Jae-Yeol Cho

Diverse Embedment Model for Steel Fiber-Reinforced Concrete in Tension: Model Development

In this paper an analysis model is presented for calculating the response of steel fiber-reinforced concrete (SFRC) members subjected to tension. To predict the tensile stress of fibers across a crack, the pullout behavior of a single fiber with both sides embedded in cracked concrete is analytically investigated, considering both frictional bond behavior and mechanical anchorage effects. Thus, the proposed Diverse Embedment Model (DEM) can be applied to end-hooked and straight fibers. The model is derived with consideration given to all possible fiber orientations and embedment lengths and as influenced by the member’s finite dimensions. The details of the experimental verification for the proposed analysis model, including the proposed fiber orientation factors, are presented and discussed in an accompanying paper.

Tension-Stiffening Model for Steel Fiber-Reinforced Concrete Containing Conventional Reinforcement

The tensile behavior of fiber-reinforced concrete (FRC) members co-reinforced with conventional deformed reinforcing bar (R/FRC members) is analytically investigated in regards to tensile stresses developed in the reinforcing bars, tensile stresses induced in the steel fibers bridging a crack, and the bond mechanism between the reinforcing bar and the concrete matrix. A tension-stiffening model for R/FRC members is developed through an analytical parametric study using a crack analysis procedure that considers the tensile behavior due to the steel fibers and the bond stress-slip relationship between the reinforcing bar and the concrete matrix. With the proposed model, the local yielding of reinforcing bars at a crack can be realistically simulated, enabling reasonably accurate predictions of the tensile behavior of R/FRC members. Analysis results obtained from the proposed model show good agreement with the test results measured by previous researchers.

Inelastic analysis of reinforced concrete columns with short lap splices subjected to reversed cyclic loads

A common deficiency in reinforced concrete columns built in the early 1970s or before is the detailing of lap splices. These splices are often short, poorly confined, and located immediately above the floor level where large inelastic demands can be expected during seismic actions. In this paper, an analytical model for estimating the seismic response of reinforced concrete columns with short lap splices is presented. The model is based on a local bond stress-slip relationship to estimate the bar stress and deformation at splice failure and includes degradation of stiffness and strength with increasing deformation amplitude and with the number of reversed cyclic lateral deformations. The proposed analysis procedure is validated against experimental data from cyclic loading tests on reinforced concrete columns with typical construction details of the early 1970s. The results show that. the strength of short lap splices can be predicted well using local bond-slip models derived from isolated anchored bars. Also, the calculated failure mode, lateral load resistance, and deformation corresponding to the splice failure were in good agreement with the measured values. A simple equation for calculating the bar stress at splice failure is presented as well. Use of the proposed equation resulted in excellent agreement between the measured and calculated strength at splice failure.

Compressive Behavior of Fiber-Reinforced Concrete with End-Hooked Steel Fibers

In this paper, the compressive behavior of fiber-reinforced concrete with end-hooked steel fibers has been investigated through a uniaxial compression test in which the variables were concrete compressive strength, fiber volumetric ratio, and fiber aspect ratio (length to diameter). In order to minimize the effect of specimen size on fiber distribution, 48 cylinder specimens 150 mm in diameter and 300 mm in height were prepared and then subjected to uniaxial compression. From the test results, it was shown that steel fiber-reinforced concrete (SFRC) specimens exhibited ductile behavior after reaching their compressive strength. It was also shown that the strain at the compressive strength generally increased along with an increase in the fiber volumetric ratio and fiber aspect ratio, while the elastic modulus decreased. With consideration for the effect of steel fibers, a model for the stress–strain relationship of SFRC under compression is proposed here. Simple formulae to predict the strain at the compressive strength and the elastic modulus of SFRC were developed as well. The proposed model and formulae will be useful for realistic predictions of the structural behavior of SFRC members or structures.

Diverse Embedment Model for Steel Fiber-Reinforced Concrete in Tension: Model Verification

Results obtained from the Diverse Embedment Model (DEM) for analysis of steel fiber-reinforced concrete, described in an accompanying paper, are compared with experimental results produced by several independent researchers. Variation of the fiber orientation factor, which accounts for the effects of finite member dimensions on fiber orientation and embedment, is also theoretically investigated and compared with experimental data. Verification studies show that the proposed model provides accurate predictions of the tensile stress and crack width relationship of uniaxial tension specimens containing straight or end-hooked steel fibers. In addition, the proposed model provides accurate calculations of the distribution of tensile stress provided by fibers. The proposed model is also shown to be useful in modeling aspects of the tensile behavior, such as crack spacing and tension stiffening, of fiber-reinforced concrete (FRC) members reinforced with ordinary steel reinforcing bars.

