A new testing configuration for shrinkage cracking of shotcrete and fiber reinforced shotcrete

Shotcrete and fiber reinforced shotcrete are commonly employed to produce layers or linings with large surface area versus volume ratios. Restrained shrinkage cracking is hence an important concern. The common test set-up used for shrinkage cracking of concrete, with a ring specimen cast around a stiff steel form, is not applicable to shotcrete. A new testing configuration, consisting of a shotcrete specimen bonded to a steel I-section and angles, is therefore proposed. In this investigation, a finite element analysis was first performed to identify member sizes that provide a good compromise between the effectiveness of constraint and weight of steel members. Restrained shrinkage tests using this new configuration were performed for plain and fiber reinforced shotcrete. Despite the simplifying assumptions in the finite element analysis, the predicted degree of restraint is in reasonable agreement with test results. From the results, the proposed set-up is shown to be a practical and viable approach for investigating the shrinkage cracking behavior of shotcrete and fiber reinforced shotcrete.     

Properties of lightweight expanded polystyrene concrete reinforced with steel fiber

Expanded polystyrene (EPS) concrete is a lightweight, low strength material with good energy-absorbing characteristics. However, due to the light weight of EPS beads and their hydrophobic surface, EPS concrete is prone to segregation during casting, which results in poor workability and lower strength. In this study, a premix method similar to the ‘sand-wrapping’ technique was utilized to make EPS concrete. Its mechanical properties were investigated as well. The research showed that EPS concrete with a density of 800-1800 kg/m³ and a compressive strength of 10-25 MPa can be made by partially replacing coarse and fine aggregate by EPS beads. Fine silica fume greatly improved the bond between the EPS beads and cement paste and increased the compressive strength of EPS concrete. In addition, adding steel fiber significantly improved the drying shrinkage.     

Durable fiber reinforced self-compacting concrete

In order to produce thin precast elements, a self-compacting concrete was prepared. When manufacturing these elements, homogenously dispersed steel fibers instead of ordinary steel-reinforcing mesh were added to the concrete mixture at a dosage of 10% by mass of cement. An adequate concrete strength class was achieved with a water to cement ratio of 0.40. Compression and flexure tests were carried out to assess the safety of these thin concrete elements. Moreover, serviceability aspects were taken into consideration. Firstly, drying shrinkage tests were carried out in order to evaluate the contribution of steel fibers in counteracting the high concrete strains due to a low aggregate-cement ratio. Secondly, the resistance to freezing and thawing cycles was investigated on concrete specimens in some cases superficially treated with a hydrophobic agent. Lastly, both carbonation and chloride penetration tests were carried out to assess durability behavior of this concrete mixture.     

Transport properties of water and glycol in an ultra high performance fiber reinforced concrete (UHPFRC) under high tensile deformation

Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability. Those characteristics make them very attractive for the rehabilitation of existing structures and the conception of new structures. To define the range of admissible tensile deformation in those materials, the influence of imposed tensile deformation and subsequent cracking on permeability and absorption was studied. The transport properties of water and glycol were assessed in order to estimate the effect of the interaction of water with a specific UHPFRC. The experimental results demonstrate that permeability and absorption increase steadily until a residual tensile deformation of 0.13% is reached in the material, then water seeping rises distinctly. During experiments, the interaction of water with the UHPFRC decreases by 1 to 3 orders of magnitude the permeability and reduces absorption by approximately 50 to 85%. Test results reveal the high capability of the material to seal cracks and improve its water-tightness with time.     

