Concrete Offsite Solutions Provide Versatility and Sustainability
21 Feb 2019
Published in: PCB, February 2019
Precast concrete is virtually unlimited in its application including the entire structure or selected elements such as frame, floors, walls, stairs or balconies. The advantages of factory production, combined with the inherent benefits of concrete, provide compelling reasons to use precast concrete.
The use of precast concrete elements is well established as a construction method throughout the world and provides solutions for a great variety and complexity of layout, shapes and façade treatments. It provides the benefits of all concrete elements, including material efficiency, quality control and speed of erection.
Precast concrete can be incorporated into any building type. Whether the building has a regular or an irregular shape, the entire structure or elements of that building, such as frame, floors, walls, stairs or balconies, can all be precast. Precast construction is virtually unlimited in its application and is suitable for single and multi-storey construction. In fact, precast building elements can be a considered option for any construction project.
Bespoke designs can be achieved using standard precast components, which need not imply a modular appearance. Precast elements, including floors, stairs and wall panels can combine seamlessly with non-precast elements to produce free-flowing spaces. Curved precast panels with a wide range of attractive and durable finishes can meet the most challenging of design requirements.
It is not necessary to take an ‘all precast or no precast approach’ to design. The key issue for designers is to identify which construction method, or mix of construction methods and materials, is most appropriate for the specific requirements of the building.
The most economical solution might well consist of a mix of cast in-situ and factory produced, precast units. The structural design might propose a solution that uses beams and floor slabs fabricated off-site sitting on in situ columns. These structural elements are then integrated as a composite structure when the in situ structural topping is placed. Thorough consideration of construction options at an early design stage is critical to optimise speed of construction, structural performance and delivery of the most economical frame package for each project.
Inherent fire resistance
Concrete has inherent fire resistance, which is present during all construction phases, and is achieved without the application of additional treatments. It is also maintenance free. Concrete has achieved the best fire rating possible under European fire standards because it does not burn and has low heat conductivity.
For concrete buildings, vibration criteria for most uses are covered without any change to the normal design. For some uses, such as laboratories or hospitals, additional measures may be needed, but these are significantly less than for other materials. Even where additional measures may be needed these will less for a concrete fabric.
The latest guidance for designers for flood resistant and resilient construction BS 85500:2015, recommends the use of a concrete structure. Concrete retains its structural integrity in flood conditions and precast elements can be designed to limit water ingress through the joints. Concrete has the strength to resist very high loads to keep water at bay and is robust enough to withstand impact from debris.
Reduction in overheating risk
The thermal mass, inherent in concrete, together with adequate ventilation, provides a simple and effective means to provide thermal comfort and reduce overheating – a growing health and wellbeing issue, particularly among the elderly.
By reducing the buildings’ heating and cooling energy load, and potentially avoiding the need for air conditioning, operational and maintenance costs are reduced. The high-quality precast concrete, whether exposed or painted are an excellent means of providing the internal surfaces of concrete necessary to optimise use of thermal mass.
High thermal performance
Thermal mass is not the only means by which precast concrete can provide excellent thermal performance. Its smooth surfaces offers an excellent solid backing for placing insulation against, either on site or in the factory, thereby reducing the risk of thermal bypass. Insulated concrete sandwich panels provide a slender, high performance wall construction, potentially combining structure, surface finish and thermal performance.
Low conductivity connectors and rigid insulation reduces thermal bridging and the concrete provides continuous protection to the insulation from damage during construction and occupation.
Precast components can meet the highest standards for resistance to sound transmission. Buildings employing precast components are included in the Robust Details under Part E of the Building Regulations.
Part L of the Building Regulations requires pre-completion pressure testing. Precast cladding improves air-tightness because the large units reduce the number of joints. These joints are also easier to seal because the edges of the units are typically flat, formed surfaces. A building failing the pre-completion tests will have to undergo a time-consuming joints and interfaces inspection process, resealing where necessary.
Durability and low maintenance
Concrete is a hard-wearing material which offers long lasting structural performance without the need of additional finishes and little maintenance. It is particularly useful for areas vulnerable to high levels of wear and tear or difficult to access. This durable finish offers additional lifecycle cost savings through reduction in maintenance (and replacement) that would be required for other ‘wearing’ finishes.
Concrete is a local material, supporting local economies and reducing emissions associated with transportation.
- 99.9% of aggregates used in the UK are sourced in the UK (80% are used within 30 miles of extraction)
- Primary aggregates are predominantly UK-sourced, their extraction is tightly regulated and adverse environmental impacts are minimised
- 90% of Ordinary Portland Cement is produced in the UK
- Concrete can use by-products from other industries, including fly ash and ground granulated blast furnace slag (ggbs)
- UK manufactured reinforcing steel is made from 100 per cent UKsourced recycled scrap
- Concrete is 100 per cent recyclable.
Author - Jenny Burridge, Head of Structural Engineering at The Concrete Centre