Heat Dissipation in Mass Concrete

[vmirat]

My client purchased an aircraft jack test stand. The manufacturer of the test stand provided an installation drawing which shows the two posts embedded in a concrete pad that is 5' thick x 20' x 20' with two mats of #8's at 6" o.c.b.w. located 5 inches from top and bottom. I asked if they considered thermal stresses in the concrete as a result of heat of hydration. They said the rebar would help dissipate the heat out to the sides of the pad.

I'm not so sure about this dissipation thing.

 

[BridgeSmith]

Sounds like a load of something stinky to me. The problem as I understand it is the difference in temperature between the center of the mass and the surfaces. Between the center of the 5' thickness and the faces is where the thermal differential will be potentially problematic, and the rebar on the faces won't help with that.

On a similar sized pour, we had them embed temperature sensors at the core and near the faces and monitor the temperature difference. The cost increase was minor.

 

[Agent666]

Yeah, I've never heard of or considered the bars as contributing to improving the situation.

A lot of it can be addressed through proper placement like insulating the concrete after placement to minimise the differential temperature and/or protect from solar heat gain, or placing pipework throughout the mass concrete to cool via circulation of refrigerated water, or placing the concrete in layers to allow for some heat dissapation. Or by addressing in the mix design for example by controlling the temperature of the water (use of chilled water or ice during the batching) or chilled aggregates or optimising the mix by using supplementary cementitious materials like fly ash or slag; or using a portland cement that generates a lower heat of hydration.

ACI 207.4 Cooling and Insulating Systems for Mass Concrete contains a lot of good strategies for addressing the issue. Also contacting a good concrete producer as they have 'been there done that' probably and can offer some good advice no doubt.

 

[cliff234]

Use minimum possible compressive strength and focus on strength at 56 days, not 28. (Will 5 ksi (at 56 days) work?)

Use maximum possible aggregate size and use well-graded aggregate in order to minimize the amount of cement needed in the mix.

Replace some of the cement with slag. (Using slag reduces heat of hydration and slows the “cooking time”. Consider replacing 50% of the cement with slag. We used 75% slag in a 17’ thick mat ten years ago.)

Pour early in the morning when temperatures are lower. Pour in cool weather (if you can wait until cooler weather).

General rules-of-thumb are to limit the maximum temperature to 165 degrees F and limit maximum differential temperature (between core and edges) to 35 degrees F.

There are several good articles available online that discuss additional tips for mix design and placement of mass concrete.