2500 psi concrete

[palves]

I have a contractor who wants to use 2500 psi concrete, but usually use a minimum of 3000 psi on all of my projects. The thing is, this has just become standard operating procedure for me over the years and I have no real reason to back it up other than "It's what I've always done." So, does anybody out there use/not use 2500 psi and have a solid reasoning behind their decision? I'm curious what people do and welcome any and all reasons so let 'em fly. Thanks!

 

[structSU10]

ACI has some minimum strength guidelines for durability considerations, many of those 4000 psi + with set w/c ratios and such. For concrete not exposed to freeze thaw, chemicals, etc. you could probably rationalize 2500 psi, otherwise likely higher. I always use 3000 psi minimum, but generally 4000 psi.

 

[hawkaz]

At footings for light framed 3 story or less buildings, SSI is not required if the design is based on 2500 psi

 

[palves]

Hawkaz, that is true per 1705.3, but it is also true that it can be exempt from SI even for f'c greater than 2500 psi as long as the walls support light frame and the footings meet the minimums of Table 1809.7. At least that is my interpretation, but their wording leaves a lot to be desired. Do 2.1, 2.2, and 2.3 ALL need to be satisfied for the exception to hold, or is it just 2.1 and EITHER 2.2 OR 2.3? Thoughts?

 

[Ingenuity]

...at 2,500 psi is it even concrete...and does the batch plant typically 'run' such a mix? I guess they do otherwise the contractor would not be requesting.

I see a lot of concrete that is 25, 30, 40+ years old - some of it is in excellent condition; the rest - not so much. Specifying 2,500 psi (assuming strength is not controlling) I think long-term durability would be your reasoning for rejecting its use.

So the contractor's main reason for requesting 2,500 psi is so that no SI is required? Maybe a red flag!

 

[JoelTXCive]

In Texas, all the production homebuilders use 2500psi concrete for residential slabs, driveways, & sidewalks. In fact, now that I think about it; they do not order any concrete that is not 2500psi.

I think everything on the commercial side and highway concrete is a minimum of 3000psi.

 

[Ingenuity]

In Texas, all the production homebuilders use 2500psi concrete for residential slabs

Post-Tensioned residential slabs too?

 

[hokie66]

Strange how this varies with location. As Ingenuity said, the big issue with low strength concrete is durability. In Australia, our minimum in commercial work is 32 MPa, about 4500 psi.

 

[rapt]

Interesting the practice in different countries.

In Australia, 3000 is the minimum recognised. Anything less is not really considered to be structural concrete! It might be used for mass concrete. And 3000 is only used for footpaths and residential driveways (by those who do not want overly good performance).

Floors without vehicular traffic would use minimum of 3500. Anything with vehicular traffic is a minimum of 4500 up to 6000 for non-pneumatic tyres.

Commercial would be a minimum of 4500 with most using about 6000 these days for durability.

 

[bridgebuster]

Was he asked to bid 2500 or 3000? You bid it; you bought it.

 

[JoelTXCive]

Ingenuity - Yes, conventional slabs and post tensioned. The only time you see a residential slab with conventional reinforcement is on custom homes (in the Houston area anyway).

All the production builders do a double-cable post tensioned system with predominately 2,500 psi concrete. There was one cable set on top of the pads and a second cable draped in the beams.

I'm sure there are exceptions, but when I was building homes 5 years ago they were all 2500psi. Prior to being an engineer, I worked for a large homebuilder and we did over a
thousand slabs a year all at 2500psi.

If I were building my own house, I would use at least 3000, but that's just me.

 

[rapt]

I thought minimum transfer strength for unbonded anchorages was 2900psi. Even if it is lower, like 2200psi, surely they want to get strength up earlier than they would with 2500psi concrete for early stressing, especially if they want to limit cracking. I thought the whole logic behind PT slabs on expansive soils was that the concrete is uncracked, providing a stiffer slab!

Then, from what we hear about US PT house slabs in practice, cracking does not appear to be a worry.

 

[S R P]

It is an interesting topic. In my construction experience in some Asian countries, a thin layer of concrete is used underneath the structural foundation, which is called screed. This is usually made with Grade 15 or 20 concrete (2200 psi or 2900 psi). The thickness varies depending on the application, from 2" to 4". This concrete is never considered for structural strength but just for making a better surface.

However, I have seen may architects in Texas prepare drawings with such small foundations for residential homes with 2500 psi concrete. Based on IRC and IBC codes, minimum requirement is 2500 psi. But I am never confident with that, especially in mid and South Texas with medium to to highly active clayey soil. When I receive those drawings for review and PE signature, I definitely revise them to minimum 3000 psi or 3500 psi depending on the application. I also do not hesitate to go for a completely different foundation design.

 

[concretemasonry]

palves -

You are hiring a contractor and you cannot tell him what strength of concrete to use?

Around here, it is rare to get anything other than 4000 psi. It it even sounds like a curb, sidewalk or driveway you get 4000 psi air entrained or sign a complete waiver/release before unloading. For a concrete supplier, they recognize that contractors that use the bottom end materials result in getting into durability disputes with the claim of poor materials when it

The cost of some cement is about the same or less than a truck going a mile or two further since the trucks, labor and time are the major cost of delivered concrete in comparison to a little cement.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[robyengIT]

table of ruling standard in Europe attached.
Minimum used (good engineering practice) for structural concrete : class C25/30

 

[XR250]

In Texas, all the production homebuilders use 2500psi concrete for residential slabs, driveways, & sidewalks

Probably once they add enough water lay it down fast, it is down to 2000 psi.

 

[dik]

You should be using 3000 psi minimum and if subject to deicing salts or freeze thaw, then consider 4500 psi for durability.

Dik

 

[cvg]

the last 3,000 psi mix I received tested at the following:

3-day avg 3,340
7-day avg 3,760
14-day avg 5,060
28-day avg 5,360

w/c was 0.4, 3 1//2 inch slump and 5.9% air
this mix contained fly ash, so long term strength gain may be even higher.

this is not that unusual, so depending on local practices, the 2,500 psi concrete (minimum) may be perfectly fine since the final product delivered on site is likely stronger than specified minimum

 

[P1ENG]

So the contractor's main reason for requesting 2,500 psi is so that no SI is required?

I think a CA reviewer once made me use 2000 psi. It was existing concrete that we were anchoring into and they didn't want to determine the strength. I don't know that I would a start a new design based on 2,500 psi. Even an average Joe mixing it in a wheelbarrow can get 3000 psi.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[rapt]

cvg,

Problem is some clever person might successfully create and deliver a 2500psi mix that is really 2500psi, and it is not really concrete!