The first reinforced concrete (RCC) was invented by Joseph Monier, a Parisian gardener who made concrete garden pots, reinforced with iron mesh. RCC combined both the tensile strength of metal and the compressive strength of concrete to withstand heavy loads and is used more than any other man-made materials in the world today. Due to the cement hydration process by which the concrete is made, employing copious amounts of water than the chemically required quantities, the substrate, thus formed, is very porous in nature and contains numerous capillaries. Though concrete looks very solid, it is a porous material and such porous structures exposed to rains and sunlight invite erosion and moisture penetration. Further, the concrete, being very tough and rigid, tends to crack on continuous exposure to sunlight and climatic conditions. So the porosity and rigidity of concrete roofs and structures call for protection by resilient materials.
Innumerable Waterproofing Technologies
The waterproofing of concrete roofs has always been a great challenge to the engineers and coating specifiers. With the increased complexity of structural movements, the growing concern for leakages in the concrete structures has carved the needs for diverse waterproofing systems since many ages. Long before the advent of polymeric technologies used in modern-day waterproofing, old generations used naturally occurring materials to impart concrete and masonry structures watertight features. The oldest material used for roof leakages was asphalt and asphalt-modified products. It was thus humankind’s relentless endeavour to find time-tested waterproofing solutions to combat water seepages and numerous technologies were emerged since then, but many went into oblivion, obviously because of the field failures. Initially, people started using special waterproof cements in which waterproofing agents like calcium stearate and aluminium stearate were added. A few people modified the concrete with admixtures by en-hancing the compactness and reducing porosities and cracking tendencies.
The most prominent among Indian contractors, brickbat coba was developed by laying lime mortar and burnt clay brick aggregates on the roofs and spreading it to give slopes for draining the rainwater away. Once water starts entering into the brickbat coba, the brick pieces absorb excess water and the roof becomes an invisible water pond, permanently causing seepages, along with increasing the dead weight of the structure.
Bitumen or Coal tar is normally employed in the form of felt formed by sandwiching jute fabric, fiber glass or polypropylene mats. These systems are slowly being disappeared from the market, as they are thermally unstable, with their unacceptable appearance and inaccessibility of the roofs for social purposes.
Integral Water Proofing Compounds
Sodium silicate solution is sold as integral water proofing agents and normally 1 Kg of this admixture is added to 50 Kg of cement in batching. Sodium silicate re-acts with CH to form C-S-H and so the set times are reduced and more compact concrete is produced. The first generation water reducers like sodium or calcium ligno-suphonate solutions are also sold as integral water proofing compounds, which reduce the water in-take for forming concrete. These products are quite popular in the retail construction market.
One of the oldest forms of waterproofing, the crystalline additive grows into the pores of the concrete substrate and densifies the concrete, making it virtually impervious to moisture intrusion. This is used for below-grade waterproofing of water-retaining structures like un-derground water tanks, sunk slabs of bathrooms and toilets, swimming pools, basements and is called as structural waterproofing compound.
Polymer Modified Cementitious Waterproofing
Latexes like acrylic emulsion, styrene-acrylic emulsion or SBR are used to modify cement in order to improve plasticity and bonding strength. These latexes are readily available from many construction chemical companies. The standard version is a 30% solution as supply form. The end-user mixes latex with cement and sand and applies as a PMC (polymer modified concrete) solution. When SBR latex is used in place of standard acrylics, the coating would give better water resistance. In order to improve flexibility of the cured films, elastomeric acrylics as binders are employed and these are known as flexibile cementitious membranes. The major disadvantage of these cementitious products is that they are not able to unravel the intrinsic rigid nature of the concrete which cannot be stretched to any desirable degree. They may be good for above grade waterproof applications in building interiors, but will not tolerate joint or crack movements.
Preformed Sheet Membranes
These sheets are either made up of modified bituminous membranes (APP, SBS), or of thermoplastics like PVC, TPO, HDPE or of EPDM and are either torch applied or stuck with adhesives. The key advantage of sheet membranes is their consistent thickness (1.5-2mm) and good elongation, but the major disadvantage is that they contain many welded seams and thereby exists the possibility of water seepage through these seams.
