Feature

Offsite Housing Systems

Offsite manufacture for onsite assembly is an established solution for residential buildings. The current drive for factory production and modern methods of construction (MMC) has led to greater investment in concrete manufacturing facilities in the UK, as well as the development of innovative precast systems that can be used to construct both high- and low-rise housing quickly and efficiently.

Concrete components tend to be limited by weight so modular concrete housing systems are usually based on flat panels – although volumetric stair and lift cores and bathroom pods are available. Pre-designed, factory-produced concrete structural elements are available for every component of a building.

These are typically made to order with dimensions to suit project requirements, although fixing details have been standardised over the years to further improve efficiency and health and safety. The surface quality of precast concrete can be good enough for a painted finish. Bespoke architectural precast concrete can be used for uniquely shaped, sized and finished elements. Working with manufacturers, designers can create a solution for individual projects, which is then delivered just-in-time for assembly onsite.

High-rise solutions

   

Crosswall is a highly efficient modular system, where precast loadbearing concrete walls are structurally tied with concrete stairs and floors using hidden rods. It is normally associated with the cellular arrangements of hotel and student accommodation, as it provides excellent acoustic and fire performance without relying on additional finishes.

An evolution of crosswall construction was used to create the structure for 206 apartments at the Lansdowne (pictured, bottom right), a 16-storey residential building recently completed in Birmingham. It took just nine days to complete each floor. The facade comprises storey-height brick-faced (and pre-grouted in the factory) insulated concrete sandwich panels, installed without scaffolding.

An offsite system based on crosswall structural principles was used for Aston Place (left), also in Birmingham, a build-to-rent project for Dandara Living of two blocks 17 and 22 storeys high, linked by bridges at each level. The development provides 324 new homes, a mixture of studio, one-bed and two-bed apartments. The system has a standardised suite of details specifically for residential developments. Internal walls are delivered with mechanical and electrical services cast in place and a pre-sprayed mist coat of decoration. The floor incorporates a recess to receive preassembled bathroom pods to create a level threshold and precast concrete balconies have a self-finished, anti-slip surface and cast-in fixings for glazing and handrails. Here an exposed “recon” finish in two different colours was chosen for the external leaf of the insulated sandwich panels of the facade, all delivered pre-glazed to site.

A very different approach was used for the 26 and 30-storey towers of Victory Plaza (top right) in east London, designed by Lifschutz Davidson Sandilands, the first use of a “rising factory” process in the UK. A lightweight, temporary “shed” covering the entire building footprint provided enclosed working conditions for the construction of the building structure for each floor, including walls, cladding, slabs, services and bathrooms. The factory was jacked up as each floor was completed, similar to jump-form rigs used for core construction, on a cycle of 55 hours. Incremental improvements enabled the final 18 floors to be delivered at a rate of one per week. The hybrid concrete structure comprised perimeter precast concrete columns, with concrete twinwall structural panels for the core and other internal structural walls, and Omnia slabs for the floor. Contractor Mace is now using the system on a number of projects.

MorgenWonen

VolkerWessels’ MorgenWonen system is made from insulated concrete sandwich panels, manufactured using a hydraulic steel mould. Houses can be erected and made watertight in less than a day by a crew of four people. In order to avoid double handling, the panels are loaded onto the lorry in the correct sequence and craned straight into place on-site. Windows, doors and most building services are installed in the factory. The only site activity is the installation of roofing over the water-tight membrane, guttering and downpipes. The house is typically ready for occupation just 10 days later, after decorating, kitchen fit-out and flooring installation.

From the outside, the homes are designed to look like traditional masonry construction. The internal perimeter loadbearing walls are as-struck precast concrete, as are the walls inside the staircore, with decoration applied directly to the concrete. The other internal walls are non-loadbearing, made of a lightweight partitioning system, and therefore layouts are flexible.

The homes have a high thermal mass of 450 kJ/m2K, as opposed to 125KJ/m2K for lightweight construction, and air leakage of just 1.14m3/m2/hr. They are sold as “energy neutral”, equipped with a very efficient air-source heat pump. As of July 2019, 1,510 homes had been constructed, predominantly in the Netherlands

Contemporary offsite housing solutions must meet high standards of building performance, in terms of energy efficiency, acoustic isolation and structural performance. These requirements are evolving with the increasing risk of overheating in a hotter climate, and new legislation related to non-combustibility. Here, concrete’s intrinsic properties are an advantage.

For example, it provides inherent acoustic damping, a key requirement of housing developments which can be difficult to meet with lightweight solutions. The thermal mass of concrete, when used properly, offers an offsite solution with in-built natural cooling and resulting energy savings, while its non-combustibility makes it a particularly appropriate material for structure, walling systems and cladding.

Innovative new systems are also being introduced, capable of delivering a range of both modern and traditional aesthetics. As the examples on these pages show, concrete continues to be able to meet the challenges our homes face, now and in the future.

Slimcrete

Apart from the roof, the entire structure of this house by Cornish Concrete Products is made from 40 bespoke precast units and can be erected in just two days. The crosswall construction comprises a 100mm reinforced structural skin, with 150mm insulation – to achieve a 0.15W/m²K U-value – and a 50mm reinforced external skin. Internally, there are 150mm-deep hollowcore floor sections, 100mm reinforced internal partitions, while the stairs are also precast. The largest panel, a wall section, measures 7.5m x 3.1m and weighs 7.5 tonnes.

Basalt fibre reinforcement in the concrete offers greater tensile strength than polypropylene or steel fibres, which means the walls can be thinner. CCP’s Thermomass connector system, meanwhile, uses non-metallic composite ties to hold the sandwich panels in place with a conductivity value of just 0.3W/m2K, preventing cold bridging.

From the outside, there is no clue that it is precast concrete: the external wall finishes are flat concrete with a 2mm-thick Parex acrylic render.

CCP has constructed eight modular homes to date, two of them using the Slimcrete system. The first Slimcrete house was constructed near Falmouth, Cornwall and the second is under construction near Wadebridge, also in Cornwall. CCP has agreed a 10-year warranty with insurer Build-Zone, and agreements with two other warranty providers are in the pipeline. There has been much interest in the system from developers of homes for private rental and shared ownership.

Images

Top: Garenne Group’s HexxHome is a flexible architectural precast system on a hexagonal plan, for one or two-storey single homes or terraces. A one-bedroom prototype has been built at Garenne’s factory in Taunton, and a three-bedroom showhome is under construction at Graven Hill in Bicester.

Above left: Garages by Tarmac for Barratt Homes, constructed from precast concrete panels at a speed of up to two per day. The panels are formed in rubber moulds with a recess pattern that resembles traditional masonry, sprayed with a reddish “Trufinish” pigment. When the concrete has set, the mould is peeled off and mortar trowelled into the 10mm-deep recess.

Above right: I-House, a collaboration between Saint-Gobain Roofspace and H+H, replaces the inner leaf of cavity walls, separating walls and internal partitions with storey-height Celcon Elements, made from aircrete. The system can deliver a watertight shell in a week, ready for follow-on trades. It has been used for over 500 homes.

Photos CCP, HexxHome, Tarmac, RoofSpace Solutions

Visualisation Dandara Living; Photos: Paul Riddle, FP McCann; MorgenWonen