SLIP-FORM CONSTRUCTION

REPORT-2

BUILDING CONSTRUCTION

NAME-APRAJEETAB.ARCH-7TH SEM

R.T.I.A.D

Slip form construction, or continuously formed construction, is a construction method in which concrete is poured into a continuously moving form.

INTRODUCTION

Basically, this method involves the continuous placing of concrete in a shallow mold having the same plan as the building to be constructed. This rigid mold, or "slip-form" as it is called, forms the working deck which is jacked slowly upwards at a controlled rate until the requiredelevation is reached.

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.

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ADVANTAGES OF SLIP FORM WORK CONSTRUCTION

Speed Of Between 7ft And 30 Ft Per 24 Hours Are Possible. Slip Forming Is A Continuous Extrusion Process. Slipforming Provides A Superior Concrete Finish With No Horizontal Joints. The Exposed Concrete Can Be Finshed At The Bottom Of The Rising Formwork. Crane Use Is Minimised.

Types of Slip-form Construction 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 formand working platform are raised by means 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.

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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.

Factor - Description1. Time - High production rates can beachieved by careful planning ofconstruction process.2. Durability - Better concrete; a joint less andwatertight structure.3. Cost - Slip forms showed cost advantagesfor more than 20 stories and largerthan 600 m2 formed area per floor.4. Maintenance - Availability of the different working platforms in the formwork systemallows the exposed concrete at the bottom of the rising formwork to be finished, making it an integral part of the construction process. 5. Flexibility - Certain formwork systems permit construction of tapered cores and towers. 6. Labor - Slip form systems require a few buthighly skilled workers. 7. Machinery - Slip form construction minimizes crane use.

• Applications of Slip-form Construction

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.

1. The formwork and the access platform areassembled on the ground.2. The assembly is raised using hydraulicjacks which are mounted on strategicallylocated steel frames to lift the formwork asthe concrete is poured into the forms.3. As the formwork rises continuously,continuous concrete and rebar supply areneeded until the operation is finished.4. As the formwork is raised, reinforcement isheld in the correct position using guides,horizontal reinforcement is tied to thevertical reinforcement.5. Concrete is poured into the forms in layersof approximately 200 mm. The settingrates of concrete are constantly monitoredto ensure that it is matched with the speedat which the forms are raised. The jacks liftthe form approximately 25mm per strokegenerally producing a slip-forming rate of300 mm per hour.

Construction Sequence

6. Slip forming can be performed on eithera continuous basis (i.e. 24 hours perday), or a discontinuous basis (i.e.pouring to a predetermined heightusually within one working day) .7. Block outs for doors and windows canbe formed with either timber or steel.These are in-stalled as the slip formproceeds and can be stripped from thetrailing decks .8. Recesses are made to host theconnections between the beams, slabsand the slip form walls9. When the formed concrete is exposedfrom the bottom of the steel formpanels it can be sponged or treated ifrequired .10. At the end of the operation theformwork is removed using a crane, theentire process is thoroughly inspectedand highly controlled.

Construction Sequence

Slip forming relies on the quick-setting properties of concrete, and requires a balancebetween quick-setting capacity and workability. Concrete needs to be workable enough tobe placed into the form and consolidated via vibration. This strength is needed because thefreshly set concrete must not only permit the form to "slip" passed the concrete withoutdisturbing it, but also support the pressure of the new concrete as well as resist collapsecaused by the vibration of the compaction machinery.

Construction Sequence

• Slip form construction is a construction methodin which concrete is poured into a continuouslymoving form.• There are two types of slip-forms; vertical andhorizontal.• Slip-form construction consumes very less timebut requires careful planning of constructionprocess to achieve high production rates.• Due to continuous concrete pouring in slip-formconstruction it produces better concrete jointless and watertight structures.• Slip form systems require a few but highly skilledworkforce.• Slip-forming is also used in the construction oftapered structures involving changingthicknesses in walls, diameters and/or shapes.• Another but similar method that is in use for thistype of construction is the “jump form method”.In this method the form work jumps up to thenext layer after the bottom layer is cast. Theconcrete pouring is not continuous as in the slip form method.

Summary