Common Mistakes Made Working With Concrete
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Being that concrete is the most widely used and versatile building materials on the planet it would serve you well to learn a little about how to work with it. Just a small amount of knowledge about how to work with concrete will advance a novice concrete worker years into the future. As a concrete contractor it surprises me how often I see trades people who occasionally work with concrete making grievous technical mistakes that should never happen at the professional level. This article is for all of these people who want to understand better how to work with concrete. If you follow the tips included in this article your chances of success with your concrete project will go up dramatically. It might not be possible or realistic to teach you everything you need to know about concrete working in one article, but this information definitely will undoubtedly improve the quality of your concrete work.
Pouring a bridge, fixing a sidewalk, concrete swimming pools, the foundation of your home...these are all different concrete applications and the mix design, consistency, strength, steel reinforcing, water resistance etc. will all be different for each of these projects. In this way the world of concrete is very deep and as such it is hard to provide generalized instruction that applies to all of these different processes. While these differences exist, these are some similarities in each of these examples as well, as the mixing, placement and curing for each would be very similar. If you needed to make a patch to any of these then that also would be a similar process. Let's look at some of these fundamental concrete processes that everyone should know if they are going to work with concrete:
Can you add water to concrete a second time?
Once the concrete begins to actuate it will emit heat and start to solidify. Some novice concrete workers can be inclined to re-wet the concrete, or add some water to the concrete a second time after it starts to harden in order to achieve a suitable placement viscosity. This is absolutely something that you must avoid. There are hundreds of different kinds of concrete, mortars and mix designs but all of them share the same quality in that adding water a second time compromises the final strength of the concrete. Not to a small degree either. Adding water to re-wet concrete that is setting up too quickly will compromise the finished strength and quality almost entirely. Never add water to concrete that has started to set up (stiffen up). If the concrete is getting hard too quickly then you should use two hands on your trowel to push hard and raise some water to the surface to facilitate finishing the top. If the concrete is already too hard for this then you have lost the load and you are better to take it out then try to add more water to actuated concrete to get it workable again. It will work, but the strength of these areas will only be a fraction of what they should be.
How much water should you mix with concrete?
Professional concrete workers all know how important water content is within a concrete mix. Part time concrete workers and home owners have the tendency to add too much water to their concrete mixes. A lot too much. What happens is that you put in water to the mix, but is appears to be absorbed immediately by the dry powder portland cement and sand, so you add more. And more. And more. Experienced concrete workers add the initial amount and then work the concrete over manually whether that be by shovel in a wheelbarrow, or letting the mix spin in a mixing drum. In any case patience and strong muscles are what it takes to mix the concrete, not more water. If you ever meet a concrete worker look at their hands and arms and note how incredibly overdeveloped the muscles are. This is because concrete work is hard work, and mixing semi-dry powders together by the hundreds-of-pounds takes an extreme amount of physical intensity. The ideal water content for the strongest concrete is a consistency similar to damp sand. Which is to say so dry that it does not even look like concrete yet. This small amount of water is the ideal amount to actuate the chemical process of hardening the mix, and any water added past this point is added to facilitate suitable placement viscosity, but will only serve to reduce the strength of the final mix. This is why professional concrete workers use water reducer admixtures to change the placement viscosity without compromising the mix strength with too much water.
Steel encapsulation in concrete
Concrete is incredibly strong in some ways and surprisingly weak in others. This is why steel grids with high tensile strength are embedded within concrete slabs. This enhances the tensile strength of the concrete dramatically, as without the steel concrete will be inclined to crack and migrate. The concrete might still crack, but the steel will prevent the concrete from migrating. In order for steel to provide strength to the concrete it must be located within the center of the slab. If you have 8" thick concrete then your steel layer should be in the middle such that you have 4" of encapsulation on both sides of the steel. Novice concrete workers often let the steel sit flat on the ground (where it does nothing). More advanced (but lazy) concrete finishers will "float" the steel into place in the "middle" of the slab thickness. This is because it is hard to walk on steel grids that are lifted on blocks or chairs. These workers prefer to walk on flat ground and then wiggle the steel mats after the concrete is on them to "lift" the steel into the right position. This is lazy concrete work and compromises the quality and strength of the finished product. Steel should be set in place with plastic rebar chairs or chunks of old concrete (never red bricks) to ensure proper encapsulation within the middle of the slab.
