Higher compressive strengths than required 1

[republic08]

Hi Guys,

I have a concern regarding excessive compressive strength of concrete for flat slab structure. We have designed one concrete structure (2 storeys) and we are getting concrete test results of almost 65MPA (9427psi) against 35MPa (5000psi) as specified per drawings for slabs.

My question is, does this higher compressive strength than required per design, harmful in anyways to the conrete behaviour in terms of brittleness, resilience, cracking etc.

Appreciate for your input.

 

[JedClampett]

Nope, as long as the higher strength is obtained by good technique and not just adding more cement. High cement content (>800 lb. per cu. yd.) causes excessive shrinkage.
It seems that an 80% overage does raise a flag, however.

 

[a2mfk]

I'm no RC expert, but could that affect some of your area of steel and ductility assumptions, especially if you are in seismic areas?

 

[Lutfi]

I always hear contactor bragging that the test breaks came way higher than the specified concrete strength. I am sure this happens more frequently than not. For example, I had a project recently that the test breaks came 2000 psi over the specified strength of 4000 psi. I think this destroys the balanced design theory and I have one heck of a time explaining it to a contractor and owners.

Any thoughts on this?

Regards,
Lutfi

 

[JAE]

I don't see a problem with this. The higher strength provides better deflection control and doesn't screw up the requirement and preference to have the reinforcement yield prior to concrete crushing (i.e. you max. steel limit will actually go up).

If your development lengths are all still based on the lower prescribed f'c, then it all should work fine.

Minimum reinforcement in beams and slabs is based upon equation 10-3 in ACI 318 and that includes an f'c parameter - but it also says you need not go over 1.33 x As(required) so that lets you off the hook.

Beyond that - yes, richer mixes sometimes shrink more but with an elevated, formed system the shrinkage stresses don't build up as much.

 

[DWHA]

If you recheck the capacity using higher concrete strengths, the code will push you into using a lower phi factor. But I bet, even with the lower phi factor you will not have a less strong system. And you will still meet code.

 

[republic08]

Thanks guys for your input.

It appears that it more like a shrinkage issue in the earliest setting time of concrete than any design issues. Concrete strength exceed with the time anyways for a certain period of time.

 

[JedClampett]

In this same vein, I've recently got a reinforcing submittal with much higher yield strengths than design. I normally expect that A615, grade 60 reinforcing is almost spot on (Between 61 and 63 ksi). But the submittal I got had yield strengths of over 80 ksi. I looked at A615 and there's no maximum. It's a low seismic area, so ductility is not a huge concern.
I ended up approving the submittal, as the structures are pretty insignificant and I really didn't have a reason to reject it.
But the whole design theory of ultimate strength of concrete is based on a well controlled yield point, giving sufficient warning of overload. There are calculations that adjust phi based on whether the section is tension or compression controlled. It's never a net negative to have stronger steel, but it makes me a little uneasy.
Anyone else seeing this trend?

 

[hokie66]

The trend has been to use the higher strength reinforcing as standard. In Australia, the standard is now 500MPa, or about 70ksi. The industry changed from 400MPa a few years ago. I didn't support the approach, but mine was a minor voice in the wilderness. Too many people dwell only on strength without thinking of the other ramifications.

 

[Lutfi]

JedcClampett, I thought ACI 318 does not allow or does not have provisions for any rebars with strength higher than 80 ksi. I need to look it up again since you brought it up.

Regards,
Lutfi

 

[JedClampett]

Lufti, as far as I know, ACI only addresses specified yield strengths. So I specified 60 ksi. They never seem to mention what to do if you get something more than that.
In 21.1.5, there's restrictions on strength of reinforcing and ratio of yield vs. tensile strength, but it's only in certain seismic design categories.
I suspect that someone had a batch of reinforcing that couldn't be used in California because of 21.1.5, so they dumped it in Arizona.