US Specification of Concrete Strengths
US Specification of Concrete Strengths
(OP)
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
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






RE: US Specification of Concrete Strengths
RE: US Specification of Concrete Strengths
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...
RE: US Specification of Concrete Strengths
RE: US Specification of Concrete Strengths
RE: US Specification of Concrete Strengths
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
RE: US Specification of Concrete Strengths
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)
RE: US Specification of Concrete Strengths
http://www.qpa.org/pdf/readymixconcrete06.pdf
John
RE: US Specification of Concrete Strengths
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.
RE: US Specification of Concrete Strengths
RE: US Specification of Concrete Strengths
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
RE: US Specification of Concrete Strengths
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.
RE: US Specification of Concrete Strengths
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).
RE: US Specification of Concrete Strengths