Over the coming weeks, farmers and building contractors will be constructing new silage pits. Silage slabs and walls must be built to a high standard.

Laying concrete is regarded as simple, yet all too often defects from poor workmanship can be seen.

You should carefully study and follow the Department of Agriculture specifications S128, S120 and S100. The main elements are:

  • Remove topsoil and lay graded hardcore. Lay the hardcore to the same gradient needed in the finished floor. Compact the hardcore with a vibrating roller to a finished depth of at least 150mm. Any soft spots should be excavated and filled with hardcore and compacted in 150mm layers. Failure to compact the hardcore material adequately will lead to subsidence and lack of support for the slab and channels, causing cracking of the concrete under the weight of machinery. This problem cannot be rectified once the slab is completed and will undoubtedly shorten its lifespan
  • Blind the compacted hardcore with sand or other blinding dust and run the vibrating roller over it to bed it in. Lay a sheet of 1,000-gauge polythene on the finished foundation
  • Order 40N20 concrete and insist on concrete manufacturer’s certificate (see Department specification S100).
  • Place and compact the concrete to a finished depth of 125mm. The concrete must be thickened to at least 150mm under the base of the channels. The slab and under-channels must be compacted using a vibrating screed and poker vibrator. Joints should be formed as recommended (see S128). Cure the concrete by covering with a new sheet of polythene to prevent premature drying out.

    • I would like to see silage bases and silage pit walls with a lifespan of up to 50 years.

    Over this lifespan, they should perform their function well from the point of view of staying leak-proof and collecting and conveying silage effluent to storage.

    Silage pit walls have to support heavy loads, so preventing corrosion of the reinforcing steel in the wall, especially at the base of the wall, is essential.

    Corroded steel at the base of the wall, if left unattended, will ultimately mean the wall will have to be replaced. There is also the possibility that the wall could fail under load. This is a safety risk too great to take.

    Some farmers are now looking at their existing slab and must decide whether it is worth repairing or needs to be replaced.

    To do remedial works, the pit must be structurally sound. If it is not structurally sound then really the only option is to take up the slab and the hardcore underneath and start again.

    The best option may be to move to a new site altogether to make room for other developments or just to create space in the yard.

    The old slab may be suitable as a concrete yard or for storing baled silage. Similarly, silage walls need to be assessed for structural damage before deciding on what to do. Typically, walls would be in a better condition than floors.