Bob van Gils
WBK Engineering Services Pvt. Ltd.
Van Boxsel Engineering Pvt. Ltd.
This article describes some of the unique capabilities of modern precast concrete technology and the flexibility it creates for constructing buildings and infrastructure. Various projects in which modern precast technologies have been utilized will be highlighted in this article.
Conventional precast forms and shapes
Precast concrete is well known for its application as an industrial building method which utilizes machinery and mould systems to efficiently prefabricate entire building structures which can be quickly assembled at a construction site. After the Second World War precast building technology was widely used in Europe for the construction of social housing. Most of the structures were built with simple flat 2D precast panels like slabs and walls because of the ease and repetition in production. For example the Plattenbau system which was used in Germany to develop entire neighborhoods as repetitive box-type houses and apartments which created a monotonous architecture. Nowadays the trend in architecture is to break the boring uniform look of straight line buildings by adding different geometries to the exterior and designing more complex shapes for aesthetic reasons.
Examples of complex geometries in precast concrete
By utilizing the latest 3D design software it has become a lot easier for architects to create building designs with complex geometries and curved shapes. Construction of such buildings can be done with concrete as the main material because it can fill almost any shape of formwork. However large complex shapes require costly formwork at site and therefore prefabrication is generally more feasible even if there is very less repetition. Some of the traditional fabrication techniques to create complex precast shapes are timber formwork, rubber formwork and polystyrene foam formwork. Examples of the application of these fabrication techniques are highlighted in the following three projects.
Example 1. Dior flagship store in Seoul
This flagship Dior store located in the Cheongdam-dong district of Seoul, South Korea, has several large glass fiber reinforced shells that form the façade of the building which was inspired by woven white cotton fabric (Picture 1). The glass fiber reinforced (GFR) panels were made as single pieces that are more than twenty meters high and seven meters wide with a texture finish resembling the pattern of weaving. The large moulds for the GFR panels were made of traditional timber formwork supported by a steel frame (Picture 2).
Example 2. Perot Museum in Dallas Texas
The Perot Museum of Nature and Science located in Dallas, Texas, USA has a precast concrete façade consisting of precast panels having convex and concave shapes as well as curved panels (Picture 3). More than 100,000 square feet of precast cladding was designed and detailed using 3D modelling software. Traditional timber frame formwork was used in combination with rubber forms to create the protruding and recessed shape of the panels (Picture 4). Carpenters built the moulds for the double curved precast panels by hand based on a series of geometric points and calculations provided by the design team (Picture 5).
Example 3. Der NeueZollhof in Dusseldorf
Der NeueZollhofdesigned by internationally renowned architect Frank Gehryis a building complex consisting of three buildings with complex curved shapes. For building B the exterior double curved walls were executed as precast concrete elements (Picture 6). Polystyrene foam blocks were used as formwork for the fabrication of the double curved precast wall panels (Picture 7). The foam blocks were cut by a CNC milling machine which received its data from accurate computer models. The polystyrene blocks were used only once and after stripping of the precast element the blocks were melted down and recycled into new polystyrene foam blocks.
3D-printed moulds for precast concrete elements
Fabrication of timber forms is a very flexible system and they can be built in almost any size or shape. However the fabrication of these timber moulds requires a lot of manpower and a high level of carpentry expertise which is not always available. With the recent developments in 3D computer modelling and 3D printing techniques it is now also possible to create 3D printed moulds. The American company Gate Precast has used this innovative technology for the first time on the One South First project located in Brooklyn, New York (Pictures 8 and 9). For the residential tower of this project a total of 993 precast punched window panels were fabricated and for the commercial tower another 612 precast panels were fabricated using 3D printed moulds. Gate Precast collaborated with Oak Ridge National Laboratories (ORNL) using Big Area Additive Manufacturing (BAAM) to print the moulds. BAAM is a 3D printing system that is sold commercially and it uses carbon fiber reinforced plastics to print objects and structures. The advantage of carbon fiber moulds is their durability and increased strength and stiffness compared to timber moulds. To build one timber mould for this project an experienced carpenter would take around 40 to 50 working hours. The 3D printing system took just 8 to 11 hours to print one mould and an additional 8 hours to complete the surface finish with a CNC machine (Picture 10). The total cost of each 3D printed mould was $9000 compared to $1800 for a timber mould. The timber moulds could be reused 10 times while the 3D printed moulds could be reused 200 times. So on an average the cost of the 3D printed moulds was $45 per pour while the timber moulds would have cost $180 per pour. The costs of 3D printed moulds can be further brought down with design optimization and cost reduction of 3D large format printers. Advantages of 3D printed moulds are the high quality, durability and savings that can be achieved in time and money to build mould sespecially for complex geometries with large repetitions. Another major advantage of 3D printed moulds is the freedom it gives to the designer to create almost any shape they want.
