Daily Archives: Wed, Feb 8th, 2012
There are some compounds, which are used as additive in plastic concrete to make it less permeable to water. These compounds are known as integral waterproofing compounds. They are based on plasticising and air-entrainment or water repellence principles. These are used as a good waterproofing precautions when other factors such as good mix-design, proper mixing/placing, compacting/curing etc are taken care of. This subject of using integral waterproofing compounds requires an in-depth discussion hence will not be taken up here. Similarly, there are many waterproofing techniques for vertical surfaces. These techniques used are also used for preserving heritage buildings by stopping/minimising the aging process of these buildings. This subject also will not be discussed here.
First of all let us review some of the old and conventional waterproofing systems and then we would discuss the modern waterproofing techniques.
Review of Conventional Waterproofing Systems
Some of the conventional water- proofing systems are as follows:
- Brick Bat Coba system or Lime Terracing
- Bituminous Treatment
- Metallic Sheet Wrapping
- Poly urethane based Water- proofing treatment
- Epoxy Based Waterproofing Treatment
- Box Type Waterproofing System
This system was developed during the initial stages of flat roof construction with lime mortar & burnt clay brick aggregate. This system involved laying of lightweight mortar on the roof and spreading it to give gentle slopes for draining away the rainwater immediately. The mortar consisted of lightweight brick pieces as aggregates and ground brick with lime as binding matrix. During British rule, this system became more popular not because of its waterproofing efficiency but because of its efficiency in keeping the interiors cool. Some applicators developed better skills in laying these systems with neatly finished top with lines engraved on top of plastic mortar now known as IPS. Some practiced embedding broken tile or ceramic pieces in the plastic mortar and called it China mosaic. This type of system remained most popular with multi-storeyed construction in all major cities. The system lasts approximately up to 15 years if done by skillful applicators. This system may be considered more from its weatherproof abilities rather than its waterproofing qualities. Once water starts entering into the brickbat coba the brick pieces absorb too much of water and the roof becomes an invisible water pond continuously causing leakage and increasing burden on the roof slab. It will be highly beneficial if brick-bat coba, is laid on a flexible waterproofing membrane as water proofing as well as economical weather proof can be achieved with this system.
Discovery of petroleum and its products and by-products has given the construction industry an indispensable product in the form of bitumen. Bitumen is more commonly used in the form of felt or flexible membrane formed by sandwiching jute fabric or fibreglass/polypropylene mats with chemically modified bitumen. These membranes are laid on the roofing system over a bitumen primer. There are two types of membranes one is cold applied and the other is hot applied which means one needs to heat the edges of the felt with a torch so that they melt and stick to the second layer in the overlap area. On the RCC flat roofs, the bitumen felts have not been successful because of the unacceptable black appearance and inaccessibility of the terrace for other social uses. Technically, it is not preferred because bitumen layer or felt on the terrace not only makes it watertight but also airtight. Concrete has the breathing property. It takes water/moisture and breathes out water vapour. Hindrance of this breathing property of concrete develops pore pressure which causes blisters in the felt. After a few seasons, the blisters multiply and eventually delaminate the felt from the concrete surface. Hindrance of breathing property of concrete, makes the concrete weak. But on the asbestos cement sheets and zinc sheets in factory roofs, this bitumen felt is the only dependable waterproofing system. Hence most of the factory roofs in India adopt this waterproofing system.
Bitumen is very effective in waterproofing of basements from outside. Bitumen primers have very successfully been used as damp-proof course in earlier days. This practice is slowly discontinued for whatever reasons now very few engineers believe that this was in practice once. As consequence of this absent DPC, we have a lot of cases of rising dampness, which we tend to attribute to wrong reasons such as the quality or salinity of sand etc. Bitumen is still the product of first choice where it is commonly recommended, in areas such as industrial roof waterproofing, basement water- proofing, and damp-proof course. Moreover, bitumen is the most economical product presently available for waterproofing. But black top over roof can absorb heat and this need to be kept in mind while deciding waterproofing system for exposed roofs.
Because of the non-existence of suitable expansion joint filling compounds before the discovery of poly-sulphides, a complex procedure used to be adopted to treat expansion joints in concrete dams and such huge structures utilising thick copper sheets. An extension of this practice was to try thin foils of copper and aluminium for wrapping the concrete surfaces with nagging leakage problems. Unavailability of common joining material for these metal foils and the concrete and mortar created weakness in the system at the joints. This discouraged the system in its infancy only. But thereafter, the metal manufacturers have been trying to market this type of waterproofing system with improved adhesives and when the metal market slumped.
Polyurethane consists of two liquid components one is called the Base component and the other is called reactor or curing agent. Base is a polyols and the reactor is an isocyanide such as TDI or MDI. There are various grades of polyols and numerous isocyanides. The combination of these two ingredients results in a formation liquid applied rigid membrane or a foam depending upon the selection.
