Modern Waterproofing Technologies

For ages, asphalt or bitumen has been the prime material for waterproofing of structures. In the ancient civilization, bitumen has been found to be the most used waterproofing material. Modified bitumen polymer chemistry replaces the air blowing process that is used to produce the asphalt used in traditional built up membranes. Air blowing is a dehydrogenation process, removing lighter oils from the asphalt as water molecules are formed and evaporated from the heated, oxygenated asphalt. For modified bitumen, air blowing is reduced or eliminated. This preserves lighter oil, thereby improving the flexibility and weather resistance. Despite their great waterproofing property, conventional bitumen viz, asphalt and coal tar pitch being thermoplastic have undesirable temperature dependent variations in their physical properties. At subfreezing temperatures, they become brittle, glasslike and too fluid at a higher temperature. Attempts were made to modify the thermoplastic properties of bitumen by using fillers like wax, and oxidation of blown bitumen led to limited success. With the passage of time, progress made in polymer technology in the development of synthetic plastic & rubber, waterproofing system got new rays of life which led to the birth of polymer modified bituminous pre-formed membrane. Two types of polymers dominate the modified membrane with their outstanding performance:
1. Atactic polypropylene (APP)
2. Styrene Butadiene styrene (SBS)
The two major polymers APP & SBS are both compatible with bitumen despite fundamental difference in their chemical nature. APP is a plastomer whereas SBS is an elastomer. This chemical difference manifests itself physically in much greater elasticity for SBS-based modified bitumen, with more nearly uniform properties through wider temperature range e.g. greater flexibility at low temperature. APP modified bitumen are generally stronger and stiffer than SBS modified. They also offer greater resistance to high temperatures.
Generic placeholder image Er. Supradip Das,
With the development of synthetic plastic & rubber, the roofing sector has made a significant leap to the present status. In India, APP & SBS modified membranes are already established products having wider acceptance with govt. agencies, consultants and engineering fraternities by displacing tarfelts / chemical coatings. In the recent past, lots have been written about the polymer modified bituminous membrane & of late these products have become a commodity. In developed countries, the selection of optimum waterproofing solution depends on several factors as per owners’ criteria whereas in India, waterproofing is still in its infant state. (Table 1)
Thermoset & Thermoplastic Membranes
Single ply synthetic roofing membrane based on thermoplastic & thermoset technology are the latest addition to the waterproofing membrane family besides polymeric modified bituminous membrane. Table 1 depicts classification of various types of membranes. Thermoset membranes are those whose principle polymers are chemically cross linked. This chemical cross-linkage is commonly referred as vulcanization. Main characteristic of thermoset polymers is once they are fully cured they can be bonded to like material with an adhesive. The four common sub-categories of thermoset roof membranes are:
1. Neoprene (CR)
2. Chlorosulfonated Polyethylene (CPSE)
3. Epichlorohydrine (ECH)
4. Ethylene Propylene Diene Monomer (EPDM)
Thermoplastic membranes are single ply flexible sheet material that are divided into five general sub categories.
1. Polyvinyl Chloride (PVC)
2. Copolymer Alloy (CPA)
3. Ethylene Interpolymer (EIP)
4. Nitrile Alloys (TPA)
5. Tripolymer Alloy (TPA)
6. Chlorinated Polyethylene (CPE)
7. Thermoplastic Olefin (TPO)
Flexible PVC membrane in the thermoplastic category & EPDM in the thermoset category are becoming quite popular though Neoprene, thermoplastic Olefins are also being used for specific requirements. These single ply roofing membranes are fixed primarily by torching or by bonding with adhesives.
feature-top Table 2: Classification of Various Types of Membrane
Molecular Structure
A polymer is a long-chain molecule consisting of many (poly) small repeating units (mer) (~103 – 106) joined end to end. like thermoset membranes, thermoplastic membrane are different because there is no chemical cross linking. (Fig. 1).
feature-top Fig.1: Schematic Molecular Configurations of (a) A Thermoplastic and (b) A Thermoset
TPs comprise long-chain molecules held together by weak bonds (Figure 1a). When heat is applied, the molecules “slide past” one another and the polymer softens. On cooling, the molecules cannot slide past each other easily and the polymer hardens. TS long-chain molecules, however, are linked together by small molecules via strong chemical bonds, a process sometimes referred to as vulcanization (Figure 1b). This three-dimensional network is so rigid that the molecules cannot move very much even when the polymer is heated. Thus, TSs do not soften when heated. Because of these differences, TP and TS membranes are bonded differently when applied. TPs can be bonded using heat welding: the hot air melts the polymer at the seam and the two strips of membrane become fused. TS membranes are usually bonded using adhesives or tapes. The curing of cross-linking process transforms the resin into a rubberized mass linking into a rigid 3D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting into a very high melting point. During the reaction, a stage is reached when the molecular weight has increased to a point so that the melting point is higher than the ambient temperature, the material turns into solid. So, thermoset material cannot be melted or reshaped after its curing. Due to this 3D structure, thermoset materials are stronger than thermoplastics and better suited to high temperature application. An attempt has been made to investigate some of the properties of these products mainly EPDM & TP based membranes such as PVC & TPO.
