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Contractors World - INTERNATIONAL
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Contractors World INTERNATIONAL - 2016 Vol 7 No 5

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Dual-Layer Paving Is First Choice - page 2 of 3

Round-the-clock paving by the Bickhardt Bau team on the A5 near Karlsruhe-Durlach

Round-the-clock paving by the Bickhardt Bau team on the A5 near Karlsruhe-Durlach

Round the clock paving

Round-the-clock paving by the Bickhardt Bau team on the A5 near Karlsruhe-Durlach where the paver hit the mark on this job: its great flexibility enabled the concrete paver to quickly be reconfigured as required after 1 km.

When the two layers of concrete have been paved, a surface retarding agent is uniformly applied to the freshly smoothed surface by a Wirtgen texture curing machine (TCM) which matches the paver. Depending on the prevailing climatic conditions, the coarse aggregate is exposed by brooming the surface just a few hours after placing the concrete pavement. The result is a non-skid surface with a large number of profile peaks and a texture which durably minimizes tyre noise.

Four typical projects highlight the advantages of this method and show which features of the SP 1500 (four-crawler model) / SP 1500 L (two-crawler model) from Wirtgen are of benefit.

Three of the projects concerned the rehabilitation of concrete roadways between 30 and 40 years old. The fourth project, in Saxony-Anhalt, involved the rehabilitation of a stretch of highway with alkali-silica reaction (ASR).

Dual-layer paving – High quality at lower material costs

Cost-efficiency is one of the factors fueling the boom in dual-layer concrete paving . This method tangibly reduces the cost of construction, as the bottom-layer concrete and the top layer of exposed aggregate concrete are designed to meet different requirements.

On the A1 near Trier, Berger Bau used the SP 1500 to pave a 20 cm-thick bottom-layer concrete pavement with 360 kg cement per m³ concrete and a maximum aggregate size of 22. Meanwhile, the 6 cm-thick top-layer concrete comprised a high-grade material mix with 420 kg cement per m³ concrete and 2/8 double broken and double screened chippings.

Material logistics is a challenge

The biggest challenge with this method lies in the logistics: Two different materials are needed and must be available in the right place at the right time and in the right quantities, since they need to be placed wet-in-wet.

The bottom-layer concrete is dumped directly in front of the paver. The top-layer concrete is delivered to the second machine via a conveyor. To this end, the concrete is either filled directly from the truck into a material hopper or intermediate container – each contractor uses its own process here.

Christoph Hofmeister, Head of Division at Max Bögl, The further procedure is then the same in all cases: Conveyors and a chute at the end of the conveyor dump the top-layer concrete on the fully compacted bottom-layer concrete behind the first paver.

A sufficient supply of both concrete grades is essential to assure the success of the paving work.
Christoph Hofmeister, Head of Division at Max Bögl, sees logistics has the biggest challenge when paving concrete in two layer, saying:

For the project on the A9, we had a steady stream of 45 articulated trucks carrying between 6,500 t and 7,000 t of sand, gravel and chippings every day during the paving phase, in addition to 27 trainloads of cement.
   This is equivalent to roughly 750 t of binder. It is currently the largest mobile plant in the world. Building this concrete roadway meant orchestrating a complex system of machines, material, weather, ambient conditions – and people.
   To assure the project’s success, not only did the technical equipment need to operate faultlessly, but the team had to work hand-in-hand, communicating swiftly and clearly.

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