Bond-Slip-Strain Relationship in Transfer Zone of Pretensioned Concrete Elements

In this paper, a novel bond-slip-strain relationship for a strand in the transfer zone of a pretensioned concrete member is presented. To draw a relationship among the bond stress, strand strain, and slip along the transfer zone, pretensioned, prestressed concrete members were fabricated, and strand strains were measured from electrical resistance steel gauges attached on the strand during prestressing force release. The test variables were the compressive strength of the concrete, the size of the cross section, the cover thickness, the diameter of the strand, the curing method, and the debonded region. Estimates obtained from the proposed model were in good agreement with the test results from other studies, as well as those from this work. Using the advantage of the strain term in the proposed formulation, the applicability of the derived model to fiber-reinforced polymer tendons was also evaluated.

STRESS-STRAIN RELATIONSHIP OF REINFORCED CONCRETE SUBJECTED TO BIAXIAL TENSION

This paper summarizes results from a research study on the tensile stress-strain relationship of reinforced concrete (RC) membrane elements. Extensive tests of RC panels with varying reinforcement ratios were performed to derive a reasonable constitutive law of the tensile stress-strain relationship of RC. Test results show that concrete carries substantial tensile stress even after initial cracking. The stress-strain relationship of concrete in biaxial tension was derived using the behavior and theory of RC members subjected to uniaxial tension. However, the application of this proposed stress-strain relationship for concrete is limited to cases where the direction of reinforcement coincides with the direction of the applied principal stresses.

Fiber Orientation Factor on Rectangular Cross-Section in Concrete Members

In order to predict the post-cracking tensile behavior of fiber reinforced concrete, it is necessary to evaluate the fiber orientation factor which indicates the number of fibers bridging a crack. For investigation of fiber orientation factor on a rectangular section, in this paper, dog-bone fiber reinforced concrete specimens were prepared with the variables of concrete compressive strength, rectangular cross-section size, fiber type, and fiber volumetric ratio. After direct tension tests, the fiber orientation factor could be evaluated through counting the number of fibers on a crack. From the test results, it was investigated that the fiber orientation factor was larger than 0.5 which is generally adopted for large members, as fibers distribution is affected by the specimen size. For rational prediction of the fiber orientation factor on a rectangular concrete section, a simple model was derived from the Diverse Embedment Model (DEM), which is a rigorous model to predict the tensile behavior of steel fiber reinforced concrete. From the comparison of the measured data and the predicted values, it was found that the actual fiber orientation factor was well predicted by the proposed model.

Simplified Diverse Embedment Model for Steel Fiber- Reinforced Concrete Elements in Tension

A simplified version of the Diverse Embedment Model (DEM) for steel fiber-reinforced concrete (SFRC) is derived by eliminating the double numerical integration, which complicates the calculation procedure of the DEM. To simplify the DEM, fiber slip on the longer embedded side is not considered in the calculation of the fiber tensile stress at a crack, while coefficients for frictional bond behavior and mechanical anchorage effect are incorporated to prevent overestimation of the tensile stress attained by fibers. The tensile stress behavior of SFRC predicted by the Simplified DEM (SDEM) shows good agreement with that obtained from the DEM; hence, the model’s accuracy has largely been retained despite the simplification. In comparisons with test results reported in the previous literature, the SDEM is shown to simulate not only the direct tensile behavior but also the flexural behavior of SFRC members. The SDEM can easily be implemented in currently available analysis models so that it can be useful in the modeling of structural behavior of SFRC members or structures.

Tension Stiffening of Reinforced High Performance Fiber Reinforced Cementitious Composites (HPFRCC)

To overcome weak and brittle tensile characteristics of concrete, many studies have been conducted on fiber reinforced concrete (FRC). Recently, high performance fiber reinforced cementitious composites (HPFRCC), which shows strain hardening behavior, has been actively investigated. However, most of the studies focused on the material behavior of HPFRCC itself. Only a few studies have been conducted on the tensile behavior of HPFRCC with steel reinforcement. Therefore, a tension stiffening test for HPFRCC members has been conducted in this study in order to investigate the effect of a reinforcing bar on the tensile behavior of HPFRCC. Tensile stress-strain relationship of HPFRCC has been derived from the tests. The HPFRCC resisted tensile stress continuously from the first cracking to the yield of reinforcing bar. Through the comparison with the tensile behavior of HPFRCC members without a reinforcement, it was shown the tensile strength and capacity of HPFRCC were reduced due to the combined effect of the high shrinkage of HPFRCC, restraining effect of steel reinforcement, and the strain hardening behavior of HPFRCC. It is expected that the tension stiffening test results can be useful for an application of HPFRCC with steel reinforcement as structural members.