This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
To give a satisfactory finish to exposed surfaces of concrete is a rather difficult problem. Usually, when the forms are taken down, the surface of the concrete shows the joints, knots, and grain of the wood. It has more the appearance of a piece of rough carpentry work than that of finished masonry. Or, failure to tamp or flat-spade the surfaces next to the forms, will result in rough places or stone pockets. Lack of homogeneity in the concrete will cause a variation in the surface texture of the concrete. Variation of color, or discoloration, is one of the most common imperfec- tions. Old concrete adhering to the forms will leave pits in the surface; or the pulling-off of the concrete in spots, as a result of it adhering to the forms when they are removed, will cause a roughness.
To guard against these imperfections, the forms must be well constructed of dressed lumber, and the pores should be well filled with soap or paraffine. The concrete should be thoroughly mixed, and, when placed, care should be taken to compact the concrete thoroughly, next to the forms. The variation in color is usually due to the leaching-out of lime, which is deposited in the form of an efflorescence on the surface; or to the use of different cements in adjacent parts of the same work. The latter case usually can be avoided by using the same brand of cement on the entire work, and the former will be treated under the heading of Efflorescence (Article 329).