Waterproofing : An Overview
Gopal Krishna & P.T.Thomas, Chembond Chemicals Ltd, Navi Mumbai.Concrete, a composite, construction material made with cement, aggregate, water and admixture, comprise the largest of all man made construction material of our time. Its plasticity, workability, ease to place, to cast and compact while wet and strong and durable when hardened, make it one of the unique material. Generally strength of concrete is considered as the most important criteria among the properties of concrete.
But when taking the account of durability of the structure, resistance to water permeability is also considered as an essential requirement of the structure for which it is designed to withstand the environmental condition for over period of time without any deterioration. Being a water based product, and due to its composition of cement-aggregate-water is often susceptible to damage and deterioration from water and chemical penetration. Although it appears to be solid material, it is porous and permeable. Normally to make concrete workable and easy to place and consolidate more water than necessary is added. This extra water will bleed out of concrete leaving behind pores and capillary tracts. Another drawback is shrinkage cracks, shrinkage is a common phenomenon generally encountered in cement based products due to contraction of total mass upon loss of moisture.
Although concrete is designed to be durable, normally it is observed that inadequate mix proportion, use of substandard material, improper compaction and placing makes the structure vulnerable to ingress of water chemicals resulting reinforcement corrosion and further deterioration of structure.
To overcome all these, to a very good extent, and make concrete reasonably impermeable, precautionary steps like, good mix design, usage of standard materials, proper supervising while placing and compacting, giving enough coverage to reinforcement, proper curing, use of flyash and other pozzolona blended cements, using permeability reducing admixtures etc are to be considered seriously. Permeability retards the durability and reduces the life span of the structure, waterproofing or damp–proofing is carried out to prevent or to seal unwanted water containing deleterious salts and chemicals to enter in the structure, resulting reinforcement corrosion and other destructive activities. The activity of waterproofing of the structure/building, which is practiced in one form or other ever since the construction started in our history. The methodology has gone through various changes, by design of the structure, availability & application of different construction materials.
There are lots of conventional and unconventional methods practiced for waterproofing in construction field. Old methods like brick bat–cobba, cement–lime based treatments, bituminous coatings are still practiced successfully. But the development of modern construction material and technology, the concept of waterproofing has changed tremendously. Nowadays integral waterproofing compounds are admixed into the plastic concrete.
These materials impart water repelling (damp-proofing) to concrete, may reduce moisture migration through capillary reaction. Surface coating application is another known method generally followed. But most of these are found to be ineffective in reducing the water passage under a positive hydrostatic pressure. Treating the concrete to retard the absorption of water or water vapor by concrete or to retard their transmission through concrete is considered as Damp-proofing.
Treatment of a structure or asurface to prevent the passage of liquid water under hydrostatic pressure is called is Waterproofing. This positive prevention of the ingress and movements of water under hydrostatic pressure distinguished waterproofing from damp-proofing. Further there are positive and negative side waterproofing. Positive side waterproofing is applied on the same side as the applied hydrostatic pressure. Negative side waterproofing is applied on the side opposite to that applied hydrostatic pressure. Due to the unavailability of access to the positive side, negative side waterproofing are also practiced.
The most common waterproofing methods:
- Brickbat coba method: This system involved in laying clay bricks with light weight lime mortar on the roof and spreading it for easy draining away of rain water. This system is popular not because of the waterproofing, but the weather proofing capabilities.
- But it adds weight to the structure and once water starts entering, the porous clay brick pieces absorbs large quantity of water, resulting continuous leakage of roof.
- Cement/lime based treatments: Coating the surface with cement lime mortar is a time proven and economical method with good insulation properties. But it is non-flexible and also increases the load of the structure. 3 Mineral slurry with polymer component is an easy method to apply. It retains the breathing capacity of concrete but with moderate flexibility. 4 Epoxy & polyurethane coating is highly abrasion resistant and resistant to UV radiation and does not add weight to the structure. But this has limited pot–life, not very flexible and stops breathing capacity of concrete.
- Elastomeric membrane forming products: It forms seamless membrane, highly flexible, UV resistant, retains breathing capacity of concrete with indirect insulation, but with low abrasion value.
- Silicon based impregnators as water repellent, easy to apply and economical, but it has no crack bridging capacity, and does not withstand pressure.
- Bituminous based products and modified bitumen are very economical, flexible, with good crack bridging capacity. But it softens under heat and brittle when cold, limited life upon solvent evaporation and other limitation due to its unpleasant black colour.
In modern construction technology developments a single product or technique is not usually enough, involvement of various bodies and techniques in co–ordination is essential for making the structure waterproof. Structure should have sufficient and efficient various control joints like expansion joints, contraction joints, etc. if proper control joints are not provided in large slabs, no waterproofing system will be successful.
Among the various methods mentioned above, two are proven successful and effective, they are: Crystalline waterproofing system Flexible membrane waterproofing system.
Crystalline waterproofing provides a quick, cost saving alternate to the traditional flexible membrane waterproofing system. In some cases, external waterproofing application requires enough space to make application outside of a structure. This can cause problems as there is not enough space for membrane application because of adjacent structures. Crystalline waterproofing can solve this problem to some extent; by negative side application as the unavailability of access to the positive side. Crystalline waterproofing when applied to a surface either as a coating or dry shake application to a freshly placed concrete slab a process called chemical diffusion takes place.
