the concrete mix
The concrete mix is critical with a good relationship and understanding between slipform contractor and concrete supplier essential.
The basic mix for barrier concrete is class C28/35 with air entrainment. Concrete design, in accordance with EN 206-1 and BS 8500 and compliance testing are detailed in the specification. Aggregate selection is important and the natural interlock a crushed angular stone provides is very helpful.
For the same reason the inclusion of polypropylene fibres also improves the cohesion and stability of the newly laid concrete. The fibres also help to control initial shrinkage cracking. Control of concrete consistence is critical.
Although the slump test is not ideal it remains the only readily available industry wide means of relaying the required workability of concrete. It is useful to think of a slipform barrier as the ultimate slump test but with a moving cone almost one metre high.
In order to maintain the exacting control of consistence it is frequently necessary to add water to the concrete at the slipform paver. This principle needs to be agreed with the concrete supplier and allowed for in the design of the mix.
