Cementing the future of the precast concrete industry

18 May 2007

Concrete, in its many forms and in one way or another, has been shaping mankind and civilisation for over two thousand years since the Romans first used it to construct bridges, buildings and complex networks of aqueducts, culverts and tunnels.

More recently, precast concrete products and structures have been continuing to shape the way we live in the UK, arguably revolutionising the building and construction industries over the last 50 years.

Today there is a greater choice of precast concrete products than there has ever been. For instance, the use of the material has become common-place in the construction of hotels and schools, and structures such as bridges and tunnels, plus much more besides.

According to the British Precast Concrete Federation (BPCF), the industry produces over 35 million tonnes of products annually in the UK worth an estimated £2 billion.

In one of the latest developments in the sector, the growing popularity of precast concrete has seen Asset International, part of Hill and Smith Holdings PLC - one of the UK’s largest suppliers of galvanised steel - diversify its range of corrugated steel products to include precast concrete solutions.

Asset has been providing steel products for use in bridges, tunnels, culverts and underpasses to a range of highway and railway clients across Britain for many years. In a recent deal with Swiss-based BEBO Arch International AG, Zurich, Asset’s International Structured Solutions (ISS) Division, has become the sole distributor of its concrete arch products across the UK, Northern Ireland and Ireland.

Richard McTavish, Asset (ISS) Divisional Director, explained: “From customer feedback and monitoring the market place we saw a demand for concrete solutions, which are a viable alternative to steel in a number of situations such as road over road, road over rail and road over waterway structures. By diversifying we are providing clients with greater flexibility and a total solutions choice ensuring Asset remains strong in a highly competitive market place.

“We wanted to work with BEBO Arch International because they have an excellent track record and, with spans of up to 31 metres their products also offer the world’s largest pre-cast concrete arches. Almost 700 BEBO structures have been built worldwide since the mid 1960s, which are all currently still in service and have required virtually no maintenance to date.

“There is a growing need for concrete solutions to meet the demands of the rail and highway industries. The two sectors are demanding greater choice to be able to pick and choose between concrete and steel structures to suit the needs of any given project. Adding the BEBO System to Asset’s portfolio not only offers our customers a globally accredited solution, but one that we believe is key to the ongoing development of bridge, tunnel and underground space infrastructure across the UK.”

Precast concrete is prepared, poured into moulds and then cast and hardened at specialist factories. The completed product is then transported and lifted into its final position on-site. The key benefits of this process include: strength, durability, speed, and overall cost savings for clients compared with insitu installations.

BEBO founder engineer and businessman, Dr Werner Heierli, set out to build concrete arches with a minimum thickness sufficient to carry heavy loads, but able to deform elastically to make use of part of the passive resistance of the soil backfill behind the sides of the arch. Using this concept, the fill can contribute to the load carrying capacity of the arch and become part of the structure rather than solely representing a loading.

Dr Heierli, explains: “The soil backfill and overfill impose vertical and horizontal loads onto the arch structure. At the un-deformed state, at-rest earth pressures are imposed. Upon deformation of the structure, the horizontal earth pressures change from at-rest condition towards active earth pressure in the case of wall movements away from the soil and towards passive earth pressure in the case of movements towards the soil respectively. This activation of a reaction of the soil mass to deformations of the structure is what we refer to as soil-structure-interaction."

“For the backfill upper and lower limits of gradation curves and a few additional requirements such as limited plasticity index and liquid limit are specified. Compaction in layers of 0.3m to 0.4m to a density of 98% of the Proctor Standard value is required for the backfill to provide the required stiffness and resistance.

“Our arches also make use of a crown joint, which is able to transfer moments – but is not a hinge, to ensure safety, strength and durability. We cast our structures with Portland cement concrete the strengths of which are within the typical range for normal concrete of 35 MPa to 50 MPa. The concrete typically conforms to: C35/45, XA1, Dmax 20, Cl 0,1, C2. The maximum aggregate size is 20mm to 32mm. Additives such as superplastizizers are selected at the discretion of the precaster to suit their procedures, equipment and previous experience. It is then up to the concrete supplier to determine the required mix details to produce a concrete that meets these specified properties.

“Corrugated metal arches normally make widest use of soil-structure-interaction in that large parts of the resistance of the soil are activated even at the service limit state. On the other hand, masonry and mass concrete arches make almost no use of such interaction because they are very stiff and the soil mass remains almost un-deformed, and therefore exercises only at-rest earth pressure onto the structure.

“BEBO arches are in between these extremes. They are flexible enough to activate considerable soil reaction i.e. part of the active and passive earth pressures, but stiff enough to keep such reactions within allowable limits, particularly under service conditions. This means that, under service conditions, our structures make use of the properties of the soil at small to very small strains, which leads to almost zero deflections under normal traffic loads for most applications,” he added.

The precast concrete industry is continuing to go from strength-to-strength, offering architects, designers, builders, engineers and the construction industry an ever-increasing array of products and structures. With Asset International now also offering BEBO concrete products, the choice of solutions available has just expanded further.

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