“No material is itself durable or non-durable, it is the interaction of the material with its in-service environment that determines its durability” - says a well-known concrete technologist Larry Masters. Thus, with reference to concrete, durability will depend on the nature of the concrete (and that means impermeability) and the aggressiveness of the in-service environment. The three important steps that ensure adequate durability of concrete in-service are:
- Compliance with current standards of good practice during construction.
- The use of new and improved materials and innovative construc- tion systems designed for increased durability at competitive costs.
- Provision of protection to existing undamaged structure against adverse environments.
- Construction practices,
- Material characteristics, and
- Exposure conditions.
The defects and the distress in the structures are manifested in several ways but generally observed manifestations are the dampness and leakages. Needless to say, though there are several independent reasons for leakages in the structures, the well thought-out methods to waterproof a structure can give long lasting rewards. Thus, it is a well identified aspect of durability to offer good waterproofing to a structure.
In this paper, an attempt has been made to bring out various near perfect methods of waterproofing based on the latest developments in the world of material science.
Waterproofing of the Structures
Modern day structures are indeed more complicated than the earlier ones, which were constructed, say 20-25 years ago. Earlier, a mere conventional waterproofing treatment of terrace, toilet, bath, sinks and an old basement needed the waterproofing job. However, these days the structures are high-rise, they have large podium, car-parking lots, basements, terrace garden at each duplex flat level, corporate office gardens, flowerbeds, swimming pools at various levels, narrow ducts (chowks) for plumbing lines and electrical lines besides conventional problems of regular large terraces. Due to unusually large heights of the structures, ensuring the quality plastering on the external surface also poses a problem, which adds to the difficulties in waterproofing. If the problems of such large structures are left unattended and uncared for at the execution stage, then the rectification becomes near impossible job adding astronomically to the cost of maintenance. Besides in several large structures like hotels or large commercial premises proper sunks are not provided in toilet and bath blocks. This creates problems while adjusting the arrangement of plumbing and other toilet lines. Sometimes even slabs are to be perforated both as an afterthought or the part of planning which can become a potential source of leakage if not handled with proper understanding of materials.
In above cited cases each waterproofing problem becomes a unique case study and needs to be addressed separately. Obviously a thorough study and absolute expertise in the field only can give a proper solution. Most of the times an additional safety measure in the course of waterproofing can help a long way to offer the relief. Looking at the very high cost per square foot of construction today and still higher costs for rectification (in case anything goes wrong) it is very wise to take a safety measure which can ensure a total waterproofing.
From economic point of view also when spread over entire cost of project per square foot which is say for example approx. Rs.1000/- per sq.ft, then the cost of this additional safety does not come to even 2 per cent. If you add the cost of interiors to the basic cost of the structures then possibly this additional cost will be further lowered down to 1 or 1.25 per cent. Further, as the number of floors increase, as is the generalised case these days, the cost per square foot is brought down to almost negligible percentage.
It is interesting and important to note that the ineffective waterproofing not only destroys the structure but also does not spare the more valuable interior decorations, which ironically is thought to be of more importance by public. Thus, wise is the builder/ constructor who acknowledges and implements the effective safety measures for waterproofing right at the conceptual stage.
Mechanism of Waterproofing by Proper Cement Hydration
One of the products of hydration of cement is cementgel or Tobermorite, which is formed upto water cement ratio 0.65 to 0.7. At lower w/c ratio, it is formed more and more and vice versa. This hydration product has absolutely least co-efficient of permeability, i.e., 7x10-14 cm/sec and by itself blocks the pores and capillary in the concrete. This can result into good waterproof concrete of 10-10 to 10-12 cm/sec. However, actually in practice at average job, we end up getting the permeability anywhere between 10-6 to 10-8 cm/sec. This is mainly due to uncontrolled w/c ratio, lack of curing and also other bad detailing.
Combination of Conventional and Non-Conventional Ways
Various ways of waterproofing the structures are recommended. However, a discerning client and the consultant need to study all the possible parameters, positive as well as negative and then come to a recommendatory conclusion. Conventional ways of waterproofing are like brickbat coba followed by IPS. Brickbat coba is generally used to provide a slope to the treatment and IPS layer works as an impermeable treatment by itself which also assumes the slope of brickbat coba. Though, otherwise found fairly successful; today due to speed of the work, quality of sand, inadequate curing to the concrete which is made from finer grade cement and many other related reasons the IPS is seen to crack and disintegrate allowing an early entry to water. The brickbat coba is not otherwise supposed to be waterproof, but on the contrary is absorbent, which accumulates the water. On saturation the brickbat coba attempts to transfer the water further, which generally penetrates into the slab, and penetrates more easily if the same is porous or full of cracks. As it is concrete is not by itself a fully impermeable material, and various compromises while construction can decrease the impermeability further. Needless to say this causes unsightly leakages to begin with followed by corrosion of steel and finally results into structural damage.
