Concrete Surface-Defects:  Slab Defects Part Two

 

In Part One of Concrete Surface Defects:  Slab Defects we shared information about two specifice concrete defects, blisters and cracking.  Although the information shared is not comprehensive, it is designed to give the reader an overview of some of the causes and preventions of concrete blistering and cracking.

 

This article entitled Concrete Surface Defects: Slab Defects Part 2 will provide information on three more types of concrete defects.  The first is referred to as concrete crazing. Crazing is a network pattern of fine cracks that do not penetrate much below the surface. Crazing is caused by minor surface shrinkage to the top and bottom of a slab (see photo below). Crazing cracks extremely thin or fine and are hardly visible except when the concrete drying after the surface has been wet.  The term map cracking is sometimes used to describe cracks that are similar to crazing cracks only they are more visible and tend to surround a much larger area.

 

Crazing cracks do effect the appearance of the concrete finish and are prone to 

collecting dirt, but crazing is not strurally serious and doesn't necessarily mean

that it's the beginning of a deterioration process. 

 

The prevention of crazing, curing should begin with minutes after the final finish

if weather permits. If temperatures are too high or the sun is out, curing should

include the use of wahter.  This will stop rapid set up of the concrete.  Concrete

should be protected from any rapid change in temperature and moisture when

possible. 

 

The next concrete defect to be discussed is what is known as concrete curling.  This term is almost self-explanatory.  It is the curiling of the slab's edges and or corners. This is caused by differences in moisture or temperature between the top and bottom of a slab. The top of the slab dries out and shrinks more than the wetter or warmer bottom portion of the slab (see image below).  Curling can be reduced by: 

 

1) Using a low-shrinkage concrete mix, 2) Use of proper control-

joint spacing, 3) Uniformity of moisture content, 4) Control of slab

temperature from top to bottom, 5) Use of thickened slab edges,

and using vacuum dewatering, shrinkage-compensating concrete

or post-tensioning.  

 

There are two other post curing options as well when curling occurs. 

Sometimes the degree of curling is reduced with time as the slab dries and the moisture content and temperature becomes more uniform. Finally, if moisture-related curling continues, an option is to pont the slab until it becomes level and cut additional controll joint where the slab curled.