Problem: Last night the temperature fell to 45 degrees. This morning I started getting calls from contractors about spider web cracking on their slabs. “I thought this kind of crack only occurred during hot weather. It never even got over 70 degrees yesterday. What happened?”
Answer: What you’re probably referring to is known as plastic shrinkage cracking. While such cracks typically occur during hot weather, they can also occur in cold weather. In fact, I get calls on this almost every fall when the first cold snap comes through. One good thing is that these cracks are only on the surface and are not structural.
To understand why a hot weather problem occurs during cold weather, we have to understand what causes it. When water evaporates from the surface of the slab faster than bleed water can replace it, the top surface starts to shrink and crack. This happens before the concrete has begun to set up, which results in the cracks being small and close together. Things that cause surface moisture to evaporate faster or delay the rise of bleed water increase the probability of plastic shrinkage cracks. (For a more thorough discussion, see ACI 305R-10, “Guide to Hot Weather Concreting,” or NRMCA CIP 5, “Plastic Shrinkage Cracking.”)
ACI 305R-10 contains a well-known nomograph that predicts the likelihood of plastic shrinkage cracks. This nomograph uses the following information to predict whether or not plastic shrinkage cracks are likely to occur:
- Air temperature
- Relative humidity
- Concrete temperature
- Wind speed
Whenever the chart predicts water will evaporate at a rate approaching 0.2 pounds/square foot/hour (lbs./sq. ft./hr.), the likelihood of plastic shrinkage cracking exists. Of course, other factors, such as elevated air content in the concrete or the presence of fly ash or microsilica, also affect the occurrence of plastic shrinkage cracks. However, ACI’s nomograph is a pretty fair indicator for everyday concrete.
So why does this hot weather phenomenon affect concrete during the fall? For the same reasons your lips dry out and crack during winter.
Relative humidity often drops during cold weather, which increases evaporation rate. In Texas, where I live, it’s not uncommon for an 85-degree day to be followed by a nighttime cold snap that drops ambient temperature to 45 degrees. Concrete placed the next morning will still be at 85 degrees to 90 degrees, but the ambient temperature will be 50 degrees with a relative humidity of 35%. Let’s say there’s a nice, brisk 15 mph breeze. According to the ACI nomograph, the concrete can be expected to lose about 0.40 lbs. of water/sq. ft./hr. – more than twice the amount needed to cause plastic shrinkage cracks.
Fortunately, you don’t have to carry around a copy of ACI 305 to do these calculations. The free “ACI Mobile” app for Apple has a section called “Job Site Weather & Curing” that does these calculations for you. In this example, the screen’s yellow background indicates that, based on the day’s weather conditions, there is a moderate probability of plastic shrinkage cracking. Thursday through Saturday are green, indicating a low probability. Red indicates a high probability.
How do you prevent plastic shrinkage cracks? The best plan for contractors is to follow ACI 305 or NRMCA CIP 5, “Plastic Shrinkage Cracking” recommendations on how to reduce excessive evaporation. One thing I have seen concrete producers do is place a pump-up sprayer filled with an evaporation retardant on each truck and rent those out to contractors as a value-added item. Note: An evaporation retardant is not the same as a curing compound. As the producer, if you have any doubts about the proper material to use, check with your local concrete chemical supplier or local ready-mix association chapter.
Plastic shrinkage cracks won’t affect the structural integrity of a slab or pavement, but they can be unsightly and ultimately affect longevity. Because you can’t do much to prevent these cracks, your best course of action is to educate contractors.
