Avoiding sealant adhesion and cohesion failure in concrete flooring joints.

Although the last element in the construction process, the correct sealing technique and sealant specification for concrete floor joints is certainly one of the most critical stages in finalising the job. However, with the pressure to fast-track projects, compounded by declining skills levels in the South African construction sector, it remains a neglected area, impacting on the longer-term performance of these structures, and creating downstream costs for building owners.

Michael Berg, national sales and marketing manager for Den Braven, a world leader in professional sealants, which provides training and product advice to industry, says that joint sealing is often perceived as an easy and quick application, but it remains a highly specialised area, underscored by the incidence of failures which occur on projects around the country.

"During concrete floor construction, we recommend that contractors plan the requirement for joint sealing at the beginning of the project to take into account the time required for concrete curing times, joint cutting and the cleaning out of joints to remove excess debris so that the final sealant stage isn't rushed," explains Berg.

"Once the joints have been washed and cleaned, the joints also need to be allowed to dry completely. In some cases, the floor may seem dry to the touch, but the insertion of a humidity meter will soon confirm the moisture content, which if too high will inhibit the accurate curing and adhesion of the sealant and increase the risk of failure."

The correct joint design formulation requires the width to be half of the depth of the joint. The dimension for floor joints also needs to be specifically measured to cater for the expansion coefficient. If this sizing is incorrect, a state-of-the-art sealant can be used but the joint is still going to fail.

As Berg explains, "The sealant, in correlation with the joint design, needs to adhere adequately to the sides to cater for expansion and contraction. This requires proper two-sided adhesion. If the joint is not primed (where required) or cleaned properly you get adhesive failure (where the sealant doesn't stick to the sides of the joint); or cohesive failure, where the material breaks or cracks from within, promoting moisture ingress over time and the growth of mildew and rot.

"This latter factor is especially prevalent where materials other than specified bond breakers (backing cords) have been used - such as timber rods or even newspaper – then followed by the sealant, with the inevitable result."

Backing cords are used as backfilling material in joints to create the right joint dimensions and depth for the sealant to be applied where joints are exposed to mechanical or water pressure – ranging from concrete floors to building perimeter weather seals.

The application of a desiccant on the floor surface in an attempt to remove moisture and accelerate curing times will also not have the desired effect. Sealants work by attracting moisture (and not wetness from the sides of the joints) during curing at specified ambient temperatures to activate the vulcanisation process. A "quick fix" is certain to fail, as an inspection will confirm.

Backing core selection and installation

In preparing the joint, after cutting and cleaning, the applicator needs to mask off the sides with masking tape. Thereafter, to obtain optimum results, the correct diameter backing cord needs to be selected to control the joint depth within the design limits. This should always be a size up from the actual size of the joint to allow for a tight fit. Backing cords (available in diameters from 6 to 30mm) are made from polyethylene foam so that conventional sealants (silicone or polyurethane) won't stick to them, creating the required bond breaker.

Once inserted to the correct depth (without puncturing the cord), the bond breaker should be left for a few minutes to settle, thereby avoiding bulges. As Berg emphasises, if the sealant is immediately applied, this potential bulging will result in bubbles forming during curing, requiring rework.

For most joint sealing tasks, a polyurethane sealant, like Den Braven's Hybriflex-LM - a low modulus product commonly used in the floor industry; or Tectane 2030, a high modulus sealant, provides the best results. The former is a harder and the latter a slightly softer material when cured. Importantly, though, both have grade tear strength resistance to counter the movement of vehicular or pedestrian traffic.

"Choosing the correct applicator gun for the specified joint diameter is essential as is the need to ensure that the sealant is extruded to the required depth to avoid trapping air underneath, which could result in lifting," Berg continues.

Why cohesion and adhesion failures occur

There are two main reasons for cohesion failures on sealants. The first is where the joint formulation is incorrect. (Triangle shape joints with connections at various building elements cause the most problems and can be avoided by using a bond breaker.)

The second cause for failures is where high temperature differences occur during sealant curing referred to as "early tear occurency". Where this happens, the skin is not capable of taking up the large movement and will tear. As a result of this, the total sealant will tear, with early evidence of this occurring within 24 hours of application. In general, the sealant tears in a diagonal direction and this situation can be avoided by not sealing on days with large temperature variations.

"Around 50 to 55% humidity at 23 degrees Celsius is the optimum ambient temperature for sealant curing," says Berg. "However, for the cold and dry winter climate typically experienced on the Highveld, for example, the curing time will take longer and provision needs to be made for this."

Curing times also depend on the joint depth and design, since sealants cure in layers. For most joints, a Part 1 sealant is sufficient. However, for anything deeper than 15mm to 20mm, or wider than 20mm wider, Den Braven recommends a Part 2 sealant - especially where time is a factor. Using a Part 1 sealant here will work, but the curing time is significantly slower.

"Where adhesion cracks take place, there are a number of causes. These include the wrong choice of sealant; or where the substrate is not dry, clean, grease or dust free; the substrate is not suitable for sealing; or where incorrect joint relations occur," Berg continues. "If in doubt, rather carry out a primary test. Adhesion can also be improved by the use of a primer /and we advise using a primer on dusty substrates and/ for exterior use on more porous surfaces. A clean, dry, degreased joint groove is in principle always necessary and this can be done by washing or with high-pressure air or a combination thereof "

Berg adds a further note in terms of final floor and building finishes, explaining that where subsequent epoxy coatings are applied following joint curing on floors, or during the painting of walls over vertical joints, Den Braven recommends checking with the paint or epoxy supplier first to ensure the optimum paint specification. For example, an inferior paint applied over an expansion joint will result in cracking if it doesn't have the required elasticity.

"During concrete floor construction, we recommend that contractors plan the requirement for joint sealing at the beginning of the project to take into account the time required for concrete curing times, joint cutting and the cleaning out of joints to remove excess debris so that the final sealant stage isn't rushed," explains Berg.

For more information contact: Michael Berg:   sales@denbraven.co.za