International Concrete Abstracts Portal

This experimental study addresses the performance of reinforced concrete slabs containing epoxy-coated reinforcement subjected to high-cycle/low stress range repeated loading typical of those encountered in bridge decks. The behavior under repeated load indicated that epoxy-coated reinforcement does not significantly increase deflections despite the larger bar slip associated with wider cracks. The wider cracks do increase the potential for increased amount of corrosive agent at the level of the top mat of reinforcement in bridge decks. The average bond strength ratios of coated to uncoated specimens support a proposed single modification factor of 1.35 for specimens with low cover.

Pullout tests were conducted on deeply embedded headed reinforcement to determine the effect of transverse reinforcement and bonded length on the side-blowout capacity and load-slip behavior of the anchorage. It was found that transverse ties or stirrups in the anchorage zone had little effect on the ultimate capacity. Increases in anchorage capacity were only observed when the head was positively anchored in contact behind a large crossing bar. Transverse reinforcement also had little effect on the load-slip behavior before failure. However, when large amounts of transverse reinforcement were placed near the head, the amount of load maintained after the blowout failure occurred was increased. Additional bonded length of a deformed reinforcing bar increased the anchorage capacity and reduced the head slip for a given load. The amount of increase in capacity can be predicted using current ACI provisions for development length. Design procedures taking into account the effects of transverse reinforcement and bonded length were developed.

This paper presents the results of an experimental investigation on the anchorage of deformed steel bars embedded in bed joints of masonry. The effect of bar diameter, anchorage length and vertical load on the mortar joint is investigated. Although the horizontal bars in masonry are under unfavourable conditions, such as low strength of mortar, small cover values, lack of bond at places where the bars pass over large holes of the masonry unit, tests have proved that embedment lengths as low as 30 to 40 times the bar diameter seem to be sufficient for the bars to develop their yield strength.

Until recently, bridge design specifications in California permitted longitudinal column reinforcement to be terminated in monolithic cap beam/column joints with straight bar development not extending fully up the joint height. Since the development length could be construed not to comply with AC1 3 18 requirements, it was suspected that the anchorage length provided for the column bars was insufficient to develop the full moment capacity of the column at the joint interface. Considering that this detailing was widely used in bridges in California, an experimental investigation was initiated at the University of California, San Diego, where response of a bridge knee joint incorporating as-built reinforcement details was examined under simulated seismic loading. Following inadequate performance of the test unit, the behavior of the knee joint was investigated based on the experimental readings, giving consideration to bond slip of the longitudinal column reinforcement. The response of the test unit indicated that the bond conditions developed when anchoring the longitudinal reinforcement of circular bridge columns is unlike that developed along the beam reinforcement anchored into building joints.

The results of the first major reevaluation of reinforcing bar geometry in the United States in nearly 50 years is described. The study involves experimental and analytical efforts designed to broaden the understanding of factors that control bond strength, improve the development characteristics of reinforcing bars, and develop practical design expressions that more accurately represent development and splice strength than existing expressions. The research has established that deformation pattern has little effect on the bond strength of uncoated bars that are not confined by transverse reinforcement. Deformation pattern, however, as represented by the relative rib area, does have a major effect on the bond strength of bars that are confined by transverse reinforcement. Increases in relative rib area, obtained with either higher ribs, closer ribs, or a combination thereof, result in an improved bond strength for confined bars. The study has also established limits on how closely ribs can be placed without resulting in a pullout failure. High relative rib area bars provide a reduction in development/splice length of 20 percent for all coated bars, independent of the presence or absence of confining transverse rein-forcement. Based on the experimental work, expressions are developed that accurately characterize development/splice strength. In the development of the expressions, f’,t/4 is shown to be superior to f$2 for characterizing the contribu-tion of concrete strength to bond. The resulting design expressions are accurate for compressive strengths between 2500 and 16,000 psi (17 and 110 MPa). The most accurate representation of the effect of transverse reinforcement on bond strength includes parameters that account for the number of transverse reinforcing bars that cross the developed/spliced bar, the area of the transverse reinforcement, the number of bars developed or spliced at one location and the relative rib area, and size of the developed/spliced bar. The yield strength of the transverse reinforce-ment does not play a measurable role. Practical comparisons illustrate reductions in splice lengths of 20 percent for conventional bars and 30 percent for high relative rib area bars compared to current requirements in AC1 3 18-95.