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THE EMPHASIS ON INNOVATION HAS DRIVEN SIGNIFICANT CHANGE IN MASONRY CONSTRUCTION

CONCRETE QUARTERLY SPRING 06

The concrete industry is developing a wide range of modern methods of construction (MMC) which, in addition to offering fast, more efficient construction, also provide inherent benefits of robustness, thermal effi ciency, fire resistance and sound insulation.

TUNNEL-FORM CONSTRUCTION Tunnel form is a formwork system that allows the casting of walls and slabs in one operation on a daily cycle. It combines the speed, quality and accuracy of off-site production with the flexibility of on-site construction. The result is a cellular reinforced-concrete structure, the surfaces of which require only minimal finishing for internal decoration. The wall ends and facades are easily completed with thermally insulated units.

The system creates an efficient load-bearing structure that is particularly suited for repetitive cellular construction, such as residential blocks. 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, offering further MMC options. In Europe, tunnel form is also a competitive option for smaller projects such as blocks of six apartments. However, it has yet to be used on that smaller scale in the UK.

During the tunnel-form process, a structural tunnel is created by pouring concrete into high-quality steel formwork to make the floor and walls. The space formed can span from 2.4m to 6.6m and can be easily subdivided to create smaller rooms.

Where longer spans of up to 11m are required, the tunnel form is extended using a mid-span section. After 24 hours, the formwork is moved horizontally so another tunnel can be formed. When the storey is 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 provide 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.

CROSSWALL CONSTRUCTION Another concrete system well suited to cellular construction is crosswall. Crosswall off-site construction offers the benefits of an efficient frame without structural downstands, resulting in a structural zone of 150-200mm. One of the main advantages of the system is the provision of an early 'dry-box' construction that allows subsequent trades access to achieve a fast-build programme using the minimum of wet trades. Load-bearing walls in the transverse direction are designed as the means of primary support, with longitudinal stability achieved by external wall panels or diaphragm action back to the lift cores or staircases. The system provides a structurally efficient building, with main division walls offering a high degree of sound insulation between dwellings.

Using vertical casting, internal walls can be manufactured to a high standard of finish.

INSULATING CONCRETE FORMWORK Insulated concrete formwork (ICF) provides permanent formwork for in situ concrete structures with the formwork left in place for the life of the building as thermal insulation. Used on the continent and in North America for many years, ICF has proved to be a robust, cost-effective method of constructing buildings.

In essence, ICF consists of twin-walled expanded polystyrene (EPS) panels or blocks that are built up to create the walls. This formwork is then filled with ready-mixed concrete to create a structure ready to accept the roof or floor construction.

The EPS remain in place to provide complete thermal insulation for the walls of the finished building and to provide a uniform surface ready for the direct application of most internal finishes and external cladding systems.

The combination of the EPS insulation and in situ concrete provides a robust building with a U-value of under 0.20W/m 2K, well below Part L Building Regulation requirements.

As well as having good thermal performance, ICF has a high thermal mass thanks to its concrete core. The combination of the insulation and concrete core also offers excellent sound insulation.

The build process is quick, tidy and precise, with lower labour and equipment requirements than other construction methods.

Typically, an experienced team of four can erect the concrete within the walls of a three-bedroomed bungalow in a day.

The provision of a variety of shapes and components creates limitless opportunities and a range of features, such as bay windows and arches, can be introduced without the need for specialist products. Floors can also be constructed using ICF.

MODERN MASONRY CONSTRUCTION The emphasis on innovation and best practice has driven significant change in masonry construction. Aircrete is well placed to answer the drive to reduce waste. Pulverised fuel ash, a byproduct of coal-burning power stations, is used for its manufacture and the waste material generated during the production process is recycled back into the manufacturing process. The high compressive strength of aircrete means that only a single blockwork leaf for external walls is necessary. This enables fast construction.

Construction times are further accelerated with the use of thin-joint mortar. The 3mm mortar joints do not need to be trowel applied and reach full bond strength within two hours.

Aircrete blocks are widely used for both load-bearing and non-load-bearing walls and as infill units in beam-and-block floor systems. The use of aircrete provides a combination of structural stability, acoustic insulation, energy conservation and fire resistance.

The Modern Masonry Alliance aims to underline the benefits of masonry as an efficient, durable, attractive and modern construction technique. It intends to disprove that off site is the only modern way forward and to demonstrate that the masonry sector has scope for further innovation and efficiency gains.

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