formwork in civil engineering

FORMWORK & SHUTTERING

Civil Engineering Drawing & Graphics

What is formwork? When concrete is fresh and in its liquid state it must be

restrained within a mould in order for it to set in its required shape. Formwork is the term used to describe this mould.

For most in-situ pours, the formwork is made of wood. A smooth outer surface provides the main support for the concrete as it sets. This is normally made of plywood.

Concrete is approximately 2.4 times as dense as water, and in its liquid state, it imposes considerable forces on the formwork containing it. Consequently it is necessary to reinforce the plywood with horizontal wooden beams or wailings.


If constructed properly, this mechanism of a plywood skin supported by timber wailings provides ample support for the setting concrete. The mechanism of transferring these loads to a solid support can be performed by numerous methods which are collectively known as falsework.


GOOD FORMWORK: There are two factors that decide how well the

concrete will look in the final result.. one is how the concrete is placed and compacted, and the other is the quality of the formwork.

Formwork is made from expensive materials, and requires great skill and experience in its manufactures. Its importance is evident when you consider that the cost of fabrication, erecting and striking the formwork, often exceeds the cost of the concrete it is designed to shape and support.


Formwork usually needs to be used many times for it to be cost efficient. This can only be done if it is carefully handled, cleaned and stored, regardless of what material it is made from.

For anything other than most smallest of concreting jobs e.g. walls over 1m in height, a drawing should be provided, showing what formwork is required. This need not be an intricate design drawing, a simple sketch is usually sufficient.


The below picture shows part of a concrete structure soon after the formwork has been struck. Not the poor finish (circled). This is an imprint off the painted plywood formwork. Some of the paint has run, and this has been transposed to the concrete. It is important to remember that the face of the concrete is effectively a negative of the formwork face. Always inspect the formwork prior to pouring.


Honey comb in concrete


Good formwork should fulfill the following criteria:

It is strong enough to support the weight of fresh concrete during placing and compacting, and any other loads it may be required to take.

The face of the formwork is of sufficient quality for its use.

It can be easily erected and struck. It is sufficiently stable in all weathers.


It can be handled safely and easily using the equipment available.

It provides suitable access for placing and compacting the concrete.

It conforms with appropriate safety regulations.

The joints between members are sound enough to prevent grout leakage.


Formwork is an ancillary construction, used as a mould for a structure. Into this mould, fresh concrete is placed only to harden subsequently. The construction of formwork takes time and involves expenditure up to 20 to 25% of the cost of the structure or even more. Design of these temporary structures are made to economic expenditure.


The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable forms are known as panel forms and non-usable are called stationary forms.

Timber is the most common material used for formwork. The disadvantage with timber formwork is that it will warp, swell and shrink. Application of water impermeable cost to the surface of wood mitigates these defects.


A good formwork should satisfy the following requirements:

It should be strong enough to withstand all types of dead and live loads.

It should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape.

The joints in the formwork should be tight against leakage of cement grout.

Construction of formwork should permit removal of various parts in desired sequences without damage to the concrete.


The material of the formwork should be cheap, easily available and should be suitable for reuse.

The formwork should be set accurately to the desired line and levels should have plane surface.

It should be as light as possible. The material of the formwork should not warp

or get distorted when exposed to the elements. It should rest on firm base.


Economy in Formwork: The plan of the building should imply minimum

number of variations in the size of rooms, floor area etc. so as to permit reuse of the formwork repeatedly.

Design should be perfect to use slender sections only in a most economical way.

Minimum sawing and cutting of wooden pieces should be made to enable reuse of the material a number of times. The quantity of surface finish depends on the quality of the formwork.


Formwork can be made out of timber, plywood, steel, precast concrete or fiber glass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary.


For small works, timber formwork proves useful. Fiber glass made of pre-cast concrete and aluminum are used in cast-in-situ construction such as slabs or members involving curved surfaces.


