Tips For Working With Concrete
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Learning how to work with concrete is one of the single most rewarding skills that I can think to name. Concrete is the most utilized building material on the planet so learning the basics for how to work with it can go a long way in terms of cost savings around the home as well as the potential to develop a business based somehow from concrete. Concrete is extremely inexpensive and is available absolutely everywhere. At the most basic end it is cement, sand, gravel and water. At the highest ends you can modify every physical property of the concrete from set up time to finished strength, color to water permeability. Best of all it is not all that hard to begin working with concrete. It definitely gets a lot harder when you start looking at larger projects like open expanses of flat work, but that is just one of many, many potential projects you could be looking at with some basic concrete skills.
There is a definitive technical process to mixing, placing and finishing concrete but once you have this basic process down you can then begin to look towards more advanced techniques and admixtures. If you attempt to get too advanced too quickly with your concrete working then you might skip over some of the critical aspects of mix design or how to mix concrete properly. These technical errors in your process will compound as you begin to explore more technically challenging concrete mixes and larger projects. Even many professional trades people lack some of the basic and most fundamental concrete working skills. If you follow the tips in this series you will learn how to work with concrete at an accelerated rate and help you to avoid the most common areas where people tend to make mistakes mixing and finishing concrete.
How hard is it to work with concrete? -
Working with concrete is a lot like baking a cake except that this cake bakes at room temperature and starts "cooking" as soon as you mix all the ingredients together. Much like baking the ingredients and the amount you use of each in the mix is critical. Cooking is a little more free in the sense you can toss in a little of this or a little of that to your taste. An experienced baker knows that carefully measured and weighed ingredients are the real secret to baking. When making concrete you must treat it like a cake mix recipe and follow the steps explicitly but this is of course a lot harder to do when we are talking about a "cake" that weighs hundreds or even thousands of pounds! Start small when working with concrete at first. Make some smaller mixes for repairing things around your home or just experimenting with mixes to see how they feel, how long they take to actuate (get hard), how long it takes for the hard concrete to cure to full strength...small projects like this are the practice you need to have confidence with larger concrete projects.
Concrete Working Tip 1) Use the least amount of water possible to achieve a uniform mix consistency
Without a doubt the first mistake that novice concrete workers will make when mixing concrete for the first time is to add too much water to the mix. This definitely facilitates easier mixing (less work if you are mixing by hand) but adding too much water to the mix will drastically reduce the final strength that the concrete is able to achieve. By the time the concrete is so wet that it is "self levelling" it is completely compromised. The ideal amount of water in order to actuate the chemical process of hardening portland based concrete mixes is similar to that of damp sand. One of the driest concrete mixes is called "dry pack" concrete which is damp sand and portland cement mixed together (often with a shovel on a piece of plywood). The damp sand holds enough moisture to actuate the concrete without any additional water being added. The only reason all concrete is not dry like this is because placing and finishing to a uniformly smooth surface would be impossible with concrete this dry. So what you need to understand is that the ideal amount of moisture or hydration for concrete is so little that you can not place and finish the concrete, and adding enough water to make the concrete more easily placed and consolidated will likely compromise the mix quality. So the goal is to use enough water such that you can actually place and finish the concrete (or use water reducer admixtures) but use no more water than absolutely necessary to achieve this goal. It takes a lot of water to mix a batch of concrete so you will be kind of used to adding it in large amounts to your mix, but once you achieve the amount of water that you need, the addition of even a small amount more water will have a drastic effect on the "slump" of the concrete, which is the system used to measure how viscous a concrete mix is. The more a cone of concrete slumps once you remove the cone, the higher the slump (2" slump is more wet than 1" slump or dry pack which has no slump). A six inch slump is standard for concrete pours like flat slab work, which is quite watery, but often this is achieved through the use of plasticizers which do not negatively affect the finished strength like too much water would. Self leveling concrete has a slump of greater than 25".
