Get Multiple Quotes
RECEIVE MULTIPLE QUOTES from our list of suppliers now!
1000 characters left

Equipment

Blockyard equipment consists essentially of a means of moulding blocks, a concrete mixer, and various general- purpose tools and equipment. These are discussed below.

Blockmaking equipment

There are two basic types of equipment, depending on the method of moulding the blocks:

• Stationary machines that mould blocks, one or more at a time, on pallets.
• “Egg-layer” machines that mould blocks on a concrete slab.

Concrete mixer

It is possible to make blocks on a small scale without a concrete mixer. Hand mixing has the advantage of reducing the amount of capital required and providing employment, but may limit output and not always be thorough. Hand mixing should be done with shovels on a concrete slab or flat steel sheet. Never mix directly on the ground because this results in contamination of the mix.

A pan mixer is the only type of machine mixer suitable for blockyards. Pan mixers, with a forced mixing action, can cope with the semi-dry mixes used for making blocks. Drum mixers do not work because they cannot mix the semi-dry concrete. The output of the mixer should match that of the brick making machine. A mixer of adequate capacity for making hollow units may have insufficient capacity for solid units.

Miscellaneous equipment

This includes wheelbarrows, batching containers, trolleys (for moving blocks), shovels, hosepipes and plastic sheeting.

Materials for blockmaking

Cement

Cement should comply with SANS 50197-1. Strength class should be 42,5N or higher because the concrete must develop strength as rapidly as possible. Note that it is illegal to sell cement which does not bear the SABS mark.

Aggregates Sand and stone are used for most block production.

Sand and stone are fragments of rock and differ only in size. Sand particles will pass through a sieve with 4,75 mm square openings. Stone particles will not. All aggregates should be clean and not contain organic matter such as roots or humus. If the aggregates contain clay it should be in a very small fraction.

The following aggregates may be considered:
• Fine sand with particles mainly smaller than 1 - mm: pit, fine river or dune sand
• Coarse sand with the biggest particles approximately 5 - mm in size: crusher, pit or coarse river sand
• Stone with a maximum size of 13 - mm for bricks or solid blocks or 10 - mm for hollow blocks It is normally possible to make blocks with coarse sand on its own.

Alternatively combinations of aggregates may be used:
• A blend of coarse sand and fine sand
• A blend of fine sand and stone
• A blend of fine sand, coarse sand and stone For small-scale production, the best aggregate or combination of aggregates is normally found by trial and error.

Water

Water that is fit for drinking is suitable. Most river and borehole water may be used.

Trail Mixes

The aim is to find a mix that will produce blocks that have an acceptable texture and are strong enough but as cheap as possible. Because cement is more expensive than aggregates, the lower the cement content the cheaper the block.

Strength of well cured blocks depends on:
• Aggregate:cement ratio
• Degree of compaction
• Size of block, solid or hollow

The degree of compaction depends on:
• Overall grading of the aggregates
• Particle shape of aggregates
• Aggregate:cement ratio
• Water content
• Compactive effort It can be seen that strength depends on a number of interrelated factors. It is therefore not possible to design a mix in a laboratory. Instead, a trial-and-error process, using the equipment of the blockyard, is followed. This process aims to arrive at the best combination of aggregates and the right aggregate:cement ratio.

Aggregates

First try coarse sand only. Then try replacing some of this by fine sand and some by stone, if these materials are available. Alternatively, if coarse sand is not available, try different blends of fine sand and stone.

Aggregate:cement ratio

Try 6:1, 8:1 and 10:1 by loose volumes (230, 300 and 380 - l of aggregate respectively per 50 - kg bag of cement).

Trials

For each combination, make up a batch of concrete with optimum water content and, using the yard’s blockmaking equipment, mould some blocks. Because block density is a good indicator of strength, blocks can be assessed by weighing them as soon as they are demoulded. Adjust the mix until the heaviest block is achieved.

The next step in assessment of strength is to look out for breakages to corners and edges of cured blocks. (If blocks break when handled, they are clearly too weak.) Strength can also be assessed by knocking together two blocks, after curing and drying out. A ringing sound indicates good strength while a hollow thud probably means that the blocks are too weak. Ideally, blocks should be laboratory tested for strength.

The National Building Regulations require nominal strengths of 7 - MPa for solid units and 3,5 - MPa for hollow units for single storey houses and buildings. Also assess the surface texture of the blocks. If the texture is too smooth, reduce the amount of fine material in the mix; if it is too coarse, increase the amount of fine material.

Water content

Water content is critical. The mixture must be wet enough to bind together when compacted, but it should not be so wet that the blocks slump (sag) when the mould is removed. A common mistake is the use of mixes that are too dry, resulting in incomplete compaction. The moisture content should be as high as possible as this allows better compaction and thus gives the best strength. Moisture content is approximately right when ripple marks form on a steel rod or the back of a shovel when it is rubbed against some of the mixture. The water content is just over optimum when ripple marks start appearing on blocks when they are demoulded.

Mixing

Hand mixing should be done, using shovels, on a level concrete slab or steel plate. First spread the aggregate out 50 to 100 - mm thick. Then distribute the cement, and stone if any, evenly over the sand. Mix 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 amount of water has been mixed in. For machine mixing, first mix aggregate and cement then add water gradually while mixing until water content is correct.

Moulding

Hand operated machines should be used as instructed by the manufacturer. The mould of a powered machine should be filled until approximately six to eight cycles of compaction are required to bring the compacting head to its stops. Too little or poor compaction should be avoided as it results in greatly reduced strengths. Demoulding or removal of the mould should be done car e fully so that the fresh blocks are not damaged. Fresh blocks should be protected from rain with plastic sheets or any suitable covering during the first day and from the drying effects of the sun and wind until curing starts.

Curing

The day after production, blocks should be removed from the production slab or pallets and stored in the stacking area, ready for curing. Stacks should be carefully built to avoid chipping edges and corners. Curing is the process of maintaining a satisfactory moisture content and a favourable temperature in the blocks to ensure hydration of the cement and development of optimum strength. In the South African climate it is normally sufficient to cover blocks with plastic sheeting to prevent moisture loss or to spray blocks with water. Blocks should be cured for at least seven days.