Now you are ready to start mixing… and placing, compacting and finishing the concrete – all of which are very important in ensuring that you produce good quality concrete.
This applies to concrete cast in place, as well as to concrete for precast items such as decorative stepping stones, concrete sleepers, birdbaths and garden ornaments.
1. Batching by volume
Batching is the process of measuring out the materials per load as accurately as possible. On small projects batching is normally carried out by volume.
• Where mixing is by hand, batches should be based on whole bags of cement, using the amounts of sand and stone as given in Concrete basics for building: Estimating and ordering materials for different stone sizes.
• For machine mixing, the amount of cement, stone, sand and water in each load should be just enough to allow the mixer to mix efficiently. Use the yield given in Concrete basics for building: Estimating and ordering materials to calculate the correct volume of each raw material per load.
Measuring vessels or containers should be loosely filled flush with the rim – part-filling is less accurate, and compacting should be avoided.
• A bag of cement contains 50 kg of cement, or approximately 33 ?. It is recommended that only whole bags be used, as cement ‘fluffs up’ when poured into another container.
• Standard builders’ buckets are useful as measures, but are rather small, their capacity being 12 ?.
• Wheelbarrows can be used to measure stone and sand. Filled flush, a wheelbarrow holds 65 ?.
• The most convenient measures are old, but clean, steel 20 or 25 ? oil or paint drums. (Note that filled to the rim, these volumes are actually about 10% more, i.e. 22 and 27 ? respectively.)
• Containers larger than about 27 ? are too heavy for easy handling.
• Provide enough containers for each material for one batch. While the mixer is mixing one batch, all the materials for the next batch can be prepared, and there is no delay in recharging the mixer.
The mixer operator or overseer can check batch quantities at a glance. Ensure that the batcher or mixer operator has clear instructions on batch quantities, either in writing, or in the form of a simple diagram.
2. Mixing concrete
Options available include mixing by hand, using a drum or pan mixer, or ordering ready-mixed concrete.
2.1 Hand mixing
Hand mixing concrete is hard physical labour. Unless great care is taken, thorough mixing is difficult to achieve. Where mixing by hand is unavoidable, use smaller sized stone, e.g. 13,2 mm instead of 19 mm.
Work on a clean, hard surface such as a concrete floor or a metal sheet. Very small batches can be mixed in a wheelbarrow.
Don’t mix concrete directly on ground:
• The ground absorbs mixing water from the fresh mix, affecting workability and potential strength.
• The mix will become contaminated with soil.
First spread out the measured materials in even layers. Mix the materials with a spade or shovel until no grey streaks are seen, then make a heap and hollow out the middle. Next, pour some of the water slowly into the hollow and shovel material from the edges of the heap into the centre, turning over each shovelful as it is dumped.
Add a little more water at a time while turning the material over, shovelling from the centre to the side and back to the centre of the heap, until the whole batch is the same colour and consistency.
2.2 Machine mixing
When loading directly into the mixer from barrows, buckets, etc., load the stone first, together with some of the water to clean the drum and prevent build-up of mortar around the blades. Load the cement next, followed by the sand and the remaining water.
If using a loading skip, place the materials in the skip in the following order: first the stone, then the cement and finally the sand to reduce cement loss and dust nuisance. Feed most of the water into the mixer quickly, starting just before raising the skip. Add more water slowly while the drum is turning until the desired consistency is reached.
In both cases, mixing time must be long enough to produce concrete of uniform colour and consistency – usually between two and three minutes.
2.3 Ready-mixed concrete
There are many benefits in ordering concrete from a reputable readymix supplier, especially when larger volumes are required.
• The readymix supplier has the resources and the technical expertise to provide a range of mixes, matching the right mix design to the application.
• The quality of the concrete is guaranteed. Site batching tends to have a higher risk of strength variability due to the lack of sophistication of small batching plants.
• Concrete can be ordered to fit in with the total construction programme, and delivery times can be changed at reasonably short notice, e.g. to avoid placing in bad weather.
• One load of ready-mixed concrete can be discharged directly to several positions on site, saving time and labour moving concrete in wheelbarrows.
• Discharge from a readymix truck is faster than from wheelbarrows.
• Site batching in residential areas raises concerns about noise levels and duration, messing of frontages/verges and potential contamination of storm water drains.
• No storage of materials on site means less pilferage, and no environmental concerns – and no clean-up operations after construction.
Most readymix suppliers also offer a pumping service. Where access is difficult, pumping moves concrete quickly across distances and heights that would otherwise be problematic.
Ready-mixed concrete offers a speedy, cost-effective alternative to site batching. When calculating relative costs, take into account the total cost of the materials delivered to site, storage, wastage and theft, the hiring or purchasing and operating costs of the necessary plant, as well as the cost of labour and supervision.
When you ask for a quotation for ready-mixed concrete, you need to specify concrete strength and workability (slump), as well as the site address. You will be given a price per m3 delivered to site. When you accept the quote, you place the order for the total amount of concrete required in m3, and give details regarding project start date, amount of time between trucks, etc. At this point, you will be given payment options and conditions.
Choose a reputable readymix supplier. The South African Readymix Association (SARMA) is a national body that sets standards and guidelines for environmental, safety and quality issues. All members must abide by these and are audited to ensure that they meet the necessary requirements.
3. Transporting fresh concrete
Transport concrete from where it is mixed or discharged from a ready-mixed concrete truck to where it is to be placed in a way that it does not cause the concrete to segregate, lose fine material, dry out or become diluted or contaminated.
