Why Does Slipform Stand Out in Very Tall Reinforced Concrete Structures?

As a reinforced concrete structure gets taller, the impact of the construction method to be used on the project also increases. Formwork cycles, transporting materials to upper elevations, concrete quality, verticality control, occupational safety, and site organization become increasingly critical.

For this reason, slipform stands out in structures that continue in the vertical direction, such as chimneys, silos, towers, shafts, and building cores. As the structure rises, the share of the system’s initial setup cost within the total construction decreases, while the time and organizational advantage provided by uninterrupted production increases.

Especially in very tall reinforced concrete chimneys, slipform is not just one of the options. If the geometry of the structure is suitable, it is the technically and economically most appropriate construction method.

Why does slipform become more advantageous as the structure gets taller?

In slipform, a formwork system approximately one story high moves upward hydraulically while concrete placement continues. After each concrete pour, there is no need to dismantle the formwork, move it to the next elevation, reassemble it, and reconnect it to the concrete.

The ability to use the same system continuously throughout the structure provides significant advantages:

Repeated formwork erection and dismantling work is reduced.
Construction joints are largely eliminated.
Concrete and reinforcement works progress within the same production sequence.
Crane, scaffold, and formwork operations at upper elevations are reduced.
The work schedule is not tied to floor or panel cycles.
The same formwork system can be used over hundreds of meters.

In low-rise structures, system setup and operational preparation may account for a larger share of the total cost. As the structure gets taller, this initial cost is spread over a longer scope of work. Therefore, in most projects, slipform becomes a more feasible and economical option as height increases.

In structures exceeding 200 meters, the repeated cycles of climbing formwork create a serious burden in terms of both duration and upper-elevation logistics. If the structure’s geometry is suitable for slipform, uninterrupted production at this scale is a much more rational solution.

Slipform is not used only in structures with a constant cross-section

It is not correct to think that slipform can only be used in straight-walled structures where the same cross-section is repeated.

Constant-section silos, shafts, and building cores are naturally suitable for slipform (e.g. the Abalıoğlu Aliağa reinforced concrete silos). However, reinforced concrete structures whose diameter, cross-section, and wall thickness change as they rise can also be constructed with this method.

One of the best examples of this is conical reinforced concrete chimneys (such as the 120 m Khabat chimney). A tall chimney generally starts wider and thicker at the base; as it rises, its diameter narrows and its wall thickness decreases. The slipform system is also designed to follow this change. The inner and outer form faces, suspension elements, and working platform are adjusted in a controlled manner according to the project geometry.

Therefore, slipform is not simply about moving a fixed cross-section upward. With the right engineering solution, changes in both cross-section and wall thickness can be managed during construction.

Why should tall conical reinforced concrete chimneys be built with slipform?

In very tall conical reinforced concrete chimneys, setting up and dismantling hundreds of formwork cycles separately is not meaningful from an economic and operational standpoint. Slipform enables the chimney geometry to be formed continuously.

While the cross-sectional diameter and wall thickness change along the chimney, the formwork system adapts to this change. In this way, the chimney shaft can be raised monolithically without forming numerous horizontal construction joints.

The advantage here is not only speed. The ability to monitor verticality, diameter, wall thickness, and surface geometry through a continuous system is also of great importance for construction quality in very tall chimneys.

In short, in very tall conical reinforced concrete chimneys, the question that should usually be asked is not, "Can slipform be used?" but rather how the system should be designed according to the structure’s geometry and the targeted production speed.

Can an existing structure be made suitable for slipform?

Not every project may have been designed with slipform in mind from the outset. However, this does not mean that the method cannot be applied.

Without compromising structural safety and architectural requirements, the structure’s geometry, details, and construction sequence can be made more suitable for slipform. For this, the slipform contractor needs to be involved in the project as early as possible.

