presence of water and subgrade or foundation material that the pumped water can carry with it. On the other hand the pumping action does not occur in the absence of some loading adjacent to the joint. From widely distributed observations the conclusion has been reached that detrimental pumping is generally associated with a substantial frequency of axle loading in excess of 14,000 pounds. Moreover, loads of that magnitude, frequently repeated, are likely to overtax the strength of pavement slabs of the usual thickness when long continued pumping has deprived them of subgrade support.
The fact that joint pumping has been observed to occur under certain conditions associated with axle loading in excess of 14,000 pounds is not a sufficient reason to suggest the limitation of all axle loads to 14,000 pounds as a maximum. It is, however, a definitely observed fact associating a particular type of road damage with a particular magnitude of axle loading. In association with the conclusions drawn from the long experience of many highway authorities, it suggests that the limitation of axle loading should be fixed at no greatly superior weight. Among the other lessons of experience are those gained from many observations of the difference in damage occurring on heavily and lightly loaded lanes of the same pavements, from the quick occurrence of more extensive damage to pavements suddenly subjected to heavier axle loading, and from the differences in maintenance expense for similar pavements lightly and heavily loaded.
Adherence to 18,000-pound limit necessary
These lessons of experience have brought highway officials almost unanimously to the conviction that the axle-load limit of 18,000 pounds presently fixed by law in 34 States should not be exceeded, but rather should be more rigidly enforced as a prudent measure of existing road preservation.
If the 18,000-pound limit should be enforced for the protection of existing roads, the need for such limitation will certainly continue for many years during the normal life of the existing roads.
The question then arises: Should new roads henceforth built be designed to support heavicr axle loads? If so, they will have to be made stronger than the great majority of existing roads. If new roads are built of such greater strength, what assurance is there that future vehicle-operating practice will not demand still greater strength? It is an incontestable fact that the highway system cannot be efficiently designed and administered to serve an uncertain and increasing axle loading.
Additional evidence supporting 18,000-pound limit
Impressed strongly by this fact, and holding the conviction that 18,000 pounds is the heaviest axle load the generality of existing roads will safely support, highway officials have also the evidence of their frequent vehicle weighing to convince them that axle loading in excess of 18,000 pounds is not an essential concomitant of greater pay loads for vehicle operators. Additional gross weight and pay load can be readily accommodated if the number of axles is increased. The evidence to this effect is extensive. Its gist is sufficiently conveyed by the following comparison of the relation between the average axle loading and gross weight of all loaded heavy trucks and combinations