Flexible formwork technologies for precast concrete elements
Research and design of flexible formwork systems for the fabrication of precast panels with complex geometries has been going on for a few decades in various parts of the world. Research projects completed by the Faculty of Civil Engineering at Delft University of Technology in the Netherlands have been promising. The objective of these R&D projects is to develop a reusable flexible and adjustable mould system for the production of single curved and double curved precast concrete elements. The ongoing research project which was initiated by H.R. Schipper (MSc, PhD) in 2009 has resulted in the development of the Kine-mould system in 2015. The developed system works in such a way that first the concrete is poured on the horizontally positioned flexible mould. This mould consists of thin flexible strips which can be moved by actuators and on top of the strips a silicone membrane is applied to smoothen the mould surface. Self-levelling, self-compacting high performance concrete with thixotropic behavior is being used to pour the elements. After some initial hardening of the concrete the flexible mould is adjusted to form the required final shape of the precast element (Picture 11). During this entire process it is important to understand the behavior of the concrete mix during the plastic stage to control flow and cracking. A prototype of the Kine-mould (Picture 12) was built and tested and showed good results for single curved precast panels and reasonable results for double curved precast panels. Further research is ongoing to build an improved larger prototype and apply this technology in a live project.
3D-printed precast concrete elements
With the introduction of 3D concrete printing no pouring of concrete will be required and instead the structure will be made by additive manufacturing techniques. With this technology no additional formwork is needed and it produces significantly less wastage of materials. The 3D printer can spray the concrete like a paste in layers on the base form.3D printing techniques can be used for on-site and off-site fabrication of buildings and construction components. The first off-site fabricated 3D printed precast concrete bridge was installed in Gemert, the Netherlands and was officially opened to public in October 2017. The bridge serves as a crossing for cyclists and has a total length of 8 meters while the clear structural span of the bridge is 6 meters. The bridge has a width of 3.5m and consists of six precast concrete 3D printed segments which are held together by prestressed tendons which are anchored in precast concrete bulkheads at each end. The cross section of the printed segments consists of a series of upside down positioned bottle shapes connected with a continuous line at the bottom (Picture 13). A 1:2 scale model was first constructed and tested in a load-controlled destructive 4 point bending test. The test results showed that the scale model carried the required loads with a large margin.
The final bridge was constructed by construction company BAM Infra (Picture 14) and manufacturing of the precast segments was done by the 3D concrete printer at the Technical University of Eindhoven. After installation of the bridge a full scale non-destructive load test was executed by placing 10 containers filled each with 500 liters of water to guarantee that the bridge is structurally safe.
Tallest precast building in the world
Another example of modern precast concrete technology and innovation is the construction of the Zalmhaven tower in Rotterdam, the Netherlands. This residential tower measures 215meters with 58 floors and a total of 257 apartments (Picture 15). Once completed it will be the tallest precast concrete building in the world. Construction started in October 2017and is scheduled to be completed by the end of 2021.
The foundation system for this high rise tower consists of a 2.5m thick foundation raft of 38m x 38m supported by a total 163 number of Tubexgrout injection piles with a length of 66m. The Tubexfoundation pile is formed by a steel thin-wall tube with an attached drill tip. The drill tip is welded to the first steel tube segment and the pile is immersed into the ground by pressing a vertical load combined with screwing and grout injection. In this project the 66m long piles are made of two steel tube segments of 33m which are welded to each other during installation. Reinforcement cages are lowered into the hollow tubes and the entire tube is filled with concrete. Tubex piles are 100% displacement piles and do not create any vibration during execution.
To create an open space for the entrance and lobby area the building system of the lower floors consists of cast in-situ reinforced concrete columns, shear walls and slabs. The 600mm thick shear walls at this level are made in concrete grade C55/67 and the RCC columns in grade C80/95. The floors above this area form a two storey high transfer girder which is created around the exterior of the building. The transfer girders are made as 500mm thick RCC walls in C55/67 concrete grade and they will be transferring the loads of the upper floors to the below column-wall system (Picture 16). The transfer structure also prevents progressive building collapse by providing a secondary load path in case one of the lower columns is removed due to a calamity.
