Where heavy traffic or other special conditions are expected, a special finish layer or hardening treatment is desirable. Heavy duty topping may be placed on the base slab while it is still in a plastic condition or after the slab has hardened. Both methods give satisfactory results provided preparation of the base has been done properly. The continuously placed two course floor is considered to be superior to unbonded or slush coat bonded types because it allows for better control of bleeding and the practical advantage that the topping can be put on after other building operations have been completed. Above all a good concrete floor depends on its base. The base of a heavy duty floor should be a impermeable membrane if it rests on the ground and be of non-compressible base material or have inadequate or widely varying bearing capacities. Until recently it was thought that the choice of coarse aggregate was the dominating factor in the design of a satisfactory wearing layer. The theory was that since coarse aggregate occupies such a large proportion of the total concrete volume it must absorb the greatest wear. However, the Journal of the American Concrete Institute has proved that there is no relationship between the hardness of the aggregate and resistance to abrasion and that all brittle aggregates in a given mix of more or less constant compressive strength give the same resistance to abrasion. The rate of wear depends on the coarse aggregates being equal or superior to the mortar matrix in abrasion resistance. Among the natural aggregates, good ones to use are, traprock, granite and quartz. Specially prepared metallic aggregates are also frequently used with great success. The actual application technique for the topping is essentially the same as that for placing the base slab. Shovel and rake the topping mix to a uniform level slightly above finished grade. Compaction may be by rollers, hand tampers or both. Floating, preferably mechanical, follows after removal and filing of the holes left by the blocks. The use of power floats allow a much stiffer mix to be placed. Curing should be started as soon as no marks will be left on the floor and should continue as long as possible.