Formwork by chakamba john

FORMWORKBY: CHAKAMBA JOHN

Is a temporary support to pre-cast or in-situ concrete structures. Is the surface, support and framing used to define the shape of

concrete until it is self-supporting. It holds the concrete until it has set and can stand on its own and

it maintains the required shape. Includes : Form on or within which the concrete is poured. The supports which carry the forms and the concrete. The bracing which may be added to ensure stability as well as

foundations and footings.

FORMWORK

It is used for Concrete : - Lintels. - Window sills. - Columns and beams. - Decking, floors and roofs. - Retaining walls, stairs, drains. - Reinforced concrete walls.

FORMWORK

Any temporary structure that is used to support a permanent structure, material, plant, equipment and personnel until the construction of permanent structure has advanced to the stage that it is self-supporting.

Includes: Foundations, footings and all structural members

supporting the permanent structural elements. Commonly used to support spanning or arched structures,

such as bridges, while they are being constructed.

FALSEWORK

All formwork must be: Sufficiently strong and rigid to support the dead load of the

concrete. Designed to allow for the temporary live load of workmen,

wheelbarrows and tamping operations. Easy to strip after concreting. Constructed within reasonable cost limits.

FUNCTIONAL REQUIREMENTS OF FORMWORK

It should : Support both live and dead loads that is, weight of wet concrete

and its reinforcement, weight of workers and their equipment as well as force of wind and rain.

Be rigid, water tight, braced and tied together so as to maintain position and shape during all construction activities.

Be constructed in such a way that it can be removed easily and safely, without damaging formed concrete.

If to be re-used, retain formwork strength during design so as to minimise deterioration of materials through use and handling.

CHARACTERISTICS OF GOOD FORMWORK

Withstand the force of internal or external vibration during the consolidation of concrete.

Have smooth surfaces and avoid warped timbers which may make the surfaces of concrete uneven.

Be Constructed and dismantled after use so that it is capable of being re-used and this is ensured by coating internal surfaces with non-staining oil, white wash or soft soap to permit easy striking off.

CHARACTERISTICS OF GOOD FORMWORK

Designers should consider the work practises necessary to carryout erection and dismantling of the formwork as well as identifying health and safety risks and controls at the design stage.

Possibility of using designs that do not require in-situ formwork, such as tilt-up or pre-cast concrete elements that may be constructed elsewhere and lifted into position.

Method and sequence of erecting and dismantling the formwork or false work and its supports as well as related risks especially manual handling.

Provision of safe access to and egress from the work areas on and around the formwork.

Minimizing the working heights for persons erecting and dismantling.

FORMWORK DESIGN

Guardrail systems including toe-boards, perimeter safety screens, scaffolding or other means should be installed when working at heights.

Fall arrest systems including safety lines can be installed if required.

Formwork drawings should include details of formwork, formwork joints, sealing procedures, ties, size and spacing of framing and details of any proprietary fittings or systems proposed to be used.

Details of the construction and erection sequence should be included on the drawings,

FORMWORK DESIGN

Choice is influenced by economy and necessity or both. Materials commonly used are timber, plywood and steel. Are used either separately or jointly. Steel formwork Merits It is durable and can be used as many times as necessary. It gives a smooth and better surface finish of concrete. It is not liable to warp. It is more economical than timber where repetitive work is

necessary. Speed of erection with fewer workmen required to assemble it

onsite.

MATERIALS FOR FORMWORK

Demerits The initial outlay of money is great. It is easily dented. It is easily affected by rust under humid conditions. Timber formworkMerits It is adaptable. It is relatively cheap. It is more economical than steel where work is non-

repetitive. It can be found locally.

MATERIALS FOR FORMWORK

It is susceptible to insect and fungal attack. It warps, especially when it is not well seasoned. It is not good for repetitive work.Timber Timber for formwork should be neither too dry nor too

wet. If too dry the timber will absorb water from the concrete

and swell resulting in distortion or decay. If too wet shrinkage may result especially during hot

weather which might result to warpage or cracking. The normal moisture content of timber used for formwork

should be 20 %.

DEMERITS

It is used where a large area of exposed concrete is to be constructed, for instance walls.

Merit The elimination of joint marks. If carefully fixed on site, the finished concrete does not

require further treatment

PLYWOOD

COLUMN FORMWORK

COLUMN FORMWORK

COLUMN FORMWORK

It is also known as a column box. Consists of vertical boards which have battens nailed across

them. The battens are called cleats usually 32mm; and the

boarding is called sheeting usually 25mm thick.

