How often have you been dealing with a qualified contractor and when they explain their proposal, you quickly realize you have no idea what they are talking about because what is basic knowledge to them is not basic to you. Sometimes it is the most common of items that we just take for granted or have given little thought to until a problem arises and then we realize that there is a serious gap in our understanding. Concrete is one of those items that we see all the time but many people have no idea what it is made of or how it is made.

The contractor starts throwing words out there like aggregate, blistering, and efflorescence as if these are words on a grade two spelling test and you are too embarrassed to say anything so instead you smile and nod. Concrete comes in all kinds of varieties depending on its application. Here is a brief explanation of what concrete is all about so hopefully it will add to your toolbox of knowledge when dealing with maintenance items.

First of all, what is the difference between concrete and cement? Concrete contains cement. Cement is mixed with water, sand and aggregate, and when it dries it becomes concrete. Aggregate is a fancy word for those stones you see in the concrete. It is called aggregate because they are various loose pebbles added to the mix. Aggregate means a collection of particles into a larger mass.

Cement is generally made from lime, iron, silica and alumina. These ingredients are heated at 1300-1550 Celsius until they fuse together. It is referred to as clinkering temperature. This new material is called clinker. The heating transforms the raw materials into a new chemical compound, much the same way burning wood transforms it into coals. This new material is then ground down into a powder called cement. Changing the ratios of the materials, the temperature, and the duration of the heating produces different types of cement.

In addition to sand, aggregate (small irregular shaped stones), cement and water, other materials such as fly ash and slag cement are added to improve the properties of the concrete but I won’t go into the specifics here.

When the concrete is poured, it contains as much as 20% water. Consider that it dries by allowing the water to evaporate. This means that where there used to be water, there is now air. Consequently concrete is very porous. Water can leach through it causing efflorescence, and oil spills will leak into it creating permanent stains.

In these first two pictures, a generator sprung a leak which went unnoticed and oil leaked through the concrete into the parkade below, dripping onto cars. Oil continued to leak through for several months after the leak was repaired and cleaned up. If oil can leak through, just imagine how easily water can.

This next picture shows the ceiling of a parkade with extensive leaching of particulates. The building burned to the ground during construction and the concrete absorbed a lot of debris as it super heated from the fire. The walls and ceilings were painted in an effort to prevent the leaching but it was to no avail. 6 years later it was still coming out.

An engineering firm was called out to check the concrete to ensure it was not going to fail and the report came back saying the structure was sound. In the coring sample, you can clearly see the aggregate.

Structural concrete is given additional strength by adding rebar which are long steel rods. In older buildings you may see red staining around cracks. This may mean that the rebar is exposed and is rusting. Over time the rebar may fail, leading to a rather serious problem.

Often when you walk through a parkade, you will see a white powder on the concrete wall (see picture at top). This is that efflorescence I mentioned earlier. As the water evaporates, it pushes out minerals in the concrete. If you have an ongoing problem with efflorescence, there is likely a problem with the membrane on the other side of the concrete letting water in. It is often completely harmless but it does look unattractive.

This link is a good glossary of concrete terms.

Glossary of Concrete Terminology

And those are some of the cold hard facts about concrete.