Concrete is the most widely used manmade building material on the planet, making its repair and upkeep vital to the integrity of most structures. Its effectiveness for structural support is virtually unrivaled, but if not properly maintained all concrete is subject to failure. In the residential housing market alone some consultants estimate the number of new homes in need of repairs to their poured concrete, block or slab foundation to be as high as ten percent. That's not even taking into consideration existing homes, commercial buildings, highways, bridges and the various other structures that rely on concrete for support.
Causes Behind the Cracking Phenomenon
Cracks are the most telltale sign that a structure is in need of repair. The cracking phenomenon is complex and depends on a variety of factors including (but not limited to): tensile strength, tensile strain, thickness of the concrete, elasticity, degree of restraint, thermal contraction, internal corrosion, creep, rate and degree of drying and drying shrinkage.
The process of drying shrinkage alone is influenced by many factors, such as: the ratio of water to cement; stiffness, shape, surface texture and grading of the aggregate; the volume, fineness and composition of cement; mineral composition; characteristics and quantity of any admixture; size and shape of the concrete and weather conditions during the drying process. Almost all concrete is mixed with more water than is necessary to hydrate the cement.
This results in the evaporation of the excess water, in turn causing the concrete to shrink. The most common cause of concrete cracking is tensile stress that develops from the resistance to shrinkage during the drying period. In many cases cracks resulting from drying shrinkage are inevitable. To minimize the damage, and predetermine the location of the cracks, control joints can be placed in the concrete.
Creep, the slow deformation of the concrete over time, is directly related to drying shrinkage. It is believed that the greater the water to cement ratio the higher the rate of creep. Typically concrete that exhibits a high degree of drying shrinkage will have a high rate of creep. Although all the causes of creep are still not fully understood it is believed to be a contributor to the structural failure of concrete.
Corrosion of embedded reinforcing steel, caused by exposure to moisture, is among the leading causes of concrete degradation. When the steel corrodes, the rust that forms has a greater volume than the steel bars themselves had. This not only weakens the supports but the resulting expansion creates tensile stresses which can eventually cause cracking and delamination in the surrounding concrete.
The National Association of Corrosion Engineers International estimates the cost of corrosion damage to concrete structures in the USA alone to be over 125 billion dollars per year. This burden lies heaviest on state and local transportation agencies who are responsible for the maintenance of bridges and other highway structures that are especially susceptible to this problem.
During the curing process the temperature of the interior concrete increases due to hydration of the cementitious materials. As this is occurring the surface concrete may be cooling and contracting. If the differential between the surface and interior temperatures becomes too great cracking will occur. This is commonly referred to as thermal cracking and is most prevalent in mass concrete structures.
Weather conditions consistently expose concrete to a variety of stresses. The freeze and thaw cycle can cause concrete to shrink or expand. In concrete slabs for example the process known as D-cracking is quite common. This occurs when water naturally accumulates in the base and subbase eventually causing the aggregate to become saturated. The saturated aggregate at the bottom of the slab then begins to crack as a result of the freeze and thaw cycles. This crack will move progressively upward until it reaches the surface.
It is important to remember that concrete deteriorates just like any other type of building material. It only takes a tiny imperfection in the structure to start the degradation process. The first small crack opens the door for moisture and other corrosive agents, which in time will cause the structure to fail from the inside out. In addition to the causes mentioned above, other environmental factors like land and water erosion, ground shifting and earth quakes can all cause concrete to fail.
Though the structural failure of concrete is ultimately inevitable, the life of a structure can be extended almost indefinitely through regular maintenance and repair. The process of structural crack injection has been proven to effectively repair failing concrete in a multitude of applications. Crack injection is a relatively simple procedure for restoring the structural integrity of concrete.
This process involves injecting a high strength epoxy polymer to fill the void space of the crack. As the epoxy travels through the crack, it flushes moisture from the inner crack surfaces. When it cures the epoxy cross links inside the crack, bonding itself to both crack faces. This provides a structural bridge across the crack that is superior in strength to the concrete. Injection repairs can be used on horizontal, vertical and overhead cracks. Structural crack injection is an effective repair method for both "working", or moving, cracks and "dormant" cracks.
Before attempting to repair cracking concrete it is important to first determine the cause of the crack and the need for a structural repair. If the crack is damp and cannot be dried out, an epoxy tolerant to moisture must be used for a successful repair. Factors such as load capacity and function of the structure, as well as width and depth of the crack should also be taken into consideration when selecting the right epoxy for the job.
ATC, Adhesives Technology Corp, is a leader in the development of products made specifically for structural concrete repair. ATC products have been approved by over forty State Departments of Transportation and are currently in use by the US military in Iraq. For more information on structural crack injection, concrete anchoring or other concrete products contact ATC's North Eastern distributor, Kelken Construction Systems.
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