Culverts may be made of plain concrete, either in the box form or of an arched type, and having very much the same general dimensions as those already given for stone box culverts. They have a great advantage over stone culverts in that they are essentially monoliths. If the side walls and top are formed in one single operation, the joint between the side walls and top becomes a source of additional strength, and the culverts are therefore much better than similar culverts made of stone. The formula developed above (Equation 8) for the thickness of the concrete slab on top of a box culvert, may be used, together with the modulus of transverse strength as given for concrete in Table XII. Tin's formula will apply, even though the slab for the cover of the culvert is laid after the side walls are built, and the slab is considered as merely resting on the side walls. If the side walls and top are constructed in one operation so that the whole structure is actually a monolith, it may be considered that there is that much additional strength in the structure; but it would hardly be wise to reduce the thickness of the concrete slab by depending upon the continuity between the top and the sides.