Materials: Concrete

Concrete is made through this process:

First, small stones or gravel are mixed with sand.
Cement powder is added.
Water is poured into the mixture, and the cement starts forming crystals.
The crystals grow until they bind the sand and stones together into a solid mass.

Bridges made with concrete can have one or more structural problems. Here they are:

Bending: compression (getting smaller) occurs on the upper part of the bridge and tension (pulling apart) occurs on the lower.
Shear: Parts of the deck slowly pass each other until a breaking point is reached.
Buckling of the piers due to compression.
(In suspension bridges only) tension (streching) occur in the cables and hangers.
Torsion (twist): happens when the weight of the traffic on each side of the deck is unequal.

Some of these problems can be overcome by placing wires, rods, or tubes in the concrete while it is drying. This keeps the concrete from changing.