Concrete is a very popular building material in the Caribbean. When done correctly, concrete structures provide more than ample protection against hurricanes and they are essentially impervious to many of the maladies which plague wood, such as termites and rot. But a concrete structure will fail if not done properly. Concrete is very rigid and will crack under compressive and shear stress if not thick enough or properly re-inforced with steel.
We will consider the two main methods of constructing with concrete. One is poured concrete and the other is concrete block. Poured concrete construction requires careful planning, heavy forms, and the mixing of large batches of concrete. For all but the smallest jobs, the last item is best handled by hiring a company to bring the large batches to the site. The second method, using prefabricated concrete blocks, can be done at a more measured pace.
Concrete blocks are manufactured in many different sizes and styles. The most common block is roughly 8 inches square on end and 16 inches long, with three holes or voids. Walls and foundations are created by stacking the blocks with a layer of mortar between the blocks. Although a simple mortar construction may be adequate for a small free-standing garden wall, it will not be sufficient for foundations, retaining walls or load-bearing walls in houses.
The strength of block construction can be improved by completely filling the voids with concrete, but it is more cost effective use steel reinforcement. Reinforcement can be achieve in several ways. The picture below shows a few examples of such reinforcement. Steel bars (usually called rebar) can be placed vertically within the blocks at evenly spaced intervals, or at piers like that shown in the first example. (Piers are the concrete equivalent of posts and beams in wood framing.) This provides resistance to flexural stress. Steel rebar or steel mesh is placed horizontally between rows of blocks to provide resistance to shear stress. Note in both examples, control joints are indicated. A flexible caulking must be placed at regular intervals in place of mortar to accommodate expansion and contraction of the walls. Without these joints, concrete block walls will inevitably develope cracks.
Electrical conduit and plumbing can be placed within the walls as the wall is constructed. The top of the divider sections can be hammered out to make room for pipes, or specially designed blocks can be used. Door and window frames (usually wood or steel) are placed in position and the wall built up and around the frames. For large openings, a lintel is needed, It can be constructed by using extra reinforcement in the blocks immediately above the frame or a prefabricated concrete, steel or wood lintel can be used provided it is properly tied into the surround blocks.
Special attention must be paid when finishing the top of the wall. If the roof is to be of standard wood construction, accommodation must be made for securely attaching the roof to the walls. Usually, the top is capped with a wood plate that is anchored to rebar or bolts that protrude from the top of the wall. If a concrete roof is to be poored, the rebar from the walls is bent over and into the forms before the poor.
In poured concrete construction, forms are put in place and concrete is poured into them. Typical foundation and wall thickness is 6 to 8 inches, depending upon the level of reinforcement. It is possible to pour thicker walls that do not need reinforcement, but at a higher overall price. All of the steel reinforcement, openings for doors and windows, electrical and plumbing elements must be in place before the pour. So greater planning is required. A modest size home will require from 30 to 40 cubic yards of concrete for the walls alone. Clearly, mixing your own concrete is not practical. A typical concrete truck can carry about 8 cubic yards of concrete, so plan on a full day and 4 to 5 truckloads.
The forms are typically constructed of 3/4" plywood framed with 2x4's. Steel pins, called wailing pins, are placed through the forms and maintain a fixed distance between the forms while the concrete sets. These forms must support the tremendous weight of the concrete while it is still in the liquid stage. Consequently, care must be taken to ensure that the support structure for the forms is adequate. It is possible to reduce the support structure needed by making pours in multiple stages, allowing the concrete to set up after each "mini-pour". The main disadvantages to this method are that it requires the additional preparations before each mini-pour and care must be taken that the interface between sections are structually sound.
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The picture to the left shows the rebar in place between the forms of a wall before the pour. The reinforcement in this example is heavier than that needed for a typical residential home. The picture to the right shows a floor/ceiling being poured. The worker to the right is using a vibrator that helps remove bubbles and voids in the concrete. Significant voids can decrease structural integrity. When you make concrete cylinders in the laboratory, you will be shown how to remove bubbles and voids using a manual method of "tamping".
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