A comparison of bending strength between adhesive and steel reinforced concrete with steel only reinforced concrete

Cracking of brittle cementitious composites subjected to excessive loading causes a potential reduction in material performance. Steel bars or metal fibers typically act as tensile reinforcing in concrete composites to increase the material's structural capacity in bending and to delay or prevent matrix cracking.The goal of this research is to determine whether the performance in bending strength and material integrity of a typically reinforced cementitious composite may be improved through the release of “healing” chemicals, such as adhesives, from hollow fibers into cracks induced by loading in addition to the metal reinforcing. Adhesive-filled repair fibers are intended to break immediately upon cracking in the concrete thereby activating the healing process with the release of a sealing or adhering substance. This self-repair occurs whenever and wherever cracks are generated.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

Torsion of steel fiber reinforced concrete members

Addition of steel fibers in concrete improves the mechanical properties of the basic matrix as they slow down the growth of crack and creates pinching forces at the tips of the cracks. Thus, the steel fiber reinforced concrete becomes a better energy absorbing material and is best material for the seismic resistant structures and blast resistant structures. Relatively little research work has been done on the behavioural aspects of this material under pure torsion compared to its behaviour under flexure or shear or under combined loading. Earlier researchers [Int. J. Cem. Compos. Lightweight Concr. 4 (1982) 45, Indian Concr. J. 55 (1981) 222-232, Int. J. Cem. Compos. 2 (1980) 85] reported that addition of fiber in concrete improves the torsional strength and ductility. However, the enhanced properties of SFRC in particular the ductility of the matrix can be achieved when a minimum volume fraction of fiber content is maintained. This investigation aims at understanding the behavioural aspects of plain SFRC members under pure torsion. An empirical formula has been proposed to predict the ultimate torsional strength of the SFRC members under pure torsion.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

Mechanical properties of hybrid fiber-reinforced concrete at low fiber volume fraction

Concretes containing different types of hybrid fibers at the same volume fraction (0.5%) were compared in terms of compressive, splitting tensile, and flexural properties. Three types of hybrid composites were constructed using fiber combinations of polypropylene (PP) and carbon, carbon and steel, and steel and PP fibers. Test results showed that the fibers, when used in a hybrid form, could result in superior composite performance compared to their individual fiber-reinforced concretes. Among the three types of hybrids, the carbon-steel combination gave concrete of the highest strength and flexural toughness because of the similar modulus and the synergistic interaction between the two reinforcing fibers.     

Influence of polypropylene fiber geometry on plastic shrinkage cracking in concrete

Plastic shrinkage cracking remains a primary concern for placements with high surface/volume ratios that are subjected to early age drying. Polypropylene fiber reinforcement controls such cracking, but the exact influence of fiber diameter, length and geometry remains unknown. A test program was carried out to understand the influence of these variables. Four commercially available polypropylene fibers were investigated at dosage rates varying from 0.1% to 0.3%. A recently developed technique of plastic shrinkage testing using a fully bonded overlay was employed. In this technique, a fiber reinforced concrete overlay is cast on a fully matured subbase with protuberances and the whole assembly is allowed to dry in an environmental chamber. Cracking in the overlay is monitored with time and characterized. Results indicate that while polypropylene fibers in general are effective in controlling plastic shrinkage cracking in concrete, a finer fiber is more effective than a coarser one, and a longer fiber is more effective than a shorter one. Further, fiber fibrillations appear to be highly effective in controlling plastic shrinkage cracking.     

Effect of sulfate solution on the frost resistance of concrete with and without steel fiber reinforcement

Properties of plain concrete (PC) and steel fiber reinforced concrete(SFRC) (with water/cement ratio of 0.44, 0.32 and 0.26) subjected to freeze-thaw cycles in 5.0% sodium sulfate solution were investigated in this paper. It was found that during the initial 300 freeze-thaw cycles, sulfate solution had little effect on the relative dynamic modulus of elasticity (E_d) of concrete. In further freeze-thaw cycling, the effect of sulfate solution on E_d was much more obvious. Both PC and SFRC specimens with w/c of 0.44 failed before 300 cycles and exhibited similar developing trends of the E_d whether freezing and thawing in sulfate solution or in fresh water. As for the concrete specimens with w/c of 0.26, the decline of E_d was more serious when freezing and thawing in sulfate solution than that in fresh water after 300 cycles. The adoption of steel fiber greatly restrained the decline of E_d and changed the failure mode of the specimen from brittle crack in midspan of PC to gradually decline of E_d up to failure under the combined action of freeze-thaw cycles and sulfate attack. Test results also demonstrated that there was an interaction effect between the action of freeze-thaw cycles and sulfate attack.     

