Experimental evaluation of fiber reinforced concrete fracture properties

Concrete is now universally recognized a construction material vital and essential for the regeneration and rehabilitation of the infrastructure of a country. The last few decades have now shown that high strength concrete, with a compressive strength of 100–120 MPa can be readily designed and manufactured. There have also been several advances made in the development of fiber reinforced concrete to control cracking and crack propagation in plain concrete, and to increase the overall ductility of the material. However, there are now many types of fibers with different material and geometric properties, and the exact fracture behavior of fiber reinforced concrete materials is not clearly understood. The overall aim of this paper is to establish the fracture properties and fracture behavior of concrete containing two widely used types of fibers, namely, steel (high modulus) and polypropylene (low modulus). The experimental investigation consisted of tests on cubes and notched prismatic specimens made from plain concrete and fiber concrete with 1% and 2% of steel or polypropylene fibers. The cube tests and the three point bending tests on notched specimens were carried out according to RILEM specifications, and extensive data on their compressive and flexural tensile behavior and fracture energy were recorded and analyzed. The results obtained from the tests are critically assessed, and it is shown that fibers contribute immensely to the structural integrity and structural stability of concrete elements and thereby improve their durable service life.     

碳纤维混杂纤维混凝土抗冲击性能研究

对比美国混凝土学会的混凝土落锤冲击试验装置,自行设计了混凝土落锤弯曲冲击试验装置,进行了不同几何尺寸及弹性模量的碳纤维、钢纤维和聚丙烯纤维混凝土的抗冲击力学性能试验研究。分析比较了不同纤维几何尺寸和弹性模量、种类和掺量对混杂纤维混凝土抗冲击性能的影响和增强机理。本文通过定义混杂效应系数,可定量评价混凝土抗冲击力学性能的正混杂效应。研究表明纤维可以明显提高混凝土的抗冲击强度,其中碳纤维混杂纤维混凝土的提高幅度更为显著。     

码头面层纤维混凝土的抗冲击性能与增韧机理

在C30素水泥混凝土中分别添加聚丙烯纤维、尼龙纤维、钢纤维配制纤维混凝土,进行抗弯曲冲击试验。以初裂次数、终裂次数、冲击韧性评价混凝土抗冲击性能的优劣。试验结果表明:添加钢纤维、聚丙烯纤维、尼龙纤维混凝土的冲击韧性分别是素混凝土冲击韧性的15.1倍、3.4倍、2.7倍。结合冲击破坏断裂特征,分析了纤维混凝土承受冲击载荷作用的几种能量吸收机制,对增韧机理进行了分析。钢纤维、聚丙烯纤维、尼龙纤维混凝土,纤维拔出功消耗能量分别是纤维断裂功消耗能量的25倍、1.72倍、0.93倍。纤维的滑移、拔出、断裂是聚丙烯纤维、尼龙纤维混凝土提高抗冲击性能的主要因素,钢纤维混凝土的抗冲击能力提高幅度最大除了纤维的滑移、拔出因素外,还归功于钢纤维对裂纹面的桥接作用。     

钢纤维对超高性能混凝土抗弯力学性能的影响

为研究长、短钢纤维对超高性能混凝土（UHPC）受弯力学性能的影响,设计并制作了13组标准养护条件下的UHPC试件,其中3组为掺单一型短钢纤维,其他组均为掺混杂型钢纤维,对其进行立方体抗压及四点抗折试验。结果表明：对于掺加单一型短钢纤维的钢纤维/UHPC,钢纤维体积掺量为5vol%时,抗折强度最大,为19.98 MPa,继续增加钢纤维掺量,抗折强度反而降低;掺混杂型钢纤维的UHPC比单一型的抗折强度高,并且当长、短钢纤维体积掺量分别为2vol%和1vol%时,抗折强度达到最大,为23.55 MPa;钢纤维/UHPC的抗弯力学性能主要受长纤维的影响,短纤维影响较小;长纤维掺量对钢纤维/UHPC的抗折强度、延性以及抗弯韧性有一定影响,但是主要取决于长、短纤维的搭配,长、短纤维体积掺量最优搭配为2vol%和1vol%。     

