Chengsheng Ouyang

Fiber-matrix interaction in microfiber-reinforced mortar

Abstract The tensile properties of mortal reinforced with polyvinyl alcohol (PVA) fibers were studied in this article. Specimens reinforced with varying volume fractions of different-sized fibers were tested in direct tension. Pull-out tests were also conducted to evaluate the interfacial properties between the PVA fiber and mortar matrix. In addition, scanning electron microscopy was used to study the failure mechanism of the fibers in the direct tension test specimens. To study fiber distribution, images of the specimen cross-sections obtained with an optical microscope were analyzed. It was found that if the influence of fiber length is taken into account, the first peak stress can be expressed as a unique function of the measured fiber distance, regardless of fiber volume fraction and diameter.

A METHOD TO PREDICT SHRINKAGE CRACKING OF CONCRETE

A theoretical model based on nonlinear fracture mechanics is developed for predicting transverse cracking of a concrete ring specimen caused by drying shrinkage. Using the measured material fracture parameters, fracture resistance curve of the ring specimen is determined. The maximum allowable tensile strain is then calculated based on energy balance during shrinking of the ring. Age at transverse cracking of the ring specimen caused by restrained drying shrinkage is predicted by equating the difference between the measured free shrinkage and the estimated creep to the maximum allowable tensile strain. The predicted age at cracking is in reasonable agreement with the experimental measurement.

Toughening of high strength cementitious matrix reinforced by discontinuous short fibers

Abstract High strength cementitious materials are more brittle than conventional cement-based materials. The brittleness of high strength cementitious materials can be reduced by using fibers. Toughening of high strength cementitious matrices reinforced by discontinuous short fibers is studied in this paper using nonlinear fracture mechanics. An R-curve-based approach is proposed to describe the matrix toughening due to fiber reinforcement. The effect of fiber bridging is modeled by a closing pressure which depends on length, diameter, and volume fraction of fibers, and the fiber-matrix interfacial bond. Results predicted by the proposed R-curve approach match quite well with experimental data from different studies. It is found that use of fibers in relatively high volume fractions not only reduces the brittle natuer of high strength cemenitious matrices, but also increases the maximum matrix strength.

Modeling of fiber toughening in cementitious materials using an R-curve approach

This paper presents the theoretical formulation describing the role of fibers in enhancing the fracture toughness of quasi-brittle cement based materials. The formulation is based on the well known R-curve approach which correlates the increase of the apparent fracture toughness of a material with the existence of a pre-critical stable crack growth region.By assuming that the critical crack length in plain matrix is a function of an initial crack length a0, a formulation for the R-curves has recently been derived and applied to predict the response of positive and negative geometry specimens of various sizes and materials. This approach is further applied to uniaxial tensile specimens containing various fiber types. Fiber reinforcement is modeled by means of applying closing pressure on crack surfaces resulting in closure of the crack faces and a decrease in the stress intensity factor at the tip of the propagating crack. Incorporation of these two factors in the energy balance equations for crack growth results in increases in both the slope and the plateau value of the R-curve representing matrix response. Enhancement in material response is shown to occur only if precritical crack growth exists, causing fibers to convert the stable cracking process into an increase in load carrying capacity of the material. Fracture response of fiber reinforced composites can be predicted up to the bend-over-point. The theoretical predictions are compared with the experimental results of cement-based composites containing unidirectional, continuous glass, steel or polypropylene fibers.

FRACTURE ENERGY APPROACH FOR PREDICTING CRACKING OF REINFORCED CONCRETE TENSILE MEMBERS

A fracture energy approach based on nonlinear fracture mechanics is proposed to predict cracking of reinforced concrete members subjected to tension. In the proposed model, concrete is considered as a quasi-brittle material, and its cohesive nature in the fracture process was taken into account by a rising fracture resistance curve (R-curve). To predict fracture response of a reinforced concrete tensile member, the fracture energy required for crack propagating in the corresponding plain concrete member with the same dimension and material was first evaluated using an R-curve approach. The strain, debonding, and sliding energy on the debonded interface of steel bars and concrete were then calculated. By balancing these energies, cracking behavior of the reinforced concrete member can be predicted. The proposed approach shows a good agreement with experimental results reported in different studies. The influence of size on cracking is discussed. A closed form solution is derived to predict the minimum reinforcement ratio for tensile members, and this minimum reinforcement ratio is shown to depend on the size of the members.

