Kiang Hwee Tan

SHEAR BEHAVIOR OF STEEL FIBER REINFORCED CONCRETE BEAMS

The behavior of steel fiber reinforced concrete beams subjected to predominant shear has been investigated. Principal stress-strain relations are proposed for the cracked steel fiber rienforced concrete. This, together with the equilibrium and compatibility equations for the cracked element, are used to predict the response of steel fiber reinforced concrete (SFRC) beams subject to shear and bending. The study found that the behavior of SFRC beams could be modeled well using the proposed principal stress strain relationships. Also, the study confirms that an increase in shear strength of as much as 70 % could be achieved by adding small quantities of steel fibers to the ordinary reinforced concrete.

Effects of Deviators and Tendon Configuration on Behavior of Externally Prestressed Beams

Six identical reinforced concrete T-beams were strengthened in flexure using external prestressing tendons (strands). Of these, three beams had identical straight external strands with a different number of deviators along each of their spans. The other three beams each had a deviator at midspan and their tendon configurations were varied by either increasing tendon eccentricity, increasing the tendon area or draping the strands. Test results indicated that the provision of a deviator at the section of maximum deflection led to satisfactory service load and a higher load-carrying capacity compared to the case where no deviators were provided. An increase in eccentricity of straight tendons with a correspondingly smaller prestressing force led to larger internal steel stresses, crack widths, and service load deflections but higher ductility, while draped tendons resulted in reduced stiffness, greater tendon stress increase and more ductile behavior near failure.

Cracking characteristics of reinforced steel fiber concrete beams under short- and long-term loadings

Abstract In this paper, an analytical method for the prediction of maximum crack width in reinforced steel fiber concrete (SFC) beams under short-term loading is first presented. The method accounts for the enhanced cracking strength, restraint against crack growth, and reduced tensile steel strains due to the presence of steel fibers. Based on a correlation analysis, a semiempirical formula for the long-term crack widths in reinforced SFC beams under sustained loads is also proposed. Tests were carried out on 10 beams to investigate the effect of steel fiber content on the cracking characteristics in both the short- and long-term. The results indicated that the use of steel fibers greatly reduced the maximum crack widths in reinforced concrete beams. Good agreement was generally obtained between the analytical predictions and test results.

Vulnerability of reinforced concrete frames in low seismic region, when designed according to BS 8110

The overstrength and ductility due to redistribution of internal forces are being investigated for three bay multi-storey reinforced concrete plane frames, using non-linear push-over analysis. These frames are designed to resist gravity loads, wind loads and a notional horizontal load in accordance with the British code BS 8110, which does not have any special provision for seismic loads. The results show that the overstrength factors for the three-, six- and ten-storey frames are respectively, 7·5, 5·6 and 2·2 times the design lateral loads, whereas, the ductility factors for the three frames are similar, and slightly greater than 2. These values yield a response modification factor of 18·0, 12·2 and 4·7 for the three-, six- and ten-storey frames, respectively. The effect of infill walls on the response modification factor is also being investigated, and a suitable response modification factor for assessing the vulnerability of reinforced concrete frames of about 10 storeys high is recommended. Copyright

Concrete with Recycled Glass as Fine Aggregates

Recycled waste glasses were used as sand replacement in concrete at 0, 25, 50, 75, and 100% replacement ratios. Diverse concrete properties were tested in fresh and hardened states with three concrete grades; that is, with compressive strengths of 30, 45, and 60 MPa (4350, 6525, and 8700 psi). The incorporation of glass sand showed no significant influence on fresh or mechanical properties of concrete. Drying shrinkage was reduced due to the negligible water absorption of glass sand, especially at lower water-cement ratios (wc). Resistance to chloride ion penetration was substantially enhanced because of the improved and densified microstructure at the interface transition zone (ITZ). Based on alkali-silica reaction (ASR) tests up to 49 days, cement mortar containing glass sand was found to exhibit innocuous expansion. Use of mineral admixtures, such as fly ash and slag cement, could further improve the durability of concrete containing waste glass particles.

