|Concrete Block Producing Equipment (GTZ, 1991, 22 p.)|
In small-scale backyard blockmaking no special equipment is generally needed for making concrete blocks, if the concrete is mixed by hand and simple wooded or steel moulds are used. But with certain equipment the production process can be facilitated and the quality of blocks improved considerably.
The quality of concrete blocks depends largely on the type of mixer and period of mixing. The free fall, revolving drum type mixers are not suitable, because of the semi-dry nature of the mix. Pan mixers have a quick moving action and are thus recommended. Trough mixers are also suitable.
Several types of machines are available, ranging form simple hand-operated ones to complex stationary or mobile plants. The simpler machines are generally mechanically operated using electric, petrol or diesel power, while the larger machines are usually electrically operated. In most of the blockmaking machines, the concrete is compacted by vibration.
1. Hand-operated moulding equipment
These are relatively inexpensive, simple and robust devices, which are especially suited for on-site production of concrete blocks. Output rates for 40 x 20 x 20 cm blocks can range from 10 to 80 blocks per hour, depending on the efficiency of the machine, rate of supply of concrete and number of workers involved. There are basically three types:
· Steel moulds that can be carried around by one person and used on a raised working surface (eg table) or on the ground; the mix is tamped with the help of special tampers that fit on the mould, but is more usually compacted by means of a vibrator fixed to the mould or to the working surface (vibrating table).
· Stationary machines with the block mould (into which a wooden pallet is inserted) at about table height; the mix is usually compacted by the tamper lid-plate, which is brought down with a few sharp blows; after compacting, the sides of the mould fold back to release the block, or it is ejected by means of a lever, which pushes the base plate upwards, so that the fresh block can be taken away on the pallet for drying. Some of these machines are equipped with a tray above the mould for preparing the mix and filling it directly into the mould.
· Stationary machines that are similar to the previous type, but have an engine operated jolting mechanism or vibrator for more efficient compaction.
Advantages of hand-operated equipment:
+ Low capital and operational costs.
+ Quick delivery (possibly available locally).
+ Low weight and small size, thus easy to transport, requires little storage space.
+ Simple to use with a little training.
+ Low maintenance needs, apart from regular cleaning and lubrication of moving parts.
+ Possibility of repairs in local workshops, no special parts required.
Problems of hand-operated equipment:
- Low rate of production.
- In case of manual tamping, possibility of non-uniform compaction of concrete; since production rate is low and the use of fresh concrete mixes is limited to the setting time, relatively few blocks are produced per mix, which can differ in quality each time.
- Tiring operation, which can lead to a drop in the quality of blocks, if the work is carried out by a single person for too long.
2. "Egg-laying" mobile machines
These are machines designed for medium-scale production, either on-site or in a factory. The name was given to these machines, because they leave the blocks to dry where they are produced on a flat production surface and move a short distance away to produce the next batch of blocks, and so on. The machines, which can be manually operated or fully automatic, have output rates for 40 x 20 x 20 cm blocks ranging from 60 to 400 blocks per hour, depending on the size of machine, the degree of automation, availability of continuous supplies of concrete and production site organization.
Advantages of egg-laying machines:
+ Relatively high output of blocks.
+ Uniform quality of blocks, since more blocks are made from each concrete mix and most of the operations are mechanized.
+ Fairly easy to operate with a little training.
+ Suitability for use on-site or in a factory.
Problems of egg-laying machines:
- Rarely available locally, usually imported.
- Higher capital and operational costs than those of hand-operated equipment.
- Requirement of large flat production area.
- Dependency on the weather, if not under a roof: in dry regions, if the blocks are not covered with plastic sheets, premature drying and cracking are inevitable; if it rains,production must cease, otherwise the green blocks will disintegrate.
- The higher the degree of automation, the greater the dependency on energy supplies.
- Repairs not likely to be possible in local workshops, if spare parts are not available.
3. Fully mechanized, stationary machines
These are automatic and very versatile machines used for the medium- and large-scale production of superior quality concrete components. They can be of various sizes, but are generally far more expensive than egg-laying machines of comparable sizes. The filling of the moulds, the compaction (vibration) and ejection of the blocks is done automatically, and output rates for 40 x 20 x 20 cm blocks can range from 200 to 800 blocks per hour. These high output rates are only possible with sophisticated ancillary equipment for transportation, handling, stacking, etc, a well-trained staff, efficient management and sound financial base. Space is saved by stacking the green blocks in shelves, where they are usually steam cured for better product quality and quicker turnover.
Advantages of fully mechanized machines:
+ Very high output rates.
+ Superior and uniform quality of products.
+ Greater adaptability to the production of special concrete products.
Problems of fully mechanized machines:
- Not available locally, have to be imported.
- Very high capital and operational costs.
- Dependency on uninterrupted energy suplies, high standard of ancillary equipment, skilled labour, good management and, above all, continuous high demand for the products.
- Limited mobility.
- Need for specialists for maintenance and repairs; spare parts usually expensive and difficult to get, or only after long delivery time.