Haiti needs good building materials in order to have any hope of building a sustainable future. One building material that is key to Haitian construction is the common “concrete-masonry-unit” or CMU. Some call them cement block, breeze-block, concrete bricks or cinder block.
Call them what you may, but never trust a CMU that was built without some sort of pressure used in the casting process. The compression of the raw materials knits the block together. You can’t get this consolidation without pressing the materials tightly in a strong mold.
In Haiti they use shovels to mix the cement and small aggregate mixture on the ground. Then they place the semi-dry material into a simple mold, tamp it lightly and then leave it to air-dry. The quality control is almost non-existent and the critical pressure-forced consolidation never happens.
In a block making factory the production steps are all tightly controlled and pressure is exerted to each block in forming a solid CMU. They also pay attention to consistent recipes and proper curing methods to insure that the block will have the compressive strength to withstand the elements.
Here are just a few of the steps that a CMU goes through during the manufacturing process:
The concrete commonly used to make concrete blocks is a mixture of powdered Portland cement, water, sand, and gravel. This produces a light gray block with a fine surface texture and a high compressive strength. A typical concrete block weighs 38-43 lb (17.2-19.5 kg). In general, the concrete mixture used for blocks has a higher percentage of sand and a lower percentage of gravel and water than the concrete mixtures used for general construction purposes. This produces a very dry, stiff mixture that holds its shape when it is removed from the block mold.
When the block molds are full, the concrete is compacted by the weight of the upper mold head coming down on the mold cavities. This compaction may be supplemented by air or hydraulic pressure cylinders acting on the mold head. Most block machines also use a short burst of mechanical vibration to further aid compaction.
The compacted blocks are pushed down and out of the molds onto a flat steel pallet. The pallet and blocks are pushed out of the machine and onto a chain conveyor. In some operations the blocks then pass under a rotating brush which removes loose material from the top of the blocks. Block curing is done in a moisture controlled environment so that they don’t receive drying cracks from curing to rapidly.
Skipping any of these important steps in the block making process can produce a lower quality product.
Haitian builders must learn to never trust a CMU that was cast from materials that were tossed together with a shovel on the ground. Odds are that the recipe is poor and one block will be very different from the one before. When cement powder is the most expensive component, you can bet that there will be shortages of this critical material from the recipe. Then if there is no compressive pressure and poor curing, the risk of failure is exacerbated.
The following videos are for comparison. One shows the “on-the-ground” method used widely in Haiti. The other is representative of block production in the developed nations. Which would you trust?
VIDEO OF A HAITI BLOCK
VIDEO OF A US BLOCK