Engineering and Contracting . nued wet weather, the more porous por-tions will absorb moisture from the sub-grade^ particu-larly if it be poorly drained, and will tend to expand con-siderably more than the relatively impervious top which enters and permeates the pavement fromthe sides or at the expansion joints, would tend to expandthe lower portion more rapidly than the upper, causingdifferential movements, apparently sufficient in extent toaccount for any of the scaling encountered during theinspection. In many cases the bond between the upper and lowerlayers was probably not
Engineering and Contracting . nued wet weather, the more porous por-tions will absorb moisture from the sub-grade^ particu-larly if it be poorly drained, and will tend to expand con-siderably more than the relatively impervious top which enters and permeates the pavement fromthe sides or at the expansion joints, would tend to expandthe lower portion more rapidly than the upper, causingdifferential movements, apparently sufficient in extent toaccount for any of the scaling encountered during theinspection. In many cases the bond between the upper and lowerlayers was probably not originally perfect, being ma-terially weakened by the method of finishing. The actionof the trowel in bringing the neat cement to the surfaceundoubtedly leaves a plane of weakness parallel to thesurface at a distance beneath the surface equivalent tothe thickness through which the effect of the trowel istransmitted. This plane is in reality a connected line ofair voids, produced in a large measure, by the action ofthe fiforr^tir A^rvc, Ce^rvr- 9y4/vo t^ffs. ^l ) •Discussion based on exact studies of concrete pavements In Iowa•and published in Technical Report Xo. 1. Iowa Highway Commission,by T. R. Agrg and C. B. McCuUough. Diagram Showing Variation in Percentage Unfilled Sand Voids forIVIortars of the Same Proportions. It is interesting to note that the action of the float ortrowel pulls away the top covering from the more porousconcrete. The fine materials are brought to the top bya process of flotation and the top surface becomes moredense, more cohesive and assumes a rubberj consistency,quaking under the action of the float and pulling awayfrom the portion underneath. Such a condition may beprevented by a thorough consolidation of the concreteprevious to or simultaneously with the striking to propercross section, followed by only sufficient troweling orfloating to smooth the surface. 210 E N G I X E E R I X GAND CONTRACTING Even if the connected line of air voi
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