Various permutations and combinations in specifications can avoid such problems, for example the following alternatives can be considered:
- Give a water impermeable coating on the finished surface of IPS – Though this method can prove effective in short term basis, may create problems subsequently. It is observed that the coatings based on pure solvent-based polymers, though effective do not function for larger duration due to its requirement for a thoroughly clean and dry surface. From economic point of view also these coatings turn to be fairly expensive and basically being a costly commodity, cannot be given in a bigger thickness. These coatings also need to be taken care of for the abuse of UV rays, which involves improvising the formulations knowledgeably with proper UV-absorbent chemical inputs.
- In order to take care of above shortcomings, these days, polymeric cementitious coatings are more preferred world over. These systems have better adhesion to the surfaces and can tolerate to an extent deficiencies of the surface like slight wetness and microdusting etc. The thickness of these coatings also is 8 to 10 times more than pure polymer coatings. However, in this type also, one needs to select the polymer judiciously so that the UV interaction and disintegration due to same can be avoided.
Though the systems based on the comatrix of polymer and cement are more preferred due to its composite properties like adhesion, waterproofing, flexibility, resistance to UV rays and improved resistance to wear and tear, a discerning specifier has to bear in mind that after all this system is a film and it is quite possible that due to any physical damage to the concrete surface underneath the coating, the coating itself can get damaged. However, by and large for inaccessible RCC slabs or terrace above, treatment is proved to be very useful and effective even in high rainfall areas like coastal region or mountain ranges.
- Another practical approach adopted is to incorporate these water-based polymers in the IPS itself. This increases the water impermeability of the IPS and at the same time decreases the extent of shrinkage cracks due to increase in the flexural strength of the matrix. Additionally the use of polymers in cement concrete/ mortar is found to increase the adhesion of the concrete mortar to the surface. This integral addition of polymer in mortar is very useful and beneficial for any high-rise structure if applied in the form of plaster, particularly for the fact that it becomes difficult for any engineer, either from client’s side or from contractor’s side to supervise the work effectively on the floors beyond 3rd or 4th floor. The permeability of polymer modified mortars has been experimentally checked by various researchers including the authors. The studies have revealed that at approximately 4 percent dosage of polymer to cement, substantial reduction in permeability is observed.
Though on long-term basis, incorporation of polymer in concrete/mortar is very useful to the structures from waterproofing point of view or even otherwise, there is also an important role of economics, which comes into play. Since the consumption of polymer in the earlier case of waterproofing film is to be considered only on the surface spread basis, and comparatively in a small thickness, the cost per unit will be substantially less. However, when the polymer is to be dispensed throughout the mass of concrete / mortar then depending on thickness the consumption of polymer substantially increases, adding to the cost. But the fact should not be overlooked that addition of polymer throughout the mass of concrete/mortar adds to the durability of the structure due to substantial increase in impermeability, adhesion, flexural strength (i.e. decrease in shrinkage cracks) and added resistance to in-service aggressive attacks by environmental pollutants or even carbonation. Once again the prevailing wisdom will decide the course of selection of option.
- Looking at the cost and benefit ratio, another method is gaining grounds and acceptance with the consultants and clients. In this method the polymer cementitious comatrix film is applied on the existing RCC slab or the mother surface. Obviously this application is done in a way that no pinholes or holidays are left in the surface. This layer is tested for impermeability by ponding test and then is sandwiched by completing subsequent brickbat coba or the IPS as the case may be. In this option, even if the IPS develops cracks due to shrinkage etc. (which can be minimized by adding anti shrinkage compounds and good curing) and the water percolates through the section, it will be totally stopped at the polymeric cementitious film which is applied on the bare slab. In this case though the water is effectively stopped, the cost would not unduly increased, as the polymer demand involved will be only for the film formation. Moreover, the film is not likely to get damaged due to wear and tear as the same is sandwiched between two strong phases viz. RCC slab and subsequent BB coba and /or IPS. In our opinion and experience this method is more acceptable due to its cost and effectiveness to fulfill the requirement of healthy and durable structures.
All the four options illustrated above can be applicable for the waterproofing of toilets, baths, swimming pools, flowerbeds, overhead and under-ground tanks, etc.
These waterproofing compounds, admixtures and coatings, etc, are non-conventional waterproofing. Today, the market place is flooded with many alternatives of serving the end of waterproofing and it is happy situation for the customer that he has a large choice. However, basic guidelines about selection of the same becomes necessary so that cost alone does not become the governing factor for the customer. This education about the generics and the performance of the product is necessary and now the time has definitely come that all the users, engineers, architects, consultants, and specialist contractors address themselves with the following four questions, viz.
- Whether there is a need for additional waterproofing material / chemical in concrete?
- Which materials (generically) are available in the market place?
- How do they function and are dispensed?
- And finally, if these products fall short of their expectations and promises, what sort of feedback should be given to further R&D efforts?
It is found that from Waterproofing, U.V. resistance and abrasion point of view, Acrylic polymers are much superior to SBR or any other material. In fact, experiments conducted have revealed that acrylate comatrix does not give any permeability even at the pressure of 70 mtrs. of water height. This test has also been specified in German Code DIN-1048.
