Prestressed Concrete
Concrete is strong in compression, but weak in tension: its tensile strength varies from 8 to 14 percent of its compressive strength. Due to such a love tensile capacity ,flexural cracks develop at early stages of loading .In order to reduce or prevent such cracks from developing, a concentric or eccentric force is imposed in the longitudinal direction of the structural element. This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical misspend and support sections at service load, thereby raising the bending,shear,and torsion capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure.
预 应 力 混 凝 土
混凝土抗压但不抗拉，其抗拉强度是其抗压强度的8%~14%。由于混凝土的抗拉能力如此低，在荷载作用早期，混凝土内部即出现弯曲裂缝。为减缓或裂缝发展，可沿结构构件纵向施加轴心力或偏心力。这种力通过消除或尽可能地减少使用荷载作用下跨中临界截面的拉应力，以防止裂缝发展，从而增大了截面抵抗弯曲、剪切和扭转的能力。当全部荷载作用在结构上时，混凝土截面表现出弹性，混凝土的极限抗压能力几乎在混凝土全截面高度得到有效利用。
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