Woo-Suk Kim

Shear Testing of Steel Fiber-Reinforced Lightweight Concrete Beams without Web Reinforcement

This study investigates the effect of steel fibers on the shear strength of lightweight concrete beams without web reinforcement. Twelve beams were tested under four-point loads, including three normalweight steel fiber-reinforced concrete (SFRC) beams and six steel fiber-reinforced lightweight concrete (SFRLC) beams. The other variables include the shear span-depth ratio (a/d) (2, 3, and 4) and steel-fiber volume fraction (Vf = 0, 0.5, and 0.75%). Results show that the addition of steel fibers with Vf of 0.75% increased the shear capacity by 30% and promoted a ductility of 5.3 or higher. The findings also indicate that the a/d adversely affects the shear capacity. The compressive strength of SFRLC was increased by 13% for Vf =0.5% and 20% for Vf=.75%, indicating that the effectiveness of steel fibers is greater in lightweight concrete than in normal weight concrete. The shear capacity of the SFRC beam was greater than that of the SFRLC beam at a given deflection due to the increased material properties of SFRC. A design shear strength model for SFRLC beams without web reinforcement is proposed based on these results.

Shear-Flexure Coupling Behavior of Steel Fiber-Reinforced Concrete Beams

Although steel fiber-reinforced concrete (SFRC) has become a popular choice in construction due to its high-performance properties, the shear-flexure coupling behavior of SFRC beams has not yet been studied adequately. This study uses generalized modeling techniques to investigate shear-flexure coupling behavior of SFRC beams. Twelve SFRC beams were tested under low-to-high shear-to-moment ratios and with different quantities of longitudinal reinforcement and steel fibers. The study included both normal- and high-strength concrete with steel fibers. The results from the tests were used to validate the nonlinear modeling techniques developed for evaluating shear-flexure coupling effects in SFRC beams. Three different cases of transverse normal stress or strain profiles in the beam shear span were used in the analytical model, depending on the presence and extent of yielding of the longitudinal steel bars. The findings suggest that the shear-flexure interaction model accurately predicted the degradation associated with the shear behavior and ductility of SFRC beams. Directions for future research are discussed.

Prediction of Flexural Capacities of Steel-Fiber Reinforced Concrete Beams

The results of previous tests by many researchers have been compiled to evaluate the flexural strength of steel-fiber reinforced concrete beams. Existing prediction equations for flexural strength of such beams were examined, and a new equation based on mechanical and empirical observations, was proposed. In other words, the constitutive models for steel fiber reinforced concrete(SFRC) were proposed, which incorporate compressive and tensile strength. A steel model might also exhibit stain-hardening characteristics. Predictions based on the model are compared with the experimental data. For the collection of tests, a variation of the Henager equations, modified to apply to fiber-reinforced concrete beams, provided reliable estimates of flexural strength. The proposed equations accounted for the influence of fiber-volume fraction, fiber aspect ratio, concrete compressive strength and flexural steel reinforcement ratio. The proposed equations gave a good estimation for 129 flexural specimens evaluated.

An Experimental Study of Reinforced Concrete Beams with Closely-Spaced Headed Bars

The use of headed bars as opposed to standard 90- or 180-degree hooked bars in beam ends, beam-column joints or other steel congested areas for anchorage and bond has become more favorable due to the fact that steel congestion is often created by large bend diameters or crossties. This research mainly focuses on evaluating the code provisions regarding the use of headed bars. Nine simply supported rectangular concrete beams with headed longitudinal reinforcement were tested under a four-point monotonic loading system. The design clear spacing, which varies from 1.5 to 4.25 times the bar diameter, was the only parameter for the experimental investigation. The test results showed that the closely-spaced headed bars were capable of developing to full yield strength without any severe brittle concrete breakout cone or pullout failure. Bond along the bar was not sufficient due to the early loss of concrete integrity. However, the headed bars were effective for anchorage with no excessive moment capacity reduction. This implies that the clear spacing of about 2 times the bar diameter for headed bars may be reasonable to ensure the development of specified yield strength of headed bars and corresponding member design strength.

Evaluation of flexural strength prediction of reinforced concrete beams with steel fibres

Abstract This paper presents experimental programme to investigate the effects of steel fibres on flexural strength of steel fibre-reinforced concrete (SFRC) beams and proposes a prediction model to evaluate flexural strength of beams which is their main contribution to literature. Four point bending test was carried out on the 11 SFRC beams which are 6 normal-strength SFRC beams and 5 high-strength SFRC beams. The variables studied in the investigation were steel fibre volume fractions (0, 0.75 and 1.50%), flexural reinforcement ratios (1.0 and 2.0%), and concrete compressive strengths (28 and 56 MPa). The test results confirmed that ultimate flexural strength increased with increasing fibre volume contents. The constitutive model for SFRC was proposed, which incorporated strength of fibre-reinforced concrete and strain-hardening of tensile reinforcement. Predictions of moment–curvature relationship based on the model matched well with the experimental data. The results of previously reported tests were combined with the results of new tests to document observed flexural failures of 129 SFRC beams. A simplified, empirical design equation to determine the flexural strength of SFRC beam was proposed and compared with existing equations. The analysis indicated that the proposed equation provided the most accurate estimates of flexural strength.

