A step by step master piece on formwork detail and application

1. BY: CHAKAMBA JOHN

2.  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.

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

4.  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.

5.  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.

6.  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.

7.  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.

8.  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.

9.  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,

10.  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.

11.  Demerits
 The initial outlay of money is great.
 It is easily dented.
 It is easily affected by rust under humid conditions.
 Timber formwork
Merits
 It is adaptable.
 It is relatively cheap.
 It is more economical than steel where work is non-
repetitive.
 It can be found locally.

12.  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 %.

13.  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

17.  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.

18.  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.

19.  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.

20.  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.

21.  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.

22.  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.

23.  Variance of actual pressure :
a. Rate of placing
b. 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.

26.  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.

27.  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.

29.  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.

30.  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.

31.  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.

34.  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

35. 2. inadequate bracing
3. vibration
4. 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 plumb
5. inadequate control of concrete placement
6. lack of attention to formwork details

36.  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.

37.  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