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Crack Formation in Concrete

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30.08.2007

Cracks in concrete structure elements can strongly prejudice their function and aesthetic appearance. Cracks cannot usually be avoided but their number and size can be controlled and it is technically feasible to close the cracks that form in many cases.

Concrete is an artificial rock produced from cement, aggrgates (sand and gravel or broken stone) and water. It cannot be considered as a material with specified properties as these develop as a function of age and continue changing over time. Cement hardening and drying is usually accompanied by shrinkage, therefore fine cracks are inevitable. Cracks exceeding 0.1 mm can be handled by optimising the concrete composition and by using reinforcement.

Crack formation caused by dynamic and static stress on concrete structure elements is due to working loads and permanent load. Thermal and chemical impacts and frost-deicing stress in the pore system of the concrete are also important. Suitably designed composition and/or preventive measures (hydrophobic treatment, coating) can long-lasting prevent weathering damages.

Permanent weathering, horizontal surfaces, dark surfaces and sharp edges increase the risk of cracking formation and/or flaking due to increased thermal strain and unfavourable stresses in the building component. Avoiding cracks greater than 0.1 mm is often planned in the design phase for technical reasons (e.g. waterproof concrete). This may also be necessary on aesthetic causes.

Before filling the cracks an appraisal has to be made to specify necessity, objectives and methods. Two techniques are applicable in principle to fill the cracks: soaking (filling the cracks without applying pressure) or injection (filling the cracks under pressure). Epoxy resins, polyurethane, cement paste or cement grouts are usually used for this purpose. The Guidelines of Concrete Repair of the German Committee for Reinforced Concrete of 2001 and/or DIN EN 1504 Part 5 and 9 regulate the relevant technical aspects.

Contact:
Dr.-Ing. Hans-Carsten Kühne
Head of Working Group "Cementitious materials:
advanced performance "
Division VII.1 Building Materials
Phone: +49 30 8104-3229
Email: hans-carsten.kuehne@bam.de

Dr. Ulrike Rockland | Source: Informationsdienst Wissenschaft
Further information: www.bam.de

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