Polyurethane Coatings for Waterproofing
The versatile polyurethane coatings have already proven excellent results in waterproofing segment because of their unique significant feature: toughness combined with film flexibility. This quality of resilience makes PU the better waterproofing medium than any other polymers or polymer modified cements. PU films are unaffected by water, oil or chemicals and have a high diffusion resistance against CO2. In a comparative test done by Bayer, Germany, it was found that PU had the highest CO2 diffusion resistance factor over other organic materials such as chlorinated rubber, acrylic solutions and epoxides. PU, having the most desirable properties such as lower degree of water absorption, adhesion, diffusion resistance, impermeability, flexibility, resilience, resistance to roots and micro-organisms, is rightly employed as liquid applied membrane to prevent moisture and chlorides from reaching the concrete.
Liquid applied PU Membranes,1K & 2K
These liquid applied membranes which cure to form rubber-like elastomeric waterproof membranes, capable of stret-ching and returning to its original shape without damage, have found wide acceptance in the market. There are two types of liquid applied PU membranes in the market: a single component (1K) system and a two component (2K) system. In 1K system, a pre-polymer is produced when one or more polyols is reacted with stoichiometric excess of isocyanate. This moisture curing urethane adduct is then mixed with fillers, antioxidants, adhesion promoters, and catalysts and when this is applied, the MCU reacts with ambient moisture to form a flexible, polyurea coating. In 2K system, the polyol and additives are blended together to make up one of the two components (Resin) and the pre-polymer (Hardener) makes up the second component. These two components are mixed in a certain volumetric ratio on site to impart an elastomeric film. A 2K system typically allows for faster reactions and better control over the curing conditions, where as the curing of 1K system depends purely on relative humidity (RH >60%).
Both 1K and 2K liquid applied membranes perform very well with excellent physical and mechanical properties, having a tensile strength of 4-8 MPa and an elongation of 300-600%. The inherent toughness and resilience enables them to bridge structural or shrinkage cracks that may develop in the concrete. They are almost 100% solids by volume (1K: 80-90 % solids, 2K: 100% solids) and able to build very thick films. These membranes are applied by conventional methods like brush, roller or single component airless spray, with single coat yielding around 450-500 microns per coat with a re-coat window of 4 hours (For 1K, the cure depends on relative humidity.) They have excellent crack bridging abilities to tackle the structural movements in the concrete and the puncture resistance to build landscaping, gardens, ballast or pavers over them, without undermining the integrity of the waterproofed layer.
They offer complete water-tightness with long life expectancy and minimal maintenance and are applied without the use of bitumen, open flames or solvents and do not contain seams or stiches. It may be noted that no bitumen should be used as root resistant layer as the roots could naturally at-tempt to penetrate the surface while seeking nutrients. These liquid applied membranes are sometimes reinforced with woven or non-woven fabric such as glass-reinforced plastic to provide enhanced tensile strength and are often protected from the traffic by cement screed overlays.
Plural Spray applied PU Membranes
Liquid applied membranes described above, have certain limitations in film thickness as single applied coats yield only 450-500 microns and hence multiple coats are needed to have a film thickness that matches with preformed membranes, which led to the development of rapid setting liquid PU membranes, applied by plural component airless spray to achieve higher film thickness of 1.5-2mm at one shot. These membranes on spraying through plural component airless equipment like Graco or WIWA, cure instantly and achieve trafficable condition within an hour. Protecting the substrate from bulk water and root penetration, these membranes create a monolithic barrier across the roofs, sidewalls, and sloping surfaces. The pot life, once mixed of resin and hardener (normally in the ratio of 1:1 or 3:1), lasts for a few seconds only and the film sets rapidly, imparting excellent mechanical properties like tensile strength ( 8-10 MPa) and elongation of 250-350%.
