Concrete is a highly valued building material because it lends itself generously for use in a variety of ways and conditions. Besides that, concrete products are like wine, becoming better -that is stronger and harder -with age. This marks them out as ideal for constructing public buildings and similar projects that should last for a hundred of years. Concrete blocks can either be dense or light-weight. Dense blocks are made from a mixture of crushed granite, limestone, gravel and water. On the other side of the coin, lightweight blocks, are manufactured by mixing sand, cement and water. Also, Lightweights blocks can be produced from clay, blast furnace slag, sintered ash and pumice. After mixing the ingredients, the mixture is cast in moulds, vibrated and comes out on the other side as blocks. These blocks must undergo a curing process before they are ready for use.
Aerated concrete blocks are manufactured by the addition of aluminium powder to fine mixture of sand, lime ,pulverized fuel ash, and Portland cement. The hydrogen gas generated by reaction of the aluminium powder with the mixture, creates a non-interconnected enclosed small spaces structure in the finished blocks.
Concrete offers a rich menu of various types of blocks and include but not limited to the following:
- Solid blocks
- Cellular blocks:-have enclosed spaces within its structure.
- Hollow blocks-Have a few holes running from one end of the block to the other end, so that you can see straight through it.
- Interlocking blocks-can be joined by virtue of parts that fits into other parts of adjacent blocks.
The standard block sizes are 9 by 9 equivalent in millimeter to (390 by 190 by 190 mm), 6 by 9 which is equivalent to (390 by 140 by 190 mm) and 4 by 9 equivalent to (390 by 90 by 190 mm). In most cases, blocks are grey in color or buff (light yellow brown color -similar to the color of sand).
Concrete blocks application and prices in Kenya
Concrete blocks are faster to build with, compared to bricks-especially when using lightweight blocks .Also they are durable , maintenance free and as we have already mentioned become harder as well as stronger with age.
Dense blocks and certain aerated blocks are suitable for below damp proof course foundations. Other types of blocks are suitable for foundation where the ground is not damp or soggy. Lastly blocks are used for constructing security walls, load bearing walls in buildings , wall capping,column capping and concrete pavement blocks in towns and driveways.
The mortar used to join individual blocks in courses should be weaker than blocks to allow for movement. Below damp proof course, the mortar should be stronger in order to give it extra capacity to the weather this damp conditions.
Plaster should be applied in two coats up to 13 mm. Fair-face block work may be left plain or painted. And Block works that is to be clad with tiles should first be rendered or covered with mixture of cement and sand
Kisumu concrete products ltd( KCP) sells its in-house manufactured solid concrete blocks beginning at Kshs 75, Kshs 70 and Kshs 55 a piece for 9 by 9, 6 by 9 and 4 by 9 block sizes respectively. Hollow brick are sold at between Kshs 90 and Kshs 110 for each piece. Finally Concrete paving blocks are sold between Kshs 750 and Kshs 1250 per piece. These prices can be used as barometer of the general price levels of concrete products in Kenya.
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Sources :Materials for Architects and Builders by Arthur Lyons PhD.