TEMPORARY WORKS
Formwork
Definition; its mould or a box into which wet concrete can be poured and compacted so that it
will flow and finally set to the inner profile of the box or mould’
Functional requirements
i. It should be strong enough to support the weight of wet concrete = 2400kg/ m³
ii. It must have grout tight joints to reduce leakage which courses honey combing on
surfaces
iii. It must not deflect under loads life/ dead
iv. Be so designed so as to necessitate easy assembly and dismantling
v. It must be accurately set out for concrete to take up the shape of the form mould.
vi. Farm sizes to be designed for ease of handling
vii. Suitable materials must be chosen for ease of fixing
Mould oils; applied to the inside surface of formwork to alleviate blow holes and uneven
colour defects. Other defects include the spoiling of the concrete face (plucking) due to the
sticking of concrete to the form
Terminologies
Brace: a member usually diagonal which act in tension or compression and stiffens. A
member act in tension or compression
Carcasing timber: timber used for any structural purpose in the support of forms but not
indirect contact with concrete.
Kicker/ kicker frame: kicker is a small concrete up stand cast above floor level to position
wall and column forms for the next lift and to assist the prevention of grout loss. Kicker form
is formwork to a kicker
Panel: prefabricated form of limited size designed for respected re-use, a number of which
may be fixed together to form a large surface
Plucking: it’s the spoiling of the concrete face due to adhesion of the concrete to the form
the reverse of form electing
Influence of striking order on the design forms
1. Forms to vertical non-load bearing surfaces e.g. column boxes, beam sides and wall
forms
2. Decking to slabs, horizontal and inclined which carry only light load e.g. slabs to roof,
floors and canopies
3. Soffit forms carrying heavy loads e.g. beams and gutter bottoms
Precautions against timber wastage
a. Stresses imposed by the fluid concrete and additional live and dead load should be
analyzed and the minimum safe sizes of timber and maximum spacing of supports
obtained
b. Timber should be ordered to the most convenient lengths to avoid wastage in cutting
c. All longer lengths should be cut first and short ones cut from short materials
d. Forms should be made to strike easily without damage, thus permitting maximum re-use
e. All loose boards and panels which are stuck should be immediately cleaned, loose nails
removed and panels stacked in neat piles for re-use
f. Small cleats, wedges, bolts and other loose parts should be thrown into separate boxes as
they become free.
Materials for formwork
Soft wood is often used for formwork
Functional requirements
1. Should be reasonably priced and readily available
2. Easily worked with hand or machine and nail easily without splitting
3. Should not be so soft so as to be damaged easily on the contact faces under normal
conditions of erecting, fixing and placing of concrete
4. Should be as light as possible to permit man handling when made up into large forms
5. Be stiff to avoid undue deflection when loaded during concreting
6. Should not be unduly liable to cast or warp when exposed to sun or rain
7. While the most expensive grade is obviously not needed the timber should be watched for
bad defects such as knot holes which would form an undesirable key on the concrete face
Use of hardwoods
Hardwoods of the type which are normally hard are heavy such as, beach, birch, ash oak, e.t.c
are not generally suitable for formwork
Why.
- Because they are very expensive
- Too heavy
- Very rarely available
But they have special uses where strength, hardness and durability are an advantage
These may be:
a. For folding wedges which have to withstand hammer blows
b. For caps interposed between vertical and horizontal bearing timbers so that the
concentrated pressure from the post may be distributed over a large area of the relatively
weaker side grain of a bearer
c. For moulds for precast work where considerable re-use is expected and where fixing is by
screws and clamps.
d. For small sections moulds such as area used to form grooves and drips where softwood
might be fragile
Timber sheeting: It’s a term applied to boards of any materials used to provide faces to form for
walls, columns e.t.c
Decking: used to describe similar facing to horizontal surfaces such as floor slabs
Plywood
Advantages
a. Rapidly and economically fixed
b. It is virtually leak proof as most of the joints which are few come on the bearer
c. It provides large level surfaces
d. No local casting or cupping of surfaces
e. It may be nailed close to the edges without spiriting
f. It have high resistance to impact load
g. It is stable does not swell or shrink to the same degree as solid timber
h. When well planned there is small waste
Form linings
Sheets of material applied to the face of the form mainly to give a special surface to the concrete
face
Types
1. Building paper
This is a stout laminated paper consisting of two crossed rayer of sisal fibers sandwiched
between two sheets of Kraft paper by means of bitumen adhesive obtained in rolls of
about 1 – 2m wide and up to 30m long
Advantages
Prevents the escape of grout retains the moisture in the cement helps the curing and keep
the form surface clear for re-use
2. Hard board
It’s a manufactured board made in sheets 1.2 to 1.5 and 1.6m by 1.2 to 5.4m long.
