May 25, 2001

10 tips for maintaining concrete

Concrete is a significant investment — an investment that’s constantly under attack from the elements. With the proper care and upbringing, concrete (like people) can live a long and fruitful life. Preserving and protecting this valuable asset today will save you time, money and headaches in the future.

Here are 10 tips to help you maintain your concrete investment:

1. Concrete requires good genetics, in the concrete industry we call it mix design. The life span and resistance to degradation, as in all life is dependent on the early formative period.

2. Proper curing is required to achieve the maximum design characteristics of the concrete mix. New research has shown that the previous rule of thumb — concrete does not need a curing method if the temperature is 70 degrees F and 70 percent relative humidity with low wind — is incorrect.

Research done by VTT engineer Erika Holt, and presented at the spring American Concrete Institute convention, clearly proves that concrete requires 100 percent humidity or curing in the first two hours and after to prevent early shrinkage cracks. High wind, temperature or lower humidity exacerbates the condition and requires immediate action.

In addition, the soon to be published ACI 308 Guide to Concrete Curing defines three distinct curing periods.

Initial cure: Actions taken after the initial screeding to prevent rapid water evaporation. Evaporation reducers can be used at this point or fogging. (Evaporation retarders should NOT be used as a finishing aid.)

Middle cure: Actions taken after final troweling such as the application of a curing liquid membrane which meets ASTM C-309 or preferably ASTM C1315 (a more stringent specification which is tested for non-yellowing characteristics, chemical resistance and topcoat adhesion).

Final cure: The application of a second coat of curing liquid membrane after the first coat is dry, or polyethylene/polyolefin sheeting, ASTM curing paper, wet burlap or fogging are among the methods used.

Concrete must be cured properly in all conditions to achieve maximum design properties and long standing, crack free durability.

3. The best defense comes from knowing the enemy. Evaluate the design criteria before choosing the type of protective coating. What kind of wear will the concrete endure?

Chemical-spills of acids, alkali or other harsh chemicals.

Mechanical-fork lift, carts, or other vehicles.

Environmental- acid rain, dirt, sand, rain, ice and snow.

4. Water is concrete’s worst enemy. Water sitting on the concrete can stain the surface, allow cracks to form and mold spores to develop making the concrete slippery. Provide for water run off from concrete surfaces. Slope to drain.

5. Eliminate dusting. A common problem in box warehouse stores with concrete flooring. To prevent dusting on an untreated concrete slab, use a penetrating, compliant hardener densifier to eliminate dusting and extend the life of the floor.

6. Concrete in all temperature conditions benefits from proper curing. Freeze-thaw cycles, a concrete hazard, cause the concrete to expand and contract, which can force cracking and leave the concrete exposed to increased water intrusion and eventual spalling.

To combat these conditions, concrete should be sealed after its 28-day cure and before it is open to foot or vehicular traffic. Modern cure and seal products which meet the requirements of ASTM C1315 will both cure and seal the concrete in one application, providing chemical resistance and a excellent base coat for adhesives, paints or coatings. If the concrete is not to be top-coated, a final seal coat should be applied after a proper cleaning. The final coat acts as a wear surface, protecting the concrete and increasing its longevity and appearance.

7. Clean concrete surface immediately after a spill to minimize any discoloration or damage. This includes exterior: fertilizers, leaves, grass clippings, bark molds and tire marks. Clean with a 28-day biodegradable cleaner and rinse thoroughly. This should be done at least every month to keep the surface appearance at its best.

8. The secret to installing a coating or stain properly is a clean surface. Before applying a top-coat or stain, clean the concrete surface with a pressure-washer type system. (Extraction type cleaning systems enable you to clean and seal your concrete in one-day.)

Use a 28-day biodegradable cleaner, allow surface to dry, and then check for water penetration. Water should suck into the concrete immediately. If it does sealing can begin. If there is considerable hesitation, an acid wash is recommended, followed by a baking soda rinse. This process will open up the pores and allow for proper penetration of the stain or high-quality coating. Always consult companies that specialize in concrete coatings for recommendations on proper sealers, water repellents and coatings. Most do-it-yourself products available at retailers are not designed for industrial and commercial markets.

9. Re-coat the concrete every three to five years to protect the surface, more often if traffic warrants it.

10. Work with a concrete maintenance partner. A maintenance partner will help you make the most of your investment and provide protection and maintenance solutions designed to fit the specific needs of your concrete. Your maintenance partner should not only know concrete, but everything involved in preparing, protecting and maintaining your investment.

By keeping these points in mind when pouring, curing, cleaning and sealing concrete, your investment will enjoy a long, healthy life.

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Norm Eckert, CSI, and Jennifer Faller, CSI, are co-principals of Seattle-based ConcreteScience Services Northwest and SurfaceTechnology Systems Inc. For more information on ConcreteScience, call (206) 937-5244 or go on-line to http://www.concretescience.net. For more information on SurfaceTechnology Systems, call (206) 937-9322 or go on-line to www.stripit.com.

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