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Tommy Walsh's DIY Guide
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Tommy Walsh's DIY Guide (2001)(Empire).iso
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t2e.txt
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1998-10-14
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Summary
Solid foundations are essential for many improvements in
the home and garden. Follow these steps to learn
about the correct procedure for laying concrete:
1 Deciding upon the slab depth
2 Calculating the slab surface area
3 Choosing the right mix
4 Preparing the site
5 Mixing concrete
6 Laying the concrete
7 About tamping down
8 About edge formwork
9 Finishing the surface
10 About the slow-drying process
11 Keeping the concrete warm
12 Removing the formwork
Tools
Spade, concrete mixer, formwork, hammer, arrising tool,
trowel.
Materials
Concrete, polythene sheeting, sand.
Step 1 - Deciding upon the slab depth
The height or depth of the slab will depend on its purpose.
For slabs less than 2 sq. metres/20 sq. ft, with only light
use and resting on a firm sub-base, m the depth can be
75mm. For softer subsoils such as clay, 2mm needs to be
added. For heavy use such as a vehicle parking bay, add
another 25mm for a total slab height of 100mm.
Step 2 - Calculating the slab surface area
The length of the slab has a maximum dimension dependent
on its width and depth. If you require a slab bigger than
these dimensions then it has to be laid in a separate bay.
These bays are to allow for the fact that concrete expands
and contracts as the temperature changes.
Step 3 - Choosing the right mix
The right mix will depend on the use. For larger areas it
will be worth getting ready-mixed concrete delivered. If
you canÆt get the concrete directly to the area where itÆs
needed specify a retarded load as this will slow the drying
process.
Step 4 - Preparing the site
Concrete is set in formwork made from either metal or wood.
For a one off job it may be better to hire the formwork.
Dig out the site, set out the formwork and prepare the
sub-base as required.
Step 5 - Mixing the concrete
If you are not having the ready-mixed concrete delivered
you will obviously have to mix it yourself. For small areas
you can do it by hand, click on the icon below for more
information, but it soon becomes practical to hire a mixer.
Motorized mixers come in several sizes. Try to keep it on
a stable surface and near the area where you need the
concrete.
Step 6 - Laying the concrete
Pour the concrete into the first bay, ensuring it is
spread evenly. Overfill the formwork by 10mm.
Step 7 - About tamping down
The concrete now needs compacting. Take a length of timber
just wider than the width of the area. Tamp down the concrete
with the timber length several times, making sure it is
thoroughly compacted.
Step 8 - About edge formwork
Run a trowel along the edge of the area in the same way you
would with a cake, to separate the concrete from the formwork.
Repeat this again after you have finished it off and as the
concrete begins to harden.
Step 9 - Finishing the surface
Obviously it is only worth finishing the surface if it is to
be exposed. Use a stiff broom to give a textured non-slip
surface. For a fish scale effect use a wooden float or back
of a shovel. A variety of effects can be achieved using
a tamping tool or piece of scaffolding.
Step 10 - About the slow-drying process
When concrete dries too quickly, possible weaknesses,
shrinkages or cracks could occur. The longer it takes to
set the harder it gets. Cover the concrete with weighted
down polythene sheeting. Curing normally takes three or
four days.
Step 11 - Keeping the concrete warm
If it is really cold it isnÆt a good idea to lay the concrete.
When there is a danger of frost add layers of sand or earth
and more polythene sheeting. You can remove the polythene
sheeting after the curing process has finished.
Step 12 - Removing the formwork
Wait a further 10 days to remove the formwork. Use an arrising
tool to neaten the edges.