Concrete is a very important building material in both residential and commercial construction. Most new houses are now built on a reinforced concrete slab with reinforced concrete footings. Reinforced concrete is also used for pathways, driveways, shed floors and outdoor areas in residential construction. Concrete is often the main material used in commercial and industrial buildings.
Basic knowledge, skills and work sequence are much the same for both large and small concrete jobs. In this chapter we will talk about:
Once again, it might be helpful to have a look at the chapter titled Planning Your Work. The general procedures described in that chapter are a good guide for planning and preparing a small concreting task. In summary, to plan and prepare for a concreting job you need to:
Concrete is made by mixing measured amounts of:
Admixtures (other ingredients added to the mix) can be used to change the properties of a concrete mix. For example, different products can be used to change the workability or setting time of a mix.
When the materials are properly mixed, the cement and water form a paste which bonds the aggregates together.
Cement
There are several types of cement which are made by kiln firing limestone, clay and other raw materials. After firing, the cement is ground to a fine powder. Type A portland cement is the type generally used in building construction.
Coarse Aggregates
Coarse aggregates such as crushed rock or gravel should be clean, strong and hard, graded with a range of sizes from around 20mm to 7mm.
Fine Aggregates
Fine aggregates such as sand and crusher fines should be clean and free of powdery material.
Water
Water must be clean, fresh and free from salt, chemicals and other matter which could affect the quality of the concrete.
If you change the proportions of materials in a mix you will change properties of the finished concrete such as its strength. Ideally, the proportions of materials used should provide sufficient strength for the job at the lowest cost.
When necessary, a concrete mix can be designed for a special purpose such as resisting extremes of heat and cold, taking the weight of very heavy trucks and machinery or just being strong enough for foot traffic.
Concrete you mix on site is generally in the proportions of 4:2:1. In other words, four parts gravel (coarse aggregates), two parts sand (fine aggregates) and one part cement. Make sure you always measure materials accurately because:
When you are mixing concrete on site with an electric or petrol driven concrete mixer, make sure you use a strong container to accurately measure materials. Never use a shovel as a measure.
Before you read further, go back and have a look at Plant & Equipment which contain useful information about the concrete mixer shown on the right. Make sure you understand the safety precautions, as well as the operating and maintenance procedures.
Square mouth shovels are generally the most efficient type to use when you are handling concrete and concrete materials. You will find useful information about different types of shovels on pages 66-68. Make sure you understand the Safe Work Methods listed on page 68.
Use the shovel to fill your measuring container, but never put the shovel into the revolving drum of the concrete mixer.
Mixing Sequence
Before you start, make sure you know how much the mixer holds. If you overload the mixer it will take much longer to properly mix each batch.
Look at the second photo in this unit and you will see a concrete truck about to enter the batching plant, where measured amounts of materials are loaded into the truck’s mixing drum. The drum is then rotated to mix the concrete.
The drum continues to rotate slowly as the truck is driven to the delivery site. The on-site photograph on the right shows the concrete being poured using a chute to place it accurately into postholes.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mr /r /f ][ What gets mixed to make concrete? ][ Water][ Cement ][ * Lime ][ Sand ][ Gravel ][ Lime was originally used to make cement before the development of portland cement. ]]
[[ mm /f ][ Match up these "halves" to make meaningful statements about concrete mixing: ][ not enough cement ~ will reduce the strength ][ too much fine aggregate ~ will give a sticky mix ][ too much coarse aggregate ~ can cause stones to protrude ][ not enough cement will reduce the strength; too much fine aggregate will give a sticky mix; too much coarse aggregate can cause stones to protrude. ]]
[[ so /f ][ Put these steps for mixing concrete in the proper order: ][ Load the measured amount of coarse aggregate. ][ Add a small amount of water. ][ Load the measured amount of fine aggregate. ][ Add the measured amount of cement. ][ Allow the contents of the drum to mix. ][ Gradually add just enough water to give a workable mix. ][ Load the coarse aggregate => Add a small amount of water => Load the fine aggregate => Add the cement => Allow the contents to mix => Gradually add water to give a workable mix. ]]
Before formwork can be erected the size, position and level of the concrete must be determined.
We will use the carport of the cottage in the chapter on Plans & Specifications as an example of finding this information.
The photograph above shows the formwork, moisture barrier and steel reinforcing prepared for a similar type of slab.
