Durkan's modern touch: Bricks, Blocks and Concrete - Specification Focus

The new west London headquarters of Octavia Housing and Care is at the centre of a mixed-use development constructed on a long-vacant canalside site. The housing association's aim was to create 28 sheltered housing units, offering 24-hour care for residents, but employment zoning meant that the new homes had to be combined with space for office or light industrial use. In the event, the development incorporated all three: 3,800m² of office space for Octavia's own use three light industrial units at basement level and 28 serviced flats.

The Octavia project is a mix of four- and five-storey construction, with a variety of brick, block and some glass curtain walling. The roof is metal clad on a steel portal frame with solar photovoltaic panels. It achieves an Ecohomes rating of 'good', in part through the installation of the grant-funded solar roof panels, which provide a portion of the building's energy needs.

MMC incorporation

Specification of a Modern Method of Construction (MMC) is a requirement for housing association funding, so contractor Durkan specified H+H Celcon's aircrete blocks with the quick-setting thin-joint system. The 100mm blocks were used to infill the reinforced concrete frame and form the inner leaf of the external walls.

The project was the first time that the contractor had used the thin-joint system. "We specified the solution as it was an MMC system," explains Paul Shadbolt, project manager at Durkan. "It was a requirement of the client and we decided that the aircrete blocks would be the best way to meet the needs of the project. It's a big scheme with varied requirements and the blocks are versatile enough to cope."

The Celcon system combines the company's Celcon Plus range of aircrete blocks with Celfix, a thin-joint mortar. As it was the first time it had been employed by Durkan, the contractor arranged for training to be given to bricklaying subcontractor, The Progressive Group. "It is a different system, so training was provided by Celcon," continues Shadbolt. "But once our subcontractors had the training they found it a very simple system to master. Basically, if you can hold a level and a saw then you should be competent enough."

Quick-setting mortar

Thin-joint blockwork enables walls to be built quickly without having to wait the conventional 24 hours for the mortar to set before further loading can be applied. The Celfix mortar used is cement-based and supplied as a dry pre-mix powder in 25kg bags and has been designed to replace the traditional sand/cement mortar. It will start to set within 10 minutes of application and approaches full design strength in just one to two hours. This enabled the blockwork at the site to be completed very quickly.

"As the mortar sets so quickly, you can put up much more blockwork compared with other blocks. It is also possible to lay it in a wider range of temperatures than traditional brick and block," says Shadbolt.

The mortar for the thin-joint system is mixed on site by adding the appropriate quantities of water and applied by a proprietary scoop or sledge. This is designed to create a consistent joint thickness of 3mm. However, this caused some problems for the contractor, as the tolerance required to lay them is acute. "With normal mortar you have a 10mm tolerance, while with this bed you only have 3mm," explains Shadbolt. "Also, you need to sand down and plane the blocks, which adds some time. Having said that, the blockwork went down relatively easily and the plus points certainly outweigh the negatives."

Having successfully applied the system for the Octavia project, Durkan is now using it on a £20m housing association project in Welwyn Garden City. It is to build 213 units of key worker accommodation for the city's QE2 hospital. The contractor expects to further exploit the benefits of the system following its initial experience, employing it on the inner leaf throughout all three phases of the project.