SPEEDY`

CONSTRUCTION

SPEEDY CONSTRUCTION:

In general concrete construction, we have seen that whatever the elements cast (beams, columns, slabs etc.) require time for setting. This is a long drawn process necessary for hardening of concrete.However, nowadays in large scale projects, newer methods of faster construction are being explored. As a result much time cannot be devoted to hardening of each member.

• Precast concrete blocks are used.• Members are cast on ground and then lifted to save on the time and money used for shuttering. eg: lift slab construction• Such members are used which do not require elaborate formwork. Eg: flat slab 

CAST -IN -SITU

REINFORCED CONCRETE FRAME

The first reinforced concrete framed building to be built was the general post office building in London which was completed in 1910.

Reinforced concrete was used in this country until the end of the second world war.

Then steel had been traditional material used for structural frames and engineers regarded the new fangled reinforced concrete with some suspicion.

The great shortage of steel that followed the end of the second world war promoted engineers to use reinforced concrete as a substitute for steel in structural building frame.

At the time the conventional method of providing fire protection to structural steel frames was to encase beams and columns in concrete that was cast in situ in framework around the steel.•The member of a reinforced concrete frame can be molded to any required shape so that they can be designed to use concrete where compressive strength is required and steel reinforcement where tensile strength is required. •When concrete is designed cast in situ it will act monolithically as a rigid structure

CAST-IN-SITU

Cast in situ construction

Cast in situ piles

WAFFLE GRIDSLAB

A building system using concrete "waffle slab" floors

supported by columns. The waffle slab is a monolithic-

poured concrete slab with a flat top surface and an

under-surface made of a rectangular grid of deep

concrete beams running at right angles. From the

underside, the slab resembles a waffle. The floor is

supported by columns spaced typically 30 feet on

center. The system is used in cleanrooms, areas that

require isolation from low-frequency vibrations and

areas that require low floor deflection.

WAFFLE SLAB SYSTEM

This type of floor is useful where a widely spaced column grid is necessary and the floor supports heavy load because of the use of intermediate cross beams If the column grid is increased from about 6.0 to about 12.0 square or near square it becomes economical to use a floor with intermediate cross beams supporting thin floor slabs.

The intermediate cross beam are cast on a regular square grid that gives the underside of the floor an interesting appearance of a waffle, hence named.

The waffle grid form of the floor may be cast around plastic or metal formers laid on timber centering , so the smooth finish of the soffit may be left exposed.The tensile stress developed in slab is partially replaced by their ribs/intermediate beams along orthogonal directions.These intermediate beams are reinforced with steel to resist flexural tensile stresses.

USES

This type of floor is used where a widely spaced column grid is necessary and floors support comparatively heavy loads.

Their profiles may be expressed architecturally, or used for heat transfer.

Waffle slab floors: (a)without band beams and (b) with band beams

TYPICAL ARRANGEMENT OF BARS IN A WAFFLE SLAB

TYPICAL ARRANGEMENT OF BARS IN A WAFFLE SLAB

• Tensile reinforcement• 75-115 slab depth, thumb rule for total depth span/24• 125 or 150 rib width• Square metal or fiberglass dome forms are available. In 485 or 760 widths & from 205-510 depths in 51 increments. Larger sizes are also available. Tapered sides allow for easier removal.

• 485 domes and 125 ribs create 610 module 760 domes and 150 ribs produce 915 module.

• For greater shear strength and moment resisting capacity, solid heads at column supports are formed by omitting dome forms. Size depends on the span & load conditions.• Suitable for spans 7 to 16m, longer spans may be possible with post tensioning.• For maximum efficiency, bays should be square or nearly square as possible. Waffle slabs can efficiently cantilevered in two directions upto 1/3 of the span. When no cantilever is present, a perimeter slab band is formed by omitting dome forms.•Coffered underside is usually left exposed.

BENEFITS OF WAFFLE SLAB SYSTEM OVER CONVENTIONAL SLAB SYSTEM

Less use of concrete eliminates concrete, quantity overruns

Greater strength and stiffness over conventional slab systems Savings in construction time over conventional slab systems

Installation during inclement weather

Insulation benefits

Effective termite protection

Column grid increases from 6m to 12 m square or near square

They support a thin floor slab and so reduce the dead weight of the floor as compared to a flush slab of similar span.

