How to Pour Concrete in Sections
Concrete walls or slabs may have to be poured in sections for a variety of reasons: not enough concrete available for delivery; small finishing crew (cannot finish a larger pour); or the contents of the pour (rebar and mechanical) may be so intensive that only sections of it are done at a time. For whatever reason, you need to know how to create a structurally sound construction joint to break the pour. A construction joint allows you to pour concrete in sections because it creates a way for separate pours to lock together. Does this Spark an idea?
Instructions
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Measure and mark on both sides of the slab or wall form the location where you want to place your construction joint. Make two marks on each side of the form, one for the top of your construction joint form board and one to line up the bottom of this board. This will help you keep the board straight when installed.
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Measure and cut your 2x4s so you have two pieces that will fit together to break the concrete. If the width or thickness of your concrete is more than 8 inches (as in most walls), cut the 2x4s but also but a strip of plywood that is as wide as the concrete you need to place a joint in to break. You will use the 2x4s to support the plywood in place.
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Place your construction joint form. For slabs, lay the wood on its edge so the two pieces are stacked together and drive stakes on the outside face of the wood (outside of the pour), then nail the stakes to the construction joint to lock it in place. For walls, nail one 2x4 to each face of the form so the inside face of the 2x4 is lined up on the outside of the marks you made. The 2x4s should be outside the pour. Slide your plywood into the form so the 2x4s are still on the outside of the pour and the plywood face is against what will be poured. Nail the plywood to the 2x4s.
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Cut the keyway. A "keyway" is an extra piece of wood strip that is added to the inside of your construction joint to create a pocket or indent in the pour. This will allow the concrete from the joining pour to lock into the existing concrete by creating a ridge or key. Cut a 2x4 so there is a 45 degree angle on each side of the board from the top of the wider face to the bottom face. Do this along the entire length of the board. You should end up with a board that is 4 inches wide on the top face and 2 inches wide on the bottom. You want enough keyway to run the entire length of your construction joint.
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Install the keyway by nailing it to the construction joint form. Place the keyway on the pour side of the form with the wide face flush to the construction joint form. The center of the narrow side of the keyway should fall in the center of the pour.
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Lay out and drill holes for any rebar that must pass through the construction joint so you can splice it to the bar that will be installed in the next section poured.
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Tips & Warnings
Make your construction joint in two pieces. Put the bottom half (from the ground the the height of the rebar or mesh) in first and place your slab steel in so it lays over it with the splice length into the next pour. Then put the top half in place, secure it with stakes and pour the concrete. If you are placing a joint in a wall, put one half coming toward the center from the face of form and then create a mating piece that will come from the opposite form.This will eliminate the need to make special cuts or holes to accommodate rebar or mesh that needs to pass through your construction joint.
Make sure you follow all the requirements for forming your construction joints as detailed in the applicable building code and blueprints. Not following them, or adjusting them at will, can cause your slab to fail inspection, resulting in a costly demolition and reconstruction.
