By Kristin Karleskint, Chad Staffileno, Michael Dickey, Zac Wilson, Alla Acheli, & Jon Choat
This entry is part of our student collaboration series.
What is porosity and how does it pertain to concrete? If you were told that a household sponge and a slab of concrete had anything in common, would you be surprised? At first glance, concrete appears to be an impenetrable solid, but in reality it’s porous much like a sponge! Porosity is a term commonly used in the concrete world and for good reason. This property refers to the amount and distribution of pores in the material. This description may present porosity as unimportant, but it actually controls many other significant concrete properties that determine the concrete’s durability! Properties including compression strength, tensile strength, flexural strength, modulus, resistivity, and permeability are all heavily influenced by the porosity. In general,when porosity increases, the strength decreases and the relative penetration increases. Both of these relationships are bad, meaning that these unseen pores could actually be detrimental to our concrete. Who would have thought that some holes in our concrete would play such a vital role in the lifespan of our infrastructure? In order to minimize our pores we must first understand how they develop and how to determine how many there are. There are three critical rules to determine the amount of pores in our cementand if used, secondary cementitious materials. These are the initial proximity of cement grains, the final proximity of cement grains, and the degree of hydration. All three of these rules are critical factors that determine the porosity of concrete and each will be discussed, coming up!
Rule number one: the initial proximity of cement grains.
It’s important to note that idealized situationsare expressed in drawings to better demonstrate theseprinciples in the real world.These schematics are extremely helpful for you if you want to understand the big picture! However,you must keep in mind that they are based on THEORY. The drawing shown here represents how initial proximity contributes to strength of concrete, but it is idealized in the sense that cement grains are not always equally distributed and that the hydration products are not always the same length. Despite this, the drawing makes it obvious that the different water to cement ratios create very different load pathsand pore systems. Loads become easier to distribute when the path can spread out within the concrete. These load paths also act as barriers for fluid penetration, whichwill increase the durability of the concrete!What is even scarier is that cracks make this concept even worse. To learn more about cracks, Dr. Ley has several videos on his YouTube page. If you love concrete and you want it to last a long time, then this information should be exciting! This concept can be defined as tortuosity, and when a system is more torturous it becomes more durable.
So if this was an ideal system, then wouldn’t it be ideal to just continually drop the water to cement ratio until there are no pores? Although this seems like a flawless plan, this isn’t actually how people design concrete mixtures. Hopefully you are asking yourself, “but, why?”
Rule number two: final proximity of cement grains.
People have asked these same questions before and plotted the cement to water ratio versus strength. They found that the strength varies linearly with an increase in cement to water, but at a certain point, itno longer follows the same linearity. In fact, they found that belowa 0.38 water to cement ratio the strength dropsoff due to a lack of ability to compact the concrete. This will lead to increased entrapped air within the concrete and an overall drop in strength.and you are benefitted less foradding more cement.Remember this rule is the final proximity; the key word is ‘final’ meaning that this part is influenced by the manual compaction that takes place after we place the wet concrete. Yes, that means that people are actually in control of how durable the concrete becomes! This also means that the amount of entrapped airair left in the concrete after compactoinin the mixture after consolidation will be higher, which will lead to a strength decrease. This shows the pertinence of properly consolidating and choosing a mix design with a high enough water to cement ratio that will allow it. Without the use of special admixturesthat will allow for the concrete to become more workable.,super low water-to-cement ratio mixes cannotbe properly consolidated and will never reach their theoretical strength potential.
Rule number three: degree of hydration.
If you are a curious person, then you must still be questioning, “but what IF we could properly consolidate?” And the answer is found here in rule number three. The answer is no, it isn’t good enough to lower the water to cement ratio and properly consolidate. If you remember, the more cement content there is in a paste, the harder it becomes to correctly consolidatedue to the decrease in water in the paste. Water allows for the concrete to flow easier because it is a low viscosity material.,Tthis means that you are potentially leaving cement grains unequally distributed and unable to optimize the creation of hydration products.In order to optimize the creation of hydration products you must cure! The good news is that through proper curing you can actually ensure that your concrete reaches its potential strength. But only for certain water to cement ratios. If YOU want to know more about how to maximize your concrete strength AND durability, then watch the video below. Dr. Ley provides and presents such useful information in such stimulating way that you don’t even have to be a concrete superfan to be engrossed. Go forth andlearn more about concrete today!He has many other videos on his YouTube page that can be useful information to anyone interested, so go check them out and don’t forget to like, subscribe, and hit that bell so you get notified when he posts new, awesome videos about concrete.
#Concrete#porosity #engineering #durability #cement #sciencestuff
Join the Concrete Freaks email list and receive exclusive tools, tips, and resources directly from Tyler.
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.