FORMWORK

SLIP FORM CONSTRUCTIONBY :14BCL03414BCL04814BCL06114BCL128

Introduction

Slip form

INDEXIntroductionProcedureTypes of slip formAdvantagePrecautionsApplication of slip form

Introduction

Slip-form Technique is one of the most productive techniques for construction of High-rise structures like communication towers, chimneys, bridge piers, silos etc. Vertical Slip forming allows concrete to be erected as a monolithic or a joint-less structure, while reducing overall project duration. Slipform paving has enabled concrete road pavements, airport aprons, taxiways, runways, canal lining etc. Slip-form Technique has also been successfully applied to various structures including offshore drilling platforms and nuclear facilities etc.

Slip form construction, or continuously formed construction, is a construction method in which concrete is poured into a continuously moving form.Basically, this method involves the continuous placing of concrete in a shallow mould having the same plan as the building to be constructed. This rigid mould, or "slip-form" as it is called, forms the working deck which is jacked slowly upwards at a controlled rate until the required elevation is reached.

Introduction

Method of vertically extruding a reinforced concrete section and is suitablefor construction of core walls in high-rise structures lift shafts, stair shafts, towers.The formwork rises continuously, at a rate of about 300 mm per hour, supporting itself on the core and not relying on support or access from other parts ofthe building or permanent works.

SLIP FORM TECHNOLOGY

Allows for the continuous pouring of concrete into walls of a structure and only stops when the full required height of the structure has been reached.The height of the formwork is designed in such a way that while the top of the formwork is being filled by concrete the lowest layer of concrete poured earlier has already gained an initial set. When the formwork is moved upwards the concrete that is then exposed remains firm.

Slip Form Technology

Procedure

Assembly can only start once the foundations are in place and the wall starter is in correct alignment.

Slip form shuttering is aligned with the help of yokes.

Horizontal crossbeams connect these yokes.

Hydraulic jacks are attached to these crossbeams for simultaneous upward movement.

Height of the slip form ranges from 1.1 to 1.5 meters.

Yokes and crossbeams also used to support the working platform.

Structure should be rigid and shape maintained at all times.

Make sure there is no lag or else it prevents the structure from free upward movement

It is also possible to reduce wall thicknesses

as the construction gains height and arrangements have to be made in the slip form structure that will enable such reduction at regular intervals.

Procedure

It is necessary to use a low slump concrete in slip forming processes where the formwork is moved horizontally in order for the slab or pavement to retain its shape as the paving machine advances. Presently, slip form pavements use "high early strength" concrete, which achieves the required strength in approximately 12 hours, as compared to conventional concrete which requires 5-14 days. The water content of this type of concrete is lower than it is for standard material, resulting in improved strength as well as improved resistance to the permeation of salt, thereby increasing the finished concrete's resistance to deterioration from chloride ions.Structural Concerns

1. VERTICAL SLIP-FORM

In vertical slip forming, the concrete form may be surrounded by a platform on which workers stand, placing steel reinforcing rods into the concrete and ensuring a smooth pour. Together, the concrete form and working platform are raised by means of hydraulic jacks. Generally, the slip-form rises at a rate which permits the concrete to harden by the time it emerges from the bottom of the form.

TYPES OF SLIP-FORM CONSTRUCTION

2. HORIZONTAL SLIP-FORM

In horizontal slip forming for pavement and traffic separation walls, concrete is cast, vibrated, worked, and settled in place while the form itself slowly moves ahead. This method was initially devised and utilized in Interstate Highway construction initiated during the 1950s.

Slip form methods of construction can also be adapted to horizontal structures and are used for paving, canals, and tunneling. The technique is more in use for structures that have continuous walls likesilos , chimneys, and piers for very tall bridges. It has also been successfully used for construction of buildings, although this requires the manner of leaving inserts for openings like doors and windows to be decided well in advance, as well as also any necessary inserts to support floor slabs after the walls are constructed.

HORIZONTAL SLIP FORM

3. TAPERED SLIP-FORMING

Slip-forming is also used in the construction of conical chimneys, cooling towers, piers and other tall concrete structures involving constant or changing thicknesses in walls, diameters and/or shapes. A form is used with sections which overlap so that one gradually slides over the other. This is commonly done in chimney construction but it is not satisfactory for architectural concrete because the lap shows. While the tapered slip-forming process is similar to that used on the standard slip-forming, it requires greater attention, contractor experience and expertise ensures the success of such projects.

ADVANTAGES

A major cost of concrete structure construction is the required formwork to retain the concrete till it can be safely de-shuttered and be able to support itself and other imposed loads.The formwork needs to be continually removed to newer locations and then re-erected. Continuous use of manpower and lifting equipment like cranes.

In the case of slip form building, the formwork is erected only once and remains intact until the entire structure is completed.Great reduction in the cost of formwork as well as time saving for re-erection.Cost effectiveThe reduction in the movement of formwork and workers also leads to far more safe working conditions that also make it a major advantage.

Advantages

PRECAUTIONS Concrete is continuously protected against loss of moisture and rapid temperature changes for 7 days Unhardened concrete is protected from rain and flowing water Prevent plastic shrinkage Plastic cracks are filled by injection of epoxy resin.

APPLICATIONS

Slip-form construction is used for tall structures, such as towers, buildings, and dams, as well as horizontal structures, such as roadway barriers. It enables continuous, non-interrupted, cast-in-place joint-less concrete structures which have superior performance characteristics over construction methods using discrete form elements. Slip forming relies on the quick setting properties of concrete, and requires a balance between quick-setting capacity and workability.

King Abdul-Aziz Airport, Jeddah, Saudi Arabia

Capital Gate Tower, Abu Dhabi, UAE

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