Liquid applied coatings systems fail for a number of reasons. Most are due to incorrect installation practices or misunderstanding of the performance capabilities of the system. One common application fault, occurs when the required membrane film thicknesses are not achieved. Many think that this is a sales pitch from membrane and adhesive manufacturers, to sell more products
Three issues could arise when applying waterproofing membranes below or above a mortar screed:
- Abrasive damage to the coatings.
- Elongation and flexibility restriction.
- Re-emulsification of water basedcoatings.
All three of the above are affected by the coating film thickness.
Film thickness and coating failure
Coating materials need to be abrasive resistant and accommodate building movement. Quite often this is misunderstood and unbonded screeds are placed over membranes that are applied too thin, resulting in mechanical abrasion. This mechanical abrasion is akin to trying to wear through the coating with a brick. Any thin spots, weak points or high spots can wear through, forming ruptures and blisters.
Differential movement when sticking the tiles directly to the coating will also impart mechanical abrasive stress at the coating interface, especially with external applications. Waterproofing systems are subjected to thermal shock movement and cyclic movement through saturation and drying phases. Maintaining an even coat of coating Dry Film Thickness (DFT), reduces the risk of abrasive blisters forming at thin spots and high points.
Movement accommodation at bond breaker junctions is severely compromised when waterproofing is applied beneath a screed. The compacted screed restricts the coating movement when releasing from the bond breaker, as there is no active release zone. This can be addressed by installing compressible foam rod to the perimeters, prior to placing the screed. This is further compromised when the required DFT is not achieved, reducing the membrane elongation tolerance drastically.
AS4858 Wet Area Membranes requires that a waterproofing system must accommodate an average building movement of 5mm at joints and junctions. A Class 3 coating has an elongation tolerance of above 300%, at the correct DFT. Twelve millimetres of an even DFT will stretch another 36mm before elongation failure. This sounds a lot, and your building is probably rolling down the hill where 36mm of movement occurs, however the same membrane at half or less film thickness may be struggling to stretch to 5mm.
Uneven film thickness is even more prone to elongation failure. Thicker coatings can restrict movement, where thinner applied coatings will tear under minimal movement. This is why filleting is not as successful as bond breaker systems. An even DFT of coating, bonded to a compatible joint sealant, is restricted from stretching as the uneven fillet section of sealant allows free movement at the thin points, and restricts movement at thicker fillet zones. Most flexible fillet systems are only aiming at the ranges around the 5mm requirement. An effective bond breaker system will allow 12mm of a Class 3 membrane to stretch to its potential where even film thickness is achieved. 36mm beats 5mm potential movement any day of the week.
Another cause of failure is where uneven film thickness occurs, resulting from ridge lines and dog licks formed in joint sealants, prior to coating application. Even though compressible joint sealants are soft and spongy, high points in the sealant result in a thinner DFT of coating even though the surrounding dry film may be at the required thickness. In many cases, these ridge lines result in DFT at the range of 0.1-0.4mm, where surrounding membrane film thicknesses reach 0.8-1.5mm. One wouldn’t consider it, but each of these ridges, with thinner coating, act as a tear line, with virtually no elongation capabilities. This little issue gets worse where fillers are used in the membrane. Sand and silica particles will act as a tear point, where paint film is not enveloping these foreign particles at the correct DFT.
Dislodged filler particles, through abrasion, leave either pin holes or very thin coating DFT. In laboratory conditions, coatings are applied over glass substrate, without imperfections. So the elongation properties are the same at 0.3mm as they are when at 1.0mm DFT, with the exception of increased tensile resistance. However, in the real world, a thin membrane is applied over substrate imperfections and contains air bubbles, which act as tear points, the same as sand aggregate. The coating will fracture at the weakest tear points.
Water based membranes can also re-emulsify where the screed is not drained. AS 3740-2010 A3.5.1 requires that where a membrane is applied under the tile bed, a drainage system be provided within the tile bed, to drain the reservoir of moisture within the bed. This requirement refers to the rebating of drainage control flanges to accept membrane drainage at the lowest level, and providing falls at level. This the coating can be achieved with proprietary levelling systems, prior to membrane application.
Correct film thickness applies here where excessively thick coating may split during curing, or re-emulsify where the membrane cannot cure out.
Getting the film thickness right
The first thing that one needs to do, is to understand the coating. Becoming familiar with the required DFT, under varied circumstances provides with a starting point. Most coating manufacturers require different ranges of minimum DFTs under different circumstances. Internal wet areas may require a minimum DFT of 0.6 to 1mm for wall applications and 1 to 1.5mm for floors and horizontal surfaces. Balconies and podiums may require a minimum DFT at the range of 1.5 to 2mm, where lift pits, pools and planter boxes may require a minimum DFT up in the ranges of 2 to 3mm, depending on the coating material used.
Once we have ascertained the required DFT through technical data sheets, or direct assistance from the manufacturer, we then need to assess the curing properties of the liquid membrane. All liquid membranes stay in solution due to a carrier. The membrane resin is called a solute, whilst in solution in the carrier. Once this carrier dissipates, we are left with a solid dry membrane.
All liquid applied coatings, therefore, have a Wet Film Thickness (WFT) that reduces to our required DFT, once the membrane reaches its cured stage. This is dependent on the solids content of the coatings, versus the carrier.
Most water basedcoatings have a solids content in the range of 50 to about 66%. With a 60% solids content, at 1mm WFT, we are left with 0.6mm DFT once the 40% of water carrier evaporates.
Wet Film Gauges are used to approximate the WFT by placing the toothed gauge into the coatings , thereby approximating our finished DFT, per coat. Most coating manufacturers advocate a 2 coat minimum, but prefer 3 coats in order to achieve the required DFT.
Low viscosity coatings must have at least 3 coats, where one cannot achieve anything greater than 0.5mm WFT, due to slump and souping of the coating. Even at 66% solids, we need to apply 3 coats to achieve 0.99mm (1mm) DFT required for floor and horizontal surfaces. An apprentice, who does not have a Wet Film Gauge, can adopt the practice of using a broom stick in the wet membrane to gauge WFT. A good practice, as we all know what 0.5mm and 1mm looks like.
Alternatively, a separate sheet of F/C can be left aside and subsequent coats of coatings applied throughout the application process. This can then be measured and tested at the end of our 3 coats. This is a good way of teaching apprentices to achieve the required DFT and become familiar with the products that they are using.
Another solution is to use the manufacturers recommended coverage rates. If we are required to apply 2litres per m2 and we have 4m2 to waterproof, we should at least be applying over half a bucket of a 20 Litrebucket. Usually waterproofing applicators will bring all empty buckets of coating products back to the store, to be stacked in the corner, and accounted for at the end of the project. This ensures that the scoped amount of coating has been applied to the job, and not used elsewhere.
Some coating manufacturers provide multiple colours in the one system. This assists in attaining a good WFT, where one cannot detect the previous coats colour, through the next coat.
Coating DFT is paramount to achieving the optimum performance as required by the manufacturer. Reduced coating thickness can result in abrasive damage and reduces elongation properties. Excessive thickness may result in re-emulsification and splitting during curing. Multiple coats at the required WFT is the answer. Detailed substrate preparation will ensure even film thickness and performance.
Questions to be asked
Ask your waterproofing applicators, how are they guaranteeing DFT. What processes are they following? Wet Film Gauges, or broom stick! Are they abiding by recommended coverage rates? Are your applicators the people that brag about how much area they can get out of one bucket? The above questions when asked on time will keep the project safe.
P. Eshwaran, Struct India Conchem (Contracting) & Concrete Technical Services (Consulting)