Thermosets Membranes (TS)
Thermosets include the commonly used ethylene propylene diene monomer [also known as ethylene propylene diene terpolymer] (EPDM). EPDM is an inert polymer made by copolymerizing Ethylene, Propylene & diene terpolymer with small amount of carbon black, oil & curing agents. It is class M rubber having a diene and saturated hydrocarbon chain. This membrane is provided in one single sheet so there is no on site joining or welding. The sheets are bonded by cold applied butyl or polyurethane prepolymer bonding adhesives. These rubber sheets are available in three thicknesses i.e. 1mm, 1.2mm & 1.5mm. EPDM sheeting found its use as a waterproofing membrane in early 80’s & the market expanded dramatically & current market estimation is around 200 million sq.m. EPDM membranes can be attributed to its unique combination of following benefits:
- Withstand extremes of temperature. The elasticity of EPDM membrane remains unchanged over a considerable period. The membrane remains unchanged & flexible at a temperature between -40° to +150°C.
- UV resistant. EPDM rubber membrane have excellent resistance to degradation due to UV radiation.
- Lightweight & Easy Installation. Installation of EPDM membrane is user friendly applied cold. Being lightweight, dead load on the structure is negligible.
- Resistant to root penetration. In a recent study by the German FLL, EPDM membranes are completely resistant to root penetration.
- High Flexibility and elongation. An EPDM membrane is highly flexible even at a temperature below - 45°C with elongation of more than 400% & takes care of movements in the structure.
- Low life cycle cost. EPDM membranes are resistant to aging & have a life expectancy of more than fifty years. - Environmentally Free. Since EPDM is made up of few inert materials, it has a very limited scope of affecting the environment.
Thermoplastic Membranes (TP)
Thermoplastics soften when heated (this process is reversible) but thermosets do not. while thermoplastics encompass a wider variety of roofing membranes, including thermoplastic polyolefins (TPOs) & Poly Vinyl Chloride (PVC). All thermoplastic roofing membranes share certain characteristics, e.g., seaming can be done by heat welding. Thermoplastic makes a monolithic, continuous sheet as the materials is heated where solid changes into semisolid state, overlaps are thermofused. However, for the most part, they have very different chemical, physical and mechanical properties & that is why there are different ASTM material-based standards for various TP products. TPO membranes are different from other TPs in several respects, and thus must be identified & specified accordingly.
PVC Membranes
PVC (Polyvinyl Chloride) is a ubiquitous polymer and has a greater demand in the construction industry as piping material, cable sheathing, wall covering, sloped roof & many other applications. Some of the main features of PVC membrane are given in Table 3.
Benefits of PVC Membrane
- Competitive installation cost
- Easy workmanship
- Uniform installation across the entire roof
- Easy of leak detection & its repair. Localised repairing possible
- Easy to install even at extreme conditions.
- Weather & UV Resistant
- High mechanical strength
- Dimensional stability
Thermoplastic Polyolefin Roofing Membranes
In late 80’s, TPO technology was first introduced to the roofing industry as an alternative to other single ply membrane such as EPDM and PVC. A typical roofing system consists of three components: a structural deck, a thermal insulation barrier and a waterproofing membrane, which consists of reinforcing fibres or fabric sandwiched between two sheets of flexible matrix. The matrix material is either asphalt - or polymer-based (Figure 1). In “single-ply” membranes, the matrix is made of flexible polymer. The reinforcement provides dimensional stability for the membrane as well as strength to resist stresses in service. It is generally made of chopped short glass fibre. Defining “thermoplastic polyolefin” is difficult. “Thermoplastic” is a generic term in polymer science; it encompasses a class of polymers that, as mentioned above, soften when heated in a reversible process. The term “olefin” is even more generic, being an old chemical name for any molecule containing carbon-carbon double bonds (the modern name for this family of molecules is alkenes). Any polymer formed by chemically linking up many olefin molecules is termed “polyolefin.”