A Study on the Shear Behavior of Recycled Aggregate Reinforced Concrete Beams without Stirrups

Dept. of Architecture and Architectural Engineering, Seoul National University, Seoul 151-744, KoreaABSTRACT Little investigations have been carried out to study the shear behaviors of RC beams with recycled aggregates. So, this experiment investigates the shear performance and suggests the possible application of Recycled Concrete Aggregate (RCA) for building structures. In general, shear strength of reinforced concrete beam without stirrups is dependent on the compressive st rength of concrete, the longitudinal steel ratio, and the shear span-to-depth ratio. In this study, total 28 recycled aggregate concrete beams without shear reinforcement were tested by two-point load and all beams were singly reinforced. The variables studied in this investigation are shear span-to-depth ratios ( a/d=2, 3 and 4), RCA replacement ratios (0, 15, 30 and 50%) and longitudinal steel ratios (0.80, 1.27 and 1.84%). The designed concrete compressive strength with a 30 MPa is used. This research will play an important r ole toward the establishment of the structural design standard for RCA concrete.Keywords : recycled concrete aggregate (RCA), construction waste, recycling, shear strength

Behavior of Steel Fiber-Reinforced Concrete Exterior Connections under Cyclic Loads

Beam-column gravity or Intermediate Moment frames subjected to unexpected large displacements are vulnerable when no seismic details are provided, which is typical. Conversely, economic efficiency of those frames is decreased if unnecessary special detailing is applied as the beam and column size becomes quite large and steel congestion is caused by joint transverse reinforcement in beam-column connections. Moderate seismic design is used in Korea for beam-column connections of buildings with structural walls, which are to be destroyed when the unexpected large earthquake occurs. Nonetheless, performance of such beamcolumn connections may be substantially improved by the addition of steel fibers. This study was conducted to investigate the effect of steel fibers in reinforced concrete exterior beam-column connections and possibility for the replacement of some joint transverse reinforcement. Ten half-scale beam-column connections with non-seismic details were tested under cyclic loads with two cycles at each drift up to 19 cycles. Main test parameters used were the volume ratio of steel fibers (0%, 1%, 1.5%) and joint transverse reinforcement amount. The test results show that maximum capacity, energy dissipation capacity, shear strength and bond condition are improved with the application of steel fibers to substitute transverse reinforcement of beam-column connections. Furthermore, several shear strength equations for exterior connections were examined, including the proposed equation for steel fiber-reinforced concrete exterior connections with non-seismic details.

Experimental and numerical simulations of prestressed Self-Consolidating-Concrete structures subjected to non-linear deformations

This paper presents experimental and numerical simulations for investigating the flexural behaviour of five prestressed Self-Consolidating-Concrete (SCC) members subjected to non-linear deformations. In this study, flowable and non-segregating SCC was used for relief of steel congestion among the prestressed concrete members. The members consisted of four reduced-scale beams with and without steel fibres, and a full-scale Type-II AASHTO bridge girder. A total of four three-point and one four-point loading tests were performed in laboratory conditions and simulated using non-linear 2D and 3D finite element analyses. The investigation results provided a quantitative measure of the reliability of the implemented theoretical models.

Characterization of air-cooled heat transfer using round and slot nozzles

Many industrial facilities, such as the widely used cooling tower, transfer heat to the atmosphere. The most common cooling media are water and air. The performance of cooling equipment depends upon such factors as flow rate, pressure at the nozzle tip, and the type of nozzle. In this article, round and slot nozzles with air as the cooling medium are studied to verify their cooling effectiveness. The heat transfer characteristics of these nozzles, compared using three different methodologies, are presented. A commercial computational fluid dynamics tool, FLUNET, is employed to model the 3D thermal-fluidic field of each nozzle. A 1D source code is also implemented in FORTRAN to achieve a faster and more reliable solution. Finally, experimental data are used to verify the two computational results. The difference between the results obtained using the FORTRAN code and FLUENT was about 0.07%. The temperatures at the cooling tower exit for the 1D and 3D codes were 431.9°C and 432.2°C (809.4°F and 810°F), respectively. Four sets of experimental data were compared with the numerical results. The maximum temperature difference between the 1D model and the measurements occurred at an inlet temperature of 450°C (842°F) and at a strip thickness of 2.28 mm. The outlet temperature from the 1D model and measurement was, respectively, 290.1°C and 278.5°C (554.2°F and 533.3°F), differing by approximately 4%. The 1D model generates good temperature predictions compared to measurements from a real cooling tower. This result would be very beneficial for industries that require fast temperature information for stable and steady operation, such as steelworks.

An Experimental Study on the Size Effect of Reinforced Concrete Beams without Stirrups Using Recycled Coarse Aggregates

The authors examined the size effect of reinforced concrete(RC) beams using natural coarse aggregates and recycled coarse aggregates. The specimens were made of concrete with a maximum coarse aggregate size of 20 mm. In this study, depth-to-width ratio, shear span-to-depth ratio, tension steel ratio and RCA replacement ratio were fixed 1.5(i.e., h/b=1.5), 3(i.e., a/d=3), 1.2%(i.e., ) and 30%(i.e., ) respectively. The test variables only include the RC beam size and aggregate type. A total of 10 simple RC beams were tested under four points loading. It has been shown that shear strength decreases as member size increases. This is associated with a phenomenon called size effect. However, the size of the RC beams does not seem to affect the shear strength proportionally. Besides, test results were analyzed and then compared with the findings and various shear design equations. To conclude, the shear equations reflect relatively well with most of experimental data.