Waffle Raft Pod standard sizes:

1090 x 1090 x 175 mm1090 x 1090 x 225 mm1090 x 1090 x 300 mm1090 x 1090 x 375 mm

Construction of Waffle Slab System

Ensure building site is level and all plumbing pipes are fixed in place as per plan.

Spread a layer of sand or metal dust approximately 50 mm thick over building area and screed level. Place 200µm plastic sheeting and tape laps

Step 1

Step 2

Set up formwork, set out Waffle Raft Pods in a grid pattern using multispacers provided which form all beams.

Cut the Waffle Raft Pods to fit around plumbing pipes etc. using sharp handsaw or power saw. Place reinforcing bars in multispacers positioned between void formers.

Step 3

Step 4

Place steel mesh on top of Waffle Raft Pods and chair steel mesh at a rate of 4 bar chairs per Waffle Raft Pod.

Pour concrete, part fill the ribs to hold the Waffle Raft Pod in position before pouring 85 mm thick slab with 25 mm minimum cover over reinforcing mesh (or as per your engineers instructions).

Step 5

Step 6

DROP SLAB

CONSTRUCTION

DROP SLAB FLOOR

• This floor construction consists of a floor slab which is thickened between columns in the form of shallow but wide beams.

• A drop slab is of same dead weight and cost as comparable to slab and beam floor.

• The depth of drop slab is generally half the depth of floor construction from top of slab to soffit of beams.

• It achieves least overall height of slab and is cost effective.

CAST-INSITU SERVICE AND

STAIR CORES

Stair Cores :This is the space in the building where the stairs are constructed. This may require special structural design so that the floors around the stairwell do not require structural walls and the stairs themselves are open to the floors.

PRECAST STAIRCASE

Section thru a precast staircase

Plan of a precast staircase

Section thru a staircase, cast in situ

Stair cores and Cross wall construction :The cross-wall system generally utilizes stair cores and lift cores for overall stability, using the floors as stiff diaphragms for the transmittal of horizontal forces into shear walls located at staircase and lift shaft positions. The floors are made up of either hollow core, solid slab, or composite construction.

LIFT SLAB

CONSTRUCTION

LIFT SLABS Lift slab is a method of erecting post tensioned reinforced concrete floor and roof slabs which have been constructed on the ground.These slabs are cast monolithically and are mainly used where the floor plans are same throughout the height of the building.Most commonly used method of construction involves steel columns; jacks placed on top of column to lift the slab, and threaded lifting rods connected to the jacks and to steel lifting collars cast into each of the slabs.

USES

The system can be employed beam and slab, and waffle grid slabs.

CONSTRUCTION

Steel or concrete columns are first fixed in position and rigidly connected to the foundation and the ground floor slab is then cast.

When it has matured it is sprayed with two-three coats of a separating medium consisting of wax dissolved in volatile spirit. As an alternative polythene sheet or building paper is also used.

Lifting collars are cast in around each of the columns. They act as shear heads to transfer forces from slabs to column.

The support of lifting collars are provided by wedges (also called weld blocks) bearing on the edges of the plates welded to column’s flange.

First floor slab is cast inside edge formwork on top of ground floor slab and when it is mature it is in turn treated with separating medium and the next floor slab is cast on it.

After successive casting of slabs these are lifted by jacks, operating on top of each column by lifting a pair of steel rod attached to each of lifting collars.

A central control system synchronises the operation of all the jacks for uniform and regular lifting.

The bases of columns are rigidly fixed to foundation so than when lifting begins, they act as vertical cantilevers. Their stiffness increases as the slabs are raised higher.

The steel lifting collars cast into slab provides a means of lifting the slab and act as shear reinforcement to the slabs around columns.

ADVANTAGES

• It is highly cost effective and a speedy construction system. • The only formwork required is to the edges of the slabs and no centering is required for the soffit of roofs or floors.  

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