Concrete Working Tip 2) The ratio of aggregates to portland cement is critical to the mix quality
When you talk about mixing concrete you are talking about ratios. The ratio of aggregates to cement is a fundamental aspect of concrete mixes. You will be using "one part" of cement in your mix to which you will add aggregates like sand and gravel. A common mortar mix ratio (concrete with only small aggregates) would be three parts of sand to one part cement. It does not matter whether your "one part" of portland cement is a shovel full or a bucket full, or any other size of scoop, so long as the ratio of aggregates to cement content is observed. Three buckets of sand mixed with one bucket of cement will make a reliably strong mortar mix in terms of the cement to aggregate ratio. 3:1 of sand to cement is the best balance between strongest mortar mix ratio versus yield volume as the more cement in the mix the more expensive the mix will be. Where 3:1 is a great mortar mix for strength and yield a mix of two parts sand to one part cement will be similarly strong, but it will have the ability to pick up more detail for casting or stamping due to the higher concentration of cement powder in the mix. If you look towards a higher yield mix like six to seven parts sand to one part cement this will be significantly weaker than a 2:1 or 3:1 mix, but still usably strong as a mortar for block wall or natural stone wall construction. The amount of sand that you need for a given mortar mix ultimately comes down to the specific application you are doing but the 2:1 casting mix, 3:1 strength mix and 7:1 yield mixes are a good starting point. If you are looking for a concrete mix instead of a mortar mix, this means you are looking for maximum yield at minimum cost. This is after all why concrete is so popular for construction around the world. The most basic high yield concrete mix recipe is the 1:2:3 mix. With the above examples (mortar mixes) there is only small aggregates, sand, however the 1:2:3 mix also uses gravel to help bulk up the yield. One part portland cement mixed with two parts sand and three parts gravel will result in a reliable concrete mix recipe that costs very little and covers the maximum possible area with usably strong concrete. The amount of water content you use in the mix will change the finished strength, but commonly a 1:2:3 mix can achieve 5000 PSI (34 MPa) or more with the correct (minimal) hydration levels.
Concrete Working Tip 3) The order to mix ingredients is water first, then the cement, followed by sand (and gravel)
One of the harder aspects of becoming proficient with mixing and batching concrete is amount of water that you use in the mix. If you have dry sand you will need way more water than a mix made with wet sand. This makes it hard to know how much water to use, but that is precisely the first thing you need to know. To give you somewhere to start a comparison would be an 80 pound bad of premixed concrete (totally dry mix, zero moisture) might need just under six pints (2.5L) of water to mix. Knowing this, or via experimentation, you can learn how much water you need to facilitate one batch of your concrete (one bucket full, one concrete mixer full etc.) and this is where your mix should start. First you add the water that you will need into the mixing drum and let it spin. This will help to clean the drum of any concrete residual left over from your last batch of concrete (not old hard concrete, the leftover from the batch you made five minutes ago). To the water you next add your portland cement component. The cement is a very fine powder and will mix instantly into the water resulting in essentially gray water. We can now also safely assume the portland cement and water are thoroughly mixed together and evenly distributed throughout the mix. To this you can add your aggregates (gravel and/or sand) however you would be well advised to not simply add all of the aggregates at once. Start by adding half the amount of aggregates that you intend to use and let these also get distributed evenly throughout the mix. With only a part of your aggregates left to add you can now more easily make a determination on the wetness of the mix to see if you have enough hydration to add the rest of the sand and gravel. If the mix remains a little too wet then it is easy to dry it up by adding small amounts more of dry sand and cement powder (observe your ratio). By starting the mix wet and drying it up as you add the aggregates you can avoid binding the mix in your mixer/bucket which is what happens if you try to spin a concrete mix which is simply too dry. By adding water, then portland, then half the aggregates and then the balance load of aggregates you can batch your concrete quickly, repetatively and with accuracy to your initial mix design.
Be sure to check back to this page in the future for more tips for working with concrete. This is just the first in a series. If you want additional information about some advanced concrete working principles then you can check out this page about how to bond concrete. If you want to see some more impressive and unusual examples of the kinds of things you can make with concrete and mortar then you surely will want to see the interesting concrete creations and colorful concrete paint jobs in this article about making fake rocks from concrete.
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Swimming Pool Steve is an award winning, second generation swimming pool specialist from Ontario Canada and one of the most trusted voices in the swimming pool industry. With over 20,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the Pleatco Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print. Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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