COMPOSITES - Materials for Civil and Construction Engineers - page 452

FIGURE 11.7 Wrapping columns of a bridge with FRP composites.

fibers are stronger than the matrix and, therefore, arrests cracks. Therefore,
instead of a worsening of the first crack that occurs in the concrete, more
cracks are developed elsewhere, and failure finally occurs when fibers pull
out or break (Neal 1977).
Entrained air in concrete can also be considered as a component in a mi-
croscopic composite material. Entrained air increases the durability of con-
crete since it releases internal stresses due to freezing of water within the
concrete. For the same water-to-cement ratio, however, air bubbles reduce
the concrete strength by about 20%. Since entrained air also improves the
workability of fresh concrete, the water-to-cement ratio can be reduced to
compensate for some of the strength reduction.

11.2 Macroscopic Composites

Macroscopic composites are used in many engineering applications. Because
macroscopic composites are relatively large, how the load is carried and how
the properties of the composite components are improved vary from one com-
posite to another. Common macroscopic composites used by civil and con-
struction engineers include plain portland cement concrete, steel-reinforced
concrete, asphalt concrete, and engineered wood such as glued-laminated
timber, and structural strand board.