Sawston College, Cambridge: Science Wing construction

Challenge

Sawston Village College used H+H UK’s Plus Blocks and the thin-joint system for its new science block extension. The extension provides eight new classrooms to develop the college’s specialism in teaching sciences to 11–16 year-olds.

The first phase – to create five classrooms over a 700m2 floor-space – was completed within 37 weeks by the Christmas holiday of 2008. The second phase – to create three classrooms over a 450m2 floor-space – was completed within 27 weeks, by the start of the summer holiday. The fittings and furnishings were installed during the holidays.

H+H UK’s Plus Blocks and the thin-joint system combine to make a fast-track construction programme, which meets the internal acoustic requirements while thermally insulating the external walls. After being used on the first phase, this construction method provided the advantage of speed against phase two’s summer holiday deadline.

Solution

Internal separating walls: 6m-high of solid aircrete blockwork to the pitched roof apex using 7N/mm2-rated Plus Block (610mmx215mmx200mm) and the thin-joint system. Murfor prefabricated stainless steel horizontal bedjoints every 2nd course. After using a three-coat plaster system at 6–8mm thickness on the first phase, the result was improved on the second phase by using a 10mm single coat spray plaster by Lafarge. Spray plaster applied beyond suspended ceiling levels to maintain acoustics.

External cavity walls: 6m-high cavity-construction walls using Plus Block (610 x 215 x 100mm) and the H+H thin-joint system with 17mm Kingspan Kooltherm insulation boards plus 100mm facing bricks. Stainless twist ties for lateral support within the 118mm cavity. Concrete ring beams on top spread the load for the 3.6N/mm2-strength Plus Blocks.

• Production

H+H aircrete consists of 80% pulverised fuel ash waste recycled from power stations that would otherwise be destined for landfill.

• Acoustic credentials

The walls are specified to achieve a minimum acoustic insulation level of 49dBRW, because the BB93 requirement is in the range of 40–45DnT (Tmf, max).

• Thermal insulation

Having a 0.15W/m2K U-value for thermal insulation, enabled the external wall specification to be lenient in other details, such as the windows and doors. This thermal rating also enabled the external wall cavity to be narrower than for concrete breezeblocks, freeing up more floorspace.

• Speed of build

The thin-joint system enables the structure to be erected faster and to a better quality, allowing follow-on trades to start work sooner in a weatherproof environment.

• Health and safety

The aircrete product presents minimal risk on site, especially in terms of its light weight for manual handling. This is not only important to the builders, but also to the architects, because they are legally liable for the H&S of handling the specified products on site.

Outcome

There were many compelling reasons for choosing H+H aircrete products for this project. The system constitutes a modern method of construction (MMC), and the product has good credentials in terms of sustainability when it comes to production, waste control, packaging and delivery/transportation.

“While most masonry manufacturers claim general acoustic insulation performance for products in housing, quantifiable substantiation of their performances was difficult to pin down against the BB93 standard for schools. From this first experience of aircrete, I will certainly specify it for future projects.” Matthew Huckelsby, Project manager, Capita Symonds

“The Thin-Joint system’s mortar for each course dries within 10 minutes, so we are building the 6m-high walls much quicker than I’ve ever known for traditionally mortared concrete. I’ve been in the trade since 1973. The Thin-Joint mortar is also good for temperatures below that of traditional mortar, which was important during the especially cold winter.” Stuart Lovesey, Site manager, Morgan Ashurst

The clients found an additional use for the completed walls: aircrete is an acceptable direct substrate for the fixings of load-bearing fittings such as shelves and cupboards, as well as for heavy plant such as heaters.