Adhesion of Stone, Brick Slips and Tiles onto Vertical Waterproofing
Introduction
Due to the porous nature of many commonly used exterior wall cladding products such as various types of stone; brick slips and tiles, it is becoming more, and more common to see the specification of a waterproofing membrane behind these products in order to meet building code and council requirements. There are of course a number of products available to meet the required criteria, but because of the ultimate limitations imposed by the use of either 'torch-on', or 'sheet' membrane products, the selection is largely narrowed to the use of a liquid applied material. While the comments contained within this document relate in a more general sense towards various types of liquid applied product, allowances for the fact that actual measurements and technical results will vary from product to product should be made.
The adhesion of the cladding, and the system as a whole, to the substrate will be dependent on a number of factors such as: type of substrate and substrate preparation, type of adhesive used, adhesive application, climatic conditions, and the overall weight per unit area of the cladding material. (This last factor can change significantly with the absorption or entrapment of rainwater, and this needs to be taken into consideration). An example is brick which gains 8% in weight when wet.
Substrate
Comparisons reveal some interesting points when adhering liquid membranes to various substrates. As would be expected, the adhesion reduces on timber substrates when exposed to high humidity, presumably due to the softening of the substrate surface. In concrete substrates the reverse is true, this time presumably due to the improvement in the cure of the surface after 30 days of exposure to moisture. This was particularly evident in the case of concrete.
Substrate Preparation
Under normal circumstances because of the relatively high weight of the various cladding materials we are talking about; the limiting factor will usually be the adhesive and cohesive capabilities of the liquid membrane to the substrate, rather than how well the stone can be adhered to the membrane itself. For this reason it is imperative that the surface of the substrate be prepared in such a way as to maximize this adhesion.
Substrate surfaces must always be clean, and free of any form of contamination that could inhibit adhesion, including dust, paint (or other surface coatings), and any oily residues. The use of a penetrating primer is recommended on any absorbent substrate.
Adhesion of stone should always commence from a solid foundation, L angle or lintel. Furthermore L angles or grout line pins should be employed at regular horizontal intervals to accommodate the severe reduction of allowable weight onto a liquid membrane. This becomes even more applicable when stone is quite thick and sits out from the wall creating a ìfulcrumî effectî. Allowable weights vary from membrane to membrane and substrate to substrate, and therefore consultation with the membrane and substrate manufacturer is advised.
Adhesive Selection and Application
As discussed above; the adhesive bond to a liquid membrane will seldom be the limiting factor in the system; however the selection of adhesive and its application will impact on the performance of the system.
Variability in moisture content of stone due to changing ambient conditions, and temperature fluctuations themselves independently of this, will cause expansion and contraction. This process places stress on the adhesive bond of the stone to the membrane, and in turn upon the adhesion of the membrane to the substrate. In order to limit the stress in the system, an adhesive that has some ability to 'flex', and take up some of this movement, should always be used.
When it comes to application of adhesive; the rule to follow is again limitation of stress, and this can only be achieved by avoiding point loading. Clearly the weight carried by a given area of membrane can be minimised by ensuring that there is complete 100% coverage of the membrane surface with adhesive, thus ensuring an even spread of the load. Any form of 'spot adhesion' must be avoided at all cost, and is not acceptable. Weight per square metre is calculated on 100% surface adhesion, and therefore should only 50% adhesion be achieved, technically the weight per square metre is doubled.
As with any rigid cladding system; flexible jointing must be incorporated at regular intervals to relieve stress at circa 3.0m maximum in any plain. Control joints must be carried through from the substrate and the stone, brick slips or tiles being adhered. These joints should then be filled with an appropriate silicone and not cement based grout or mortar.
Rules on adhesive Open Time, Wet Tack and Pot Life should also be observed
Climatic Conditions
Ambient conditions will play a part in the performance of the installed system. Thermal cycling, along with seasonal and daily changes in moisture content will introduce stress to the system, but moisture can have other more catastrophic effects in areas that are frost prone, or experience freezing winter conditions. When water freezes, it expands approximately 10% in volume, and if this occurs in a confined area such as where water is trapped in trowel notches in adhesive behind stone etc,or in 'fissures' between the stone and the adhesive; the stone can literally be blown off the substrate by the pressure that is created. This is yet another reason to ensure full coverage of adhesive. At the other end of the scale, water entrapped in voids or notched trowel lines can have a softening effect on a liquid membrane, and on hot days act like a pressure cooker.
Summary
Many factors other than the weight per unit area of the stone, and the adhesive used, can affect the performance of a stone, brick slip or tile exterior cladding system adhered to a liquid membrane on a wall.
It is recommended:
Substrates are properly prepared.
Adhesion commences from a solid foundation.
L angles or grout line pins are employed at regular intervals to accommodate weight reduction and the fulcrum effect when adhering to a membrane.
Control joints are followed through.
No evidence of notch trowel lines between stone / tile and substrate.
100% adhesion with definitely no spot adhesion.
Observe adhesive rules on Open Time, Wet Tack and Pot Life.
Note:
Flexco (NZ) Ltd can only give recommendations on the use of its adhesive, and cannot control its use by tradesmen. Flexco (NZ) Ltd is not responsible for engineering, and specific or architectural design and specification
Flexco (NZ) LTD will not be responsible for adhesive washout due to water ingress from rain or sprinklers prior to cure of the adhesive.
It is the subcontractors responsibility to ensure there has been sufficient cure time of the adhesive allowed prior to traffic, and that all precautions are taken to avoid ingress of water from rain etc during the cure phase.