History of Concrete Strengths
History of Concrete Strengths
(OP)
I'm noticing that the standard concrete design strengths are edging up, from 4000 to 4500 psi. I can remember when 3500 psi was the state of the art, then it was 4000 psi, now it looks like 4500 psi will be typical.
Is there a chart that roughly indicates the standard mix strengths through time?
Anyone else noticing this?
Is there a chart that roughly indicates the standard mix strengths through time?
Anyone else noticing this?





RE: History of Concrete Strengths
When I started in Australia in the 1970's, standard for suspended RC was about 3000 - 3500psi (20 - 25Mpa). PT slabs used about 4500psi (30MPa) then. Minimum is now about 4500psi (32MPa) and common would be about 6000psi (40Mpa) which is required by Australian codes as an absolute minimum for durability when anywhere near salt water wind zones and most of Australian construction is near salt water!
RE: History of Concrete Strengths
RE: History of Concrete Strengths
Since that time most structural concrete has been 4,000 psi with slabs-on-grade running 50-50 with 3,000 and 4,000 psi.
Precast used to be 5,000 psi and seems like it might be creeping up a bit higher in practice but still a lot of 5,000.
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RE: History of Concrete Strengths
Mike McCann, PE, SE (WA)
RE: History of Concrete Strengths
1. Until after World War II, ready-mix concrete was not commonly available - except in cities. Variable quality local aggregate was used, and a Contractor's on-site concrete mixer was "charged" with measurement by volume (e.g. 1 wheelbarrow cement, 2 wheelbarrows fine aggregate, and 4 wheelbarrows coarse aggregate... add water - by experience, mix, and place.)
2. Before concrete strength design was formally introduced by ACI in 1963, the safe working stress was arbitrary set low. In 1940 the value was 45% of the (laboratory) 28-day compressive strength. This was to compensate for variations in field-mixed concrete.
From first-hand experience, in the 1970's we refurbished and widened, highway bridges that were constructed in rural locations in the 1930's. That 1930's concrete was every bit as good, or better, than modern 4000 psi concrete. The earlier methods were crude, but workmanship and attention to details (accurate charging of the mixer, vibration, curing, etc.) were superior.
IMHO, if the design strength of a pre-1960 structure is known, test samples from the existing structure. The actual concrete strength is likely much higher than the design strength.
For structures from before 1930, testing samples is more important - quality of cement was a variable also.
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RE: History of Concrete Strengths
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: History of Concrete Strengths
RE: History of Concrete Strengths
NYSDOT still uses 3000 psi for bridge decks and substructures, although the mixes will typically result in much higher strengths. The idea being that the strength will never fall below 3,000 regardless of the time of year when it's placed. Precast panels have a minimum f'c of 5,000 minimum & prestressed is 8,000 minimum. There is a 4,000 psi mix for decks and substructures if needed.
I've seen test results from cores of bridges built in the 1940's and 50's on the order of 4,000+ to 8,000+.
RE: History of Concrete Strengths
There are a few other exposure categories that require it also, but the freeze thaw condition usually controls in the midwest for those following ACI318-08.
Have not found a way around it yet......still looking, but its usually not a huge bump in cost to increase the strength by 500 psi.