FORMWORK FOR A COLUMN

Cleats used at the bottom are usually larger so as to withstand the pressure which is exerted by the weight of the concrete.

The sides of the box are secured firmly together by means of yokes of about 50 by 75mm which may be replaced by use of sash clamps.

The column box should be located against a 75mm high plinth or kicker which has been cast monolithic with base or floor.


The reinforcement rods and stirrups must be set and well plumbed up before erecting the column.

The box is held in position by means of braces fastened to sole pieces on the floor.

If the corners of the columns are to be shaped, triangular fillets or mouldings of the desired shapes are nailed to the corners of the formwork.

Concrete is poured as soon as the form base has been cleaned.


Vertical mould- resist considerable horizontal pressures in the early stages of casting.

Kicker – accurately positions the formwork onto the base or floor. Also prevents the loss of grout from the bottom edge of the form.

Soldiers – vertical studs used to strengthen panels forming the column sides.

Annexures to the column formwork.

Cleats – horizontal battens used to strengthen panels forming the column sides or mould.

Margin piece – piece of plank fixed around the opening of a cut out to provide sufficient bearing for the beam boxes.

Yokes – pieces of timber used to firmly secure the sides of the box together.


Yokes – collars of timber or metal used to hold together column forms timber yokes are purpose made whereas steel column clamps are adjustable within limits of the blades.

Spacing of yokes and clamps should vary with the anticipated pressures, the greatest occurring at the base of the column box.


Variance of actual pressure :

a. Rate of placingb. Type of mix being used- generally the richer the mix the

greater the pressure.c. Method of placing- if vibrators are used pressures can

increase up to 50 % over hand placing and compacting.d. Air temperature – the lower the temperature the slower

is hydration process and consequently higher pressures are encountered.


BEAM FORMWORK

Consist of two side boards, soffit board and a head tree. Two side boards 32mm thick are tongue and grooved and secured

by cleats of 32 by 150mm which are placed at 1200mm centres. The soffit boards are thicker than the side boards since they bear

the greater part of the weight of the concrete. The soffit board rests directly on the head tree. The head tree of 75 by 100 mm timber is placed centrally on a

prop

The column box may also provide support to the beam box in framed buildings.

FORMWORK OF A BEAM AND SLAB

The head tree is secured firmly to the props by braces. The props rests on a wedge which in turn rests on a sole

plate.

Functions of wedge To allow the props to be removed easily by knocking out the

wedge when dismantling the formwork. To help in achieving the required height.

FORMWORK OF A BEAM AND SLAB

BEAM FORMWORK

Some concrete lintels are pre-cast while some are cast in-situ.

In-situ casting: Consist of two side boards and a soffit board which are

sometimes supported by the jambs and, if necessary immediately between the jambs by wooden posts on a pair of folding wedges.

Walls are normally built to lintel level before setting lintel formwork.

FORMWORK FOR A LINTEL

A racking back of about 225 mm on the wall will provide space where the concrete lintel will rest.

The soffit rests on the head tree of the props.

Props are cut little less than the height of wall but required height will be obtained by striking sole pieces and folding wedges into the base of the props.


The side boards are placed so that they are flush with the wall.

Boards are held by struts nailed to the head tree or by sloping struts firmly secured to the ground.

Sometimes side boards are held together by cross-battens nailed on the top of the side boards at suitable intervals.


LINTEL FORMWORK

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Formwork failures are the cause of many accidents and building failures that occur during concrete construction, usually when fresh concrete is being placed.

Generally some unexpected event causes one member to fail, then others become overloaded or misaligned and the entire formwork structure collapses.

The main causes of formwork failure are: 1. improper stripping and shore removal

CAUSES OF FORMWORK FAILURE

2. inadequate bracing3. vibration4. unstable soil under mudsills (A plank, frame, or small footing on the ground used as a base for a shore or post in formwork), shoring not plumb5. inadequate control of concrete placement6. lack of attention to formwork details

CAUSES OF FORMWORK FAILURE

It is one in which the form is left as an integral part of the structure.

Becomes apparent when pouring concrete into the ground Can be utilised as the facing materials of in-situ reinforced

concrete. They can be of two types participating and non-

participating. The materials used for this form must be durable and of

sufficient strength. Commonly used materials include polyvinyl chloride,

galvanised coiled, fabricated steel.

PERMANENT FORMWORK

low cost of transportation and installation, precise form design, maximum flexibility, greater durability with reduced long term maintenance and Versatile.

DISADVANTAGES high initial cost of design and installation lack of familiarity for installation and maintenance

ADVANTAGES

Engineering

Formwork by chakamba john