Effects of clays on fracture properties of cement-based materials

The effects of alumino-silicate clays on the fracture properties of cement-based materials are reported in this paper. Two clays, illite and kaolinite, were used to replace 20% of Type I portland cement in different mixtures. Pore size distributions of hardened specimens were measured using mercury intrusion porosimetry, and compressive and fracture tests were conducted. It was found that when alumino- silicate clays are incorporated as substitutes for a fraction of the cement, the total porosity of hardened mixtures increases compared to that of hardened cement paste with no clay addition. The increase occurred primarily in the fraction of finer pores. However, the addition of clays decreased porosity for mortars. The addition of illite clay in cement-based mortars slightly decreased the compressive strength and the critical stress intensity factor (K(sub IC)), but increased the compressive toughness and the critical crack tip opening displacement (CTOD(sub C)). The replacement of 20% cement with 12% silica fume and 8% illitic clay increased the values of K(sub IC) , CTOD(sub C) and the compressive strength. These results indicate that appropriate use of silica fume and alumino-silicate clays may make cement-based materials stronger and more ductile. (A)

Response of Reinforced Concrete Panels under Uniaxial Tension

This paper summarizes the results of an experimental project aimed at improving the current understanding of the response of cracked concrete. Twenty-three reinforced concrete panels of normal and high-strength concrete were tested in uniaxial tension. Three variables were examined : 1) reinforcement ratio ; 2) reinforcement (reinforcing bar) distribution ; and 3) concrete strength. Average stress-strain curves were recorded for all panels. Stress-strain curves of the reinforcing bars were determined by testing tensile coupons. Matrix strength at first cracking was calculated. Average contribution of the concrete matrix was obtained by subtracting the contribution of reinforcing bars from the response of the panels. Additional information on the cracking behavior in the panels was obtained from the measurements of the crack mouth opening displacement ofa single crack The matrix strength at first cracking was found to increase with decreasing spacing of reinforcing bars, but was independent of the reinforcement ratio. Reinforcing bar spacing did not influence the average contribution of concrete. For high-strength concrete panels, the concrete contribution decreased with increasing reinforcement ratio. Compared to normal-strength concrete, the average contribution of high-strength concrete was found to be higher during the multiple cracking phase, but decreased faster with increasing strains.

DESIGN OF HYBRID-FIBER REINFORCEMENT FOR SHRINKAGE CRACKING BY CRACK WIDTH PREDICTION

ABSTRACT The use of fiber reinforcement to reduce shrinkage cracking is becoming increasingly common. The effectiveness of fiber reinforcement can be enhanced using multiple types of fibers. A means of predicting the shrinkage performance of hybrid-fiber reinforcement would facilitate its design. A model to predict shrinkage crack widths in restrained ring shrinkage tests is being developed. The ability of the fibers to transfer stress across a crack and free shrinkage behavior are used to determine crack width. To facilitate calibration of the model, a procedure for establishing tensile performance from flexural test data using fracture mechanics has been developed. The crack width model and the procedure for predicting tensile performance are presented. The initial phase of calibrating the model, verification of the tensile performance prediction, is discussed.

Fracture behavior of cement-based materials

The characterization of fracture behavior is a continuing challenge to the cement and concrete community. The performance of a material can be evaluated by its stress-strain response. For an ideally brittle material, elastic response is terminated when stress suddenly drops to zero, as shown in Figure 1a. However, cement-based materials are considered quasi-brittle because they respond nonlinearly prior to peak stress, and their stress gradually decreases after reaching a peak, as indicated in Figure 1b. To make cement-based materials stronger and tougher, one needs to understand the fracture mechanisms associated with nonlinear stress-strain behavior and to characterize material fracture properties based on these fracture mechanisms. Three novel techniques are being used at the Center for Advanced Cement-Based Materials (ACBM) to detect the quasi-brittle nature of cement-based materials. These three techniques are laser holographic interferometry, acoustic emission, and microscopic surface analysis. This article summarizes both the fracture mechanisms in cement-based materials and the application of the three techniques to characterize and measure fracture properties.