In waterproofing, this rigid liquid membrane was tried with fibreglass reinforcing mats. The systems failed because coefficients of thermal expansion of concrete and rigid PU membrane being different lateral movement or creep occurred with the passage on one working climatic cycle. When exposed to ultra violet rays or direct sunlight, most polyurethane rigid membranes became brittle and crumbled. Apart from this, the application of polyurethane coating needed very rigorous surface preparation. The surface needed to be neutralized by removing alkalinity from the concrete surface through acid itching then washing and blowtorching to make the surface bone dry. This kind of surface preparation with acids angered, the civil engineering community and the product ceased to be used as waterproofing material apart from its several failures. Never the less continuous research in the polyurethane technology gave the construction industry excellent sealant for glazing industry and foams for thermal insulations. The new generation polyurethanes which are alkali stable and water-based, may find better applications in waterproofing industry.
Like polyurethane expoxy is also a two-component system having a base resin and a reactor or curing agent. Base resin is obtained by dissolving Bis-phenol ‘A’ flakes in epichlorohydrin. This base is available in various viscosity ranges to suit different application conditions. The curing agent is an amine/polyamine aliphatic or aromatic or an amine-adduct for general applications and polyamide or an amino-amide for coating purposes. After mixing base and reactor components, the resultant viscous liquid or paste if some fillers are added to it can be brush applied like a paint or trowel applied like a mortar.
Not withstanding, the alkalinity of concrete and the concrete needs to be acid washed and neutralised which the civil engineers donot agree. Here again the coefficient of thermal expansion of concrete and epoxy being different the compatibility of epoxy in waterproofing exposed concrete surfaces such as roofs became limited. Later the use of epoxy in waterproofing was discarded. But epoxies have come to stay in civil engineering industry as bonding agents, floor & wall coatings, coatings for food processing units, operation theatres and computer and pharmaceutical industries.
This type of waterproofing system is used only for basement waterproofing or structures below the ground level from outside to prevent leakages of subsoil water into the basement.
In this method, limestone slabs (Shahabad Stones) are first laid in the excavated pit over blinding concrete in a staggered joint fashion to avoid the continuity of the mortar joints. The joints are effectively filled with rich cement, sand mortar admixed with integral waterproofing compound and cured. Over this, the raft is laid and shear/brick walls constructed. The limestone slabs are erected around the walls in a similar fashion leaving a gap of one to two inches between the external surface of the wall and the inner face of the stone surface. The joints again effectively sealed with rich admixed mortar and the same mortar is filled in the gap between the wall and the stones. This stonework is continued up to ground level. In this system, the raft and the sidewalls are protected from direct exposure to sub soil water.
This system works on two principles of common sense. First the area exposed to subsoil water is only the area of the joint where as the whole stone is impervious to water, hence only a fraction of area, that is, that of the joint is exposed to subsoil water when the joint itself is filled with rich and quality mortar. Secondly, the path of water to reach the raft or the sidewall is elongated. This elongated path is through quality mortar. This system seeks to delay the occurrence of leakages in the basements. A lot of building structures are waterproofed using this system. A few notable successes are to its credit especially in five star hotels and of-course there are a few failures as well.
Modern Techniques in Waterproofing
Modern technique aims to understand the functional behaviour of the structure, understand the properties of the available materials to arrive at a system, which is best suited for the structure and incorporate the system at the design stage itself.
A single product or technique is not usually enough, involvement of various bodies and techniques in coordination is essential for making structures waterproof. For success of any system, the building structure should have sufficient and efficient control joints if the slabs dimensions are more than twenty meter in any direction. Control joints are structural engineering design features. These joints are supposed to be designed and their configuration marked on the drawings and their detailing needs to be given with respect to waterproofing system proposed in large buildings. Different types of control joints are Construction joints or day joint, crack inducer joint, contraction joint and expansion joint. Coefficient of thermal expansion of concrete is between 6 to 10 X 10-6 mm/mm/oC. If proper control joints are not provided in large slabs, no waterproofing system will work. Modern technique relies basically on two main waterproofing systems, which are fool proof and simple.
They are as follows:
- Crystalline waterproofing system
- Flexible membrane waterproofing system
This system involves blocking the water bearing capillaries with insoluble crystals. This method is used for waterproofing of water-retaining structures like overhead/underground water tanks, sunk slabs of bathrooms and toilets, swimming pools, basements, terrace gardens etc. The main product in this system is a grey or brown looking powder.
The method of treatment involves saturating with water the surface to be treated. Then mixing two and half volume of the powder with one volume of water to form neat consistency hot slurry. This slurry is brush applied on the saturated surface. The active ingredients in the slurry pass through the water bearing capillaries and react with calcium oxide present in the concrete to form insoluble crystals, which effectively block the capillaries. Further the chemical ingredients of the product remain in concrete to reactivate the process of crystallisation and when a new capillary is developed. This system of waterproofing, the water retaining structures is practiced in most of the developed countries. Typical specification for this type of system is as follows:
- Clean the surface to remove all loose particles, grease shuttering oil and such deleterious materials to get a sound concrete surface.
- Cracks if any should be cut into ‘V’ shaped grooves one inch wide and proper size width & depth Cut honey combed areas back to sound concrete.
