Advice on Concrete Brick-and-Block Manufacture for Small Businesses

EXCAVATORS-KUBOTA-KX-80-3

AfriSam’s commitment to superior performance gives its customers the peace of mind that comes with guaranteed technical excellence, top quality products, sustainability and continuous innovation. Not only does AfriSam provide quality concrete and cement, it also provides its customers with expertise and technical assistance.

Small-scale concrete brick and block manu­facture is well suited to small businesses, as production can take place outdoors, while the process itself is relatively simple with minimal outlay on equipment. Producing units of the high­ est quality depends on using the correct cement at the outset and following the correct procedure.

Note that aggregates and cement should be ordered in good time. Stocks should be sufficient to prevent stoppage due to lack of material. As a rough guide, using an aggregate-to-cement ratio of 8:1 by loose volume, three and a half bags of cement and a cubic metre of aggregate will be enough to make about 400 bricks. The number of blocks produced from the same quantity depends on the size of the blocks and whether they are solid or hollow.

Batching

Bagged cement should be batched by the full bag. It is preferable that cement supplied in bulk be weighed, or batched by loose volume. It is im­portant to batch all materials accurately. Batching containers such as wheelbarrows, buckets, drums and wooden boxes should be filled loosely to the brim and struck off flush.

To avoid errors, there should be enough containers for a full batch to be made without using any con­tainer more than once. It is also important to ensure that the containers used are not dented or broken.

Water content is critical. The mixture must be wet enough to bind together when compacted, but should not be so wet that the blocks slump or sag when the mould is removed. A common mistake is using mixes that are too dry, resulting in incomplete compaction.

The moisture content is about right when ripple marks form on a steel rod or the back of a shovel when it is rubbed against some of the mixture. Note that the water content is just optimum when ripple marks start appearing on the blocks when they are removed from the mould.

Hand mixing using shovels should be carried out on a level concrete slab or steel plate. First spread out the aggregate to a thickness of 50mm to l 00mm. Thereafter distribute the cement and stone, if any, evenly over the sand. Mix the aggregate and ce­ment until the colour is uniform. Spread the mixture out, sprinkle water over the surface and mix.
Continue with this process until the right quantity of water has been mixed in. For machine mixing, first mix the aggregate and cement, and then gradually add the water while mixing, until the correct water content is attained.

Ready for curing

The concrete mix should be used within two hours of mixing, and must never be retempered by mix­ing in additional water, as this reduces the resultant strength of the mix. Adequate care must be taken when compacting the concrete in the moulds of the block-making machine. Too little or poor com­paction will also reduce the strength.
The moulds should be removed carefully so as not to damage the fresh b lo c ks. In addition, during the first day, fresh blocks should be protected from rain as well as from drying out due to sun or wind by means of plastic sheeting or any suitable covering material.

The day after production, blocks should be removed from the production slab and stored in the stack­ ing area, ready for curing. Stacks should be built carefully to avoid chipping the edges and corners. Curing is the process of maintaining adequate moisture content and a favourable temperature in the blocks to ensure hydration of the cement and optimum strength. Blocks should be cured for at least seven days.

Three aspects

In terms of quality control, it is critical to monitor the following three aspects to ensure quality masonry units:

Strength: The quality of the blocks should be controlled so that the strength is adequate to avoid breakages or rejection by customers. This will also mean that the mixes used are prepared as economically as possible.

 

Dimensions: The length and width of the units are determined by the mould and will not vary greatly. However, height can vary and should be monitored using a simple gauge. Units of in­ consistent height will lead to difficulties during building, and possibly even result in penetration by rain.

Shrinkage: Concrete masonry units shrink slightly after manufacture. In order to avoid this from occurring in the wall itself, cured blocks should be permitted to dry out for at least seven days before being used for construction.

AfriSam
Maxine Nel
Tel: (011) 670-5893
Email: maxine.nel@za.afrisam.com
www.afrisam.com