Available in two qualities, standard and super hard board used for formwork in thickness
of 3 and 4mm. not capable of many re-use although the super hard board is harder and
more moisture resistance
3. Thin plywood
Thin plywood up to 6mm thick is used. As this has appreciable stiffness the supporting
boards if used may be more widely spaced according to the maximum pressure involved.
For concrete slab up to 250mm thick boards may be spaced up to 125mm apart while for
wall forms poured in 1m lifts, they may be spaced at 63mm apart using 6mm plywood in
each case
4. Plastic lining
Have successfully been used as lining, they are nailed or glued to the sheeting. They give
a smooth finish to concrete face. They enable the sheet to be used many times
Disadvantages
- It imports a shine to the concrete surface causing crazing
- It has a totally impervious face which prevents the disposition of surface air
collecting on the face creates small cavities which vary in size and easily seen
5. Insulation board
A soft and absorbent material which, when applied to the form sheeting absorbs the
surplus moisture from the face of the concrete minimizes the risk of blow holes and
improves surface texture
Permanent formwork lining
This is linings which are left permanently as an integral part of the wall when the forms are
striped. Those commonly used are
- Precast concrete blocks and re-constructed stone
- Separate surface finishes
Ref: Barry vol 1 and 2
Wall formwork
Consists of standard panel tied together over their backs with horizontal members called walling
fulfill the same function as yokes providing resistance to the horizontal force of the wet concrete.
A 75mm high concrete kicker is formed at the base of the proposed wall to:
i. Enable the forms to be accurately positioned
j. Help prevent the loss of grout at base of the form
Fig vii 1 and 2
Climbing formwork
This is a method of casting a wall in set vertical lift heights using the same forms in a repetitive
fashion thus obtaining max usage from a minimum number of forms. The first lift is positioned
against the kicker in the inverted position, the concrete is poured and allowed to cure after which
the forms are removed, reversed and fixed to the newly cast concrete, the forms are removed and
raised to form the next lift until the required lift has been reached
Fig vii 3
Sliding formwork
This is a system of formwork which slides continuously up the face of the wall being cast by
climbing up and being supported by a series of hydraulic jacks operating on jacking rods. The
whole is therefore cast as a monolithic and joint less structure making the method suitable for
structure such as water towers, chimneys and cares of multi storey buildings with repetitive
forms.
SHORING NEXT RTI DCE MAY 2021
Shoring is the temporary support required in case of faulty foundation, building walls in case of
overturning or where alteration proves necessary. The requirements for shoring are in each case
individual. The extent and direction of movement to be expected in a wall are highly variable
and the duration for which shoring are needed ranges from weeks to years. Shoring may be
carried out in timber or a combination of timber and steel. These members should be of sound
capability to offer strength to the building the stability of which is threatened by damage or
adjacent building operations
There are three types of shoring
1. Dead shores
2. Raking shores
3. Flying shores
Common situations where shoring may be required
- During demolition work for support the adjacent building
- Support upper part of wall for formation of opening in the lower wall
- To avoid failure of sound walls near basement construction
- Give support to the building likely to become unstable due to subsidence building or
leaning
- Give support to a floor/roof to enable a support wall to be removed and replaced by
beam
Shoring systems
1. Dead shoring: supports dead loads which act vertically downwards. It consist of the
following
i. Vertical prop or ( shore leg)
ii. Head plate
iii. Sole plate
iv. Some means of adjustment
Sequence of operation
1. Site investigation to ascertain
- Number of shores, possible loading and window positions
- Bearing capacity of soils
- Location of underground services
2. Fix as close to the wall as possible ceiling struts between suitable head and sole plate to
relieve the wall off floor and roof load
3. Strut all window openings within the vicinity of the shores to prevent distortion of the
opening
4. Cut holes through the wall slightly longer in size than the needles
5. Cut holes through ceiling floors for shore legs
6. Position and level sleeper on a firm base
7. Erect wedge and secure shoring
8. Leave the shoring in position at least 7 days
2. Rake shores: it transfers the floor and wall loads to the ground by means of sloping
struts or rakes
Operation of sequence
- Site investigation as for dead shore
- Mark and cut mortises and housing in wall plates
- Set out and cut holes for needles in external wall
- On a firm base lay grillage platform and sole plate
- Cut and erect rakes beginning with the bottom shore
- Fix cleats, distance blocks, binding e.t.c to the backs of shores
3. Flying shores
Fulfill the same functions as raking shores but have advantage of providing clear working
space under shoring. Used between any parallel wall surfaces up to 12m span Short spans
up to 9m have a single horizontal member while large spans require two horizontal shores
to keep the section sizes within the timber range commercially available
