On the combined axial and bending capacity of FRP bar reinforced concrete

One of the common approaches to creating a soft eye for a TBM to drive through is the use of GRP reinforcement. It’s been a while since I’ve designed a structure with GRP reinforcement so I’ve spent a bit of time over the last couple weeks refreshing myself on the current state of the art in concrete reinforced with GRP bars.

The first thing I needed to know is how much a capacity reduction is there when using GRP and possibly more interestingly when looking at our classic moment thrust envelopes, how does the reduction exhibit itself. fib have published a very good bulletin looking at how standards around the world undertake GRP design and makes its own recommendations about how to design for GRP rod reinforcement.

The images below show the results of an assessment I've undertaken using the Eurocode 2 combined with the fib recomendations. I have derived capacity curves for unreinforced concrete, steel rod reinforced concrete and two different cases with GRP. The first case is a typical low performance GRP bar and the second is a more modern high performance bar, both of which have been used for soft eyes on major UK projects. I’ve based these curves on a 1.2m thick diaphragm wall reinforced with 40mm bars at 150mm c/c.

Capacity curve comparison for lower specification GRP bar reinforced concrete with steel reinforced and unreinforced concrete. 

Capacity curve comparison for lower specification GRP bar reinforced concrete with steel reinforced and unreinforced concrete. 

Capacity curve comparison for higher specification GRP bar reinforced concrete with steel reinforced and unreinforced concrete. 

Capacity curve comparison for higher specification GRP bar reinforced concrete with steel reinforced and unreinforced concrete. 

It is obvious that there is a big capacity difference between the low spec and the high spec GRP bars. This improved performance is through a combination of the ultimate tensile strength, a reduced time related degradation and the improved elastic modulus. It is also interesting how the high performance bars differ in capacity from the steel. At high and low axial forces the behaviour is very similar, but in the middle range you can’t achieve the same peak capacity with GRP reinforcement that you can with steel.

So the relative performance of GRP bars depends very much on the specification of the bars. High spec bars can get very similar performance to steel, whereas the lower spec bars replacing something like a 40mm steel bar with a bundle of 3 32mm bars seems an entirely reasonable outcome.