Overheated concrete in mass concrete foundation

[Osama Mubarak]

Hello Everyone,

I have a question about overheated concrete in a mass concrete foundation. It is a compressor foundation with dimensions of (13.5 X 2.25 X 5.8)m. During the construction, our contractor has added a pipe of cooling pipes to control the concrete temperature during the hydration process and inserted 9 no. of sensors to monitor the concrete temperature levels. The concrete has reached high records of temperatures due to the hydration process up to 160 Celcius degrees at the center of the foundation block, but lesser levels were observed at other places which are around the acceptable limits.
Knowing that the foundation is only a damping mass, in this case, I am looking for your ideas and feedback on what would be the potential consequences and what should I do to further assess the foundation conditions -if needed-.

I have attached to this thread a PPT slide showing the temperature records curves at different sensors in the foundation block.

 

[centondollar]

Presumably, the slab in most parts of the cross-section is cracked due to the massive temperature gradient. Your powerpoint did not show the exact locations (with coordinates), so it cannot be used to infer much about the severity of the temperature gradient. If the temperature exceeds 70 degrees anywhere, you have a problem, and if the gradient between mean ambient temperature and concrete temperature near the surface exceeds 20-25 degrees, cracking will occur.

Furthermore, the foundation is probably not a damping mass if it is the only thing separating the compressor and the soil, since there is no way to control the stiffness of the slab you mention - depending on soil conditions (Winkler springs or a more advanced model) and the time-history of the concrete (creep and shrinkage have a massive effect on Young's modulus and thus on stiffness), the stiffness is completely uncontrollable.

A tuned mass damper would normally be installed onto the compressor rig or onto a smaller slab located between this "mass concrete foundation" you mentioned, and it would consist of a spring (e.g. a steel beam mounted into concrete) and a dashpot (some form of hydraulic cylinder).

PS. I'm curious to know why the slab was made so thick (2.25 meters) to begin with? Surely the compressor does not weigh enough to actually require that thick a slab (massive inner lever arm between the top and bottom rebar nets)?

 

[Osama Mubarak]

Thanks @Cenondollar, I appreciate your comments. My responses are as below:

I have attached to this response, the locations of the sensors in the foundation block.

The temperatures have exceeded 70 degrees in some of the locations (especially at the middle-level sensors). See the brown limit in the graphs, the Red limit indicates 100 degrees in which the concrete mixture starts to lose the hydration water. I agree that the given high temperatures will cause cracking due to the internal stresses. This will be looked at in a more detailed approach following the removal of the formwork. my focus at the moment is to assess the foundation integrity from the functional point of view.

The foundation is a damping mass following the guidance provided in ACI 351.3R-27 Clause 4.1.2.1 Rule-of-thumb method. And the foundation was sized upon this.

 

[BigH]

That is a huge temperature and cooling pipes were installed?
Was an analysis done of the anticipated temperature rise?
Did the Contractor actually use the pipes? (No gender is to be presumed due to the the use of "he")
Did he use cooled water? with ice?
Did he reverse the inlet/outlet say at 4 to 6 hour intervals?
How much cement was put into the mix?
What was the required mix strength?
Did the mix contain fly ash?
What at the contractual issues with respect to the concrete and temperature rise? If out of compliance - the Contractor may have a major issue on his hands - or, also, the structural designer.

On one Reinforced concrete job we used cooling pipes, reversed cooling inlet/outlet, 35 MPa (cylinder) strength and 50% flyash and never got temperatures even close to that? Similarly for mass concrete - albeit strengths in the order of 25 MPa, again, using 50% fly ash with no cooling pipes, temperatures didn't rise above 45 degC.

