Demolition of any existing concrete structure can be expensive and dangerous. But both the expense and the danger can be minimized by careful planning and correct choice of technique. The first step then should always be a reconnaissance of the job- the structure, the site and any adjoining structures. Once the survey is complete, a plan of procedure should be established. This plan should be adhered to strictly, unless progress reveals the need for changes. Demolition by mechanical or thermic means will normally start at the top of the structure and proceed down to the foundations. For most jobs this will prove both the safest and most efficient approach. The earliest attempts to use mechanical aids for breaking up concrete relied on percussive or impact drilling- the use of jack hammers. Today the rotary drill has replaced the jack hammer because it is comparatively quiet, and much cleaner and less exhausting to operate. The action of rotary drilling is an abrasive or cutting one. With percussive drilling each impact provides its own pressure and the drill need only be restrained enough to prevent chatter at the bit. With rotary drilling, on the other hand, the operator must provide the necessary cutting pressure himself. For this reason rotary drills are usually mounted on a rigid stand to enable the operator to keep the bit forced into place hydraulically, or with the aid of a mechanical linkage. For demolition of concrete, the most efficient type of drill is the hollow type through which the main part of the waste can be removed as a solid core. Thermic cutting, in its most primitive form, uses a lance or steel tube filled tightly with iron wires, through which oxygen under pressure from cylinders is forced. Thermic cutting has several advantages over mechanical methods. It is often faster, the equipment is lightweight and inexpensive, and operation can be significantly cheaper for many jobs. There is also no vibration or dust problem. The big disadvantages lie with the smoke evolved, and the fire danger if combustible products are in the work area.