What Causes Sealant to Fail?
As sealant is used to frequently during the construction of a build, there are many opportunities for it to fail. In the following blog we will identify a broad array of these potential failures, equipping you to identify and avoid them during your development.
Poorly Designed Joints
This is the most common cause of joint failure.
If a sealant joint is properly designed it should have a 2:1 width-to-depth ratio, which is the configuration which allows the joint to accommodate movement most effectively. Manufacturers of sealant provide maximum and minimum dimensions.
Not allowing for sufficient building movement, can cause even correctly proportioned joints to fail.
Sites subject to vandalism, water and weather conditions demand sealant with superior abrasion resistance. High-heeled shoes are known to be a particular bane for sealant and are notorious for puncturing it. Sealants have been designed to withstand vandalism, these are harder materials and tend not to accommodate much movement.
Substrate compatibility is very important. Some products can leach chemicals, causing staining, and sometimes degrading of porous materials (such as Brick, masonry, stone) surfaces or in some cases, degrading them.
If a sealant is stronger than the substrate it can cause cracks and spalls as the sealant dissipates forces to the weaker materials. This particular phenomenon is common within exterior insulation and finish systems (EIFS / ETICS).
If the substrates have existing coatings this can also cause problems, fully removing these coatings is advised. Alternatively undertaking an adhesion test and finding a sealant which is compatible with the existing product will be necessary. Note: many surface sealers are transparent and on existing buildings it is important to always conduct a field adhesion test before starting work.
Sealant failures are very often due to poor surface preparation. The thorough cleaning and priming of a joints surfaces prior to application is of primary importance, manufacturer’s recommendations for preparing the substrate should always be followed. The use of dirty rags, incorrect or contaminated solvent, lint or even residue from existing sealants are just some of the ways a sealant joint can be compromised.
Some sealant types require surface primers to be applied to the substrate prior to the application of the sealant. These are designed ensure the correct adhesion is achieved and / or prevent the sealant from diffusing into the substrate and bind with the dirt particles remaining on the surface.
Weather conditions on the day of application must be correct. An ideal sealant installation temperature would be at the median of the design range. This means that the product retains its flexibility the ability to elongate or compress to accommodate future fluctuations in temperature.
The viscosity of the sealant will also vary with the ambient temperature. Hot temperatures may cause the sealant to sag and cold temperatures may make it too thick to tool properly.
High levels of humidity, frost, dew or general dampness can also lead to failure as the majority of sealants will not adhere to a surface unless it is dry.
3 sided adhesion must be prevented, and this can be done by utilising a backer rod or release tape. Three-sided adhesion will prevent sealants from moving freely resulting in adhesive or cohesive failure (or sometimes, both).
Poor or sloppy tooling can result in voids, gaps and irregular sealant thickness which can cause stresses to act unevenly along the joint. Sealant should follow the curve of the backing rod, ideally with a concave tooled outer surface, so that is resembles an hourglass in a cross-section.
Other things you should try and avoid are:
- Making use of sealant which has reached or gone past its shelf life
- Storing sealant in a location which is subject to extreme temperatures
- Mixing multicomponent sealant incorrectly
- Applying irregular pressure and flow with the sealant gun.
A team member should be tasked with quality control on site so as to ensure installation meets the manufacturer’s warranty requirements for testing and inspection
Some organic based sealants, especially those derived from polyurethanes, have a potential for reversion failure in which the revert to an uncured or gummy state in response to moisture or UV light exposure.
Modern sealants do not suffer from this, but older sealants on existing buildings may suffer from this.
Careless specification of sealant products and unclear drawings or documents can leave the installation open to guesswork which of course has a high potential for failure.
Site conditions do vary and it is a good idea to test the products onsite whenever possible.
Using peel tests to check adhesion on multiple test areas. This is especially important for older buildings.
Testing at a laboratory can provide more in depth and detail information than site testing, but should not substituted for site testing. In the lab it is possible for conditions (such as movement, stresses, temperature, cure times, product, etc) to be varied to recreate many different scenarios.
Sealant joints demand care and attention if the building envelope is to perform like it is meant to. A good point of reference would be ASTM C1193, Standard Guide for Use of Joint Sealants, which gives an in-depth set of information on joint design and sealant installation.
Sealant Joint Replacement
Sealants have a limited lifespan and that means that, regardless of how well you install them in the first place, they will eventually need to be replaced. When replacing sealants always utilise the experience of a sealant manufacturer.