indicating that it has carried away some of the lime or other alkali content of the cement. Coupled with this is an erosive action which, while slight, is nevertheless present. Particularly are both solvent and erosive actions aggressive at the construction joints, as is evidenced by the excrescence of magnesia and other salts on the under surface at these places. This is because at the joints there is almost always found a film or deposit of laitance, which is loose in texture, non-coherent, chalky, and very porous. Water, if allowed to pass through the concrete at the said joints, will both leach and erode the joint walls very rapidly, thus exposing the reinforcing steel to the air with consequent corrosion, and opening the way in temperate and cold climates to the disruptive effect of frost. The result of these combined actions on concrete is certainly the weakening of the structure, as well as the making ready for further and more drastic effects of water disintegration.

In climates where freezing temperatures prevail in winter, there are peculiarly forceful reasons for water-proofing bridge floors. The disruptive force of freezing water is one of the most destructive agencies operating against masonry of whatever nature, whether its mass be mainly natural or artificial. It is particularly harmful where the masonry contains cracks, however small or shallow they may be, into which water can penetrate freely. This again has a most direct bearing on concrete bridge floors. In common with most flat-slab constructions, whether the slab rest on arches or beams, bridge floors are almost certain to develop cracks. They may be merely shrinkage cracks incident to the setting of the concrete, and hence more or less superficial, or they may be expansion cracks extending through the full depth of the slab; but the ultimate results will be practically the same. If only surface cracks exist, these will fill with water, which, on freezing, will break down the surrounding walls, causing the concrete to spall. It will thus be weakened along the line of the crack, which must inevitably be deepened with each repetition of the process, until the injury has extended through the entire thickness of the slab. Furthermore, since the concrete has been weakened along this line, any unusual stresses incident to expansion, or other force, are apt to split the slab at the weakened section.

Where the crack extends completely through the slab, the action is still more serious. In such a case, the same frost action may force the walls of the crack apart a material distance, exposing the reinforcing metal to unin[ter]rupted and unavoidable corrosion, thus weakening the entire structure. If electricity be present, the condition will be aggravated by electrolysis, which will not only accelerate the corrosion of the steel but also will soften the concrete, providing it be moist or wet. (See Tech. Paper, No. 19, U. S. Bureau of Standards.) If electrolytic action does occur, the concrete is certain to be split away from the reinforcing material by the mechanical pressure of the forming rust scale, which pressure has been recorded as high as 4,700 pounds to the square inch.