Retro 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:

1. Vertical twist ties were found to have a life expectancy of only 30 years - half of that originally intended.
2. Wire ties were found to have a service life of only 15 years.
3. Mortar is alkaline which actually protects and enhances the working life of wall ties.
4. 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 both 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
The Retro Flex system of wall tie replacement is available in three different diameters of 6mm, 8mm & 10mm. It offers the advantages of a non-expanding mechanical fixing on the far leaf and a mortar fixing on the near leaf. Proof testing of the far leaf using a Target Universal Test Unit can be performed randomly as installation proceeds. Because the fixing method employed does not induce additional stresses into the substrate Retro Flex can be used in many and varied materials, from poured concrete columns to aircrete blocks, with satisfactory results and there is no concern to achieve the required edge distance spacings necessary with any expansion fixing.

The design of the Retro Flex remedial tie ensures that any potential for installer error can be minimised. The multiple drip design of each fin allows the Retro 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 Retro Flex is installed horizontally.

Testing
It is recommended that testing is performed in accordance with the requirements of BRE Digest 401. This publication gives a wind zone chart and the various proof test requirements for different parts of the country in differing situations. Most of the information is in table form which negates the need for complicated calculations. 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 Retro 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 fixings to ensure that the overall loading per m² is achieved. Lowering the density below the standard is not recommended.

The Materials
Retro Flex ties are manufactured from Grade 304 or, for more aggressive atmospheres, Grade 316 austenitic stainless steel. The 6mm diameter Retro Flex has a tensile strength in excess of 9.7kN. The manufacturing process produces very hard fins that are able to cut a thread into the hardest concrete, and a very soft and flexible core.

The near leaf fixing can be achieved by the use of Target Polyester Resin or Bond Flex Cementitious Grout. Epoxy resins are not recommended for normal installation as it has been noted that many of these resins are not reliable due to their accurate mixing requirements.

Where there is a need for a fire resistance in excess of 30 minutes, i.e. multi occupied buildings over 15m high, only Bond Flex cementitious grout should be used.