Efficient, well-planned water-proofing is the only sure preventive of these evils.

As provocative of cracks in concrete, the effect of moisture-changes merits attention. It has been fully demonstrated by some most-carefully-conducted tests that concrete expands in volume on becoming wet. This fact may well produce cross stresses in the slab, which will result in the cracking of the surface. If, for example, we have a floor slab 10 inches thick, and if sufficient water falls upon it to wet it to a depth of two inches, the upper two inches will have a tendency to swell or expand, while the lower 8 inches will remain fixed. Along the planes separating these two, therefore, there will be produced a stress which no amount of provision could guard against. Should this cross stress produce surface fissures or cracks, the way is opened for the ultimate disintegration which has been mentioned.

On this point, Hool & Johnson, in their "Concrete Engineers' Hand Book" made the following statement:

"The expansion and contraction of mortars and concretes, subjected to variations of temperature and moisture conditions, are responsible for practically all failures of these materials under conditions of exposure to the weather. Either temperature effects or moisture effects may be alone operative, or both effects may be combined.... In the average situation the introduction of dangerous stresses caused by a tendency to expand or contract is more apt to be due to moisture changes than to temperature changes, because the volumetric variations in the latter cases are less marked."

There is still another factor entering into the subject, to which engineers generally are not inclined to pay sufficient consideration, largely because it is one which is not directly or immediately measurable in dollars and cents, viz., the matter of appearance.

There is no concrete structure which is designed for a greater degree of permanency than a reinforced-concrete viaduct. It is almost invariably an important, indeed a vital, link in a railway or a national or state highway, the line of which does not change in a generation; and it is usually designed to carry many times the load which present conditions render requisite, thus making improbable the necessity for renewal by reason of changes in transportation methods. It stands, therefore, through many years a monument to the man who designed it, as well as an indication of the progressive spirit of the community; consequently, both from the stand-point of the designing engineer and from that of the community, every reasonable precaution should be taken to preserve the appearance of what is naturally a beautiful structure.

If water is permitted to flow freely through a bridge floor, the result will invariably be the excrescence of magnesia and other salts, which appear on the surface  in  the  form  of  a  white  "bloom."  This  is  particularly  in  evidence  at  the  construction  joints,  and  notably  at  the  joints  between  successive  arch  ribs. Nothing is  more  disfiguring  to  a  concrete  bridge,  nothing  is  more  indicative  of  careless  or  incomplete  work,   than  the  discoloration  of