According to the latest draft ASTM standard for TPOs, the composition is very non-specific. The standard states that TPOs must contain more than 95% by mass of TPO polymer. The polymer itself is not defined within the standard, which states only that the sheet shall contain the “appropriate” polymers. Because of this loose definition, there is an endless list of chemicals that would fall under this standard (e.g., polyethylene, polypropylene, and isobutylene, as well as their derivatives). Ideally, manufacturers would specify the exact polymer in terms of marketing and labelling. There are a few published papers that attempt to explain the different types of TPOs. One point is clear, however: unlike plasticized thermoplastic membranes, TPOs do not contain plasticizers (small molecules added during compounding to increase the flexibility of the product). Therefore, the problem of plasticizer loss associated with some plasticized membranes is eliminated. TPO roofing membranes have been in service in Europe for more than ten years. The first appearance of a “TPO-type” roofing product in the United States was around 1987. Yet, little is known about their durability. The confusion associated with TPOs comes from both the chemical terminology and the product marketing. The latter has promoted mainly the EPDM-like characteristics of TPOs, i.e., it being like rubber with the added benefits of welded seams (which EPDMs do not have). Also promoted has been the chemical resistance attributed to the olefin component of the polymers. Unfortunately, some confusion has occurred, especially regarding the use of the term thermoplastic (TP). It is important to remember that TPOs are thermoplastic (i.e., TPs) but only some TPs are TPOs.
Advantages of TPOs
In general, TPO membranes are being marketed as a product that combines the properties of EPDM and PVC, without the associated drawbacks that the latter two materials have. In other words, they are supposed to be as UV-resistant and as heat-resistant as EPDM, and as heat-weldable as PVC. The following benefits and characteristics have been reported for TPOs:
- Environment-friendly and recyclable
- Seams can be heat welded
- Available in many colours
- Resistant to heat and UV degradation
- Resistant to many chemicals
- Good cold-temperature flexibility
- No external plasticizers added.
Performance of TPOs
TPO membranes are lighter in weight and easier to handle than the other thermoplastic membranes. But, while flexible, they have a rather rigid feel: they tend to hold their shape, and do not relax quickly. Contractors appear to be adjusting slowly to them and some say they are not “contractor-friendly.” Their comments about TPOs include the following:
- Hot-air-welded seams are easy and clean;
- Costs are lower than for other hot- air welded membranes;
- Mechanically fastened systems (as opposed to loose-laid or fully adhered systems) work well in recover applications without adding extra load;
- Non-reinforced flashing membranes are easy to form for detailing;
- Noticeable changes in colour and texture occur over time;
- Membranes respond dramatically (expansion and contraction) to temperature changes;
- Cold welds (i.e., welds at temperatures that are not hot enough) occur frequently; the start and stop positions of the robotic welder are especially critical, as are the positions of t-seams;
- Narrow welding window exists between cold welds and scorch/ burn-through (i.e., welds at temperatures that are too hot);
- Failure of substrate bonding adhesive is common (i.e., not sticking to membrane);
- Membranes sometimes require solvent wipe (to clean or prime) before welding;
- Re-welding membranes (in repair) is problematic after exposure to sun;
- Black membranes are more difficult to weld than white ones. Conventional thermoplastic waterproofing membranes also requires an expensive grid anchoring system to isolate water infiltration due to an installation defect or puncture. Off late some advanced technology has developed in case of any puncture on the membrane, its Active Polymer Core (APC) activates with the water to seal the breach thus preventing water infiltration in to the structure. Active Polymer Core Technology activates and seals water breach through the thermoplastic membrane – automatically and reliably.
Liquid Membrane
The concept of liquid roofing has existed since (at least) the early 1800s, when natural bitumen was combined with jute, straw, rag felt and other man made materials to provide a waterproofing solution for roofs. In the early twentieth century the manufacture of liquid roof coatings became a commercial activity, with the earliest coatings being based on liquefied rubber. The 1960s and 1970s saw the introduction of acrylics, acrylic emulsions, styrene butadienes and unsaturated polyesters, which led to improved quality and durability of the coatings. In 1970s, the first water based elastomeric roof coatings were introduced. In the late 1980s, single component moisture-cured polyurethane based liquid membranes were developed and remain the basis of most of today’s cold applied liquid roof coating technologies. Liquid roofing involves the application of a monolithic, fully bonded, liquid based coating to a roof. The coating cures to form a rubber-like elastomeric waterproof membrane, capable of stretching and returning to its original shape without damage. Such coating systems are usually reinforced with secondary materials such as glass-reinforced plastic to provide additional tensile strength. The coatings can be applied over most traditional roofing materials, including felt, asphalt, bitumen, and concrete.
