Placement of Mass Concrete to Control Thermal Cracking

[emilywalters]

When placing concrete for a large mass concrete gravity retaining wall - what is the best placement method for control of thermal cracking?

I typically deal with smaller reinforced slabs which I know benefit from a sequential bay construction rather than an alternate bay construction / "hit-and-miss" because I know that pouring an infill slab means that external restraint can be very high. However, in this case it is likely that internal restraint (as the volume of the pours is likely to be large) will be a key issue. Is is therefore better to follow a sequential pour method (to minimise external restraint) or an alternate bay method to allow formwork to be left on as long as possible for insulation purposes (to minimise internal restraint).

I can't find information on what is the traditionally accepted methodology for concrete placement in this scenario and there are differing opinions in my office. The Contractor would prefer sequential pours as he would like to use formwork on rails which he will push along for each subsequent pour.

I have also heard that there can be issues regarding bond between pours and that sometimes a diagonal pour is recommended to increase the surface area between pours. Refer to diagram:

http://s16.postimg.org/6gb6g4sad/Thermal_Cracking_Sea_Wall_Concrete_Placement_p.jpg

Any input would be very much appreciated...thanks!

 

[bridgebuster]

There's no simple answer. In a mass placement thermal cracking is a function of several aria less, including mix design,ambient air temperature, placement size, flyash, etc; but you probably know that already. The thing with mass concrete is to keep the core concrete temperature and the surface temperature of the concrete within 40 degrees F of each other, but staying below 160 degrees. It is tricky but not impossible; been done plenty of times.

There is free software available at

http://www.texasconcreteworks.com

to perform the analysis. Also, look at ACI 207.

I don't like the diagonal placement because of acute corners. There's nothing wrong with a construction joint. You just need to size it properly and make sure you call for a water stop and proper surface prep before placing subsequent placement. I would go with staggering the placements.

 

[PilebuckEngineer]

Bridge buster is right on. Various methods can be used to keep differencial temperatures below 40-degrees and upper temperature below 160-degrees. Some ideas:

-Lower the starting temperature of the concrete and you lower the upper limit. If you are pouring in the summer adding ice can help.
-Cover the slabs with cure blankets, regardless if it is cold outside, will insulate and keep differencial temperatures down.
-Cooling tubes can be used to recirculate cold water in the footing
-Install temperature monitoring, or maturity meters than can be monitored. This allows you to react to anything out of the ordinary.

It is usually good practice to make a couple of test pours to see how your specific mix is going to kick off.

 

[BigH]

Have you considered using fly ash substitution for the cement content? We used 50% fly ash and 50% cement on a mass concrete job - placed at 27 to 29 degC and max temperature in 900 mm thick lifts (sometimes containing 1500 m3 of concrete) was only to 43 degC.