Foundation Concrete

The type of foundation concrete and the proportion of the ingredients used in its making depend upon the nature of the structure, quality of the materials used and the site conditions. Since lime is fairly cheap, lime concrete is generally used for foundations in dry-subgrade. Lime concrete is produced by mixing one Cu. m of wet ground lime mortar with 25 cu. m of ballast. The proportion of ingredients in lime mortar may be 1 lime : 2 sand or 1 lime : 1 surkhi : 1 sand or 1 lime : 2 surkhi. The ballast may be of brick, stone or shingle. The size of the ballast is generally restricted to 40 mm.

For moist subgrade with high sub-soil water (usually 15 in. or less below the foundation level) cement concrete should always be used. In such situations the foundation concrete should not be leaner than 1:4:8 (1 cement 4 sand : 8 stone ballast). For less important work in dry sub-grade (subs oil water level below 1.5 m from foundation level) a leaner cement concrete such as 1:8:16 may be adopted. In normal practice, however, cement concrete 1:5:10 is recommended for lean concrete.

The mixing of concrete can be done either by hand or in mechanical mixer. In case of hand mixing of concrete, the concrete should be mixed on a clean dry and water-tight platform. Concrete should be laid (and not thrown) in layers not exceeding 15 cm in thickness. Each layer should be thoroughly rammed and consolidated before the succeeding layer is laid.

In case of lime concrete, the curing should start (by keeping the concrete damp with moist gunny bags, sand etc.) after 24 hours of its laying and should be continued for a minimum period of 7 days. The masonry work over the foundation lime concrete should be started only after 7 days. In case of cement concrete, however, the masonry work over the foundation concrete may be started after 48 hours of its laying. The curing of the cement concrete, which starts 24 hours after its laying, is continued along with the masonry for at least 10 days.