In the base, the arrangement of U-shaped bars must be firmly tied. Straight bars are used both to hold the U-bars in place and prevent cracking due to shrinkage. Extra steel may be required at the top of the base slab if upward pressure from ground water is expected. A kicker or small wall section 75mm high should be cast as part of the base slab to provide a convenient starting point for the wall and to ensure integrity at this important junction. The top surface of the kicker should be roughened to ensure an adequate bond with the wall concrete. Alternatively water bars can be used as described in FDS specifications. The formwork for the walls needs to be firmly fixed and braced to withstand the pressures of the fresh concrete. It should be oiled with a proprietary mould oil to allow easy removal. Diesel oil should not be used. The full height of the wall should be poured in one operation and the concrete brought up in layers of 600-800mm. Use of a poker vibrator is essential to achieve full compaction and removal of entrapped air. The poker should be placed in the concrete to such a depth that it reaches the layer beneath. This will ensure full compaction at the interface between layers.   In order to produce a durable concrete which is low in permeability the concrete needs to be protected to prevent moisture loss especially for the first 7 days. This is achieved by keeping the surface damp through covering it with polythene or damp sacking. Alternatively the surface can be treated with a suitable spray-on curing membrane. Backfilling should not occur to full height until at least three weeks after construction. Concrete tanks should not be filled with effluent for a month after construction.   Tanks should be constructed in accordance with a detailed specification. Steel should be carefully placed in accordance with the drawings. Ensure the correct mix is ordered and keep a strict control on the addition of water. Compact concrete with minimum of delay using a poker vibrator. Protect freshly placed concrete from rapidly drying out and from frost. The readymixed supplier is responsible for providing material as ordered. Do not add extra water as this weakens the concrete and may invalidate the supplier’s guarantee. Advisory Engineers are always available to help solve problems related to the use of concrete on the farm.