It seems probable that under some conditions progressive changes in dimension may take place over a long period, though it must not be inferred that work in which the concrete is restrained from such changes is subjected to the stresses which would be imposed by the necessity of resisting them all at once. Concrete restrained, as in reinforced work, from yielding to the tendency to contract., probably becomes adjusted to the situation so that it would not contract if the restraint were removed.
Contraction joints are commonly used to prevent the cracking of concrete by shrinkage. The compressive strength of concrete is usually sufficient to take up the stresses clue to expansion without injury to the structure, but tensions clue to contraction may be sufficient to crack the conctete.
Thin concrete walls usually need contraction joints 20 to 30 feet apart; in heavy walls, they may be 50 or 60 feet apart. The use of light reinforcement in the exposed surfaces between expansion joints may prevent disfiguring surface cracks.
Ordinarily these joints may be formed by building the work in sections and allowing one section to set before the adjoining one is placed. This introduces planes of weakness through the work which will yield when the wall contracts. To bond the ends together, grooves may be left in the sections first constructed and filled in placing the new work. Joints are sometimes made by inserting strips of roofing paper and placing the new concrete against these, or where water-tight work is necessary by filling a thin opening in the concrete with asphalt cement.
86. Finishing Concrete Surfaces.—The appearance of a con crete structure depends largely upon the way in which the surfaces are finished. When the forms are removed, the marks of the lumber of the forms are plainly visible, and lines between successive layers of concrete are usually seen. When the concrete next the form has been carefully spaded in placing the concrete, the surface should be fairly smooth with no vacant spaces which require filling, and if the forms are smooth, a quite even, uniform appearance may be obtained. For certain classes of structures, such as retaining walls and bridge abutments in certain locations, the appearance may be satisfactory without further treatment, although the dead color of the smooth surface skin is not particularly pleasing.
A smooth surface is sometimes obtained by plastering with cement mortar—a method not usually satisfactory, as the mortar is apt to scale off. A rough appearance is usually more suitable to
the material, and the surface of the concrete itself should be used. When a smooth mortar surface is desired, it should be obtained by placing the mortar at the same time as the concrete, which may he done by using a movable form for the mortar. The form slides inside the main form and separates the mortar from the concrete, and is removed as the materials are placed so that they may be tamped together.
A pleasing appearance may often be made by scrubbing the surface with a stiff brush and water as soon as it has set sufficiently to remove the forms, which removes the marks of the forms and brings the pieces of larger aggregate into view. Scrubbing must be done before the concrete has hardened too much, usually within twenty-four hours of placing the concrete, and immediately after removing the forms, as the surface hardens rapidly after the forms are taken off. In removing forms for this purpose, care must be used to prevent breaking the corners of the concrete, as, to present a good appearance, the edges must be straight and sharp. Scrubbing involves comparatively little labor and is an inexpensive method of finishing.
After the concrete surface is hard it may be scrubbed, and the skin removed, by the use of a solution of about one part hydro chloric acid to five parts water, though this method is quite laborious and rather expensive.
Concrete surfaces are sometimes finished by tooling, using the axe, bush-hammer, or point. The concrete may thus be made to show a very uniformly roughened surface which is very pleasing. If neatly done, this is rather slow and expensive, although a roughly pointed effect may be produced with less work.
The appearance of the finished surface may be controlled by the choice of aggregates. If a uniform appearance is desired, small aggregates may be used on the surface. If a more rough appearance is wanted, larger and less uniform material may be employed. Pleas ing color effects may often be had by care in the choice of aggregates to he used on the surface, of mortar colors may be used for the pur pose. White Portland cement may also be used where special effects are desired.
Breaking the continuity of a surface by introducing panels may frequently improve its appearance. These are made by nailing boards of proper shape to the inside of the forms. The surface may be broken by lines indented into the concrete by nailing strips of triangular section to the inside of the forms.