What is slipform?
- slipform is a self climbing formwork system.
- Supported on a recoverable vertical support medium.
- Steel faced formwork shutter configured in a single sided support fashion, i.e. no tie bars.
- Structural steel frame to form a rigid structural system where the formwork shutter is an integral part of the structure .
- Slipform does not cast concrete, it extrudes concrete.
- Slipform relies on a disciplined placement methodology akin to a manufacturing process in an in-situ concrete environment.
- Slipform is the fastest method of forming in- situ vertical concrete available.
What we can slipform?
- Vertical in-situ concrete structures.
- Preferable to have regular wall plan throughout the height of the structure.
- The taller the better.
- Preferable to avoid high strength concrete mix designs.
- A multitude of different planar structural shapes can be accommodated.
- Preferable to have a minimum wall thickness of 200 mm.
- Tapered structures can be slip formed.
Typical Assembly of Slipform
- Slipform typical assembly consists of mainly 3 parts
- Top deck
- which acts as a storage place.
- Reinforcement-verticals are placed.
- weather protection.
- It is the middle deck.
- Reinforcement - horizontal are placed.
- Placing of concrete.
- Which is the last deck used for finishing and curing.
Factors considered for Slipform:
- The main important factor to look is the slip area and the sliprate/day.
- Assembling and dismantling of slip takes more time.
- Having the right Design mix is the success of slipform.
- Optimum setting time of concrete is equality important for smooth and consistent slipping of slipform.
- No Glenium products, micro silica or PC based admixtures to avoid sticky concrete.
- Use of GGBS limited to 30% to achieve required setting time.
- Slump close to 200mm.
- Water Cement Ratio close to 0.35.
- Linear stiffening through to initial set, and final set, reaching initial set (also referred to as initial stiffening) at around 6 hours from placing, depending on site conditions and placement methods.
On Field Concrete trails :
8-10 Nos field trials were conducted to determine the design mix with right setting time for field conditions.the desired setting time around 8-10 Hrs which was obtained through these field trials. However taking in to consideration learning curve in the initial periods up to attaining required slip rate, the setting time of 10-12 hours was designed. Also there was potential opportunity to achieve slip rate higher than the planned Slip rate, a setting time of 6-8 hours was designed. So before we started the 1st concrete we had M60 grade of design mix with 3 varieties of setting time depending on the Slip rate of Cores which gave us flexibility of slowing down in case of requirement or accelerating in case of possibility. To start with the concreting in, design mix with 8-10 hours setting time was implemented.
Existing conventional banana type bucket was not suitable for slipform concreting due to very stiff handle which was manually operated to open and close. However the with huge pressure of concrete flow, the gate hardly can be closed resulting spilling of concrete.Wastage, housekeeping,loose of productivity etc. were issue with this bucket, instead of this we used a concrete bucket of 1 Cum with automatically closing discharge gate by spring re tractors with handle-lever and rope to open the gate. Concreting can be done with bucket suspended above the working platform. Easy operation of this bucket improved efficiency of concreting tremendously.
Operation's involved in Slipform:
Step 1: Fill concrete bucket
Step-2 : Lift bucket to top deck
Step-3: Pour concrete in shutter layer eise of 250mm each.
Step-4: Vibrate concrete
Initial filling of forms up to top of shutter with concrete.the forms are filled in a controlled manner and the concrete is placed in layers of approx. 250 mm evenly around all part of the shutter. This operation takes approx. 5 hours depending upon ambient conditions.The concrete does not have to have achieved great strength before next work of the project commence. The weight of the Slipform rig is carried on the climbing tubes and not the concrete. First lift is taken is dependent upon the rate of set of concrete mix adopted,and is established using probes within the concrete to determine when the initial set of the bottom 100mm of concrete has taken place. the shutter is lifted an initial 50 mm following with the inspection in the concrete surface is made before any further movement in slipform.Jacking Including Placing of Concrete - Concrete is placed in layers 250 - 300 mm deep to a set sequence and is compacted immediately using poker vibrators. The rate of climb is governed by the initial setting time of the concrete and its condition as it emerges from the Slipform shutter. Jacking operations are regulated according to regular inspections made of the emerging concrete from the lower inspection platform.
Allowable tolerance for verticality of cores: +/- 25 mm.
Equipment for measuring verticality comprises of strategically positioned Plummet Instruments & or Vertical Lasers around the base of the structure, to targets fixed on the slipform shutter.