The nature of concrete that should be used for a foundation depends on the nature of the soil it is to be laid in : the object in all cases being to get as nearly as possible a homogeneous bed under the structure. If the soil is dry, a concrete of sand, gravel, and as much ordinary lime as is necessary to produce a coherence of it altogether is sufficient; as it is little more than a bed of coherent gravel; but then it must be spread over such an area that it might be sloped at an angle of 45° from the outside of the footings of the walls, down to the bottom of the foundation; and of such a thickness that it will not be liable to crack under the pressure. For ordinary buildings probably 2-3 ft. is sufficient. If the soil is wet, or the building is of great weight or special character, the concrete should be made of hydraulic lime and sand and broken stones, in about the same proportions as would bo used in rabble masonry; that is to Bay, the lime should be about 1/7, the sand about 2/7, and the broken stones about 4/7. These, however, must be considered only as average proportions for medium hydraulic lime and ordinary wet soils; the proportion of lime must be varied inversely as its quality is better or worse, or as the circumstances are more or less important.

In such cases the concrete, if properly constituted and laid, may bo considered as a solid coherent mass, capable of bearing without crushing the weight per sq. ft. mentioned in recognized tables as the crushing resistance of different kinds of concrete, a proper coefficient of safety being used. The bed of concrete must also be thick enough not to break by transverse strain, but so as to settle in one mass if the subsoil yields. These 2 considerations will determine the area of the bed for the foundation.

With moderate hydraulic limes and common limes there will be an expansion of the mixed concrete, consequent on the slaking; in some cases the lime increases to double its original bulk; this may be almost entirely provided for by allowing time for the lime to be thoroughly slaked before laying the concrete; in some eases, however, the lime, or parts of it at least, will take so long to slake, that the process is completed after the concrete is laid, and it is therefore generally desirable to consider this expansion in preparing the site for the concrete.

As the principal object in laying a bed of concrete is to form a solid cohesive mass when it hardens, it has been sometimes recommended that it should be thrown in from a height to consolidate it; this practice, however, has the disadvantage of separating the fine from the coarse particles: it is better to lay the concrete from barrows or boxes on the level of the site, and to consolidate it afterwards by ramming; in ordinary foundations, to effect this properly and to allow the lime to set, the concrete should be laid in strata of not more than 1 ft. thick each; it is very desirable to bond these strata into each other in the process of laying, as the joint between 2 days' work is always a weak part in the mass. In large foundations, or with strong hydraulic lime, it is better to make the strata 2-3 ft. thick; on that account, for the same reason, the whole of one stratum should be laid as quickly as possible.