What Are the Causes of Pitting in Concrete?
Pitting in concrete is caused by a wide variety of conditions. Some pitting and spalling, as it is sometimes called, is from natural aging and sometimes it can be caused by abuse or misuse.
In some circumstances it occurs from faulty aggregates in the concrete, and there are examples of concrete that has degraded prematurely due to improper placement and finishing techniques. To properly diagnose degradation in concrete, it helps to know a list of details which are, at times, hard to assemble. Age, climate, mix details of the concrete, conditions under which the slab was poured and patterns of maintenance, such as snow removal, salting in winter and application of sealer, are all important.
Concrete and Climate
Climate in the parts of the United States that have snow and ice in the winter can be especially brutal on concrete. Ice and snow that are left on the surface can cause pitting from the cycle of freezing and melting from night to day. In the sunlight, ice may melt and the water penetrates the surface of the concrete, and will freeze again at night. The expansion of the ice within the top surface of the concrete slowly ruptures, forming tiny craters which continue to expand. Over subsequent seasons, those pockets continue to grow until they are visible and ultimately can start breaking apart the structure. The freeze-thaw cycle, as it is known, is the most common cause of pitting in concrete.
Concrete Pitting From Inadequate Mix
Under circumstances in which concrete was placed using an inadequate mix design for the local climate, pitting and degradation can occur prematurely. If the age of the slab is known to be relatively new, less than five years for example, this can be the cause. A low psi mix design can degrade faster in the less temperate climates than one of a higher strength. Local building codes usually specify strength of mixes for their climate conditions. A lower strength concrete is less waterproof, and less resistant to the effects of winter ice and snow detailed in the first section of this article.
Concrete Degradation From Improper Placement and Finishing Techniques
Concrete can begin to break down on the surface if it was placed, finished or cured improperly. If a slab was not cured, too much water can escape via evaporation from the surface as the concrete sets up, especially if it was poured on a hot day. If the concrete finisher added water to the surface to aid in finishing, that surface is weakened and can degrade sooner. The sub-grade can affect the concrete's strength by drawing water from it prematurely if was not sprayed down with water prior to concrete placement. These three effects have their greatest implications on the hottest summer days.
Aggregates and Concrete Degradation
On rare occasions, premature pitting and degradation of a concrete slab can be attributed to an aggregate that is not suitable for use in concrete. This scenario typically happens quickly and in a variety of conditions and can usually only be determined in a lab. A bad aggregate will either create a chemical reaction that breaks down the concrete or has excess porosity that poorly resists the freeze-thaw cycle. Concrete materials are standardized and tested. If something slips through, many times it is from a small vein of material in the rock quarry that goes unnoticed.
Age, Maintenance and Prevention
All concrete eventually can succumb to the rigors of winter and use. There are preventative methods that can increase its life span. Winter is the most brutal stress on concrete, and is ultimately the main overall cause of pitting in concrete. Snow and ice removal can drastically reduce pitting over the years. Salt and other ice melting compounds can be an obvious necessity for safety, but use in limited amounts after physical removal of the bulk of the snow and ice is best. Applying a concrete sealer to the slab on a periodic basis reduces the amount of water it can absorb. This protective action can add years to a concrete slab.
- "Design and Control of Concrete Mixes"; Steven H. Kosmatka, Beatrix Kerkoff, and William C. Panarese; 2002
- "Masonry and Concrete Construction"; Ken Nolan: 1998
Dave Root is an arts and crafts professional specializing in concrete, clay, wood and metal. He has written for the Concrete Network and his craftwork has appeared in "Metropolitan Home," the "Kansas City Star" and other local publications in Kansas City, Mo. Root holds a B.F.A. in design with an emphasis in ceramics.
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