Within the scope of this work:

Wall axes and cross-section transitions can be simplified.
Sudden geometric changes can be made gradual to the extent permitted by the project.
The elevations of openings, recesses, and embedded elements can be arranged according to the production sequence.
Reinforcement splices and lap zones can be planned in a way suitable for the slipping operation.
The structure can be divided into independent slipform zones.
Special transition elevations can be resolved with different formwork systems.

The objective here is not to force the structure to fit the formwork system. It is to develop a more feasible solution by evaluating the structural system, architecture, formwork method, and site organization together.

In this context, SİBA offers not only slipform application but also feasibility, construction method development, segmentation, system design, and reinforced concrete project coordination services.

Building cores and large structures can be divided into sections

One of the areas where slipform is used most efficiently in tall buildings is reinforced concrete cores. The core, consisting of elevator shafts, stairwells, and technical shafts, can be planned independently from the rest of the structure.

Core construction can progress ahead of the floor slabs or the steel structural system. Dowels, sleeves, plates, and blockouts required for slab and structural system connections are left at the relevant elevations.

Not all reinforced concrete parts in the same building need to be built with slipform. A large structure can be divided into:

independent building cores,
shaft groups,
blocks or wings,
sections to be slipped at different times,
separate construction zones to be built with slipform and climbing formwork

Thanks to this approach, it is possible to benefit from the advantages of the method even in projects that do not appear suitable for slipform at first glance. What is important here is that segmentation is considered together with the structural design and the construction schedule.

Is it possible to rise more than 6 meters per day with slipform?

If project and site conditions are suitable, reaching 6 meters per day with slipform, and even exceeding this level, is possible. However, this speed cannot be explained only by the capacity of the hydraulic system.

As the formwork rises, concrete placement, reinforcement installation, placement of embedded elements, surveying, surface finishing, material supply, and shift changes also continue uninterrupted. Any disruption in any of these tasks affects the speed of the entire production.

Of the two reinforced concrete chimney shafts 120 meters high built with slipform by SİBA in the Tarsus district of Mersin, one was completed in 25 days and the other in 23 days. These durations correspond to average climbing speeds of approximately 4.8 and 5.2 meters/day, respectively. On some days, daily climbing exceeded 6 meters.

This performance should not be regarded as a standard speed commitment for every project. Geometry, reinforcement density, concrete production capacity, openings, weather conditions, and working area directly affect the result.

What made the production speed in Tarsus possible was not only the rapid movement of the formwork. The fact that the entire process, from the concrete batching plant to the working platform, was carried out at the same pace, together with experienced shift crews and prompt decision-making on site, was decisive in this result.

In slipform, speed is achieved not by lifting the formwork faster, but by making the entire production chain work at the same rhythm.

Concrete and reinforcement design together determine the production speed

In slipform, the concrete mix cannot be selected solely according to the targeted compressive strength. The concrete must have sufficient workability when being placed in the formwork, and when it emerges from below the formwork, it must have reached early strength that will allow it to retain its shape.

Concrete that sets too quickly may make placement and surface finishing more difficult. Concrete that sets slowly, on the other hand, may leave the formwork before reaching sufficient strength, leading to surface deterioration and loss of shape.

For this reason, cement type, water/cement ratio, aggregate gradation, chemical admixtures, consistency, and concrete temperature must be evaluated together with transport time, weather conditions, and the targeted slipping speed.

Concrete is not the only factor determining production speed. Reinforcement design and detailing are at least as important as the concrete mix.

The arrangement of vertical and horizontal reinforcement, lap lengths, mechanical couplers, reinforcement around openings, and embedded parts must be planned in a way suitable for the uninterrupted movement of the slipform. A highly congested reinforcement detail, or one prepared without considering the application sequence, can slow installation on the platform and disrupt the entire operation.

The objective is not to compromise on structural reinforcement, but to detail the required reinforcement so that it can be installed safely and continuously.

In a successful slipform project, the concrete mix design, reinforcement design, formwork system, and targeted climbing speed are not independent decisions. All of them are parts of the same construction plan.

