Alkali-Aggregate Reactivity

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Alkali-Aggregate Reactivity

Background

The phenomenon of alkali-aggregate reactivity (AAR) has been known for many years, but remains a current subject in concrete technology.  Typified by the presence of polygonal cracks on the surface of exposed concrete, such as on bridge parapets, columns, or retaining walls, this pattern cracking often contains white secondary deposits resulting from AAR.

When the concrete element is subjected to freeze-thaw or wet-dry cycles, the humidity contained in the cement matrix acts as a catalyst for the reaction. The crystalline silica present in concrete aggregates (fine or coarse aggregates) reacts with the alkali contained in the cement to form a silica gel that expands in the presence of humidity.  This expansion causes tensile stresses leading to the cracking of the concrete element.  Such tensile stresses allow for an additional space for the production of more silica gel, which will expand and create more cracking.  It is a vicious cycle that will lead to the destruction of the concrete element.

Several repair solutions are possible; however, it is recommended to minimize the use of such aggregates.

Our Equipment

CRA possesses the equipment to determine the reaction potential of concrete aggregates.  The equipment includes mixing and molding devices, two rooms at controlled humidity and temperature conditions (38^oC and 80^oC according to the specific test standard), and volumetric change measuring devices to determine the expansion of concrete subjected to AAR.

CRA Services

Our services related to the verification of AAR include:

• Assisting with the development of new borrow pits for the production of fine and coarse aggregates
• Renewal of conformity certificates for producers of concrete aggregate producers
• Expertise to determine the presence of AAR on cores extracted from concrete elements