Hydrophobic concrete is concrete in which water is repelled by making the concrete hydrophobic. Hydrophobic concrete also meets the standards outlined in the definition of waterproof concrete. Started in Australia hydrophobic concrete has a 45-year success story before coming to the United States in 1999. Millions of cubic yards have been laid in Australia, Asia, and Europe. Its effective use in hundreds of structures has contributed to its large acceptance and growing use in the past two decades.
Contents
Structure
Typical concrete is quite hydrophilic. This comes from the intricate systems of tiny "capillaries", which work to 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. Dan DArcy's Coefficient, refers to the ability of liquefied water, under pressure, to flow through the pores and capillaries that are present. Note that a lower DArcy's constant correlates with a higher quality material.
There are many different structures within a regular concrete mixture that companies use to create hydrophobic concrete; however all of them involve filling the porous concrete mixture somehow. Some of the most commonly used methods include polymer formation, small speck infusion, or crystalline formations. Of the three, crystalline formation is the most widely used.
The way polymer formation works is by having a water soluble pre-polymer polymerize via ion exchange with di-valent metal ions such as Ca and Fe to form rubbery insoluble particles. These small particles migrate and concentrate in the small fissures and capillaries of the concrete as it goes through a drying cycle. As these small particles concentrate in these capillaries and fissures the polymerization proceeds until rubber plugs form permanently sealing these water pathways greatly reducing both water absorption and water permeability.
The way crystalline technology works in hydrophobic concrete, is through the formation of crystal structures in the tiny capillaries, pores and other air pockets left behind in the curing process of concrete. When this forms there are by-products left behind in the capillaries and pores of the freshly cured concrete. These by-products are typically calcium hydroxide, sulfates,sodium carbonates, potassium, calcium, hydrated and unhydrated cement particles. These crystal structures then plug the pores and capillaries preventing water from getting through. Once the crystalline chemicals are added to the concrete mixture, through either an admixture or coating, these crystalline chemicals 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 moves through the process of chemical diffusion. This is the process of a high chemical density solution migrating towards the low density chemical solution until these two come to equilibrium. Soaking the concrete in water creates low chemical density in the pores, and applying the crystalline chemical as a coating then creates a high chemical density, and these two then diffuse through the inner structure of the concrete until they reach equilibrium throughout the inner structure. Once 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 easily attract to partially positive or partially negative charges. However, when on a very neutral surface, water molecules will bunch up and attract to each other, creating a nice, spherical “droplet” of water. These droplets can then be evaporated off the concrete’s surface rather than being absorbed into the capillaries of the concrete. The exact structure and composition of the crystals used in hydrophobic concrete is not shared with the public, however, due to its properties, it can be assumed that it is a non-polar molecule.
The property to repel water gives hydrophobic concrete the ability to avoid contamination 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 rebar rusting that so often compromises the strength of concrete with iron bars running through it. Standard commercial concrete has an average water absorption of 4-10%. In contrast, hydrophobic concrete has an average of .3-1%.
An overlooked property of hydrophobic concrete is its ability to repel humidity in the air as well. Contrasting liquid water, water molecules in the air are 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.
Processing
Hydrophobic concrete is achieved through a variety of ways that fall under two categories; coatings or admixtures. Both ways allow the crystal structures to form in the presence of water.
