Crack Stitching, Christchurch

The pictured two storey brick building was seismically retrofitted in 2007.

Alterations to the internal layout were made. Selected internal walls were removed and replaced with secondary moment resisting steel frames. These strengthened the building and created large openings and a new dynamic to the ground floor operations. The design won a prestigious architectural prize and helped the building withstand the Canterbury earthquake.(1)

Cracked brickwork was among the short list of observed earthquake damage. Crack stitching was specified as part of a repair programme designed to mend damaged areas and restore strength to the cracked brickwork.

Slots spaced at regular vertical intervals were cut into the mortar joints at each location where cracking had occurred. These were then cleaned and primed. A bead of HeliBond grout was then injected to the back of each slot, before lengths of HeliBar reinforcement were pushed into position and a second bead of HeliBond injected over the top to cover. Slots cut into the external brickwork were repointed to finish. Plaster covered those cut into the internal walls and completed the hidden repair.

Many thousands of aftershocks have since tested the performance of the installed HeliBars at this location, with none larger (to date of publication) than the M6.3 aftershock of 13 June 2011. This event was large enough to cause significant new damage to much of Christchurch’s remaining building stock, including new damage to this building. Cracking was observed once again in several areas. Significantly, however, no new damage was observed in any of the areas where Helifix repairs had taken place. Not surprisingly, the specification for a second round of repairs includes Helifix crack stitching.

The Helifix Crack Stitching System was also used on the large commercial building pictured.

Crack stitching repairs designed to restore integrity to sections of cracked brickwork around this building had been underway for several weeks prior to the 13 June earthquake and following the buildings closure on the day of the devastating M6.3, 22 February earthquake.

The sequence of events allowed the project engineers and contractors alike the opportunity to scrutinise the performance of the Helifix Crack Stitching System against strong, real world seismic conditions. The system performed extremely well, and the same engineers have since specified crack stitching to be used in several other projects on their books.

Cracks in masonry may lengthen, open and close as a result of numerous everyday factors, including on-going shrinkage and heave in foundation soils, and seasonal wet and dry, hot and cold environmental conditions. Under conditions such as these, crack stitching contributes improved masonry stability and the redistribution of tensile loads and wall movement to help minimise the chance of crack development. Under seismic conditions, Helifix Crack Stitching contributes further by adding strength to areas that might otherwise act as paths of least resistance.

(1) Ingham, J. and Griffiths, M. (2011). The performance of unreinforced masonry buildings in the 2010/2011 Canterbury earthquake swarm. Report to the Royal Commission of Inquiry.