Last weekend we celebrated Easter Sunday and, much like a lot of you, I enjoyed some excellent ham for the main dish. As a person who is almost constantly thinking about concrete and how we can make it better I was thinking about what ham and concrete might have in common. I was reminded of this sign. The best concrete and the best ham both have this in common: they are properly cured.
Concrete is made up of many different ingredients but the key ingredient is cement. Concrete and cement are NOT the same thing. You wouldn’t pour a cement sidewalk anymore than you would bake a loaf of flour.
So what is cement and why is it essential?
Cement is the essential binder that makes concrete a solid thing after the chemical reaction is complete. For the sake of simplicity (it is actually far more complicated) cement is made by taking limestone and super heating it to remove all of the hydrogen and oxygen from it. When we, as concrete suppliers, get it in powder form it is starved for those two elements. We then add those two elements back to it in the form of water (H2O), form it into the shape we desire and let it get hard as rock again.
This chemical reaction is called hydration. Controlling hydration by adding the correct amount of water throughout the entire reaction process is key to making good concrete. Adding too much water will dilute the mixture, creating weakness, while not enough water will leave un-hydrated cement which also causes weakness along with a serious lack of economic value.
Back to ham:
This post is not about water to cement ratios in concrete mixes. That’s for a different day. This post is about what can be done later in the hydration process which continues for a very long time after this step. Just like good ham, proper curing is essential to utilizing this chemical reaction to create the best final product.
When the concrete is placed and finished to the desired shape and look it will reach initial set, meaning it is hard to the touch but does not have any structural integrity. From this point on the hydration process is continuing but the strength can no longer be impacted by adding water. Proper concrete curing means providing enough water to the placement to keep feeding hydrogen and oxygen to the chemical reaction and make the most of each grain of cement.
There are many concrete curing methods. I will cover some of the best I have seen but this is most definitely not an exhaustive list of concrete curing options.
All different types of concrete aggregate have some level of absorption. When proper aggregate handling is happening at the batch plant, the aggregate brings water to the hydration party. As the cement particles inside the placement are trying to reach equilibrium (think of your high school chemistry class) and use up all of the available hydrogen and oxygen the water is available to the cement from inside the aggregate. This often takes a long time. If done properly, the compressive strength gain from internal curing will be seen between 7 and 28 days after placement. Although aggregates with high absorption can be difficult to handle for a concrete supplier the benefits from internal curing far outweigh the extra work.
Whenever it is possible to setup a dam or a dike and flood the placement this should be done. This will provide constant water to the placement and allow the cement to pull all of that is necessary to completely hydrate it. The Hoover Dam was built between 1931 and 1936. Many people believe it is still hydrating. Due to its thickness and constant supply of water from the lake, the cement is still pulling hydrogen and oxygen.
Fogging can happen a little bit earlier in the curing process. Vaporized water is presented over the top of the placement to combat evaporation from heat and wind. It allows the right amount of water to enter to surface of concrete without adding to much water to degrade the surface.
Hoses and Felt Backed Sheets
This is one of the most effective curing methods I have seen. A hose with many small holes put on top of the slab then a felt backed vinyl sheet is placed over the entirety of the placement. The hose provides a slow and even release of water while the sheets prevent it from evaporating from sun and wind.
Concrete curing is an essential part of any concrete placement. The picture below (and some good Easter ham) have sparked this blog post. This sign placed by the city to prevent people from walking or driving on recently placed concrete is actually quite ironic. There is some truth to it but there is no external curing method being implemented. As a result, when I see this sign I ask “Is it? Is it, REALLY?” I think not.
Just like your honey cured Easter ham, make sure and properly cure your concrete!