Toughness Of Fiber Reinforced Concrete Research Articles

Applicability of CMOD to Obtain the Actual Fracture Toughness of Rightly-Cracked Fibrous Concrete Beams

Unfortunately, most of the previous work studying the fracture toughness of fibrous composites has deliberately ignored bridging the fiber onto the pre-crack/notch surfaces by creating such a crack as a through-thickness crack (TTC). Furthermore, no standard specifications for measuring the fracture toughness of fibrous composites have considered the fiber bridging through the pre-notch. Only a few pieces of research, no more than fingers on one hand, have addressed this problem by creating an actual crack, i.e., a matrix crack (MC) instead of a TTC. The challenge these researchers face is the inability to calculate the fracture toughness directly through the stress intensity factor (SIF) relationship because there is no geometry correction factor equation, f(a/d), for an MC. The main objective of the present work is to calculate f(a/d) and ascertain a relationship between the SIF and crack mouth opening displacement (CMOD) for an MC numerically using 3-D finite element analysis. An experimental program was also conducted to measure the fracture toughness of three types of concrete beams: high-strength concrete (HSC) beams with a TTC, HSC beams with an MC, and fiber-reinforced concrete (FRC) beams with an MC. The results showed that FRC beams with an MC have the highest fracture toughness and, subsequently, the highest resistance to crack growth. The numerical results revealed a suggested relationship between the SIF and CMOD of FRC beams with an MC. This relation was used to predict the fracture toughness of FRC with an MC by the critical value of CMOD measured experimentally.

Real fracture toughness of FRC and FGC: size and boundary effects

The present dilemma is how to simulate the real crack in full depth (FD) fiber-reinforced concrete (FRC), FD FRC, to get the actual fracture toughness of such fibrous composites, i.e., through-thickness pre-cracks are inappropriate for such materials. To overcome this dilemma, a new technique was adopted to create a pre-matrix crack (MC) without cutting the fibers bridging the two surfaces of the pre-crack. The main objective of the present work is to study the size and boundary effects on the real fracture toughness of MC-FD FRC and functionally graded concrete (FGC). Forty-eight MC-FD FRC and MC-FGC beams with three different span to depth ratios L/d equal 4, 5, and 6, and three different beam depths of the same beam span have been tested under three-point bending. All beams have the same pre-MC length to beam depth ratio (ao/d) of 1/3. Hooked end steel fibers of 1% fiber volume fraction produced FRC. FGC beams consist of three equal layers, FRC layer at the tension side, normal strength concrete layer at the middle of the beam, and high strength concrete layer at the compression side. The applied load versus all beams' crack mouth opening displacement (CMOD) curves have been analyzed. The present load/CMOD results showed that beams having constant L/d ratios are recommended to capture independent size effect parameters. The size effect law (SEL) and boundary effect model (BEM) are good candidates to predict the size effect. According to the maximum non-damaged defect concept, the SEL is more reliable in predicting MC FD FRC fracture toughness than BEM.

Persistence of Strength/Toughness in Modified-Olefin-Fiber- and Hybrid-Fiber-Reinforced Concrete

Abstract This paper investigates the flexural performance of modified-olefin-fiber- (MOF) and hybrid-fiber-reinforced concrete. The standards and methodologies that are commonly used to evaluate the flexural performance of fiber-reinforced concrete are also discussed. Persistence of strength/toughness (PST) was introduced to evaluate the safety and economics of fiber-reinforced concrete. By comparing the modulus of rupture and the limit of proportionality, the PST can be easily and qualitatively obtained. This method agrees with the toughness index. PST can serve as an excellent supplement to the traditional standards and methods that evaluate the strength and toughness of fiber-reinforced concrete. The test results demonstrate that using hooked steel fibers increased the flexural strength and toughness most. The hybridization of the MOFs and the hooked steel fibers, and the addition of MOFs improved the PST.

