Hydrophobic concrete is concrete that repels water. It meets the standards outlined in the definition of waterproof concrete. Developed in Australia in the mid-20th century, millions of cubic yards of hydrophobic concrete have been laid in Australia, Asia, and Europe, and in the United States since 1999. Its effective use in hundreds of structures has contributed to its large acceptance and growing use.
Structure Typical concrete is quite hydrophilic. This comes from its intricate system of tiny capillaries, which suck water through the microcrack network within a concrete slab. This hardened matrix creates a continuous "source to sink" cycle, meaning water from above is constantly pulled to an area of lower elevation. Darcy's coefficient refers to the ability of liquefied water under pressure to flow through any pores and capillaries that are present. A lower Darcy's constant correlates with a higher quality material. Commercial companies use different approaches to modify a regular concrete mixture in order to create hydrophobic concrete, all of which involve somehow filling the porous concrete mixture. Some of the most commonly used methods include polymer formation, small speck infusion, and crystalline formations, the latter being the most widely used. Polymer formation works by having a water-soluble pre-polymer polymerize via ion exchange with di-valent metal ions such as Ca and Fe ions to form rubbery insoluble particles. These small particles migrate and concentrate in the small fissures and capillaries formed in the concrete as it dries. As polymerization proceeds, rubber plugs form and permanently seal these water pathways, greatly reducing both water absorption and water permeability. Crystalline technology is used to create hydrophobic concrete by causing crystal structures to form in the tiny capillaries, pores and other air pockets left behind in the concrete curing process. During this formation, by-products are left behind in the capillaries and pores of the freshly cured concrete, typically calcium hydroxide, sulfates, sodium carbonates, potassium, calcium, and hydrated and unhydrated cement particles. These crystal structures then plug the pores and capillaries, preventing water from flowing through them. Once the crystalline chemicals are added to the concrete mixture, through either an admixture or coating, they react with the by-products in the presence of water. This reaction then forms an insoluble crystal structure that clogs the pores. This process continues until all the chemicals have reacted. When applied as a coating, the chemical reaction proceeds through the process of chemical diffusion. This is a process of a high chemical density solution migrating towards the low density chemical solution until the two come into equilibrium. Soaking the concrete in water creates a low chemical density in the pores, and applying the crystalline chemical as a coating then creates a high chemical density. These two fluids diffuse through the inner structure of the concrete until they reach equilibrium throughout the inner structure. When this process is finished, the hydrophobic concrete's crystal structure is complete.
Properties
The ultimate goal when forming a hydrophobic material is to reduce the polarity of the molecules. Because water molecules are very polar, they are easily attract to partially positive or partially negative charges. On a neutral surface, water molecules bunch up and attract each other, creating a spherical droplet of water. These droplets can then evaporate off the concrete surface rather than be absorbed into the capillaries of the concrete. The exact structure and composition of the crystals used in hydrophobic concrete is not public information; due to its properties, however, it can be assumed that it is a non-polar molecule. The property to repel water gives hydrophobic concrete the ability to avoid contamination by particles dissolved in water drops. Because the crystals themselves are not polar, there is little interaction between the crystals and dissolved oxygen. This allows the concrete to withstand the rebar rusting that so often compromises the strength of concrete that has iron bars running through it. Standard commercial concrete has an average water absorption of 4-10%. In contrast, hydrophobic concrete has an average of 0.3-1%. An overlooked property of hydrophobic concrete is its ability to repel humidity in the air as well. In contrast to liquid water, water molecules in the air moving with a higher kinetic energy and ultimately exist in a gas-like form. The crystal structures in hydrophobic concrete are compact enough to prevent humidity from moving through the capillaries of the concrete.
… excerpt ends here. Continue reading the full article.
