[cmsmith]

The bridge looks to be precast concrete box-girders (hollow rectangular beams with plenty of steel reinforcement around the edges). The 'gravel' is the asphalt roadway (often this will be cast-in-place concrete) that's poured on top of the precast girders. The purpose of pouring the asphalt/concrete roadway on top is as a sacrificial wear element to prevent the structural beams being worn down by traffic.