Evaluation And Minimization Of Bruising (Microcracking) In Concrete Repair (ACI 364.7T-02(11))

Description

Bruising (microcracking) induced in a concrete substrate during the concrete removal or surface preparation process prior to repair is detrimental to the bond between the repair material and the substrate. Preventing or minimizing bruising and its effect on bond is an important requirement for a successful repair.

Concrete repair projects commonly involve the removal of unsound concrete and replacement with new material.The concrete removal process can damage the substrate surface, resulting in varying degrees of microcracks and fractures, commonly referred to as “bruising” (Fig. 1). A bruised surface is a surface layer weakened by interconnected microcracks.Concrete can be removed using a variety of methods such as chipping hammers, milling, abrasive blasting, and hydrodemolition. Removal of unsound and sound concrete subjects the concrete substrate to a wide range of impact and dynamic loads, and the resulting bruising will depend on the method used and the quality of the concrete.

The depth of the bruised layer varies, but is usually superficial (typical depth of the order of 1/8 in. [3 mm]). There are no criteria yet for the degree of bruising that reduces service life.Pull-off testing of the repair system (surface repair and substrate) can be conducted to determine the bond strength.1-4Bruising translates into a weakened concrete surface layer and will result in lower recorded pull-off strengths,5,6 with failures occurring predominantly in the substrate. The incidence, severity, and depth of bruising can be identified by microscopic examination (such as petrographic examination using ASTM C8567 methods) of the concrete. Microscopic examination can further be used to assess remedial actions taken to eliminate bruising of the concrete. To identify bruising, a polished surface needs to be magnified 20 to 100 times, depending on the width of the cracks (Fig. 1)