Concrete Strength Substantially Above Required Strength
Concrete Strength Substantially Above Required Strength
(OP)
My question is in regards to a project that we just worked on that required 6,000 psi concrete for the building columns, and elevated slabs. The concrete strengths came back substantially higher, most of them on the order of 9,500 to 10,000 psi after 28 days. The owner is concerned that this will lead to more cracking in the structure. There is a substantial amount of concrete that is architecturally exposed and cracking wouldn't be good for the visual appeal.
Are there any references on what to do if the concrete strength is substantially higher than assumed in design? The only issue that I can see is that the minimum rebar for beams would be increased due to equation 10-3 in section 10.5 of ACI 318-11.
Thanks,
Scott B.
Are there any references on what to do if the concrete strength is substantially higher than assumed in design? The only issue that I can see is that the minimum rebar for beams would be increased due to equation 10-3 in section 10.5 of ACI 318-11.
Thanks,
Scott B.





RE: Concrete Strength Substantially Above Required Strength
I think f'c by itself may be an inadequate measure of a slab's tendency to crack. Other factors, which affect f'c, are more related to cracking. w/cm (which is also directly related to f'c, but not the sole determining factor) is associated with shrinkage cracking. Low w/c ratio leads to less cracking (and higher f'c), so I would assume you're actually be on the winning side on that one.
I think curing would have a much larger effect on surface cracks than f'c.
RE: Concrete Strength Substantially Above Required Strength
Thanks for your response. Currently it appears that the slabs/columns look great, I have not seen any cracking anywhere.
RE: Concrete Strength Substantially Above Required Strength
RE: Concrete Strength Substantially Above Required Strength
RE: Concrete Strength Substantially Above Required Strength
RE: Concrete Strength Substantially Above Required Strength
Since the material is exposed, you should verify things like the sensitivity of the aggregates to potentially higher alkalinity (higher strength concrete frequently has more cementitious material). There may be a durability issue down the road, but not strictly due to the strength. If the aggregates are too reactive, you can potentially reduce reaction by excluding some moisture with a sealer or other water repellent.
For next time, if you are exposing concrete and want very good crack control, use a tighter grid of small bar (or wire), meeting ACI 350 (environmental structures). We tend to design with minimum stirrups and ties, but in exposed members, meeting the minimum T&S requirements of ACI 350 (twice those of ACI 318) for transverse steel will help control nuisance cracking.
RE: Concrete Strength Substantially Above Required Strength
I've actually never encountered this, so I'm not sure what the engineer's responsibility for ensuring adequate confinement under ACTUAL strength vs DESIGN strength is.
RE: Concrete Strength Substantially Above Required Strength