 |  | Carpentry for Vocational Schools - A Teacher's handbook (GTZ, 252 p.) | ||
 |  | 4. CONCRETE FORMWORK | ||
|  | (introduction...) | ||
| | 4.1. Formwork for concrete slab | ||
| | 4.2. Formwork for concrete wall | ||
| | 4.3. Formwork for concrete stairs | ||
| | 4.4. Formwork for septic tank | ||
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TOPIC: 4. CONCRETE FORMWORK
INTRODUCTION: This topic teaches students the different types of concrete formwork which must be strong enough to withstand the pressure of the wet concrete.
Carefully select the chapters which are needed to meet the aim and the syllabus of the school.
OBJECTIVES:
4.1. Students must be able to make a simple formwork for a concrete slab. When possible demonstrate it on a small project. E.g. a watertank foundation.
4.2. Students should be able to identify the parts of a formwork for a concrete wall. They also should be able to describe the process of making a formwork for a concrete wall.
4.3. Students roust be able to name the parts of a stair, its correct measurements and describe the process of making the formwork for concrete stairs.
4.4. Students should know the correct names for the parts of the formwork for septic tanks and the correct measurements.
METHOD: Prepare photocopies of the drawings taken from this topic for the students for better understanding. After introducing the objectives of this topic, students take notes from the blackboard and glue the copies of the drawings in their trade theory books.
This topic might be difficult to demonstrate on an practical example because of the large amount of timber needed to do such a demonstration. However, if there is a building project within the school going on, take this opportunity to show the students the correct procedure for making such a formwork.
NOTE: At the end of this topic prepare a worksheet for the students to reinforce their knowledge.
Formwork is a temporary construction used to hold concrete in place while it sets. As concrete is very heavy, formwork must be solidly constructed and firmly braced and supported. To ease the removal of the formwork after the concrete is dry the sheating must be well oiled before pouring the concrete. The material used for sheating are boards, which have a thickness of 25 mm. In order to get a smooth surface or if a high wall is required, nowadays 19 mm plywood is often used. Wet the timber of the sheating before pouring concrete; this will, swell the timber and tighten the joints and thus prevent the escape of water from the mix. It will also prevent the too rapid drying of the outer part of the concrete by absorption of water by the boards.
A slab is made for concrete path, driveways, tank stands, floors etc... As slabs are relatively thin the pressure on the formwork is low, demanding a less solid construction. The soil is slightly excavated and the pegs driven into the ground at approximately 600 mm intervals. The boards are nailed on the pegs.
a) Procedure for making a concrete slab:
First place the pre-assembled wooden forms with the aid of building rope to the correct height and line. If it is necessary brace the forms strongly so they will not weaken when they are filled with concrete. Measure diagonals to ensure squareness. Fill in the wooden forms with concrete.
Figure
b) Joining concrete slab:
Very seldom can any type of concrete construction be built without joints. There are two types of joints in concrete work - construction joint and contraction joint.
- Construction joint appear wherever concreting is stopped or delayed to the extend where fresh concrete has to be placed against hardened concrete.
Figure
- Concrete expands and contracts with extremes of temperature or with variations in temperature. It may shrink during the hardening process. To eliminate random cracks, contraction joints are build into the concrete.
Figure
Formwork for a concrete wall is normally built up on both sides of the wall. Reinforcement bars are laid on wires before the spreaders are placed and the wall is tied. The studs are approximately 600 mm apart. All studs are braced.
a) Formwork for high concrete wall:
There are two methods of fixing wall forms, one with "wales" and the other without wales.
- Wall forms without wales: The sheating is made in panels of a size convenient to handle and are placed between two rows of studs. Wire ties are used to hold the sheating, with spreaders between to keep them in the correct distance apart.
Figure
- Wall forms with wales: If there is danger of the wall forms getting out of line, the load of the concrete, then wales should be used. In the waling method, horizontal members (wales) are fixed and bolted trough at the side of the studs. Spacing pieces are again required to keep the sheating the correct distance apart (thickness of concrete wall). As the bolts have to be removed from the concrete they should be greased before pouring concrete.
Figure
b) Formwork for a concrete wall an a slop:
On sloping ground an earth face may form one side of the formwork.
Figure
c) Formwork for a footing wall:
Figure
Concrete stairs also require temporary formwork and the necessary check timber supports. As with timber stairs, check the height from floor to floor as the first step in the preparation of a set out. Divide this total rise into a suitable number of risers and then calculate the proportionate size to the go, as described in the chapter dealing with timber stairs. A flight of stairs should be easy and comfortable to climb. Certain dimensions must be followed to ensure this.
- Maximum rise is 190 mm.
- Minimum going is 255 mm.
- One going plus two rises should equal 585 mm to 625 mm.
- Before pouring the concrete reinforcing mesh is laid in position.
Figure
- External concrete stairs should also have a 6 mm fall on the top of the step to allow rainwater to run off.
Figure
The excavation for a septic tank must be as accurate as possible (sides must be plumb and hole square) as the sides of the hole form one side of the formwork. Cut and test assemble the formwork above ground. The sides should be made in two sections to make handling easier. Remember to make provisions for drain pipes. The bottom of the septic tank is sloping. Use the bottom edge of the side panel to get the exact slope. The bottom slab is poured first using bottom edge of side panel for accuracy. When concrete has set slightly the formwork is placed and the sides poured. The top edge of the tank is 100 mm above ground level to prevent rainwater entering. The sheating usually used for septic tank is 19 mm plywood.
Septic
tank
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