The applications of these polymer-modified cementitious acrylic coatings can be very easily extended to overhead water tanks, terraces, chajjas, parapets, etc. Generally, the cost incurred for this treatment is approximately Rs.12/- to Rs.14/- per sq.ft. based on today’s contemporary market with good assurance of performance.
Some Important Case Studies -
- Laboratory Secondary School, Kirtipur, Nepal is one such example where these elastomeric acrylic cementitious waterproofing coatings were applied on the sloping roof slab covering more than 80,000 sq.ft. The structure was constructed 45 years ago and the intresting part of the roof was that it was an Hyperbolic Paraboloid structure- Hypar Roof.
The following specifications were adopted-
- Break open the existing water- proofing layer on the sloping roof.
- Clean the surface thoroughly using an air blower and make it free from any dust, dirt, grease, oil etc.
- Prepare and apply three coats of elastomeric polymeric acrylic coating- POLYALK WP and Cement mixed in the ratio 1:1.25 for the first coat and 1:1.5 for the second coat at twenty four hour intervals by brush.
- The slurry should be uniformly mixed and consistent in brushing. This has to be done using an portable electric operated stirrer.
- Mist Cure the coating for 3-4 days after 24 hours of application of the coating.
- During the application of the third coat ensure that the I.P.S topping is laid on the third coat while it is still wet and tacky.
- Cure the I.P.S as per standard construction practice.
- Around 12 nos. Ground storage reservoirs (GSR) and 9 nos. Elevated storage reservoirs (ESR) were restored and waterproofed using these treatments. The capacities of G.S.R ranged form 1.1.5 to 5.5 ML and E.S. R from 0.6 to 2.5 ML. One such case study of an ESR is mentioned-
- Support the R.C.C. members by steel props & spans as per loading conditions & extent of damage of the members to be repaired. Expose the R.C.C. member by means of chisel & hammer. Remove the loose concrete beyond the reinforcement plaster, including scaffolding etc. complete.
- Remove loose rust from reinforcement by means of tacha & wire brush (mechanical /manually). Apply Sulphate and Chloride free rust remover- RUSTICIDE solution to the reinforcement by brush or cotton waste. After 24 hours clean the reinforcement etc complete.
- Provide a slurry of 1 part of alkaline elastomeric rust preventor – POLYALK FIXOPRIME with 1.25 part of fresh cement & mix the above solution by means of stirrer & apply it on the reinforcement by paint brush. After 24 hours apply similar second coat of such slurry. Make sure no area is left uncovered.
- Repair the damaged R.C.C. members using acrylic based polymer- POLYALK EP in ratio 1:5:15 (POLYMER: Cement: Quartz Sand) & required proportion of water. Apply a bond coat of ACRYLIC-BASED POLYMER & cement in 1:1 proportion to R.C.C. members & apply polymer mortar to build up the thickness in layers. Cure the same by spray pump after 24 hours.
- Apply a waterproof coat of an elastomeric polymeric cementitious slurry using non degradable and U.V. Resistant acrylic polymer – POLYALK WP and cement prepared in 1:1.25 ratio to the internal and external concrete areas Curing the same by spray after 24 hours including scaffolding etc. complete.
- Applying a polymeric bond coat of a non-ionic bonding agent- HACK AID PLAST for the plaster, admixed with about 150ml dosage of superplasticizer - POLYTANCRETE NGT per bag of cement.
In conclusion it is always advisable to prevent a possible attack on the durability, water leakage and eventual distress in structure due to corrosion rather than curing the same after its manifestation. In such eventuality advances in material science can help to serve the cause. It is thought that basically protection of the structures by waterproofing them is an important aspect of assuring durability. Various methods to achieve good results are discussed in the article and also a point of view based on experimental trials as well as site results is expressed for the general awareness. Certainly this is not the last word in the matter and it is quite likely that many more useful innovations will be made available to mankind to increase the health of the structure in the coming future.
About the author
Dr. S. K. Manjrekar (Ph.D. Bombay University - 1977), Dr. S. K. Manjrekar is Chairman and Managing Director of a well-known construction chemicals company in India - M/s. Sunanda Speciality Coatings Pvt. Ltd.,for last 40 years. Operations in UAE, Oman, Nepal, Tanzania and USA.
He is actively working on several technical and administrative committees of American Concrete Institute (ACI) in USA.
He teaches concrete field testing course of ACI to engineers of India as a ‘Train the Trainer’ initiative.
He has published more than 200 papers in various national and international journals.
He has given more than 100 key note lectures in the field of Concrete, Corrosion Prevention, Waterproofing and Nano materials etc.
Lectured extensively internationally in Countries - United Kingdom - London, Leeds University, United States of America - Pittsburgh, Malaysia, Oman, Dubai, Sharjah, Kuwait, Romania, Hong Kong – Hong Kong University and many more as an invited keynote speaker.
He is the guest editor of International journals in UK, USA and Mexico.