Cracks in walls are the result of movement beyond that which can be accommodated by the masonry construction materials. A wall tends to crack where brickwork is over-stressed and least restrained, such as at window and door openings or at corners. Stress and movement is usually caused by variations in moisture content in the walls, or in the ground that supports them, temperature fluctuations or chemical reactions. Some movements are cyclic and reversible, many are permanent.
Unless one monitors the crack over a period of time it can be difficult to determine its cause or the appropriate method of repairing it. Generally, cracks that are passive or cyclic and less than 5mm in width can be repaired by simple brick stitching techniques that improve the tensile and flexural strength of masonry walls such as to accommodate small and/or cyclic movements. Repairing wall cracks that are active or that are greater than 5mm in width should only be carried out under the guidance of a structural engineer.
Shaped and twisted stainless steel rods are bonded into the walls, stitching the masonry across the cracks at regular intervals. The brick stitching system allows the cracked wall to behave as a reinforced non-fractured unit.
Having a helical configuration the crack stitch rods physically interlock with the bonding agent and exhibit a unique and resilient torsional spring-like quality that allows small amounts of wall movement and recovery to occur without brittle failures.
Installation is straight-forward. A horizontal channel is cut in the masonry, usually in the bed joint where installation can be easiest disguised. The slots are flushed with clean water immediately prior to the installation of a high performance shrink-compensated cementitious grout. Brick stitching bars are simply pushed into the grout, ensuring full encasulation of the bars along their length. The helical bars extend 500mm each side of the cracks to dissapate loads and disperse them evenly into the wall structure.
Chase slots at 300mm intervals along a length of wall that extends 500mm each side of crack.
Flush the slots thoroughly with clean water to remove loose material and to reduce the tendency of porous brickwork to suck moisture from the grout.
Working from the back of the slot fill to two thirds full with WHO60® grout, leaving room to repoint and match existing finishes.
Push the crack stitching bars into the slot ensuring they are fully encapsulated in the grout. The bars must extend 500mm each side of the crack to allow them to absorb loads and to disperse them evenly into the wall structure.
Make good to disguise slot.
Note: It is important to ensure all cracks in walls are filled and sealed to keep out water. In cold weather water in the cracks may freeze, turning to ice and increasing in volume by 9%. This expansion is a major cause of producing further stresses within the brickwork.
The use of resins for masonry crack stitching is discouraged unless the load potential on the helical bars is wholly axial (parallel to the helical rods). When used as a fully encapsilated bar-bonding agent for repairing cracked walls, resins do not cope well with shear forces (perpendicular to the rod) and should be avoided where there is cross-plane movement potential.
On its own a masonry crack stitch repair system should not be relied upon to solve problems associated with active cracks. Brick walls having fractures which are increasing in width and/or length or having fractures that open by nore than 15mm require further investigation by an engineering professional. In such cases brick stitching bars should only be used for repairing cracks in walls when done so in conjunction with a full structural engineers repair specification that addresses the source of wall movement.