Effects of exceeding maximum concrete temperature

[structural-eng]

I have project in Texas where the concrete test revealed that the concrete temperature of at least one of the trucks was 98 degrees F during the pour. The contractor was 3/4 into pouring multiple large piers and decided to use the concrete to finish the pour even though the temperature exceeded the 95 degree ACI limit. I didn't find out about this until almost 2 weeks after the pour. They submitted concrete strength result tests for us to review in hopes we would say the high concrete temperature wasn't a concern. I explained that strength tests weren't helpful because they were lab cured and aren't representative of the in-place concrete so they won't reveal if there is damage from the high temperature. I recommended the contractor discuss with Terracon whether cores could be taken and inspected to confirm there was no damage to the concrete. I recently got compressive test results for cores that Terracon took from each of the 3 piers. Two of the cores broke fairly close to the strength that the 28-day lab cured cylinders broke at (~4000 psi). The third cylinder broke at 2180 psi. I have no idea if the low break is a sign of thermal damage or possibly for some other reason (like the core being damaged when it was obtained).

I can't find much information on what to do when the concrete temperature is exceeded. I was expecting Terracon to inspect the samples under a microscope for signs of thermal damage but was not involved in the conversations with Terracon on what tests they would recommend for this situation. Should further inspection be required? Are there other tests that can be done to allow the concrete to remain? The structures have been installed on the piers so repairing or replacing them will start a battle. I have checked the structural capacity of the piers and even at 2000 psi concrete, there is not a structural concern provided the concrete isn't damaged.

 

[dik]

Ron should drop in for this... Thin section examination will likely show if there was any damage to the matrix, realising that the test area is only a small part of the structure. The big problem with concrete testing is determining what to do with the test results when they come back. You may want to check with a testing agency about what the results might be, before you actually do the testing. A clear methodology has to be in place to accept or reject the work, and that's the hard part.

In my experience high temperatures have not been a problem, except I've not encountered mix temperatures of that magnitude.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[nackra]

reduction in strength and durability problems down the road.

https://www.forconstructionpros.com...8/twining-inc-hot-weather-effects-on-concrete

 

[hokie66]

So this concrete is in large bored piers? There should be some penalty for using concrete of that temperature, but that won't entail replacement.

Isn't there inspection at the site? What process does the concrete plant have to reduce the temperature of the fresh concrete? You mention the 95 degree ACI limit, which may be the absolute maximum, but what does your project specification say? Mine usually said 30 degrees C, or 86 degrees F. In Texas, you would think ice plants would be mandatory for concrete suppliers.

 

[structural-eng]

The piers are 4' diameter x 23'. There is some inspection and testing on site (hence the caught the high temperature) but the project is an electrical substation so there's no building official to hold anyone accountable (only a really picky owner who will surely backcharge them for reduced durability at a minimum). We're working for the general contractor so there's no spec other than the owners spec. Our documents and the owners spec both refer to ACI 301 and the ACI Guide for Hot-weather concreting. The concrete contractor and ready-mix supplier have since taken steps to prevent future temperature problems (using ice, pouring at 3 am, etc).

 

[dik]

That says a bunch... I'd ask the contractor to get an engineer's report that says it's acceptable. This has 'court case' written all over it... it's best to side step this if you can.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hokie66]

dik,

Didn't you read the part about the OP being the contractor's engineer?

 

[dik]

Sorry Hokie... I didn't. I missed he was working for the General... His first post referred to the contractor as a third party... In any event, it doesn't change the reply much. I guess it's a matter of finding out what the owner wants and if it is acceptable to get an independent consultant to prepare a report, or get testing done, with a clear understanding of what will happen as a result of the testing results or the report.

Judging from the positon of the owner... it still has 'court case' written into it... unless the contractor is prepared to pay the 'ransom' or to remove the offending concrete.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BigH]

I'll put my 1 somoni into this discussion (Tajikistan currency) . . . Let's obtain some information, if I may. What was the design compressive strength of the mix? Did the mix's cementitious content based solely on cement or was there a percentage of fly ash involved? What kg of cement? of fly ash if used? What grade of cement was used? Temperature rise is in the order of 5 to 7 degC per 50 kg of cement - so if you have 350 kg of cement, then one would expect a rise in temperature of between 35 to 50 degC. The other thing is that the piers are relatively small - it is not mass concrete where the dimensions are very large - with the piers you will have the effect of being in the ground - typical temperature of say 20 degC?? with a maximum "radius" of 2 ft (600 mm) from the center of the pier to the surrounding soil. In a mass concrete the dimensions can be up to 2 m per placement thickness but very large in area. Your lab should be able to provide you some guidance as to the nature of the temperature profile across the pier and if the rate difference from center of pier to edge is too much. Whilst ACI gives a limit, in general, actuality might be much different and I have suspicions that you do not really have a major problem. But Ron is the MAN in these cases! By the way - see Neville's seminal paper on the interpretation of cores.

As a bit of recent history, the contractor on the job here in Central Asia placed about 2 m lift of concrete at a temperature of 25 degC - the maximum temperature rise 1.5 m from the open side face and at mid height of the lift was to 48 deg using sulphate resistant cement - this represents a rise of about 3.5 degC per 50 kg cement (360 kg). The air temperature averaged about 25 degC which I think would be similar to yours. Note, though that the concrete dimensions beyond the thermocouple was like 15 m . . . so there was no dissipation of heat due to outside temperature.

 

[structural-eng]

The mix ID listed on the test report from the testing agency doesn't match the approved mix ID. They're close so I'm not sure what the change is but assuming they're the same (which I'll have to confirm) the design strength of the mix was 5000 psi (which they didn't meet). We don't need 5000 psi structurally but the concrete contractor wanted to use 5000 psi to reduce the required air. The ready-mix suppliers in TX don't typically provide air entrained mixes but the owner was requiring air because if you read ACI durability requirements without using some common sense you can conclude that air is needed. The air battle is a whole other story. Cementitious material was all C150 type I/II LA (658 lb). Ambient air temperature at time of testing was listed as 90 degree F (when the concrete temp was 98 degrees F).

I agree that most of the pier is in the ground and it's not large enough to be concerned about mass concreting. The "hot" concrete was at the top of the pier and the most the piers are extending above grade is 18".

 

[dik]

I should have added that over speccing concrete is really bad for carbon footprint... concrete has a 'big one' to start with, The concrete agencies are actively working on providing a concrete design requirement and not a strength requirement, because of this.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[driftLimiter]

Do you have/have access to Properties of Concrete by A.M. Neville? Link

Chapter 8 of this text contains very good information and references on how temperature can impact curing time and strength relationships.
I don't know if I can post snapshots of his book but there a few graphs that I would post to help add to the conversation.

The general trend is that higher early temperatures spur faster development of early concrete strength. I.e. 7 day strength goes higher.

But this 'early setup' leads to a weaker formation of the concrete matrix and results in lower 28 day strengths.

From what I see in this literature you could expect a lower 28day f'c than if the temperature was controlled to a lower values before pour. Seems to me that the point is moot structurally but if the concrete spec is for 5000 psi and there is concern that it isn't achieved why not just core a few samples and test them. The in-situ strength of the concrete should be compared with the specification.