Placing, Finishing and Jointing Concrete
How should I place concrete?
Flat, Even Sites - Start placing concrete at the furthermost corner of the formwork from where the concrete is mixed or delivered; work towards the
source of concrete supply, never away from it.
Always place new concrete into the face of concrete already in place. Be careful not to damage or move the formwork. Avoid damage or movement of steel reinforcement.
Sloping Sites -
Start placing at the lowest point, working upwards to the highest point. Pre-plan how concrete will be supplied to the site; where it is mixed or delivered.
If attempting to place a large area of concrete where the gradient is steep and/or site access is difficult, pumping the concrete may offer the most
practical solution in terms of time, energy and convenience.
If placing concrete directly from a truck or concrete pump, place concrete vertically into the face of concrete already in place. Never allow the concrete to fall more than 1 to 1.5 metres.
You must compact concrete during placement.
How should I compact concrete?
Compaction is a necessary part of concrete placement and must be done while concrete is in a workable (plastic) state. Compaction is the first step to ensuring that the hardened concrete will be strong and durable.
For most small household projects, compaction would typically be achieved by first working the concrete well into the formed area, particularly alongside the formwork, with a spade or shovel as the concrete is progressively placed. Use a spade or shovel in poker action to consolidate the concrete to a consistent depth throughout the entire job.
The second step in the compaction process is tamping the placed concrete with a screed board. A screed board can be a straight-edged piece of timber placed horizontally across the formwork. First tamp the concrete with the screed board, followed by sliding the screed board back and forth across the surface of the concrete. Continue this motion while travelling along the formwork in the same direction in which the concrete is being placed. The concrete should now be compacted and levelled to the top edge of the formwork.
Always keep a small amount of concrete in front of the screed board to produce an even surface across the entire job.
If intending to construct a floor slab for a house, seek expert advice. It will require mechanical compaction using a poker vibrator for the deeper sections of concrete.
How do I finish concrete?
There are five simple steps to be undertaken to finish concrete.
Following compaction of the placed concrete, for most household projects, the initial finish would be achieved by screeding the concrete surface a second time to further compact the concrete.
This will also help to correct any minor unevenness across the surface and remove small potholes and blemishes which may have occurred during earlier screeding.
Check that the placed concrete is level with the top edge of the formwork. Again, keep a small amount of workable (plastic) concrete in front of the screed board when screeding the concrete surface.
Bleed water will usually appear on the surface of the concrete after the initial screeding. Only when it has evaporated may final finishing be commenced.
Do not try to dry up excessive bleed water by using sand or cement as this only weakens the hardened surface. It can be removed by dragging a garden hose across the surface of the concrete.
Once the concrete can support the weight of a person, leaving only slight surface imprints, the final finishing can begin.
Final finishing involves floating or trowelling the surface of the concrete, jointing and edging. If a special surface finish is required such as a broomed or exposed aggregate finish, this is undertaken at the final finish stage. To select an appropriate finish refer to Table 4.
Floats are usually made of timber, trowels of steel. Steel trowels produce a smooth surface finish. Always bear in mind that steel trowelled concrete surfaces can become slippery when wet and are better suited to internal use. Wood floats produce a slightly textured surface finish, are less likely to be slippery when wet and are better suited for external use.
Using either a wood float or steel trowel, place the tool flat on the surface. Incline slightly upwards in the direction of motion. Run the tool smoothly and evenly over the surface of the concrete, right to left in an arc, keeping it slightly inclined in the direction of each motion and moving outwards or inwards to cover the surface.
INSERT Table 4 Applications of Various Finishes
To produce the desired texture, it may be necessary to trowel the surface a second time after further hardening of the concrete has taken place.
An edging tool, which can be purchased from most hardware stores, should be run around the perimeter of the slab adjacent to the formwork to produce a neat, rounded concrete edge finish.
Keep the edging tool firmly against the formwork to prevent it from biting into the concrete.
Broomed finishes are attractive, slip-resistant textures achieved by PULLING a damp broom across the concrete surface after it has been wood floated or steel trowelled. Coarse textures suitable for steeply sloping paved surfaces are produced by stiff bristle brooms.
Medium textures are obtained using soft bristle brooms. For best results, the broom should be rinsed in water after each pass and tapped to remove any excess water.
A broomed texture can be applied in many ways - straight lines, wavy lines. Driveways and paths are normally broomed at right angles to the direction of traffic.
Fine to medium broomed textures are commonly used to make slip-resistant surfaces on exterior entertainment areas, garage and shed floors, and gently sloping and flat walking paths and driveways. An edging tool should be used to neaten each side of the area that has been broomed.
Exposed Aggregate Finishes
Exposed aggregate finishes also provide an attractive slip-resistant texture and can be achieved either by 'seeding' the surface of the concrete with decorative gravel, or by exposing the structural aggregate used in the concrete.
When 'seeding' the surface, the gravel (usually 20-mm) is scattered evenly over the surface and embedded by trowelling or screeding until just covered with mortar. When the concrete has hardened sufficiently to bear the weight of a person, the stones are exposed by gently water washing and brushing with a stiff nylon broom several times more vigorously as the concrete hardens.
Alternatively, this method of water washing and brushing can be used to expose the structuraI aggregate in the slab, but in this case the pre-mixed concrete supplier should be consulted prior to supply of the concrete so that an appropriate mix can be supplied.
However, it should be stressed that this finish is best executed by experts and can be achieved over hardened concrete by professionals applying a thin decorative topping slab.
Why do we need joints in concrete?
There are a number of reasons why joints are needed in concrete.
Firstly, as concrete dries, it shrinks. If this shrinkage is restrained, then the concrete is liable to crack. Joints allow the shrinkage to occur and minimise the risk of cracks occurring randomly through the member.
