This is the second installment in my blog series "Know Your Construction Materials". Last week we talked about Concrete and what goes into making concrete. Next we move onto what is commonly called concrete block or concrete masonry units (CMU).
CMU is literally the basic building block of construction in Liberia, and much of the world. Being from the Northwest, where residential and light commercial construction typically use wood, I didn't come to Liberia with much experience using CMU. Because it is easy to make and relatively inexpensive, almost every structure in Liberia is made of CMU.
Like the cement depots that dot the side of the road, you can't travel far without passing a micro block factory. These factories have 2-5 young men who are trying to "hustle" and earn money by casting blocks in a country with high unemployment. The number you hear most of time in Liberia is 85% unemployment. That number is a little deceptive though, because 85% of the people in Liberia may not have official jobs, but they are working doing things like making blocks or busting rock to make gravel as you saw last week.
So the basic ingredients in CMU blocks are the same as concrete, just without the coarse aggregate. You have:
* Water
* Sand
* Cement
Pretty basic.
Your average block making establishment will either buy a dump truck load of sand and stockpile it, or if they are close to the beach they use wheelbarrows to bring sand from the beach. They have a round "bowl" area that they've used so many times to mix the sand, cement and water together that it is now sometimes up to a foot thick in hardened cement and sand. So the three elements are mixed together on the Bowl and one by one, the work crew starts to make block. The mixture is shoveled into the metal block mold that the block makers can buy from their local steel fabrication shop for around $50. The mixture is tamped down with a shovel, smoothed off, and then the form is removed leaving the block to dry in the sun.
The block dimensions are 8" tall x 16" long x either 4", 6" or 8" deep. On your typical 8" block the crew would mix about 3 wheelbarrows of sand with one bag of cement. That mixture will yield approximately 25 blocks if they are making 8" block. Of course there are no set standards and a crew that might try to maximize their profit might mix a little more sand in and stretch their yield to 28...30...or even more blocks. The more you thin the cement the weaker the blocks though.
When selecting their block a good mason will go and feel how strong the blocks are by rubbing their fingers on them to see how much sand comes falling off, or take the block and drop it from head height and see if it breaks.
Block making is a hot job with long hours in the African sun. The block crew gets paid $100 Liberian dollars per bag of cement they mix. In a day a 4 man crew will typically work through 10-16 bags of cement, netting $250 - $400 Liberian Dollars per day (about $3-$5 USD per day).
Those who can afford to make or buy a block making machine use the manually operated machine to make blocks that are more dense and stronger. The machines are rare though...for every block making machine you probably have 50 guys making them by hand on the side of the road. The process with a machine is about the same...the steel form is filled with the cement-sand mixture, a packing ram is pulled down repeatedly by hand to pack the block, the form is lifted and block removed. Here are some pictures of the block making at the site:
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Here are some pictures of a two man crew making block with a typical 6" form just outside the fence at ELWA.
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CMU is literally the basic building block of construction in Liberia, and much of the world. Being from the Northwest, where residential and light commercial construction typically use wood, I didn't come to Liberia with much experience using CMU. Because it is easy to make and relatively inexpensive, almost every structure in Liberia is made of CMU.
Like the cement depots that dot the side of the road, you can't travel far without passing a micro block factory. These factories have 2-5 young men who are trying to "hustle" and earn money by casting blocks in a country with high unemployment. The number you hear most of time in Liberia is 85% unemployment. That number is a little deceptive though, because 85% of the people in Liberia may not have official jobs, but they are working doing things like making blocks or busting rock to make gravel as you saw last week.
So the basic ingredients in CMU blocks are the same as concrete, just without the coarse aggregate. You have:
* Water
* Sand
* Cement
Pretty basic.
Your average block making establishment will either buy a dump truck load of sand and stockpile it, or if they are close to the beach they use wheelbarrows to bring sand from the beach. They have a round "bowl" area that they've used so many times to mix the sand, cement and water together that it is now sometimes up to a foot thick in hardened cement and sand. So the three elements are mixed together on the Bowl and one by one, the work crew starts to make block. The mixture is shoveled into the metal block mold that the block makers can buy from their local steel fabrication shop for around $50. The mixture is tamped down with a shovel, smoothed off, and then the form is removed leaving the block to dry in the sun.
The block dimensions are 8" tall x 16" long x either 4", 6" or 8" deep. On your typical 8" block the crew would mix about 3 wheelbarrows of sand with one bag of cement. That mixture will yield approximately 25 blocks if they are making 8" block. Of course there are no set standards and a crew that might try to maximize their profit might mix a little more sand in and stretch their yield to 28...30...or even more blocks. The more you thin the cement the weaker the blocks though.
When selecting their block a good mason will go and feel how strong the blocks are by rubbing their fingers on them to see how much sand comes falling off, or take the block and drop it from head height and see if it breaks.
Block making is a hot job with long hours in the African sun. The block crew gets paid $100 Liberian dollars per bag of cement they mix. In a day a 4 man crew will typically work through 10-16 bags of cement, netting $250 - $400 Liberian Dollars per day (about $3-$5 USD per day).
Those who can afford to make or buy a block making machine use the manually operated machine to make blocks that are more dense and stronger. The machines are rare though...for every block making machine you probably have 50 guys making them by hand on the side of the road. The process with a machine is about the same...the steel form is filled with the cement-sand mixture, a packing ram is pulled down repeatedly by hand to pack the block, the form is lifted and block removed. Here are some pictures of the block making at the site:
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| The form is filled with sand. |
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| The metal ram is pulled down to pack the block. |
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| The form is pulled off the block. |
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| And the block is taken out into the sun to dry. |
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| A typical block in Liberia has a solid bottom. The blocks that our project is using are the more typical "American Style" blocks that are hollow all the way through. The contractor had to custom make the mold because they are not available in Liberia. |
Here are some pictures of a two man crew making block with a typical 6" form just outside the fence at ELWA.
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| The mold. |
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And this weeks video, making blocks for the hospital.
Up next, Steel
3 comments:
Maybe I can help you with this website that I will recommend to you. this company is a cement distributor in the Philipppines, maybe you can get an idea from them.
Your post is soundful. You can also visit @ Everon Impex. We are Manufacturers and Exporters of Concrete Brick and Block Making Machinery.
Nice information! We are offering both manual and automatic block making plants in various ranges and also manufacturing fly ash brick making machine, concrete paver machine, tiles machines.
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