 

[Osama Mubarak]

Thanks BigH, Answers as below:

That is a huge temperature and cooling pipes were installed? ==> Yes. Though I am unsure about the basis he used to install the pipes (If he designed it/analyzed the temperature raise before installation). We are checking with him at the moment.
Was an analysis done of the anticipated temperature rise? ==> I am unaware of such analysis if being done in the field. We are checking with him at the moment.
Did the Contractor actually use the pipes? (No gender is to be presumed due to the the use of "he") ==> Yes as above.
Did he use cooled water? with ice? ==> I do not have the answer.
Did he reverse the inlet/outlet say at 4 to 6 hour intervals? ==> I do not have the answer.
How much cement was put into the mix? ==> I do not have the answer.
What was the required mix strength? ==> C28 cylindrical strength.
Did the mix contain fly ash? ==> Not sure. but I do not think.
What at the contractual issues with respect to the concrete and temperature rise? ==> I am not sure. We did not go into this yet. We are only assessing the situation from a technical perspective.

 

[SlideRuleEra]

Osama Mubarak - I believe the sensor reading is faulty. The only heat transfer path is conduction through surrounding concrete. Nearby sensors would have shown somewhat elevated temperature... none of them show elevated readings. But that is just my opinion, the high reading should not be ignored. I would core the block at the high sensor location to determine concrete condition. I expect the results will indicate the inertia block is good.

http://www.SlideRuleEra.net

 

[BigH]

SRE - but, in my humble opinion, there are too many unanswered questions - all part of design ... The block may be good for its intended purpose but, still, one doesn't install pipes without knowing the need and how they are used . . . . very strange centre reading, I agree compared to the others at 60 deg C or so. . . still

 

[dik]

What was the slump, and the strength? Can you use 6" or 12" aggregate (I don't know if there is a limit in size). Can you use a large percentage of flyash? I've actually used insulation to minimise the heat differential. With the Pinawa project in Manitoba, they used a technique called Intrusion Prepacked; this is where they place large aggregate and pressure grout the concrete to place it. They used rock sized aggregate.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[SlideRuleEra]

BigH - I'm looking at this as a (mechanical engineering) heat transfer problem... temperatures (and their rate of change) at the center sensor just do not make sense compared the other 8 sensors. I'll take a better look at it, but right now a faulty sensor seems most likely.

IMHO, coring the block is the next thing to do... whatever the cause.

http://www.SlideRuleEra.net

 

[LittleWheels]

I agree with SlideRuleEra that the 160 degree sensor reading is likely incorrect for the obvious reasons - energy demands of pore water phase change from liquid to gas, loss of hydration as water boils, ridiculous step in temperature gain, etc.

Cracking due to temperature differentials is one issue and others have addressed that, but there is another aspect that might not be clear to some folk. The reason for the 70 degree temperature limit (some variations between codes) is to limit the risk of Delayed Ettringite Formation (DEF), a form of sulphate attack. Above 85 degrees, DEF is extremely likely. If DEF occurs, the concrete can slowly demolish itself due to internal expansion, in the right conditions. I would not expect petrography to be able to identify DEF at this very early stage but it is worth asking a petrography specialist the question as I am often wrong.

https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=932

GGBS and fly ash mixes tend to reduce the likelihood of DEF at temperatures somewhat above 70 degrees but there are a lot of influencing factors. CIRIA C766 is the most useful guide to DEF (and avoiding early age concrete cracking) that I have read but no doubt there are other references that I've not read yet.

 

[Osama Mubarak]

Thanks Everyone, I appreciate your interest and time to advise me on the issue. We have new set of readings from the second foundation which have the exact same foundation properties and I have been told that they are better than the first batch of readings. I will share the updates once they are available with me.

 

[chicopee]

The bridge contractor which I worked for was constructing round piers for an elevated bypass about20 years ago. To the best of my recollection, I estimate the pier sizes at 10 feet in diameter and of lengths ranging 40 feet to 70 feet. When the formwork was removed a week or more after the pours, surface cracks appeared a few days later. The STATE allowed the drilling of holes for concrete epoxy injection to fill the cracks. When the repairs were completed the masons smoothed out the surfaces where the cracks were visible. After twenty years of use the piers have shown no signs of problems. So would this type of repair be acceptable on the OP's foundation.