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1) INTRODUCTION
1.1 Basics of Bond Strength
a) Bond:-Bond in reinforcement refers to the adhesion between the reinforcing steel and the surrounding concrete.
b) Importance of Bond b/w steel and concrete:-The bond between the reinforcing steel and the surrounding concrete is responsible for the transfer of axial force from a reinforcing bar to surrounding concrete, thereby providing strain compatibility and composite action of concrete and steel. If this bond is in adequate, slipping of the reinforcing bar will occur, destroying full composite action. Hence the fundamental assumption of the theory of flexure, viz., plane sections remains plane even after bending, becomes valid in reinforced concrete only if the mechanism of bond is fully effective
c) Mechanism of Bond resistance:-
1) Chemical adhesion
2) Frictional resistance
3) Mechanical interlock
d) Types of bonds:-There are two types of loading situations which induce bond stresses, and accordingly bond is characterized as
a) Flexural bond &
b) Anchorage bond or development bond
Flexural bond:-flexural bond is that which arises in flexural members on account of shear or variation in bending moment which in turn causes a variation in axial tension along the length of a reinforcing bar. Flexural bond is critical at points where shear is significant
Anchorage bond or development bond:-is that which arises over the length of anchorage provided for a bar or near the end of a reinforcing bar; this bond resists the ‘pulling out’ of the bar if it is in tension, or conversely, the ‘pushing in ‘ of the bar if it is in compression
e) Bond Tests:-
Pullout Test:-Pullout tests are the easiest and least expensive bond tests to conduct; however,
they provide the least realistic results of bond strength. In these tests, tension is applied directly to a bar which has been embedded in a block of concrete. A schematic of a pullout test is shown in Figure 1.
Fig 1.Schematic of pullout test (adapted from ACI 408)
Results of pullout tests are not a good indicator of actual bond strength because they do not represent realistic loading conditions found in structural members. The concrete block is placed in compression during the test while bars being anchored as tension reinforcement are usually surrounded by concrete in tension. Compression struts also form between the end reaction and the reinforcing bar which place the bar in lateral compression. In actual structural members, compression between the bar and the surrounding concrete is produced as the lugs of the bar bear on the concrete after adhesion is overcome and initial slip of the bar occurs. To prevent crushing failure of the concrete block, pullout specimens usually contain a high level of confining transverse reinforcement. As described previously, transverse reinforcement adds significantly to the bond strength by preventing the growth of splitting cracks. Due to these shortcomings, ACI Committee 408 does not recommend pullout tests as a sole indicator of bond strength.
Beam-End Test
Beam-end tests are the simplest tests that reflect realistic boundary conditions and bond strength results. In these tests, tension is applied to a reinforcing bar that has been eccentrically embedded in a block of concrete. A schematic of a beam-end test is shown in Figure 2.
Fig 2.Schematic of beam-end test (adapted from ACI 408)
Unlike the pullout test, beam-end tests more accurately represent actual loading conditions in structural members. Both the bar and the surrounding concrete are placed in tension due to eccentric placement of the reinforcing bar in the concrete block. The effect of the end reactions can be negated if the supports are located at a distance of at least the embedment length of the bar from the end of the reinforcing bar. Shear reinforcement can be detailed such that it does not provide confinement to the bar being developed or it may enclose the reinforcing bar in order to study the effect of transverse reinforcement
Beam-Splice Test
Full scale beam tests provide the most accurate results for bond strength since they best duplicate the actual stress state around the reinforcing bars being developed. A popular full scale test used for bond research is the beam-splice test. In these tests, bars are lap spliced within a constant moment region at the center of the beam span. A schematic of a beam splice test is shown in Figure 3. Because of its simplicity of design and fabrication and because of the accuracy of the test results, beam splice tests have provided the majority of data for the development of descriptive and design code equations for bond and anchorage of reinforcement
Fig 3.Schematic of beam-splice test (adapted from ACI 408R-03)
2)Effect of Consolidation on Bond of Reinforcement in Concrete of Different Workabilities
(by Yin-Wen Chan, Yong-Guo Chen, and Yi-Shi Liu; ACI Materials Journal/July-August 2003)
2.1 Introduction:- In the construction of reinforced concrete structures, consolidation has to be applied to the concrete mixture to ensure the quality of concrete placement. Insufficient consolidation may result in defects, such as honeycombs, voids, or vacancies, in reinforced concrete members. These construction deficiencies would lead to reductions in durability and structural performance. In members with congested reinforcement or with large dimensions, however, consolidation may not always be an easy task. The selection of concrete with proper workability is usually governed to resolve the difficulties in concrete placement. With the increasing use of congested reinforcement in moment-resisting members, for example, due to seismic consideration, there is a growing interest in specifying high-workability concrete such as high-performance concrete (HPC).
In general, HPC possesses high strength as well as high workability and is used as a solution to the situation that demands particularly high durability. To resolve the problems with concrete placement, a special type of concrete, called self-compacting concrete (SCC) has been developed that requires no consolidation at all. It has been verified that with non vibration construction, the application of self-compacting concrete can enhance the uniformity of materials in reinforced concrete (RC) members and thus can help to ensure the designed structural performance. The practice of non vibration concreting, however, is totally against the conventional practice of construction that demands sufficient consolidation during concrete placement. Apparently, the application of SCC requires a certain extent of training and adjustment in the construction practices of workers. For this purpose, it is desired to know how the task of consolidation should be adjusted in the placement of SCC or what the effect would be if vibration is still applied during the placement of SCC.
Several factors affecting the bond strength have been studied by a number of researchers. These factors include the composition materials, compressive strength of concrete, and testing methods and apparatus. Research on the influence of workability of concrete on bond properties is very limited.
In this paper, the effect of internal vibration by electrical motor-driven vibrator on the bond strength of reinforcing bars in concretes of different workability was investigated. Different consolidation conditions were adopted to simulate various situations that might occur during practical concrete placement, including standard vibration, over-vibration, no vibration, and improper vibration. During the casting of specimens, the concrete mixtures were subjected to various conditions of internal vibration including no vibration, vibration in different durations, and vibration directly in contact with the reinforcing bars. The experimental program varied the vibration duration to quantify the degree of vibration. Additionally, two more conditions that are likely to occur in practice were checked, including no vibration for simulating the case of insufficient consolidation, and vibration against reinforcing bar for simulating the case of improper practice in applying vibration.
2.2) RESEARCH SIGNIFICANCE
This research has special significance in construction practice and materials specification for reinforced concrete projects. The experimental program varies the vibration duration to quantify the degree of vibration. Additionally, two more conditions that are likely to occur in practical Situations are adopted, including no vibration for simulating the case of insufficient consolidation and vibration against reinforcing bar for simulating the case of improper practice in applying vibration. The experimental results clearly indicate that the degree of vibration as well as the workability of concrete may tremendously affect the bond strength between reinforcing bars and concrete. For special concretes such as HPC and SCC, necessary adjustments in consolidation practice are particularly important and essential.
2.3 EXPERIMENTAL PROGRAM
Single reinforcing bar pullout tests were conducted to measure the bond strength of horizontal deformed reinforcing steel bars in various concretes of different vibration conditions
2.3a Materials
Three types of concrete of different levels of workability were used in the experimental program, including normal concrete (NC), with a slump less than 75 mm, as the comparatively low-slump concrete; HPC, with a slump of over 200 mm, as the high-slump concrete; and SCC as an extremely high workability concrete.