subject: Thor Helical Remedial are Wall tie Experts [print this page] Thor Helical Remedial are Wall tie Experts
Wall ties are an essential element for the stability of a cavity wall structure, tying its weather protecting brick facade to the main body of a building. An effective brick tie system transfers static and live loads across the cavity, enabling load sharing by both inner and outer walls. Typically cavity wall ties are bedded in a mortar bed joint as a building is constructed.
Cavity tie failure can be a consequence of a construction defect; for example where the original 'built in' cavity wall ties have been omitted, incorrectly fixed or fitted with brick ties that are too short. Alternatively failure may be a result of a buildings aging process, whereby wall tie corrosion may have compromised the walls load sharing capabilities.
Over time mortar joints, which host the wall ties, undergo a chemical change through carbonation. The mortar becomes aggressive to the steel base and its protective coatings, reducing the life expectancy of the cavity tie systems to as little as 26 years. The design life of the building is typically much longer than this period and it therefore follows that at some point a remedial retrofit replacement of the brick ties may be necessary if the stability and load sharing capacity of the wall is to be maintained.
As wall tie corrosion sets in, the mild steel brick ties generate a build up of iron oxide layers which occupy a greater volume than that of non-corroded steel. In most cases, particularly when wire wall ties have been used, the increase in the volume is accommodated in the mortar bed as the cavity ties corrode. In such cases there may be little sign that the outer facade is free standing and the walls not supported, save the wall may appear bulged.
In other case, particularly when sheet steel or vertical twist ties have been used in less forgiving mortars, the iron oxide build up may have the effect of lifting the masonry above a line of insitu brick ties. If expansion is widespread and present along several rows of corroding cavity wall ties, a tell-tale pattern of horizontal cracks may be produced along each tie course.
Designing Remedial Works
Remedial Cavity Wall Tie Systems can be employed to reinstate stability to existing walls; damaged by expansion of corroding mild steel vertical twist ties; loss of wire ties by corrosion; insufficient ties; ineffective ties and or unrestrained masonry.
The choice and validation of a suitable remedial wall tie system and their density must provide for fixing in both inner and outer leafs required by the imposed loads (refer to topographic tables) and conform with BRE Digest 329, 401 and BS 5628.
All Thor Helical Remedial wall ties are manufactured from 304 or 316 stainless steel. Thor Helical remedial wall ties offer a range of fixings to choose from to provide the right fixing for all situations.
Testing
Testing should be undertaken as a minimum on one or two fixings to each elevation to determine the minimum pull load obtainable from both leafs. On large projects 5% of ties should be tested as part of the installation routine. The tensile load results should be used to determine the suitability of the fixing and density in accordance with BRE Digest 401.
It is not practical to carry out compressive resistance tests on site. Thor Helical Remedial wall ties can be considered adequate in compression as compression required is usually 80% of that needed in tension when tested in accordance with DD140: part 1.
All Thor Helical remedial Wall Ties can be pull tested.
