Stone Cladding with Spot Bonding

June 4, 2003

By Sean Boyle

Cladding with stone provides an architect with endless design possibilities. Storefronts, office and hotel lobbies, shopping malls and building facades are enhanced by the elegance of stone.
The use of a high-strength construction epoxy adhesive (or chemical anchor) — referred to as the spot-bonding method of installation – allows the designer/architect a much-wider design range and offers tremendous savings in both material costs and installation time. Traditionally, exterior facades have been installed via mechanical anchors, which can have many drawbacks.
High-performance epoxy adhesives applied by the spot-bonding (or dab) method are used on approximately 10 percent of the veneer surface, thus providing a ventilation gap behind the stone. This is an important advantage, because it will reduce the potential for water staining and efflorescence.

QUICK AND EASY
These specialized epoxies are strong (shear bond strengths in excess of 1100 psi), and yet flexible enough to handle building movement and stress. The two main advantages of high strength epoxy adhesives are:
1) Speed. Unquestionably, the spot-bond method (developed by LATICRETE incorporating high-strength epoxies) is up to five times faster than mechanical fastening. Traditionally, a three-man crew installing, let’s say a 2’x2’ granite veneer panel, would be expected to install, at most, 150 ft² per day. Using the spot-bond method, a two-man crew can easily and realistically install 350 ft² per day.
This number will dramatically increase to more than 500 ft² per day with the use of a cordless mixer to dispense the adhesive at an even-faster rate, thus eliminating the need for hand mixing. In addition to the time savings from installation, set-up and clean-up time takes only a couple of minutes … therefore saving the contractor even more time.
2) Ease of Installation. Adhering stone with mechanical anchors isn’t exactly an easy process. There are a myriad of sizes, shapes and techniques used to adhere the stone to the substrate, along with the need for many other supporting products used in conjunction with the anchor.
From drilling holes into the substrate, setting pins or bolts, assembling anchors, and even having to cut kerfs in the stone, it’s no wonder the average three-man crew can only set 8-12 stones per day. Often, the installation of just one stone requires a few “dry sets” and a lot of lifting until the proper position is obtained. In addition, breakage of stone is common, and can add cost as well as slow down the installation.
The spot-bond method via high strength epoxy adhesives is far easier and provides tremendous jobsite advantages. First, the contractor only needs to scarify the areas on the back of the stone to accept the bonding adhesive, which takes literally only seconds to prep. Then, the adhesive can be applied either by hand mixing or with a cordless mixing gun to the prepped areas.
The veneer can then be installed over the substrate; this is where the real benefit is noticed. Many substrates are not perfectly smooth or plumb and, in addition, stone thickness varies quite a bit. Both of these situations are easily adjusted for with the use of the high strength epoxy adhesive.
With the non-sag epoxy on the stone, it is placed against the substrate and then simply adjusted until plumb. The contractor can pull out or push in one side or corner until the desired position is achieved. The non-sag performance and high strength of the adhesive allows the veneer to stay in place until fully set (usually, in just about 30 minutes).
Using a high-strength epoxy adhesive has considerable advantages that translate directly into cost savings for the contractor, architect and customer. The adhesives, however, must meet all building-code requirements and norms; it’s important to use only those products with a track record of performance and those approved by the building community.

ON THE OUTSIDE
Exterior applications are certainly the most critical as they involve numerous compatibility factors. These include:
Thermal Movement. The entire system must be able to accommodate the expansion /contraction occurring through wide temperature shifts. For instance, a sudden rainstorm in the middle of a hot, sunny day can affects the cladding – a building-surface temperature swing of more than 80° F can happen in a matter of seconds, putting tremendous stress on the system.
Shock and Vibration Resistance – All buildings move, whether in California or Kalamazoo. While an earthquake provides a dramatic example of building movement, consider that wind loads, building settlement, moisture shrinkage/expansion and creep are more-routine stresses placed on the cladding system.
Freeze / Thaw Durability – In cold climates, the ability to withstand freeze/thaw cycles is important. Water expands approximately nine percent of its volume when it freezes, so building materials must be able to withstand this stress.
And, the substrate is just as important; material must be clean and sound for a direct-adhered system.
Clean means that all bond breakers or contaminants must be removed. This includes dirt, dust, curing compounds, oils and sealers; all of which are best removed by mechanical scarification.
Sound refers to the ability of the substrate to support the weight of the veneer, and to perform for the job conditions specified. For example, gypsum wallboard is a common substrate for tiles for interior, dry area applications.
However, that same gypsum wallboard should never be used for exteriors, because moisture would deteriorate the board, therefore causing failure. When specifying a manufactured substrate, it is most important to check with the manufacturer as to the suitability of its product for the intended purpose.
Similarly, the natural stone must be appropriate for the intended application. For example, certain types of fissile stone (such as slate) can delaminate within its layered structure due to moisture/freezing expansion. For exteriors especially, it is critical to address the absorption rate, thermal movement compatibility with the adhesive, breaking strength, dimensional stability and frost resistance.
For certain applications, spot bonding can be used as part of a team approach. The system, used in conjunction with mechanical anchors, provides a “belts-and-suspenders” approach to successful wall cladding.
The spot-bond/chemical-anchor method of installing stone is a recognized, proven method designed to perform under the conditions described above. Certainly, building-code compliance is an important consideration as well.
When utilizing spot bonding, use only those installation products approved by the various code bodies as acceptable for exterior or interior applications. It’s also imperative that building owners and specifiers check with all manufacturers involved and get as much information as possible.
Everyone involved should lean on the manufacturers with a successful track record with installations, and use the best system possible for your application. The consequences of improper materials or poor workmanship are too severe to cut corners.
Sean Boyle is a Product Manager at LATICRETE International. He holds a Bachelor of Science Degree in Management from Central Connecticut State University and a Master of Business Administration degree in Marketing from the University of Hartford.