In this production process as well, the concrete is removed from the forms immediately after compacting the concrete. Here, however – and similar to late form removal – the pre-tensioning is applied to the tendons with respect to a frame. After the concrete has achieved sufficient strength, the pre-tensioning with respect to the frame is released, which introduces the tensioning forces into the sleeper via a direct bond between the concrete and the tendons. Here as well, this process requires only a small number of forms, since only the frames with the sleepers need to be placed into intermediate storage until application of the pre-tensioning forces.

The methods with late form removal offer advantages from concrete-engineering standpoints, since it is possible – according to the vibration techniques applied – to select the working properties of the concrete throughout a broad spectrum, from stiff to self-compacting. These options offer many possibilities in concrete production and delivery. The techniques with immediate form removal, on the other hand, require the use of very stiff concrete with high green strength – which does not allow great leeway, either in compacting techniques or in concrete delivery. On the other hand, immediate form removal does in fact offer great advantages with respect to flexibility in production – and in the small number of forms required.

In this process, the sleeper forms are arranged in lines over 100 metres in length. Special devices are used to close the ends of the forms. The pre-stressing forces are introduced into the pre-stressing tendons via abutments at the end of the pre-stressing beds. When the concrete has reached sufficient strength, the pre-stressing forces are released at these abutments, which directly introduces the forces by direct bond into the concrete of the sleeper. During the entire process, the forms remain in the pre-stressing bed. Turnout sleepers are also produced in a pre-stressing bed: but with these sleepers, pre-assembled bottom plates with the required anchoring points for the rail fastenings are mounted in the pre-stressing bed, instead of the forms for normal sleepers.

For further information, please consult the Web site www.travipos.com

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