masonry crack repair

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CRACKED MASONRY WALLS

THE PROBLEM
Cracks in masonry are the result of movement beyond that which can be accommodated by the construction materials. Buildings tend to crack where masonry is over-stressed and least restrained, such as at window and door openings or at corners. The movement is usually caused by variations in moisture content in the walls, or in the ground that supports them, temperature variations or by chemical reactions. Some movements are cyclic and reversible, many are permanent.

Cracked Masonry

Cracks in brick walls can be divided into three categories:

active (those which are increasing in width and/or length)
passive (those which are no longer increasing in width or length) and
cyclic (those which open and close seasonally).

Unless you know the history of the building, and know that the cracks fall into the passive or cyclic categories, determining the kind a crack can be difficult and will require the input of a structural engineer who will regularly monitor the cracked brickwork over an extended period of time before arriving at a conclusion as to how an active crack can be made passive.

CRACK REPAIR SOLUTION
A cracked masonry wall cannot function as the composite unit for which it was designed until its integrity has been restored and the wall strengthened to at least its pre-cracked state.

Generally, cracks that are passive or cyclic and less than 5mm in width can be repaired by simple crack stitching techniques, which use bed joint reinforcement rods that are bonded into mortar joints to improve the tensile and flexural strength of the masonry.

	Mortar joints are mechanically raked out to form 30-40mm deep slots that extend 500mm either side of the crack in the wall. Slots are located at regular intervals, usually 300mm to 450mm apart

	Each slot is cleaned out and the bed joint is thoroughly soaked with clean water, making ready to implement the crack-repair

	A proprietary non-shrink cementitious bonding agent is pumped into the slot filling it from the back of the slot to approximately two thirds deep, along its full length

	A highly deformed stainless steel crack repair rod is pushed two thirds of the way into the cementitious bonding agent such as to fully embed the rod and tie the masonry on each side of the crack

	The slots are pointed up to match the existing facade

This effective masonry repair technique fully restores the integrity of the cracked wall to its pre-cracked state without costly rebuilding work and with minimum inconvenience to the occupants. The strengthened masonry is able to provide resilience to small and/or cyclic movements and to function as a reinforced non-fractured unit.

Repaired wall

MASONRY STITCHING PRODUCTS

stainless steel helical rods

Profiled and twisted stainless steel helical rods have a nominal ultimate tensile strength that is twice that of rebar, four times that of epoxy glass-fibre rods and seven times that of flat twisted plate [ref: BRE GBG 62].
Having a helical configuration the rods physically interlock with the bonding agent, exhibiting a unique and resilient torsional spring-like quality that allows small amounts of cyclic movement and recovery to occur without brittle failures.
The angular faces of the helix redistribute tensile forces over the reinforced area to stabilise the structure.

Cementitious grouts are the preferred bonding agents for retrospective masonry reinforcement. The use of resins for crack stitching is discouraged unless the load potential on the rod is wholly axial (parallel to the helical rod). When used as a full length rod-bonding agent, brittle resins do not cope well with shear forces (perpendicular to the rod) and should be avoided where there is cross-plane movement potential.

For free assistance in matters relating to masonry crack repairs in domestic housing phone the Twistfix technical help and advice line on 0845 123 6006 or visit www.twistfix.co.uk