The admixture industry in India is an evolved one. With usage history in mainstream projects of more than three decades, admixture usage has seen a tremendous upswing. Also with the advent of Ready-Mix Concrete, concrete pumps and placing machinery, the usage of admixtures has become inevitable. The trend is quite clear and it is certain that in future concreting operations, the admixtures shall become an integral part of the concrete mix. The earlier attitude of taking recourse to the use of admixtures only after facing problems is changing fast, and now, in most of the large projects, the admixtures are already included in the specifications.
As the infrastructure projects are more specific and need dependent, specific concretes will have to be designed to suit individual requirements. The durability would be the prime consideration as these structures cannot be easily replaced or repaired. Depending upon the appropriate selection of exposure classes, specialized specifications for concrete mix designs would need to evolve. The mix designs are getting relatively complex on account of interaction of several materials and mix design calls for expertise in concrete technology and materials. High Performance Concretes will have to be adopted considering special properties as well as low cost maintenance strategies. In this regard, integral waterproofing additives are gaining momentum as a solution to help mitigate durability problems.
Concrete has an affinity to water and therefore is a wettable material. Water enters the concrete through continuous capillaries either under hydrostatic pressure or by capillary action. The capillary suction forces in dry or partially saturated concrete can be equivalent to hydrostatic head of several metres. The water cement ratio has a direct bearing on capillary discontinuity. The porosity of concrete is one of the major factors contributing to ingress of water into the concrete. As waterproofing cannot be better than the base concrete, it is imperative that the concrete should be produced with a low water cement ratio and minimum permeability. That is where usage of Integral Waterproofing / damp proofing additives comes into the picture.
How Does Water Enter Concrete?
The ingress of water in concrete depends upon the degree of saturation of concrete and mechanisms of entry. The rate of water entry depends upon the microstructure of the concrete paste. If unsaturated concrete is exposed to water, it will be absorbed into concrete even in absence of pressure due to capillary suction. Permeation properties of near surface concretes are very important while determining the entry of water into the concrete. Liquid, Gas or Ions can migrate in concrete by diffusivity because of a concentration gradient.
The permeability approach is more relevant to concrete under saturated conditions. The concept of sorptivity is more associated with flow of water in unsaturated concretes. The cover should be highly impermeable as well as with a very low rate of water absorption. In most of the circumstances capillary action rather than permeability controls the passage of water and this should be a major consideration in design of waterproofing systems.
Functioning of Integral Waterproofing Additives
The aim of integral waterproofing is to densify the concrete to prevent water ingress and / or convert wettable capillaries to non-wettable types that would eventually lower the penetration of water into the system. Figure 1 shows the concept of hydrophillic and hydrophobic capillary action. The theory of capillary and the concept of capillary rise and capillary depression provide a basis for working of hydrophobic waterproofing materials. Wettable surfaces have low contact angles and difficult to wet surfaces have higher contact angles. The high contact angles have a two-fold effect on concrete. Firstly the pressure required to enter the concrete is positive where by capillary action is nil and a high water pressure (approximately 14 m head of water) would be required to penetrate the concrete surface. Hydrophobic surfaces exhibit high contact angles of water as shown in Figure 2. Integral Waterproofing Additives use this theory to affect permeability reduction in concrete / mortars.
In Short, waterproofing admixtures belong to a class of admixtures that improve concrete durability by controlling water and moisture movement and by reducing permeability and hence chloride ingress. This class of admixtures is referred to in IS: 2645. EN-934 refers them as water-resisting admixture and ACI 212.3R refers to the same as permeability reducing admixture. These admixtures are generally sub-divided into two categories (ACI Comm.212), viz., permeability reducing admixtures for concretes exposed to non-hydrostatic conditions (PRAN) and concrete exposed to hydrostatic conditions (PRAH).
Types / Bases of Integral Waterproofing Additives
These admixtures can be based of different chemistries depending on the application of the base concrete, mortar or plaster. Damp-proofing admixtures or PRAN are water-repellents / hydrophobic. These are more suited for use in situations where the base concrete is not subjected to high hydrostatic heads. These for example can be used in external above ground walls in buildings, plasters, mortars, architectural concrete, concrete blocks, etc. Integral-waterproofing admixtures or PRAH may be hydrophobic (not necessarily). These are more suited for use in concrete / mortars subject to hydrostatic pressure heads. These for example can be used in water storage tanks, wet areas, basements, STPs, Tunnelling Concrete, Underground structures and other similar structures. Classification and materials used in Integral Waterproofing / Damp-proofing Admixtures is given in Table 1 below.
Mode of Action
Integral Waterproofing Additives generally function on the basis of one or a combination of the three mechanisms listed below.
- Reduction of Capillarity by w/c ratio reduction
- Hydrophobising Capillaries
- Physical or Chemical Pore Blocking
- Reduction of Capillarity: This is achieved using Conventional water-reducers or Air Entraining Agents or by using reactive pozzolans and silicates. These additives may improve workability (increases slump) at the same w/c ratio or provide a reduction in w/c at the same workability levels. This property helps concrete achieve excellent compaction and denser hydration (C-S-H Gel Formation) thereby reducing capillary pores. Less capillary pores means less transport of water through the cement matrix.