Crystalline waterproofing compound reacts with various chemicals and moisture in the concrete to form insoluble crystals which seal the capillaries and shrinkage cracks. Its action as by filling and plugging pores, capillaries, micro-cracks and other voids with a non-soluble/insoluble highly resistant crystalline formation makes waterproofing more effective. As the crystalline waterproofing chemicals continue to migrate through water, a crystalline structure is formed. This reaction will continue until the crystalline chemicals are either depleted or run out of water. Some manufacturers are claiming that this chemical diffusion takes place about 12 inches into the concrete.
If water has soaked only 2 inches and stop but, they have the potential to travel 10 inches further, if water re-enter the concrete at some point in future and reactivate the chemicals. Thus the crystalline formation engages the material filling and plug the voids in the concrete to became an integral and permenant part of the structure. Because this crystalline formation are within the concrete and are not exposed at the surface, they cannot be punctured or damaged like membrane or surface coatings.
Crystalline waterproofing system can be executed in three different ways as per the requirement and the situation demands. Surface coating is most common, other are dry shake powder application and as an admixture added at the time of batching. In dry shake powder application for horizontal set concrete and structural slabs, the Crystalline waterproofing compound is spread across uniformily to fresh concrete after initial set and power trowelled. Unset concrete matrix contains an abundance of moisture for optimal penetration of crystalline waterproofing compounds. As an admixture, when added at the time of batching, crystalline waterproofing compounds reacts with moisture in fresh concrete and the bi-products of cement hydration to cause catalytic reaction, which generates a nonsoluble crystalline formation through the pores and capillary tracts of the concrete.
Advantages
This integral CWP system has so many advantages compared to other waterproofing systems. When used as a negative side waterproofing application, which is as effective as positive side waterproofing, allow to expedite the construction schedules by back filling sooner. It is highly resistant to abrasion, wear and tear, no risk of tearing or puncturing. Cost of material and labor are lower. As there is no problem like insufficient seam coverage, poor surface preparation and inadequate adhesion as there in the membrane system. The integral crystalline waterproofing system become a part of concrete matrix, are permanent can self seal hair line cracks, resistant to hydrostatic pressure, allows concrete to breath, protect for a life time.Waterproofing by Surface Applied Membranes
Surface applied membrane type waterproofing system currently accepted worldwide. The recent progress in polymer technology with the development of polymer modified bitumen like ATACTIC POLYPROPYLENE (APP) modified bitumen improves the physical property of bitumen. This modified bitumen coating has overcome to a very good extent, to remove the drawback of conventional bitumen membrane of its undesirable temp; related variations like to become brittle at freezing temps and soft at high temps.Since waterproofing connot be carried out using a single material, i.e. suitable for one structure, may not apt for another. The use of various materials in combination and methodologies are vital for effective waterproofing to meet the specified design and durability requirements has significantly revolutionized waterproofing as waterproofing system. This system includes combination of materials, application techniques, specified requirements like providing efficient drainage system, using various construction joints, water stops etc. To demonstrate waterproofing as system one of the very recent example is construction of Delhi metro, underground structure. Here the various material in combination to meet or to conform, international & Indian standards, techniques, test methods for performance guarantee etc made this particular project a typical example to describe waterproofing as a system.
Application of APP Membrane for Base Slab
Base slabs are constructed over leveling concrete which is applied with waterproofing membrane. External walls and roofs are covered with waterproofing membrane, which is bonded with parent concrete, and these membranes are further protected with concrete blocks protection layers coverage for any possible tearing or puncturing from external sources. Besides there are provided construction joints and water-stops and other drainage systems as per the design requirements.
Following is a brief description of the materials used application techniques executed on the waterproofing treatment of under ground structures like base slab; walls and roof of all under ground structures built with cut and cover method of Delhi metro projects. Here all the cut and cover underground structures are provided with a fully tanked waterproofing membrane fully bonded with external faces of structure.
Vertical Application of APP Membrane
On sufficiently hardened, surface leaned, blinding concrete a bituminous primer is applied. A waterproofing membrane–4mm thick homogeneous thermoplasticblend of Atatic polypropylene (APP) distilled between with a reinforcement of 160 gm per square meter of non-woven polyester, top surface is covered with mineral protection with silica and the bottom surface is covered with a flamable polyethylene film, is then thermo fused by flame torching of inside face of membrane, which is embossed and protected by a seven micron thick polyethylene film. On this membrane applied surface a high abrasion resistant polymeric mortar is laid up to 3 mm thickness. At least three days curing is provided to this mortar layer before laying reinforcement.
The recent economic boom in India generated the upcoming of lot of mega projects, like, metro rail projects, express high ways, multi-storied building complexes with large capacity underground facility for vehicle parking and other utilities. Unavailability of surface land at crowded cities, shortening of distance of rail and roads, maximal utilization of available space etc are forced construction agencies to consider seriously for underground projects.
Metro rail projects, tunnels through mountains and under the bed of water, and deep basements are typical examples. For all these projects waterproofing has a vital role for the designed durability especially surface applied membrane system to prevent passage of water under hydrostatic pressure to form a continuous impermeable membrane barrier to prevent leakage into usable space or to prevent loss of water from a retaining structure. So waterproofing nowadays revolutionized as a major preventive measure than a cosmetic treatment.
NBM&CW April 2008
