PREPARATION OF THE BASE. The base may be the sub-floor or the structural slab. Normally a screeded bed of concrete is laid on the hardened base, which is cleaned and prepared for receiving the upper layer. The terrazzo mix is then laid as the wearing coat over the screeded bed. The screeded bed should preferably be laid after the building operations are completed. The bond between the screeded bed and the base depends to a very large extent upon the conditions of the surface of the base at the time of laying the screeded bed. The surface should be absolutely free from dust or contamination from oil, grease, etc. and attempt should be made to obtain a good, mechanical key. A good bond decreases the tendency for shrinkage cracking. In case where the surface is contaminated with oil, grease etc and the area of this contamination is small, it may be cut out and made good before laying the screeded bed. Otherwise the superficial contamination may be removed by using a suitable detergent. Dilute hydrochloric acid may be used for this purpose. The base should then be thoroughly washed clean with water. The surplus water should be removed by mopping after the surface is left wet overnight. A cement mortar grout of 1:1 proportion is then brushed onto this surface and this operation should be kept just ahead of laying the screeded bed. If this screeded bed is laid on dry slurry, it will result in poor bonding. The surface of the base concrete (at the time of its laying) should be brushed with a stiff broom before it hardens. This operation will remove all the laitance and loose aggregate from the surface and will incidentally help to roughen the surface. It is preferable to close back the concrete surface in four different directions by using wire brushes. Where this is not possible, the hardened surface will have to be suitably chipped to give required indentations, and in this case, care should be taken not to damage the structural base.
