Concrete block construction is gaining importance in developing countries, even in low-cost housing, and has become a valid alternative to fired clay bricks, stabilized soil, stone, timber and other common constructions, providing the ingredients are available locally, are of good quality and economically viable.
The essential ingredients of concrete are cement, aggregate (sand, gravel) and water, but the physical characteristics of the material can be extremely diverse, depending on the type and relative proportions of these ingredients, the addition of other ingredients and components, and also the production method. Concrete is thus a very versatile material and can be made to satisfy a large variety of requirements, whether it is used for foundations, floor slabs monolithic walls cast in situ, or for prefabricating concrete blocks.
Assuming that the ingredients and workmanship are of average quality, the main characteristics of the most common types of concrete are:
· high compressive strength, resistance to weathering,
impact and abrasion;
· low tensile strength (but can be overcome with steel reinforcement);
· capability of being moulded into components of any shape and size;
· good fire resistance up to about 400°C.
The main problems, particularly with regard to developing countries, are:
· the need for a relatively large amount of cement, which
can be expensive and difficult to obtain;
· the need for a relatively large amount of clean water for mixing and curing, which can be a serious problem in dry regions;
· the need for special knowledge and experience in the production process;
· the risk of deterioration through sulphates in the soil or water to which the concrete is exposed.
Entrepreneurs wishing to start the production of concrete blocks will not only have to consider all these technical and economic aspects, but also a number of environmental, social and administrative factors, in comparison to other alternative building materials, before undertaking further steps towards the establishment of a manufacturing plant.
The information on concrete block production presented on this folder should, however, be regarded only as a brief introduction to the technology and criteria for the selection and purchase of equipment. The reader is advised to refer to the Select Bibliography for detailed information.
Types of Concrete Blocks
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.
· Block sizes are usually referred to by their nominal dimensions, which are the actual block length, width and height plus 10 mm of mortar bed thickness added to each dimension. These are normally based on the modular coordination of design with the 10cm module as its basic unit. The most commonly used concrete blocks are the stretcher blocks with a nominal length of 40 cm (half blocks: 20 cm) nominal height of 20 cm, and nominal widths of 8, 10, 15 and 20 cm. In addition, a wide variety of non-modular blocks and special shapes are available, such as corner, jamb, lintel, pilaster and interlocking blocks, to name only a few.
· Solid blocks have no cavities, or - according to US standards - have voids amounting to not more than 25 % of the gross cross-sectional area. Thinner blocks of less than 75 mm (3") width are essentially solid, because of the difficulty of forming cavities.
· Cellular blocks have one or more voids with one bed [ace closed, and are laid with this 'blind end' upwards, preventing wastage of bedding mortar, which would otherwise drop into the cavities.
· Hollow blocks are the most common types of concrete blocks, having one or more holes that are open at both sides. The total void area can amount to 50 % of the gross cross-sectional area, and - according to British Standards - the external wall thickness must be at least 15 mm or 1.75 x nominal maximum size of aggregate, whichever is greater. The use of concrete hollow blocks has several advantages:
+ they can be made larger than solid blocks, and if lightweight aggregate is used, can be very light, without forfeiting much of their load-bearing capacity;
+ they require far less mortar than solid blocks (because of the cavities and less proportion of joints, due to large size), and construction of walls is easier and quicker;
+ the voids can be filled with steel bars and concrete, achieving high seismic resistance;
+ the air-space provides good thermal insulation, which is of advantage in most climatic regions, except warm-humid zones; if desirable, the cavities can also be filled with thermal insulation material;
+ the cavities can be used as ducts for electrical installation and plumbing.
· Dense concretes are normal concretes with a density exceeding 2000 kg/m3, while the densities of lightweight concretes can be as low as 160 kg/m'. The former are produced with well graded aggregates (with a large amount of fines to fill all voids) and full compaction, while the latter comprise lightweight aggregates and/ or a high proportion of single-sized particles of coarse aggregate (no-fees concrete) in a lean mix, which is not fully compacted, or comprise a sand-cement mix with a foaming agent to aerate the mixture. Lightweight concrete is generally used for concrete blocks, provided that the ingredients are available and the strengths obtained are acceptable.
. Air curing is the standard procedure for the strength
development of concrete, by which the concrete is kept wet for at least 7 days
and then allowed to dry at ambient temperature. With steam curing, by which the
concrete is exposed to low or high pressure steam (in autoclaves),
high early strengths can be achieved (with autoclaving the 28 day strength of air-cured concrete can be obtained in 24 hours). However, in developing countries, steam curing is unlikely to be implemented, because of its high cost and sophistication.
· The definition of loadbearing and non-loadbearing blocks is fairly complex and depends not only on the compressive strengths of the blocks, but also on the ratio of their height to thickness, their density and proportion of voids.
· Manual block production is the cheapest but most laborious method, and the blocks are not likely to attain the superior qualities that are achieved by the far more expensive mechanized production.
