There appear to be confusion and opinions when it comes to preventing water penetration through concrete. Terms such as “waterproof,” “watertight and “water-repellent” seem to be used interchangeably. However, these words can describe some very different conditions. Susanne M. Papas, Senior Associate, Wiss, Janney, Elstner Associates, Inc, Illinois, USA, demystifies the use of admixtures in concrete mix and chemical barriers applied to the hardened concrete surface to prevent water penetration.
Concrete that is well proportioned has a low water/cement (w/c) ratio, is well cured, is of durable material and has low permeability. It will have resistance to liquid water penetration but it cannot totally prevent water from passing through it. Water can and will pass through concrete due to two main mechanisms:
• Capillary absorption
• Hydrostatic pressure
Capillary absorption is the movement of water through the pores and micro cracks in the concrete itself. Capillary absorption can occur when the exterior surface of the concrete is wetted, such as rain hitting a vertical exterior wall of a building. Hydrostatic pressure occurs when water moves through concrete due to pressure differences. Examples of water moving due to hydrostatic pressure are groundwater surrounding an underground vault or tank, or water ponding on the roof of a structure.
Systems that provide reduction in the passage of liquid water through concrete fall into two categories:
• Damp proofing
Waterproofing systems prevent the passing of liquid water under hydrostatic pressure. Damp proofing systems resist the passage of liquid water when hydrostatic pressure is absent. Neither system will prevent water from moving through larger cracks (macrocracks) in concrete.
Cold joints form in concrete when it is cast against existing hardened concrete. Cold joints can also occur during casting of concrete where the time between pours was excessive or if new concrete is poured without preparing the existing concrete.
Concrete shrinks as it cures, so if a new slab or section of concrete is poured against an older or existing section of concrete, the joint between the two will appear initially to be filled; however, as the new concrete cures, shrinkage will leave a space between the two slabs or sections of concrete.
Microcracks are fine cracks up to 0.02mm (0.74mil), typically around 0.004mm (0.16mil). Microcracks include most shrinkage cracks as well as some settlement cracking. Mesocracks are typically 0.1mm (3.94mil). Macrocracks are larger still, ranging from 0.15-0.3mm (5.9-11.8mil)
The degree of shrinkage is dependent on several factors—cement content, w/c ratio and aggregate content. For example, for a concrete with a w/c of 0.5 and an aggregate content of 70%, shrinkage would be approximately 0.0008 in./in. (0.02 mm/mm) of concrete. For a 25-foot slab with this w/c and aggregate content, shrinkage would be 0.24 in. (6.1 mm).
Gaps in concrete systems due to cold joints can therefore be significant sources of water intrusion. Experience shows that neither damp proofing systems nor waterproofing systems have been effective in the prevention of water moving through cold joints. There are two basic methods for preventing or stopping water from passing through concrete:
• Admixtures integral to the concrete itself
• Barriers applied to the exterior surface of concrete
The American Concrete Institute (ACI) classifies admixtures that control moisture or water movement through concrete as permeability-reducing admixtures (PRA). There are three main types: hydrophobic chemicals, finely divided solids, and crystalline admixtures.
Hydrophobic chemicals represent the largest group of these admixtures. This group consists of materials made from long-chain fatty-acid derivatives, vegetable oils, and petroleum-based products. Hydrophobic chemicals create a water-repellent layer along the concrete pores, but the concrete pores remain physically open. These products though useful for damp proofing, will not prevent the transmission of water through concrete when hydrostatic pressure is present.
The most common type of hydrophobic admixture is a stearate-based system such as calcium stearate. Calcium hydroxide produced by the hydration of Portland cement reacts with the stearate admixture to form insoluble calcium stearate and water. Hydrophobic admixtures also include polymers, which coalesce within the concrete to form a water-repellent film. While polymer admixtures effectively block the concrete pores, they lack the ability to bridge cracks within the concrete and, therefore, can allow liquid water to pass through the concrete. Integral petroleum-based products do not react with the cement in the concrete system and like polymer admixtures, have limited or no ability to bridge cracks, again allowing liquid water to pass through the concrete.
The second type of PRA is finely divided solids. Some are chemically inert, and some are chemically active. This type of admixture works by filling the pores of the concrete and restricting the movement of water through the concrete. Examples of these are talc, bentonite clay, lime, silicate, and colloidal silica. This type of admixture works by increasing density or filling the voids in the concrete; however, they only provide damp proofing or water-repellency rather than completely blocking the passage of liquid water through concrete. This type of admixture should only be used where hydrostatic pressure is absent.
The third type of PRA for concrete is a crystalline admixture. These admixtures, like the stearate-type admixtures, react with the calcium hydroxide produced by hydration of the Portland cement. Crystalline admixtures are commonly provided in a mixture of cement and sand. This type of admixture reacts with the water and the calcium hydroxide in the hydrating cement to form calcium silicate hydrates and pore-blocking precipitates. This type of admixture has passed the plastic state. If water and calcium hydroxide are present (concrete is not fully carbonated), they can provide some level of microcrack bridging and capillary-pore blocking for many years after initial placement of the concrete. These admixtures will resist water penetration when some hydrostatic pressure is present but cannot bridge meso- and macro-cracks that form in concrete with time.
Barriers for Concrete Surface
Barriers applied to the surface of concrete can contain similar compounds to those used as integral admixtures. Barriers can consist of hydrophobic materials such as stearates, pore-blocking materials such as bentonite, and crystalline admixtures such as sodium metasilicate (water-glass systems). The performance issues with these admixtures are similar when applied as a barrier as when used integrally in concrete. They can provide damp proofing and some degree of water-repellency. However, the same limitations—the absence of hydrostatic pressure and limited crack-bridging capacity also apply.
The key to a successful water-protection
It is essential to know whether the substrate will be exposed to hydrostatic pressure or not and then selecting an appropriate system as integral PRAs can affect properties of plastic concrete. Levels of air entrainment, setting time and water reduction can be increased or decreased, depending on the admixture used.
These types of admixtures can also affect concrete finishing, mix consistency and scheduling of placement. Once hardened, PRA concrete can exhibit changes to compressive strength, freeze-thaw resistance, and shrinkage. It is critical to review product data information as well as to perform trial batches with a selected PRA to ensure that the selected concrete mix will meet expectations. Working directly with a PRA admixture manufacturer can provide needed information on addition rate, order of addition, mixing time, and compatibility with other materials in the cementitious system.
Success of barrier PRA systems also requires careful review of product data sheets to determine if a given system is appropriate for a given application. Surface preparation is a key factor in the success or failure of a PRA barrier system. Strict adherence to the manufacturer’s instructions on surface preparation, coating film thickness, cure time before application of subsequent coats, and ultraviolet protection (if needed) are necessary for proper system performance. In addition, barrier systems may require periodic reapplication to perform properly.
Both integral permeability-reducing admixtures and barrier permeability-reducing admixtures can have an effect on the performance of subsequently applied membrane systems. The surface preparation including the potential need for a surface applied primer or bonding agent may be required for good performance of the membrane.
It is critical that communication with the membrane vendor include the type of permeability-reducing system used as well as the approximate age of the concrete system. Mock-ups should be considered to determine if the permeability-reducing system is compatible with the proposed membrane system.
Reproduced from the original published in Interface Journal of RCI Inc., USA