When creating hydrophobic concrete through a coating process, it is sprayed or brushed onto a porous surfaces. In most cases, it is applied to a regular concrete slab that then undergoes a corrosive process to expose more of the concrete’s capillaries. This can be achieved by water blasting the surface at about 3,000-4,000 psi. Sandblasting and acid etching are also suitable processes. The addition of water is the next step. Regardless of a vertical or horizontal application, temperatures should not go below 33 degrees Fahrenheit to prevent water from freezing. Excessive evaporation should also be avoided. In areas with high evaporation rates, this process is often done overnight.The main goal is to saturate the pores as much as possible. Once saturated with water, the coating can then be applied. Hydrophobic chemicals are in a powder form, and mixed with water at a ratio of five parts powder to two parts water for application by brush. For spray application, the ratio is five parts powder to three parts water. The coating is applied between 1.25-1.5 lb per square yard. This coating process is continued until the whole surface is covered. If the surface requires another coat, it must be applied within forty eight hours of the initial application of the hydrophobic mixture. Once applied, the concrete must cure in a moist environment two to three hours after the application. This is achieved by spraying the surface with water at least three times a day for a few days. Occasionally evaporation retardants are also used. Depending on the climate, the curing processes may take more time and require more frequent wettings. Once the concrete is cured, it sits for two to three weeks before the process is complete.
When hydrophobic concrete is made through the use of an admixture, a powder with the hydrophobic chemicals is added during the batching process. In other words, it is added to the concrete mixture itself at the time that the concrete is laid.The usual dosage is two to three percent of the concrete mixture. Because water is apart of the batching process, it does not need an additional curing process. This process is a lot easier and less labor-intensive, however it can only be used when new concrete is laid.
Uses
Hydrophobic concrete is often used in exactly the same applications as regular concrete. It is used more often on projects where regular concrete is dangerous to repair or the cost of structural damage would be detrimental. Tunnel work is a major application of hydrophobic concrete as repairs underground are difficult to do. It is also a favorite choice for laying foundations for buildings and sidewalks in places below the water table.
Under water use of hydrophobic concrete is a major application in marine facilities. Is often used to hold water to create pools and ponds. In fact Nasa used hydrophobic concrete to build the swimming pool used to train astronauts for walking on the moon. Hydrophobic concrete is also used in applications that are exposed to rain or rain puddling, such as green roofs and parking structures, as well as plazas and other rooftops as well.
Pros
Amongst the many benefits of using hydrophobic concrete it reduces installation time and lowers costs. Use of hydrophobic concrete can shave three months of labor off an industrial project. (Hycrete in S. Cal)The time is saved because normal concrete involves a corrosion proofing period as well as a waterproofing period. With hydrophobic concrete, both corrosion proofing and waterproofing are done at the same time.
Likewise, time reduction reduces the cost of installation. Regular, membrane-backed concrete can cost around $5 a square foot. Prices can vary based on application.The one-step installation process of hydrophobic concrete brings the cost to about $3.20 a square foot. As one can imagine, a $1.80 per square foot savings can add up quickly over the course of a project. (Hycrete in S. Cal)
An estimated five billion dollars were spent in Western Europe alone on hydrophilic cement repairs in 1998. Most of the repairs are necessary due to the damage of water corrosion on urban areas. Because water corrosion is little to none, hydrophobic concrete remains more preserved than regular concrete, which typically looks worn and aged after a few years.
From an environmental standpoint, hydrophobic concrete is also beneficial because it is "green". Its ability to be re-crushed makes hydrophobic concrete easily reusable. Regular concrete can also be re-crushed, however it involves a very costly process. This means most concrete just ends up in a landfill. The properties of hydrophobic concrete enables a cost efficient process for reuse of it in a future project. (Hycrete in S. Cal)
Cons
Hydrophobic concrete has a lengthy installation process. It can only be achieved under certain weather conditions. Generally the concrete has to be poured on dry, above freezing days. Leaks in hydrophobic concrete often occur, which are problematic where there is restricted accessibility. Cracks often expand causing the concrete to deteriorate.
Some other cons to hydrophobic concrete, comes from the application process. When applied as a coating, it will only penetrate up to 12 inches into the material. Also, the coating process is extremely labor-intensive. So if the structure is thicker than 12 inches, or it is a large project, one would have better results applying as an admixture.
Another con to using this crystalline technology to produce hydrophobic concrete is that it will only work when water is present. If you try to use it as a coating without wetting the surface before hand, it will not work.