A friend’s mother – let’s call her Helen – was about to have work done on her condo balcony as part of a major restoration. Helen didn’t understand what was about to happen, nor did the friends she talked to. The “construction people” would explain things in technical terms with words like “anodes” and “dissimilar metal corrosion” but all she wanted was a basic understanding.

The average person, who does not deal with concrete repairs on a regular basis, will have little insight into the basics of concrete. Nor will they understand the problems that can occur in a “cast in place” concrete system or how repairs are made to such a system.

Concrete is a hard, strong building material made by mixing a cementing material (Portland cement) and a mineral aggregate (sand or gravel) with enough water to make the cement set and bind the entire mass. Think of the cement as the glue or paste. The aggregate gives strength and the water starts the chemical reaction. When the water evaporates the concrete hardens or “cures”. The standard time for concrete to reach its full cured strength is 28 days. Concrete may be poured into wood forms (or frames) to create different shapes and sizes. For example, you may have a concrete balcony at your condominium that has been formed and poured into a shape 10-feet wide by 20-feet long by six inches deep. This is referred to as “cast in place” concrete.

Reinforcement bars – commonly called rebar – are steel bars that come in different diameters. Rebar has an irregular surface that allows the concrete to adhere to it better. Rebar is strategically placed in concrete to increase the strength of the concrete slab. The combination of concrete and rebar is called reinforced concrete.

A reinforced concrete slab has an unfinished surface, which is not waterproof. Think of concrete as a hard sponge. Moisture can soak below the surface, but will usually evaporate after the source of the moisture is removed and therefore doesn’t normally present a problem. However, problems arise when the moisture, along with salts from the air, penetrate the surface of the concrete slab and reach the reinforcement bar. Fasteners for railings, screen enclosures, shutters and surface cracks allow additional moisture to penetrate the slab. When rebar gets wet it will form rust. The salts accelerate the corrosion further, forming a brittle coating on the steel. As this coating of rust grows it expands and causes the concrete to crack, a process known as “spalling.” This allows more moisture and salts to reach the rebar and causes more rust and expansion, leading to an accelerating cycle of deterioration of the reinforced concrete system and the loss of strength.

To repair this situation, the concrete is cut, chiseled or chipped and removed to expose all of the rusted rebar. Enough concrete is removed to ensure all of the deteriorated rebar is replaced or repaired. At this point, chemicals can be applied to the steel to help reduce future corrosion. New concrete is then poured in place to replace the concrete that was removed for the repairs.

A good way to prevent moisture and salts from migrating through the slab to the steel reinforcing bar is to install a waterproofing system on the surface and edges of the concrete. A good waterproofing system will add years of life to a reinforced concrete slab. One common type of waterproofing is a fluid membrane that bridges cracks and seals holes, preventing moisture and salt intrusion. Regardless of whether the concrete is waterproofed or not, it is best not to install anything that will retain moisture, such as carpet or planters.

There’s much more to be learned about the science of concrete repairs and maintenance. Many books, articles and publications are available, depending on your interest level. But knowing these basics will help you the next time a concrete installation or repair project comes up for discussion.

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Author : William Clarke
Company : Jenkins & Charland Inc.
William Clarke is the restoration project manager for Jenkins & Charland Inc.

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