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Site applied Structural Glazing - are the risks being ignored?

The early 1990's saw the emergence of structural glazing perceived as a high-technology form of the ever increasing curtain wall construction of modern buildings. As the method became more widely accepted specifiers began to see the aesthetic and performance benefits of structural glazing.

Four-sided structural glazing is only installed by specialist fabricators with the international experience of their systems designers to draw on, but two-sided structural glazing is considered simpler to install and has thus become the most popular curtain wall system used today. The method of installing two-sided structural glazing most commonly used is site-applied, variously referred to as site bonding or field applied structural glazing, using one-part, cartridge applied silicone sealants.

The structural silicone sealant manufacturers will not permit site application of four-sided structural glazing systems but accept this for two-sided systems. The concern is that as this practice has become so commonplace specifiers and installers have lost sight of the inherent risks of this method of application of structural bonding.

In the late 1980's a European technical authority stated "On-site bonding is to be considered as an exceptional measure, reserved for cases where in-shop bonding proves impossible. Where used, special provisions must apply" It is these special provisions, or procedures for successful site bonding that are now being regularly overlooked, or even ignored in some cases. Issues to be considered are:

Site conditions at time of bonding regarding dirt, dust and other contamination. Areas of glazing and aluminium to be bonded must be correctly cleaned just prior to the application of the structural bonding. Failure to ensure the total cleanliness of both surfaces to be bonded(glass and aluminium)could lead to the structural silicone sealant not achieving full adhesion to both surfaces, thus presenting a risk to the integrity of the structural bond in service.

Most cleaners are solvent based, which means high temperatures will cause them to evaporate very quickly, known as "flash off". This condition leaves an undissolved residue on the surface of the glass and/or aluminium surfaces to be bonded, thus seriously threatening the adhesive bond of the sealant. This condition can occur in a factory environment but the in-plant controls and temperature and humidity conditions minimize this.

Moisture is one of the biggest threats to silicone sealant adhesion therefore no site bonding can be carried out in rainy or high humidity conditions.

The presence of atmospheric moisture is a pre-requisite for all one-part RTV silicone sealants to begin their curing process. Therefore extremely dry climates may significantly delay the sealant's achievement of sufficient cure strength and potential adhesion qualities.

High temperatures can mean that the sealant is installed when the construction components are at their largest due to thermal expansion and thus the joint openings at their smallest. This means that the sealant will be in permanent extension stress throughout its service life, greatly reducing its ability to withstand the negative/positive windloads that the building will be subjected to. Low temperatures create the opposite condition.

Another aspect of applying the sealant at high temperatures is that it could cause the sealant to cure faster than intended, thus reducing its surface wet-out time and its subsequent mechanical adhesion strength development.

Site storage conditions of sealants, primers, adhesive tapes, etc., should follow manufacturer's guidelines regarding temperature and humidity, usually a maximum of 25(+/-2)degrees C and 50%(+/-5%)relative humidity. On many some projects it is quite possible that materials are being applied that are not at their best, another threat to their long term performance. The opposite condition of high temperatures, freezing, also causes problems with sealant storage. The sealant will have problems with shelf-life, adhesion and cure strength when thawed out.

Site application of sealants invariably utilises one-part cartridge applied sealants installed manually, not mechanically by metered pump machinery as in-plant applications are. This means that there will be variances in the pressure applied to install the sealant into small joints, thereby not guaranteeing 100% uniformity of fill.

The accurate placement of spacer tapes is not so easily achieved in site conditions as it is in the controlled factory environment, thus potentially jeopardising the correct size of silicone tensile bead. This dimension is critical for the security of the structurally bonded system and is dependant on glass size and windloads

One-part sealants take much longer to achieve satisfactory cure strength than factory applied two-part sealants, typically manufacturers specify 14-21 days cure, dependent on bead size, temperature, humidity, etc. This means that temporary fixings must be installed to support the structurally glazed panels until the sealant has cured sufficiently, typically 14-21 days. Construction programmes can be delayed awaiting the removal of scaffolding and access equipment which has to be left in place for this time.

Almost all temporary fixings secure the structurally bonded panels from the outside. This means that the sealant bead will not be significantly disturbed by negative pressures during its curing period. However, these fixings will not prevent the uncured bead being compressed when subjected to positive windloads, potentially disturbing or weakening the sealant's ultimate cure strength.

If adhesion tests show that the aluminium finish requires the use of primers for the sealant to gain satisfactory adhesion, correct site application becomes very difficult. Most primers have an open time which means they must be left on the surface for maybe 20 minutes before the sealant is applied, protecting the "wet" primer from accumulating dirt, dust and other contamination during this period is almost impossible, thus further threatening satisfactory adhesion of the sealant. High temperatures may also cause the primer to evaporate and leave an undissolved residue on the bonding surfaces, similar to the issue with cleaners described earlier.

There are other issues regarding site applied structural bonding, the above only represents the main concerns common to most applications, but it is clear from this that the original comment that "site bonding is to be considered an exceptional measure" in Europe and is even more relevant in hotter climates. In most projects in the Gulf, for example, some, if not all, of the above concerns are present, thus greatly jeopardising the security in service of the building fašade.

The obvious solution is the use of two-sided structural glazing systems which are designed to be silicone bonded in factory conditions, thus removing the need for any site application of structural bonding. Although many systems are not designed to allow this there are some systems available on the market today which are engineered to preclude any site bonding, one of these being Britals SG-2-56V & H curtainwall systems. The specification of such systems is the only way for architects, building owners and contractors to be fully confident of the service and security of their 2-sided structurally glazed curtain walls.