www.kondiningroup.com.au No. 254 March 2013 Farming Ahead 25

There 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

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

Cheat sheet for concrete slab construction

DIY concrete Workshop |

Table 1 Recommended concrete strengthStrength 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 finishesapplication 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

Phot

os: J

osh

Gium

elli

and

Ben

Whi

te

Farming Ahead March 2013 No. 254 www.kondiningroup.com.au26

| Workshop DIY concrete

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).

5 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.4 Here we have used the front end

loader on the tractor to grade the site reasonably flat.

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.

7 Remember to check all formwork for squareness.

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.

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.

2 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.

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.

www.kondiningroup.com.au No. 254 March 2013 Farming Ahead 27

DIY concrete Workshop |

10 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 post in a hole, and a series of star pickets and rope have been used to hold it vertical.

14 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 forming as the slab cools and shrinks.

15 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.

11 While you can use a magnetic level to make sure the pole is perfectly vertical, this handy post level

makes the job just that little bit easier, and only costs $10.

13 With all the formwork completed, it is now time to add underlay to the site. We are using this heavy-

duty poly sheet purchased from a hardware store. A 4m x 10m roll costs about $60.

12It is secured to the post with an elastic band, and has spirit levels on two sides to ensure the post is

vertical in both directions.

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Farming Ahead March 2013 No. 254 www.kondiningroup.com.au28

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.

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.

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.

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.

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).

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.

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.

| Workshop DIY concrete

www.kondiningroup.com.au No. 254 March 2013 Farming Ahead 29

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.

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.

32 Use an edging trowel to round the corners of the pad as shown.

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.

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.

30 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.

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.