Learn A Bit More About Concrete

Working The Concrete

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 Tables 1 or 2 of page Estimating & Ordering Materials for different stone sizes.

Tip: Where mixing by hand is unavoidable, use smaller sized stone, e.g.13,2 mm  instead of 19 mm if at all possible as hand mixing concrete is very hard physical labour.

Note: A bag of cement contains 50 kg of cement, or approximately 33 litres. It is recommended that only whole bags be used, as cement "fluffs up" when poured into another container.

Volume batching of sand can never be precise because the quantity (mass) of sand in a container varies with the moisture content of the sand, and with the degree to which it is compacted in the container.

Tip: Don't mix concrete directly on ground:

• The ground absorbs mixing water from the fresh mix, affecting workability and potential strength, and

• The mix will become contaminated with soil.

Work on a clean, hard surface such as a concrete floor or a metal sheet. Very small batches can be mixed in the wheelbarrow.

First spread out the measured materials in even layers. Mix the materials with a spade or shovel until no gray 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.

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. Mixing time must be long enough to produce concrete of uniform colour and consistency - usually between two and three minutes. (Inadequate mixing leads to colour differences, variable strengths and lack of durability.)

Ready-mixed concrete

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.

There are many benefits in ordering concrete from a readymix supplier, especially when larger volumes are required. Here are a few:

• 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 stormwater 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. This helps one avoid the concrete to segregate, lose fine material, dry out or become diluted or contaminated.

Tips

• When 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.

• Prior to mixing or ordering concrete, check that the foundations have been excavated to the correct depth and width and that any required formwork and/ or reinforcement is in place.

• If there is steel reinforcement involved 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.

• NB - Avoid getting release agent on reinforcing bars - this will  stop them bonding with the concrete.

• 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.

• When hand compacting:

• 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.

• 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.

• 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.

Curing

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.

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 atomizer 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.