Secondly, paths and footings, driveways and garages, support different loads and will behave in a different manner from each other. Therefore, joints should be provided between these elements to allow this to occur. Joints are required to separate the extent of the work which can be completed in a day.
Where should joints be placed?
In paths and driveways where the concrete is unreinforced, the joint spacing should not exceed 2 m. In addition, the ratio of the long to short side of any panel should not exceed 1.5:1. Joints should also be formed adjacent to existing structures.
Joints are formed by using a grooving tool available from hardware stores.
A grooving tool can be used to form control joints
Why should concrete be cured?
Curing is essential to the
achievement of good quality
concrete. It is a simple process,
frequently overlooked to the
detriment of long-term durability
and the wear resistance of the slab.
The purpose of curing is to ensure that the concrete does not dry out prematurely, but retains moisture so that it will build up strength and gain
durability and resistance to wear. The concrete should be kept continuously damp for at least 7 days to achieve satisfactory curing. The easiest method is to cover the concrete with plastic sheeting immediately after finishing.
Alternatively, a commercially produced curing compound may be sprayed, brushed or rolled onto the surface. If using a curing compound, read the
manufacturer's specifications and use as directed.
Remember Curing is essential.
When are topping slabs used?
Topping slabs are used to surface either new or old concrete and are frequently used as levelling slabs to aid water drainage. Topping slabs from 25 to 50 mm thick can be placed over concrete that is at least one day old and has a suitably roughened and clean surface.
The clean surface should be saturated with water, preferably for several hours prior to placing the topping. When beginning to place the topping slab, ensure that the surface is moist but that there is no water lying on the slab.
A suitable mix to achieve a hard-wearing surface for household applications is 1 bag of cement: 55 litres of sharp sand: 80 litres of 10-mm-diameter gravel or crushed rock. Water is added sufficient only to obtain a plastic and workable mix. The amount of water will depend on the dampness of the sand and gravel.
Toppings can be applied in a minimum thickness of 25 mm, but require a rough surface to achieve good bond. Generally 40-mm-thick toppings are recommended. If the topping exceeds 50 mm thick, then use of reinforcement should be considered.
The topping should be placed and compacted as soon as possible after mixing to ensure a well mixed concrete. The topping should be compacted
and finished as described previously.
Any contraction joint in the base concrete should be repeated in the same position in the topping slab.
Ensure that the topping slab is properly cured.
Why use a protective coating?
The stain resistance of clean concrete can be enhanced by the periodical application of a suitable clear sealer or paint-coat finish.
Seek advice from paint suppliers regarding suitable protective coatings for concrete surfaces recommended for your particular application. Follow manufacturer's instructions.
Coloured Concrete Finishes
How do I colour concrete?
Concrete may be coloured by painting or staining, or by incorporating colouring pigments into the concrete mix. Paints or stains should be applied strictly in accordance with the manufacturer's instructions.
Painting and Staining
Paint finishes are either water-based or mineral-based, are susceptible to abrasion and will need to be reapplied periodically. Stains are epoxy- or silicon-based and penetrate the concrete surface. Stains often need several applications before an even colour is obtained.
Colouring pigments are expensive, with some colours more costly than others. As a result, they are rarely incorporated into the slab or pavement but are applied subsequently in a topping.
Coloured toppings can be achieved in one of three ways. By applying a topping to the slab before it has hardened and when the surface water that
appears after the slab has been floated, has disappeared; or by applying a topping to hardened concrete which has a roughened (broomed) surface; or by applying a pre-mixed dry-shake colouring pigment to the concrete before it has hardened but after the surface water has evaporated.
About two-thirds of the dry-shake mixture is applied uniformly to the surface which is then refloated, edged and jointed. The remaining dry shake is then applied and floating, edging and jointing repeated. The topping is then cured in the normal way.
The timing of the application is critical. If the surface of the concrete is too wet when the dry shake is applied, the surface will not be strong. If the dry shake is applied too late, it will not bond to the base concrete.
If pre-mixed dry shake is unavailable, colouring pigments can be mixed dry with cement to the manufacturer's instructions. This combination is then mixed with sharp clean sand in the proportion by volume of 1 cement: 2 sand.
Whereas a competent handyman can gain reasonable results with integral colouring, particularly when used in applications around the house such as garden paths and borders, if large areas of patio or driveway are to be coloured, a skilled and experienced concreter should be employed. It may be advantageous to investigate the wide range of colours, patterns and textures available from the suppliers of stamped and patterned paving.
If mixing concrete on site, it is recommended that a 1- x 1-m trial area is formed to establish the desired colour, the timing of the application and the correct trowelling technique. Use a suitable container to measure the exact proportion of colouring pigment in each batch of concrete or topping mixed, otherwise the colour will not be consistent.
Pre-mixed concrete suppliers may supply coloured concrete mixes subject to minimum quantity orders and the purchaser's acceptance of surcharge rates.
Integral colouring is commonly provided by the dry-shake method.
What can you tell me about decorative concrete finishes? How do I select an experienced concreter and, how do I specify the type of finish I want?
Permanent decorative finishes can be built into concrete during construction to enhance the prestige and value of any home.
Decorative variations, from natural exposed-aggregate finishes, colourful textured treatments to geometric patterns which are either stamped, scored or rolled into the concrete to resemble stone, brick or tile paving, are limited only by the skill of the concreter and the scope of available designs, colours and natural aggregates (see Finishing Concrete).
This type of work would be undertaken by a concrete paving contractor and in the case of exposed aggregate finishes, these are often undertaken by concrete pebble paving contractors.
Both categories are listed in Yellow Pages telephone directories.