Experimental study on staggered lapped bars in fiber reinforced concrete beams

The paper presents experimental results on lap splices in Fiber Reinforced Concrete (FRC). Several full-scale beams reinforced with rebars of different diameters were tested with all or part of the longitudinal reinforcement lap spliced at mid-span. A traditional volume fraction of hooked steel fibers (equal to 0.5%) was adopted for FRC. Experimental results show that the toughness of FRC can enhance the behavior of the weak joint with only part of the rebars lapped at the section. The capability of FRC to control the flexural and splitting crack opening and propagation can increase the strength of staggered lapped splices, thus allowing a reduction of lap length.

Effect of Mixing Method on Mechanical Properties of Fiber Reinforced Concrete

Fiber reinforced concrete (FRC) has been successfully used to enhance the flexural toughness of concrete. As fibers are randomly oriented in FRC, they sometimes produce clumps that reduce the mechanical performance, and a properly chosen mixing protocol can be a way to minimize this problem. In this research, the effects of mixing method on the mechanical properties of FRC were investigated. The compressive strength, flexural strength, and flexural toughness were measured using three different mixing methods. It was shown from the results that the compressive strength and peak flexural load were not affected by changes in mixing method. However, in terms of flexural toughness, the changes in mixing method clearly affected the flexural toughness of FRC. The truck-mixed FRC outperformed two pan-mixed FRCs.

High-rate tensile behavior of steel fiber-reinforced concrete for nuclear power plants

The direct tensile behavior of fiber-reinforced concrete (FRC) at high strain rates were investigated for their potential to enhance the resistance of the containment building of nuclear power plants (NPPs) against aircraft impact. Two types of deformed steel, hooked (H) and twisted (T) fibers were employed. To improve the tensile resistance of FRCs even at higher rates by adding more fibers, the mixture of concrete was modified by either increasing the sand-to-coarse aggregate ratio or decreasing the maximum size of coarse aggregate. All FRC specimens produced two to six times greater tensile strength and one to five times higher toughness at high strain rates (4–53s−1) than those at a static rate (0.000167s−1). T-fiber generally produced higher tensile strength and toughness than H-fiber at both static and high rates. Although both fibers showed favorable rate sensitivity, T-fiber produced much greater enhancement, at higher strain rates, in tensile strength and slightly lower enhancement in toughness than H-fiber. As the maximum size of coarse aggregate decreased from 19 to 5mm, the tensile strength and toughness of FRCs with T-fibers noticeably increased at both static and high strain rates.

Experimental study of dynamic compressive properties of fibre reinforced concrete material with different fibres

This research conducts drop weight impact tests to study the dynamic compressive properties of fibre reinforced concrete (FRC) material with different types of fibres. The impact tests are conducted with an instrumented drop-weight impact system consisting of a hard steel drop weight, two 180 t fast response loadcells, a high-speed video camera, and a fast response data acquisition system. Seven fibre types with different shapes and material properties are considered in the study. They are synthetic fibres, undulated, cold rolled, flattened, hooked end, and two new spiral shape steel fibres developed in this study. A volume fraction of 1% fibre is used in all specimens. The concrete matrix for all FRC specimens is mixed to obtain a compressive strength of 35 MPa. The drop-weight impact experiments are conducted with two different drop heights in order to study the dynamic material properties at different strain rates. The impact forces on top and bottom of specimens are measured to investigate the axial inertia effects and the stress wave propagation effect. The high-speed video camera is used to capture the failure process, displacement and velocity responses of specimens, which are used to estimate the strain and strain rates of the specimen under impact loading. Strain gages are also used for direct strain measurements. The dynamic stress–strain relations and impact resistance of the tested specimens are compared. The influence of fibre shapes on the failure modes, strength and energy absorbing capability of FRC is discussed. The rate sensitivities of the compressive strength, Young’s modulus and toughness of FRC are also examined. The testing results demonstrate that the new spiral steel fibre proposed in this study provides better confinement to concrete matrix and thus better bonding to concrete material, therefore increases the dynamic resistance and energy absorption capacity (toughness) of FRC.

