SMOOTH SURFACE OF SUBGRADE IS COMPACTED AND A LAYER OF THE WATER PROOF PAPER IS LAID DIRECTLY OVER THE SOIL TO REDUCE FRICTION BETWEEN SLAB AND THE UNDER SURFACE (SUBGRADE) .

Cement concrete roads should not be laid on black topped surface having soft spots caused by excessive bitumen or where thick premixed carpets have been rutted under traffic. In such cases entire surfacing material should be removed up to the top of the compacted macadam surfacing. The subgrade is prepared and checked at least two days in advance of concreting. The under layer should be in wet condition but not muddy at the time the concrete is laid .A clay must not be too wet when it is laid over the subgrade. Smooth surface is compacted sand , gravel and clinker, stabilised soil ,rough foundation covered with water proof paper .It is desirable to lay a layer of water proof paper whenever concrete is laid directly over the soil subgrade. Such an arrangement reduces the friction between the slab and the under surface.The use of water proof paper is particularly desired for the slabs laid in hot weather, when considerable contraction may take place before the full strength of the concrete is developed .A layer of sand about 20 mm may be laid over the old surface and wetted list by way and virtue of which there is an advantage of water proofing layer between the slab and the subgrade is loss of cement paste to a porous base or subgrade is prevented .The interposed layer will also prevent any harmful salts present in the subgrade rising up and attacking the concrete slabs. Concrete road slabs can usually be laid in single layer if good aggregate is available to produce hard and dense surface and where the traffic is not very heavy or the thickness in not so high to produce any difficulty in appropriate laying of the slab thickness provided on the sub grade.