How Concrete Works

 

While the pages of job pictures on this website have captions that explain a bit of what I do, this section is a comprehensive guide as to what I know about concrete and how it works, to educate you as a potential customer. Iíll tell you some of the wisdom Iíve gained from the hands on process of dealing with the stuff over the years. This is where I break down the process and answer the questions that people ask, as well as the ones they donít, but should know about. One question people always ask is "when does it get hard"? A concrete slab or structure is like a human being, is somewhat organic, and has a lifespan. It gets harder as time goes on, reaches it's prime, and inevitably turns back to dust. Concrete provides a subtle to exotic form and function. Nobody notices the basic concrete slab unless it's yours and it's brand new, then out comes the magnifying glass. Like a person, it's never absolutely perfect, and there may be flaws. The beauty is skin deep, so you have to look at it as a whole and ask yourself what you like about it.

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1) What I specialize in:

Flatwork refers to all concrete besides walls. This is my specialty since Iím a finisher. Although I will do some smaller walls or retaining walls, I typically donít do major foundation work. Itís a whole different ball game and there's more demand for good finishers. Typically I replace the old front entries, steps, walks, drives, patios, garage/basement floors, slabs and the like. New construction is much nicer and easier, but not very common. Therefore it's sometimes a guessing game as to how the job will unfold when replacing that 100 year old front entry.

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2) Types of finishes. There are not a whole lot of options, but here they are:

Broom Finish Ė Common and preferred since it leaves the most traction. Itís the easiest to repair since it can be brushed on after its set. Typical for steps especially, as well as drives, walks and patios. Works well on slabs that have more than the standard slope.

Sponge Float Ė This is similar to the broom, but the surface is textured with a big, soft concrete sponge, if the lines from a brush arenít desired.

Exposed Aggregate Ė Molasses is sprayed on the top at the right time, and a whole lot of sand and slurry is washed off that creates a big mess. Itís common for new construction out there in the sub divisions because itís the one finish that can be done in the rain and in large volume on any given day, but many circumstances donít allow the mess.

Hard Trowel Ė Smooth concrete. Pretty much standard for the garage or basement floor. Iíve done this for other indoor floors that donít get covered as well as patios and occasionally steps. A bit more labor intensive.

Acid Wash Ė Done after hard troweling and requires scrubbing the top layer off with acid to expose the sand rather than the rock. Not too common. Itís the same effect as several years of erosion.

Stamped Ė Tough to do on steps with the stone patterns. Not too conventional for big drives, but more so on patios and smaller slabs. Lots of patterns to chose from, but only a few are common, such as the cobblestone and slates. Having a single color is a much cleaner and easier process, but rare. Typically people like the contrasts of gray or brown shades.

Stamp Texture Ė Similar to the stone patterns, but just the slate texture throughout instead. Itís cheaper and way easier than lining up the mats, especially on stairs. Looks good too.

Decorative Ė The skyís the limit, but I donít usually go down this road unless someone has a specific idea, and understands there's no guarantee that some special finish will match what's in their mind after the fact. There are patterns for stenciling which I donít typically get asked to do. Thereís broadcasting anything from rock salt for a pitted finish to pieces of colored glass, setting tiles or whatever. As a rule I don't like to get too fancy. It's just concrete, but I'd sooner let you be the artist if you wanted.

Rat Slab - Refers to concrete that has a rough, ugly finish for the purpose of setting tiles or whatever to the surface. Sometimes people have a need for a slab and for whatever reason don't need or want it to look good.

Resurfacing Ė Resurfacing is not concrete. Itís any of a number of different polymer compounds mixed from a bag and used to cover over concrete in a thin layer. The finish is usually broomed or textured. This is common for walls and steps, but not big slabs so much, unless you can cap over at least 2" or more with concrete, which is the way to go. Concrete is cheaper and easier to work with.  Thereís a lot of variables for repair, but mainly if the old concrete is wasted, then thereís usually nothing to stick to after its grinded clean. Providing a new skin can be beneficial, but if the old stuff underneath is rotten, it's basically "polishing a turd".

