The other day I found myself poring over the Kenya building code for 1997. I was consulting it because it’s the law that ensures order ,sound building practice and a level playing field is this niche. One drawback about the document however ,is that its written in 1963 convoluted legal language which I suppose render the document barely accessible to contractors and Masons, not to mention many Kenyan with an interest in the built environment. After a lot of behind the scene heavy lifting that demanded the patience of Job , I gleaned some insights that clarified a few issues.
According to this building code foundations and walls for the built environment may be constructed using fired clay bricks, building stones and concrete blocks. In addition to these materials, Masons are also required to use good quality building sand, Gravel, Ballast, Hard-core rocks and Murrum. In the next ten minutes, I shine a beam of light on these materials that are important in Mason’s work.
Bricks are manufactured by molding moist clay according to specifications in moulds. These are then allowed to dry slowly-in sheltered conditions. When they ready, the green bricks undergo a firing process in a Kiln at a temperature of between 940 degrees Celsius to 1200 degrees Celsius .The oxygen content in the Kiln has an effect on the color of the finished bricks. High temperature and low oxygen produces blue bricks while on the other hand, higher oxygen level will produce red bricks. The red coloration is due to the chemical reaction between the higher amounts of oxygen and iron oxide in clay.
It worth noting that bricks can and are produced with other coloration such grey, yellow, and black. These colorations depend on type of clay used, firing conditions and type of pigments added to the clay
Large producers use a highly automated and controlled manufacturing process. After the firing process, bricks absorb moisture from the atmosphere irreversibly up to a maximum of 0.1%.Its therefore recommended that bricks should not be used for at least two weeks after firing.
The standard bricks sizes are (a) 6 inches by 6 inches by 12 inches, (b) 5 inches by 6 inches by 12 inches and (c) 4 inches by 6 inches by 12 inches. These brick sizes weighs between 2 and 4 kg, can easily held by hand and sell at between Kshs 6/ and Kshs 12/.
Three types of bricks
There are variety of brick types to choose from depending on the soil condition of the construction site, the depth of your pockets, and type of building and architectural design preference. That said, the four brick types that are commonly used by contractors and Masons in Kenya are:
- Common bricks –These bricks have no visual finish and are used for general building work.
- Facing bricks-These bricks have a surface rendered with a designed texture, sand faced or made smooth.
- Engineering bricks-they are dense and vitreous(look like glass) with high load bearing and low water absorption capacities. They are more expensive because they require high firing temperatures.
- Standard specials—these bricks have special shapes to enhance the architectural quality of the brick work. For instance we have bullnose and radial bricks.
Bricks are known to absorb water. It’s therefore advisable to use building stones or concrete blocks for the foundation in damp grounds, and then switch to bricks for the wall above the ground or damp -proof course.
The building stones, in Kenya are sourced from Juja quarry in Kiambu County. Other sources include but not limited to Narok, Kisumu and Eldoret. The stones can come in any size so long they are easy for the mason to handle and the resulting wall width can withstand the building load. Having said that I will add that in Kenya the standard building stones come in three sizes: 4 by 9, 6 by 9 and 9 by 9 (measurement in inches). Price start from Kshs 25 per piece exclusive of transportation.
Concrete blocks are prepared by mixing water, cement, sand with or without gravel and molded into three dimensions rectangular shapes. Concrete mixer machines can hired to mix the ingredient to make the process run like clockwork. The mixture forms a hard rock like material when it dries up. Mortar is a mixture of cement, sand and water, which can be used to make concrete blocks or bond courses of say bricks when constructing a wall.
Standard solid concrete block size in Kenya is 16 inches by 8 inches by 8 inches .These blocks are more expensive than building stones, selling as they do at between Kshs 80 and Kshs 90. However since most of them may be manufactured on site, there may be zero transport cost from the supplier to the construction site.
This is what is commonly called Kokoto and many people confuse it with ballast. Gravel may be machine cut and manual cut. Building gravel is produced by quarrying and crushing very hard rocks such sandstone, limestone or basalt. In Kenya gravel is in produced by crushing basalt rocks.
Gravel is expensive, selling at between Kshs 1500 and Kshs 1450 per ton. The total cost is even higher when you add up the transport cost of the gravel to the construction site. Tipper trucks are preferred mode of transport for gravel. Gravel is used to make concrete for foundation and slabs, concrete blocks and mortar.
Natural occurring gravel that accumulate in large deposit in river beds or mountain side can also be used in construction in place of commercially produced gravel.
Sand is readily available material and cheaper than gravel. A common source of sand in Nairobi is Makueni County and Naivasha.In others areas, a lot of high quality sand is harvested from rivers beds and even streams. Developers usually Transport sand using Tractors, or Canter Isuzu Trucks. In Naivasha, sand is sold at Kshs 35,000 per 25 ton tipper (price inclusive of transport cost to Nairobi).
There are three type of building sands
- Grey black sand-fine to medium grained and with a shade of greyish black color
- Coarse sand- Its coarse grained and brown in color
- Fine sand- Its fine grained and brown in color .This sand is the best.
Sand is used to make Mortar and concrete.
Ballast is composite building material. It is a mixture of sand and gravel and is used to make high density concrete. Concrete is manufactured by mixing sand, Gravel, and cement plus water. In other words concrete is mixture of Ballast, cement and water. Concrete will dries up within a few hours after mixing the ingredients together. The amount of cement, sand and gravel used will determine the strength of the concrete. Ballast is more often than not, used to make mixing concrete for foundation and slabs.
This are required for filling the base of a foundation in order to raise the floor above certain desired level. Hardcore stones are granite rocks , readily available and cheap. However raising the foundation to given height can consume a lot of these hardcore stones.
Murrum is made up of disintegrating sandstone or other rock types .It is used to spread on hardcore stones in the base of foundation to create an even surface. Concrete slab is then poured and spread over Murrum surface of the foundation.A vibrator is usually used to in spread the concrete slab more thoroughly and systematically over the foundation base before it hardens up within a short time. Murrum is cheap and readily available building material.
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