Before we divulge into the details of concrete, what exactly is it? Let us establish, however, it is not the same as cement, cement makes up a part of concrete. The following ingredients are combined to make concrete:
- general purpose cement (which is a type of Portland cement)
- water
- rocks, sand, or gravel (this is known as aggregates).
The cement element of concrete, which is a powder, is like a binding agent when it is mixed with the water and aggregates (between 60 and 80% aggregate is in concrete) . Once all the elements have been combined, it creates a concrete mix that will be poured, hardened and voila! Concrete. 6
Concrete is one of the oldest and most cost-efficient materials around. The possibilities of what concrete can be used for is limitless. Concrete is a durable flexible material that is used for a range of purposes, in fact, 70% of the world’s population either lives or works in a structure or building that contains concrete. When looking at the building industry in Australia, concrete is considered the most cost-effective material. The aggregates used are likely to be locally sourced because they lower the cost of transporting, and it helps to support local suppliers.
While concrete has a wide range of applications, we will focus on some of the simpler uses of concrete including the following: 3
Slabs
A slab is a concrete pad which is built on the ground. A slab has a range of purposes and there are a number of types of slabs. Slabs can range from a small pad for a BBQ or outdoor setting in a garden to a strong, level platform for a building to sit on. The slab is one of the most important aspects of a building project, especially when looking at a house or a building. If the slab is not poured correctly, it will not set correctly. If it does not set correctly, the entire build is at risk because the slab is the foundation of the build.
Concrete beams
Concrete beams, sometimes called thickening beams or footings, run through raft slabs to provide added strength. They can also reduce concrete tensioning, a concept we will explore a little
Concrete aprons
Aprons are the extension of a slab. You will often see an apron between the front of a garage and a driveway. You may also see them around the sides of a house, shed or garage. They can be used to drain water away from the building structure.
Channels
Channels are used as part of a water drainage system. They are shaped like a square ‘U’. Sometimes drainage grates sit at the top of the U.
Formwork
Formwork is essentially a mould. Wet concrete is poured into these supportive temporary moulds which are then removed once the concrete has set and can support itself. Commonly, formworks are made from timber, though they are available in reinforced plastic, steel, or fibreglass.
Post holes
Concrete is used in post holes to hold the posts securely in place. Once dug, the hole is wetted, and the water allowed to drain into the soil before adding gravel. Rapid set concrete is recommended, of which half should be poured in and tamped, then the remaining half. The post hole depth may alter according to the job at hand.
Pathways
Pathways are set out using formwork. Concrete is then poured into the formwork.
Garden edges
Garden edges are set out using formwork. They are made by setting out the placement and shape (for example, will it curve around garden beds, will it remain straight, etc) and then setting up the formwork and pegs. Floating with a trowel is done to the smooth the surface, as well as edges and control joints. Concrete is then poured into the framework.
Beam thickeners
Beam thickeners are used to strengthen slabs and reduce concrete tensioning.
Fun fact!
Aside from water, concrete is the most used product in the world!
As with any new knowledge, many of the materials and techniques used in concreting may be unfamiliar to you along with the various terms that are associated with it. Let us take some time to go through the terminology you will start to add to your vocabulary toolbelt.
Materials used in concreting
There are a range of components and materials that you will need to become familiar with when working with concrete. For example:
CAN THE GRAPHICS TEAM PLEASE DRAW UP A GRAPHIC THAT HAS ALL OF THE BELOW COMPONENTS IN IT. THEN, NUMBER THEM AS A REFERENCE TO THE BELOW TABLE.
|
No. |
Components / Materials |
Description |
|
1 |
Formwork |
Formwork is the term which describes the combination of components that are used to make the mould that concrete is added to. The formwork is removed once the concrete has hardened. |
|
2 |
Bar chairs |
Bar chairs are used to support rebar and reinforcement mesh at the correct height in slabs. They are generally made from plastic (although 3concrete and steel bar chairs are also available). They come in d4ifferent shapes, sizes and heights. |
|
3 |
Braces |
Braces are used to keep formwork stable and in place. They provide additional strength and support to the edge boards to prevent bowing and movement. They may also be used in areas where the formwork could be disturbed. Braces are generally made from steel or timb9er and are propped against the formwork to keep it secure. Steel braces are often adjustable. |
|
4 |
Edge boards |
Edge boards are used to create the temporary mould into which concrete is poured. Edge boards are expensive to replace, therefore should not be cut to size. Edge boards can be purchased in different sizes and are manufactured straight and true. Some edge boards are made from laminated veneer timber, thus being referred to as LVL, and some are made from aluminium. |
|
5 |
Pegs/pins |
Pegs, which are driven into the ground to provide support to edge boards and keep them securely in place. Pegs are made out of timber and steel (often called pins). |
|
6 |
Reinforcement mesh |
Reinforcement mesh is used to give strength to concrete and reduce cracks and breakage. Reinforcement mesh sits on top of bar chairs to keep it clear from the ground. Reinforcement mesh comes in large sheets and will need to be cut to size for smaller projects. |
|
7 |
Spacers |
Spacers are used to hold reinforcement in the right position and ensure correct concrete cover. Spacers are made from concrete, plastic and metal and serve the same purpose as bar chairs. |
|
8 |
Reinforcing bars (often called rebars) |
Rebars are individual rods used to provide strength to concrete under tension, to reduce the risk of cracking and breaking, and to absorb stresses on the concrete. Reinforcing bars perform the same job as reinforcement mesh. |
|
9 |
Vapour barriers |
Vapour barriers are sheets of plastic placed beneath reinforcement materials. Vapour barriers stop moisture from getting into the slab and inhibit mould growth and concrete cancer. |
Concreting techniques
Along with the terminology used, there are various concerting techniques you will need to familiarise yourself with. The following terms underpin each of these concreting techniques
Finishing
Finishing is the process used to apply different surfaces or textures to the concrete. For example, in some applications, you might want a very smooth finish, in other instances, you might need a rougher surface to create a non-slip surface.
