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To meet the objectives, 12 full-scale high strength concrete beam specimens were tested Each beam was designed with bars spliced in a constant moment region at midspan. The variables were bar size 20, 25, or 32 mm, and the amount of steel fiber reinforcement added in the splice region during casting: Vf = 0 5, 0 5, 1.0, or 2 0 %--The test results indicated that the use of steel fibers in the splice region increased the bond strength and the ductility of the mode of failure of the beam--specimens The increase in bond strength with high fiber content exceeded 3 square root fc, the maximum increase in bond strength of a reinforced concrete beam that could be achieved by using transverse reinforcement in the splice region.
The deterioration of reinforced concrete structures is increasingly becoming a serious problem facing the infrastructure worldwide. To prolong the service life of existing structures, fiber reinforced polymer (FRP) sheets are being used. On the other hand, research reported in the literature indicates that a mechanism should exists which would confine the tension lap splices in high strength concrete (HSC) in order to have a more ductile failure and to increase the capacity of the lap splice. The main objective of this research is to assess the effect of FRP wraps in improving the serviceability and ultimate response of bond-critical regions in reinforced concrete members. The lack of research reported in the literature on the effect of the FRP wraps on bond strength makes this study significant. Moreover, the research will provide important design information that facilitates the introduction of FRP into design codes and encourage the use of this new technology. To meet the objective, 10 full-scale high strength concrete beam specimen were tested. Each beam was designed with 20-mm bars spliced in a constant moment region at midspan. The variables were: FRP type (glass or carbon), number of layers of FRP (1 or 2), and the configuration of FRP wraps placed in the splice region. Results of the study indicated that FRP wraps have similar effects to those of steel fibers and transverse steel reinforcement in confining the splices in HSC beam specimen. The brittle mode of failure is modified to a more ductile one. More bar lugs along the spliced bars are allowed to participate in the stress transfer between steel and concrete, and the average splitting bond strength is increased. Finally, a new index, Ktr, f, accounting for the presence and amount of FRP confining tension lap splices in HSC beams was proposed.
With the more frequent use of FRC in earthquake resistant structures as a means for improving energy absorption and dissipation capacity, understanding the infl uence of steel fiber reinforcement on the bond strength between steel bars and c oncrete becomes of particular interest. While several experimental and analytical studies have concentrated on the bond characteristics under static load conditions, data on the bond stress characteri stics of steel bars in plain concrete or concrete applied with steel fiber reinf orcement is still very limited, particularly when the mode of bond failure is by splitting. This shortage of data makes it difficult at present to establish gen eral recommendations for computing the minimum volume of steel-fiber reinforceme nt needed to improve the seismic performance of reinforced beams, taking into ac count the bond parameters of the beam spliced reinforcement and the distribution of this reinforcement in the section. Experimentally investigating the bond cha racteristics of reinforcing steel bars embedded in FRC under seismic loading for better understanding of the mechanism by which fiber reinforcement improves the bond strength and seismic performance of spliced bars in tension constitutes th e primary objective of this proposed investigation. Also based on the results of this investigation, the main parameters that influence the response will be eva luated and discussed, and existing models for predicting the bond strength will be further validated or refined. To meet the objectives, 12 full-scale normal and high strength concrete beam spe cimens were tested. Each beam was designed with bar splices (20db) placed in a c onstant moment region at midspan. No transverse reinforcement will used in the s plice region. The design variables were the bar size (20 and 25 mm), ratio of co ncrete cover to bar diameter (c/db of 2.0 and 1.4), and the volume of fraction o f fibers (Vf = 0.0%, 0.5%, 1.0%, 1.5%). The test results indicated that the use of steel fibers in the splice region inc reased the ultimate load capacity, bond strength, reduced bond deterioration, im proved ductility, increased energy absorption capacity and also verified in part the equation proposed by Harajli and Mabsout2000 that accounts for the increase in bond strength of beams due to the presence of fibers.