- Repair these areas first by priming with crystalline waterproofing slurry and then by filling them with crystalline modified mortar produced by mixing crystalline waterproofing powder OPC and zone II sand in the proportion of 1:1:4 and water enough to produce a stiff consistence paste. Allow the repair mortar to dry.
- Thoroughly sprinkle water on the area to be treated till the surface becomes saturated. Some recommend overnight ponding for effective results
- Prepare crystalline waterproofing slurry by mixing the powder and water in the proportion of two and half volume of powder to one volume of water to form a hot slurry. Brush apply this slurry on the water saturated surface in two coats.
- When the second coat is still wet plaster the surface with 1 : 4 cement sand mortar admixed with an integral waterproofing compound and cure properly.
The main product used in this system, comprises two components one liquid and the other is a powder packed roughly in the proportion of 1:4 by weight. The liquid component is an acrylic emulsion and the powder component is a polymer-modified cement with film forming chemicals and fillers, and some manufacturers add reinforcing fibres to make the product extra effective.
When these two ingredients namely the pre-weighed powder and the liquid are mixed together an uniform slurry results. This slurry is brush applied on the roof surface, which upon drying forms a flexible film. Since there is cement in this product, its compatibility with concrete is excellent. The film further allows the breathing of the concrete without any hindrance hence there is no problem of its de-lamination from the concrete surface. This film being flexible takes care of the deflections in the slab and the movements caused because of primary or secondary settlements, movements due to wind loads and temperature stresses developed in the concrete. This system can be used on surfaces which had bituminous treatment earlier. In European countries, this type of product is used as coating on pre-stressed concrete bridge girders to protect them from aggressive chemical atmosphere. A typical waterproofing specification using flexible membrane system will be as follows:
- Clean the surface to remove all loose particles, grease shuttering oil and such deleterious materials to get a sound concrete surface.
- Cracks if any should be cut into ‘V’ shaped grooves of a proper size in depth. Cut honey combed areas back to sound concrete.
- Repair these areas with a stiff paste of fibre reinforced polymer repair mortar generally sold in the name of crack fill.
- Mix the powder and the liquid components of the product to form an uniform slurry and brush apply this slurry on the surface to be waterproofed.
- When the waterproofing coating is still tacky apply 1 : 4 cement sand screed and cure normally.
Ferrocement Waterproofing System
Ferrocement is a cement based high strength Semi Flexible Composite used for construction of water retaining and resisting structures, boats, roofs, pipes etc. In SERC Roorkee the ferrocement group headed by Er. P.C. Sharma developed ferrocement lining technology for waterproofing and jacketing of RCC (for strengthening) & masonary structures. The structures like Calico mills Ahmedabad, Khatima Power Station, BHEL Turbine Testing Lab Caliberation Tank at Haridwar done more than 30 years ago have behaved well. The system is simple but need skills and proper training. NBM&CW has published many articles on this topic providing total know how.
Ferrocement has great bond with cement concrete and brick surface and is reinforced with at least two woven mesh layers. Thickness of this treatment is generally between 12 to 20 mm. Large number of basements, overhead tanks, underground reservoirs and important roofs have been treated using this technique. When life cycle cost is considered it is the cheapest waterproofing system today which add strength to the original structure in addition to waterproofing.
- Especially in large buildings, waterproofing systems fail if the control joints are not in position or not properly executed and maintained. This failure occurs even after one complete year when the structure has passed through one complete summer and the winter cycle. One can’t repair this type of failures. It needs a different type of approach involving provision of crack inducer joints and then tackling the waterproofing in movement restricted bays.
- Application of a waterproofing product in non-specified areas, such as using a crystalline waterproofing system in waterproofing the terrace or using a flexible membrane system in the water tank results in failures even though the products themselves may be genuine.
- Stretching the coverage of the products beyond the specified limits by the manufacturer results in inefficiency of the product and hence a failure.
Precautions to be Taken by the Client and Consultant.
- Selection of an effective waterproofing system for a structure taking all the performance criteria of the structure into account and also taking into the consideration, the performance of the materials in the system is of prime importance for any success of the waterproofing job.
- Waterproofing jobs awarded to waterproofing contractors purely on economical price considerations often fail.
- It is always advisable to involve the architect or the structural engineer in selection of a system and requisite performance guarantees should be taken from the contractors.
- It is better to avoid bargaining for the job value out of contractor’s margins. This could result in stretching the product beyond the coverage specified by the manufacturer or substitution of cheaper material in the system to cover the cost.
- Always supervise the job and the incoming materials for the intact tamperproof seals and quantities.
- Use the services of an engineer or architect in selection of materials. That too only procured from well know manufacturing firm.
- Generally, architects in India, do not specify in depth waterproofing details and leave this aspect generally to ignorant client to chose whatever system he likes, and many structural designers give least importance to the control joints. As a result of this, many systems have failed in several prestigious buildings and the blame went to waterproofing contractors or the product manufacturer.
- There is a need for the architects and the structural designers to understand the various systems available and specify them clearly and in sufficient detail, taking into consideration the in-service conditions of the structure.
- The client should also insist on the architect to provide waterproofing design details in advance so that no ambiguity remains till the end. This would give enough time in selecting the specified material.