Question: We have had a number of slabs placed in our area lately that are experiencing severe surface cracking. The details for each of these slabs are as follows:
- Ambient temp: 20s at time of pour, low- to mid-30s high during the day, 20s again overnight
- Concrete temp: 50° F to 60° F
- Mix: 3000 psi non-air-entrained for interior slabs
- Additives: Hot water, 2% nonchloride accelerator or 2% calcium chloride (depending on whether wire mesh was used)
- Compressive strength specified: >3000 psi at 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 or heated)
No additional efforts were made regarding cold-weather concreting other than the addition of hot water and accelerator to the mix. 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?
Answer: This question drew many responses to the ARI Forum—here is a summary.
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 extending the lines it appears that you are right at the 0.2 lbs./sq. ft./hr. 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.
The ready-mix concrete producer is responsible for delivering 50° F to 60° F concrete to the jobsite during winter weather conditions. It is then the sole responsibility of the concrete contractor/finisher to keep the concrete at a minimum of 50° F for at least two days if either Type III cement or accelerator was used in the concrete mix, and for at least three days if Type I or II cement and no accelerator was used. Refer to ACI 301, Table 3.1 for the specifics on this. Bottom line is that your concrete finisher is trying to shirk his responsibilities and blame someone else for his lack of knowledge and professionalism.
My first thought is that the low subgrade temperature, as well as a low concrete temperature, are causing a slow rate of hydration and slower initial set; all this while the surface is being desicated by the 15-mph wind. If you have to continue these types of placements, I recommend increasing the percentage of accelerator and the concrete temperature and using wind barriers and surface finishing aids to reduce evaporation.
You should not be placing concrete on frozen ground—period. Ground heating blankets can be placed on the subgrade to thaw it out. But the concrete that is placed must be protected from freezing and kept above 50° F during the curing period (three days minimum, seven days preferred). This can be done with insulation blankets to keep the heat of hydration in the concrete. The colder the concrete, the slower it gains strength.
- It appears that you have more problems than surface cracking. The fact that the concrete was poured on frozen ground is one—when the ground thaws, it will recede, leaving a void. In addition, the lack of a cold weather cure has probably frozen the concrete before it could cure and reach its design strength. I would advise removal and replacement, then follow the cold weather standards in ACI 306.