The method you should use to repair cracks depends on the visibility of the crack and the available budget. If the cracks are highly visible, I recommend replacing the cracked brick units along the length of the crack and grinding and pointing cracked mortar joints full depth. In multi-wythe walls, it may be necessary to repair the crack in one or more of the backup wythes to prevent cracks from recurring.
If the crack is in a relatively concealed area, or if budget constraints do not permit rebuilding the crack, grinding out cracks and sealing them with a good quality sealant can be an effective repair. Prior to installing sealant, grind the cracks to a width of approximately 1/4 to 3/8 inch and a depth of roughly twice the width. Install a backer rod to create a good joint profile and to avoid creating a sealant joint that is bonded along three sides. Sealant joints bonded along three sides are prone to failure when there is any movement across the crack. In many cases, it may be necessary to prime the masonry bond surfaces to ensure good bond between the sealant and the existing masonry.
In some cases, the appearance of these sealant joints can be improved by applying sand onto the surface of the uncured sealant. This procedure may have some impact on the movement capability of the sealant joint; however, if appropriate repairs are implemented to address the cause of the cracking, significant movement across the crack should not occur. Where significant movement is expected, the sealant manufacturer should be contacted to evaluate the suitability of this approach. Placing mortar or grout in cracks will typically not be an effective long-term repair. The mortar will not accommodate any movement across the crack. For this reason, cracks filled with mortar will likely crack again, increasing water penetration and deterioration.
Before making any crack repair, it is important to determine the cause of the cracking. In most cases, the repair program will need to include adding appropriate vertical or horizontal expansion joints to accommodate the differential movement of the masonry that caused the crack in the first place. If the crack is related to settlement, it may be necessary to repair or underpin the foundation walls before repairing the crack in the brick masonry.
In historic buildings, it is important to preserve as much of the original materials as possible. Grinding cracks to widen them is generally not appropriate. In these buildings, many people will grout cracks or apply sealants over the top surface of cracks in limited areas. Although these repairs may not be as durable, they are relatively reversible and do not damage the original building materials.