In sprucing up my house for her 100th birthday, I couldn’t ignore the need for a new driveway. The driveway wasn’t original to the house but according to old photos, it was at least 60 years old. It lasted through Iowa winters and at least two families with teenagers parking on it, bikes going round and round over it, and incessant basketball games. It was made from a nice aggregate and held up well until the past few years. At last, time took an unbearable toll.
I decided to replace it with concrete with less interesting aggregate in it for one reason–cost. Aggregate can double the price of a driveway and as you can see, this is a big driveway.
Besides materials, one concern about a driveway is the slope of it so that water runs down. Our driveway has just barely enough slope because when Main Street was redone, it was made 6″ too high due to an error in reading the instructions. We would need a storm drain if we hadn’t made the grade.
Before we could get a new driveway, we had to stabilize our garage floor. It had cracks from the years but replacing it was impractical because of its ultra deep footings. Without stabilization, the pressure of the new concrete would crack it more.
Here are before and after shots of that:
The first step involved in making a driveway included removing the old slabs–easy in our case because they were so broken. The workers found an old sewer pipe in the curb of the old driveway and they kindly took it away. Next came building a frame, leveling the surface below the frame, adding reinforcing rods, and pouring the new concrete. Then the concrete needed to be smoothed so that water doesn’t pool on it and finally, joints were cut. Concrete shrinks when it dries and when it shrinks it cracks so cutting joints gives it a set of already made aesthetically pleasing cracks.
It took just two days to have the old driveway removed and the new one poured. Following that came patience as the “cement” (concrete is the proper term) cured for a week until it became tough enough to drive on.
Concrete is a calcium silicate made from mixing limestone (lots of calcium), clay (lots of silicates), and water. Water brings the calcium and silicates together. As cement dries the water–the matchmaker–leaves and calcium silicate bonds form to give the final product strength. One interesting thing about concrete is that once it is mixed up, it needs to be poured within a few hours before bonds start forming prematurely.
Extra water is added to the mixture so that it can be poured easily. Once that evaporates, the cement can begin to set up and bond. Heat is evolved and the process is fairly slow. Some say that it can take years for all the bonds to form. As with the formation of crystals (as opposed to nanoparticles) the slower the better. Stronger concrete with bigger crystals forms more slowly at cooler temperatures and higher humidity. In order to keep your cement crystals growing bigger and stronger, it’s best to leave it alone to cure for at least a week before driving on it and at least two weeks before parking on it. It will slowly cure over years but most of it is formed within 28 days. Deicers are tough on concrete and particularly new concrete so we’ll have to avoid those and give our new drive all the time it needs to form those chemical bonds that make it such a strong material.
Concrete has been around since the days of the Roman Empire but working with it takes some skill and patience. My description of the process is pretty simple. Here is a great website for more detail about concrete chemistry. Cement
One downside of concrete is its carbon footprint. Concrete is responsible for 8% of the world’s anthropogenic carbon dioxide. Heating the limestone to 1,450 C ( 2642 F) to make cement is an energy intensive process. 30 billion metric tons of concrete is produced each year. Fortunately, new concretes are being developed, some using Portland cement, which is clay based.
One thought on “Concrete Details”
I found your chemist perspective of cement interesting. Never thought about cement in terms of its chemical properties.