Following are the materials which are used for formwork: Plywood: This is by far the most common material used for the

facing panel. It is easily cut to shape on site, and if handled and stored carefully, it can be used many times.

Note the different thickness: A standard plywood thickness on site is 18mm. this is usually sufficient for most pours. However, if the formwork is curved, a thinner plywood is used to facilitate bending. Thicker plywood may be used when the weight of concrete causes a standard thickness plywood to bow out, distorting the concrete face.


Timber: This is the most common material used

for bracing members to the form face. Like plywood, it can be easily cut to size on site. Formwork made from timber is called traditional formwork. The construction methods using timber formwork have been used on site for years, and all well understood by trained operators.


Ply wood Timber


Steel: Steel is also used in pre-fabricated

formwork. Purpose made steel forms are fabricated when dimensional tolerances are critical, or when the forms are planned to be re-used. Steel forms become cost-efficient after about a dozen uses, although they can be used up to 100 pours if they are carefully cleaned and stored.


STEEL FORMWORK


Aluminum: Often used in pre-fabricated formwork,

that is put together on site. Aluminum is strong and light, and consequently fewer supports and ties are required. The lighter sections will deflect more, but this can be avoided by simply following the manufacturers recommendations.


Plastics: Glass reinforced plastics (GRP) and vacuum

formed plastics are used when complicated concrete shapes are required (e.g. waffle floors). Although vacuum formed plastics will always need support, GRP can be fabricated with integral bearers making it self supporting. Like steel, plastic formwork can be re-used many times, as long as care is taken not to scour the surface whilst vibrating the concrete.


Timber Formwork: Timber for formwork should satisfy the following

requirement:It should be,1. Well seasoned2. Light in weight3. Easily workable with nails without splitting4. Free from loose knots Timber used for shuttering for exposed concrete

work should have smooth and even surface on all faces which come in contact with concrete.


Normal sizes of members for timber formwork:

Sheeting for slabs, beam, column side and beam bottom

25 mm to 40mm thick Joints, ledges 50 x 70 mm to 50 x 150 mm Posts 75 x 100mm to 100 x 100 mm


Plywood Formwork: Resin bonded plywood sheets are

attached to timber frames to make up panels of required sizes. The cost of plywood formwork compares favorably with that of timber shuttering and it may even prove cheaper in certain cases in view of the following considerations:


It is possible to have smooth finish in which case on cost in surface finishing is there.

By use of large size panels it is possible to effect saving in the labor cost of fixing and dismantling.

Number of reuses are more as compared with timber shuttering. For estimation purpose, number of reuses can be taken as 20 to 25.


Steel Formwork: This consist of panels fabricated out of

thin steel plates stiffened along the edges by small steel angles. The panel units can be held together through the use of suitable clamps or bolts and nuts. The panels can be fabricated in large number in any desired modular shape or size.


Steel forms are largely used in large projects or in situation where large number reuses of the shuttering is possible. This type of shuttering is considered most suitable for circular or curved structures.


Steel forms compared with timber formwork: Steel forms are stronger, durable and have longer

life than timber formwork and their reuses are more in number.

Steel forms can be installed and dismantled with greater ease and speed.

The quality of exposed concrete surface by using steel forms is good and such surfaces need no further treatment.

Steel formwork does not absorb moisture from concrete.

Steel formwork does not shrink or warp.


Construction of formwork:This normally involves the following

operations:1. Propping and centering2. Shuttering3. Provision of camber4. Cleaning and surface treatment


Order and method of removing formwork: The sequence of orders and method of removal

of formwork are as follows: Shuttering forming the vertical faces of walls,

beams and column sides should be removed first as they bear no load but only retain the concrete.

Shuttering forming soffit of slabs should be removed next.

Shuttering forming soffit of beams, girders or other heavily loaded shuttering should be removed in the end.