Types Of Liquid Membrane
- Hot Applied Polymer Modified Bitumen. More generally referred to as Hot Melts, these products are based on hot applied polymer modified bitumen which is usually applied in two layers incorporating a polyester reinforcement. Due to the relative softness and tacky surface of the material a surface protection membrane is applied to the top coat. The protection membrane varies dependent upon roof usage and access required. The system can only be used as an inverted/buried roof membrane application or in roof garden/green roof design and therefore can only be used on roofs of up to 15 degrees pitch. The material is sensitive to UV light and at all upstands/details where the membrane becomes exposed; a protective membrane must be applied to prevent UV degradation.
- Polyurethane Liquid Membrane. High solid content elastomeric polyurethane liquid membrane basically available in single / two component system. These systems are aliphatic polyurethane consisting of Polyether Polyol with Isocynurate for rendering flame resisting characteristic.
Polyurethanes may be single or two component, aliphatic or aromatic and cure/dry by reaction, or moisture trigger from latent hardeners, or by moisture cure from the air – possibly some of these in combination. They are formulated to give good fire performance and are highly resistant to UV light with aliphatic based product having better colour fastness. Whilst single component polyurethanes usually contain solvents, two component systems can be formulated solvent free. Reinforcement is used either locally or overall to produce tough, long-lasting systems; although unreinforced polyurethane is a very tough material and reinforcement may be added selectively. PU liquid membranes are available as per ASTM C 836 & are laid as per ASTM C 898.
Application Methodology
Application includes polymerized mastic over the roof surface to achieve smooth surface. Once the roof has been cured & dried, application of polyurethane based primer to be carried out prior to the application of liquid membrane. Stir the single component to achieve a uniform consistency. The liquid is then brushed or sprayed on the primed surface to be waterproofed. For two coats, second coat to be applied over the first coat after 12 hours but within 48 hours as it becomes touch dry. Care shall be taken as the pot life of the liquid is short & ensure that correct amount to be mixed at one go for pouring to avoid wastage.
feature-top Fig 2: Laying of Liquid Membrane
Some of the features of PU based liquid membrane as per ASTM C 836 are given in Table 4.
Polyurea Liquid Applied Waterproofing Systems (LAWS) are chemically like polyurethanes but have key chemical differences that can result in quite different application characteristics and physical properties. Due to the typically high tensile strength and elongation of polyurea LAWS, they are usually applied in one coat. Fabric reinforcement is not usually required. Polyurea are typically very fast cure, durable, damage resistant and have good long-term resistance to UV light and infra-red radiation.
Sometimes described as Acrylics, MMA or PMMA, these products are formulated to achieve the required physical performance by using
various polymers dissolved in reactive methacrylate and acrylate monomers. These two-component, solvent-free products are generally applied by spray, although they can be formulated for roller, brush, trowel or squeegee applications. They cure by an extremely rapid in-situ polymerisation, which is activated by the addition of a peroxide catalyst. This curing proceeds rapidly even at temperatures around 0°C. All methacrylate coats chemically react with each other to produce a fully-bonded composite. The systems incorporate primers to ensure adhesion on a variety of substrates and reinforcement can be incorporated if required. The use of Bond Coats can allow them to be overlaid with asphaltic surfacing. On exposure to UV light, they display very good retention of physical properties.
Acrylic Roof Coating Systems (Solvent Based)
Acrylic Roof Coating Systems is one-component polymeric composition for application in repairing, coating leaking roofs, gutters & similar structures. They are specifically useful in an emergency repair situation in adverse weather conditions and will cure under water. Applied by roller or brush direct onto a clean, dustfree surface with any wide gaps being reinforced with glass-fibre tape. Most sound surfaces are suitable without a priming coat, but priming is required on fresh bitumen, metal or asbestos.
Polymer Modified Liquid Waterproofing Membrane
In the recent past, acrylic polymer based waterproofing membranes experienced fast development and formed several product lines, such as flexible polymer modified membranes, polymer modified cement-based coatings etc. Among these, flexible acrylic membrane is the main product. The rigid waterproofing materials are modified by adding an organic polymer, a waterproofing admixture, a water reducer, and an expansion admixture to reduce and compensate for cement shrinkage, to prevent cracking, to reduce and refine the capillary pores of the materials, and to make the concrete denser and improve the impermeability remarkably. Although these materials have good aging resistance, they cannot accommodate extensive deformation of the base because of their rigidity. If dry shrinkage or expansion and contraction due to temperature changes lead to cracking of the base layer, the waterproofing layer may also crack or delaminate, which will affect the waterproofing performance. Their application will be limited greatly, and they cannot be applied to large-area waterproofing engineering of floors and roofs.
