Water ingress is one of the major causes of the deterioration of concrete structures due to the corrosion of the steel reinforcement and degradation of the concrete itself. The waterproofing mortars are cement mortars to which powders, latexes or emulsions have been added as modifiers. The modifiers can be non-polymeric or polymeric.
Waterproofing mortars are applied on roof tops, bathroom floors, basements and wet areas to keep the structural concrete dry. Such mortars should possess good compatibility by ensuring that moisture does not pass through, not cracking due to the loads or shrinkage and by bonding well to the base concrete. These systems are quite cost-effective since the material components are economical, the application does not require special surface preparation, and they can be applied even when the structural concrete is not completely dry.
Non-polymeric modifiers are usually dry powders or liquids that function as plasticizers or pore blockers. Plasticizers such as lignosulphonates act by making the mortar easier to place and more compact. Pore blockers (some are also called as crystalline admixtures) are expected to improve the impermeability of the cement mortar by reacting with calcium hydroxide in the presence of water to fill and plug pores and micro cracks. However, their effectiveness is yet to be scientifically established.
Polymer based Waterproofing:
Polymer-based waterproofing modifiers are widely used for providing effective and long term solutions as they result in systems that bond well to the base concrete, have low permeability, good workability and strength, and low shrinkage. The most effective polymer based modifiers are latex emulsions of styrene butadiene rubber (SBR), acrylics, acrylonitrile rubber and styrene acrylic ester. The most popular and probably the most effective are SBR based modifiers, which form films, and bridge voids and micro cracks in the hydrated cement paste (as seen in the figure below), thereby improving the waterproofing ability of the modified mortar.
Assessment of cement mortar waterproofing systems
The most desirable properties of a polymer modified cement mortar for waterproofing are low permeability and shrinkage, and good bonding with the base concrete, as well as resistance to weathering. Since the performance of a waterproofing mortar is dependent on the chemical nature of polymer, as well as the composition of the mortar, it is essential that the mortar be assessed appropriately, especially for major projects. Test methodologies that can be used for determining various strength and waterproofing properties are described below.
The strength of a waterproofing mortar is not critical but is indicative of the quality and consistency of the system. Moreover, loads due to traffic and installations could cause stresses in the mortar. Also, higher strength mortars would resist abrasion and wear-and-tear better in field conditions. More important, however, is the bonding strength between the mortar and the base concrete, which is critical for the durability and efficiency of the waterproofing system, with a weak interface leading to delamination and ingress of water. An appropriate procedure for checking bond strength is that of ASTM C 1583, where the force required to pull-off a metal disc glued on to the mortar applied on a concrete surface is measured.
Obviously, the most important property is water permeability, which can be determined through a procedure such as that of DIN 1048, where 150 mm cubes of the mortar are pressurized at 5 bars for 72 hours and the subsequent depth of permeation is measured. Capillary absorption is also important as it reflects the extent of pore connectivity and moisture absorption. This property can be determined through the procedure of RILEM CPC 11.2, where the end of a dry mortar prism is immersed 5 mm deep and the intake of water over a week is quantified.
Whenever the waterproofing mortar is exposed to a dry environment, it experiences shrinkage, which can lead to cracking, if excessive. It is, therefore, essential to use a mortar that has low shrinkage strains, especially during the initial ages, when it could be left exposed. To characterize the drying shrinkage and propensity for cracking, mortar prisms exposed to drying, and the consequent strains are measured, say, according to the procedure of ASTM C 596.
IIT Chennai study
Waterproofing mortars with different modifiers commonly used in the Indian scenario have been studied in the Construction Materials Laboratory over the past seven years. The materials evaluated include SBR, acrylic polymer, lignosulphonate, silicone, fatty acid and neutralized amino alcohol. Investigations cover the strength and waterproofing properties, as well as weathering due to UV light, wetting-drying cycles and sea water exposure.
Some interesting conclusions from the studies are that the lignosulphonate based integral waterproofing agent performs well in areas protected from weathering (such as toilets and other interior wet areas); the SBR latex is most versatile under conditions expected in India (i.e., high temperatures and humidity), even in areas exposed to sunlight; and a carboxylated SBR could be the most adequate in harsh environments such as tidal zones in marine applications.
Dr. Ravindra Gettu
Professor of Civil Engineering at IIT Chennai. His research interests are in the areas of the effective use of chemical admixtures, fibre reinforced concrete, self-compacting concrete and mechanical characterization.
Dr. Priya S. Nair
Post-Doctoral Researcher in the Building Technology and Construction Management Division of the Department of Civil Engineering, IIT Chennai. She is currently involved in the evaluation of commercially available polymer modifiers used in cement mortar based waterproofing systems.