Over troweling the concrete
When you place and finish your concrete you will be inclined to trowel it too much. This is because it takes many thousands of hours on a trowel to develop master level finishing skill. As a result of you trying to make the concrete look good, but taking too many passes with the trowel to get it there, is that you will raise too much water in the mix. The more you trowel the more water will work itself to the surface of the concrete. When utilized properly this is a tool to help with finishing as you raise a small amount of concrete "cream" by working your trowel back and forth, and this gives you some material to fill small gaps and imperfections as you are going. If you trowel too much and raise too much water accidentally then the top layer of the concrete will be significantly weakened and will deteriorate at an advanced rate. You are further ahead to place wet concrete and try to finish it smooth as best as you can in a few passes at most. Even if not perfect you would be better to leave it than keep trying to trowel it. Once the concrete loses the shiny net look and begins to look hazed over you can try working that area again. There is a sweet spot when the concrete has hazed over slightly, but before it is fully starting to actuate where you can get the best possible finish with the least trowel passes. As an amateur this will be hard if not impossible to do, but the takeaway here is to know to avoid overworking and raising too much water, and sometimes you have to walk away for a bit to let the concrete haze over to get the finish you are looking for.
Apply concrete over damp surfaces
Whether you are pouring a fresh new concrete pad or you are preparing some old concrete for a repair patch, you must make the area damp. Concrete requires water to actuate and if you were to pour new concrete over a dry area, like sand or old dry concrete, then the substrate layer will leech moisture from your fresh mix and this is definitely something you want to avoid. The goal with concrete is to allow the water to chemically actuate the concrete before it evaporates in the sun or is leeched into the surrounding soils. This is why concrete workers often start extremely early in the day to allow the concrete time to start setting up before it is under the direct sun and heat of the day which can cause the concrete to dry out too quickly. After the concrete has set up and is now hard, most concrete mixes benefit from wet curing. This means periodically re-wetting the concrete down over the first week or so a few times per day. Some admixtures do not require wet curing of the concrete, but the vast majority of concrete mix designs and applications will benefit from wet curing the concrete especially over the first week.
How long do you wait before sealing new concrete?
Concrete takes 28 days to cure to approximately full strength. During this time the concrete is still "green" which means there is moisture trapped within the concrete, and the strength has not yet fully developed. During this period the concrete is much more inclined to scratch, crack and stain. After 28 days the concrete is usually considered safe for staining or sealing purposes. If you attempt to seal before this point you will trap moisture in the concrete which will likely cause the sealer layer to fail and become white / hazy looking. You are better to wait an additional week or more if there is any concern that the slab is still green of holding moisture rather than sealing too early. One thing that people often forget to do is re-seal concrete. Sealing concrete to limit water absorption regularly is the single best thing you can do to extend the life (and the look) of your concrete surfaces. When water permeates concrete it dissolves small amounts of the portland cement component which then washes away with the water leaving behind the aggregates which are sand and gravel. This eventually weakens the concrete and regular sealing is how you protect against this slow degrading process.
Far from an inclusive list about how to work with concrete, this is just the very first things that I would want you to know if you are just getting started learning about how to work with concrete, or you already work with concrete but want to step up your game from a technical perspective. Along those lines if you want to learn more about how deep the work of concrete working is, then you should learn more about concrete bonding processes in this article about how to bond new concrete to old concrete. If you want to see some unusual examples of what you can do with concrete, especially some fancy concrete paints and coloring you should take a look at this article about making fake rocks from concrete.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 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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