Thermal Effects of Mass Concrete Pour 3

[MkAustin]

We are performing materials testing on a 4-ft thick concrete slab pour with 1500 cy. The specifications are loose and the contractor is going to do one continuous pour. The maximum specified placement temp is 95 deg and there are no specific requirements for cooling. The pour will start in early evening and go all night. Night time temp is 75 deg F and daytime temp is 100 deg F. No chance to delay project until cooler weather. We didn't design this but are concerned the heat of hydration may cause problems. I am a geotech with no experience with thick concrete pours. Any comments on whether there may be problems with this thick of a pour and ways to address this? Thanks.

 

[dik]

I assume the ACI has some thermal requirements and it is a matter of insulating the formwork and surfaces and cooling the mix prior to placing.

Temperatures can be monitored using thermocouple devices for this purpose.

Dik

 

[Ron]

You need to monitor the internal temperature of the concrete during placement and in the next few hours afterward. The temperature will rise quite a bit. This will cause early strength, increased cracking potential, and will decrease the ultimate strength.

ACI has guidelines for hot weather placement and for mass concrete placement.

 

[bb29510]

4 ft thick is nothing, just place and get on with life, even if it does heat up, not much you can do about it. we have place twenty foot thick mats. not much you can do

 

[Ron]

brownbagg...I disagree. There's plenty you can do about it if you plan it on the front end. If you predict the probably hydration temperatures and they are too high, then you can do some things to lower the placement temperature to keep the heat of hydration down, such as using a low heat of hydration cement (Type IV), using ice place of part of the water or all of the water during mixing, cooling the mixing with liquid nitrogen and a variety of other methods to help this situation.

Four feet is a thickness that can cause problems in the climate noted by mesaus.

 

[MkAustin]

We can add thermocouples to measure the temperature. ACI is very general, but they do say the concrete should not rise in temp more than 25 deg F above the mean annual ambient temp for mass pours. That would be about 115 deg. Does anyone have a maximum temperature before problems arise? I know that depends a lot on the mix. Appreciate the practical comment Brownbagg. You are likely right. As a curiosity, what type of project do you use a 20 ft mat on?

 

[MkAustin]

They will be using ice to cool the mix, but that may only get it down to 85 to 90 deg at placement. The stockpiled materials are in the heat and sun.

 

[conceng]

mesaus,

There are two temperatures you need to worry about. The first is the maximum internal temperature measured near the geometric center of the placement. If it rises higher 160 F, there is risk of delayed ettringite formation, significantly reducing the service life of the structure. I've seen agenciess specify a maximum temperature as low as 145 F.

The other is the temperature differential between the center of mass and the outer faces. If the temperature differential gets too high, there is significant risk of thermal cracking. Some agencies specify a differential as low as 30 F, but I think the standard is 40 F. As the concrete gains strength, this number becomes less of an issue. I would place more thermocouples 2" below the top surface near the center.

I typically place a minimum of two thermocouples per location, several feet apart to provide redundancy in the equipment, and far enough apart so that if they blast an area with the pump hose and it takes out the thermocouple, it shouldn't take them both out at the same time.

Greg

 

[hokie66]

I agree with doing everything possible to lower the initial temperature and to limit the differentials, but what would you do if the thermocouple monitoring suggested a problem with the curing concrete?

 

[conceng]

If the temperatures are getting too high, typically you won't be have a differential problem at the same time, so you take steps to cool the slab w/water to a point where you are not exceeding the differential.

If the differential is getting too great, the slab may need to blanketed. Either way it should probably be blanketed or at least covered in some manor for curing and temperature control.

Starting with a cool initial temp is key, and 4-feet thick shouldn't be too much of a problem with too much heat as long as the initial mix design is prepared with mass concrete in mind.

Greg

 

[BigH]

As others have indicated, there may be problems with your placement and have given good advice on how to control heat of hydration, monitor, etc. (don't forget you can use fly ash in the mix which will help to minimize the heat of hydration). We have a very stringent maximum heat of hydration temperature of 50degC (120degF) on our job with ambient a constant 30 to 32degC with a maximum placement temperature of 30degC (85degF) - so we cool cool cool and place in thin lifts - BUT, our specifications reflect the need to maintain specific temperature maxima. Your specifications apparently do not. Unless you can find a "liku-liku" (wiggly) way to force the contractor to do something that isn't specifically stated (like using preamble specs that ACI or AASHTO or BS govern - or there is a clause that the client can order certain things not specifically mentioned) - what are you going to do so long as he places at an ambient temperature less than 95degF? You have very little to control the contractor - hopefully, your contractor will show care and want to do the right thing to protect his reputation and ensure good quality concrete and he will do the right thing.

