Concrete Slip Forming

Topic Summary

Description and Uses

Slip forming is an on-site process that involves moving the formwork as the concrete is poured, simultaneously consolidaing and finishing it in a continuous or near-continuous operation.  The formwork may be moved vertically with a hydraulic jacking system to construct walls, silos, and large tanks.  The formwork can also be moved horizontally using a paving machine, a piece of equipment that continuously extrudes and finishes the concrete.

Methods and Materials

Slip forming uses a shallow form, fabricated specifically to the required concrete profile.  To move the formwork vertically, a hydraulic jacking system is used.  As the pour progresses, the formwork is supported by a slip form rig that is also raised.  The rate of lifting is determined primarily by the concrete pour rate, the time required to add reinforcing steel, and the setting time of the concrete.

The slip form rig includes three working levels, or decks.  The lower deck provides access to the newly exposed concrete so that surface finishing processes can be completed and design specifications can be met.  The middle deck is the main working platform.   Usually, it supports the hydraulic lifting system as well as provides an area where reinforcing steel is assembled and concrete is poured and vibrated.  The middle deck also provides the locations where gates, ladder platforms, and maintenance decks are formed.  The uppermost deck is used to install vertical reinforcing steel and to facilitate placement of the concrete.

Paving machines, used in the slip forming of concrete pavements, ride on treads over the area to be paved.  The formwork is attached to the paving machine and, therefore, it moves with the machine as the concrete is placed in front.  The machine spreads, shapes, consolidates, screeds, and finishes the concrete in one continuous operation, requiring close coordination between placement of the concrete and the forward speed of the paver.  Special machine attachments are available for paving machine construction of median barriers or curb and gutter combinations.

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.

Environmental Considerations

As with most construction projects, weather conditions at the site during slip form operations are important considerations.  In winter weather’s low temperatures, the hardening of most concrete is slower, delaying the removal of the formwork.  Cold weather can also affect concrete's strength.  In hot weather, concrete is susceptible to rapid slump loss, reduced air content, premature stiffening, and thermal cracking.

Vertical slip forming can be affected by wind.  High winds can make the operation difficult, produce unsafe working conditions, or require more strength to be obtained in the cast concrete prior to additional vertical lift. Horizontal slip forming is less affected by wind.  However, rain, hail, or debris blown in the wind can adversely affect the surface of the recently cast concrete.

Design Basics

A slip form typically consists of a wood-faced or steel-faced form attached to a steel frame. In a vertical application, the frame is mounted onto hydraulic jacks.  For the construction of pavements and other horizontal applications, the forms are attached to the paving machine.  If the surface of the structure is to be permanently visible, form liners may be used to create an aesthetically pleasing appearance or to create an architectural finish.

Formwork that is to be moved vertically should be designed to meet the estimated loads, lateral pressures, and stresses that will be imposed on the system as the concrete is poured and as it hardens.  The concrete mix and the reinforcing design must take all loads into consideration, including: the weight of any concrete that has already been placed above and is in the curing process; the weight of the slip form system; the weight of the personnel and materials on the assembly (platform); the weight of the freshly poured concrete and reinforcing; lateral loads, including wind and earthquake (including those vibrations caused by the forms); and any miscellaneous loads.

Slip forms, especially the vertical forming systems, are engineered systems with a high degree of sophistication.  Shop drawings of the system, the installation, the concrete and reinforcing design, the sequence of construction (including the curing time), and the continuous concrete source must be submitted to and reviewed by an expert and local jurisdictional authorities, as appropriate, prior to the start of construction.

Preparation

The majority of vertical slip forming is started from a level grade or level foundation.  The slip form contractor should inspect the surfaces for conformance with both the design parameters and the practical needs of the forming system.  Adequate time must be allowed for corrective measures.

As with all formwork, the components of the system should be inspected carefully to ensure that the surfaces of the forms and/or form liners are free of defects.  Prior to installation, the surfaces should be sprayed with a release agent.  Repeated applications of the release agent may be needed during the slip forming operation.

For slip forming concrete pavements, a base is compacted to the required density and areas are graded to support the paving machine at the planned elevation.

Since most slip forming requires the continuous casting of concrete, it is recommended that the team review responsibilities and processes prior to starting work in order to assure a continuous concrete pour.

Applications and Installation

The setup time for the slip forming process is typically longer than would be required for conventional formwork, but once the system is operational, the completion time can be significantly reduced.  In constructing walls, tanks, and silos, the form is raised as the concrete is poured, so that by the time the form is full, the concrete at the bottom has hardened. More concrete and more steel reinforcement is added and the process is essentially continuous. The rate of raising the form is not only dependent upon the rate at which the concrete hardens, but also upon the time required to install block outs, such as the rough openings for gates, doors, and windows.

In the slip forming of concrete pavements and median barriers, various power screeds and vibratory screed machines can be used.  Laser screeds to control elevation can shorten the placement time and reduce labor costs.

Other Considerations

The manufacturers of the slip forms, slip form systems, and associated machinery can provide more in-depth knowledge and practical applications for their products.

Slip forming can be more economical than standard forming, and this is often the reason it is used.  Also, it can be much quicker to use a slip forming process than standard forms.

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