Concrete Working & Hating

Rev. 2001-09-06, 2003-11-13

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The thing I HATE most about concrete is that I keep doing it myself. This last time was a 5.5' x 6.5' x 3" slab for my furnace and gloryhole, done on the hottest day of the summer so far. The latter happened because of several stupid choices, including taking a truck that was only available 9-5 instead of working in the evening, and not hiring someone to help, which in this neighborhood wouldn't have cost all that much.

Among the ways of doing concrete, there are three that I have done, plus  I will mention a couple more, then I will discuss doing it.

  • For small batches, like a 2x2 foot square footer, the least complicated way is to buy 80 pound bags of Sackrete, Quikrete, etc. ready to use mixes of cement, gravel and sand. These are most easily mixed with a hoe, mixed and moved in a wheel barrow. The sacks also give a good set of directions for concrete mixing and working. Each sack supposedly does 2/3 cubic foot, but I have found that optimistic, especially since calculating the cubic feet needed is often also optimistic. Two thirds of a cubic foot is a block 4 inches thick by 1 foot by 2 feet. Yet when framing that area, it is very easy on uneven ground to add 1/2-3/4" in thickness, taking the volume towards 3/4 cubic foot or more (3/4" more increases to 0.79 ft3).  The normal solution is smooth sand, graded from the top to even depth, but even that in a home job can end up deeper.
  • For middling batches, like my 5x6 foot slab, renting a power mixer, which come in several sizes, makes life a whole lot easier. There are still those 80 pound bags, but the mixers are on wheels and dump reasonably well.  In order to dump well, the bowl opening must be chest high - awkward lift. Most run off electric motors and Home Depot rental, for example, provides a heavy extension cord to protect their motor.
  • For bigger slabs, like my garage floor, done in sections, when I had a van capable of pulling the thing, a trailer full of concrete, 1/4 to 1/2 cubic yard, seemed efficient and cost effective to me. I bought the concrete from a mix place about 3 miles away and the trailer was free for about enough time to get home, unload into a wheelbarrow, spread and strike the stuff, clean the trailer and return it (2 hours I think) although it always seemed a rush. One of the links above suggests that this is a bad method as the aggregate (stones) settle. Now that I am reminded, I recall having to drag the gravel out while unloading and at the end, putting it on top of the previously poured and working it in.
  • Beyond that, one rents a bigger mixer and hires help. Here it is possible to save money by buying the ingredients in bulk -bags of cement, loads of sand and of gravel dumped beside the drive, instead of premix bags.
  • At the high end, one frames everything first and gets a concrete truck to deliver. The framing must be strong, and right, there will be little time to fix it and a real mess if it collapses.  If the slab is in a really awkward location, like uphill from the street, behind buildings, it may be necessary to hire a concrete pumper to get the concrete where it is needed. Talk to the concrete mix guys about special mixes for higher temps, etc. The trucks give you a certain amount of time to unload after they arrive - not very long - then start charging. Have enough people with wheelbarrows and push tools to get the concrete out of the truck and into the forms.
  • [And I suppose I should mention - one hires a firm which does everything so the artist's job is writing checks.]

The first absolute requirement of concrete work is carpentry - the slab or wall has got to be framed and strongly framed. Concrete weighs a lot - 150 pounds/cubic foot,.(2400 kg/m3 (high density) 4045 pounds/cubic yard)  Framing needs to be screwed and nailed to itself and to strong stakes driven in the ground.  Flat panels need to be through wired to keep them parallel.

A major trick of building flat (slab) forms is making sure that the posts/stakes supporting the forms do not extend above the forms, so when the form is leveled, the top of it can be used for leveling the surface inside, for striking the concrete and for finishing it. The frame should be made of lumber and stakes strong enough to keep the concrete in place and the right width for the slab, which is why a lot of slabs are 3.5" (the width of a 1x4 or 2x4). If the stakes, after pounding and nailing/screwing the form to them, extend above the form, saw them off level. Forms are made of 1x (3/4") wood if they have to be gently curved, 2x (1.5") for strength.  Special curves take thin material - masonite, etc.- built up to thickness. Posts to support the forms can be 1x2, I bought a bunch of hard pine stakes (like those used for campaign signs) that I save and reuse; or 2x4 in heavier work. Forms should be strong enough to walk along the edge and withstand rolling a wheelbarrow over - ramped with a board laid on top.

