By Kevin Yuers
Water is the enemy of hardened concrete. It causes expansive damage when it freezes, carries corrosive salts to attack the reinforcing steel, reacts with certain aggregates to cause disruptive expansion, and provides an essential ingredient for the growth of mould. Water-retaining structures like reservoirs, dams, and waste treatment facilities must prevent water from escaping; other structures such as tunnels and buildings must prevent water from entering.
For as long as anyone can remember, the construction industry has used the word ‘waterproof’ to describe construction materials. People commonly refer to something as being waterproof if it holds water in or out and does not leak. However, the word waterproof is technically not defined this way. Most dictionaries define it as being impervious to water, that water cannot penetrate it at all. This raises a serious question: Can anything really be completely impervious to water?
The American Concrete Institute (ACI) is an international organization dedicated to the advancement of knowledge about concrete. Recognizing the problematic nature of the term ‘waterproof,’ ACI has discouraged its use, stating:
Because nothing can be completely ‘impervious’ to water under infinite pressure over infinite time, this term should not be used.
Instead, ACI has over the years preferred to use the term ‘watertight.’ However, its definition of this word is very similar to that of waterproof (which, in practice, remains far more frequently employed). Another commonly used industry term is ‘dampproofing,’ which is defined by ACI to mean:
Treatment of concrete or mortar to retard the passage or absorption of water.
The word is typically used to describe liquid coatings or plastic sheets applied to the outside of concrete in contact with damp soil. Its goal is to prevent the absorption/wicking of moisture by the porous concrete.
Waterproof, watertight, dampproof… the trouble has been all three of these terms are imprecisely defined and tend to overlap each other in common use. This is especially problematic when they are used to define admixture products because testing methods and performance standards are relatively new and still being developed. Where does the performance of a dampproofing admixture end and a waterproofing admixture begin? How can a professional expect to write a proper specification using such terms?
Advent of permeability-reducing admixtures
Permeability-reducing admixtures are not new; people have been adding things to concrete to reduce its permeability for centuries. These range from plant and animal products to modern plasticizers. Additionally, supplementary cementitious materials (SCMs)—such as silica fume, fly ash, and slag—are technically not categorized as admixtures, but can nonetheless be added to a concrete mixture to reduce permeability.
The ACI sub-committee responsible for concrete admixtures is Technical Committee 212, Chemical Admixtures. Its members recognized something needed to be done to clarify any confusion. Professions needed more precise language with clearly understood meanings. Standardized testing methods and performance criteria that could be included in written specifications was also necessary.
The revised technical document, ACI 212.3R-10, Report on Chemical Admixtures for Concrete, contains a completely new section specifically written to address issues relating to permeability-reducing admixtures. This Chapter 15 describes three general categories for these materials:
- hydrophobic or water-repellent chemicals designed to increase water repellency and reduce absorption (e.g. stearates, soaps, and oils);
- finely divided solids to fill up space and densify the concrete (e.g. clay, silica, silicates, and polymers); and
- crystalline chemicals, which are hydrophilic chemicals that react with cement and water to fill the pores with crystalline structures that offer permanent water resistance through self-sealing.
Since products may contain one or more of these materials, they cannot simply be classified based on their ingredients alone. Instead, Chapter 15 classifies permeability-reducing admixtures by their ability to resist hydrostatic pressure:
- permeability-reducing admixtures for non-hydrostatic conditions (PRANs); and
- permeability-reducing admixtures for hydrostatic conditions (PRAHs).
Most products will fall into the PRAN classification. Water-repelling or hydrophobic materials can be very effective at preventing water absorption into concrete. They work by way of surface tension in the same way fabric treatment repels spills on clothing and furniture. They can be easy to use and cost-effective for applications not subjected to hydrostatic conditions.
However, even a modest amount of this pressure can overcome and push past the surface tension created by these materials. If acted on by water under pressure, concrete protected by only a PRAN may allow water to pass through.
Another term, ‘finely divided solids,’ refers to materials that improve the packing of the concrete’s ingredients, causing its pores to be as small as possible. These materials may also act to block the pores with loose particles. The category includes:
- clay materials that swell in contact with water (e.g. bentonite);
- polymer admixtures that form globules meant to plug the concrete’s pores; and
- silicates such as sodium silicate, which also works by reducing pore size or plugging pores.
All these materials can significantly reduce permeability. However, because they cannot reliably plug all the pores and because they are unable to bridge cracks, they cannot be counted on to withstand hydrostatic pressure, especially over extended periods. For these reasons, finely divided solids are also classified as PRAN.
Crystalline chemicals react with water and the cementing materials in concrete or mortar to form distinct crystalline structures within the pores and small cracks of the concrete. These crystals effectively block the concrete’s pores in a similar way to the finely divided solids. Additionally, these crystalline structures have the ability to bridge small cracks. Since any concrete structure has a high likelihood of developing cracks, this bridging ability is critical to successfully creating a watertight structure.
Further, since crystal formation only takes a small amount of crystalline materials in reaction with a larger amount of water and cementing materials, the admixture is not used up. This means when new cracks form later, and moisture begins to penetrate the concrete, more crystals grow to seal the crack. This self-sealing ability is unique to crystalline materials. Consequently, crystalline products have been shown to withstand very high hydrostatic pressures over long periods.
Specifiers should be aware not all products calling themselves ‘crystalline’ actually fall into this category—some merely crystallize as they harden or dry. For example, sodium silicate is a solution that forms a crystalline structure as it dries, whereas ‘true’ crystalline materials are PRAHs that cause a chemical reaction to form distinctly new crystals. More importantly, the material remains continuously reactive, allowing new crystal formation in the face of future moisture penetration. To be a PRAH, the crystalline material must possess this self-sealing ability.