The whitish or yellowish stains that occasionally appear on a concrete surface exposed to action of the weather are due to the leaching out of lime or other salts. The more waterproof the concrete the less probable these are. Unsightly appearances are also due to leakage through construction or shrinkage joints.
Surface Surface of concrete may be treated to improve its appearance by scrubbing off the film of mortar when green or tooling it when hard, to expose the aggregate, which may be selected for aesthetic effect. A wash of 10 per cent acid solution is also used. Plastering or stucco on concrete must be applied with care. (See Srucco). For large and important structures, smooth forms and careful spading of dense concrete against these forms is preferable to any after effects. For ornamental work, best effects are produced by use of dry selected aggregate next to forms and by setting-in tiling. By the use of the cement gun fine aggregate and cement and water are blown by air pressure through a nozzle onto a surface to be covered.
Cost—The cost of concrete depends upon cost of cement, sand and stone, upon conditions W of work and size of work. here forms are simple, cost ranges from $4 to $7 per yard.
Thin sections, with expensive forms and handling, will bring this up to from $10 to $20 per yard. A reliable estimate must take ac count of all of the circumstances.
Applications of Massive Mas sive concrete, or plain concrete not reinforced with steel, as used up to about 1850, is applied to foundations, walls, piers, dams, retaining walls, short arches, etc. In these uses, the principal stresses are compressive. No tension is expected on the concrete. For instance in a massive retaining wall, the resultant force of the lateral pressure of the earth on the wall, and the weight of the wall is directed outwardly downward and acts within the middle-third of the horizontal width of the wall. Consequently no bending action exists and no tension exists in the concrete on the interior face of the wall. Structures of massive concrete are the old stone masonry forms built in concrete.
With metal reinforcement to carry prin cipal tensile stresses, concrete structures can become more constructed in cross-section, more skeletonized, and the members can withstand bending action.
The advantages of massive concrete over stone are availability of local materials; use of unskilled labor and machinery in mixing and placing; adaptability for placing in small spaces; monolithic character of resulting struc ture. The following elements need careful con sideration: Cost of forms, expansion and con traction; uncertainty of quality of materials, demanding skilled inspection; appearance of surfaces; necessity of extra good foundations to prevent cracks due to settlement in an un yielding material.
Application of Plain Sidewalks usually are 4 to 5 inches thick on a foundation of porous material 10 inches thick. For a fl inch walk, the base is a dry tamped concrete 3 to 3% inches thick, and laid in blocks about 6 feet long, or the width of the walk, separated by a joint, formed by tarred paper or sand. After a few of these blocks have been laid, and protected from dirt and dust, and before they have set, the wearing surface of mortar from to 1 inch is placed, is leveled with a straight edge and floated with a plasterer's trowel. The final floating is performed from two to five hours afterwards. Grooves are made in the wearing surface just above the joints in the base. Skill is necessary in prop erly floating the surface at the right time. The finished wall must be protected from the hot sun. Basement floors are constructed in the same manner except that a wetter concrete may be used for the base, and in cellars the joints omitted. Driveways are similar to side walks, and are from one inch thick for ordinary traffic up to five inches for heavy trucking.
Concrete roads are now established con struction. Knowledge of the necessity of skilled contractors, very best materials, for difficult service conditions has been gained at expense of early failures. National confer ences on concrete road building in 1914 and 1916 crystallized practice. The travel on rural and suburban roads has increased rapidly in recent years and changed in character to motor driven vehicles that destroy the surface of ordinary water bound roads; so that the maintenance of these throughout the entire year to meet the demands of modern conditions has become impracticable from an economic stand point.