When designing the formwork for a wall panel, plan a continuous peripheral edge form, either of lumber or steel. Before placing begins, be sure that the form is clean and free from all foreign matter, and that the bond-breaking membrane is thoroughly dry. Direct chuting by ready mix truck into the forms will be the most economical method of delivery, but the use of a crane and bucket does allow the placing of a stiffer mix. The method of placing should be decided upon along with the mix design. There is one characteristic difference between finishing a tilt-up panel and finishing a slab cast on grade. Normally, water movement in a slab proceeds both up and down- the smaller proportion being absorbed by the subgrade. Casting of a tilt-up slab, however, is almost invariably done over a membrane-sealed surface and this means that the only direction in which the water can travel is upwards. On its way up some of this water hits the reinforcing steel an although most of it will continue past the steel, some water has a tendency to collect. The net result is that, as hardening begins, these small channels show up as tiny cracks in the plastic concrete, particularly if more moisture is being absorbed into the atmosphere than is being supplied to the surface. The finisher must then close these cracks with his wood float as soon as possible. This means that there must be enough finishers available to do the job quickly or that the amount of concrete placed be restricted to that which the available finishers can safely handle.
