5 Ways to Prevent Plastic Shrinkage Cracks

As freshly poured concrete cures, water rises up toward the surface in a process known as bleeding. Simultaneously, water evaporates out of the surface into the surrounding air. When evaporation happens faster than bleeding, the concrete tends to form plastic shrinkage cracks, which mar both the appearance and the stability of the concrete.

Fortunately, professional concrete contractors have a variety of ways to minimize the formation of plastic shrinkage cracks. If you would like to improve your knowledge of concrete installation techniques, keep reading. This article outlines five effective strategies for preventing plastic shrinkage cracks.

1. Environmental Shielding

As noted above, plastic shrinkage cracks occur when water evaporates more quickly than it rises to the top of the slab. This difference dries out the surface of the concrete, causing its volume to shrink. Cracks open up as the shrinking concrete pulls apart from itself.

Environmental factors play a huge role in the formation of shrinkage cracks. Under ideal installation circumstances — cool, humid, overcast weather — the likelihood of cracks forming remains relatively low. Yet as temperature increases and humidity drops, the rate of evaporation correspondingly goes up.

Contractors must take precautions when installing concrete in especially hot, dry, sunny, or windy weather. Temporary sun shades protect the surface of the concrete from direct exposure to the sun. Since high wind exposure also increases evaporation, contractors often put up wind breaks as well. 

2. Pour Timing

Extreme weather can pose a serious threat to even well-shielded concrete. If possible, consider putting off the installation until cooler weather arrives. Otherwise, contractors can reduce the threat by installing the concrete either early in the morning or in the evening, since the afternoon hours pose the greatest threat to the curing concrete.  

3. Fog Spraying

Plastic shrinkage cracks can still occur in periods of relatively mild weather because not all concrete mixes have the same bleed rate. Even if rates of evaporation remain low, a retarded bleed rate may still cause cracks to appear. In this case, shielding the concrete from sun and wind exposure may not deliver the desired results.

Instead, contractors must ensure that the surface of the concrete remains adequately moist. One of the most effective ways to do this involves installing fog machines around the perimeter of the slab. These machines emit a steady flow of mist, which ensures that the surface of the concrete never becomes dry enough for shrinkage cracks to form.

4. Aliphatic Alcohol

Contractors also prevent shrinkage cracks through the use of compounds known as evaporation retarders. The most common evaporation retarder goes by the name of aliphatic alcohol. This water-based compound reduces the rate of evaporation, ensuring that the surface of the concrete retains more moisture.

Workers apply aliphatic alcohol by spraying it across the concrete, where it forms a protective film. When working in especially hot conditions, the concrete may need multiple applications, since the alcohol also evaporates as time goes on.

5. Polypropylene Fibers

Contractors can also reduce shrinkage by incorporating polypropylene fibers in the concrete mix. These fibers increase the tensile strength of the concrete. Even if the surface becomes dry, the additional strength provided by the fibers can overcome the shrinkage forces. For best results, however, contractors must add a sufficient amount of fibers.

In addition, the length of the fibers plays a key role in their effectiveness. Generally speaking, longer fibers provide a greater degree of reinforcement. Yet if fiber length becomes too great, they tend to clump together in the mix. As a result, not all of the concrete receives an equal degree of protection.

Contractors have a variety of tools and techniques to prevent plastic shrinkage cracks. For more information about how to install concrete the right way, contact Orlando's industry professionals at Accu-Krete.

Ashley Zona