It should come as no surprise to learn that since the commercial availability of high-performance concrete, the problems associated with autogenous shrinkage have become more widespread and profound. Certainly, modern concrete, has all the ‘ingredients’ for dramatic increases in potential autogenous shrinkage and is more susceptible to excessive early and later-age cracking when compared to more traditional concrete mixtures. Hopefully, once the combination of all the factors that exacerbate adverse reactions associated with autogenous shrinkage are better understood—by parties involved with the design and construction of concrete structures—then perhaps problems and conflict can be avoided and the potential for cracking can be reduced to a nominal, more
| TYPES OF CRACKS
|Terms to describe types of cracks vary from region to region, but cracks resulting from hydration reactions or moisture transfer are most commonly classified by their cause or by their appearance.
Plastic shrinkage cracks
Plastic shrinkage cracks occur within the surface of fresh concrete while it is still plastic, and are characterized by their random pattern. They are usually discontinuous and rarely extend to a free edge. The cracks are caused by the rapid drying of the surface, particularly during warm and wind-drying conditions. The potential for cracking increases with workability—particularly when inadequate curing measures are taken.
Drying shrinkage cracks
Drying shrinkage cracks occur when concrete is subjected to a lower environmental relative humidity (RH) than its internal relative humidity. The mechanism is generally accepted to be due to the development of capillary tension as water is lost from the pores and menisci are formed. Shrinkage of the cement paste of a concrete will be larger the higher the w/c ratio because the latter determines the amount of water in the cement paste and the rate at which water can move toward the surface.
Carbonation shrinkage cracks
Carbonation shrinkage cracks can occur because of atmospheric carbon dioxide diffusing through the cement matrix pore structure to react, in the presence of moisture, with calcium hydroxide. Carbonic acid is formed during the initial reactions that subsequently result in the formation of calcium carbonate. Cracks form as the result of dissolution of the calcium hydroxide crystals while the crystals are under pressure, and deposition of calcium carbonate in places where the carbonate is not under pressure.
Surface crazing and map cracksinadequate curing practices are carried out and rapid drying conditions prevail. The result is that the top surface cracks in a random manner, sometimes referred to as alligator cracking or mud-cracking, with
the cracks usually only extending a few millimetres below the surface.
Thermal stress cracks
Thermal stress cracks occur as a result of the evolution of heat during the cement hydration process. The potential will increase with increased thickness of section and cement content. Unlike drying shrinkage or carbonation shrinkage, contraction during cooling of mass concrete is greater within the interior.
Paul Jeffs has more than 45 years of experience in the construction industry around the world. He is principal of PJ Materials Consultants Ltd., a Guelph-based company that provides consulting and sub-consulting services across Canada for the investigation, construction, and restoration of masonry and concrete structures. He can be reached at email@example.com.
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