From fifth floor level onwards the apartment units are starting for which the structural system is formed by load bearing precast concrete solid walls in combination with a precast half slab system. The precast half slab system consists of 100mm thick precast slabs with lattice girders on which a 170mm thick RCC screed will be poured. All the balconies will be made as precast concrete solid slabs which are partially cantilevered and connected to the structure by steel angle connections. The lateral load resisting system is formed by coupled precast shear walls in T-shapes running in x-direction and y-direction. To form the T-shape the interior precast solid walls are connected to the exterior precast sandwich panels (Picture 17). Furthermore the interior precast walls are connected at the intersections by wet concrete connections with protruding reinforcement which is lapped to create a fully monolithic joint. Thickness of the interior load bearing precast concrete walls is 500mm for 5th to 20th floor and 400mm for the 21st to 40th floor and thickness further reduces to 300mm for the 41st to 52nd floor. The structure of the 53rd to 58thfloor consists of 250mm thick interior precast walls in combination with a steel structure. The thickness of the exterior precast sandwich panels varies from 400mm to 300mm. The lift and staircase walls are also fully made as precast concrete solid walls but are not part of the lateral load resisting system (Picture 18).
– Red = Interior precast walls
– Purple = Lift and staircase precast walls
– Orange = Exterior precast sandwich panels
– Green = Exterior precast sandwich panels
– Cyan = Precast 3D element (solid slab + wall)
– Gray = Precast balconies (solid slabs)
– Yellow = Precast half slabs with lattice girders and screed
Climbing factory for precast erection
The precast structure of the Zalmhaven tower shall be erected by using a climbing factory. The climbing factory system has a large steel frame structure which shall be placed on top and around the perimeter of the under construction building. This steel structure forms a temporary tent which is wind and waterproof and serves as a platform for the construction workers to build one floor at a time. The factory has two overhead gantry cranes for material handling out of which one can be used for vertical lifting and the other for horizontal movement and installation of precast elements (Picture 19). As soon as one floor has been completed the entire factory will “jump” to the next storey by the use of hydraulic jacks. A similar system was used in the past for the construction of the DelftsePoort (1990) and Erasmus MC (2013) both located in Rotterdam as well as for the East Village building No.8 at Stratford, London (2017). However the climbing technology for the Zalmhaven tower will be different because for the first time external columns with hydraulic jacks will be used (Picture 20). Major advantages of the climbing factory are the improved working and safety conditions as well as the high construction speed that can be achieved. The erection cycle for the Zalmhaven tower will be one floor per week and the entire project will be completed and handed over by the end of 2021.
1. Flagship Dior Store, www.christiandeportzamparc.com
2. How Morphosis Harnessed CAD and Concrete for the Perot Museum, architizer.com
3. Morphosis’ Museum of Nature & Science Facade: Gate Precast, Fabrikator September 30, 2011.
4. Designing and Manufacturing Architecture in theDigital Age, KOLAREVIC, Branko, University of Pennsylvania, USA.
5. 3-D printing shapes building industry, creates rapid construction potential, Oak Ridge National Laboratory.
6. 3D-printed precast concrete molds for redeveloped NYC landmark, ConstructionDive, April 3, 2019.
7. Precast concrete molds fabricated with big area additive manufacturing, Solid Freeform Fabrication Symposium.
8. Double-curved precast concrete elements, Proefschrift, H.R. Schipper, September 2015.
9. Production of Curved Precast Concrete Elements for Shell Structures andFree-form Architecture using the Flexible Mould Method, Ir. H.R. Schipper; et al.
10. Flexibeleherbruikbaremal, RoelSchipper en Peter Eigenraam, Tektoniek.
11. Salet, T. A. M., Ahmed, Z. Y., Bos, F. P., &Laagland, H. L. M. (2018). Design of a 3D printed concrete bridge bytesting. Virtual and Physical Prototyping, 13(3), 222-236.
12. De Zalmhaven, Rotterdam, Uitvoeringsaspectenhoogbouw in stedelijkegebieden, presentation on 5th Feb 2019 by Ir. J.J.H. (Joris) Hesselink PMSE, BAM Advies& Engineering.
13. Hoogbouwnaar 200m en hoger? Presentation on 1st November 2018 by Ir. Sander van Eerden RO, Zonneveld Ingenieurs.
14. Hoogtepuntvoorbinnenstedelijkewoningbouw, Ir. Sander van Eerden RO, Zonneveld Ingenieurs, Cement 2017-1.
Mr. Bob van Gils is director and co-founder of the structural engineering firm WBK Engineering Services Pvt. Ltd. which is a precast design company based in Gurgaon, India. The company is involved in some of the state-of-the-art precast building projects in Europe, USA, India and other countries.