Sealing Ė This isnít a finish, rather itís an option that works for some surfaces and not others. Itís common for stamp work to bring out the color, but can be slick on a wet day in bare feet, especially if thereís too much slope, or steps of course. Sealing does protect the surface from erosion and is a good idea every year or two. There are many different products out there, ranging from water based, to heavy-duty acrylic for a heavy shellacking. Anybody can spray or roll it on, preferably sooner than later. Re-sealing will prevent the concrete from eroding, thereby keeping it young and beautiful for many years.

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3) Color in Concrete:

Iíd really like to take this time to explain that if you have a particular color in mind, forget about it because it has a mind of itís own. The surface is not going to look like that little square on the color chart. People have this misconception that pouring concrete is like opening a can of paint. It has little to do with the finish, but everything to do with the mix, the weather,  the temperature, the humidity, where the sun was shining or not and even the wind. Effervescence is calcium leaching to the surface making white splotchy spots or areas that can come to light the next day or two after the pour. It usually always goes away in a few days to a few months, nobody knows. A finisher can control the form and texture, but not the color.

Concrete is usually light gray to white, and gets darker from dirt and stuff as it erodes over time. An integral color can be added to the mix at the plant. Black is common, but bear in mind the likely mottled appearance will only have that much more contrast, which isnít really noticeable on the stamp work, by the way. A dark red can double the cost of the concrete truck, so itís really not worth it. Thereís powdered color hardener that can be troweled on top, which adds a lot of work to get consistent, though it can give the stamp work another shade. There are acid stains that easily and permanently change the color, much like staining wood. Some are very expensive and act more like paint. Then of course thereís paint, but don't use the stuff that flakes off after a few years, right? Sealing can help too. Again, color is beyond the finisher's control. Almost always, people come to find that given some time, the surface does in fact even out to a consistent shade. After all, it's just concrete.

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4) When to Pour or Not:

Rain never does any favors for a finish (well, a little is nice on warm days), though with exposed aggregate, thereís a better chance of escaping disaster. The typical, common passing drizzle isnít usually a problem. I'll take the average of several internet weather forecasts and make a recommendation on whether to pour or not. I always let the customer make the decision, since they ultimately have to live with it. So far I've been lucky, but large slabs out in the open are risky when the weather may be unfavorable.

Another misconception about concrete is that it canít be poured in the winter. You can actually pour below freezing since the curing slab creates enough heat for a couple days. After then youíll want the temperature to be above 32į F. Insulation blankets can be used if absolutely necessary, but pouring on frozen ground is a bad idea. Hot weather can be a bigger problem. Besides the obvious setting up too fast and thereby not having enough time to shape the stuff, it simply cures too fast and can crack all over from shrinkage. Here's a hint, use plasticizer or a retardant to save the slab and yourself from a heart attack, and get paid. Concrete work tends to be seasonal since itís outdoors, and therefore the summer months are busy. However, winter here is mild enough to be just fine for pouring.

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5) Types of Mixes:

There are many variations of concrete mix, depending on the finish and application. Mainly thereís the ratio of different sized rock and sand to cement and water. Thereís additives such as retardants and accelerants depending on the temperature, and plasticizer to increase the fluidity without adding water.  Concrete is a matter of timing, and you really donít want to get behind, or waiting all day for it either. Every load is different depending on many factors, even if you get two trucks with the same mix from the same plant on the same day. Itís not like opening that can of paint.

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6) Placing the Concrete:

The preferred method is to back the truck right up to the spot and dump it out the chute and work your way out, or tailgate itís called. Otherwise youíll need either a boom pump or line pump, depending. If thereís power lines in the way or youíre in a basement, a boom isnít going to help much. Wheel barrowing is doable, but not for large amounts, or uphill. The stuff is heavy. The idea is to get the mud down, on the ground as fast as possible. The goal is to make the job easier, not more difficult than it already is, or you fall into the category of getting behind or having a heart attack, and nobody likes that. Also, if you think hand mixing bags is an option, itís not one for me unless youíre talking a 3X3 pad. With small jobs, you tend to pay a lot for the minimum, so you'd be wise to make it worthwhile for a bit more volume and area.

