Cooling concerns for concrete

[Structures33]

Hey - We've got a retaining wall that will be 8' thick (36' high, ~300' long) and will be constructed in New Orleans. Is this thick enough to worry about cooling in the summer heat in that area? I am fairly certain it would not require cooling pipes or other involved measures, but does it warrant suggesting/specifying construction in cooler months? Thanks

 

[SlideRuleEra]

Eight feet thick should be ok without special cooling, even with Type I cement in the concrete mix. You could use Type IV cement to slow the heat of hydation, if desired.

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[JedClampett]

Fly ash slows down the hydration and makes the concrete cheaper, too.

 

[lkjh345]

Oddly enough, It will actually be better to construct the wall during the summer than during the winter. Problems arise when there is a steep tempature gradient between the interior core of the wall, and its exterior surfaces. The closer all parts of the cross section of the wall are in tempature, the less stress will be induced in the concrete. It is the difference in tempature that can casue problems.

You will want to keep the exterior surfaces warm untill the interior cools sufficiently. You may want to think about using concrete blankets to keep the exterior surface of the concrete warm. Though this sounds like it might be pretty tough given the size of the wall.

See ACI 207.1 for some additional info, and talk with a concrete supplier and contractor who have done 'mass' concrete pours.

 

[civilperson]

The rule of thumb for mass concrete requiring thermal design criteria is the smallest dimension of greater than four feet. You fit in this category. Reseach ACI for mass concrete specs.

 

[kslee1000]

I have involved in a concrete dam construction in Georgia, ice was used for onsite concrete mix (with fly ash) without special cooling devices. Limiting the lift hight may help also. lkjh is correct, the form could be remain in place to minimize the evaporation of surface water, or a coat of curing compond will do as well.

 

[Zambo]

It is certainly a large enough section to warrant concern about the heat of hydration and possible cracking.

The problem with casting during the summer is that the concrete should be placed at as cool a temperature as possible. You should check with the concrete batching plant what measures they have available including reducing the heat of the aggregates, the use of ice or chilled water and peferably PFA in replacement for some OPC .

Instead of casting in cooler months you could cast at night, or preferably very early morning when the aggregates are at their coolest and the sun isn't heating up the drums on the truck mixers.

 

[oneintheeye]

ikjh341, this is true maybe for thick sections, not for thinner where it is better to let the heat escape.

 

[Zambo]

herewegothen,

you don't need the heat to escape. You need to limit the heat differential between the centre of the pour and the surface.

There are many ways to achieve this including reducing the temperature of the concrete at the time of casting and keeping the formwork in position longer.

For "thinner" pours, which you mention, there should not be a problem as it is a mass of concrete (i.e a large quantity of cement) which generates the heat of hydration which may result in cracking.

 

[concretemasonry]

This is not really mass concrete or even hot weather concrete since it only New Orleans with uniform warmth and more humidity than real heat. There are several refernces available through ACI.

They are ACI 305R-99 and ACI 305.1-06.

They provide some guidance into the concrete aspects.

The realatively narrow temperature variations (with relatively warm uniform temperatures) lend a certain amount on consistancy with aggregates, that really do not cool in the evenings.

Spray on curing componds hold in moisture and do not offer cooling to a thin 8" concrete wall.

If you are concerned about the concrete temperatures during and after placement, use cold mixing water, fly ash and slow curing cement. There is a required chemical reaction when concrete hydrates and firtunately you do not have a mass concrete problem or overheating, but a constructability situation that can be controlled.

You are not going to pour the entire length at once, so look to the joints to mainatain the integrity you desire.

Dick

 

[Zambo]

Usually concreting early in the morning is an efficient way to reduce the temperature of concrete at the time of placement. I expect this will also be the case in New Orleans.

The wall is 8' think not 8". Heat of hydration is a concern which needs to be looked at.

 

[oneintheeye]

zambo i have to disagree slightly, in thinner sections there will be little temperature differential through the section (and hence no internal restraint to contraction of the outer parts). Therefore by using steel shutters (or non insulted) and/or removing earlier will allow the heat to escape from the section meaning lower temperature gains from hydration and less shrinkage strain all around. On thicker sections it is better to use insulted formwork and leave in place until peak temp has passed to, as you say limit temp differential.