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Concrete block construction is gaining importance in developing countries, even in low-cost housing, and has become a valid alternative to fired clay bricks. They are gaining importance and being widely used because of the several benefits they offers such as durability, strength and structural stability, fire resistance, insulation and sound absorption.

Concrete blocks are produced in a large variety of shapes and sizes, either solid, cellular or hollow, dense or lightweight, air-cured or steam-cured, loadbearing or non-loadbearing, and can be produced manually or with the help of machines.

Production Process

Batching and Mixing

All ingredients for making concrete block viz. aggregates, sand, cement and water can be batched by volume or by weight, but the latter is more accurate. In backyard block production, with less stringent quality standards, batching by volume using buckets, tins, wooden boxes or wheelbarrows is quite acceptable, if done with care to ensure uniform proportions of mix. For high scale mechanized production batching plant is used having hoppers and conveyors with load cells for weighing.

Since concretes begin to set within 30 to 60 minutes, depending on the type of cement and ambient temperature, only so much concrete must be prepared as can be used up before that happens. In hot climates, the fresh mix must be shaded from the sun to avoid premature setting.

In case of hand mixing, it must be done on a level, smooth, hard surface (eg. concrete slab or steel plate). Because of the relatively low cement content of the concrete and the need for a cohesive mix, thorough mixing is essential. Thus the best mixes are obtained with mechanically operated mixers.

Moulding

Concrete blocks can be moulded by several methods, ranging from manually tamping the concrete in wooden or steel mould boxes to large-scale production with 'egg-laying' mobile machines and fully automatic stationary machines. The quality of blocks generally increases with the degree of mechanization, but medium standards are normally adequate for most construction purposes. In all cases, the blocks are de-moulded immediately after compaction, so that they have to maintain their shape even before the concrete hardens.

Curing

The blocks are either left to set and harden where they were moulded, or carried away on pallets to the curing place. In all cases it is important to keep the concrete moist, for example, by regularly spraying with water, until the concrete has obtained sufficient strength. This can take 7 days or more, depending on the type and quality of cement. Quicker strength development is achieved by exposing the blocks to steam, but this is only viable in large scale factory production.

Equipment

In small-scale backyard block making 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.

Mixers

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.

Block Making Machines

Several types of machines are available, ranging from 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 block making 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.

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.

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 or may be more. 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.