Forum Message



Name: Jay  Shilstone
Heading: 1.1.5RE:help! - cold weather thermal cracking?
Message:
We have had a number of slabs placed in our area lately that are experiencing signs of what we have been told is "thermal cracking." The pour details for each of these slabs is as follows:
Ambient Temp: 20's deg F at time of pour, low to mid 30's high during the day, 20's again over night
Concrete Temp: 50's to 60's deg F
Mix: 3000 psi non-air for interior slabs
Additives: hot water, 2% Non-chloride or 2% calcium chloride (depending on wire mesh)
Compressive Strengths: >3000 psi @ 28 days
External conditions at time of placement: ground frozen (or near frozen), winds in excess of 15 mph, slabs open to air (not enclosed and/or heated)
No additional efforts were made regarding cold weather concreting other than the addition of hot water and accelerator to the mix.
THE PROBLEM: The slabs are cracking before they can even be finished. We have been told that the slabs are exhibiting "thermal cracking" caused by the concrete setting up underneath prior to setting up on the top surface due to a large temperature gradient. Our finisher says it's the concrete's (and therefore the ready mix producer's) fault. The ready mix producer says it is a result of failure to follow appropriate cold-weather concreting techniques. What's really going on?




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Jason Wimberly


Jason,

There is a guide to cold weather concreting available from BASF Construction Chemicals - Admixture Systems that may be of interest to you. You can retreive a copy of this document at their website or by using this link - http://www.basf-admixtures.com/NR/rdonlyres/1BC095FA-3F3F-46CF-8FB6-6C75F9E16898/0/PIP_ColdWeather.pdf

Regards,

Phil Graham
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Phil Graham


Jason,

What I THINK you are experiencing is plastic shrinkage cracking, not THERMAL cracking. A lot of people think plastic shrinkage is a hot weather condition, but we see a lot of it in Dallas when a cold front moves through after a warm period. ACI 308, Guide to Curing, deals with this issue. Figure 4.1 is a nomograph describing the rate of evaporation from a concrete surface. Your conditions are actually off the nomograph, but by estimating it appears that you are right at the 0.2 lbs/sqft evaporation rate that is an indicator of plastic shrinkage.

Plastic shrinkage occurs when the surface moisture evaporates faster than it can be replaced by bleed water. Low humidity with a cold breeze is an ideal scenario for plastic shrinkage. Depending on the amount of accelerator used and the radiant solar energy, the concrete temperature can become higher than the initial temperature, making matters worse.

Thermal cracking occurs in 2 ways - when the interior of the concrete is hotter than the exterior (typically by more than 35 degrees F) or when there is a substantial overall change in temperature, such as concrete placed in Arizona in the summer, when curing concrete can approach 180 degrees, which is then exposed to a 5 degree ambient temperature during the winter.

Another factor making matters worse is that, as the concrete in contact with the ground cools down, set time will be increased, allowing for longer bleeding. If the slab is finished too soon, the top surface may delaminate.

The concrete producer may be able to help matters by reducing his concrete temperature somewhat and using a coarser sand, but it appears that most of the problem lies with the placing, finishing and curing. Check out ACI 306 on Cold Weather Concrete.

Why are you even placing concrete in these conditions? It violates almost every guide to cold weather concreting.

Good luck,
Jay Shilstone
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Jay Shilstone
 
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