Go back to page 162 and read the dimensions of the carport slab off the floor plan. This will give you the measurements to the inside of the forms. By the way, you might hear this kind of formwork called ‘boxing’ on a construction site.
Can you find two pieces of information about levels for the carport slab in the footing detail below?
You should now have the inside measurements for the formwork, the level (height) of the slab in relation to the cottage floor and the direction of fall. What is fall?
External slabs should never be perfectly level. They should have a slight slope to run the water off. Section C-C shows that the carport slab falls away from the cottage. For this job the forms could be set up with 40 to 50mm fall in the width of the slab.
You would still need one more measurement before you could set out the position of the slab on the site. Where will you find it? You will also need to know the thickness of the slab so you can select the correct width material for the forms.
To help you understand some of the important task requirements, we will follow the progress of a concreting project which consists of a driveway and paths around a small cottage.
The photograph below shows the ground excavated to the required depth. The level for the concrete surface has been taken from the existing slab outside the front door.
Note that the construction worker has used a mattock to break up the compacted ground before shovelling the spoil out of the excavation. In the photograph above he is removing loose material from the excavation.
The hole has been made wide enough to allow for the formwork and deep enough for a layer of fine crushed rock to be spread as a base for the concrete and to level out the subgrade.
The photograph above shows the construction worker spreading the crushed rock.
The photograph on the right shows the construction worker using a screed to smooth and compact the layer of crushed rock.
He will need to make sure that the surface allows for the correct thickness of concrete from the top of the formwork and the chalk line on the other side.
The photographs show that levels for the top of the formwork and the chalk line were taken off an existing slab.
Both these levels will need to be transferred as the formwork is erected for the path around the front of the cottage.
The worker in the photograph on the left is using a laser level to transfer the height of the formwork. Below he is using a spirit level to check that the formwork is level.
Have you noticed the plastic safety caps on the steel pegs that hold the formwork in place? Steel pegs and exposed reinforcing steel are a hazard. If the worker fell on an unprotected steel peg he could be seriously injured. In the photograph on the right the construction worker is using the spirit level and a measuring tape to check the fall from the chalk line to the top of the formwork.
The formwork has been set up to give the surface of the concrete about 20mm fall so that rain water will run away from the cottage.
The steel form at the side of the cottage wasn’t quite long enough to reach the corner.
In the photograph on the left the construction worker is using a piece of timber to complete the formwork in the corner.
He will need to make sure that it lines up accurately with the steel form so the edge of the concrete will be straight.
The formwork will also need to be pegged strongly enough so it doesn’t move when the concrete is poured against it.
The formwork for the path runs across the front of the cottage to join up with the driveway on the other side. Later you will see the concrete pour starting at the driveway, continuing along the path in front of the cottage and then down the righthand side of the cottage.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ What determines the width of your concrete formworking material? ][ The thickness of the slab. ][ * The amount of concrete to be poured. ][ * The area of the slab. ][ * The amount of reinforcing steel in the slab. ][ The thickness of the slab determines the width of the formwork you need. ]]
[[ mr /f ][ What is the purpose of the yellow caps that are placed on top of top of formwork pegs? ][ To make the pegs easily visible. ][ To avoid injury by falling over or on the pegs. ][ To reduce the chances of pegs being bumped and moved. ][ * To stop the steel pegs from rusting. ][ * To make them easier to hammer into place. ][ Yellow caps won't prevent rusting; you can't hammer pegs through the yellow caps. ]]
[[ mc /f ][ When would you deliberately make a concrete slab that wasn't level? ][ * When the ground itself isn't level. ][ When the surface needs to drain water off of it. ][ * When you need thicker concrete under the walls. ][ * When you expect some parts to shrink more than others. ][ You usually only slope a slab if you want it to drain water away. ]]
Reinforcement is used to increase the strength of concrete and to help control cracking. Small paths that carry only light foot traffic sometimes may not require reinforcement. However, all concrete on our construction site is to be reinforced. Concrete slabs are to be reinforced with welded steel fabric or mesh. Common mesh sizes are F72, F82 and F92. For example, F means welded fabric, 8 stands for 8mm diameter steel in the welded fabric and 2 means that the steel is welded in 200mm squares (2x100).
Go back to pages 156 to 159 in the section on Footing Layout if you need to find out more about other types of reinforcing steel for your concreting project.
On our construction site all concrete is to be placed on a sheet of plastic which stops ground moisture from being absorbed into the finished concrete.
