Classification and mechanism of action of waterproofing agents

When people encounter the problem of water permeability in concrete, there will always be a lot of questions and solutions. The terms "waterproof", "watertight" and "hydrophobic" are often used interchangeably, but in fact, These words can describe some very different situations. The purpose of this paper is to focus on incorporated water repellents used to enhance the impermeability of concrete and water repellants that can form chemical barriers on hardened concrete surfaces. Membrane systems such as membranes or coatings for physical barriers are beyond the scope of this paper.

Concrete with a low water-cement ratio (w/c) and a good mix ratio can become a material with excellent durability after adequate curing, with low permeability and resistance to penetration by liquid water; nonetheless, such concrete Nor is it always possible to block water from passing through it. Water is able to pass through the interior of concrete due to two main factors: capillary adsorption and hydrostatic pressure.

Capillary adsorption is a phenomenon in which water migrates through the pores and microscopic cracks of concrete itself. The general size of microscopic cracks is about 0.004 mm, and the maximum size can be 0.02 mm. Microscopic cracks include most of the shrinkage cracks and part of the settlement cracks. The mesoscopic crack size is generally around 0.1 mm. The macroscopic cracks are larger, ranging in size from 0.15 to 0.3 mm. Halvorsen believes that due to surface texture and other factors, the smallest crack that the human eye can detect is 0.13 mm, which is right between mesoscopic and macroscopic cracks.

When the outer surface of concrete is damp (such as when rainwater hits the vertical outer wall of a building), a capillary adsorption effect occurs. The difference in pressure creates hydrostatic pressure as water passes through the concrete. Groundwater around the cellar or storage tank, roof water, etc. will cause water movement due to hydrostatic pressure.

There are two broad categories of methods for reducing the penetration of water in concrete: waterproofing and moisture-proofing. Waterproof systems impede the passage of water under hydrostatic pressure; moisture-proof systems prevent the penetration of water under non-hydrostatic pressure. But neither can stop the penetration of water in large cracks (macrocracks) in concrete.

When fresh concrete is poured over hardened concrete, cold joints are formed in the concrete; in other cases, cold joints sometimes occur.

Concrete shrinks during curing, and because of this, when new concrete is poured over old concrete, the joint between the old and new concrete appears to be dense at first; void. The degree of shrinkage is related to several important factors: cement amount, w/c ratio and aggregate amount. For example, for concrete with a w/c ratio of 0.5 and an aggregate content of 70 %, the concrete shrinkage is approximately 0.02 mm/mm. A 25 foot (about 7.62 m) long piece of concrete will shrink by 6.1 mm using the above w/c ratio and aggregate amount. Therefore, gaps caused by cold seams are an important channel for water intrusion. Experience has taught us that neither damp proofing systems nor waterproofing systems are completely effective in preventing water penetration through cold seams.

Here are two basic methods for preventing water from penetrating concrete: admixtures (impermeables), which are integral to the concrete itself, and barrier-type water repellents applied to the concrete surface.

1 Admixture (impermeable agent)

Impervious agents (PRAs) in concrete admixtures control the movement of moisture or water through the concrete. The American Concrete Institute (ACI) divides them into three broad categories: hydrophobic water repellants, fine dispersions, and crystalline admixtures.

Hydrophobic water repellents are the largest group of such impermeable agents. This group includes materials made from long-chain fatty acid derivatives, vegetable oils, and petroleum by-products. These hydrophobic water repellents can form a hydrophobic film along the pores of the concrete, but the pores of the concrete are still physically open. These products are very useful for damp proofing works, but still do not stop the migration of water in the concrete under hydrostatic pressure.

The most commonly used hydrophobic waterproofing agent belongs to stearic acid substances, such as calcium stearate, and its mechanism of action is shown in the following formula.

Ca(OH)2+RCOOH→Ca+COOR-+H2O

Portland cement hydration produces calcium hydroxide, which reacts with stearate to form insoluble calcium stearate and water. Hydrophobic water repellents also contain polymers that interpenetrate with the concrete to form a continuous hydrophobic film within it. Polymeric admixtures can plug the pores of concrete, but they do not act as a "bridging crack" in concrete cracks, so water can still penetrate through the concrete. Petroleum-based products do not react with concrete, like polymeric admixtures, they cannot bridge cracks, and water is able to pass through the inside of the concrete again.

The second type of PRA is a fine dispersion. Some of them are chemically inert and some are chemically active. Admixtures such as talc, bentonite, lime, silicate substances, and silica sols restrict the movement of water by filling the internal pores of the concrete. Finely dispersed materials increase the density of concrete and fill the pores inside the concrete; however, they are limited to moisture-proof or hydrophobic, and cannot completely block the seepage channels inside the concrete. They can only be used in the absence of hydrostatic pressure.

The third concrete PRA is a crystalline admixture. BASF's Rheomac 300D and IPA's Ipanex are crystalline admixtures. These admixtures, like stearic acids, react with the calcium hydroxide produced by the hydration of Portland cement. Crystalline admixtures are usually made into mixture products with cement and sand. It can react with water and calcium hydroxide in cement to form calcium silicate hydrate and pore-clogging substances. It can continue to react after the concrete plastic state. As long as water and calcium hydroxide are present (the concrete is not fully carbonized), they can continue to function to bridge microscopic cracks and plug concrete capillaries years after construction. Under certain hydrostatic pressures, such admixtures can resist the penetration of water; however, they cannot bridge mesoscopic and macroscopic cracks in concrete over time.

2 Barrier water repellent for concrete surfaces

Barrier water repellents for concrete surfaces contain compounds similar to those used in concrete admixtures. Barrier water repellents are composed of hydrophobic materials such as stearate, pore blocking materials such as bentonite, and crystalline admixtures such as sodium metasilicate (water glass system). These admixtures perform similarly when used in surface barriers and when incorporated into concrete. They are moisture resistant and somewhat hydrophobic; nonetheless, they suffer from the same limitations - they can only be used in the absence of hydrostatic pressure, and they have limited ability to bridge cracks.

3 Summary

The key to a successful waterproofing solution is to figure out if the foundation is under hydrostatic pressure, and then choose the right waterproofing system.

The addition of PRA (impermeable agent) will affect the performance of plastic concrete. Be sure to carefully browse the data and information of the product and conduct trial mixing to ensure that the concrete mixture can achieve the expected performance. Manufacturers of PRA admixtures can generally provide us with information about the dosage, order of addition, stirring time, and compatibility with other materials in the gelling system.

The success of a barrier waterproofing system also requires careful reading of the product data sheet to determine whether the system solution is suitable. Surface preparation is a key factor in the success of a barrier waterproofing system. Follow the product instructions, strictly control the surface treatment process, coating thickness, curing time before the next coating process, and UV protection if necessary.

Both externally mixed impermeable and barrier water repellents have an impact on the performance of the subsequently applied coating system. Certain high-performance coatings such as polyureas require rigorous concrete surface preparation, including the possible need for surface treatment primers or binders. It is necessary to communicate with the coating supplier in time, including the type of impermeability system and the service life of the concrete structure, and establish a relevant model to determine whether the impermeability system is compatible with the coating system.