Concrete as a Construction Material

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Concrete is a human-made material composed primarily of sand, stone aggregate (gravel), Portland cement and water. The ratio of these materials dictates the strength and performance of the concrete. Because concrete lacks significant tensile strength, reinforcement, often in the form of a cage of deformed metal bars or metal wire mesh, is encased within the concrete mass to allow the concrete to carry loadings while spanning across supports. This reinforcement will also allow the concrete mass to undergo thermal expansions and contractions without developing excessive cracking.

Cracking of concrete, corrosion of reinforcement, spalling of the concrete cover, and surface scaling are the four most common and important types of deterioration of reinforced concrete. Deterioration of concrete can result from:

• environmental factors including moisture levels, temperature levels, the presence of chlorides, and carbon dioxide;
• the original materials and workmanship, including aggregate material, level of consolidation of the concrete during placement, amount of reinforcement, presence of cold joints, location and number of crack control joints; and
• improper maintenance such as prolonged exposure to moisture, application of waterproofing coatings that inadvertently trap moisture, saturation with chlorides due to the spreading of road de-icing salts on or nearby the concrete.

Concrete dating from the early part of the 20th century was often built to low construction standards relative to the standards common today. Designers and fabricators from that period often had little knowledge of the properties and characteristics of the concrete. Early instances of concrete construction are thus often in poor condition and can require a significant degree of conservation work.

Virtually all concrete will crack with time. Cracks can be a result of natural shrinkage of the concrete during curing, thermal expansion and contraction, flexure or shear from overloading, and adverse reactions between the alkalis in the cement and some aggregates known as alkali-silica reactivity. Proper design and placement of the reinforcement can provide the necessary tensile strength to counteract the shrinkage, thermal, and overload cracking. Crack control joints can also be introduced at regular intervals, to force the concrete to crack at pre-determined locations to accommodate shrinkage. Shrinkage cracks are dormant and will not change with time. Thermal cracks will tend to widen and narrow with the cycles of the ambient temperature. Structural cracks are active only if the overload condition is continued or if settlement is occurring. Cracking due to alkali-silica reactivity has the appearance of lines on a road map and over time will develop a white crust on the surface.

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Concrete is a porous material and absorbs water. To protect the reinforcement metal from corrosion, it is typically buried within the concrete mass and covered by some minimum thickness of concrete. The thickness of the concrete cover provides a physical barrier to moisture to protect the metal, while the high alkalinity of the concrete also provides a degree protection. Cracking of the concrete can expose the reinforcement resulting in the premature corrosion of the metal. Insufficient concrete cover over the reinforcement also often results in the premature corrosion. Corrosion of the reinforcement metal resulting from contact with moisture will result in a reduction in the strength of the concrete structure, the loss of the concrete cover material, and possible staining of the concrete. Exposure of the concrete to carbon dioxide can also neutralize its alkalinity, thereby eliminating the chemical protection afforded the metal. This is known as carbonation. In the 1970s, the use of steel reinforcement that was coated with an epoxy to increase its resistance to corrosion became relatively common in high exposure situations.

Spalling is the loss of the surface concrete material and is typically a symptom of the corrosion of the underlying metal reinforcement. As the metal corrodes, rust is produced which occupies significantly more space than the original metal and causes expansive forces within the concrete cover which can produce the spalls. Spalls can reduce the strength of the concrete structure due to the loss of concrete, and the loss of bond between the concrete and reinforcing, and can expose the reinforcement to an even greater risk of corrosion.

Surface scaling can result from freeze-thaw actions of moisture trapped within the concrete surface. As the moisture expands when it freezes, it can break off layers of the concrete, resulting in a pitted uneven surface. Scaling can also result from the introduction of high stresses in the concrete produced by thermal expansion forces acting in a concentrated manner on a small area of concrete.

Additional information can be obtained in the United States National Park Service's preservation brief, Preservation of Historic Concrete Problems and General Approaches, published by the United States National Park Service, Technical Preservation Services. This document is available online at: http://www.cr.nps.gov/hps/tps/briefs/brief15.htm