This plastic sheeting is called a moisture barrier or vapour barrier.
The photograph above shows a construction worker cutting the plastic to size with a safety knife. The plastic should fit neatly into the area of the slab.
The reinforcing mesh is cut with bolt cutters to fit inside the formwork. Pieces of mesh should overlap each other and be securely tied with wire ties to prevent movement of the mesh during the concrete pour.
The photograph below shows the worker tying the mesh at the overlap.
Steel reinforcing mesh should be covered all around by concrete to protect it from rusting.
Note that the mesh has been cut smaller than the formwork so the ends of the steel will be covered. The amount of cover may vary with the engineer’s design. In most domestic work, the minimum top cover for interior slabs is usually 20 to 30mm and exterior 30 to 40mm.
Bar chairs are used to lift the reinforcing mesh off the ground and position it to give the right amount of top cover.
The photograph above shows plastic bar chairs supporting the reinforcing mesh. Have another look at the Footing Details on page 159 for minimum side and bottom cover for reinforced footings and beams.
If you look carefully at the photograph above you might notice that a strip of flexible material has been placed against the edge of the existing slab. Concrete will expand with change in temperature. The expansion joint compresses and prevents damage to the concrete as it expands.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ What is the main purpose of the black plastic sheet placed under the reinforcing steel? ][ It stops ground moisture entering the slab. ][ * It stops moisture from the concrete soaking into the ground. ][ * It stops dirt getting into the concrete. ][ * It prevents termites getting into the house frame. ][ It helps the concrete to cure faster. ][ Its main purpose is to prevent ground moisture soaking into the finished slab. ]]
[[ mc /f ][ Why is the reinforcing steel / mesh overlapped and tied together? ][ * To give it extra strength. ][ To prevent it moving during the pour. ][ * To make it easier to walk over. ][ * To prevent it from rusting in the slab. ][ * To make it easier to vibrate the concrete. ][ Mesh is tied together to keep it still during the pour. ]]
[[ mc /r /f ][ What would you normally use to cut mesh to size? ][ * Angle grinder. ][ Bolt cutters. ][ * Pliers. ][ * Aviation snips. ][ * Wire cutters. ][ Mesh usually needs bolt cutters to cut it easily and safely. ]]
Before concrete is placed, you should check that the formwork and work area is clean and clear of debris or excess material that could hinder workers during the concrete pour. A release agent could be applied to the formwork if necessary.
If the concrete is to be delivered by truck, make sure there is unrestricted access to the site before the truck arrives.
Decide where you want the driver to position the truck so you can give clear directions when the truck arrives on the site.
The photograph at the bottom of the previous page shows the concrete truck in a good position to start placing concrete for the driveway. The workers are controlling the chute to place the concrete where it is required.
When you are controlling the chute you should try to place just enough concrete to give the required depth. If you place too much in one spot it will have to be moved and spread with a shovel. This can waste a lot of time and hold up the job.
The photograph on the right shows the construction worker using a concrete vibrator to compact the wet concrete.
The vibrating poker is pushed into the wet concrete for about ten seconds at a time, working in a pattern to cover the whole area.
Air is pushed out of the concrete as the vibrator shakes it. This helps the concrete to settle and become denser, stronger and more durable.
Refer to pages 106-107 more information and maintenance for the concrete vibrator.
The photograph on the right shows the worker using a float to compact the concrete with a patting action.
He is also forming the surface of the concrete, working to a chalk line on the timber retaining wall.
A similar surface will need to be established on the other side of the driveway as well because it was not possible to erect formwork for this part of the job.
This photograph shows the concreter using a screed to straighten out the concrete, working to the surfaces formed on each side.
He moves the screed from side to side as he drags excess concrete away from the formed surface.
The concrete truck should always be positioned so the pour can proceed as efficiently as possible. The truck was moved so the chute could be used to directly place the concrete for the path in front of the cottage.
The truck couldn’t be moved into a position where the chute could directly place the concrete for the whole of the path on the righthand side of the cottage. Some of the concrete had to be transported and placed using a wheelbarrow.
The photograph below shows the truck repositioned for this part of the pour and the workers adjusting the chute to discharge concrete into the wheelbarrow.
Note that the concrete for the far end of the path will have to be transported in the wheelbarrow before the nearer end is poured from the chute.
