How To Concrete
Unlike the Roman recipe, a modern concrete mix typically consists of 4 parts aggregate (small stones), 3 parts washed sand and 1 part cement. Because of the weight of the materials, all but the smallest of quantities are usually mixed in a cement mixer which can be hired for short periods. Larger wet pre-mixed quantities can be delivered by truck and (access permitting) can be pumped to the site where it’s required.
The Pantheon in Rome, built by Marcus Agrippa some 2000 years ago is a living testament to the qualities of concrete. It’s no small wonder that concrete is the accepted material of choice when it comes to strength and hardness in any application you can imagine.
Site Preparation & Steel reinforcing
The first stage of a typical concreting job, like a concrete driveway or concrete slab, is your site preparation which will consist of the following;
- Measure, mark out, dig out and level the area (erect string lines).
- Allow for drainage where necessary with channel, agricutural (Ag) pipe, and drainage gravel.
- Plan for wheelbarrow access to the site – planks may be required.
- Measure cut and place heavy duty plastic within the area to be concreted. This will slow the amount of moisture loss, help curing and provide a moisture barrier. For a path or driveway where moisture is not an issue – simply wet down the area (not to wet) prior to laying the concrete mix.
- Measure for steel reinforcing mesh and rebar – this should cover the area to be concreted to within 30mm of the perimeter. Builders merchants will advise on the gauge depending on the area and application. Mesh will need to be cut to fit and wired together where more than one piece is used.
- Mesh reinforcement must be approx the middle of concrete with the mix over and under. Plastic stirrups (bar chairs) are available or you can used small rocks, brick etc to suspend the mesh.
Important; Any monolithic (one continuous area, like a house slab) slab larger than 2 metres requires reinforcing steel which will transfer weight and stress across the whole area and minimise cracking. This is especially important if the slab is load bearing ie. a water tank base or is to be built on fill or soft ground which is likely to compress.
Form work is a temporary structure built to contain, shape and finish the edges of your concrete.
- Formwork is built to the finished height of your concrete – check for levels using stringline and level on timber.
- Formwork for a slab must be able to withstand the pressure of the concrete that it contains without warping or changing shape. Use plenty of pegs and nail as required.
- Form work is usually made from timber (jarrah) and an ideal size for a slab for a shed would be 75 to 100mm x 25mm. Other timber can be used including plywood but all formwork must be securely pegged so it does not move. Nails should be used instead of screws as the formwork will need to be taken apart after the concrete has cured for about 48 hours.
- Make sure the top of your formwork is straight and flat as it will be used to screed your concrete.
Concrete cost is a trade off between the amount of concrete required for a job and the amount of work you are prepared to do. Mixing concrete is hard work so my advice is to pay for convenience. Mini and micro concrete delivery is a very well serviced area.
- Mix by hand – In a wheelbarrow or flat surface (sheeted) 4 parts aggregate, 3 parts washed sand, 1 part cement. Mix dry then make a pile and form a hole in the middle. Slowly pour in water (not to wet) and work toward the centre.- Smaller quantities only suitable for small jobs.
- Concrete mixer – Suitable for small slabs and driveways. Allows you to work at your own pace. Tip; Try and have a mate load the mixer while you are laying the concrete down as it starts hardening in as little as 10 minutes and can be walked on in about an hour or so.
- Delivered Pre-Mix Concrete – Available in very small to very large loads. If you are laying down a large amount, have 1 or 2 mates give you a hand or you could have a problem laying it down and finishing before it hardens.
Tip; When the concrete arrives it’s all hands on deck until the job is done so plan for it.
Before you start laying down your concrete make sure you have a straight edge (a straight length of timber long enough to span your formwork about 100mm wide (so you can grip it) and heavy enough to stay down flat. A bucket of water and a scrubbing brush will be useful to clean off any concrete buildup.
- Empty concrete out of the wheelbarrow in mounds close enough together to fill that area when spread.
- Roughly level it with a spade or shovel allowing slightly more than you think you may need.
- Lay your straightedge across the concrete so it overhangs the formwork by about 300mm and, with you mate at the other end, use a sawing motion to level the concrete out leaving flat, slightly ridged concrete behind.
- Concrete buildup in front of the straightedge can be used to fill any holes you have passed over. Go over these filled holes with the straightedge again and continue.
- The concrete that the straightedge has passed over should be flat with slight ridging which should be fine for a pathway or drive. your finished.
- If you require a smooth finish – as the concrete drys, float the surface with a steel float until there are no lines and the surface is flat.
Video – Example of reinforcing steel and rebar in a driveway
Video – Laying and finishing concrete
Contraction (Control) Joints
Control joints provide a point of weakness where the concrete can crack as it expands and contracts. This is critical if the concrete is constrained in any way , for instance -being tied in to the house or other structure like a wall or roadside curb.
- Control joints are usually applied after the concrete is layed but still wet by making a grove along a straight line with a grooving tool or piece of metal or wood 6mm wide.
- The groove needs to be about 30% of the depth of the concrete.
- The groove can be finished by working an edging tool up both sides. Be patient as aggregate rich mixes can be difficult to finish. Tip; Try working some slurry into the gap and edge – then try the edging tool again.
Expansion or Contraction Joints
Expansion joints provide a similar function to control joints but are designed to accomodate concrete expansion and contraction without cracking by having a wider join which is usually about 10mm. This join is filled with a flexible compound like bitchumen tar or heavy duty caulking compound which will absorb any movement. Expansion join foam is available in roles of different sizes to accommodate every application and is installed prior to concreting.
Tip; Important: Ensure your contractor installs suitable expansion joints around ajoining structures like swimming pools and your house as this is often overlooked and can cause major problems.
Video – making control joints with the right tools