Concrete Floor Cycle Case Study

Concrete Floor Cycle Case Study
DESCRIPTION OF THE PROJECT
This project was being built in a city centre location. It is a 50-storey tower built off of a
three-storey podium deck. There are two levels of basement. The central core is of
concrete construction and is being built ahead of the floor slabs. The system for the
core is jump form. There is a central tower crane that is both used for the core
construction and the floors. The floor slabs are following the core construction about six
to eight floors behind. The whole building is of reinforced concrete construction.
THE FLOOR CYCLE CONSTRUCTION
The floors are supported on columns and attached to the centre core structure. A
perimeter ring beam connects the columns and band beams span between the
perimeter beam and the core. The basic floor plan is symmetrical and repetitive.
Additional features to the end bays are that the corners are splayed but with projecting
areas to allow a straight unit based cladding system to be attached. The end bays have
a slight projection to add interest to the end elevation.
Construction elements of the concrete floor
To provide both fall protection, external access for column forming and protection to the
public a three storey height perimeter screen has been used. This is lifted on its own
jacks once the lower fixings have been disconnected. The screen also incorporates two
hoists that are used to lift the struck forms from one floor to the next. (Figure 1)
The slab is 250mm thick and the beams are 1200mm wide x 300mm deep. The beam
formwork is plywood and a special system is used to provide the needed support. A
timber piece is nailed to the top of the beam side support to receive the metal floor deck.
The floor deck is standard metal deck nailed to the timber plate and fixed to the beam
sides. This is the only fixing for the deck until the concrete is cast. A system of frame
supports from below the deck is provided. (Figure 2)
The splay ends and projections at the edge of the slabs are made from timber and ply.
The metal deck is cut on site with power saws to shape the slab edge. (Figure 3)
The deck is lightly reinforced with the main reinforcement in the beams. These are posttensioned by cables within conduit built into the beam reinforcement. Tensioning is done
in two stages prior to the screen being finally moved from the floor (Figure 4). The forms
are struck in the conventional way by lowering and dismantling the forms into the
constituent parts.
Two decks of forms are in use. The crane is used for steel deck and reinforcement
materials delivery to each floor. Reinforcement is supplied in steel boxes. Other
materials are loaded onto the hoists and raised to the next floor.
Construction programme of the concrete floor
The floor construction cycle is four-day programme. The target is to concrete the whole
slab in one pour. The first two days are used to prepare the forms the third day final
reinforcement and the final day concreting.
Contracting and management structure
The overall project is construction management project. The main packages are:
 Formwork
 Reinforcement
 Stressing
 Concrete
 Screens
 Crane
Issues and considerations
The screens
The screens have to be lifted by 11 am in the morning otherwise there is a whole day
delay. To maintain the screens four men are needed. The screens are quite old and it
takes three hours to lift to a new position and the hoists take two hours. The hoists are
lifted every other lift of the screens.
Formwork
Because the steel floor deck is nailed to the beam sides there is a 30% to 50% remake
of the beam sides on every floor. There is not a planned order for striking, collecting the
used material and for sending it up to the next floor. Striking is the usual pull down into a
heap that has then to be sorted, stacked and sits waiting for a suitable opportunity to be
taken up on the form hoists (Figure 5). The stacking should consider the grouting
operation to the stressing cables so that it will not get in the way.
Where the floor is steel deck the mould oil on the beam forms spreads onto the steel
deck and makes it very slippery and dangerous. Therefore a major cleaning process
should be conducted prior to concreting.
Concreting
The floor is concreted as a whole. This is dictated by the needs of the stressing system.
There is an initial stress once the floor has been cast. The final stress must be
completed before the third floor above is commenced.
Figures
Figure 1: Materials hoist and screen assembly

Fixing the metal decks to the sides of the
beams is essential to keep the formwork
in place when pouring the concrete. The
beams and the slabs are constructed in
the following sequence:
 beam base,
 supports,
 beam sides,
 metal deck is nailed into the beam
side.

Concrete
Nail Beam base
Beam side
Prob
Support
Metal deck
Figure 2: Beam and slab formwork
The splay is a 450 angle that
is used at the corners of the
floors.
The slab is “cut” across at
the corners of the building,
so they used pre-made
shutters to pour the last pit of
the in situ slab.
Figure 3: Edge of slab construction
Figure 4: Reinforcement
Figure 5: Stacked formwork
Shutter
Shutter
Slab
edge
Splay
In situ
concrete