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7) Forming and Reinforcement:

Iíve often said you need to be a finisher first before properly forming a large set of steps, to fully understand the physics involved. Thereís a fine line between having the forms sturdy enough to hold the weight and pressure, especially if the concrete is too wet, versus being able to strip the forms to finish the steps soon after. Walls arenít as critical, but obviously they have to be sturdy. Any form does, and if not then you have whatís called a ďblowoutĒ, and in the case of walls or steps, that can spell very bad news. Forming needs to be done right every time. Lots of guys like to use those steel pins as stakes, but I think they're a complete pain in the butt unless the ground is rock hard. Wood stakes offer a better grip and can be cut off.

Reinforcement is essential for longevity. Thereís fibermesh that can be added to the mix. Itís like little tiny hairs that work great on the micro scale, but won't stop a major crack. I never add it unless someone requests. Itís come a long ways since the clumps of ďrabbit hairĒ that would bunch up and inhibit a nice hard trowel. Wire mesh is ok. Itís not nearly as rigid as rebar, and if you ever have to break out the slab for any reason, you have to pulverize it. Itís cheaper than rebar, and I do use it for thin slabs or over existing, which by the way shouldnít be any thinner than 2Ē. At least 4Ē is standard, by the way. Wire is flimsy, but much better than nothing.

Rebar is where itís at, and I almost always put it in, except for city sidewalks where itís not allowed. While itís essential in your foundation walls, many builders skip it in the slabs because theyíre cheap and nobody ever knows, until later when it's too late. Like concrete, the cost has gone way up in recent years. However, itís the best insurance, since it makes the slab many times stronger on a macro scale. It's like your skeleton. Think of the slab as like a dinner plate, hard and brittle. It will crack the day after pouring or not for a while, but the ribbed rebar will prevent it from separating. It doesnít need to be overkill, but a grid averaging 20Ē squares more or less is ideal. Let me repeat something here. Your new concrete slab will crack, but with the steel in there, you don't have to worry about it moving around..

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8) Designing the Concrete Layout:

Occasionally Iíll have a customer with a specific plan for the layout. This is nice, but rarely happens. The main consideration is with the elevations, keeping in mind that the ideal slope is about 1/6, or one inch of fall for every six feet of distance. An 1/8 is the absolute minimum, and of course the slope goes up from there. The idea is to avoid the puddles, which nobody wants to have on their new slab.

I always welcome a plan, which makes my bidding process a bit easier, though this is easier to visualize on new construction. Usually the customer just wants me to replace whatís there. The problem is that the ugly old concrete that was there is usually out of whack. It's at different angles, cracked to pieces, sloping to the house, sunk into the ground after 100 years and what not. I have to come up with a layout that works with whatís there, regardless of the yard. On a front entry for example, I have to go from point A, the front door threshold, to point B, the city sidewalk, and make it work in between. Steps need to be of consistent height and tread width, and the yard isnít permanent like the concrete. The basic idea with your concrete is to make sure it makes sense. Keep it logical, reasonable and straightforward. I tend to let folks know if something doesn't seem right. It is somewhat permanent.

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9) Bidding the Job:

As aforementioned, a plan or some ideas obviously help, but it depends on the job. Itís anywhere from simple and straightforward where I donít even need to meet the homeowner, to complicated and extremely technical. I usually do need to meet for consultation, and so it helps me to know the potential customerís schedule. Appointments donít work so well because my job is anything but a 9 to 5. Feel free to send pictures and dimensions to expedite the process. Itís common for me to send multiple estimates depending on the finishes and scope of the job. Any time I bid a job I have to make assumptions, so the clearer I am on whatís needed the better, since this work is always more than we think.