Price Correlation to Concrete Strength (and other properties)

[trou33]

Hello. Long time, first time.

Until recently, I've understood that 3ksi and 4ksi concrete mixes were comparable from a price perspective. I had a design manager with a GC indicate that their pricing exercise may have been completed with 3ksi mix and using 4ksi would be an unnecessary cost. He assured me my previously mentioned understanding was incorrect. The 3ksi mix will work, there is nothing extreme about this project, so this is to satisfy my own curiosity.

I know there are situations in which exposure and other factors will drive cost, but for typical mix designs for use in spread footings, grade beams, and ground supported slab (not using the same mix for the footings and slab), what kind of factors go into the difference in cost and what might the actual cost delta be on a yard of 3ksi vs 4ksi?

Thanks for the input.

Make no little plans.

 

[EZBuilding]

My understanding is that it's approximately 10 dollars per cubic yard per 1 ksi increase.

 

[BridgeSmith]

In our area, the standard mix from the ready-mix suppliers is a 4.5 ksi mix. The only difference between specifying 3ksi vs. 4ksi is the few days it takes for the concrete to achieve the design strength. The only potential impact is to the construction schedule, if some components are required to achieve the design strength before pouring others.

 

[trou33]

It's similar in most regions my projects are in. I don't recall a 3ksi mix submitted that wasn't reaching 4ksi or higher. So I'm not sure how the GC in this case priced a 3ksi mix if most suppliers would be providing the same mix either way.

Any estimating personnel out there have input?

Make no little plans.

 

[MrHershey]

I don't recall a 3ksi mix submitted that wasn't reaching 4ksi or higher

You always have to overshoot your mix strength so that even if you're a standard deviation below average, you still hit the required strength. Plants can't shoot for exactly what the specifications say because that means it's basically a coin flip on whether the mix makes strength or not. So their 3 ksi mixes have to come in at 4 ksi in the mix history so that even on a bad day they still hit strength.

So your 4 ksi mixes should be coming in closer to 5-6 ksi on a regular basis.

 

[dauwerda]

The main difference affecting the strength of concrete is the cement content. A 3000 psi mix will typically be 5.5 sack (517 lbs of cement per yard). A 4000 psi mix will typically be 6.5 sack (611 lbs of cement per yard). Cement is more expensive than the rock and sand aggregate that the extra cement replaces, therefore making the raw material cost go up (maybe $5 per yard). (Many more factors come into play for extra high strength concrete)

The other thing to consider is location/supplier. To technically qualify as an approved mix the batch plant must have records of the mix and breaks producing the design strength. Some smaller batch plants may have this for 3000 psi mixes but not 4000 psi mixes which may cause them to put a premium price on a non-typical (for them) mix design.

 

[Tomfh]

It’s a little more expensive. Not hugely, but no one happily pays more even if it’s only a little bit more.

So if you up the concrete grade you will often get pushback.

 

[BridgeSmith]

Yeah MrHershey, that's why I mentioned "a few days" as being the difference in time to reach the design strength. In our case, the typically mix design was to achieve 4.5ksi minimum. The cylinder tests were reaching our specified "class B" strength of 3.25 ksi in 3-4 days, and our "class A" strength of 3.75ksi in about 7 days. We recently changed our specs to 3.5ksi and 4.0ksi design strength.

 

[cvg]

recently finished a project. the 3,000 psi shotcrete came in as high as 4,700 and the 4,000 psi structural concrete came in as high as 7,800 (mainly to shorten the schedule and reduce contractor risk due to weather)
the shortened construction time saved money

 

[txeng91]

I’ve been told $7/yard by a local supplier for upping from 4 to 5 ksi . That said, I spec 3 ksi a lot and can’t remember the last time I received a mix design that qualified for under 4 ksi per ACI. Minimum average 28 day break I usually see is around 4.5 ksi.