If the nearer end was poured first the worker would have to push the wheelbarrow through the concrete.
The photograph on the right shows a construction worker wheeling a barrow of concrete.
Because access to this part of the site is difficult, he is wheeling the barrow over the reinforcing. This is not the best practice but sometimes it can’t be helped.
Never put more concrete than you can handle comfortably in a wheelbarrow. This can place a lot of strain on your body as well as causing waste through spillage of the concrete.
Make sure you read all the safety precautions and operating hints for the wheelbarrow on pages 105-106.
In the previous example the concreter was screeding the driveway, working to surfaces he had formed to chalk lines on each side. In the photograph below this worker is screeding to the boxing on one side and a surface he formed to a chalk line on the other side.
The easiest concrete to straighten out is a job where both ends of the screed can rest on the formwork.
Screeding large areas of concrete, such as a floor slab for a house, requires a lot of skill and experience.
It is always better to have a little concrete left over than not to have enough for the job. So what do you do with leftover concrete?
Excess concrete should be used wherever possible to avoid waste, or disposed of according to the site environmental plan.
The photograph below shows the construction worker placing leftover concrete at the back of the cottage.
The formwork was prepared for two small slabs which were to be part of the next pour, just in case there was some concrete left over from the first pour. Wherever possible, make provision for leftover concrete when you are planning and preparing for a concrete job.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mr /f ][ Which of the following are reasonable preparations to make before a concrete pour? ][ Make sure the work area is clear of obstructions. ][ Unrestricted access for the concrete truck. ][ Know where you want the truck to be on the site. ][ Have wheelbarrows ready to move concrete to difficult locations. ][ * Know where your shovels are in case they are needed. ][ * Have the concrete vibrator on standby in case it is needed. ][ Your shovels should be within easy reach; the concrete vibrator should be ready to run - they will both be needed. ]]
[[ mr /4 /r /f ][ What tools will be needed while the pour is happening? ][ Wheelbarrow. ][ Screed. ][ Shovel. ][ Vibrator. ][ * Float. ][ * Helicopter. ][ Floats and helicopters are finishing rather than pouring tools. ]]
After screeding, concrete is generally finished in two stages. Finishing further compacts and hardens the surface of concrete as well as giving it the required appearance.
The first stage, or initial finishing, often involves bull floating the surface and then leaving it to set for a while.
The photograph above shows the concreter bull floating the driveway surface on our concreting job.
This photograph shows the concreter using a fresno float in the first stage of finishing the concrete surface.
The fresno float is smaller than the bull float and is generally tilted a little to the surface of the concrete.
In the first stage of finishing, water appears on the surface of the concrete. This is called bleed or sheen water. Final finishing cannot begin until the bleed water has dried up.
You can begin the second stage of finishing when the bleed water has dried up and the concrete can support your weight with only a slight marking of the surface. This could take from half an hour to two hours depending on the weather conditions.
The way you do your final finishing depends on the type of surface you want on the concrete.
You could use a wood float to give a rough, non-slip finish and a steel float to give a smooth, dense surface.
The Wood Float
The photograph on the right shows the concreter using a wood float in a continuous circular motion.
Using a wood float in this way pushes down any coarse aggregates and brings fine aggregates and cement paste to the surface. Floating also fills any small gaps and is your last chance to do any straightening of the surface.
The Steel Float
This photograph shows the concreter using a pointed steel float (sometimes called a flooring trowel) in the last stage of floating.
Steel floating is usually done with a wide sweeping action. Your instructor will show you the correct technique.
The float in the concreter’s other hand is a magnesium float which is light but not flexible like the thin steel float. It can be used like the wood float but gives a smoother finish. You can find a picture of a common steel float used by concreters and plasterers on page 68.
Trowelled Finish
The finish produced by a steel float is sometimes called a trowelled finish. On larger concrete jobs a trowelling machine is used to give this kind of finish. Have a look at the top photograph on page 160 and you will see a trowelling machine on the floor slab. Trowelling machines are often called helicopters on the construction site.
The photograph on the right shows the concreter using an edging tool to round off the edges of the slab.
Edging gives a neater and stronger edge that is less likely to chip.
Edging is usually done in two stages. You do the first pass during the first stage of floating and finish the edge during the final float.
Did you notice the expansion joint between the existing slab and the new concrete in the previous photo? Note also the barricade mesh used in the photograph below.
