History of Concrete Strengths

[JedClampett]

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?

 

[rapt]

Jed,

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!

 

[hokie66]

It sounds like each of us has similar experience, and I think the current practice is much for the better. As to the timelines, I don't know. But some of the buildings here in Brisbane from the 1920's have 800 and 1000 psi concrete.

 

[JAE]

My recollection is that up until the early 1970's, in the US, concrete was generally 3,000 psi.

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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[msquared48]

I recall specifying a lot of 2500 psi mixes in the late 70's and early 80's to get around the need for special inspection with the higher mixes.

Mike McCann, PE, SE (WA)

 

[SlideRuleEra]

Before the 1960's there were two "wild cards" that required the design compressive strength to be set relatively low:

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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[ornerynorsk]

One of my first jobs as a youth was construction laborer, when I didn't know a whole lot about a whole lot. But I do remember 3000 psi as being bantered about by the ready mix drivers and the sight super as being the best stuff money could buy, that the owner commissioning the project must be lighting his cigars with 100 dollar bills. I'd be very curious, has anyone ever had the opportunity to sample and test any of the old Roman concrete? Or medieval concrete, for that matter?

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.

 

[mikeofBelAir]

About 20 years ago, we hired a concrete technician from Mother Russia. He watched me break about a dozen cylinders--3000 PSI spec, but the actual breaks were in the 3500 to 4000 PSI range. He just shook his head and remarked, In my country, someone would be going to prison for wasting a valuable State resource..." His experience was that 3000 PSi concrete broke at 3000 PSI, if it broke higher, then there was too much cement in the mix and cement was a critical resource.

 

[bridgebuster]

Mike - was that the 1870's & 1880's?

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+.

 

[MotorCity]

If you are following ACI 318-08, you are required to use a minimum f'c = 4500 psi for freezing and thawing category F1 and higher according to Table 4.2.1. and Table 4.3.1

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.