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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.

RE: Concrete Strength Substantially Above Required Strength

The requirement of 10.5.2 is a nominal requirement to ensure that there is a minimum amount of reinforcing beyond the cracking moment.

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

(OP)
frv,

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

Why worry?  Be happy.

RE: Concrete Strength Substantially Above Required Strength

(OP)
The architect/owner is worried about additional cracks appearing in the exposed columns/slabs. The strength of the concrete is great, seeing as the foundations came in 1500 psi low in the early going of the job.

RE: Concrete Strength Substantially Above Required Strength

Strength and tendency to crack are not closely related.  Concrete cracks when its tensile strength is exceeded, and concrete with high compressive strength also has relatively high tensile strength.  You are concerned about the wrong thing.  If the concrete cracks, it is for other reasons than too much strength.

RE: Concrete Strength Substantially Above Required Strength

2
If you don't have cracks yet (mist water on it and you get dark lines at cracks first), you won't get more drying/shrinkage or curing-related cracking.  These happen very early in the process.  This would happen if you had poor curing practices, excessive cement paste fraction, or too much water.

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 wouldn't so much be concerned with cracking, presuming adequate placement/curing methods were used. But if the significantly over-strength concrete was present in high seismic lateral elements, could run into confinement problems. Required transverse reinforcement is dependent on concrete strength for axial/flexural members of special concrete moment frames and for boundary elements in special concrete shear walls. So 10 ksi concrete is going to require a lot more confinement than 6 ksi concrete.

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

Whoa there TX....drying shrinkage occurs for years! Initial drying shrinkage is the usual cause of early cracking, but drying shrinkage cracks can occur long term and certainly get wider with age.

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