Behavior of high strength concrete with and without steel fiber reinforcement in triaxial compression

Based on an extensive experimental program, this paper studies the behavior of high strength concrete and steel fiber reinforced high strength concrete under uniaxial and triaxial compression. Triaxial stress-strain relations and failure criteria are used to evaluate the effect of steel fiber reinforcement on the mechanical properties of high strength concrete in triaxial compression, which is found to be insignificant.     

Engineered cementitious composite with characteristic of low drying shrinkage

This paper reports a new class of engineered cementitious composite (ECC) with characteristics of low drying shrinkage, tight crack opening and high tensile strain capacity. Research emphasis is placed on the influence of different cementitious matrix on drying shrinkage, tensile property and early age cracking behavior of the composites. Experimental results show that drying shrinkage of the composite is greatly reduced as using the low shrinkage cementitious material in matrix, while the composite remains strain-hardening and multiple cracking characteristics. The measured drying shrinkage strain at 28 days is only 109 × 10^− 6 to 242 × 10^− 6 for low shrinkage ECCs. For traditional ECC, the shrinkage strain at 28 days is nearly 1200 × 10^− 6. The average tensile strain capacity after 28 days curing is 2.5% of the low shrinkage ECC with tensile strength of 4-5 MPa. Further, in the strain-hardening and multiple cracking stage, cracks with much smaller width compared to the traditional ECC are formed in the low shrinkage ECC.     

Interaction between loading, freeze-thaw cycles, and chloride salt attack of concrete with and without steel fiber reinforcement

In this paper, the deterioration of concrete subjected to the combined action of four-point bending—loading, freeze-thaw cycles, and chloride salt attack—is discussed. Test results show that concrete tested in chloride salt solution scaled much more severely than in fresh water, and its weight loss in chloride salt solution was twice that in water. However, dynamic modulus of elasticity (DME) of concrete in chloride salt solution dropped more slowly than that in water due to supercooling resulting from chloride salt. It is also shown that the degradation process of concrete simultaneously exposed to loading, freeze-thaw cycles, and chloride salt attack was significantly accelerated. The higher the stress ratio exerted, the lesser the freeze-thaw cycles that concrete could resist and, consequently, the shorter the service life. When a relatively high steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the three damaging processes is considerably reduced.     

Strength properties of nylon- and polypropylene-fiber-reinforced concretes

The strength potential of nylon-fiber-reinforced concrete was investigated versus that of the polypropylene-fiber-reinforced concrete, at a fiber content of 0.6 kg/m³. The compressive and splitting tensile strengths and modulus of rupture (MOR) of the nylon fiber concrete improved by 6.3%, 6.7%, and 4.3%, respectively, over those of the polypropylene fiber concrete. On the impact resistance, the first-crack and failure strengths and the percentage increase in the postfirst-crack blows improved more for the nylon fiber concrete than for its polypropylene counterpart. In addition, the shrinkage crack reduction potential also improved more for the nylon-fiber-reinforced mortar. The above-listed improvements stemmed from the nylon fibers registering a higher tensile strength and possibly due to its better distribution in concrete.     

Description and modeling of fiber orientation in injection molding of fiber reinforced thermoplastics

As mechanical properties of short fiber reinforced thermoplastic injected components depend on flow induced fiber orientation, there is considerable interest in validating and improving models which link the flow field and fiber orientations to mechanical properties. The present paper concerns firstly the observation and quantification of fiber orientation in a rectangular plaque with adjustable thickness and molded with 30 and 50 wt% short fiber reinforced polyarylamide. An automated 2D optical technique has been used to determine fiber orientations. A classical skin (with orientation parallel to the flow)-core (with orientation perpendicular) structure is observed for thick plaques (thickness greater than 3 mm) but the core region is fragmentary for thickness less than 1.7 mm. It is shown that the gate design and different levels of fiber interactions, due to different fiber concentrations, are responsible for these observations. Secondly, computer simulations of flow and fiber orientation are shown. The agreement with the actual data is good, except in the case of the core for thin plaques. The limitations that have to be resolved come not only from the standard fiber orientation equations, but also from the flow kinematics computation.     

