Concrete for Condos
Bob Burns, P.E., R.S.
Concrete used outdoors can provide an extremely durable, versatile and attractive component to any condominium’s landscaping scheme. In our reserve studies, we typically assign it a thirty to forty year useful life. It’s a long-range component in your capital reserve fund plan. If you have concrete driveways, patios, walks on your property you know, however, that it does sometimes need some help in achieving that two score and ten goal. Concrete is an engineering amalgam of Portland cement with fine (sand) and coarse (stone) aggregate. The cement paste bonds to the aggregate and bridges across it. It’s that bridging action that provides the strength we all recognize. Where concrete can get in trouble outdoors though is when water is allowed to penetrate the surface. That saturates the coarse aggregate triggering the damaging freeze and thaw act. It’s that movement that triggers the cracking, opening the door for more water entry and accelerating damage. One way to combat the process is to include entrained air admixture to the concrete at the batch plant. The entrained air admixture provides little bubbles in the cement paste that allow the aggregate to expand without damaging the paste. For durability, it’s essential that the concrete mixture have a low water-cement ratio when placed. Once the concrete is in service, epoxy based sealers can be used as added protection against water penetration to the aggregate. Cracked or heaved sections of condominium concrete walks will need to be removed, of course, and new sections formed up to replace them. Make sure the soil sub-base is well compacted before pouring the concrete. If the work is done in hot weather, wet the sub-base to prevent rapid loss of the mix water into the soil. In hot weather, the concrete should be misted occasionally as it cures to prevent rapid hydration of the mix water to the air. Concrete is fussy about curing. It takes about 28 days to reach its full strength. Ideally, gradual loss of mix water during hydration (curing) is what we like to see happen. Driveways require essentially the same precautions with an additional caveat. They need help accepting the weight of vehicles. Wire mesh is the typical reinforcement used here. Reinforcement is also a good idea for walkways. Control joints are cuts or slots provided in new concrete work to weaken the concrete locally. This tells the concrete to crack minutely beneath the cut / groove above as it expands during hydration. Otherwise it will crack randomly and deeply where it chooses to, creating an unacceptable piece of work. If your existing concrete is pitted and worn, it can be re-surfaced. This amounts to pouring a thin application of topping material over the damaged surfaces. The best preparation consists of scarifying the existing surface, removing of all loose material, and then etching the old concrete with hydrochloric acid. The finish topping coat can provide a skid proof surface suitable for pedestrian traffic. Moss can be removed by washing the area with a solution of household bleach and water. Now all of this is common sense technology to competent contractors. But a little friendly conversation along these lines beforehand will let him know that you expect him to make use of that common sense for your project.