Smooth concrete can be very slippery when it is wet. For safety reasons, exterior concrete paving should have a non-slip surface.
The photograph below shows the worker using a long handled concreter’s broom to give a textured surface.
You could use an ordinary hair broom to do the same job. The stiffer the bristles in the broom you use, the coarser the textured finish on the concrete.
The photograph below shows a close-up look at the finished job.
The second pass with the edging tool was made just before the concrete surface was broomed.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mr /f ][ Which of these tools are used during initial finishing of a slab? ][ Bull Float ][ Fresno Float ][ * Wood Float ][ * Steel Float ][ * Concretor's Broom ][ Bulla and Fresno Floats are used udring initial finishing. ]]
[[ mc /f ][ What has to happen before final finishing can be done? ][ Bleed water must evaporate. ][ * The sun has to set. ][ * You need shade over the slab. ][ * Forwmwork must be removed. ][ The bleed water must have dried up before final finishing. ]]
[[ mc /f ][ What is used to produce a non-slip surface on a slab? ][ * A wood float. ][ * A steel float. ][ A concretor's broom. ][ * An edging tool. ][ * A trowel. ][ A long-handled broom can be use to give a textured, non-slip surface. ]]
Concrete doesn’t harden and strengthen simply by drying out. The cement in the concrete mix gradually takes up water (hydrates) over a period of time. Concrete reaches its full strength when the cement is fully hydrated. This can take three or four weeks. To help cure concrete you can slow the moisture loss for a few days.
There are several ways to cure concrete by slowing down the moisture loss. A couple of simple and cheap methods are:
When curing is necessary, formwork could be left on for a few days to stop moisture loss from the edges of the slab.
[[ sh /n ][ Stripping Formwork ]]If it is not specified that formwork is to be left in place for a few days most concreters will strip the forms soon after the job is finished.
Where there is more work to be done on the site the formwork is stripped, cleaned and edge boards de-nailed before moving to the new location.
The photographs show the concrete worker carefully removing the steel pegs and the forms soon after the job was finished.
When you are stripping steel forms or edge boards you must be very careful not to damage the edges or surface of the ‘green’ concrete.
You should never lever steel forms and edge boards away from the edge of ‘green’ concrete or drag them upward. This is likely to cause damage. Release the forms by carefully striking downward then pull them away from the edge of the concrete.
Formwork materials are high cost items so they should be used as many times as possible before being replaced. As an employee you will be responsible for the proper use, care and maintenance of these materials.
Make sure you scrape or brush all forms clean of slurry and concrete spills after they have been stripped, then stack them tidily ready to be moved to the next job or re-used at the current location.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mc /f ][ How should you remove formwork from around "green" concrete? ][ * Lever it away from the edge of the slab. ][ Strike the formwork down then pull it away. ][ * Strike the formwork up and pull it away. ][ * Don't disturb it for at least 48 hours. ][ * Hit it with a lump hammer to break it away from the slab. ][ you should always strike the formwork downwards to avoid chipping the edge of the green slab. ]]
Collect all pegs, safety caps, braces, outriggers and other materials used on the job and de-nail any timber items. Clean items to be re-used and store them tidily in the proper place or move them to the next job. Dispose of all rubbish properly and leave the work area in a tidy and safe condition.
Never let concrete set hard on tools and equipment such as screeds, floats, buckets, wheelbarrows, site mixers and other hand tools. It may even be necessary for you to clean them occasionally during the job. When the job is finished you should clean all tools and equipment by scraping, brushing and washing down thoroughly.
When you have finished cleaning the tools and equipment, you should make sure that everything is in good working order before you put them away or move them to the next job.
[[ sh /n ][ Personal Protection ]]Cement and lime products can cause skin irritation and infection of open wounds. Make sure you protect yourself properly by wearing watertight boots, applying protective creams to exposed areas and wearing suitable gloves when necessary.
Most concrete work is outside so you will need to protect yourself from the sun. Wear a hat when you are in the sun, apply sunscreen when necessary and protect your eyes from glare by wearing shaded safety glasses.
Before leaving this page, think of what you've just been reading, and test yourself with these questions.
[[ mr /f ][ What PPE is appropriate when working concrete? ][ Broad-brimmed hat. ][ Heavy duty boots. ][ * Gloves. ][ Sunblock. ][ Protective goggles of glasses. ][ Gloves would probably not be helpful while working concrete - all other items would be beneficial. ]]