PLAIN CONCRETE Concrete consists of mortar in which are embedded pebbles or pieces of stone, broken brick, etc. At present the mortar used in making concrete is invariably cement, although in ancient times lime was so used. Of course, common lime is wholly unfit for use in large masses of concrete, since it does not set when excluded from the air. The lime used by the ancients usually had some hydraulic properties.
Concrete has been in use from remote antiquity, but it is only comparatively recently that it has been used to any considerable extent. The development of the American portland cement industry has greatly stimulated the use of concrete in this country in recent years; and at present concrete occupies a peculiar and preeminent position in structural work.
"Concrete is admirably adapted to a variety of most important uses. For foundations in damp and yielding soils and for subter ranean and submarine masonry, under almost every combination of circumstances likely to be met in practice, it is superior to brick masonry in strength, hardness, and durability; is more economical, and in some cases is a safe substitute for the best natural stone, while it is almost always preferable to the poorer varieties. For submarine mr.sonry, concrete possesses the advantage that it can be laid, under certain precautions, without exhausting the water and without the use of a diving-bell or submarine armor. On account of
its continuity and its impermeability to water, it is an excellent material to form a substratum in soils infested with springs; for sewers and conduits; for basement and retaining walls; for piers and abutments; for the hearting and backing of walls faced with bricks, rubble, or ashlar work; for pavements in areas, basements, sidewalks, and cellars; for the walls and floors of cisterns, vaults, eta. Groined and vaulted arches, and even entire bridges, dwelling houses, and factories, in single monolithic masses, with suitable ornamentation, have been constructed of this material alone." The use of concrete enables the engineer to build his super structure on a monolith as long, as wide, and as deep as he may think best, which can not fail in parts, but, if. rightly proportioned, must go all together—if it fails at all.
This chapter will treat of plain concrete, i.e., of concrete without steel reinforcement; and the next chapter will consider reinforced concrete, i.e., a combination of concrete and steel.
This chapter is divided into five articles which treat respectively of: (1) the materials; (2) the laws of proportions; (3) the forms; (4) mixing and placing, and matters related thereto; and (5) strength and cost.