Only fill wheelbarrows to about ? with fresh concrete to avoid spillage en route.
• Check that wheelbarrows are grout-tight.
• If the constituents segregate in the wheelbarrow, remix the concrete by giving it a few turns with a shovel before tipping it out.
4. Site Preparation and Formwork
Prior to mixing or ordering concrete, check that:
• Foundations have been excavated to the correct depth and width.
• Any required formwork and/or reinforcement is in place.
• If you are using ready-mixed concrete, make sure that access to the site is clear, that the roadway is strong enough to support the weight of the full mixer truck and that the truck will be able to safely discharge concrete as close as possible to where it is required.
Formwork is commonly constructed of timber, plywood or steel and should be:
• Strong and rigid enough to withstand its own weight, the weight and pressure of the fresh concrete and the weight of the workmen and equipment during concreting.
• Able to be stripped or dismantled easily and safely without damaging the concrete or the forms.
• Accurate in both dimension and position.
• Grout-tight to prevent leakage of cement paste from the fresh concrete.
• Of acceptable surface texture.
Moulds for precast elements, e.g. stepping stones, should be tapered to allow easy de-moulding.
Ensure that sufficient space between the steel or other reinforcement and the edges of the mould or formwork is left so that there will be enough concrete ‘cover to steel’ to inhibit rusting.
Before use, cover all form or mould faces in contact with the concrete with a recognised release agent as thinly, but as thoroughly as possible to assist in securing a clean release without damage to the concrete, or to the mould or shutter.
After stripping, carefully remove any adhering concrete from the formwork – use a hardwood wedge for timber formwork, a metal scraper for steel.
5. Placing and compaction
Before placing concrete in foundations, or against earth or other materials which may draw water from the fresh mix, wet the area thoroughly, but not so much that there is free water standing where the concrete is placed.
Deposit concrete as near as possible to its final position, in layers (not more than 150 mm thick for hand compaction, or more than 450 mm thick for poker vibration).
Air becomes trapped in fresh concrete during placing. To ensure that the concrete attains its full potential strength, as much of this trapped air as possible must be expelled. Pay special attention to compacting the edges of slabs, corners of paving, etc.
5.1 Hand compaction
As soon as the first layer has been placed, rod, spade and/or tamp the concrete, taking care to work the concrete well against formwork, into corners and around any reinforcement.
Level and compact slabs such as floors with a heavy wooden beam fitted with a handle at each end. Use a chopping action first, then a sawing motion to strike off the surface flush with the side forms.
On thin slabs use a pipe roller to compact the concrete and finish the surface in one operation.
5.2 Compaction by vibration
This is usually done with immersion or poker vibrators.
Do not use the vibrator to move the concrete from the point of deposit to its final position as this can cause segregation.
• Vibrator operators must be properly trained and supervised.
• Vibrators must be of the appropriate size and capacity and in good working order.
• Place concrete in shallow layers (no more than 450 mm) which must be vibrated before subsequent layers are placed.
• Insert the vibrator at points close enough together to allow complete compaction.
• Insert the vibrator quickly into the concrete and withdraw it slowly.
• Provide additional vibration in corners and next to construction joints.
• Avoid touching the formwork with the vibrator.
If cracks develop in the concrete while it is still plastic, close them up by reworking or re-compacting the surface before the concrete sets – usually within three to four hours of placing.
Blowholes result when air bubbles are trapped against formwork surfaces when the fresh concrete is placed and compacted.
• Foundations: tamp to level
• Slabs: to provide a non-slip surface, finish the concrete using a wood float. Where an especially non-slip surface is required, the concrete can be textured with a stiff broom or coarse sand particles floated into the surface.
• For an ‘organic’ texture on garden stepping stones, stairs, birdbaths, etc. trowel and texture the surface to suggest weathered irregularities. This is especially effective on fully pigmented items.
Newly cast concrete must be cured to ensure that hydration continues until the full potential strength of the hardened concrete is achieved and to minimise any tendency to crack. This means ensuring that the concrete is kept damp and not allowed to freeze.
The more extender in the mix, the slower the strength development, and the hardened concrete will only reach its full potential strength if it is properly cured.
Concrete can be cured by:
• Covering the surface with a water-retaining material such as sand, earth, straw or hessian that is kept continuously damp.
• Sprinkling or spraying with water often enough to keep the concrete continuously moist.
• Ponding water on the surface.
• Covering with plastic sheeting or waterproof paper. The covering must be held in place at its edges in a way that does not damage the concrete, and be sufficiently overlapped at joins.
• Using a manufactured spray-on membrane (curing compound/agent).
• Leaving formwork in place and covering any exposed concrete surfaces.
Small precast elements (e.g. decorative paving stones) can be demoulded and cured by immersion in water.
If freshly placed concrete is exposed to hot sunshine or drying winds, prevent evaporation by covering with plastic sheeting immediately after placing and finishing. If the plastic sheeting could damage the surface, use a water-filled atomiser spray of the type used for spraying insecticides on fruit trees to produce a mist over the fresh concrete until the surface is hard enough to permit one of the above curing methods.
It may also be necessary to provide adequate wind breaks for the concrete during cooler parts of the day.
In cold weather, protect newly placed concrete from frost by covering it with an insulating material such as sacking or straw.
Continue curing for at least five days after placing concrete, and longer (seven days) in cold weather.
Adequate curing results in concrete attaining its potential compressive strength at 28 days.