By Chad Staffileno, Kristin Karleskint, Michael Dickey, Zac Wilson, Alla Acheli, Jon Choate
This entry is part of our student collaboration series.
Have you ever seen concrete that is cracking, shrinking, or even falling apart? A large part of why this happens is due to the pore structure in our concrete. A pore is basically a void inside concrete. Think about a sponge, you will see all the gaps and holes. That is pore. If we can control the pore structure to help reduce cracks and permeability of our concrete, we will create durable long lasting structures that will last generations. In order to understand why the pore structure of our concrete is so important, we first must take a look at the types of pores that actually form in concrete. In a typical mixture there will four different types of voids. From smallest to largest, there is CSH interlayer space, Capillary pores, entrained air, and entrapped air. The CSH interlayer pore is the space between the CSH particles. This space really cannot be avoided because there are formed during the creation of CSH and do not affect the concrete as much. These voids are really small, about .5-2.5nm and really do not affect the permeability or strength of the concrete. Next is what we call capillary voids, these pores are the space between the cement grains themselves, usually about 2.5nm -5μm in diameter. To compare these to the CSH voids imagine that you are at a crowded bar. The space between your fingers, arms, and legs would be the CSH interlayer space and the Space between another person and you would be capillary voids. The volume of these voids is largely dependent on the water-to-cement ratio(w/c), and the degree of hydrationand other factors talked about in other videos.If you know how to control these pores you can have better control of the creep, shrinkage, andpermeability and other properties of your concrete. The next pore is an air entrained void which is actually formed by putting a soap like material called a surfactant (air entrainer) into your concrete mixture. These voids are generally 10μm –to 1mm in diameterand actually have a distinct spherical shape. The construction industry refers to this add mixture as adding “air” into your concrete. The reason we add air into the concrete is to get good freeze-thaw durability. Finally, we have entrapped air which is air that naturally gets trapped in your concrete while mixing. These entrapped pores are usually larger the 1mm. All of thesevoids can contain water, air, or both depending on the conditions your concrete is in. Having pores in your concrete can both help and hurt your durability which no one wants. If you have too many poresthat are interconnected, this allows outside chemicals to penetrate your concrete which can over time decrease the structural integrity of your concrete
Now before we move on, we must attack this crazy concept called shrinkage! There are a few types of shrinkage, which can be found in other videos. The first is called differential shrinkage. This happens when concrete is not cured properly and the top of the concrete shrinks while the bottom does not. This will cause your concrete to bow upwards. This bowing is called curling. Curling can cause cracking in your concrete and you will lose durability in your concrete, which is bad. This type of shrinkage can be prevented by proper curing of your concrete such as putting something like a curing tarp so there are equal amounts of water in the top and bottom of your structure. Now what if your shrinkage is restrained? Let’s say you have a strong base. Then your concrete will still shrink causing tension in your concrete and of course this will lead to cracking as well. The next type of shrinkage occurs during the hydration process, when you have low water-to-cement ratio, anywhere below .4. What happens in this case is that while your concrete paste is hydrating it will consume water, which as you can probably guess, causes your pores to lose water. This can happen right when your start mixing or several days after the concrete sets. When this happens during early on in the mixing process this is known as chemical shrinkage but when it happens after set it is called autogenous shrinkage. They are similar and occur throughout the hydration process but happen at different times. So how can we combat chemical and autogenous type of shrinkage? We simply replace about 20 percent of regular sand with saturated light weight sand, reduce w/cm, or use admixtures.The paste is thirsty and needs water to hydrate. That is why we saturate the lightweight sand because is like having little stock piles of water in your aggregates that are ready to provide your paste with a great water supply. You want this sand to be well displaced throughout your concrete with pores that are not too small or too large.
What have we learned? There are several different types of pores in concrete, each having their own purpose for being there. Whether that purpose is good or bad depends on how you want your concrete to perform in certain conditions. Next we learned all about the different types of shrinkage, and we know that shrinkage can cause cracking which of certainly will decrease the durability of your concrete. There are ways to combat shrinkage chemical and autogenous shrinkage by adding light weight aggregate at about 20 percent replacement of your normal sand. In order to have great long lasting structures we must have a detail understanding of what is happening in our concrete. Now if you really want get into the nitty gritty of why pores are crucial for concrete durability then there is great video on YouTube called “The Importance of Pores in Concrete” by Dr. Tyler Ley.
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Dr. Tyler Ley, Ph.D, P.E., is a professor of structural engineering at Oklahoma State University. He was named a Most Influential People in the Concrete Industry by Concrete Construction Magazine in 2019, and was named the outstanding professor at a research university by the Oklahoma Foundation of Excellence in 2018. He has a passion for researching and educating people about what he considers to be the greatest material in the world.