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

FRP an innovative material

FRP is a durable replacement to steel in reinforced traditional slabs. In the past the severe corrosion of steel reinforcement has occurred and the cost to rectify the damages sometimes exceeded the original cost of the structure. The implementation of FRP reinforcement in civil engineering would help to produce sustainable and durable structures.

Compressive Membrane Action (CMA) is an enhanced strength development which occurs in laterally restrained slabs. Previous investigations into laterally restrained steel reinforced slabs showed that those slabs demonstrated a different type of behaviour and developing strength far in excess of their traditional design strength. So far FRP reinforced slabs have been purposely designed with excessive reinforcement to avoid catastrophic failure due to FRP fracture and also to improve the stiffness of the slab. However using CMA behaviour, the amount of FRP in the slab can be lowered, allows for the design of economical and high performance slabs without compromising its strength and serviceability.

FRP reinforced laterally restrained slabs are very sustainable. Sustainability is expected to be achieved in the UK by emphasising key tasks such as sustainable consumption, lower energy consumption and using sustainable resources etc. Introducing FRP reinforcement and incorporating CMA with the FRP reinforced slabs can obtain three major sustainable achievements.

I. Since FRP is a very durable material, unlike steel reinforced structures the FRP reinforced structures will have longer life span.
II. As a result of considering CMA action, it is possible to reduce the amount of reinforcement required for laterally restrained slab without compromising its performance. Therefore this achieves more with less resource.
III. FRP is a very light weight material compare to steel reinforcement. Therefore associated energy consumption for mobilization and handling in the site is considerably low.

The successful development of FRP reinforced laterally restrained slabs is not only advantageous in achieving good structural design but also help to create a more sustainable world.