FRP Reinforcement

FRP bars are made from fibres and resin using a pultrusion method. The mechanical properties of FRP bars depend on type of fibre, resin and the proportion of those two in the material. Therefore the strength of FRP bars varies from manufacturer to manufacturer since different manufacturer use different type of fibres, resins and compositions. Despite the different strengths and modulus of elasticity, all FRP bars share some common attributes and behaviours.

* FRP bars are brittle in nature
* Modulus of Elasticity of any FRP bars is lower compared to steel. In general glass FRP has

25% and Carbon FRP has 75% of modulus elasticity of steel.
*FRP bars cannot be bent on site, so should be customised in the factory.

The typical stress strain curves for Glass Fibre Reinforced Plastic (GFRP), Carbon Fibre Reinforced Plastic (CFRP) and Aramid Fibre Reinforced Plastic (AFRP) is shown in Figure 1.

Figure 1: Typical Stress-Strain behaviour of different FRP materials and Steel (Lees, J.M and Burgoyne, C.J., 1999).

Among these three FRP types, GFRP has the lowest modulus of elasticity and CFRP has the highest modulus of Elasticity. However, CFRP is ten times more expensive than GFRP, so industry prefers GFRP bars/rods (Hughes Brothers, 2008).

FRP Rebar Manufacturers on both CFRP and GFRP:

Schoeck Combar : A specialist GFRP rebar manufacturer http://www.schoeck-combar.com/

Pultrall : A Canadian rebar manufacturer : http://www.pultrall.com/Site2008/eng/V-ROD.htm

Hughes Brothers: An American manufacturer :http://www.hughesbros.com/Aslan_FRP.html

Compressive Membrane Action

What is Compressive Membrane Action?
Compressive Membrane Action is an inherent strength enhancement which develops when a slab tends to move against lateral restriction.
Although this is the definition given for compressive membrane action, I know this is not clear enough to understand.

Well, lets take an example!









Picture 1 : (Pictures Courtesy: http://www.answer.com/)

From the above example we can see that neutral axis of a simply supported slab strip is located near the mid depth. What will happen when the slab strip started to crack? The slab will fail.

However, if a slab is laterally restrained, then it behaves differently. The initial behaviour prior to crack formation illustrated in the Figure1.

Since the tensile strength capacity of the slab is lower than compressive strength capacity, cracks will occur at tension zones. In the fixed end slab, the tension zones are in three locations. Top face of the slab strip near the supports and mid span bottom face.

Figure 1: Before cracks

As a result of cracks, the neutral axis migrates to the very compressive zone. Thus the resultant structure can be explained as it is on Figure 2.

Figure 2: After cracks

The neutral axis shift leads to a new structural form. The difference between the structure prior and after the neutral axis shift can be explained in Figure 3. Due to the cracks hinges forms. Thus structure tends to expand from the loading point towards the in plane. Thus if there is a lateral restraint then it induce compressive membrane action.

Figure 3:

When the structure loaded, any vertical deflection should be accompanied by a lateral expansion. If the lateral expansion is restrained with a fixed end, then there will be lateral force induced and thus compressive membrane action induce (See Figure 4).

Figure 4: CMA