Materials for Concrete Blocks
Since the ingredients of concrete can be of very different types and qualities, not only depending on their local availability, but also on the desired properties of block, equipment and production method, it is not possible to give detailed recommendations on materials and mix proportions, other than very general guidelines. It is up to the manufacturer to select the most suitable materials and design of mixes by trial and error, and making tests with the available equipment under the conditions of full-scale production.
· The following cements are commonly used in concrete blockmaking:
Ordinary Portland cement (OPC). cheapest and most common type used.
Rapid hardening Portland cement (RHPC): more finely ground cement, which hardens much faster than OPC. It is especially useful:
- where storage space is limited,
- when rapid production is important, and
- to produce good strength blocks despite poor gradation of aggregate.
Block mix cement: marketed especially for blockmaking, but can vary from one manufacturer to another. It has the high early strength qualities of RHPC, but is lower in price.
Special cements: such as Portland blast furnace cement, sulphate-resisting Portland cement and others, used where special properties are of importance. The partial replacement of cement by apozzolana, eg rice husk ash, fly ash, may be acceptable in certain cases, but should not be implemented without prior laboratory testing.
· In order to facilitate transportation, handling and laying concrete blocks, it is necessary to reduce their density. This is achieved by reducing compaction, ensuring a relatively high proportion of air gaps between the aggregate particles and/or using lightweight aggregates. Hence it is important to have a relatively high proportion of coarser particles, because too much fine aggregate would fill the gaps and increase the density. However, a carefully measured amount of very fine particles is necessary to produce the cement paste required to bind the coarser particles.
· The maximum particle size of coarser aggregates is 13 mm (or 10 mm for hollow blocks). Rounded stones produce a concrete that flows more easily than angular (broken) particles, but the latter give higher 'green strength' to the newly demoulded block, because the particles interlock. This is very important for concrete block production.
· Suitable aggregates are usually obtained from natural sources (eg river beds, gravel pits, stone quarries, volcanic deposits) or from industrial by-processes (eg expanded clay, aircooled, granulated or foamed blast furnace slag, sintered fly ash, etc). All aggregates, whether fine or coarse, must be free from silt, clay, dust, organic matter, salts or other chemical impurities, that could interfere with the bond between cement and aggregate or cause deleterious chemical reactions.
· After determining the correct blend of aggregates, the proportion of aggregate to cement must be found by trials with different ratios, eg 6:1, 8:1,10:1, up to 16:1 by weight, end testing the qualities of blocks produced.
· The proportion of fine aggregate to cement is of special importance: if the ratio is too high, the mortar will lack the cohesiveness needed for green strength and will be too weak to impart enough strength to the matured blocks; if the proportion is too low, the mortar will be very cohesive and the mix may not flow easily in handling and filling the mould.
· Only water that is fit for drinking should be used to mix the concrete. The correct amount of water to be added to the mix depends on the types and mix proportions of aggregates and cement, the required strength of the block, and the production method and equipment used. The concrete must contain just enough water to facilitate production without any slumping of blocks occuring after demoulding. If the aggregates are dry, they may absorb some of the water (lightweight aggregates may absorb up to 20 % by weight), but if the aggregates are wet, the blocks will take longer to dry out.
· As a simple test for cohesiveness, no excess water should be visible when a lump of concrete is squeezed in the hand, but if the sample is rubbed quickly on a smooth round metal bar or tube (2 to 4 cm in diameter) a slight film or paste should be brought to the surface.
Batching and Mixing
· Aggregates can be batched by volume or by weight, but the latter is more accurate. For this reason, cement should only be batched by weight, or preferrably by using only whole bags of 50 kg. 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.
· 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.
· 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 demoulded immediately after compaction, so that they have to maintain their shape even before the concrete hardens.
· 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.
Building with Concrete Blocks
· In order to minimize the need for cutting concrete blocks, all horizontal dimensions of walls should be multiples of nominal half blocks (most commonly 20 cm) and all vertical dimensions should be multiples of nominal full-heights (20 cm). This also applies to the positioning of doors and windows.
· In order to minimize the risk of cracking, the lengths of individual wall sections should not be greater than one-and-a-half times the height.
· Hollow blocks should be specified when good thermal insulation is required. These blocks are also useful when additional structural stability is needed, eg in earthquake areas, because the cavities align vertically and can be filled with reinforcing steel and concrete.
· Blocks with a rough surface (open textured), as in the
case of most lightweight blocks, are advantageous, because they
- provide a better key for bedding mortar and applied finishes,
- have less capillary attraction for water and dry out more quickly after rains.
· Concrete blocks must be dried out thoroughly before use, otherwise drying will continue after building the wall and shrinkage cracks may develop. Only dry blocks should be used and they should not be wetted before laying. Instead the preparation of the mortar must take into consideration that the blocks absorb some of the water.
· Mortars used for bedding should not be too rich in cement. Cement: hydrated lime: sand mixes of 1: 2: 9 or 1: 1: 6 have a high water retention and good workability. It is important that the strength of the mortar does not exceed that of the blocks, so that the joints can absorb a limited amount of movement, preventing the blocks from cracking.