Flexural impact response of textile-reinforced aerated concrete sandwich panels

Mechanical response of textile-reinforced aerated concrete sandwich panels was investigated using an instrumented three-point bending experiment under static and low-velocity dynamic loading. Two types of aerated concrete: autoclaved aerated concrete (AAC) and polymeric Fiber-Reinforced Aerated Concrete (FRAC) were used as the core material. Skin layer consisted of two layers of Alkali Resistant Glass (ARG) textiles and a cementitious binder. Performance of ductile skin-brittle core (TRC–AAC) and ductile skin-ductile core (TRC–FRAC) composites was evaluated in terms of flexural stiffness, strength, and energy absorption capacity. The effect of impact energy on the mechanical properties was measured at various drop heights on two different cross-sections using energy levels up to 40 J and intermediate strain rates up to 20 s^− 1. The externally bonded textile layers significantly improved the mechanical properties of light-weight low-strength aerated concrete core under both loading modes. Dynamic flexural strength was greater than the static flexural strength by as much as 4 times. For specimens with larger cross-sections, unreinforced-autoclaved AAC core had a 15% higher apparent flexural capacity. With 0.5% volume of polypropylene fibers in the core, the flexural toughness however increased by 25%. Cracking mechanisms were studied using high speed image acquisition and digital image correlation (DIC) technique.     

Residual flexural behavior of fiber reinforced concrete after heating

In this study, the effects of fire on the flexural performance and residual strength of plain and fiber reinforced concrete are investigated. Three types of concrete are tested: plain, polypropylene (PFRC) and steel fiber reinforced concrete (SFRC). Prior to the flexural test, the specimens were exposed to fire for 15, 30, 45, and 60 min on a furnace. The burnt specimens were then tested under flexural load to measure their toughness and residual strength. Results indicate the reduction of flexural strength for both plain and FRC after being subjected to fire. For FRC, the effect of fire on the flexural response depends mainly on the fiber type and fire exposure duration. For PFRC, the flexural strength is found to drop significantly for every exposure duration, while toughness is found to increase at short exposure duration and then, drop quickly after long exposure duration due to the fiber evaporation effect. For SFRC, the flexural strength and toughness are found to drop gradually for every exposure duration due to the deterioration of cement paste and reduction in bond strength. SFRC exhibits a more consistent ability to maintain load carrying capacity after long exposure to fire than PFRC.     

Flexural performance of fiber-reinforced ultra lightweight cement composites with low fiber content

This paper presents an experimental study on flexural performance of ultra lightweight cement composites (ULCC) with 0.5 vol% fibers. Low density of the ULCC is achieved by using cenospheres from coal-fired power plants as micro aggregates. Effects of shrinkage reducing admixture (SRA) and fiber types on compressive strength and flexural performance of the ULCC are investigated. ULCC with density of 1474 kg/m³, compressive strengths of 68.2 MPa, flexural strength of 8 MPa, and deflection hardening behavior can be produced. Such good performance could be attributed primarily to the SRA which reduced entrapped air in paste matrix and densified fiber–matrix interface. The improvement on the flexural performance of the ULCC depends on fibers used and bond between fibers and matrix. Improvement of the flexural performance of the steel fiber (coated with brass) reinforced ULCC due to the densification effect by SRA was more significant than that of the PE fiber reinforced ULCC.     

