Tall Construction: Powered by Precast

As the population grows there is a greater demand for homes, particularly in urban areas.  One solution to this is building tall with precast concrete and hybrid design providing many of the answers.

To realise its full potential, a tall building must satisfy many varying objectives, including efficient layout, minimal circulation space and structural footprint, density of use, efficient services provision, speed of construction, cost, appearance and safety.

Offsite concrete solutions can provide everything from whole building and structural systems, to precast construction components, including walls, stairs and floors, that are essential to meeting the need for multi-storey construction. Thorough consideration of construction options at an early design stage is critical to optimising the speed of construction, structural performance and delivery of the most economical frame for each project. It is not necessary to take an ‘all precast or no precast approach’ to design. The most economical solution might well be a hybrid concrete solution, using a mix of cast, in situ and factory-produced precast units. 

When building tall, there are some key performance characteristics to consider including fire performance, thermal mass and vibration. Concrete has inherent fire resistance – it does not burn and has low heat conductivity and therefore does not need additional treatments to achieve fire performance standards. Concrete achieves the best fire rating possible under European fire standards and maintains structural capacity longer than other materials in a fire. Following recent fires, at Grenfell Tower and in Barking, there are now restrictions on using combustible materials for cladding and balconies. Concrete is non-combustible and continues to be permitted, even for high-risk residential buildings. 

Concrete meets the vibration criteria for most buildings without any change to the normal design, this saves cost and programme. 

A concrete structure has a high thermal mass. By reducing the building's heating and cooling energy load, and potentially avoiding the need for air conditioning, there is a reduction in operational and maintenance carbon emissions and energy costs. Exposed concrete, typically slab soffits, allows the fabric energy storage to regulate temperature swings. The high-quality finish of precast concrete, whether exposed or painted, is an excellent means of providing the internal surfaces of concrete necessary to optimise the use of thermal mass. 

Thermal mass is not the only means by which precast concrete can provide excellent thermal performance. Its smooth surfaces offer an excellent solid backing for placing insulation against, either on-site or in the factory, thereby reducing the risk of thermal bridging. Insulated concrete sandwich panels provide a slender, high performance wall construction that can deliver structure, surface finish and thermal performance in one element. Low conductivity connectors and rigid insulation reduce thermal bridging and the concrete provides continuous protection to the insulation from damage during construction and occupation.

Structures up to and including 22 storeys have been completed in the UK using crosswall construction. This efficient method of construction uses precast floors and load-bearing walls, and is ideal for buildings of a cellular nature, for example hotels, student accommodation and apartments. Load-bearing walls across the building provide the means of primary vertical support and lateral stability, with longitudinal stability achieved by external wall panels or diaphragm action taken through the slab to the lift cores or staircases. Crosswall construction provides an efficient frame without structural downstands, resulting in a structural floor zone typically of 200 to 250mm, helping to maintain a high degree of sound insulation between floors.

Crosswall construction normally includes staircases and core walls as part of the precast system, allowing safe and easy access to all floors as the construction progresses. The cost savings from these and other benefits should be fully considered when comparing the costs with alternative structures.

Crosswall buildings are quick to erect, offering the client a rapid room occupancy programme. As an indication, six to eight rooms can be erected in a day and the lead time will be 12 weeks. Generally, the walls are sized to be as thin as possible to increase the net floor area. There may be occasions when a thicker wall is required, such as in tall buildings where there are high compressive loads, or adjacent to long floor spans where there are high bending moments due to the notional eccentricity of the wall.

Typically, as soon as the ceiling units at roof level are erected, external cladding/brickwork, window installation and internal first fix can commence. It is also possible for following trades to start before the precast erection is complete. Minimal finishing is needed within each room, which reduces the overall construction programme. Exterior walls can be constructed as fully-finished sandwich panels, which removes the need for scaffolding. The wall finishes can include brick, concrete or render, giving a wide range of options to choose from.

There are many advantages to building tall in concrete, from inherent fire resistance to thermal mass. Additionally, precast concrete solutions, such as crosswall, can be used to deliver good quality finishes and faster construction times, meaning the need for high performing homes can be met sooner.

For more information visit: www.concretecentre.com

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