Because of the better durability and less environmental pollution during their production and application, cement-based waterproofing materials can be widely used on different base materials and are very convenient in application. A great deal of effort has been put into the studying and comparing of these materials. By improving the formulation and the production process, a polymer modified cement material with a lower ratio of polymer/cement and water/cement evenly mixed and dispersed under the impact of temperatures and high shear force can be made. It combines the advantages of a rigid cement waterproofing material with those of a flexible waterproofing material (such as rubber and plastic). Using a calendaring process, a flexible cement-based waterproofing material with a denser structure, better waterproofing performance, excellent durability, and capable of accommodating deformation of the base can be obtained. A Few salient points are:
- Suitable for any climate - remains flexible in arctic cold; will not crack or dry out in desert heat; unaffected by ultraviolet radiation resulting from long-term sun exposure; resistant to ozone, acid rain, and most other air pollution
- Environment-friendly - water-based coatings contain no toxic dispersants or tints; ideal for roofs used for rainwater collection; safety precautions for applicators are same as for water-base house paints
- Bonds to almost anything - suitable substrates include asphalt, wood, metal, concrete, and rigid foam insulation boards
- Durable colours - architectural colours such as reds, greens, and greys match traditional metal roofs; white keeps buildings significantly cooler in the summer, saves energy, and minimizes urban heat-island effect
- Affordable - cost for materials is less than $2.00 per square foot delivered; any contractor or homeowner can obtain excellent results without prior experience; repair patches are just painted on
feature-top Fig. 3: Preferable application method for liquid membrane
Polymer Modified Bitumen Emulsions and Solutions
In these systems, bitumen is modified with suitable polymers or co-polymers to enhance durability, flexibility and elasticity to the dried coating. These products can be supplied as water based emulsions or solvent based solutions. Water based products are normally applied in appropriate weather conditions as recommended by the manufacturer. In addition to bitumen and polymer modifiers, both the emulsion and solutions may also contain inert fibres and fillers. The products are designed for application by brushing or spraying or may be suitable for spreading. The systems may also require primers, reinforcements and solar reflective or protective finishes.
- Cost-effectiveness. The process of liquid roofing provides a cost-effective method of making a new or existing roof waterproof. It can deliver up to 25 years performance depending on the coating system employed. It is estimated that liquid roofing is 70% less expensive than overall roof replacement in refurbishment situations.
- High performance. Liquid roofing can be undertaken with high performance materials. Many of the products used in the liquid roofing process have been independently tested and their performance verified. In the United Kingdom, the leading testing and approvals house the British Board of Agréement (BBA) has been issuing approvals for liquid roofing systems since 1975.
- Safety. Unlike the installation processes of pre-formed membranes, liquid roofing does not involve hot works. Hot work on roofs is an extremely high-risk process, that poses a significant fire risk to contractors as well as the building and its occupants. The coating material used in the liquid roofing process is applied cold, thus negating any fire risk.
- Encapsulation. When applied, liquid roofing systems encapsulate the surface they are applied to, preserving whatever is underneath and protecting it from weathering. This is especially desirable when dealing with asbestos roofs, as attempted removal of asbestos roofing can damage it, releasing material that can trigger asbestosis. Using a liquid coating system instead seals the asbestos, therefore making disturbance of the material more difficult.
Importance of waterproofing treatment using various innovative methods or systems are gaining momentum, till recently the treatment was treated by the architects/consultants as secondary issues. In most of the cases, this aspect of building construction lies with the client, who is ignorant and goes for the material readily available in the market that too without having basic information about the products or systems. With globalization, these new generation products are now accessible in our country & our engineering fraternity should generate some awareness among the users.
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3. Beer, Hans-Rudolph. “Longevity and Ecology of Polyolefin Roof Membranes,” Proceedings of the Fourth International Symposium on Roofing Technology, Gaithersburg, MD, 1997, pp. 14-21.
4. Das Supradip, “Polymeric Membrane – Recent Developments in Waterproofing”. Civil Engineering & Construction Review. Sept’01.
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8. Das Supradip: “Polyurethane Based Elastomeric Liquid Membrane - A New Concept As Waterproofing System” Civil Engineering & Construction Review Vol. 29, No.11 (74-76), Nov’ 2016