Seong-Cheol Lee

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.

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.

Crack Model for Steel Fiber-Reinforced Concrete Members Containing Conventional Reinforcement

This paper proposes a new model for the calculation of crack spacings and crack widths in steel fiber-reinforced concrete members containing conventional steel reinforcing bars (R/SFRC). The model considers the effects of various fiber and conventional reinforcement parameters. Predictions are compared against the test results of 17 plain reinforced concrete (RC) and 53 large-scale R/SFRC specimens subjected to uniaxial tension available in the literature. It is found that the proposed model predicts the crack spacings and widths of R/SFRC with reasonable accuracy and outperforms other steel fiber-reinforced concrete (SFRC) crack spacing models currently available. The model is expanded to include biaxial stress conditions to facilitate the analysis of elements such as SFRC panels subjected to shear. Here, too, the model is found to give sufficiently accurate predictions of the average crack conditions.

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.

Shear Behavior of Large-Scale Post-Tensioned Girders with Small Shear Span-Depth Ratio

Although there have been several studies on the shear behavior of prestressed concrete beams, these generally focused on shear capacity rather than shear deformation. The present study investigated the shear deformation of large-scale reinforced I-shaped girders and post-tensioned prestressed concrete girders with a small shear span-depth ratio of 2.5. The test variables were the compressive strength of the concrete, the stirrup ratio, and the prestressing force. This large-scale experimental study enabled the investigation of diagonal cracking behavior, namely crack zones, patterns, principal strain direction, and crack width, as well as ultimate shear capacity. This extensive information can be used to establish the formation of compressive struts and force-resisting mechanisms in shear analysis. The experimental results show that the ultimate shear capacity of concrete girders increased with an increase in the concrete compressive strength, the stirrup ratio, and the prestressing force. The effect of concrete strength in the girders with stirrups and prestressing force, however, was not much as in those without stirrup and prestressing force. The stirrup was highly effective for controlling diagonal crack width, while the prestressing force is only effective at delaying cracking load. With the comparison of the crack zones of the test girders, the stirrup ratio was revealed as the dominant factor in the arch action of a beam member with a short shear span-depth ratio.

Analysis of Steel Fiber-Reinforced Concrete Elements Subjected to Shear

In this paper, a rational analysis procedure is presented for modeling the shear behavior of steel fiber-reinforced concrete (SFRC) elements. In the development of the analysis procedure, the Disturbed Stress Field Model (DSFM), based on the Modified Compression Field Theory (MCFT), is modified by implementing constitutive models for SFRC, which are derived from the Diverse Embedment Model (DEM). For the contribution of steel fibers, a local stiffness matrix for fibers has been developed separately from those for concrete matrix and conventional reinforcement. The composite element stiffness matrix for an SFRC element with conventional reinforcement is then derived by superposing the three local stiffness matrixes. In the element stiffness matrix, the effect of shear slip at a crack is also taken into account by considering the resistance due to steel fibers against shear stress on crack surface. Through comparisons with the test results of SFRC panels previously reported in the literature, it is shown that the actual shear behavior of SFRC panels are accurately predicted by the proposed analysis procedure, not only for the shear strength but also for the shear strain at the failure. Through implementation into finite element analysis programs, the analysis procedure devel oped in this paper can be useful in the modeling of SFRC members and structures also containing conventional reinforcement.

Flexural Behavior of HPFRCC Members with Inhomogeneous Material Properties

In this paper, the flexural behavior of High-performance Fiber-Reinforced Cementitious Composite (HPFRCC) has been investigated, especially focusing on the localization of cracks, which significantly governs the flexural behavior of HPFRCC members. From four points bending tests with HPFRCC members, it was observed that almost evenly distributed cracks formed gradually, followed by a localized crack that determined the failure of the members. In order to investigate the effect of a localized crack on the flexural behavior of HPFRCC members, an analytical procedure has been developed with the consideration of intrinsic inhomogeneous material properties of HPFRCC such as cracking and ultimate tensile strengths. From the comparison, while the predictions with homogeneous material properties overestimated flexural strength and ductility of HPFRCC members, it was found that the analysis results considering localization effect with inhomogeneous material properties showed good agreement with the test results, not only the flexural strength and ductility but also the crack widths. The test results and the developed analysis procedure presented in this paper can be usefully applied for the prediction of flexural behaviors of HPFRCC members by considering the effect of localized cracking behavior.