Material and bond properties of ultra high performance fiber reinforced concrete with micro steel fibers

For investigating the effect of fiber content on the material and interfacial bond properties of ultra high performance fiber reinforced concrete (UHPFRC), four different volume ratios of micro steel fibers (V_f = 1%, 2%, 3%, and 4%) were used within an identical mortar matrix. Test results showed that 3% steel fiber by volume yielded the best performance in terms of compressive strength, elastic modulus, shrinkage behavior, and interfacial bond strength. These parameters improved as the fiber content was increased up to 3 vol.%. Flexural behaviors such as flexural strength, deflection, and crack mouth opening displacement at peak load had pseudo-linear relationships with the fiber content. Through inverse analysis, it was shown that fracture parameters including cohesive stress and fracture energy are significantly influenced by the fiber content: higher cohesive stress and fracture energy were achieved with higher fiber content. The analytical models for the ascending branch of bond stress-slip response suggested in the literature were considered for UHPFRC, and appropriate parameters were derived from the present test data.     

Effects of steel fibers and mineral filler on the water-tightness of concrete pipes

Water-tightness of concrete and reinforced concrete pipes used to convey sewage flow of any kind is extremely important from the aspects of (1) groundwater contamination and (2) durability of the pipes. In this study, water-tightness tests were applied on plain concrete, reinforced concrete, and steel fiber concrete pipes of 500 mm diameter. Standard seepage tests were applied on many pipes of various materials and combinations with the objective of determining the pipes with the best water-tightness. Tests on plain concrete pipes whose concrete included finely ground limestone passing the no. 100 sieve (d < 0.15 mm) at an amount of 7% by dry weight of the total aggregates revealed that the water-tightness of former was 57% better than that of pipes manufactured without the filler. The water-tightness values measured on steel fiber concrete pipes with a fiber dosage of 25 kg/m³ turned out to be 47% and 15% better than those of plain concrete pipes and reinforced concrete pipes, respectively. Those findings tangibly reveal that the addition of steel fibers and mineral filler in concrete pipes improve their seepage property.     

Some of the physical and mechanical properties of cement composites manufactured from carbon nanotubes and bagasse fiber

The objective of this investigation was to evaluate physical and mechanical properties of experimental cement type panels made from multi-wall carbon nanotubes (MWCNTs) and bagasse fiber. Three levels of MWCNTs, namely 0.5 wt.%, 1 wt.% and 1.5 wt.% were mixed with 10 wt.% and 20 wt.% of bagasse fiber in rotary type mixer. Thickness swelling, water absorption, bending characteristics and impact strength of the samples were evaluated. Based on the findings in this work the water absorption and thickness swelling of the nanocomposites decreased with increasing amount of the multi-wall carbon nanotubes content in the panels from 0.5% to 1.5%. On the other hand flexural modulus and impact strength of the panels were enhanced with increased percentage of carbon nanotubes. Panels having 0.5% MWCNTs exhibited the highest impact strength. Overall dimensional stability and strength properties of the samples were adversely influenced with increased amount bagasse fiber in the samples. It appears that using lower percentage of bagasse fiber or application of heat or chemical treatment to the raw material should be considered to improve negative influence of bagasse fiber on properties of the panels.     

Repairing reinforced concrete slabs using composite layers

There are several strengthening methods for rehabilitation of RC structural elements. The efficiency of these methods has been demonstrated by many researchers. Due to their mechanical properties, using fibrous materials in rehabilitation applications is growing fast. Therefore, this study presents rehabilitation of slabs in such a way that plain concrete layers on top, on bottom, on the entire cross section are replaced by reinforced concrete layers. In order to reinforce the concrete, Polypropylene (PP) and steel fibers were used by 0.5%, 1% and 2% fiber volume fractions. Nineteen slabs were studied under flexural loadings and fibrous material effects on the initial crack force, the maximum loading carrying capacity, absorbed energy and ductility were investigated. The obtained results demonstrated that increasing the fiber volume fraction or using reinforced concrete layer on top, bottom, or at the entire cross section of the slabs not only always leads to improvement in the slab performance, but also sometimes debilitates the slab performance. Hence, this study will propose the best positioning of reinforced concrete layer, fiber volume fraction and fiber type to achieve the best flexural performance of slabs.     

