I usually design precast concrete members for the common concrete strength of f'c=4,000 psi at 28 days. Occasionally, a mix design will be submitted that has a very high strength. In one case, the mix that was submitted had a history of cylinder breaks above 7,000 psi. This is 175% of the specified strength!
Normally in engineering extra strength is good, but this seems excessive. Steel grades typically specify a strength range (min and max), but I have never seen a maximum compressive strength in a concrete specification.
Should I reject these very-high strength concrete mixes?
What could be the complications of too high of a compressive strength?
There are only two possible areas of concerns that I can think of:
- Modulus of Elasticity is directly related to strength. A high strength may cause the concrete to be more prone to cracking (serviceability concern).
- Un-designed high strength may be detrimental in seismic design where you want members to fail in certain locations and in certain modes. Note, my situations typically include moments (either beams or beam-columns). There likely wouldn't be any concern in a purely compression member.
The reason for submitting such a high strength mix seems to be that the precaster wants to get the piece out of the form quickly, so they add lots of cement to insure that the strength requirement is met.