The Science of Concrete
While concrete has been around for thousands of years – the building material of choice for ancient Egyptians and Romans – the quest to make concrete stronger, more resilient and longer lasting continues.
Providing the highest quality concrete for our commercial and residential construction projects is paramount to Custom Concrete. We work with reputable, long-standing ready-mix concrete companies like Shelby Materials to provide us with the best product for our foundations.
Chris Wolf, director of technical support for Shelby Materials, weighs in on “The Science of Concrete” this month.
Think of concrete as a very slow setting, two-part epoxy … and by “very slow,” I mean close to four weeks. The sand and stone are just fillers; the action is all about the water. When we make concrete, we add water and cement in the correct proportions. If the job requires more fluidity, we add water, but we don’t always add cement. Adding water is like adding more hardener than the epoxy needs for balance. The qualities are reduced.
The same thing is true when we fail to take steps to keep the water in place during curing. The cement is bound by gravity and never leaves, but the water can evaporate. Some days are cloudy or humid so the water leaves very slowly. Other days are dry, sunny and windy. On those days, the crews are adding water as fast as they can. In my position, I hear owners ask, “Why is my concrete different than everyone else’s?” Water is the “why.”
There is one more problem. The two-part epoxy of cement and water is a chemical reaction. Like all chemical reactions, it is dependent on temperature. So colder is slower, and hotter is faster. In summer, hydration is very fast. We often have full design strength in a few days. However, crews struggle to keep up with the setting of the concrete. A good foreman has extra man power to keep up with demands of the concrete. When we make summer concrete, we add ingredients to help them like fly ash and admixtures. We cannot reduce the cement content or that would upset the balance of the “epoxy.” Fly ash is a very effective retarder. It is also very cementitious. Fly ash is a waste product of our coal fired power stations. Power stations are very efficient, so the fly ash we receive is very consistent. Using fly ash is environmentally friendly and beneficial.
The big problem is in the winter. In winter, the hydration is very slow, and thus, strength gain is slow. Set time is slow. The need for curing and protection is critical. The concrete is “green” longer. It is more likely to lose water to dry winter air. Furthermore, winter includes freezing rain, ice, and snow. All of these damage concrete that is not yet strong. As a producer, we have sand heaters, we remove fly ash, we add more cement, and we have accelerators. All of these tools help the crews give the concrete a head start.
The thing to remember is that water is part of the strength of the concrete. We must take steps to keep water in the concrete so that it can hydrate with the cement to give us the strength and durability we paid for. Curing is critical.