The Economy of Concrete Compressive Strength 1

[elinwood]

I've never really thought about concrete strength and have always used the standard 4000 psi concrete. Recently, I have been reading up on some of the factors to take into consideration when specifying a concrete strength, such as the inherent safety in designing for a lower concrete strength - such as 3000 psi and then specifying a 4000 psi concrete for peace of mind during construction ( In case the concrete does not pass the compressive test) .

How economical is high strength concrete ? The cost of concrete that I have found is as follows (I live in the Southeast Asia so prices are a lot lower):

3000 psi - $67 / M3
4000 psi - $77 / M3
5000 psi - $85 / M3

What would your justification be for selecting 4000 psi concrete as opposed to 3000 psi one ? or even 5000 psi as opposed to 4000 psi concrete

 

[Lion06]

I never spec less than 4,000 psi concrete. I recently had a discussion with a concrete contractor and the price difference in price from 4,000 to 5,000; from 5,000 to 6,000; and from 6,000 to 7,000 is about 5%.

I just recently spec'd 5,000 psi for foundation walls where I would normally use 4,000. I typically use 4,000 as a minimum and increase from there if I need to.

 

[hokie66]

32 MPa concrete, about 4500 psi, is now the norm in Australia for structural floor systems. Higher if exposure conditions dictate. For columns, it is more economical to use higher strength concrete than to increase the amount of reinforcement. 50, 65, 80, even 100 MPa concrete strength is standard practice in high rise buildings here.

 

[asixth]

For in-situ floors I don't think there is any benefit for anything higher than 40Mpa which I think would be 5000-6000psi. Mainly they are driven by shear and deflection requirements so driving up the the grade drives up the costs without saving much materials. Column, walls and footing sizes can be saved which are compressive strength driven. Sometimes precast elements as well.

 

[OHIOMatt]

Omitting concrete properties other can compressive strength is not advisable. Typically, higher compressive strengths are achieved with higher concentrations of cement in the batch. More cement means more water which results in more shrinkage and cracking. This can be especially worrisome in slabs and walls. In the past I used the philosophy that stronger is better, but eventually that caught up to me. My philosophy today is to use the concrete strength required for the application.

 

[rowingengineer]

I am on a different court to the above.

I can't see many reason to spec footings above 25mPa, and if it was a mass pier than I would probably spec 20mPa.

for slabs that are on ground with low loadings I prefer lower cement content concrete, for a house slab I would prefer 20mPa over 40 mPa any day of the week.

As for spec'ng a higher strength concrete for the fun of it, it can be interesting if your pushing Mcr. I prefer to get the concrete I specified delivered and require.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[SteelPE]

Interesting.

I worked on a project where we needed some additional strength in the walls from the standard 3,000 psi concrete so we specified 4,000 psi concrete. The project was phase 2 with phase 1 complete under the same circumstances. The contract was design-build with the contractor as the lead (who I was hired by). I received a call from the contractor complaining about the increase to go from 3,000 to 4,000 psi concrete which they didn't "budget" for. I explained that the specifications for phase 2 were exactly the same as phase 1 and that he never complained on phase 1 and should have known about the switch in phase 2 after constructing phase 1 (which we had compressive test on that met the needs of the specifications). I never knew increasing the compressive strength would cause that much of a problem.

We usually use a compressive strength of 3,000 psi. Most of the concrete work we do is for foundations and suspended slabs on metal decking. We have had complaints about SOG cracks and cracks on suspended slabs when we have specified higher strength concrete and complaints from clients about the cost. In the end I always explain to my client that the only way to keep concrete from cracking is to keep it in the bag.... or reinforce the s**t out of it (within limits of the code of course).

 

[JedClampett]

We always, always, always, use 4000 psi or greater. It's better for wear and absorption, besides all the strength advantages. The code mandates 4000 psi or greater for all concrete in contact with all but the least aggressive soils (ACI 318 Table 4.3.1).
It looks like your concrete cost is directly related to strength. But if you consider total costs, with formwork, reinforcing and finishing, the concrete cost is at most one third of the total placed costs. And for elevated work or walls it's about 10%. So your $10 a cubic meter is maybe 1% to 3% of the placed concrete costs.

 

[ztengguy]

@ SteelPE, If I read that right, it was spec'd 4000, he used 3000, but the strengths came up to 4000?

 

[SteelPE]

That's not what happened. In the test reports they list the compressive strength that needs to be attained in the specifications. Everything matched. I think he was complaining just for the sake of complaining.

 

[shaneelliss]

Concrete costs vary quite a bit depending on where you are, but the installed cost is so much more than the material cost that it should be virtually no cost difference, unless the job is huge. Our costs for delivered concrete are $107/yard for 4000 psi and $113/yard for 5000 psi (6% difference). However, our installed costs for concrete (including rebar) range from $700/yard to $1200/yard depending on the complexity. So the cost difference even at the $700/yard range is less than 1% for a 25% increase in strength. It seems pretty economical to me.

And I don't agree that higher strength concrete always means more shrinkage and cracking. There is more cement but the W/C ratio for higher strength concrete is usually lower. If mixed and placed correctly, the higher strength concrete is stronger, more durable, and provides greater corrosion protection for your rebar.

 

[Ron]

The durability of concrete is related to its compressive strength. Consider durability needs first and almost every time you'll meet or exceed the strength need. Cost differential can be negotiated. It is not significant except for very large placements.