Plural Spray Applied Polyurea Membranes
The top most line in these rapid setting liquid applied membranes is Polyurea with its specific advantages. Even at very low temperatures (as low as minus 200C) the polyurea coating will cure without producing any coating defects associated with polyurethane coatings such as bubbles, foams and moisture-caused blisters. Polyurea coatings have distinct advantages over PU membranes such as fast cure, moisture insensitivity, hydrolysis resistance, higher tear strength, higher tensile strength, higher abrasion resistance, auto catalytic nature and rapid return to service. The most obvious advantage of polyurea is its excellent tear strength, because of which the cured film cannot be punctured or torn under any sort of pressure.These membranes are applied by Graco or WIWA plural component machines, as the pot-life is as short as 10 seconds.
Cool green roofs with plural spray app-lied coatings
U.S. Green Building Council (USGBC) is the eminent green building rating system, which awards LEED credits to construction projects for implementing environmentally conscious practices. A cool roof system is the one that reflects solar radiation and emits thermal radiation as well which keeps the building cooler and helps reducing air-conditioning costs. Both the plural spray applied PU and polyurea membranes are predominantly employed for cool green roofs which can earn LEED (Leadership in Energy & Environment Design) points. And for these high performing cool roofing systems, we need to employ a mixed bag of products to assure functional harmony and maximize performance and the most important among them is the selection of the right liquid applied membrane which serves as the backbone of the cool green roofs. The roofs and podiums meant for landscaping, fountains, gardening and tree plantations need to be adequately protected by spray applied polyurethanes or polyureas, which should meet the following criteria:
– Excellent barrier to water ingress and continuous exposure to water
– Resistant to root penetration even for intensive roots
– Should enable for sand filling directly on the membrane without protection of any screed
– Resistant to pesticides
– Resistant to dynamic and impact loads
– Very high impact and puncture resistance
– Able to sustain mechanical abuse during construction
– Zero VOC, environmental-friendly, to meet the norms of green products for waterproofing.
Cool roof waterproofing system consists of a moisture compatible primer that will have tenacious bond strength with the concrete, followed by spraying of PU or polyurea membrane by plural component spray to a thickness of 1.5-2mm. The thermal insulation for the roofs is achieved by spraying rigid, insitu, PU foam having a density of 45-50 Kg/m3. The PU foam thickness varies from 50 to 100 mm. Now a new PU foam, called as Vydro foam, has been developed for green roofs by Huntsman, which is 60% lighter than the present foams and can take up to 30 times its weight in water. This foam swells ±15% after 24 hours of soaking and has the ability to hydrate and rehydrate.
The final component of this green cool roof system is a solar reflective PU alipha-tic coating applied over the foam layer. This solar reflective top coating is a specially formulated aliphatic polyurethane white coating which on curing imparts a flexible, UV resistant film having excellent waterproofing characteristics. It contains glass microspheres in order to achieve remarkable energy efficiency through superior insulating properties which works by refracting, reflecting and dissipating radiant heat thus reducing heat build-up and heat transfer through building walls, ceilings and roofs. The comparison of Cipy Cool roof systems versus conventional SRI roof tiles is given in Table1.
An effective and durable waterproofing system is absolutely critical to ensure the long-term durability of the concrete slabs and to keep maintenance costs low by avoiding costly repairs. At Cipy, we have a complete portfolio of water proofing products to choose from a myriad of technologies such as cementitious waterproofing products (Hydrokrete, Hydrokrete SBR, Flexikrete), liquid applied 1K PU membrane (Cipthane 100), 2K PU membrane (Terraflex 100), Spray applied PU membrane (Durathane 100 WP) and spray applied polyurea membrane (Securea SC). The high- end waterproofing systems like plural spray applied polyurethane and polyurea membranes, being tough, flexible and puncture resistant, don’t allow the intensive roots of the plants to pierce through and ensure that they keep performing for a very long period of time, even beyond the warranty period to provide maximum value and comfort to the end-user.
For further details:
Cipy Polyurethanes Pvt Ltd.
T-127, MIDC, Bhosari, Pune-411026