Mechanical Fix or Adhesive Bonding – Which is Best?

Mechanical Fix or Adhesive Bonding – Which is Best?

The answer to this question is broadly driven by the type of brick that you wish to use, the level of intricacy  and supporting structure.

Mechanical fix systems can be broadly divided into two categories, those that use an extruded brick effect tile which is manufactured to incorporate a key for the rails, or those that use slips cut from full bricks which then have keys machine cut into the slip to allow a connection to a rail.

Similarly, adhered systems can be divided into site bonded and factory bonded systems, both of which are capable of utilising slips cut from real bricks and possibly extruded pre-formed slips.

Mechanical Fix

Mechanical fix systems are at their best when utilising extruded pre-keyed slips specifically formed for the rails to which they will be connected. They are particularly suited due to their manufactured dimensional tolerances, plus the keys are often radiused meaning that stress points are not induced into the slip:

The key in the top of the machined slip is cut oversize to allow the slip to be introduced into the two horizontal rails:

Adhesive Bonding

Adhesive bonding can be carried out on site using grab type adhesives which are generally Modified Silane Polymer type adhesives or factory bonded where, for example, at FastClad we use Two Part Epoxy adhesives, where the need for a grab fix is not required.

Adhesives will spread the load across the rear face of the slip and do not have point loads applied to the slip allowing more fragile slips to be used than mechanical fix systems can safely cope with.

Site bonding is always reliant on the installation team to ensure that the bond between the slip and the substrate has been properly achieved and that the weather conditions are suitable at that time.

Slips applied in factory conditions are generally bonded whilst the backing board is horizontal which negates the need for a grab adhesive. The biggest advantage of using an epoxy adhesive over an MS polymer grab adhesive is longevity, where epoxy adhesives once cured remain stable, MS polymers will lose their flexibility over time and do not achieve the same high bond strengths as epoxy.

Slips bonded with grab adhesives can only achieve a minimum 30 year certified design life, whereas epoxy bonded systems can exceed the 60 year requirement, meaning that they are more readily accepted by building warranty providers who demand the 60 year certification.

Brick Tolerances

Mechanical fix systems should be restricted to using bricks classified as T2 for tolerance and T1 or TM bricks must not be used. For example, a T2 brick may differ in height by 4mm (+/-2mm) therefore if the maximum amount of top rail going into the top key is 6mm, then it could be as little as 2mm for the lowest height brick. T1 bricks can vary by as much as 6mm (+/- 3mm) meaning that many of the slips may not even touch the top rail. TM tolerance bricks have no defined variation and should definitely be avoided.

Bonded systems are not constrained by brick dimensional tolerances and can even use reclaim bricks.

Utilising Fragile Bricks

The actual tensile strength of cut slip should also be considered but this is not something that is generally published for a cut slip, but bricks that are clamp fired or demonstrate a very low compressive strength are generally not suitable for mechanical fix systems for two reasons, one the cutting of the initial slip then followed by cutting two grooves into the slip edges can cause the slip to deteriorate and become weak, consequently their ability to withstand the point loading of rails can be seriously compromised.

Which System to Choose

In summary, both systems have their applications and both methods, when specified, manufactured and installed correctly, will exceed their certified service life, and the ‘bonded slips systems fall off’ is an opinion based on incorrectly installed projects of the past.

The key to a successful install relies on the manufacturer, installer and specifier to ensure that it is the correct solution for their project.