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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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1.1 General

Reinforced concrete structures is one of the most commonly used construction material which consists of two materials (concrete and steel) with different mechanical properties. Concrete represents a material with heterogeneous properties, which change in magnitude, nature and duration of loading. In the presence of steel reinforcement, the behavior is even more influenced by loading. So this can cause a non-linear behavior in some reinforced concrete structures.

Beams are very important parts of reinforced concrete structures. Beams resist the loading and distribute these loads to the columns or supports. The behavior of reinforced concrete beams at shear failure is distinctly different from their behavior in flexure. Shear failure may happen abruptly without sufficient warning, and the diagonal cracks that develop are considerably wider than flexural cracks.

Strengthening of structural members  as slabs, beams, and columns using FRP composite materials as carbon fiber reinforced polymer (CFRP) has wide acceptance over the last two decades due to its high strength to weight ratio,  light weight, flexibility, high stiffness, ease of installation, and resistance to corrosion as compared to other materials. Carbon fiber reinforcement polymer composite sheets are widely used to externally strengthen RC beams in flexure and shear. However, premature failures such as laminate separation and peeling failure can significantly limit the capacity enhancement and prevent the full ultimate flexural capacity of the retrofitted beams from being acquired. Laminate separation or peeling may occur due to the high longitudinal shear and transverse normal stresses at the laminates end resulting from the abrupt curtailment of the laminates.

1.2 Failure Modes in Shear

The various failure modes in shear without shear reinforcement are described in this section.

1.2.1 Diagonal Failure

Many types of structural concrete members other than beams have been reported to fail due to shear distress or diagonal failure e.g. slabs, columns, foundation, corbels and shear walls. It is believed that the shear transfer mechanism is very similar or the same in all the cases but the cracking pattern may differ. A combination of shearing force and moment is the fundamental cause of diagonal failure.

1.2.2 Diagonal Tension Failure

The diagonal crack initiates from the last flexural crack formed. The failure occurs in beams when the ratio a/d is approximately 2.5 – 6.0 in the shear span “a”. The crack propagates through the beam until it reaches the compression zone. When the beam reaches a critical point it will fail as a result of splitting of the compression concrete. Often this happens almost without a warning and the failure becomes sudden and brittle shown in           Fig. (1-1).

Fig.(1-1) Diagonal tension failure.

1.2.3 Shear Tension Failure

This type of failure is similar to diagonal tension failure but applies to short beams. The shear crack propagates through the beam but doesn’t cause failure of the beam on its own. Secondary crack travel along the longitudinal reinforcement and the concrete or anchorage failure shown in Fig. (1-2). When the beam reaches a critical point it will fail as a result of splitting of the compression concrete.

Fig.(1-2) Shear tension failure.

1.2.4 Shear Compression Failure

On the other hand, if the diagonal shear crack propagates through the beam, causing failure when it reaches the compression zone without any sign of secondary cracks as is described in shear tension failure, it’s referred to as a shear compression failure shown in Fig. (1-3). This failure mode applies to short beams. The ultimate load at failure can be considerably more than at diagonal cracking as a result of arch action.

Fig.(1-3) Shear compression failure.

1.3 Fiber Reinforcement Polymer (FRP)

The need to develop economic and efficient methods to upgrade, or strengthen existing structures has received a considerable attention recently. The motivation to strengthen and repair an existing structure typically comes from changes in design, loading increases and a desire to repair deterioration that has taken place over the years of use. In such circumstances, there are two possible solutions, which are to demolish and rebuild or carry out a program of strengthening, the first solution is not attractive and may not be economically feasible to replace an outdated structure with a new one.

Advances in the fields of plastics and composites have resulted in the development of HS, fiber reinforced polymer (FRP). This FRP fastly becomes the preferred choice in the strengthening and rehabilitation of existing structures, which it has been used to repair and strengthen concrete members such as slabs, columns, beams and girders in structures such as bridges, parking, decks, and buildings. Depending on the member type, the objective of strengthening may be one or a combination of several of the following;

• Increment stiffness to reduce deflections under service and design loads.

• Increment axial, shear or flexural load capacities.

• Increment durability against environmental effects.

• Increment the remaining fatigue life.

The term composite often refers to a material composed of two or more distinct parts working together. Often one of the parts is harder and stronger, while the other is more of a force transferring material. FRP is an abbreviation of fiber reinforced polymers and is a composite of fibers and adhesive, see Fig. (1-4).  

Fig.(1-4) Typical composition of FRP material.

1.3.1 Fiber Materials

Several materials are available for the fibers, for example glass (GFRP), aramid (AFRP), carbon (CFRP). Almost 95 percent of all applications for strengthening purposes in civil engineering are by carbon fibers. Fig. (1-5) shows some typical response of uniaxially loaded fiber materials and steel. HM and HS are abbreviations of high modulus of elasticity and high strength, respectively.

FRP materials exhibit a linear elastic stress strain behavior when loaded in direct tension until failure which is brittle. Table (1-1) compares the material properties of carbon fiber, concrete and steel.

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