WATERTIGHT CONCRETE and its uses

Concrete structures can never be completely impermeable. There is a certain degree of porosity in concrete that makes it impossible. As a result, external environmental agencies and ingress of chlorides, sulphate, and moisture can deteriorate the structure. Hence, it is desirable to impart water tightness to the structure. Instead of waterproofing the structure, the solution may lie in making it watertight, giving it the ability to keep the water in or out. The aim here is to boost its water resistance, which is defined by the amount of water or moisture that appears on the side that is away from any water contact.

Concrete and water ingress

Porosity and permeability are often used interchangeably while defining the microstructure of concrete. However, both have their own significance. Porosity of concrete is the virtue to hold fluids, whereas permeability refers to its ability to convey or transfer fluids. Concrete is inherently porous as it is impossible to make it absolutely impermeable given the gel and capillary pores in its structure. The smaller gel pores are the part of glue that holds aggregates together. Capillary pores are formed after hydration due to the evaporation of excess water in the system. These are larger and interconnected. These cracks form a channel for the flow of moisture and water. Thus capillary voids, cracks and construction joints are the major sources of leakage in a concrete structure. Thermal stresses, shrinkage, external loading and structural movements lead to cracking of concrete. These sources will be a major threat for water retaining and watertight structures. Hence, it is desirable to produce a dense and durable concrete having sufficient resistance against hydrostatic pressure without seepage of water through the concrete elements.

Sources of leakage

Making a watertight concrete structure is easier said than done. Other than the porous media, cracks and construction joints are the major sources of leakage in a structure. Water is one of the essential components in concrete. Ideally, 25-28% water goes into the hydration of cement. The remaining water is used to impart the desirable workability and ease. During hydration, this excess water evaporates and forms the pores.

To bring down the water content, concrete has to be designed with very low permeability. Fluids usually penetrate concrete in three ways: through cement paste, aggregates, or an interfacial transition zone. Paste contains pores formed from air and water. These pores can be controlled by minimising the water to binder ratio (w/b). Aggregates have lower porosity compared to paste, however they may be more permeable if the pores are of larger size. The interfacial transition zone (ITZ) is a thin zone, surrounding the aggregates. This zone consists of fewer cement particles compared to water, thus leading to more porosity. The ITZs are interlinked and provide a high permeability zone, which can be controlled by using certain admixtures and supplementary cementitious materials. This zone has to be strengthened and made watertight. All these factors make ready-mix concrete instrumental in protecting from water ingress or for retaining water within a structure.

Innovative solutions for watertight concrete

It is important to design the concrete mixes at a micro-structural level while considering particle packing of individual raw materials.

Maintaining Water-binder (w/b) ratio: The w/b ratio is the most important factor in making concrete watertight. A lower w/b ratio will ensure reduced porosity and permeability. The availability of water reducers and the ongoing technological advances in the development of these products have led to overall reductions in water-cement (w/c) ratios, which have resulted in improved watertight precast concrete structures.

Using appropriate admixtures: Two types of permeability reducing admixtures are gaining prominence in concrete mixes. They are classified as PRANs (non-hydrostatic) & PRAHs (hydrostatic). PRANs are hydrophobic in nature and act mostly like water repellents. PRAHs react with water, being hydrophilic in nature. Crystalline waterproofing admixtures fall under this category. These admixtures typically use a crystallisation technology (Crystalline Waterproofing) that reacts with water or moisture to create insoluble crystals that block the pores and capillaries within the concrete, leaving it impervious to liquid. These crystalline additives consist of silica sand and specialised chemical additives. Crystalline chemicals react in the presence of calcium hydroxide and moisture. They perform exceptionally well in below-grade structures or when facing hydrostatic pressure. This blockage will resist water ingress under hydrostatic pressure.

Using supplementary cementitious materials: Materials such as fly ash, slag and silica fume can also increase concrete's density, thus reducing capillary porosity and permeability. Nuvoco mixes are a proportionate blend of water reducing admixtures, crystalline water proofing compounds, suitable proportion of supplementary cementitious materials (SCMs) which aid to sub divide the capillary pores and make them continuous.

Manufacturing process: This is critical for the production of durable, watertight concrete products. Proper attention to important pre-pour activities such as maintaining prescribed mix proportions, form cleanliness, and specified reinforcement placement with minimum cover is very important. Durability of concrete plays a critical role in controlling its serviceability.

Applicability

Watertight concrete finds applicability in structures where we need to keep water either in or out such as swimming pools and tunnel galleries. Concrete structures need to be protected from aggressive environments like chloride, sea water, aggressive chemicals etc. Basements, reservoirs, dams, tunnels, liquid storage tanks etc., are some of the structures that must be made watertight.

Because of its formation, it is not possible to completely waterproof concrete. However, we can work towards making it watertight by controlling its ability to keep water in or out. Watertight concrete helps in building structures where we need to contain water without the latter weakening the entire structure. The use of innovative materials and manufacturing processes has allowed us to make near complete-watertight structures.

- Pranav Desai

Vice President, and Head Construction Development and Innovation Center (CDIC)

- Samidha Pathak

Manager- R&D, Construction Development and Innovation Center (CDIC)

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