More and more during recent years architect's specifications are calling for finely finished concrete surfaces on walls, ceilings, and floors. At first, contractors used hand rubbing bricks, but finding this technique to be slow and costly for large areas, they turned to various sorts of powered tools. One of the first tools to be used for grinding concrete was the concrete vibrator. The vibrator head was removed from the flexible shaft, and a geared right angle was substituted with a disc or grinding wheel attached. This is still true to a great extant. There are basically two methods used to smooth off hardened concrete surfaces. They are dry grinding and wet rubbing. Dry grinding is done of dry concrete surfaces to remove fins and marks caused by forms. Formerly resinoid abrasive cup grinding wheels were used for this, but recently abrasive discs have been developed that are much lighter than the grinding wheels an do an excellent job. These discs are made up of multiple layer of abrasive-impregnated cotton fiber material bonded together under heat and pressure with strong resin bond. A silicion- carbide abrasive is used since this has been found to be the best abrasive for concrete. These discs are usually about one-fourth of an inch thick, and come in diameters of 7 to 9 inches. They can be applied to the work at an angle of 30 to 40 degrees using the weight of the tool for pressure. These discs can be attached to any type of portable tool and should be operated at from 3,000 to 4,000 rpm. Wet rubbing is accomplished on green concrete by a slow speed grinding wheel or disc, at the same time applying water. The water, plus the rubbing action, brings up a cement paste which results in a very smooth, plaster-like surface. The grinding wheel or disc is used with a right angle head, which has various gear reductions to reduce the grinding speed to 300 to 500 rpm. This wet angle head is provided with an attachment for a water hose so that water can run through the center of the grinding wheel onto the concrete.
