In commercial applications carbon fiber strengthening is designed to help strengthen an under reinforced concrete wall or beam. These straps are not intended to be the only reinforcement. Normal reinforced concrete design is a delicate balance between the crushing strength of concrete and the tensile strength of reinforcing steel. To better illustrate this lets look at the graphic below:
When a beam deflects under loading the the top is under compression and the bottom is in tension. Normal concrete is good in compression, but poor in tension. The rule of thumb is the tensile strength of concrete is 10% of its compressive strength. If this beam was constructed with 3000psi concrete it will have a tensile strength of 300psi. With out reinforcing steel the beam is very weak, so reinforcing steel is added in the tensile area to compensate for the low tensile strength of the concrete. The more reinforcing steel you add the more load the beam can carry up to a point where the compression zone reaches our maximum compressive strength of 3000psi in this example. Any additional loading will crush the concrete in the compression zone causing a brittle failure of the beam. Brittle failure for a lack of a better word would be a sudden and explosive event with little or no warning. To avoid this engineers intentionally under reinforce concrete beams, which results in large beam deflections long before the concrete reaches 3000psi in the compression zone. This large deflection creates noticeable cracks in the tension zone, which is another visual sign beam failure has occurred. Basically over reinforced beams cause sudden brittle failure with little or nor warning and kills people, under reinforced beams have noticeable deflection and cracking giving ample time to make repairs and prevent death. That’s why concrete design codes limit the amount of reinforcing steel in a beam.
Now back to carbon fiber straps applied to beams or foundation walls. If a beam or wall already has the proper amount of reinforcing steel adding carbon fiber straps to the tension side of the wall will result in an over reinforced wall, which can result in brittle failure, i.e. the wall basically explodes inward, NOT GOOD. Carbon fiber straps in commercial applications will strengthen walls or beams that were under reinforced, so adding carbon fiber provides the additional reinforcement needed to increase the bending strength. But engineers do the math to ensure the wall or beam is NOT over reinforced.
If you haven’t fallen asleep yet…These foundation companies that apply carbon fiber straps don’t know the concrete strength of the wall, they don’t know how much reinforcing steel is in the wall, if any. Without this information you can’t properly design the amount of carbon fiber you need, if any. If too much is applied the brittle failure I described could occur, NOT GOOD.
