PLASTIC SHRINKAGE CRACKS
Shrinkage is the cause of most cracks we find in concrete pavements. Shrinkage is essentially and simply the shortening of the cast member. This change in dimension can occur in the whole thick-ness of a concrete slab or only in the surface layer. When the crack occurs only in the surface of the slab, it is called a “plastic shrinkage crack”, because these always occur when the concrete is still workable or plastic. Generally, these types of cracks are always oriented perpendicular to the wind direction, however exceptions do occur and cracks can occur in any directions.
The cause of plastic shrinkage cracks is always the result of the loss of water from the surface of the concrete. The loss of the mix water present throughout the concrete occurs only at the surface where the evaporation occurs, thus these crack are usually quite shallow. The water loss is acceler-ated on days when the weather is warm, the wind is blowing and/or the humidity is low. However, it can happen when it is not generally expected, also.
As soon as concrete is dumped on the ground, evaporation accelerates due to the increase in sur-face area exposed to the air. Finishing of concrete should be performed as soon as possible. Plus, protection of the mix water in the finished concrete should also be protected as soon as possible. Plastic shrinkage cracks have been known to show up within minutes after finishing during extreme conditions. In any case, effective curing must start before sufficient water evaporates to cause shrinkage cracks. If curing procedures cannot start right away, interim applications of evaporation retarders should be applied until the concrete can be cured. Alternatively, wait until a later time when conditions are less aggravating for evaporation while placing and finishing concrete.
While shrinkage cracks can be somewhat unsightly, they do not generally cause a loss of durabil-ity. This is due to the fact that the cracks are tight, which is nominally considered a condition that is not detrimental. It has been shown that strength of the member is not affected either, as the loss of water is only near the surface of the concrete. Since we all want concrete which is crack-free, if possible, efforts to minimize loss of water from the finished surface should always be undertaken to prevent plastic shrinkage cracks.
Contact your NDRMCPA concrete supplier member for additional information on prevention of plastic shrinkage cracks through use of evaporation retarders and effective curing. Or contact Dave Sethre, P.E., Marketing Director of the NDRMCPA for information or engineering assistance.