Precast concrete construction goes modular
Shaun Brown, chairman of the Structural Precast Association, explains the ancient material of concrete is delivering ultra-modern buildings.
We owe a lot to the Romans but without doubt their greatest contribution to today’s construction industry is their invention of concrete. When it comes to versatility, mouldability, strength, durability, sustainability, recyclability and fire resistance, there is no other material that can match it.
Indeed, it is impossible to imagine what today’s world would be like without this ubiquitous material.
Fresh concrete is an extremely mouldable material, allowing complex shapes to be formed. Panels can incorporate cill, coping, soffit, window reveal and special sections, especially where repetitive. Large panels avoid the need for secondary structures for their vertical support and lateral restraint, and grid-width units can reduce midspan loadings on the structural slab edge. In addition, reduced wall thicknesses with precast cladding panels can increase net lettable floor area.
Precast concrete has its benefits both below and above ground but as soon as construction moves above foundation stages, attention switches to the benefits of its precast form, most obviously:
• No need for storage of materials on site
• Reduced need for site labour
• Opportunity for early design value engineering
• Greater consistent quality control at the manufacturing stage
• Complements lean build programmes and shortens programme times
• Guaranteed long life and intrinsic fire protection of concrete components
• Nominal material waste on site
• Just-in-time deliveries, further reducing need for labour and increasing speed of construction
• Less disruption to other trades in restricted working environments and earlier access for follow-on trades
• Improved safety and reduced environmental risks
• Fewer logistical problems, especially on restricted access sites
But prefabrication alone is not the end of the construction story. The above list can be extended and made even more appealing both physically and from a design perspective by adopting modular construction, in which a range of standardised components are prefabricated - and often fitted out - off-site before being quickly assembled by a skilled workforce on site.
Some of the first applications of such modules were hotel rooms and bathroom pods but the scope is almost limitless.
A good example of such modular construction can be seen in the structural precast rooms system designed and supplied by Bell & Webster for student accommodation at the University of Essex, Southend. This proved an ideal choice for contractor Hollybrook. The project involved 561 student bedrooms, requiring 2,251 factory-engineered concrete units. With 1,296 wall units and a height of ten storeys, the building was constructed to create a strong, robust structure that could be installed and fixed far faster than most alternative systems buildings of this scale.
The engineered wall units were manufactured and installed by the company’s specialist installation teams and site costs were kept low by ensuring that the initial phase - including the installation of bathroom pods - would be completed on time, making way for following trades to install the services.
The construction consists of load-bearing crosswalls supporting single-span floor slabs in reinforced concrete with pre-stressed concrete for the longer span common rooms and kitchens. The walls are laterally restrained by the floor slabs and external walls laterally restrain the building at right-angles to the crosswalls.
Floor slabs act as a diaphragm and their plate action carries all lateral loads from wind and notional loads down through each floor level to foundation level in shear and moment actions. Progressive collapse and floor-plate action is provided through cast-in vertical and horizontal ties.
The selection of this particular system was based on extensive research, visiting similar contracts and the factory to see the design and manufacturing processes. The system suited the university scheme because it offers offsite modular benefits combined with speed of erection, while close liaison at design stage and during construction helped produce a fast-track build.
Peter Bowes, operations manager for Bell & Webster, commented: “Owing to the congested town centre location, there were no facilities to hold delivery vehicles and no opportunity to use forklift trucks.
By providing weekly updated delivery schedules, we were able to ensure that the precast elements could be delivered to site in a just-in-time manner.”
To sum up, precast modular concrete offers an offsite manufacture approach to construction. Rooms are constructed using factoryengineered precast concrete components, each individually designed and manufactured, and offering excellent acoustic properties and high thermal mass. They are also robust, virtually maintenancefree and quick to erect, so offering earlier occupancy.
Once assembled, units are typically tied together by a series of reinforced hidden joints that are grouted as work progresses, with vertical ties incorporated to meet Building Regulations progressive collapse criteria.
Room components can include party walls, floors, ceilings, lift shafts, stairs and ducted risers, all manufactured to tolerances well within BS8110 and delivered to site ready for final minimal preparation allowing direct decoration.
It could therefore be said that modular precast concrete is miles ahead - an appropriate description since it was the Romans who gave us the word ‘mile’, derived as it is from ‘mille passuum’ or a 1,000 paces, one pace being two strides.