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Why is exterior waterproofing important for concrete and masonry surfaces? What product options do I have for waterproofing? Are there any tips for selecting the right products?
J.C., Kansas City, Mo.
Due to their exposure to the elements, exterior concrete and masonry surfaces are particularly susceptible to the damaging effects of water. They are porous and absorb water to varying degrees. As water enters the surface, it sets up conditions that can result in a variety of problems including deterioration of the surface caused by freeze-thaw and efflorescence; the loss of structural strength caused by rusting of rebar, nails and fasteners; mold and mildew growth in interior wall cavities; and damage to interior drywall, carpeting and flooring.
The absorption of water, and therefore water damage, can be prevented by applying waterproof coatings and can be significantly reduced by applying water-repellent sealers. But, what’s the difference
between waterproofers and water repellents?
Waterproofers vs. Water Repellents
Waterproofers are opaque coatings with similar application properties to paint. They form a relatively thick film on the surface to provide a barrier against water penetration. Generally white in color, they change the appearance of the surface to which they are applied. Many can be tinted for a more decorative appearance, a popular choice on interior basement waterproofing jobs. Elastomerics and basement waterproofing paints are good examples of waterproofers.
A water repellent is a transparent coating that allows the natural beauty and texture of decorative concrete to show through. It penetrates the surface to reduce the porosity and create hydrophobic, or waterrepelling, conditions that reduce the amount and depth of the water absorption.
It provides protection without changing the texture or hiding the underlying substrate of Concrete Masonry Units (CMU) such as light, medium and normal weight block, smooth-face block, split-face or fluted block, burnished or ground-face block, and slump block; concrete substrates such as cast-in-place, tiltup panels, precast, slabs and stucco; and masonry surfaces such as concrete brick, clay brick, stone and brick veneer.
Water repellent has different application properties than paint. Its coverage rate is significantly less – only 50 to 200 square feet per gallon – and it is spray applied with large tip sizes at very low pressure. Examples include hydrophobic micro-emulsions, silicones and siloxanes. Water repellent acts as a barrier between the substrate and liquid water, but it also prevents moisture from building up in the concrete, by allowing the moisture to escape through the water repellent and the concrete to the outside.
Selecting the Right Product for the Job
When it comes to selecting the right waterproofer or water-repellent product for the job, contractors have a myriad of choices. Selecting the wrong product could result in moisture intrusion or potential job failure. Either way it can be an expensive mistake to go back and correct.
First, determine whether you want to use an opaque waterproofer or a clear water repellent. Next, determine the type of concrete or masonry surface to be coated and measure the porosity of the surface.
Knowing whether the surface is very porous, porous or dense will help in product selection. An easy method of determining surface porosity on vertical concrete or masonry surfaces is with a low pressure tube test, also known as a rilem tube test.
Test the Porosity of the Surface
A rilem tube test simulates a 49 to 95 mph wind-driven rainstorm on the concrete surface. It involves attaching a rilem tube, a plastic tube that looks like a test tube with a reservoir at the bottom to collect water, to a dry concrete or masonry surface with putty and filling the tube with water. The time it takes for the water to drain from the tube indicates the porosity of the substrate. In fact, many water-repellent manufacturer warranties require rilem tube testing both before and after coating application.
To perform a rilem tube test, select a variety of test locations around the surface you are testing, including both the concrete block and mortar joints. Attach a rilem tube to the surface with putty. Roll the putty into a snakelike piece approximately 1/4 inches in diameter and 6 inches in length to fit around the brim of the tube. Firmly press the tube with the attached putty to the surface forming a watertight seal between the tube and the masonry.
Next, fill the tube with water and measure three minutes’ worth of water loss. Most tubes hold 5 mL of water graduated in 0.1 mL increments from 0 to 5 mL. If the water starts to immediately fall from the tube and soak into the surface, it is a candidate for a water protective sealer. The more porous the surface is, the more aggressive a water repellent it requires.
If the tube holds the water for three minutes, it means an uncoated block is very dense. A coated block that loses no water in this time has excellent protection already and does not require additional water repellent. There is, however, a benefit to treating these dense surfaces with a water repellent for the added protection it can provide. When water hits the surface, it wets the concrete, which can lead to staining, erosion or rusting. By treating the concrete and making the surface hydrophobic (water-repellent), these potential problems can be avoided.
The chart below includes information about the measuring surface porosity rating on both coated and uncoated surfaces, using rilem tube testing. Once you determine the whether the surface is very porous, porous or dense, it will help you in selecting the right water-repellent product for the job and the best way to protect your exterior concrete or masonry surface.
Liz Krauthammer is the director of high performance coatings for Rust-Oleum Corporation.