Impact resistance of fibre-reinforced concrete

Fibre reinforcement increases the toughness of plain concrete under static compressive loading. The compressive toughness of fibre-reinforced concrete (FRC) under impact loading has not, however, previously been investigated. The current paper reviews the behaviour of high-strength fibre-reinforced concrete under uniaxial compressive impact loading. An instrumented drop weight impact machine was used to carry out compressive impact tests on various FRC systems with compressive strengths ranging from about 60 MPa to 120 MPa. The deformations of the FRC cylinders were determined using a high-speed video camera system. As expected, the compressive strength was found to increase with increasing drop height (or impact velocity). The dynamic compressive toughness was also found to increase with increasing drop height and with increasing matrix strength. It was observed that the mode of failure of the FRC was dependent upon the properties of the matrix and of the fibres, as well as on the drop height. Thus, the dynamic compressive toughness of FRC appeared to be dependent on the constitutive behaviour of the matrix, the fibre type and volume, the impact velocity and the mode of failure.

Toughness enhancement in steel fiber reinforced concrete through fiber hybridization

Crimped steel fibers with large diameters are often used in concrete as reinforcement. Such large diameter fibers are inexpensive, disperse easily and do not unduly reduce the workability of concrete. However, due to their large diameters, such fibers also tend to be inefficient and the toughness of the resulting fiber reinforced concrete (FRC) tends to be low. An experimental program was carried out to investigate if the toughness of FRC with large diameter crimped fibers can be enhanced by hybridization with smaller diameter crimped fibers while maintaining workability, fiber dispersability and low cost. The results show that such hybridization indeed is a promising concept and replacing a portion of the large diameters crimped fibers with smaller diameter crimped fibers can significantly enhance toughness. The results also suggest, however, that such hybrid FRCs fail to reach the toughness levels demonstrated by the smaller diameter fibers alone.

A New Test Method for Fiber-Reinforced Concrete

Abstract It has long been recognized that the principal benefit gained by using fiber reinforcement in portland cement concrete is a conversion of the material from brittle to relatively ductile behavior. The apparent ductility, or toughness, is primarily related to the improved tensile strength of fiber-reinforced concrete (FRC) especially and even after significant matrix cracking has occurred. However, the methods used to measure tensile strength and toughness of FRC have been, at best, elaborate and controversial. A practical and much needed testing methodology for fiber-reinforced concrete has been developed and adopted as an ASTM standard. It was conceived from the need to have a test that is relatively simple and inexpensive to conduct and is yet capable of assessing the tensile strength of cracked FRC. The test method, ASTM C 1399, and an interlaboratory testing program conducted to help formulate a precision statement for the new methodology is discussed.

Concrete and Fiber Reinforced Concrete Subjected to Impact Loading

ABSTRACTDespite its extensive use, low tensile strength has been recognized as one of the major drawbacks of concrete. Although one has learned to avoid exposing concrete structures to adverse static tensile load, these cannot be shielded from short duration dynamic tensile stresses. Such loads originate from sources such as impact from missiles and projectiles, wind gusts, earthquakes and machine vibrations. The need to accurately predict the structural response and reserve capacity under such loading has led to an interest in the mechanical properties of the component materials at high rates of straining.One method to improve the resistance of concrete when subjected to impact and/or impulsive loading is by the incorporation of randomly distributed short fibers. Concrete (or Mortar) so reinforced is termed fiber reinforced concrete (FRC). Moderate increase in tensile strength and significant increases in energy absorption (toughness or impact-resistance) have been reported by several investigators in static tests on concrete reinforced with randomly distributed short steel fibers. A theoretical model to predict fracture toughness of FRC is proposed. This model is based on the concept of nonlinear elastic fracture mechanics.As yet no standard test methods are available to quantify the impact resistance of such composites, although several investigators have employed a variety of tests including drop weight, swinging pendulums and the detonation of explosives. These tests though useful in ascertaining the relative merits of different composites do not yield basic material characteristics which can be used for design.The author has recently developed an instrumented Charpy type of impact test to obtain basic information such as load-deflection relationship, fracture toughness, crack velocity and load-strain history during an impact event. From this information, a damage based constitutive model was proposed. Relative improvements in performance due to the addition of fibers as observed in the instrumented tests are also compared with other conventional methods.