DIY concrete Workshop |
Photos: Josh Giumelli and Ben White
Cheat sheet for
concrete slab
construction
As one of the handiest building materials, concrete has a variety
of uses around the farm — from shed floors and cattle yards to
silo pads and driveways. Kondinin Group engineer Josh Giumelli
writes anyone can lay concrete (as proven by this article) but
there are a number of steps to be followed if the job is to be
done properly
Table 1 Recommended concrete strength
Strength grade
Application
Curing time
MPa
Days
N20
Footpaths, driveways, light footings, residential floors
3
N25
Industrial floors (light traffic)
3
N32
Industrial floors (medium traffic)
7
N40
External concrete forms within one kilometer of the coast
7
Source: Kondinin Group
Table 2 Recommended uses for reinforcing mesh (common sizes)
Code
Wire spacing
Wire diameter
Sheet weight
Recommended use
mm x mm
mm
kg
SL52
200 x 200
5.0
23
Garden paths, lightweight pads,
garden sheds
SL62
200 x 200
6.3
36
Heavier paths, patios, carports,
garages
SL72
200 x 200
7.1
45
Garages, light duty machinery,
house pads, tank and silo slabs
SL82
200 x 200
8.0
58
Heavy garage, medium duty
machinery, house slabs, larger silos
Source: Kondinin Group
Table 3 Concrete surface finishes
Application
Method of finishing
Cattle yards, steep driveways, heavy non-slip areas
Sweep with a coarse broom or form grooves
Surfaces suitable for tiling
Screed
General non-slip areas
Light sweep
Carpeted areas
wooden float
Smooth surfaces, areas for vinyl or carpet
Mechanical or hand steel trowel
Source: Kondinin Group
www.kondiningroup.com.au
T
here are many concreting jobs
around the farm just waiting for
some spare time, but they never
seem to get done.
Concreting is hard work, and the raw
product is not particularly cheap, so it is
little wonder some of these jobs get put
off in favour of other urgent tasks
around the farm (of which there are
always plenty).
But the difference a concrete floor can
make over dirt or gravel in the machinery
shed is massive.
It can reduce dust, eliminate lost tools
and generally create a much cleaner,
more organised work environment.
This month, we concreted a parking/
maintenance bay in front of the farm
office and workshop.
While none of us are particularly skilled
with trowel or screed, the results are
perfectly suitable for most pads needed
around the farm which don’t require
engineered strength, such as silo pads or
large shed footings.
Building a reinforced concrete slab is
not difficult work, provided you put the
preparation in beforehand.
A nice smooth slab may be pleasing to
the eye, but if it is not well constructed or
cured, it will crack when machinery is
driven over it.
While concrete is a hard and tough
material, it contains small, invisible
imperfections and cracks, which reduce
its tensile strength, or elasticity.
Reinforcing mesh provides tensile
strength to the slab, and expands and
contracts at the same rate as the concrete.
Given the cost of the concrete, it makes
little sense to skimp on the reinforcing
mesh to save a few extra dollars.
Also, make sure you order concrete of
sufficient strength for the job. Concrete is
classified with an N number (for
‘normal’), with the strength of the cured
product in megapascals (see table 1). We
have used N25 for this slab. Contact ▶ Josh Giumelli
josh.giumelli@kondinin.com.au
No. 254 March 2013 Farming Ahead
25
| Workshop DIY concrete
2
1
The site for your slab should be clean
of all refuse and organic material.
Any decay over time may lead to
subsidence and cracking of the slab.
Assess the slope of the site. This is
easily performed with string lines
and levels, or long rigid timber or
steel beams as shown. While the majority
of slabs should be perfectly level, any
slabs exposed to the weather should slope
slightly towards the drainage point. Even
though this slab will be fitted with a roof,
rain can come in from the sides, and will
need to drain to the outside of the slab.
5
4
7
26
Here we have used the front end
loader on the tractor to grade the
site reasonably flat.
Remember to check all formwork
for squareness.
Farming Ahead March 2013 No. 254
Compaction of the base is essential
for any concrete slab, as any
movement will lead to cracking
down the track. We are fortunate in this
case the concrete slab is being poured over
a previously compacted gravel driveway,
but areas of fill still need compaction. Here
we are using a makeshift, but effective
‘arm-strong’ compactor made from pipe and
a section of heavy I-beam. You wouldn’t
want to compact a whole pad with it, but it
is perfect for small areas.
8
Short sections of formwork will
need securing into place to prevent
movement when the concrete
is poured. Timber formwork will require
staking at much closer intervals to prevent it
from bulging out.
3
Remove any high spots to achieve
the minimum required slab
thickness. This is easily gauged by
passing a beam or string level over the site
and noting high or low areas (top). Likewise,
fill any low areas with clean sand or other
appropriate material. While you leave low
points to be filled with concrete, you will
use more, and it becomes much harder to
calculate the volume of concrete required
for the job (bottom).
6
When you are satisfied the slab is
level, install the formwork. If you
are pouring into an existing shed,
you will most likely need little formwork as
the concrete can flow right up to the walls.
Timber makes ideal formwork, provided
it is straight. Here we are using some
downgrade 50x100mm RHS tube, which is
ideal for the job due to its stiffness. Peg it
into place using timber or steel stakes. The
100mm thick slab is ideal for driveways,
shed floors and garages that aren’t exposed
to overly heavy machinery. Otherwise, it is
best to pour a 150mm thick slab.
9
This section of formwork is used
to provide a channel for a sliding
door to run in. While we could have
placed it flush against the shed wall, it may
be very tricky to remove once the concrete
has set. A better idea is to use some timber
wedges, which can be extracted later to
allow easier removal of the formwork.
www.kondiningroup.com.au
PLe
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As we are installing a pole for
the shed roof at the same time,
it is imperative it is placed in
the correct position before the concrete is
poured, as there is no adjusting it later on.
