Art and Science of Concrete
The art and science of concrete construction has a very long history dating back to at least ancient Rome. Reinforced concrete construction has a much shorter history dating back only some one hundred years. Today we have many highly specialized and advanced techniques for making concrete. There is even a concrete canoe race held every year to apply new concrete technologies.
In order to understand the basic principals of any technology, it is important to understand the terminology. Cement and concrete are two different things yet many times these terms are used interchangeably. Cement is a powder and is the central ingredient of concrete. Cement mixed with water, is referred to as stucco for exterior use, yet, is called plaster for interior use. It can also be used to construct swimming pools. A wet, high-pressure air applied mix is called shotcrete. However a dry, high-pressure air applied mix is called gunite. The same basic mixture is also referred to as mortar in masonry block and brickwork. If you dilute this same mixture with more water into a flowable form it is called grout.
Even though all of these applications and terms are distinctly different, the actual chemical process that takes place and the principles that apply are the same. Therefore, if you take this small amount of time to understand the basics of concrete, it will be invaluable to you in the future.
Water - Comes in three forms depending on its temperature. A liquid (water), a gas (water vapor), a solid (ice).
Waterproof concrete is impervious or unaffected by the liquid form of water. It will prevent the penetration of water. The term “waterproof” is frequently used inaccurately in the flooring industry.
Waterproof concrete or waterproofing concrete does not stop water vapor movement. Concrete must be “vaporized” before you apply flooring materials and/or surface coatings.
Vapor-proof - Vapor-proof concrete restricts or prevents the passage of water vapor. Waterproof concrete is not necessarily vapor-proof but vapor-proof concrete is always waterproof.
Water Vapor - Water vapor moves much faster and much more readily through concrete than water. Water vapor moves through the capillaries of the concrete left behind in the concrete making process (curing).
Waterproof concrete does not stop water vapor. Water will not move through waterproof concrete, however water vapor will. Additionally, when water vapor reaches the dew point under flooring materials, it condenses, becoming water. This is extremely undesirable because the water will deteriorate the flooring adhesive or perhaps the flooring itself. Therefore, to waterproof concrete is not the issue with regard to the installation of flooring materials. The issue is to make concrete vapor proof. When you do it will be inherently waterproof.
Moisture Vapor, Moisture Vapor Emissions - Moisture Vapor and Moisture Vapor Emissions are terminologies that the flooring industry uses to define water vapor. Moisture vapor emission is the direct result of a number of factors. Moisture vapor emissions, which are higher than the flooring industry's maximum allowable levels will cause flooring installations to fail.
Water To Cement Ratio
Water to cement ratio, or water cement ratio, or w/c ratio, is the weight of the water used divided by the weight of the cement
used when making concrete. This ratio is expressed as a decimal fraction. It is the second most important factor in the quality of the end product, just behind the curing process. At this point we are forced to jump ahead a little because the two factors mentioned above - water to cement ratio and the curing process are intimately connected.
Curing is the process whereby this mixture of cement, water, sand and aggregates become concrete. This does not happen all at once, but is a chemical process that takes time. The time it takes depends primarily on the temperature and humidity (water vapor content) of the air. Because we have little control over these environmental conditions, we need to control the process by other means. The reason we want to control the process is that if curing takes place too quickly, the concrete created will be filled with numerous capillaries. The same is true if you use a high water to cement ratio. Either way, capillaries are extremely undesirable because they render the concrete weak and porous.
Capillaries are formed during the curing process. They are unwanted small tunnels left behind after excess water (water not needed chemically, but needed for flowability), has evaporated from the concrete. No matter how high a quality of concrete you make, there will always be capillaries formed if no other measures are taken. The goal is to keep them to a minimum, and of course eliminate them all together if possible. Luckily this last option is now available to us, as we will see later.
To keep capillary formation to a minimum you would:
1. Keep the water to cement ratio to a minimum (less water).
2. Control the curing and cure for as long as possible.
3. Add a high tech substance to the concrete (Admixture)
4. All of the above.
Why Do Capillaries Make Concrete Weak?
It is because they take up space that should be filled with concrete. The same would be true of steel. A solid piece of steel is stronger than one that has a lot of holes drilled through it.
Why Do Capillaries Make Concrete Porous?
Because they are filled with air until water or water vapor comes in contact with them from the ground or air. They will actually wick moisture from the surrounding environment and deliver it to the top of the concrete slab just underneath flooring materials. There, the water or water vapor when it condenses, attacks flooring adhesives. Capillaries form the transit system for water and water vapor (moisture) migration.
The Trade Off
Life is a compromise. For any given goal, if you compromise somewhere, you will need to compensate for it somewhere else. We make these types of decisions every day. If all you needed was a medium quality concrete slab, and you needed to put it into use quickly, what should you do?
Using the guidelines above, you should:
1. Use a low water to cement ratio to keep capillary formation to a minimum.
2. Use a fast curing time so you can use it more quickly. By the same token, if you needed to place he concrete quickly to meet a pouring schedule, but did not need to use it right away you would:
3. Use a high water to cement ratio to place it more easily.
4. Use a slow curing time to keep capillary formation to a minimum.
These two approaches would generally yield the same results for the same quality of raw materials.
The amount of water necessary for reasonable ease of placement of concrete is higher than is necessary for the completion of the chemical process. However, the less water you use, the stiffer the concrete will be. This property of consistency or stiffness is called the slump.
Slump is a convenient way to approximate the water to cement ratio in the field. Slump is measured with a slump cone. A slump cone is an inverted cone, 12” tall and open on both ends. The top is 4” wide and the bottom is 8” wide. Fresh concrete is placed in the cone and “rodded” with a steel rod to compact the concrete. The cone is removed and placed next to the pile of concrete. The difference between the top of the slump cone and the top of freshly molded concrete is the slump. More flowable concrete is said to have a higher slump. A higher slump generally indicates a higher water content (higher water to cement ratio). The desirable characteristic of a high slump is that it is easier to work with when it is placed. The undesirable side is that it will create an inferior product unless the concrete is allowed to cure for a longer time. The reason that the concrete will be inferior, if not allowed to cure for the proper amount of time, is because the concrete will develop an unwanted amount of capillaries. In other words, if the concrete is allowed to cure (dry) too fast, you allow the water to evaporate quickly leaving numerous capillaries behind. Again, this translates into a less waterproof, less water vapor-proof and weaker concrete.
Bring It All Together
Water is an essential chemical ingredient in the making of concrete; however we need to use more of it than is chemically necessary in order to place the concrete with some degree of ease. How we handle this extra water after the concrete is placed is of extreme importance. As was stated before, even if you were to make the best concrete you possibly could, (low water to cement ratio and a long controlled curing process) without an additive or “admixture”, the concrete would still have some degree of capillary formation.
What We're Doing at Olde World to Improve Our Formulas
Here at Olde World Enterprises, Inc. we are constantly testing and evaluating our stone and tile formulas due to the rapid progress being made in the industry to improve concrete with admixtures. We are currently testing two new additives, and will keep our Producers advised of formula updates as they are made.
Basic Refresher Tutorial and Updated Information About Concrete and Cement
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