|Carpentry for Vocational Schools - A Teachers Handbook|
a) Striking off excess concrete:
After the formwork has been filled with fresh concrete just above the formwork the concrete is consolidated in order to get out all air pockets inclosed into the concrete.
The excess concrete is striked off with a specially prepared wooden board or magnesium rod across the concrete, starting on one side of the concrete. It normally requires two people to do it properly.
Consolidation and strike off are often combined into a single operation by lifting the board above the fresh concrete consolidate the mixture by bouncing the concrete using the bottom edge of the board.
Return the board to the starting point and strike off by moving forward with saw-like motions across the top of the form.
Keep the top-edge of the formwork and the edge of the strike-board clean and repeat the process forming a true surface.
b) Hand floating concrete
The size and the dimension of the concrete slab will determine the procedure of hand floating concrete.
After striking off the excess concrete, there must not be water on the surface. If the concrete has been mixed too wet you have to wait for some time till the water has gone. If necessary speed up this process by spreading cement powder on the wet surface.
When surface has dried up, start floating on the farthest corner. To avoid stepping in the wet concrete, place timber boards on the places you have to stand on.
When concrete is too dry you may have to wet it with water in order to produce a smooth finish.
Floating is done by moving the float in short circular motions. Keep a trowel near by to fill in or to remove stones from the surface.
c) Control joints:
When having large areas of concrete slab, it is advised to divide the whole slab into smaller sections, because of the shrinkage of the concrete during the drying process.
The planned spacing of the control joint weakens the slab at these joints. In case the concrete cracks because of the shrinkage, it will crack on the weakened sections.
The joints do not extend to the full depth of the slab. They are usually cut one quarter of the depth of the slab and are made with a GROOVER.
Put a straight board across the concrete and push the groover alongside the board in the fresh concrete. Push the groover back and forth until a smooth groove has been formed into the concrete.
d) Edging Concrete:
Edging concrete is done when all water and sheen (glossy appearance) have left the surface. Rounding all the edges of the concrete prevents breaking off. This is necessary on steps or other parts where there is danger of breaking off corners.
Rounding off the edges is performed by using an EDGER.
It should be run back and forth until the contour is shaped. This operation can be done easily if it is done easily when the concrete is firm but still moist.