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Cement Surface: The resulting cement surface, produced from the formerly discarded grappiers, was of much higher quality than that obtained from the unsintered material. This fact was firmly established by the English cement surface manufacturer L. C. Johnson in 1845, and the term "portland cement surface" has since been applied solely to the cement surface made from the sintered material. This period marks the real beginning of the portland cement surface industry.
On a large flight—more than, say, about 10 steps—it is advisable to cast a cement surface footing in a trench at the base to support the bottom riser and prevent the entire flight from sliding down the bank. Dig the trench under the position of the bottom riser, about twice the front-to-back measurement of the riser, about 4in deep. Ram rubble into the base of the trench and top up to ground level with fresh cement surface. Compact the cement surface, level it and allow to set overnight before building on the surface.
More specifically, the fineness is specified by a minimum average surface area of 1,600 to 1,800 sq cm per gram, depending on the type of cement surface being made. The purpose of the gypsum is to make the mixture of the cement surface with water and aggregate (as in fresh concrete mix) remain fluid and workable over a period of several hours. Without the addition of gypsum, the mixture might set before it had been properly placed in the forms. |
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