Physical and mechanical characterization of Fiber-Reinforced Aerated Concrete (FRAC)

Fiber-Reinforced Aerated Concrete (FRAC) is a novel lightweight aerated concrete that includes internal reinforcement with short polymeric fibers. The autoclaving process is eliminated from the production of FRAC and curing is performed at room temperature. Several instrumented experiments were performed to characterize FRAC blocks for their physical and mechanical properties. This work includes the study of pore-structure at micro-scale and macro-scale; the variations of density and compressive strength within a block; compressive, flexural and tensile properties; impact resistance; and thermal conductivity. Furthermore, the effect of fiber content on the mechanical characteristics of FRAC was studied at three volume fractions and compared to plain Autoclaved Aerated Concrete (AAC). The instrumented experimental results for the highest fiber content FRAC indicated compressive strength of approximately 3 MPa, flexural strength of 0.56 MPa, flexural toughness of more than 25 N m, and thermal conductivity of 0.15 W/K m.     

Mechanical properties of ceramic fiber-reinforced concrete under quasi-static and dynamic compression

The research herein is made on the quasi-static and dynamic mechanical properties of ceramic fiber reinforced concrete (CRFRC for short) through the adoption of a hydraulically-driven testing system as well as a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. As test results have turned out, such quasi-static properties as compressive strength, splitting tensile strength and flexural strength of CRFRC increase with the rise in the volume fraction of fiber. Within the strain range of 20–120 s⁻¹, the effect of the axial strain acceleration on the dynamic strength of CRFRC could be ignored. Therefore, the dynamic increase ratio (DIF) derived from SHPB tests can truly reflect the dynamic enhancement of CRFRC. The dynamic strength, critical strain and specific energy absorption (SEA) of CRFRC are sensitive to the strain rate. The addition of ceramic fiber to plain concrete can significantly improve its properties—dynamic strength, critical strain and energy absorption. And also, an analysis is conducted of the mechanism for strengthening and toughening the concrete.     

Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.     

A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites

The flexural strength and ductility properties of cementitious composites (mortar) under high temperature may be significantly improved by incorporating different types of fibers. In this study, four different types of fibers are added to cement mortars with the aim to investigate their mechanical contributions to mortars under high temperature, comparatively. Polypropylene (PP), carbon (CF), glass (GF) and polyvinyl alcohol (PVA) fibers are chosen for research. These fibers are added into mortars in five different ratios (0.0%, 0.5%, 1.0%, 1.50% and 2.0%) by volume. The mortars are subjected to the following temperatures: 21 °C (normal conditions), 100 °C (oven dry), 450 °C and 650 °C. The mechanical properties investigated are flexural strength, deflection and compressive strength of the cement mortars. In addition, thin sections of mortars are investigated to obtain changes in mortar because of high temperature. It is concluded that all fiber types contribute to the flexural strengths of mortars under high temperature. However, this contribution decreases with an increase in temperature. The samples with PVA show the best flexural performance (75–150%) under high temperature. CF which does not melt under high temperature also gives high flexural strength (11–85%). The compressive strengths of the mortars reduce under high temperature or with fiber addition. The highest increase in flexural strength and the lowest decrease in compressive strength is at 0.5–1.5% for CF if all temperature conditions are taken into consideration. The optimum fiber addition ratios of the samples containing PP and GF are 0.5% by volume. And for PVA, it is between 0.5% and 1.5% by volume.     

Application of high performance polypropylene fibers in concrete lining of water tunnels

In this study, the application of high performance polypropylene fibers (HPP fibers) in concrete lining of water tunnels, was investigated experimentally. A comparison between the behavior of steel fiber reinforced concrete and HPP fiber reinforced concrete with ordinary concrete is drawn. Advantages and shortcomings of HPP fibers used for concrete lining of water tunnels are also presented.The obtained results showed that the HPP fibers were not effective in compressive strength when compared to steel fibers, but the effects of HPP fibers on tensile strength, flexural strength, toughness and energy absorption of concrete were significant. Based on the results, the effects of HPP fibers on concrete characteristics such as the flexural toughness, concrete permeability and resistance to chloride penetration were higher than those of steel fibers. The results also showed that with application of HPP fibers, durability and serviceability of the concrete linings can be improved.     

