Dri Flex from Target Fixings Ltd

Dri Flex - More Details

Introduction
During the 1970’s research was conducted into the need for replacement wall ties. The actual durability of the protective zinc or galvanised coatings of the original built in wall ties was studied and this now affects the design and use of wall ties for both new build and replacement.

The results of the research was found to be quite beyond any previous expectations:

Vertical twist ties were found to have a life expectancy of only 30 years - half of that originally intended.
Wire ties were found to have a service life of only 15 years.
Mortar is alkaline which actually protects and enhances the working life of wall ties.
A reaction between mortar and the air causes a process called carbonation which turns the mortar acidic which then attacks the wall ties.

Early attempts at producing a method for replacing existing wall ties showed many of the pitfalls that were to be encountered. The expansion type of tie has been found to induce additional stresses into the masonry - similar to the expansion caused by a rusting existing wall tie - and were costly to make and fit. The use of heavy section re-bars was soon outlawed because of the need for flexibility to allow the structure to move. Generally a connection between each leaf using a bar of 8mm diameter or above was found to act like mini crowbars and would eventually work themselves loose The introduction of BSI DD140, BRE Digest 329 and the more recent BRE Digest 401 at last gives guidance for manufacturers and specifiers of remedial wall ties.

The System
Dri Flex ties were developed as a result of the identified need to drill smaller holes in masonry to control the amount of spalling when drilling the near leaf. A large hole drilled through a brick using a hammer drill can result in up to half of it spalling away. This material can cause bridging of the cavity and moisture transfer, and leaves a much reduced fixing area for the near leaf connection. Drilling for the Dri Flex tie is performed using a rotary percussion drill, which dramatically reduces the spalling, with hole sizes reduced to 5-7mm. Dri Flex offers the advantage of a non-expanding mechanical fixing to both the far and near leaf. Pre-installation testing should be performed to the requirements of local standards and technical help and advice is available. Because the fixing method employed does not induce additional stresses into the substrate Dri Flex can be used in many and varied materials, from poured concrete columns to aircrete blocks, with satisfactory results. Edge distance spacings, so necessary with any expansion fixing, are not a requirement with Dri Flex.

The design of the Dri Flex remedial tie ensures that any potential for installer error can be minimised. The multiple drip design of each fin allows the Dri Flex to be installed at an angle of up to 25^o towards the inner leaf without the possibility of any water transfer across the cavity. It is recommended that each Dri Flex is installed horizontally.

Testing
Because Dri Flex Ties cannot be randomly tested on site it is necessary to perform comprehensive pre-installation tests.

There are several different methods of performing tests, but the most straightforward way is to drill into both leaves as described in 1 on the installation page. The near leaf hole may then be over-drilled to 12mm. The Dri Flex Tie should then be installed into the far leaf and a test performed using the Target Load Test Unit. The fixing capability into the near leaf material may ascertained by drilling into the near leaf as in 1 on the installation page and inserting the Tie to a maximum depth of 80mm and then tested as above.

It is recommended that testing is performed in accordance with the requirements of BRE Digest 401. It must be understood that wall ties are designed as load sharing devices and as such there is no necessity to have a high point loading on any individual tie. Only in exceptional circumstances does the proof test load requirement exceed 1kN.

Fixing Densities
In general terms the fixing densities for Target Dri Flex remedial wall ties would be the same as new build - 2.5 per m² or 450mm vertically and 900mm horizontally in a domino five pattern. This density would, however, be subject to on site testing to ensure that the required tensile loadings are being achieved. It should also be borne in mind that around all the openings - doors and windows - ties should be installed at no more than 300mm vertical centre and no more than 225mm from the edge of the opening. BRE Digest 401 gives more information on the proof test load requirements.

If the masonry is so weak or friable that the required proof test load for standard density fixings cannot be achieved, it is quite acceptable to increase the density of Dri Flex Ties to ensure that the overall loading per m² is achieved. Lowering the density below the standard is not recommended.

The Materials
Dri Flex ties are manufactured from Grade 304 or, for more aggressive atmospheres, Grade 316 austenitic stainless steel. The 8mm diameter Dri Flex has a tensile strength in excess of 11kN. The manufacturing process produces very hard fins that are able to cut a thread into the hardest concrete, but maintains a very soft and flexible core. Dri Flex Ties may also be used where there is a need for a fire resistance in excess of 30 minutes, i.e. multi occupied buildings over 15m high.