Placing
Placing is the process of adding the concrete to the mould. This may involve shovelling from a barrow for smaller jobs or pouring from a mixer for larger jobs. It involves using equipment such as wheelbarrows, buckets, chutes, pumps and so on to place concrete into the mould. The technique requires concrete to be placed carefully and in a quantity that the person can efficiently work with. Concrete should be placed side by side and worked together, starting from the perimeter and working in. Concrete should not be placed by dropping from heights of more than one metre.
Compacting
Compacting is the process of vibrating the concrete to remove air pockets and to ensure that it reaches into every corner of the formwork.
Screeding
Screeding is the process of levelling surface or the top layer of the concrete. Screeding involves using a straight edge or a screeding tool (which may be manually or mechanically operated) to smooth and flatten the concrete, as well as removing any excess. Screeding occurs after the concrete has been placed and spread. Screeding uses a sawing and scraping motion to smooth the concrete and draw back the excess.
Vibration
Concrete vibration is an integral part of the concrete pour because it removes air pockets. This helps to maximise its strength and density along with avoidance of cracking as it sets. To vibrate the concrete, there are three key categories for vibrating concrete, these include:
Immersion vibrators
This type of vibrator rotates with eccentric weight causing the vibrations to ripple from the casing and concrete. The Immersion vibrator, also known as poker or needle vibrator is immersed into the concrete, approximately 150mm into the layers of concrete. This process should take 15-20seconds when sedentarily inserted, and the air bubbles will stop surfacing.
Surface vibrators
This type of concrete vibrator uses an external downward force onto the concrete, which enables an easier work process for larger and flat areas of concrete pours. They are best used for compacting concrete slabs, roads, and industrial floors. Surface vibrators are best used in conjunction with immersion vibrators as they do not penetrate the concrete as much.
Form vibrators
This type of vibrator is used for high density reinforcement. Also known as external vibrators that have been designed to clamp onto a particular type of formwork. They consistently vibrate through the formwork to create concrete compaction. Concrete compaction is achieved once there are no more visible air bubbles on the surface of the concrete pour. Form vibrators are best used on concrete floors, beams and one sided blind walls. 10
Over vibration
Over vibration can lead to segregation, where the aggregate sinks to the bottom and a thick film of water sits on top of the cement. This can lead to the concrete losing strength and sinking.
Floating
Floating is another process which helps level and compact the surface of the concrete. A concrete float is moved backwards and forwards across the surface to further smooth it out and remove any gaps or ridges. Excess water must have evaporated from the concrete before floating begins. Floating is performed after screeding to remove any imperfections.
Curing
Curing is a process used to slow down the setting of the concrete by keeping it moist. This allows it to harden properly.
The following are some examples of techniques that can be applied to the curing process:
- covering with a sheet to maintain moisture as soon as the surface is set (mechanical barrier)
- covering with hessian or damp sand and spraying with water to ensure it is kept damp (absorptive cover)
- using curing compounds by either spraying them on or using a roller (membrane forming curing compounds).
The ‘states’ of concrete
The state of the concrete means the stage that it is in the setting process.
Plastic state
Plastic state refers to concrete that has not set (i.e. wet) and is still able to be moulded. This is the form concrete is in at the time of placing and compacting. You will sink into concrete when it is in the plastic state.
Setting state
Concrete in this state is starting to stiffen, which means that placing and screeding starts to become harder as you can feel the properties of the concrete changing. You will be able to leave marks, handprints, and footprints in concrete during the setting state.
Hardened state
Concrete in this state is gaining strength. You will not be able to mould it, you will not be able to leave footprints or handprints in it and it will be easier to walk on.
Workability
Workability relates to the ability to place, handle, compact and finish. This will relate to factors such as the current state of the concrete, how well it has been mixed and the coarseness of aggregate used.
Cohesiveness
Cohesiveness relates to the consistency of the concrete during the plastic state. Too much water will lead to a sloppy mix that cannot be worked with and can lead to segregation. Coarser aggregate leads to concrete that is less workable.
Strength and durability
The strength and durability of concrete is impacted by a number of factors, including the correct setting of reinforcements, compaction (well-compacted concrete equals less voids), how long the curing process takes etc. The stronger the concrete, the more durable it will be.
Formwork for brick veneer slab on ground video
Before slabs can be poured out onto a site, the ground needs to be levelled. Remember, the concrete will set according to how it is poured, if the ground is not even, the concrete will set into the crevices and it will set unevenly. The uneven set of the slab will cause all sorts of complications later in the building process.
The first part of this video from Buildsum shows the formwork and slab for a brick veneer, slab on ground building.
Consider safety
As always, safety must come first, which is why it is a part of the planning and preparing phase. When concrete is in its dry form, the dry mix will come with an SDS/ MSDS- Safety/ Material Safety Data Sheet. The SDS needs to be read carefully before opening the materials as they have instructions and information about the ingredients.
Along with the SDS, you also need to factor in the manual handling of this task. Once you receive a concrete delivery, you will need to be careful with how you transport it. This includes applying appropriate lifting techniques and use of additional equipment such as trolleys.
PPE
PPE is a standard precautionary measure you need to take no matter what task you carry out on a building site. When using concrete, the following PPE is recommended:
- safety hat
- disposable mask of dealing with a small amount of dust
- mask with respirator for medium dust levels
- safety boots
- hi-vis clothing
- gloves
- eye protection may also be required to prevent dust particles penetrating the eyes.
We are now going to go through the 6-step process required when using concrete.