Several studies are reported in the literature on the effect of transverse reinforcement on bond and anchorage characteristics of reinforcing bars in normal and high strength concrete specimens. Transverse reinforcement provides concrete with confinement and increases its tensile resistance against splitting. When used to confine splices, transverse reinforcement leads to an increase in the bond capacity and to a more ductile and gradual mode of failure.--In 2000, a research program was carried out by Joumaa to study the effect of fiber reinforcement on bond strength and mode of failure of tension lap splices in high strength concrete. In this research, the effect of transverse reinforcement on bond strength and mode of failure of tension lap splices in beam specimens similar to those tested by Joumaa, will be investigated.
"In 1993, the CEB Commission 2 Material and Behavior Modelling established the Task Group 2.5 Bond Models. It's terms of reference were ... to write a state-of-art report concerning bond of reinforcement in concrete and later recommend how the knowledge could be applied in practice (Model Code like text proposal)... {This work} covers the first part ... the state-of-art report."--Pref.
To meet the objectives, 32 NSC full-scale beam specimens were tested. Each beam was designed with a small splice length (5 times the bar diameter) at midspan to simulate local bond conditions. The design variables were bar size 16, 20, 25, or 32 mm, ratio of bar cover to bar diameter c/db = 0.56, 0.88, 1.0, 1.34, 1.5, 2.0, or 2.1 and the amount of steel fiber reinforcement added in the splice region: Vf = 0.0, 0.5, 1.0, or 2.0 %.--The test results indicated that the use of steel fibers in the splice region increased the bond strength and the ductility of the mode of failure of the beam specimens. These results were combined with other results reported in technical literature and used to develop a model to describe the local bond stress-slip relationship of reinforcing bars embedded in plain and FRC under splitting type failure. The model accounts for all of the important parameters that tend to influence the bond characteristics of reinforcing bars as observed in the experiment.
Many reinforced concrete structures containing lap splices were constructed before modern bond and fatigue design codes came into existence and are subjected to fatigue loading, which may lead to a bond failure even when the applied load is far below the ultimate load for a bond failure under a monotonic loading. Fatigue loads result in a deterioration of the bond interaction between the steel and concrete and interrupt the force transfer mechanism resulting in an increased deflection, an increased number of cracks and their widths, and a decreased load carrying capacity of reinforced concrete elements of structures. Some of these structures require strengthening to enhance their bond strength at lap splices. This study was aimed at increasing our understanding of the behaviour of the bond between the steel bar and the concrete along the lap splice region for structures subjected to cyclic loading. An additional aim of the study was to investigate the effect of fatigue loading on the bond between concrete and steel, and the ability of the new high and low modulus fiber-reinforced polymer (FRP) sheets to enhance the fatigue performance of a tension lap splice.
The use of silica fume has been steadily increasing all over the world as designers utilize the resulting high performance concrete (HPC) to increase the concrete compressive strength and to reduce or eliminate durability problems such as sulfate attack and corrosion. Very few researchers investigated the effect of silica fume on bond-slip characteristics of deformed bars in high performance concrete. Researchers who worked on this topic like Itani and Azizinamini, concluded that the presence of silica fume reduced the bond strength and induced a brittle mode of failure. They suggested the use of transverse reinforcement over the splice region in order to increase the bond strength and provide a ductile mode of failure. The objectives of the proposed research program were to investigate the effect of transverse reinforcement on the bond-slip characteristics of tension lap splices in high performance silica fume concrete, to study the validity of the upper limit of 70 MPa (10,000 psi) imposed by the ACI Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of deformed bars embedded in high strength concrete. Twelve beam specimens were tested. Each beam specimen included two bars in tension, spliced at the center of the span. The beams were designed in way that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume and the amount of confinement over the splice region. Test results indicated that silica fume decreased the bond strength and that specimens containing silica fume but without any transverse reinforcement in the splice region had a brittle, sudden and noisy mode of failure. The use of transverse reinforcement in the splice region increased the bond strength and the ductility of the mode of failure of the beam specimens. Minimum amounts of transverse reinforcement were recommended.