String lines have been used to position the
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11
While you can use a magnetic level
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It is secured to the post with an
elastic band, and has spirit levels
on two sides to ensure the post is
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13
With all the formwork completed,
it is now time to add underlay to
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Plastic sheet helps the concrete
cure more slowly and strongly as
moisture from the setting concrete
is not sucked away by the sand underneath.
It also prevents rising moisture in the slab,
and limits friction between the slab and the
ground, reducing the chance of any cracks
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Overlap any joins in the plastic
by at least 200mm, and run the
plastic down into any trenches in
the slab. If there are any drainage risers
in the slab, cut and tape the plastic to the
pipe, and tape over the open end.
No. 254 March 2013 Farming Ahead
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| Workshop DIY concrete
16
Next comes the reinforcing mesh,
which is vital to the strength of
the finished slab. Reinforcing
mesh comes in sheet sizes of 6x2.4m, and
is usually designated with an SL number
followed by two digits. The first number
is the steel diameter in millimetres, and
the second refers to the size of square. For
example, an SL72 mesh is made from 7mm
wire and has 200x200mm squares. See
table 2 for more information.
19
Note the chairs can be used in
two different orientations to give
50mm mesh height (100mm thick
slab, top) or 75mm mesh height (150mm
thick slab, bottom).
17
Cut the mesh to size with bolt
cutters. Make sure there is a
little gap between the edge of
the formwork and the mesh, or the mesh
may corrode, splitting chunks off the edge
of the pad. Where sheets of mesh join,
overlap by one row of squares and tie
together with wire.
20
Next, add rows of wooden pegs,
roughly the distance of your
screed apart. These are used to
provide a reference point when screeding
the concrete in the centre of the pad.
18
The mesh must sit in the middle
of the slab. Plastic bar chairs cost
about $0.20 each and are ideal
for holding the mesh clear of the plastic.
Place the chairs at around 800mm intervals.
Here we have installed chairs around three
sides of the slab, allowing us to use a
wheelbarrow over the remaining section
of mesh. Additional chairs will be added to
these areas as the pour progresses.
21
We have used a steel beam sitting
on the formwork to ensure the
pegs are at the correct height. For
larger areas you can use a string level. The
pegs will be pulled out as the concrete is
screeded level.
22
The easiest way to place the
concrete is straight from the
chute, but most concrete trucks
will have a maximum chute length of
4m, so a wheelbarrow is needed to place
concrete further away. It is generally unwise
to remove formwork and reverse the truck
into the slab. For large jobs, use two helpers
with wheelbarrows to keep up a constant
supply. If the concrete is too stiff, ask the
driver to add water to the mix. It is far
easier to work the concrete too sloppy than
too dry, and it will cure slower as a benefit.
28
Farming Ahead March 2013 No. 254
23
Place the concrete into the
furthest position from the truck
first. Screed off the formwork and
the pegs to establish a level pad of concrete.
Use a sawing and chopping motion with the
screed, as this helps compact the concrete,
releasing any air bubbles trapped inside.
24
As the pour progresses, screed off
the previously screeded sections,
using the level pegs as reference
points. Don’t forget to add in bar chairs to any
remaining sections of unsupported mesh.
www.kondiningroup.com.au
DIY concrete Workshop |
25
When the pour has reached the
length of the chute, you can
dispense with the barrow and pour
directly into the pad. Take care not to dump
too much concrete, as it can come out very
quickly. Spread the concrete around with a
shovel as shown, and continue screeding.
26
When you are nearing the end
of the pad, have the truck drive
forward to pour right up to the
edge of the formwork. Remember to pull out
the level pegs as you go. They can easily get
lost in the wet concrete. The pad now needs
floating to smooth it, and then a second
time when drier to achieve the desired
finish. See table 3 for surface finishes.
27
With the pour completed and
the concrete screeded off, use a
telescopic bull float to smooth
the concrete. Twist the handle clockwise as
you push forward, and twist the opposite
way when you pull backwards. You will
quickly get the hang of the tool as you
smooth out the pad. Bleed water will rise
to the surface, which will help improve
workability. Bull floats can be difficult to
use inside a shed as there is often not
enough room for the handle.
30
28
Two wooden floats can be used to
smooth out areas near the edge of
the slab. Use one float to support
your weight so you can reach further. Entire
pads can be satisfactorily finished with just
wooden floats if desired. Use sections of ply
board to spread your weight when working
in the centre of the slab.
29
If the weather is hot or the wind
is strong, the pad may cure too
quickly, reducing workability
and decreasing the strength of the finished
job. You may need to run a sprinkler or
periodically wet the surface. Alternatively,
a retarding compound can be added to the
concrete to increase curing times.
31
Note the blades can be angled
more aggressively by turning the
centre handle clockwise. This is
good for initial working of the concrete.
Rotate the handle back to lower the blades
for final smoothing of the pad. Work the
entire pad again until you are satisfied with
the surface finish.
www.kondiningroup.com.au
32
Use an edging trowel to round the
corners of the pad as shown.
When most of the bleed water
has evaporated, the pad can be
finished. By this stage you should
be able to stand on the surface without
sinking more than a few millimetres. Here
we are using a rotary trowelling machine to
smooth the surface. They can be hired for
around $60 a day and are a must if working
on a large pad, but are not essential for
all jobs. Start with the troweller where
the concrete was first laid, and make a
pass over the entire slab, working towards
the concrete laid last. Smooth out your
footmarks as you go.
33
The finished pad should be allowed
to cure for several days, but can be
walked on much sooner. Remove
all formwork and you are done.
No. 254 March 2013 Farming Ahead
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