Concrete cellular construction

Cellular construction offers a range of inherent concrete benefits that are particularly suited to high-density residential development, such as robustness, fire resistance, sound insulation and thermal efficiency.

In-situ tunnel form

One method of cellular construction is in-situ tunnel

form. Recognised as being a modern method of construction, the method has reportedly interested the Prince of Wales for his Poundbury development. Tunnel form is a formwork system that allows the casting of walls and slabs in one operation on a daily cycle. The result is a cellular reinforced concrete structure, the surfaces of which are of sufficiently high quality to require only minimal finishing for internal decoration. The wall ends and fa?ades are easily completed with thermally insulated units.

Tunnel form creates an efficient load-bearing structure. The solid monolithic structure can be 40 or more storeys high and the accuracy of the system suits the installation of prefabricated elements such as cladding panels and bathroom pods. The cellular space formed can span widths from 2.4 to 6.6m and can be easily subdivided to create smaller rooms. Where longer spans of up to 11m are required, the tunnel form formwork can be simply extended using a mid-span section. After 24 hours, the formwork is moved horizontally so that another identical tunnel can be formed. When the storey has been completed, the process is repeated on the next floor.

Tunnel form offers significant cost and time savings with exceptional quality. The large bays constructed using tunnel form offer design and layout flexibility and allow a high degree of freedom in the final appearance. The elevations can be adapted by using extendable formwork to create cantilevered balconies and the exterior can be finished as required.

Precast crosswall

Similar benefits of speed and high performance are offered by precast crosswall construction, which provides an efficient frame without structural downloads resulting in a structural floor zone of 150-200mm. 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 back to the lift cores or staircases. Structures up to and including 16 storeys have been completed in the UK using crosswall construction. Building Regulations Part A requirements for robustness are provided by vertical, horizontal and peripheral ties.

Crosswall construction is fast and cost-effective as there is a minimisation or elimination of other items due to the fact that the crosswalk and floors do more than just structural load carrying. Partitions and party walls are largely eliminated. In some cases, the internal leaf of cladding is also eliminated as it is part of the crosswall structure. As with tunnel form construction, acoustic separation and additional finishes with crosswall are minimised as the floor and wall panels provide the required airborne noise separation. The acoustic performance of both systems is excellent because of their mass and acoustic damping.

Concluding remarks

Fast overall project times are achievable with both construction methods. Once part of the structure is erected, follow-on trades can quickly commence work. This is because the wall and floor units provide a semi-internal environment. Service runs can be pre-installed before the units are cast. Other facilities, such as bathroom pods, can be installed as completed units during frame erection.

Good surface finishes to wall and floor units minimise site works and any additional finishes. This has the benefit of fast and cost-effective construction and skim-finished walls and ceilings require a minimum of ongoing maintenance. Tight tolerances enable reliable fitting of bathroom pods, prefabricated built-in furniture and carpets. This enables fast and cost-effective fitting out.

Concrete provides thermal mass which is optimised with tunnel form and crosswall construction. Independent research shows that the benefits in winter through passive solar design can reduce heating energy by 11% and offset the marginally higher embodied CO2 of concrete construction compared with timber in only 11 years(1). In terms of summer performance, it is clear that thermal mass reduces the risk of overheating and the need to retrofit air-conditioning in the future.