 

[dik]

With 4' thick, it's unlikely that the overall temperature will be too high, a bigger problem will be the difference in temperature. That's why I suggested using insulation to minimise the heat loss. The thermocouples, will give you an indication on the difference in temperature.

Regarding the aggregate, it may be possible to spray it with water to drop the temperature a bit and also saturate the surface. This can have an impact on the water in the mix design.

Dik

 

[MkAustin]

Great comments. Thanks everyone. I had another comment from a concrete person outside this forum that says the differential temperature is the more critical in our situation. He said that we should be OK in the summer with the ambient temp being high, but could have problems in the winter where the differential would be greater. This is reflected in many of your comments above. We spoke with the contractor and he bid the job as one pour so he will not do anything that is more expensive without a change order. He says liquid nitrogen can be used to cool the mix, but costs $1000 per load or $150,000 and that would be an extra. We are going to install thermocouples in the center and near the top as conceng suggests. Hokie66 has a point about what will happen if thermocouples indicate a problem. I think we will just look at it as a learning experience to gain information and let the chips fall where they may.

 

[DRC1]

There are several things you can do that will not greatly effect the cost. Nitrogen for a four foot pour does sound a little overkill, but even adding ice is usually a prohibativley expensive operation. Starting at night is good. Often we will add retarder to the mix so we can slow the pour with out a false set. The concrete supplier will probably wet the large aggergate if you explain the circunmstances. Check the placement temperature carefully and besure it is less than 95deg F. Talk to the contractor and see what his plan is, he may have incorperated procedures to minimze the heat. Another problem is that 1500 yds 4ft thick leaves about 10,00 sf. This is big for one pour and jointig needs to be considered, or random crackig will develop from shrinkage.

 

[Ron]

mesaus...I'm not sure where you're located, but "letting the chips fall where they may" is a relatively dangerous proposition. 1500 cy of concrete will cost about $450,000. If it goes bad, you'll have at least twice that amount to remove and replace it. So now you're looking at $1.45 million. That could be YOUR liability, both personally and corporately depending on where you're located. Don't treat such issues trivially. Yes, most of the time they work themselves out...but being in the forensic side of things, I see many that do not work out.....and yes, we can usually figure out what went wrong and why....which leads to whom is responsible. If you're in the loop, that could be you.

Your call.

 

[dik]

You might check the drawings and specifications to determine what the Contractor is responsible for. If compliance with ACI or whatever mass concrete... then ask him how he proposes to achieve this.

Dik

 

[hokie66]

DRC1 suggested that jointing may be required, but I don't think that should even be considered. 10,000 square feet is not a large area to cast. Jointing would present more problems than it solves.

 

[BigH]

I think that perhaps you need a good concrete man on your team - he can view the specs, the contractor's requirements, determine the dangers, off mitigation techniques that are within your purview and the contract specifications. This forum can give advice, but as Ron says, you will be on the hook and with little expereience as you say, you need some professional help.

 

[Zambo]

You should start early in the mroning when the aggregates are at their coolest. Ice can be used to cool the water if a chiller isn't available. You should also check the cement, probably in this economic climate it isn't a problem but previously I have received cement which is still hot from production. Changing some of the cement to PFA would help to slow the initial heat of hydration.

The trucks need to be shaded during delivery and in general the aim has to be to get the concrete into the forms as cool as possible.

As you state the main problem is differential temperature differences so althouh ponding would be ok for curing try not to have flowing water which will cool the surface as the core temperature is still rising.

 

[MkAustin]

Since we are only doing the testing and were not involved with the design, we have to be careful. I do not think we will be on the hook for problems. And we may be over stepping our bounds by officially bring up concerns about the design. We have done it informally, though.

RESULTS. The pour was friday night/sat morning. Took about 10 hours. Concrete placement temp = 90 - 95 deg. Started at 9:00 pm - 85 deg air temp. Finished at 7:30 am - 80 deg air temp. afternoon air temp = 101 deg. Two thermocouples worked and 2 did not. One was in the center of the slab. End of pour = 130 deg. 12 hrs was max temp at 152 deg. 24 hrs = 151 deg. 48 hrs = 140 deg.

A second thermocouple was 1 ft from side edge and in the middle vertically. Temp was 10 to 20 deg less than the other thermocouple.

So based on all the comments and our results, I would recommend the specs have explicit requirements. The bid docs have to have specifics for the contractor. I don't like just referenceing ACI as they are thick documents and usually very general. I would say 40 deg differential in concrete. Place 8 thermocouplers - 4 near edge and 4 in center. Assume half will not work. Maximum temp = 145 deg. As always, local specs/codes take precidence. Hot weather vs. cold weather temp must be taken into account, but the differential should handle that.

Strategies to help achieve the above would be retarder in mix, cool the mix before placement, or place sequentially in checkerboard squares. Other comments?