An important advantage in terms of occupational safety

In slipform, the working platform rises together with the structure. There is no need to dismantle the formwork every few meters, transport it by crane, reassemble it, and anchor it again.

The reduction of repeated dismantling, transport, and assembly work at high elevations provides an important advantage in terms of occupational safety. Of course, the slipform system also requires specialized design, regular inspection, and disciplined execution. However, in a properly set up system, the crew works on an integrated platform that rises together with the structure instead of constantly changing temporary work areas.

The margin for error narrows as the structure rises

Slipform is a continuously progressing construction process. Once the concrete emerges from below the formwork and gains strength, it is not easy to reverse the completed work.

A large opening left at the wrong elevation, a missing embedded part, incorrect reinforcement placement, or a geometric deviation that gradually increases cannot be corrected at a later stage with a simple formwork adjustment. Such errors may lead to repairs requiring engineering calculations, concrete cutting, or local demolition.

While these operations are difficult even at ground level, they are much more costly and risky at heights of hundreds of meters. In addition, if a small verticality or rotation error is not detected early, its effect may grow as the structure rises.

For this reason, in very tall structures, the selection of a slipform contractor should not be based only on equipment and unit price. Formwork design, knowledge of concrete and reinforcement, surveying system, site organization, shift management, and experience in applications at similar heights should all be evaluated together.

Rather than looking for a solution after an error occurs, it is necessary to work with a team that can set up the system correctly from the very beginning.

SİBA’s approach: designing the construction method first

SİBA’s slipform service is not limited to bringing the hydraulic system to site and operating it.

Reviewing the project’s suitability for slipform, dividing the structure into sections when necessary, designing the formwork system for variable geometry, evaluating concrete and reinforcement details from an application perspective, determining the surveying method, and preparing the uninterrupted site organization are all parts of this service.

SİBA’s 45 years of accumulated expertise and experience in reinforced concrete chimneys exceeding 200 meters become especially important in structures where it is very difficult to remedy an error. The company’s references also include the 252-meter ZETES III reinforced concrete chimney and the 208-meter Soma Kolin reinforced concrete chimney.

The advantage provided by slipform in very tall structures is clear. However, this advantage does not emerge automatically without the right project and a well-planned site organization.

This is exactly where SİBA’s contribution begins: handling the structure, the formwork system, the concrete and reinforcement design within a single production plan, and implementing this plan on site without interruption.

Frequently Asked Questions

Does slipform become more advantageous as the structure gets taller? Yes, if the structure’s geometry is suitable for slipform. As height increases, the system’s initial setup cost is spread over a longer scope of work. Since repeated formwork cycles and dismantling-reassembly work at upper elevations are eliminated, the time and cost advantage of slipform becomes stronger.

Which method should be used for tall conical reinforced concrete chimneys? Tall conical reinforced concrete chimneys should be built with slipform. The slipform system can be designed to follow the changing cross-sectional diameter and wall thickness as the chimney rises. In this way, the chimney shaft is constructed continuously and in a controlled manner.

Can an existing project be made suitable for slipform? Yes. While preserving the safety of the structural system, wall geometry, cross-section transitions, openings, reinforcement details, and construction sequence can be made more suitable for slipform. The earlier this work is done, the more applicable solution options there will be.

Can building cores be built with slipform? Yes. Reinforced concrete cores consisting of elevator, stair, and service shafts are highly suitable for slipform. In large projects, cores or shaft groups can be planned as independent sections.

How many meters per day is it possible to rise with slipform? The daily climbing speed depends on project conditions. In some well-planned and properly organized applications, it is possible to rise 6 meters per day and above. Concrete mix, reinforcement density, geometry, weather conditions, and site organization jointly determine this speed.

Why is an experienced slipform contractor needed for very tall structures? Because in slipform, concrete placement and climbing progress continuously, errors are difficult and costly to correct later. As the structure rises, access becomes more difficult and the effects of small geometric deviations become greater. Therefore, the system must be designed correctly from day one and implemented by a team with experience at a similar scale.