The Joy of Concrete

Published : 10/06/2010 | Author: Popular Mechanics | Merle Henkenius

Nearly 2 000 years ago, the Romans constructed a temple of a size and design that engineers still look upon with awe. It was a massive circular building, 43,5 m in diameter, covered by a monolithic dome. The building still stands. It’s called the Pantheon – and it’s made of concrete.

Although mortar had been used in stone construction by the Egyptians, Roman concrete was something new. It relied on a paste made of ground limestone, water and volcanic ash, or pozzuolana, that was poured into forms where it hardened and bound together a rock filler, or aggregate. It was literally mouldable stone – and with the fall of the Roman Empire, the secret of concrete disappeared for centuries.

Master concrete and you can build a backyard empire – starting with a spot to fire up your braai.

Hard core

Today, nearly 6,9 billion cubic metres of concrete is poured each year around the world. It’s big business for a humble mix of sand, gravel, water and portland cement. But you don’t have to build the Hoover Dam to get involved; anyone with a back yard can do it. Pouring a slab for a landing or a small patio for your braai is a great way to learn how to mix and finish concrete. And there’s nothing quite like making something that will be around for a long time. It may not be immortality, but it’s close.

Of course, on big projects, ready-mix concrete – the kind delivered to your job site – is the way to go. But it has about a 45-minute delivery limit and there are places the trucks can’t reach. Partial loads carry surcharges, so the less you order, the more you’ll pay. A full load is about 6 cubic metres; a cubic metre weighs about 2 400 kg and is enough to pour a 100 mm-thick, 5x2-metre (or 10m²) slab.

For small jobs, you can buy bags of cement that already contain sand and gravel – you just add water. On larger jobs, it’s more economical to buy cement and aggregate, and combine the ingredients in a motorised mixer. These rent for around R120 a day and can be towed home or delivered for about R35. Cement comes in 50 kg bags; about six will make a cubic metre. The sand-and-gravel aggregate should include a range of sizes from grains to 19 mm stones. If the aggregate in your area is too fine, add some coarse river sand. As for time, you can easily pour a small slab in an afternoon. Here are the steps to get the job done.

The form

Build the form out of sturdy planks, nailed at the corners with large nails. Then use a shovel to slice the earth into manageable squares. Rough out the rest of the form bed, but be careful to not dig too deeply. Backfilled soil settles, so over-excavation will require compacted sand, which adds work and expense.

Set the form in place, shave away soil from high spots, and check with a level. A slight slope of about 20 mm per metre helps shed water. Drive stakes around the perimeter to just below the top of the form (see image) and fasten the stakes with drywall screws.

Mixing the ingredients

First pour 7-8 litres of water into the mixer drum and then add aggregate to get it wet. If you have a sand-and-gravel combination, throw in one shovel of cement for every five shovels of aggregate. If you have a separate pile of coarse aggregate, make it 2½ shovels of sand, 2½ shovels of coarse gravel and one shovel of cement. Keep the shovel topped off so the quantities are uniform.

When the drum is about three-quarters full, gradually add more water – the exact amount will vary, depending on the moisture content of the aggregate. When the mixture reaches a gray/green colour with hardly any of the rock colour showing through, stop the mixer and pull out a small handful. Form it into an 8 cm ball and toss it from one hand to the other. If it crumbles, it’s too dry. If it splatters, it’s too wet. Cautiously add a little water or more aggregate and cement to correct the consistency, then run the mixer for 3 more minutes.

The pour

Load the mix into a wheelbarrow and deliver it to the form. Spread it around with a shovel and press it against the sides. Then mix another batch.

When you have about half the form filled, use a screed board to level the surface, but leave it slightly high. Fill the other half of the form in the same way. A larger slab would require rebar or steel mesh to help control cracking, but our small slab can do without. Then, steadily saw the screed board side to side while moving it along the form. If you find a low spot, fill it and saw through that area again.

Finishing

Finishing is all about timing – and surface water is the timer. Each pass with a tool brings up water and cement, but overworking the surface can weaken it. When the water from the previous pass is no longer visible, it’s okay to move to the next operation.

After screeding, wait a few minutes before using the float. When the surface appears dull, hold the float on the concrete with extra pressure on the trailing edge. Run it along the pad edges and across the surface in wide arcs. Then use an edging tool to round the slab edges.

Trowelling will give you a smoother surface, if desired. Again, wait for the surface water to disappear. Trowel the edges first, then sweep the surface in wide arcs. If you can’t avoid sweep lines, wait 15 minutes and trowel again.

Concrete cures best between 10 and 20 degrees Celsius. Keep the job damp for five days by covering it evenly with a plastic vapour barrier or by using a sprinkler. Alternatively, apply a curing-and-sealing compound.

Safety

Wet concrete is strongly alkaline and can start a chemical burn if it contacts bare skin. Wear long-sleeve shirts, long pants and gloves when working with concrete, and don rubber boots when you have to walk in it. And don’t forget goggles to shield the eyes from airborne cement dust.