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cement psi question
- Thread starter lt1driver
- Start date
timewarp
Well-known member
Unless it's a large commercial building you don't test it, if you want to test it you cast some cylinders when you do the original pour and then crush them to see how strong they are, one at 7 days to see how it's doing, then another at 14 days and the remaining at 28 days, by 28 days the concrete should be at it's designed strength. Most people just buy the concrete mix from a reputable concrete company and take their word for it that they made the mix correctly for the strength you ordered.
Torque1st
MEMBER EMERITUS
Sample cylinders are one way. You can also have some core samples drilled from the pad and tested. The amount of water added by the contractor also affects the strength.
A device called a windsor probe can test the compressive strength of concrete in place. It is essentially a stud gun which fires a pin into the concrete. The depth of penetration determines compressive strength.
http://civil-online2010.blogspot.com/2010/10/penetration-resistance-or-windsor-probe.html
http://civil-online2010.blogspot.com/2010/10/penetration-resistance-or-windsor-probe.html
ConCretin
Well-known member
As mentioned, there are a number of ways to determine the compressive strength of concrete. In any case you'll likely need to hire a testing lab to perform the work. You should be able to find a local lab in the phone book.
Test cylinders typically achieve 70% of design strength in 7 days and should hit design strength in 28 days. The standard for determining the acceptability of concrete tested in place is based on the applicable ACI specification.
Test cylinders typically achieve 70% of design strength in 7 days and should hit design strength in 28 days. The standard for determining the acceptability of concrete tested in place is based on the applicable ACI specification.
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sberry
Banned
Its not really an issue for new concrete, most of it is above 4k, they basically want to make sure you are not bolting to some old crumbling material.
ConCretin
Well-known member
Its not really an issue for new concrete, most of it is above 4k, they basically want to make sure you are not bolting to some old crumbling material.
Not sure I would agree with this statement. 3,000 psi concrete is far more common than 4,000 in residential construction.
Before I hung a car over my head, I'd want to know my concrete was adequate.
Dan in Pasadena
Well-known member
Most of our lowest strength concrete (3000psi) develops well more than that after 28 days anyway.
To the lt1driver, You don't need to worry about this if the concrete was batched by a reputable batch plant and placed by a reputable contractor. The batch ticket indicates the maximum amount of water that can be added and not violate the water/cement ratio - one of the primary concerns. If the placement wasn't on a super hot day where the contractor MAY have felt the need to re-temper the concrete more than he should have (add too much water), you're fine.
To the lt1driver, You don't need to worry about this if the concrete was batched by a reputable batch plant and placed by a reputable contractor. The batch ticket indicates the maximum amount of water that can be added and not violate the water/cement ratio - one of the primary concerns. If the placement wasn't on a super hot day where the contractor MAY have felt the need to re-temper the concrete more than he should have (add too much water), you're fine.
Wardster
Well-known member
after you have the pad poured to meet 3500 psi, how do you know/test to be sure it meets the specs lift maker required. thanks.
If you had a copy of the batch ticket and new how much water they may have added during transit or after it showed up, then one could make a reasonable assumption of its strength based on the mix design.
The windsor probe is another alternative, but I have only witnessed it being used one time in my 15 years of heavy civil construction because we make test cylinders to verify strength.
Falcon67
Well-known member
Around here it's more like 3500~4500 or better from what the builders tell me. I asked for 3500 min on the shop and the plant said their 5 sack mix would go 5K in 30 days easy. The guy with the plant says it's easy - we're a small area and everybody knows everybody. You rip somebody on a pour or provide bad service - words gets around quick, the phone stops ringing and you're out. I used the "#2" guys here that want to eat #1's lunch on price and service.Not sure I would agree with this statement. 3,000 psi concrete is far more common than 4,000 in residential construction.
Before I hung a car over my head, I'd want to know my concrete was adequate.
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Around here it's more like 3500~4500 or better from what the builders tell me. I asked for 3500 min on the shop and the plant said their 5 sack mix would go 5K in 30 days easy. The guy with the plant says it's easy - we're a small area and everybody knows everybody. You rip somebody on a pour or provide bad service - words gets around quick, the phone stops ringing and you're out. I used the "#2" guys here that want to eat #1's lunch on price and service.
I'd be aprehensive to believe that they easily see 5000 psi in 30 days from a 5 bag mix, especially in the "real world". Here's my reasoning: A 5 bag is typically considered to be in the same strength class as a 3000 psi mix. Although it may break at 3500 - 5000 psi from their specimens, that does not mean the concrete in your floor is 3500- 5000 psi. Why? Because the test cylinders see teh absolute optimum conditions, they're sealed inside plastic containers for the first day or so, when they're hauled very carefully to the test lab, and then submersed in a 70 degree water bath until their break dates (usually 7, 14, & 2@28 days). Unless you have your entire slab enveloped in a sealed plastic "container" and have it submersed in a temperature controlled water bath, you're cast-in-place concrete will never achieve the strength as the test specimens. Thus, most decent ready-mix suppliers shoot for a 10-20% margin in additional strength to give the end user some margin of error for the environment, etc........
ConCretin
Well-known member
Joes169 has it about right. If someone is claiming 5000 psi concrete from a 5 sack mix, they are full of it. Compressive strength is a factor of water cement ratio, period. To get 5000 psi out of a typical 3000 psi mix design, you wouldn't be able to add enough water to get the concrete out of the chute. That's not going to happen in the real world.
To the OP's original question; If you haven't placed the slab yet, buy from a reputable company, keep the slump under 4" or use a mid range water reducer and cure the slab properly. Hire a testing lab to take a couple cylinders if you want to be certain.
If the slab is already in, hire a testing agency to do an in place test. It will cost a couple hundred bucks and your life - or at least your piece of mind - is worth more than that.
To the OP's original question; If you haven't placed the slab yet, buy from a reputable company, keep the slump under 4" or use a mid range water reducer and cure the slab properly. Hire a testing lab to take a couple cylinders if you want to be certain.
If the slab is already in, hire a testing agency to do an in place test. It will cost a couple hundred bucks and your life - or at least your piece of mind - is worth more than that.