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Chapter 17 Constructing Structures
one layer. Asphalt is generally applied in
two layers: a coarse undercoat and a finer
topcoat. Finally, the shoulders, or edges, of
the road are prepared. The shoulders can
be gravel or asphalt.
Bridges
Another constructed structure vital
for transportation is the bridge. Bridges
provide a path for vehicles to move over
obstacles. These obstacles include marshy
areas, ravines, other roads, and bodies of
water. Bridges can carry a number of trans-
portation systems. These systems include
highways, railroads, canals, pipelines, and
footpaths.
Generally, there are fixed and mov-
able bridges. A fixed bridge does not move.
Once the bridge is set in place, it stays there.
Movable bridges can change their positions
to accommodate traffic below them. This
type of bridge is used to span ship chan-
nels and rivers. The bridge is drawn up or
swung out of the way so ships can pass.
Bridges have two parts. See Figure 17-24.
The substructure spreads the load of the
bridge to the soil. The abutments and the
piers are parts of the substructure. The
superstructure carries the loads of the
deck to the substructure. The deck is the
part used for the movement of vehicles and
people across the bridge.
Figure 17-23. Road building is a step-by-step process.
Grade the Right-of-Way Set the Forms Lay the Base
Ready for Use Place the Concrete Place the Shoulders
Building a
Concrete Highway
Mix the Concrete
Shoulder
Concrete
Fine gravel
Coarse gravel
Earth