To make the amount of hard to handle and costly concrete predictable, to provide a uniform slab, etc., cheap sand is used to make a level bottom to the form. Take a piece of wood (1x2 or 1x4) long enough to span the edges of the form, nail/screw a wider board to it, extending down the desired depth for the slab. By moving the guide back and forth, sand can be added (or removed) to make a level even base. The guide can be used as dozer blade on smaller projects. This becomes more difficult if doing the slab in stages, as I did in the garage, where the reinforcement wire from previous pours interferes with leveling. The sand should be tamped down firm and dampened to keep from absorbing water from the concrete.

Above the sand is normally put a sheet of 6 mil plastic. This prevents moisture from wicking through the slab from the ground, making the inside of the building damp and musty. If the slab is to be exposed, there is less reason for using the plastic.

The next layer is reinforcement. Concrete is good in compression, but weak in tension, so steel (usually) is added to take the tension. This may be in the form of rebar - steel rod made for the purpose with a rough surface - or rebar mesh, usually 6x6" welded wire. On smaller jobs I have used 2x4 and 1x2 welded wire hardware cloth. The garage used 6x6; the outside slab 1x1. The reinforcement must be enclosed in the concrete, so it is either propped up - with rocks on smaller jobs, with plastic props on commercial jobs - or pulled up into the concrete with a rake or the hook on the back of a special concrete hoe. If the job is done in stages, the rebar from the previous stage should overlap the new stage - which means running under or through the framing, which complicates life (a lot, I think.)

My experience has been that accurately estimating how much concrete is needed has not gone well. I have taken care of this by providing a moveable section to my framing, so if I am short (or long) on concrete, I can move the section, (which I did repeatedly on my garage) or by having enough bags of mix (2 extra it turned out) to keep mixing until the form was filled. I am sure that most of my problem is the condition of the ground in my forms, I don't take enough time leveling it and firming the sand.

Putting down the concrete is a heavy, time filling process. If you are going to do the heavy work yourself, it is still a good idea to get someone to assist - for example to drive the trailer back to the dealer or to wet down areas or vaguely (or strongly) push concrete around. This site Subject: Concrete Floor Instructions gives a lot of good suggestions and comments, even saying my choice of trailers was wrong (and reminding me of what I did to bypass some of the problems.)

Putting down concrete is also a messy job. If you can do a section over 3' wide without spilling concrete on your feet/legs/etc. and/or without walking in it then you are better person than I am. Have shoes or boots on that can be washed off and a hose and brushes available to scrub pants, shoes, tools, striker, forms, etc. If concrete is left to dry on any of these it is tough to get it off. Forms (and tools?) should be coated with paraffin, linseed oil, or other separator. If your hands are sensitive, wear gloves, as concrete is alkali and will damage skin more or less.

If you are going to put concrete over there, and you can only get the mixer, trailer or truck to here, then someone is going to have to move the concrete from here to there - usually in a wheelbarrow. If you distrust wheelbarrows and think something else would be better, you haven't tried something else in similar situations. The overwhelming advantage of a wheelbarrow is that it has ONE squishy tire on the ground. This means that the ground is not trying to tip the barrow (as it would with 2 or more wheels) or break the wagon (as it would with 4) and the wide wheel is not going to dig in. It is perfectly possible to run a loaded wheelbarrow up a ramp made of a flat 2x4 (although a 2x6 is easier, of course) while making a reliable ramp for a 2 wheeled cart becomes a major construction job. It is possible to rent power wheelbarrows from commercial rental sites but a level walkway of some kind is virtually required..

When I did my garage, I thought I was going to flow the concrete from near the front to the back. But the bottom of the trailer was way too low. I had a few minutes to solve the problem and grabbing a couple of planks to run the wheelbarrow over the edges of foundation and forms was the solution.

Of course, using a wheelbarrow is a new experience. I am tall and can run the cross brace in front of the wheel into the ground and other things without half trying. I have used a smaller flatter barrow and a steep sided bigger one and the latter is much easier for moving concrete without slopping out (which is why I bought it) but it is also very, very easy to get it very, very heavy. Concrete weighs about 2.2 times the weight of water and comes close to slopping like water. If you can't move your wheelbarrow pretty full of water, you haven't got a chance of moving it half full of concrete.

By the way, most wheelbarrows we see are sold with tubeless tires.  If they are left sitting around on the tire, it will go flat and getting it re-inflated and stuck on requires a high volume air compressor we don't have.  Storing it upside down or hanging on a wall saves the flattening when air is lost.  Sooner or later, the tubeless starts losing air in a couple of days.  The best answer is putting in a tube, which will cost about $8 and hold air for months.  For those who watch money drain due to flat tires, there are solid soft tires the same shape as the original for $45-55.  For those of us who buy our barrows on sale at $34.99, a tube is a better idea.



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