Mixing And Pouring Concrete for the Do It Your Self Enthusiast
Concrete in one form or another has been around dating back to the ancient Egyptians. Although not popular at that time, it wasn’t until the ancient Romans concrete really took off. The Romans used concrete to build the aqueducts, roads, and many of their buildings. Modern cement in concrete was given credit to Joseph Aspdin in 1824 when he patented his mixture known as Portland cement. Concrete is the most used man made material in the history of the world. Everywhere you look you will find something made out of concrete.
Modern concrete in its basic form is nothing more than 3 items; Portland cement, aggregate, and water. There are many who confuse concrete and cement but Portland cement is just one component to make concrete. Cement is the binder that holds everything together. It’s kind of like glue, but it needs the two other components to make it stick.
Aggregate is the material used to add filler and help give strength to the mix. The larger the aggregate the stronger the mix. Aggregate can be anything from sand for fine detail forms of concrete such as mortar, to sand and gravel or crushed stone, used in making sidewalks, foundations, and supports for fence poles. Sand is in general grade concrete with crushed stone, but crushed stone is not in the fine mix like mortar.
Water is the activation ingredient in the mix. The water causes a chemical reaction with the Portland cement to make it workable and thus start the process of hardening and strengthening. The reaction is called hydration. Its important to note that concrete does not get its strength from the drying process.
Concrete gets strong from the chemical reaction when water mixes with the cement that causes it to harden. Knowing this, it is always better to allow concrete to “slow cure” over a few days. The longer the water and cement can react to each other the stronger the concrete becomes. Have you ever noticed when road crews are making a concrete roadway or laying the surface on a bridge, that they have sprinklers spraying the poured concrete about a ½ a day after the pour? They know that keeping the concrete moist allows the reaction to get stronger. DO NOT spray concrete with water during or right after a fresh pour. You need the hardening process to take effect for 2 to 4 hours before water misting is recommended. Another technique to help keep the water in is to cover the project with a tarp. This will help seal the moisture in and allow for that slower cure time. After about 3 days you can remove the tarp.
When you purchase a bag of concrete, the cement, sand, and stone aggregate are all ready premixed to the proper ratios for the type of concrete you have purchased. It is up to you to properly mix the last ingredient; water. Please take note, that you should wear a dust mask and safety goggles when mixing concrete. When you open a bag and pour, the dust from the mixture will rise in plumes, and Portland cement is not the best thing to breath in or get into your eyes. Always take safety precautions.
Concrete is sensitive to the amount of water you add. The less water in the mix, the stronger the concrete and the more water in the mix, the weaker the concrete. Keep in mind though that in order to cause the reaction of the cement, the mix has to be saturated. No dry powder can be left. Dry powder will cause pockets in your mix and thus create weak spots that will eventually lead to the failure of your project. When reading the instructions on a bag of concrete it will usually tell you how much water in quarts to add to bag of concrete. (Bags of concrete are usually 80 pounds). But the true measure of concrete mix to water is a pound for pound mixture. One gallon of water is about 8.35 pounds. In a typical sidewalk mix, for every pound of concrete mix, there is about a ½ pound of water. So its about a 2:1 ratio. Broken down, 1 pound of concrete would be mixed with about a ½ pint (or one cup) of water. It takes very little water to get the ball rolling making concrete.
If you decide not to take the scientific approach to measuring out the concrete to water ratio, the best advice is add the dry mix first then slowly add water and stir until your mix is moist but thick. If mixed properly, you should be able to make a concrete “mud ball” set it down and have it slowly flatten into a pie shape after about 45 seconds. If you get soup, add more concrete mix until its clearly too dry then add a little bit of water to get the right consistency. Believe it or not, some of the old masons would take a mouth full of water and spit it into the bucket of concrete mix. Again, it takes very little water and they always knew how much they held in their mouths. If your doing a small project (1 bag of concrete or less) You can mix it in a sturdy bucket. Anything more you should probably rent a portable concrete mixer. When mixing you should stir with a hand spade shovel and fold it over as if you were mixing a cake batter. You should mix it for 6-10 minutes and until you are quite sure that all the mix is wet.
Concrete has a pot life (working time). The average pot life is about 20 minutes. Read the instructions on the bag of mix for specific times. What pot life means is, once you start adding water the chemical reaction to harden has begun. After a certain amount of time, the concrete mix has passed the point of being able to be poured and spread for your project. The idea is to only mix what you can use in the recommended working time.
Once you pour your concrete onto your project surface, you can use a concrete rake (A wide steel blade with small or no teeth attached to a handle), or a shovel to work the concrete into every nook and cranny of the project surface. After its all poured and leveled out, you can smooth the surface using various length trowels. Hold your trowel at a 45 degree angle to level it, then reduce the angle to about 25 degrees to give it a smooth surface. If the concrete starts sticking to your trowel, the mix is getting to dry. Try dipping your trowel in a bucket of water, or if necessary, very lightly mist the surface of your project.
If you think your project is too big to work in the allotted time, then it is recommended to bring in professionals to do the job. Projects such as pouring an entire sidewalk, or a concrete pad in your garage or a new foundation for an addition, require concrete by the truckload, and a team of people to get it spread. Some of the tools that are required for the big jobs include gas powered vibrators to get all the air pockets out of the poured concrete, re-bar to strengthen the concrete, large smoothing trowels or power levelers to smooth and level the surface of the project.
Some projects well within the range of a Do It Yourself enthusiast is repairing or replacing a section of sidewalk, making concrete steps, laying decorative walkways with brick and a concrete binder, making decorative concrete block edging, pouring to set fence posts, etc.
Concrete cant flex and has no tensile strength. Concrete is strong, but it is also very brittle. Concrete on its own does not make a good structural support. When doing a sidewalk, or stairs, or any other project that the concrete would be supporting weight, you must have a firm solid even surface. Before a sidewalk gets poured, the ground is dug down, graded, then crushed stone or gravel is placed on the working surface. It is usually rolled to make it solid and firm. Through the seasons, the ground heaves and shrinks based on water and temperature. Since there’s no flex to concrete, it will crack very quickly. Concrete is also not water proof either. Water can absorb into the concrete and freeze which causes expansion and creates cracks.
To help protect concrete there are sealers you can purchase to help with water resistance, giving your project extended life. If you are doing a larger pour, like a sidewalk slab, it is important to have expansion joints in your work. I’m sure you have noticed the lines separating a sidewalk into slabs. Those lines are expansion joints. Concrete will expand and shrink slightly with water absorption. These cuts give the concrete room to do this without breaking. If it was just one long slab, the concrete would crack everywhere in just one season, and all that hard work and expense would be wasted. You can cut in an expansion joint once the concrete starts to firm (About 45 minutes to an hour) then use a thin piece of metal (such as a painters trim blade) or thin wood (like a piece of paneling) and work it straight down using a gentle back and forth sawing motion. Go slow as not to tear up your work.
Another way to counter the weaknesses of concrete, metal rods or “re-bar”. The re-bar helps absorb the flex and tensile pressures allowing the concrete to stand firm. Most do it yourself projects wont require the use of re-bar, so I am not going to get into detail on its use.