Design of Reinforced Concrete Beams with Circular Openings

This paper investigates the adequacy of the ACI Code approach, modified for the inclusion of transverse openings and for shear design of a beam with circular openings. Seven T-beams with circular web openings were designed for moderate to high shear force and tested in an inverted position to simulate conditions that exist in the negative moment region of a continuous beam. Test results show that crack control and preservation of ultimate strength may be attained by providing reinforcement around the opening. Diagonal bars were found to reduce the high stress in the compression chord, thus avoiding premature crushing of the concrete. The modified ACI approach has been found to give conservative design provided the reinforcement is properly detailed near the opening region.

Ten-Year Study on Steel Fiber-Reinforced Concrete Beams Under Sustained Loads

This article reports on a study of the behavior of steel fiber-reinforced concrete (SFRC) beams with different fiber contents under different sustained loads for a period of 10 years. Nine reinforced concrete beams with discrete steel fiber contents ranging from 0 to 2% were subjected to sustained flexural loading between 0.35 to 0.8 times the flexural capacity. The results showed that the addition of steel fibers to concrete is effective in containing long-term deflection and restraining crack widening. The deflections and crack widths were smaller for lower sustained loads. The modified ACI approach and effective modulus method (EMM) predict the long-term deflections very well, while the adjusted effective modulus method slightly underestimates the same. The authors conclude that, except for the permanent deformation and reduced initial stiffness due to sustained loading, the aged beams showed the same load-deflection behavior as those of the unsustained beams tested before, both in terms of strength and stiffness.

EFFECT OF SHEAR IN EXTERNALLY PRESTRESSED BEAMS

Seven T-beams, post-tensioned with straight tendons, were tested to failure to investigate the effect of concrete strength, shear reinforcement, and shear span on the failure mode of externally prestressed beams. Each beam was fabricated with a deviator at the midspan and tested under simply supported conditions. The beams were each subjected to equal concentrated loads at third-points, except for one, which was subjected to a concentrated load at the midspan. Test results indicated that decreasing the concrete strength or the amount of shear reinforcement leads to shear-type failure of the beam. When an appropriate concrete strength and amount of shear reinforcement are provided, flexural-type failure prevails even for shear span to effective depth ratio of as low as 2.5. Analytical considerations based on a strut-and-tie model that was proposed earlier showed good predictions of the ultimate strength and failure mode of the test beams.

Reinforced Concrete T-Beams with Large Web Openings inPositive and Negative Moment Regions

Fifteen test specimens, each simulating either the negative or the positive moment regions of a reinforced concrete continuous T-beam, were fabricated with large openings through the web and tested to failure. Test results indicated that the presence of web openings leads to a decrease in both cracking and ultimate strength as well as postcracking stillness similar to earlier observations on simply-supported rectangular beams. For the same passageway, beams with multiple openings were found to perform better in terms of strength and serviceability than those with a single opening. Test results confirmed Vierendeel panel behavior at the opening segment of the beam. It has been found that apportioning of the applied shear between the chord members in accordance with their flexural stiffnesses gives a good reflection of actual distribution. An analytical procedure for both the ultimate strength and service load analysis incorporating the use of equivalent stiffnesses for the opening segment was found to predict the test results well.

Performance of FRP-Strengthened Beams Subjected to Elevated Temperatures

The flexural behavior of fiber-reinforced polymer (FRP)–strengthened beams after exposure to elevated temperatures in an electrical furnace was investigated. Twenty-five specimens making up unstrengthened beams and FRP-strengthened beams were fabricated. Glass and basalt FRP systems were used with and without protective systems, which included a cement mortar overlay and two types of commercially available intumescent coatings. Typical temperature-time histories at the surface of FRP laminates, FRP-concrete interface, internal steel bars, and center of beams were monitored by using two specimens. The other specimens were tested to failure under three-point bending after subjecting them to elevated temperatures. Test results indicated a general decrease in the initial stiffness and ultimate strength of the specimens with an increase in the exposed temperature. The protective systems appeared to preserve the structural integrity of glass FRP systems when the elevated temperature was less than approximately ...