Determination of Pull Out Capacity of Steel Bar in Concrete

Topics: Concrete, Reinforced concrete, Rebar Pages: 24 (7570 words) Published: February 17, 2013


1.1 Introduction of Research

Steel-reinforced concrete is a widely used structural material. The effectiveness of the steel reinforcement depends on the bond between the steel reinforcing bar and the concrete. Reinforced concrete is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars and is usually embedded passively in the concrete before it sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and structural failure.

For concrete is a mixture of coarse (stone or brick chips) and fine (generally sand or crushed stone) aggregates with a binder material like usually Portland cement. When mixed with a small amount of water, the cement hydrates to form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure. Typical concrete mixes have high resistance to compressive stresses about 28 MPa. However, any appreciable tension (due to bending) will break the microscopic rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

The utility of reinforced concrete as a structural material is derived from the combination of concrete that is strong and relatively durable in compression with reinforcing steel that is strong and ductile in tension. Maintaining composite action requires transfer of load between the concrete and steel. This load transfer is referred to as bond and is idealized as a continuous stress field that develops in the vicinity of the steel-concrete interface. For reinforced concrete structures subjected to moderate loading, the bond stress capacity of the system exceeds the demand and there is relatively little movement between the reinforcing steel and the surrounding concrete. However, for systems subjected to severe loading, localized bond demand may exceed capacity, resulting in localized damage and significant movement between the reinforcing steel and the surrounding concrete.

A good bond between steel deformed reinforcing bar and concrete in concrete structures is crucial for structural and serviceability performance. If this bond is inadequate, behavior and failure characteristics will be altered. The bond mechanism allows the forces to be transferred between the concrete and steel.

1.3 Statement of problem

In concrete construction, many types of contaminants are present on the site, such as form oil for coating the forms and bond breaker used in tilt-up construction. The reinforcement could be contaminated during construction if care is not taken. If contaminated, there is concern regarding the bond strength and specifications are in place to guide the action to be taken. When constructing reinforced concrete structures, inspectors have the duty to enforce several specifications dealing with concrete construction. One of the goals of the specifications is to maintain a clean and safe construction environment.

Often, reinforcing bar is subjected to various construction contaminants, such as form oil or mud during concrete construction, and the specifications require the reinforcing bar to be cleaned prior to placing the concrete. The primary concern of these specifications is assuring a good bond, but do the contaminants reduce the bond strength enough to warrant these specifications. Without detailed research into this issue, most construction specifications are conservative and require the removal of these contaminants from the reinforcing bar. This is time-consuming, costly, and may be unnecessary.

Previously, several studies have been performed in regard to the variables that affect...
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