钢纤维轻骨料混凝土抗冻性试验研究

我国混凝土结构量大面广,自建国以来兴建了大量的混凝土建筑物.但是,随着运行时间的增长,混凝土建筑物的老化问题日益突出,即混凝土结构的耐久性问题.本文对钢纤维轻骨料混凝土的杭冻融性能进行了试脸研究.冻融试验分为两种强度等级(LC30,LC35),LC35冻融循环25次、50次、75次;LC30冻融循环50次、100次.以试脸数据为依据,分析了冻融循环次数、混凝土强度子级、钢纤维体积率等因素对轻骨料混凝土冻融后抗压强度及其质量损失的影响规律.研究结果表明,随着钢纤维休积掺量的增加,混凝土的抗冻性能也有所提高.     

Flexural behavior of fiber and nanoparticle reinforced concrete at high temperatures

In this study, the flexural tests were conducted to investigate the effects of temperature, steel fiber, nano‐SiO₂, and nano‐CaCO₃ on flexural behavior of concrete at high temperatures. The load‐deflection curves of fiber and nanoparticle reinforced concrete (FNRC) were measured both at room and high temperatures. Test results show that the load‐deflection curves become flatter, and the flexural strength, peak deflection, and energy absorption capacity decrease seriously with the increase of temperature. Both steel fiber and nanoparticles could significantly improve the flexural behavior of the concrete at room and high temperatures. The energy absorption capacity of FNRC before the peak point increases with the increase of steel fiber volume fraction. The improvement of nano‐SiO₂ on flexural strength of FNRC at high temperature is better than that at room temperature, but the enhancement on energy absorption capacity is reverse. Nano‐SiO₂ is more effective than nano‐CaCO₃ in improving flexural behavior of concrete both at room and high temperatures.     

The macro- and micro properties of cement pastes with silica-rich materials cured by wet-mixed steaming injection

This research used cement pastes with a low water/blaine ratio (W/b=0.27). Rice husk ashes (RHA) burned at 700 and 850 °C, silica fume, silica sand (Ottawa standard sand), etc., were the added ingredients. Wet-mixed steam injection (WMSI) was at five different temperatures: 65, 80, 120, 150 and 180 °C. We investigated cement pastes with added silica-rich materials. For different WMSI temperatures and times, we explored the relations between compressive strength, hydration products, and pozzolanic reaction mechanism. From scanning electron microscopy (SEM) and EDS, we know that hydration products become very complicated, depending on the WMSI temperatures and times. It is difficult to determine the direct effects on the strength based on changes in the products. Experimental results, however, clearly showed that the compressive strength was worst for 80 °C and best for 180 °C. High-temperature WMSI is best with 4-h presteaming period and 8-h retention time. Curing in saturated limewater for 28 days did not increase the strength. The three types of silica-rich materials used in this research all participated in the reaction during high-temperature WMSI; they helped to increase the strength. Addition of Ottawa standard sand resulted in the best strength, followed by addition of RHA, while addition of silica fume was worse than the others. Specimens treated with high-temperature WMSI would swell slightly if they were placed in air. This was different from normal-temperature curing.     

钢纤维与仿钢纤维增强混凝土性能研究

本文通过在混凝土中掺加不同种类、不同掺量的钢纤维和仿钢纤维,研究其对混凝土的工作性能、抗压强度、抗折强度及抗裂性能的影响。研究表明：钢纤维和仿钢纤维均可以显著提高混凝土的抗折强度,使混凝土在受到力学破坏后的试件形态完整度更好,并且在混凝土受力变形后起到的桥接作用,混凝土没有出现完全的断裂和折断。纤维混凝土综合力学性能优异顺序为：端钩型钢纤维>粗合成仿钢纤维>铣削型钢纤维>竹节型仿钢纤维,在应用中应综合考虑,选择合适的纤维种类。