Rapid hardening cement, warm weather and light loading conditions allow early removal of formwork. The formwork should under no circumstances be allowed to be removed until all the concrete reaches strength of at least twice the stresses to which the concrete may be subjected at the time of removal of formwork. All formworks should be eased gradually and carefully in order to prevent the load being suddenly transferred to concrete.


Details of timber formwork for RCC beam and slab floor

Details at section shown in figure


Elevation Details of timber formwork for circular RCC column


3D View Details of timber formwork for square or rectangular RCC column


Sectional plan showing details of timber formwork for an octagonal column Details of formwork for stair

Timber formwork for RCC wall


Period of removal of formwork:

Walls, columns and vertical sides of beams1 to 2 days

Slabs (props left under)3 days

Beam soffits (props left under)7 days


Removal of props to slabs (a) For slabs spanning up to 4.5 m 7 days (b) For slabs spanning over 4.5 m 14 days

Removal of props to beams and arches (a) Spanning up to 6 m 14 days (b) spanning over 6 m 21 days


PRE-CONCRETE CHECKS FOR FORMWORK:

Before the concrete is poured into the formwork, it must be checked by someone who has been trained to inspect formwork. Depending on how big or complicated the pour is, the inspection may just take few minutes or it could take hours. Only when the formwork has been approved, may the pour take place.


Formwork pressures are function of height (including the height from which concrete is dropped into the forms) and are affected by concrete workability, rate of stiffening and rate of placing. One task of the temporary works coordinator is to consider such factors as ambient temperatures and concrete composition, when calculating maximum permissible rate of concrete placing.


Exceeding this limit may lead to unacceptable formwork deflections, loss of grout / concrete at joints, or even collapse. The cost of remedial work due to formwork deflection will usually exceed the original cost of doing the job properly.


Below are the checks that should be verified before pouring begins:

Is the formwork erected in accordance with the approved drawings?

Is the formwork restrained against movement in all directions?

Is it correctly aligned and leveled? Are all the props plum, and at the right spacing? Are bolts and wedges secure against any possible

loosening? Has the correct number of ties been used? Are they

in the right places and properly tightened?


Are all inserts and cast-in fixings in the right position and secure?

Have all stop ends been properly secured?

Have all the joints been sealed to stop grout loss (especially where the formwork is against the kicker)?

Can the formwork be struck without damaging the concrete?


Are the forms clean and free from rubbish such as tie wire cuttings, and odd bits of timber or metal?

Has the release agents been applied, and is it the correct one?

Are all projecting bars straight and correctly positioned?

Is there proper access for placing the concrete and compacting?

Have all the toe-boards and guard rails been provided?


RELEASE AGENTS FOR FORMWORK: Formwork needs to be treated with a

release agent so that it can be removed easily after the concrete has set. Failure to use a release agent can result in the formwork sticking to the concrete, which may lead to damage of the concrete surface.


A single application of release agent is all that is required when forms are then used. Care must be taken to cover all the surface that will come in contact with the surface of concrete. However, if there is an excess of release agent, it may cause staining or retardation of the concrete.


There are different release agents depending on what material is used for the formwork.

The three (03) most common release agents for formwork are:

Neat oils with surfactants: used mainly on steel surfaces, but also suitable for timber and plywood.


Mould cream emulsions: good general purpose release agents for use on timber and plywood.

Chemical release agents: recommended for high quality work, applied by spray to all types of form face.


ASSIGNMENT (A REPORT)

MiNi-Project In M.S Word, do make a brief report of at least

20 pages having in it “Pictures” from original sites covering 18 chapters of the referred text book (Preferably 01 page for each chapter). Try to make report understandable by providing sequential pictures of the phenomenon and a minimum of 4 to 6 lines in each page and relevant topic.

formwork in civil engineering

Engineering

formwork face

formwork isknown

stripped formwork

painted plywood formwork

anintricate design drawing

howthe concrete

weightof fresh concrete

common material