The chairman of the Structural Precast Association, Shaun Brown, explains how the ancient material of concrete is delivering ultra-modern buildings.
We owe a lot to the Romans, but their greatest contribution to today’s construction industry is without a doubt the invention of concrete. There is no other material that compares in terms of versatility, mouldability, strength, durability, sustainability, recyclability, and fire resistance.
Indeed, it is difficult to imagine today’s world without this ubiquitous material.
Fresh concrete is an extremely moldable material that can be formed into complex shapes. Panels can include cill, coping, soffit, window reveal, and special sections, particularly where they are repeated. Large panels eliminate the need for secondary structures for vertical and lateral support, and grid-width units can reduce midspan loadings on the structural slab edge. Furthermore, using precast cladding panels with reduced wall thicknesses can increase net lettable floor area.
Precast concrete has advantages both below and above ground, but once construction progresses beyond the foundation stages, the emphasis shifts to the advantages of its precast form, most notably:
- No need for on-site material storage • Less need for on-site labour
- Possibility of early design value engineering
- Improved quality control at the manufacturing stage
- Complements lean construction programmes and shortens programme times
- Concrete components with guaranteed long life and intrinsic fire protection
- On-site nominal material waste
- Just-in-time deliveries reduce labour requirements while increasing construction speed.
- Less disruption to other trades in constrained working environments, as well as earlier access to follow-on trades
- Increased safety and decreased environmental risks
- Fewer logistical issues, particularly on restricted access sites
However, prefabrication is not the end of the construction storey. The list above can be expanded and made even more appealing, both physically and visually, by using modular construction, in which a variety of standardised components are prefabricated – and often fitted out – off-site before being quickly assembled on site by a skilled workforce.
The first applications of such modules were hotel rooms and bathroom pods, but the possibilities are nearly limitless.
A good example of such modular construction is Bell & Webster’s structural precast rooms system, which was designed and supplied for student housing at the University of Essex, Southend. Hollybrook, the contractor, found this to be an excellent choice. The project included 561 student bedrooms, which necessitated the use of 2,251 factory-engineered concrete units. The building was built with 1,296 wall units and a ten-story height to create a strong, robust structure that could be installed and fixed much 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, which included the installation of bathroom pods, was completed on time, allowing subsequent trades to install the services.
The structure is made up of load-bearing crosswalls that support single-span reinforced concrete floor slabs, with pre-stressed concrete for the longer span common rooms and kitchens. The floor slabs restrain the walls laterally, and external walls restrain the building at right angles to the crosswalls.
Floor slabs act as a diaphragm, carrying all lateral loads from wind and notional loads down through each floor level to the foundation level in shear and moment actions. Cast-in vertical and horizontal ties provide progressive collapse and floor-plate action.
This system was chosen after extensive research, including visits to similar contracts and the factory to observe the design and manufacturing processes. The system was appropriate for the university scheme because it combines offsite modular benefits with speed of erection, and close collaboration at the design and construction stages aided in the production of a fast-track build.
“Because of the congested town centre location, there were no facilities to hold delivery vehicles and no opportunity to use forklift trucks,” said Peter Bowes, operations manager for Bell & Webster.
We were able to ensure that the precast elements were delivered to site in a just-in-time manner by providing weekly updated delivery schedules.”
To summarise, precast modular concrete is a construction method that is manufactured offsite. Rooms are built with factory-engineered precast concrete components that are individually designed and manufactured and have excellent acoustic and thermal mass properties. They are also strong, almost maintenance-free, and quick to erect, allowing for earlier occupancy.
Once assembled, units are typically held together by a series of reinforced hidden joints that are grouted as work progresses, along with vertical ties to meet Building Regulations progressive collapse criteria.
Party walls, floors, ceilings, lift shafts, stairs, and ducted risers are all examples of room components that can be manufactured to BS8110 tolerances and delivered to site ready for final minimal preparation and direct decoration.
As a result, modular precast concrete is miles ahead – an apt description given that the Romans coined the term “mile,” which is derived from “mille passuum,” or “a thousand paces,” with one pace equaling two strides.