Waterproofing of Concrete
Often when a discussion turns to watertight concrete, we think strictly in terms of hydraulic structures. In reality, there are many other applications of concrete in which permeability plays an important part in the performance picture. For example, virtually every home basement is a job in need of watertight concrete. In fact, concrete must be impermeable for nearly every below – ground application. Above ground, too—as in parking garage floor slabs—concrete must often withstand water or vapor pressure.
The term watertightness is often tossed about rather loosely. Sometimes it is used in reference to the ability of concrete to withstand hydrostatic pressures. At other times reference is made to the ability to withstand the passage of water vapor. Naturally, it is more difficult to achieve a concrete that successfully withstands the pressure of a liquid than one that resists the passage of bothersome amounts of water vapor. Unless otherwise noted, we will refer in this article to the resistance to hydrostatic pressures.
While the structure is still on the design table, the ground work should be laid for watertight concrete. Joints must be carefully located and waterstops indicated. If joints are incorrectly positioned, free cracking will occur, and waterstops will be of no value.
The lay of the land will also have an important bearing on concrete watertightness. If possible, the structure should be located where natural drainage will carry water away from the structure. Drain tile and other water diversionary techniques also should be employed.
In effect, the structure should be designed as a vessel or tank with careful attention paid to the maintenance of its structural integrity and minimization of the buildup of hydrostatic pressures.
Concrete mix design
Even if the plans are properly prepared to provide watertight construction — correct jointing, use of waterstops, adequate thickness, and proper reinforcement—the structure will be permeable if the concrete itself is of poor quality.
First, the concrete ingredients should be of uniformly high quality. This means sound, well graded aggregates held together by an impermeable, hardened cement paste. The bearing that aggregate quality has on the impermeability of concrete is multi-faceted. With a well –graded aggregate, the amount of cement paste, often the culprit in leaky concrete, is minimized. Also, the resulting concrete is easier to handle and consolidate. The aggregates must be sound and inert to obviate popouts or alkali-aggregate reactivity which would open passageways for water through the concrete. The aggregates themselves should have a low absorption factor and they should be reasonably free of organic matter.
Waterproofed portland cements are available which are customarily made by adding a small amount of calcium, aluminum, or other stearate to the cement clinker during final grinding.
In areas subject to chemical attack or reactions between cement and aggregates a sulfate-resistant or low-alkali cement should be used. Type II portland cement has greater resistance to sulfate attack than Type I and Type V has even greater resistance than Type II. Portland – pozzolan cement, a blend of pozzolan consisting of silicious and aluminous material and ground portland cement clinker, although developing strengths more slowly during the first three months than Type I portland cement, is often stronger and more watertight after that time.
A number of “waterproofing admixtures” have been marketed over the years. These admixtures include the air-entraining agents, gluconates and lignosulfonates, finely divided pozzolanic materials, and fatty acids of the stearic acid group.
The billions of air bubbles found in air-entrained concrete, being distinct entities without any interconnections, theoretically set up a wall which renders percolation of water through the concrete mass more difficult. However, it is difficult to prove this in tests. Air entrainment does greatly suppress freeze/thaw – induced disintegration and for this reason at least it should be used in all exposed applications.
Water-reducing agents facilitate watertight concrete by making possible a concrete mix of exceptionally low unit water content and yet one which has sufficient workability to be thoroughly consolidated.
As portland cement hydrates, one of the by-products of the chemical interaction of the cement and water is hydrated lime (calcium hydroxide) to the extent of about 7 to 17 percent. This chemical, being soluble in water, is easily leached out of the concrete mass, there by undermining its watertightness. The use of finely divided pozzolanic materials such as fly ash, shales, and diatomaceous earth helps to minimize the amount of hydrated lime in concrete.
High molecular weight saturated fatty acids of the stearic acid group are used as an aid in achieving watertight concrete. These powders, usually used as a liquid or paste dispersion, are intermixed and entrapped throughout the cement paste to result in a counteraction to capillary attraction. They react with the free lime released during cement hydration to form calcium stearate, palmitate, and myristate. As a rule, these waterproofing admixtures are effective against water vapor only—not in pre venting the passage of water under pressure.
A concrete mix design to be watertight should (1) have a low water/cement ratio; (2) have as low a unit water content as possible; (3) have the optimum amount of entrained air; and (4) incorporate high-quality materials appropriate for the application.
For a truly waterproof barrier an impenetrable mass is needed. Any substance such as concrete that has as part of its setting process the release of one or more of its components through, for example, evaporation or bleeding, has a disadvantage in achieving maximum density. Bleeding results in capillaries through the cement paste and just as these channels permit the escape of water from the concrete mass, they will later allow the infiltration of water into and/or through the mass.
Handling and placing practices must be such that segregation is avoided. The concrete should not be allowed to free-drop more than 6 feet. Vibration should not be used to move the concrete horizontally. In high members, the concrete should be placed in lifts with access to the bottom portions of the form work through openings in the sides of the forms.
It has already been mentioned that construction or cold joints should be avoided whenever possible. Where it is impossible to do so, care should be taken to minimize bleeding which would weaken the surface of the concrete and therefore its bond potential. If laitance forms, the surface should be cut back by grinding or by acid-etching to sound, high-strength concrete. When concreting is resumed, the surface should be clean, moist, and free of contamination.
Curing is highly important in achieving watertight concrete. Often it can mean the difference between a troublesome leaky structure and one that is impermeable. The concrete should not be subjected to hydrostatic pressures before the curing period has been completed. Watertight concrete must be cured at least seven days and preferably 14 days or more. It is important that curing be started as soon as possible and be continued without interruption.
A number of treatments have been used to improve watertightness after concrete has hardened. In general, these treatments are most effective when applied to the face of the concrete member directly subjected to the hydrostatic pressure rather than to the inside face. This often entails extra work, for example, excavation of dirt around a foundation, but usually the results are more satisfactory if this is done. In some cases, it is the only means where by leakage can be stopped or appreciably stemmed.
One protective treatment is the cementing of a fabric membrane to the exterior surface of the concrete with hot asphalt. A bone-dry condition of the concrete is necessary when the membrane is to be applied. A membrane will seal porous concrete and it should be flexible enough to bridge fine cracks.
Another effective waterproofing treatment is the application of a mortar incorporating metallic aggregates and an oxidizing catalyst. When mixed with water, controlled enlargement of the metallic aggregates takes place, resulting in a dense, crack-free barrier to penetration by water. Another method involves the use of bentonite expanding clay which can be applied in bulk as a cove where the footing and wall of a building meet, or is available sandwiched between cardboard panels which are cemented or stapled to exterior basement walls. Bentonite is said to stay active indefinitely, expanding each time it is wetted to provide a seal against water.
There is not room in this article to include all of the treatments which have been developed to waterproof concrete. It must be kept in mind, however, that no applied treatment will be able to compensate for large random cracks and other problems arising from poor design and/or construction practices.
In summation, to achieve watertight concrete the following points should be heeded: (1) use high quality concrete ingredients appropriate for the application; (2) prepare a mix design that will result in sufficient strength and work ability; (3) include entrained air; (4) try to channel water away from the structure; (5) design the structure correctly; especially in regard to joints; (6) adhere to recommended handling and placing practices; (7) thoroughly cure the concrete; and (8) if needed, apply a high quality waterproofing treatment to the exterior walls subjected to water vapor or hydrostatic pressures.
Afzir engineers provide you the best solutions for waterproofing of concrete structural & non-structural elements including concrete tanks, retaining walls, pools and etc.