US Specification of Concrete Strengths 2

[jonse]

Simple Question

Are design strengths of concrete in the US based on Cylinder Strength or Cube Crushing Strength.

I am doing some fairly simple design work for a friend building his own house in the US (I am in the UK). The concrete strength he quoted me (3000psi = 20N/m2) seems low compared to what we would use in the UK for reinforced concrete (At least 30N/mm2 = 4500psi)

Thanks in advance

Jon

 

[whyun]

Cylinder strength at 28 days is standard.

 

[whyun]

Oh... 3000 psi is a typical compressive strength we specify here in the US. If we need higher strength, we may go 4000 psi, 4500 psi, etc. I have not specified higher than 5000 psi so far.

If the volume of work is small, why not specify 4500 psi if it will make you feel more comfortable. Cost impact won't be too significant...

 

[JAE]

For house construction in the US - 3000 psi is very much the standard.

 

[UcfSE]

Is there a reason why the UK uses a much higher typical strength concrete than the US? Is it maybe environmental or some other concern, code requirement, or is that just how they do it across the pond?

 

[apsix]

The 30 N/mm2 cube strength quoted is approx. equal to 3600 psi cylinder strength, so the strength increase is not as large as it first appears.
The british concrete code BS8110 lists 30 N/mm2 as the lowest grade required to achieve adequate durability for 'mild' exposure classification ie. concrete protected against weather or aggressive conditions (Tables 3.2 & 3.3).

John

 

[BigH]

Just for those who haven't caught it in other threads - the rule of thumb to convert cube strength to cylinder strength is:

cube strength times 0.8 = cylinder strength
cylinder strength times 1.25 = cube strength

This is a rule of thumb - actually there is variation depending on the strengths involved. According to Shetty the ratio of cylinder to cube is in the order of 0.9 for cube strengths greater than 30MPa - 0.8 is good for cube strengths of less than 25 or so. Others have a slightly different take on this. There is one older thread that gives some BS standard details on this aspect - sorry I don't have the thread number handy - may try a search.

(Shetty Concrete Technology Theory and Practice 5th Revised Edition 2002)

 

[apsix]

The official conversions (european)can be found here;

http://www.qpa.org/pdf/readymixconcrete06.pdf

John

 

[PT999]

For one and two story homes:

The IBC (I am looking at the Residential Code of New York Stated which is part of the IBC code group but has maps and figures relating to the entire US ) states that even less compressive strength is required to comply with the code's min requirements:

Table 402.2
2500 psi for foundations not exposed to the weather.........
based upon weathering probabilites in the US

The code has a map of the US with 3 different weathering probability areas, Severe (northern tier of US), Moderate (southern states such as North Carolina, most of Oklahoma and New Mexico, and Negliglble areas (Florida, southern Texas, Southern California. But 2500 psi is good for all three for the "not exposed" case.

This code also states 3500 psi for severe areas of the US for specific areas such as porches, carports and garage floor slabs, and steps exposed to the weather

So, it depends on which State!

But go with 3000 psi, it is standard, except for the specific areas stated above where 3500 psi is required, and provided that the specific location uses the IBC Code.

 

[BFPartners]

Just remember something when designing homes... every concrete truck which is turned away at an inspector controled site (say a commercial, industrial or government job) usually ends up at a residential site where there is no inspector. If you specify too high a concrete strength you might not get what you want. Hire an inspector for some quality control...

 

[jonse]

Thanks for the responses - The confirmation that the US uses cylinder strength as the basis for specifying concrete was what I needed to know.

It is curious that even accounting for the Cylinder Strength - Cube strength conversion, Concrete in the US appears to be generally weaker than in the UK.

Cheers

J

 

[Ron]

jonse...It would appear that the British Standards, relative to the US Standards, place a greater emphasis on durability rather than compressive strength. Glad to see that. I have long been a proponent that if you design concrete mixes for the durability required for the application, compressive strength becomes a benefit, not a requirement.

While the minimum standard in the UK might be a bit higher than the minimum in the US, both are adequate, by historical performance, for the application.

 

[SlideRuleEra]

On many commercial and industrial jobs contractors have other things to do besides waiting for concrete compressive strength to come up to spec. Bids are often prepared with a mix design that meets spec in 10 to 14 days (instead of the 28 day standard). The result is concete that has signicantly higher "design" compressive strength than required.

There are sometimes reasons to consider limiting MAXIMUM compressive strength (within reason). More cement in the mix - more shrinkage - more tendancy to crack. I have heard of this being done for applications such as unreinforced sidewalks (in mild climates).

 

[BigH]

jonse - don't think the US concrete is "weaker" - it is just that both countries specify different compressive strengths for a similar application. Sort of like the dollar and the pound sterling. When I go to McDonalds in the US I pay 4.59 or so for a Quarter Pounder Meal Deal - in London, I paid 4.29 or so for the same thing. Does that make the US dollar stronger? Couldn't see someone in US paying nearly US$8 at McDonalds for one deal meal - maybe in 10 years!