- Accumulation of ice and packed snow.
- Emergency movement of special construction and industrial equipment.
- The springing action of vehicles.
AXLE LOADS DETERMINE REQUIRED ROAD-SURFACE STRENGTH
So far as vehicular effects are concerned, the load carried on a single axle is the principal determinant of the required load-supporting capacity of roads. Road load-supporting capacity is a composite result of the composition and depth of the subgrade or foundation, base, and surface or pavement provided. The supporting capacity of any combination of foundation, base, and surface is affected by variations of water or moisture content in any of the three elements of the structure. Supporting capacity of some of the materials employed, particularly portland cement concrete, is also affected by variations of temperature. The numerous combinations of variables involve complexities of relation and result which are imperfectly understood.
Static loads
It is known that the effect of two loads statically applied upon a road surface is not greater than the effect of either of the loads singly if the two loads are separated by not less than about 40 inches. Hence, for the estimation of vehicular effects the axle-load determinant is considered to be the total load transmitted to the road by all wheels the centers of which are included between two parallel transverse vertical planes 40 inches apart, extending across the full width of the vehicle.
Whether, and in what degree, the road effect of closely spaced loads dynamically applied differs from the known effect of static application, is not established.
Effect of weather
Roads which in certain conditions (especially conditions of moisture content) will support a particular axle load, under more adverse conditions will fail when subjected to considerably lighter axle loads; hence the common necessity to reduce axle loads in the spring when, as a result of the melting of frost, elements of the road structure are heavy with moisture.
Warping of concrete
Concrete pavements warp or bend downward at their edges when, as commonly on a warm day, the temperature of the upper surface is higher than that of the bottom of the slab. Conversely, when the temperature of the upper surface is lower than that of the bottom of the slab, as often it is at night, the edges of the slab bend or warp upward to such an extent that they are no longer in firm contact with the foundation. Hence, a given load applied at the edge of a concrete road slab may differently affect the slab according as the slab at the time is or is not in intimate contact with its foundation.
Action of repetitive loads
A road which will support a particular axle load in single or very infrequent application may fail or rapidly deteriorate under frequent