- Hydrophobising Capillaries: This is achieved using hydrophobic additives such as Soaps, Bitumen, Veg. Oils & Fats, Mineral Oils (Old Technology) OR Long Chain Fatty Acids, Fine Wax Emulsions, Silicones, Silanes-Siloxanes, etc. (New Technology). These materials react with the cement constituents and form insoluble hydrophobic by-products that line the pores. This lining converts the water-absorbing capillary forces into water repellant capillary forces due to the hydrophobic effect of the constituent. Thus water is pushed out of the capillary pores and the concrete is kept dry. Fox e.g.: the stearates react with the calcium hydroxide in concrete to form insoluble calcium stearates that forms a hydrophobic layer on the walls of the pores in concrete. These admixtures are effective at reducing the capillary absorption under non-hydrostatic conditions only.
- Pore Blocking (Physically or Chemically): Physical Pore Blocking can be achieved using inert powder fillers such as talc, bentonite, etc. or by using finely divided waxes, bitumens, or acrylic / SBR emulsions. When Hydrostatic Pressure is applied, these finely divided materials or emulsion globules are pushed into the capillaries, until they jam together, forming a physical plug, preventing further penetration of water.
Chemical Pore Blocking is achieved using advanced micro-fine latent hydraulic materials or pozzolanic Nanoparticles or reactive silicates to affect continuous hydration to get the densest packing of C-S-H gel in the cement matrix on a nano-scale. The strength and permeability of a cement-based material is based on hydration of the clinker components, to form C-S-H gel and free lime (Ca(OH)2). The materials above use this Ca(OH)2, in presence of water, to recrystallize and form new more stable, foil-like C-S-H and C-A-S-H phases. These materials when used as admixtures are also referred to as crystalline admixtures.
For e.g. In case of use of specific high reactivity pozzolans, the cement matrix would be densified by a complex process. Therefore the most porous part of the cementitious matrix, (Ca(OH)2), is converted to very stable, impermeable C-S-H and C-A-S-H type crystals. This crystalline mineralization process continues over time in presence of water, and the structure of the matrix becomes more refined, overall porosity decreases, micro-cracks are sealed and pore volume is minimized. This process thus makes concrete watertight. Figure 3: shows the Working Mechanisms of IWP Admixtures
Effects on Properties of Concrete
- On Plastic Concrete
Integral waterproofing admixtures are formulated to affect the properties of the hardened concrete, and not those of concrete in its plastic state. Water repelling admixtures, conventional admixtures and air entrainers, may increase the workability of the plastic mixes slightly. On the other hand, inert pore fillers or reactive pore fillers, due to their finely divided particle sizes may de-crease workability slightly. These materials mostly do not affect setting time or stability of the mixes. However, in all cases, it is advisable to test the material for required properties prior to use.
- On Hardened Concrete
Integral waterproofing admixtures are designed not to have any significant impact on the mechanical properties of concrete. With most materials there is no significant changes in compressive / flexural strengths or on elastic modulus of the mixes. The only properties they impact are related to permeability of the concrete. This can be measured by means of initial surface absorption (BS: 1881 or Equivalent) or water penetration under pressure (DIN 1048 or equivalent).
- Provides an efficient and durable barrier against rainwater, moisture and ground water and makes concrete waterproof
- No external application, simply mix it into existing concrete
- Save costs on material and application costs for external membranes
- Speeds up construction, no waiting for external membrane application
- Permanent protection from water penetration and absorption
- Makes the mix more workable (improves the slump) at lower w/c
- Does not change setting time nor adversely effect the reinforcement.
- Disperses rapidly and makes a homogenous mix.
- Free from chlorides
- Generally safe for use in contact with water intended for Human Consumption
- The use of prevention as a means to waterproof structures can be a cost effective measure, especially for structural elements as it prevents future deterioration and repair cycles.
- Not a substitute to bad mix design and manufacture, placing and curing practices.
- These may not be capable of preventing water ingress through deep cracks or structural cracks in the concrete.
- Requires high degree of quality control during concrete manufacture, placing and curing.
- In case of integral waterproofing or any other waterproofing treatment for underground structures, joints become the weakest link in entry of water and should be adequately treated.
- Application of Integral Waterproofing Compounds should be considered after characterizing its performance and requirements as per site conditions that the concrete would be subject to.
Areas of Application
- Basement slabs & Walls
- Cast structural slabs
- Elevator pits
- Concrete foundations
- Underground pipes
- Deep pile foundations
- Water-retaining structures
- Swimming pools
- Cooling towers
- Podium decks
- Outdoor balconies
- Potable water tanks
- Wet Areas
Practical Usage Guidelines
Integral waterproofing additives are easy to use, either in the concrete batching plant or in a transit mixer at site. These materials should be added to the concrete after all other components of the mix have been added. Concrete should be mixed for atleast a minute after addition of Integral Waterproofing Additives. Dosage is generally at 0.5 to 2.0% by weight of cement or as per manufacturer’s recommendations. Complete dispersion of the material should be ensured prior to placing. Concrete placement should be followed by proper finishing, compaction and curing practices to get best benefits.
Water-resisting admixtures have a positive effect on durability of concrete by mitigating water ingress and hence chlorides, etc. Surface absorption is also reduced and when concrete is exposed to harsh environs this admixture has a beneficial effect on increasing the life span of a concrete structure. However, it is to be remembered proper mixing is essential for the performance of the waterproofing admixture. Excess dosage of the admixture has to be avoided and when used with other admixtures compatibility check has to be done.