Placing concrete is the hardest part of concreting. I will class it as getting the concrete to the job, placing it into the formwork and leveling so that the finishing can start.
Before this of course you should have done all the preparation work. This should be done in the previous days, leaving the placing and finishing to be done on a separate day.
So here are the jobs that I like to see finished completely before I order the concrete.
- Remove vegetable matter. I am not talking about market gardening here, I just mean that top layer that contains grass or weeds and other crap. Get down to subsoil. By hand or with a machine ( “Bobcat” or similar skid steer loader). Cut out tree roots and so on.
- Compaction. If you then have to add fill to get the correct levels, make sure that it is compacted. This could be as simple as watered sand or sand or gravel fill compacted with a machine, say a plate compactor.
- Formwork. You will need formwork of some sort to contain the concrete, to define the edges. On small jobs like shown here, it also is a guide for the top of the concrete.
- Vapour barrier. I like to see a plastic membrane laid under the concrete. On house slabs it is mandatory, to stop dampness rising through the concrete slab, but for driveways it is optional, and so you don’t have to do it, but using a membrane underlay stops the plastic concrete (that’s concrete that is still in a wet, manageable state) from drying out too quickly or unevenly. If you pour concrete onto dry earth, a percentage of the water in the concrete will soak into the soil, so changing your precious water cement ratio. At the very least if you are not using a vapour barrier, water the earth down to stop too much water being sucked out of the concrete.
- You will need your rebar or reinforcing mesh in place, if it is needed. Packed up to the correct height with bar chairs.
That is the main work done but then I also:-
- Tidy up the site, as I have said before, things get a bit intense when pouring concrete, so do yourself and your mates a favour and shift the crap from out of your way.
- If you are using wheelbarrows, pump up the tyres. If you are a first time concreter, make sure you have a decent solid wheelbarrow. Garden barrows just won’t handle the loads.
- Make sure the tools etc. that you need are on site and good to go. Fuel up vibrators etc.
- Make sure that the access for the concrete truck is clear, I have on occasions parked my own vehicle in it, to stop a stranger parking there and then disappearing.
Placing The Concrete
|Placing concrete – Charley and Andrew started at the bottom of the slope working uphill. Note the height of the chute and the consistency of the concrete. Savvas is tapping the forms with a hammer, now and again he heaves up on the mesh if it is sagging.|
OK, so we are all ready to go, nice and early in the morning. All the preparation work has been done the previous day, to avoid any unnecessary delay when the truck arrives. The correct mix has just arrived in a transit mixer truck. The guys in the photos were going to do an exposed aggregate concrete finish, and so had ordered a mix with a maximum 10mm aggregate size.
|Placing concrete – A hand full of the first concrete out of the chute. Experienced concreters get a feel for the concrete and can tell by the look and feel if it is a good mix. This is a 10mm paving mix that will have an exposed aggregate finish.|
- The concrete that arrives at the job will normally be as good as it gets. You can of course stuff it up, by adding extra water, and I have already said that adding extra water is equivalent to throwing $50 bills into it. There are exceptions of course, where conditions change, like you have underestimated the time it will take you to place the concrete, but on a well organised job there is no reason to add extra water.
- Segregation. You can stuff up a good mix in another way too. That is allowing the concrete to segregate into it’s component parts.
- Concrete should not be allowed to fall freely more than a 1.5M.
- You are pouring a basement floor, you must not just dump the concrete in there. Either provide extra chutes or hire a pump.
- Pouring concrete stairs, start at the bottom and place the fresh concrete on top of the already placed stuff. Don’t let it slide downhill.
- The reason that the transit mixer always has the bowl slowly turning is to keep the correct consistency of the concrete.
- Transporting wet concrete over a distance, say in the bucket of a loader, or in a trailer, just does not work. What happens is that the heavy solids gravitate to the bottom, the lighter sand and cement is in the middle and the water is on the top. So much so that it becomes almost impossible to dig out with a shovel. This is concrete segregation at it’s worst.
- The following points all apply to whatever is the physical method of getting the concrete into place. That is b pump, by the chutes of the transit mixer or a wheelbarrow.
- So, back to our normal job, Start at the low end. Work uphill.
- Keep the chute low and close to the placed concrete. Try not to drop it or drag it too far.
- Place fresh concrete up to the older concrete.
- Keep one face working at a time, that is don’t do odd patches here and there.