulation properties. Numerical simulations verified that the fiber insulation mortar has good thermal insulation properties in high-temperature tunnels.

Impact of using oat flour and erythritol on characteristics and glycemic index of chiffon cakes and study the effect of erythritol on cavity- causing bacteria

Oats have a high content of β-glucan. Its consumption lowers cholesterol and the glycemic response. One of the main components of many baked confectionery goods is sucrose. Reducing the amount of refined sugar in food has become urgently necessary as a result of the growing understanding of the link between excessive sugar consumption and the emergence of chronic diseases. Oat flour in the chiffon cake formulation was replaced at three levels, 50, 75 and 100% with wheat flour and had sucrose replaced with erythritol. Raw materials and prepared chiffon cake samples were analyzed for their chemical composition. Final product was evaluated for baking quality, glycemic index (GI) and organoleptic Properties of chiffon cake. The inhibitory effects of erythritol on Streptococcus strains were examined. Results suggest that, it could be seen that a significant increase in fiber and fat contents (10.1 and 6.52%, respectively) and a decrease in carbohydrate content 68.79%, in oat flour compared to wheat flour. Our results indicated that in the presence of erythritol was decreased the growth of S. sanguinis by 73%. Chiffon cake produced with oat flours and erythritol it was characterized with their higher content of moisture, ash and fiber. The highest weight (360.0 g) was given by adding 100% oat flour to the chiffon cake sample compared to 342.6 g for control cake. The GI were decreased as the levels of oat flour were increased and substituted sucrose with erythritol. Chiffon cakes prepared with 50% oat flour and 100% erythritol was decreased in sweetness

Industrial approach to circularity of polymer composites: Processing, characterization, mechanical testing, and wear regression

Cotton, polyester, and polyethylene terephthalate are the most common types of polymers that produce huge wastes. The circularity of these post-consumer (PC) waste faces operational problems during processing. In this innovative research, the relationship between circularity, surface characterization, mechanical and tribological testing of fiber reinforced (cotton, polyester), and particulate (polyethylene terephthalate) composites is explored for industrial pilot production. Cutting model can control the size of fibers during grinding. The fiber reinforced composites (FRCs) with 10% (by weight) fiber loadings are found as operational candidates for structural, automotive, and medical applications due to suitable tensile strength (26–29 MPa), percentage of extension (10%) and abrasive wear (3 × 10−6 mm3/Nm). An increase in fiber content produces micro-defects like asperities, rough areas, voids, cracks, and pits in recycled composites. Therefore, the particulate and FRCs with 40% (by weight) fiber loadings become hard and brittle. However, these composites (especially with 40% wt. fiber loadings) exhibit reasonable elastic modulus (1526–2751 MPa) and abrasive wear (6.5 × 10−6 mm3/Nm). The ductile to brittle transition effect has appeared in all composites (with 30% wt. fiber content) due to continuous fiber addition, micro-defects creation and dual phase presence. In conclusion, natural and synthetic PC wastes can be utilized for sustainable processing of commercial polymer composites. Moreover, injection molding, polymer characterization, tensile testing, abrasion evaluation, and regression analysis can be introduced for the transformation of open-loop into closed-loop manufacturing.

Combined effect of using steel fibers and demolition waste aggregates on the performance of fly ash/slag based geopolymer concrete

Developing sustainable construction products has been showing a rising trend recently. Geopolymer composites are remarkable binding materials fabricated based on recycling and sustainability. In this paper, an experimental investigation was conducted to study some mechanical and durability characteristics of steel fiber-reinforced fly ash/slag-based geopolymer concrete (GPC) with different proportions of recycled coarse aggregate. Steel fiber-reinforced geopolymer concrete mixtures incorporating recycled coarse aggregates of up to 40% at the interval of 10% were prepared, and the mixtures without recycled coarse aggregate were used as reference samples. The steel fiber ratio of 0.3% and 0.6% were used and the combined effect of steel fiber and recycled coarse aggregate on the geopolymer composites’ behavior regarding strength properties, sulfate resistance, elevated temperature resistance, abrasion resistance, and freezing-thawing resistance was addressed. A significant improvement in strength properties was observed in steel fiber reinforced recycled aggregate geopolymer concrete with increasing fiber content; however, the strength properties of geopolymer concrete were decreased with the increase of recycled coarse aggregate amount. Also, the properties such as abrasion resistance, resistance to elevated temperature, sulfate resistance, and freeze-thaw resistance were improved with the addition of steel fiber. It indicates that the synergetic effect of steel fiber and recycled aggregate helps to produce ecologically sound geopolymer concrete with good engineering properties and durability characteristics that will bring sustainability to the concrete sector. Generally, the results show that the recycled coarse aggregate ratio of up to 30% and 0.6% of steel fiber can be considered an ideal combination to make geopolymer concrete with better overall properties.

Experimental and numerical investigations on direct shear performance of UHPC dry joints

Ultra-high performance concrete (UHPC) has great prospects in the field of precast segmental bridges. The excellent mechanical properties of dry joints between the UHPC segmental have become the focus of scholars all over the world. Currently, there are few studies on UHPC dry joints with a compressive strength between 120 MPa and 140 MPa. In this study, the direct shear performance of UHPC dry joints was tested and simulated with parameters of steel fiber content, confining stress, key number, shear key interlocking depth-height ratio (the depth-height ratio was used later), and key height. The results showed that the cracking resistances and ultimate strength of UHPC joints could be significantly improved with the increase of steel fiber content. All the UHPC specimens showed a little brittleness behavior with the average cracking load being 78.4% of the average ultimate load. Different cracking patterns occurred in single-keyed UHPC dry joints, while sequential failure arose for multi-keyed UHPC dry joints. Compared with monolithic specimens, the direct shear strength of keyed UHPC dry joints was reduced with a range of 51.4%–66.0%, and the decreasing trend was alleviated with the increase of the key number. The direct shear capacity of UHPC dry joints increased with the increase of key height and depth-height ratio, and the optimized depth-height ratio was 0.40–0.50. Compared the calculated results of existing formulae with the test results incorporated with the previous studies, it was found that Jiang's formula predicted the shear capacities of UHPC dry joints more accurately than the other formulae. In addition, the influence of the depth-height ratio on the direct shear capacities of UHPC dry joints cannot be reflected by the existing formulae, which need further improvement.

Torsional capacity of ultra-high-performance concrete beams using rectangle stirrup

This study studies the torsional capacity of rectangular ultra-high-performance-concrete (UHPC) beams. Nine UHPC beams were tested under pure torsion load. The failure modes were presented and the influences of parameters, including reinforcement ratio, stirrup ratio and steel fiber content were discussed. The experimental results showed that longitudinal reinforcement ratio had little effect on crack width and plastic stiffness in serviceability stage. Increasing the stirrup ratio and steel fiber content effectively inhibited the development of crack width and improved plastic stiffness. The torsional ductility of UHPC beams decreased with the increase of longitudinal reinforcement ratio but increased with the increase of stirrup ratio and steel fiber content. The cracking torque and elastic stiffness of UHPC beams were less affected by the reinforcement ratio and increased with the increase of steel fiber content. The twist, energy consumption, and ultimate torque of UHPC beams increased with the increase of longitudinal reinforcement ratio, stirrup ratio, and steel fiber content. The damage index of UHPC beams was 0.85–0.90, and the plastic stiffness accounts for 1/25–1/10 of the elastic stiffness.