WATERPROOFING OF THE STRUCTURES | BY SUNANDA GLOBAL (PART 2)

by Sunanda g. SUNANDA SPECIALITY COATINGS PVT LTD

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 a 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 the 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 a near-impossible job adding astronomically to the cost of maintenance. Besides in several large structures like hotels or large commercial premises, proper sinks 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 a proper understanding of materials.

In the 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 time 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 that can ensure total waterproofing.

From an economic point of view also when spread over the entire cost of the project per square foot which is to say for example approx. Rs.1000/- per sq. ft, then the cost of this additional safety does not come to even 2 percent. 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 percent. Further, as the number of floors increases, as is the generalized case these days, the cost per square foot is brought down to an 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 to the 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 cement gel or Tobermorite, which is formed up to a water-cement ratio of 0.65 to 0.7. At a lower w/c ratio, it is formed more and more and vice versa. This hydration product has absolutely the 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 in good waterproof concrete of 10-10 to 10-12 cm/sec. However, actually, in practice at the 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 the 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 the 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 in structural damage.

OPTIONS AVAILABLE FOR WATERPROOFING

Various permutations and combinations in specifications can avoid such problems, for example, the following alternatives can be considered:

1.Give a water impermeable coating on the finished surface of IPS – Though this method can prove effective on a 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 a larger duration due to their requirement for a thoroughly clean and dry surface. From an 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.

2.In order to take care of the above shortcomings, these days, polymeric cementitious coatings are more preferred the world over. These systems have better adhesion to the surfaces and can tolerate to an extent deficiency of the surface like slight wetness and micro dusting 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 the same can be avoided.

Though the systems based on the comatrix of polymer and cement are more preferred due to their 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 terraces above, treatment is proved to be very useful and effective even in high rainfall areas like coastal regions or mountain ranges.

3.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 an 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 the client’s side or from the 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, a substantial reduction in permeability is observed.

Though on a long-term basis, incorporation of polymer in concrete/mortar is very useful to the structures from a 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, the 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 dispersed 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 the addition of polymer throughout the mass of concrete/mortar adds to the durability of the structure due to a 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 the option.

4.Looking at the cost and benefit ratio, another method is gaining ground and acceptance with the consultants and clients. In this method, the polymer cementitious comatrix film is applied to the existing RCC slab or the mother surface. Obviously, this application is done in a way that no pinholes or holidays are left on 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 increase, 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 applied for the waterproofing of toilets, baths, swimming pools, flowerbeds, overhead and under-ground tanks, etc.