CFRC梁拉敏性试验研究

碳纤维混凝土（CFRC）是在普通混凝土中均匀加入一定形状、尺寸和掺量的短切碳纤维而构成的一种同时具有良好力学性能和机敏性能的本征型智能材料．由于碳纤维具有导电性能，把碳纤维加入混凝土中能改善混凝土的导电性。通过试验，研究了将碳纤维加入普通的钢筋混凝土梁后构件的拉敏性能，试验结果表明，CFRC构件的电阻变化与荷载、应变、挠度、裂缝发展具有相应的对应关系，并通过试验数据，回归出CFRC电阻变化与构件荷载之间的对应关系，为CFRC在结构健康监测中的应用提供了理论依据和量化关系。     

Low-velocity flexural impact response of fiber-reinforced aerated concrete

Impact response of fiber-reinforced aerated concrete was investigated under a three-point bending configuration based on free-fall of an instrumented impact device. Two types of aerated concrete: plain autoclaved aerated concrete (AAC) and polymeric fiber-reinforced aerated concrete (FRAC) were tested. Comparisons were made in terms of stiffness, flexural strength, deformation capacity and energy absorption capacity. The effect of impact energy on the mechanical properties was investigated for various drop heights and different specimen sizes. It was observed that dynamic flexural strength under impact was more than 1.5 times higher than the static flexural strength. Both materials showed similar flexural load carrying capacity under impact, however, use of 0.5% volume fraction of polypropylene fibers resulted in more than three times higher flexural toughness. The performed instrumented impact test was found to be a good method for quantifying the impact resistance of cement-based materials such as aerated concrete masonry products.     

Interlaminar shear strength of SiC matrix composites reinforced by continuous fibers at 900 °C in air

To reveal the shear properties of SiC matrix composites, interlaminar shear strength (ILSS) of three kinds of silicon carbide matrix composites was investigated by compression of the double notched shear specimen (DNS) at 900 °C in air. The investigated composites included a woven plain carbon fiber reinforced silicon carbide composite (2D-C/SiC), a two-and-a-half-dimensional carbon fiber-reinforced silicon carbide composite (2.5D-C/SiC) and a woven plain silicon carbon fiber reinforced silicon carbide composite (2D-SiC/SiC). A scanning electron microscope was employed to observe the microstructure and fracture morphologies. It can be found that the fiber type and reinforcement architecture have significant impacts on the ILSS of the SiC matrix composites. Great anisotropy of ILSS can be found for 2.5D-C/SiC because of the different fracture resistance of the warp fibers. Larger ILSS can be obtained when the specimens was loaded along the weft direction. In addition, the SiC fibers could enhance the ILSS, compared with carbon fibers. The improvement is attributed to the higher oxidation resistance of SiC fibers and the similar thermal expansion coefficients between the matrix and the fibers.     

Fiber synergy in Hybrid Fiber Reinforced Concrete (HyFRC) in flexure and direct shear

In most cases, fiber reinforced concrete (FRC) contains only one type of fiber. The use of two or more types of fibers in a suitable combination has the potential to improve the mechanical properties of concrete, and result in performance synergy. This combining of fibers, often called hybridization, is investigated in this paper under flexure and direct shear. Along with a reference plain concrete mix, several single-fiber reinforced concrete mixes and two-fiber reinforced hybrid composite mixes were cast using diverse fiber combinations. Two types of macro-steel fibers and a micro-cellulose fiber were examined. Flexural and direct shear tests were performed and the results were analyzed to identify synergy, if any, associated with various fiber combinations. The paper highlights the influence of load configuration on fiber synergy.