When concrete loses moisture by evaporation it shrinks. Shrinkage strains are mdependent of the stress conditions in the concrete. If restrained, shrinkage strains can cause cracking of concrete and will generally cause the deflection of structural members to increase with time. The calculation of stress and deformations due to shrinkage is deferred until Chapter 10.
A curve showing the increase in shrinkage strain with time appears inFig.2.21. The shrinkage occurs at a decreasing rate with time appeard in shrinkage strains vary greatly, being generally in the range 0.0002 to 0.0006 but sometime as much as 0.0010.
當(dāng)混凝土由蒸發(fā)丟失水分時(shí)便產(chǎn)生了收縮現(xiàn)象。 混凝土的收縮是一種自發(fā)的變形，并不依靠作用在混凝土上的外力。因此當(dāng)收縮變形不能自由進(jìn)行時(shí)，有可能導(dǎo)致混凝土產(chǎn)生裂縫，并且裂縫的數(shù)量隨著時(shí)間的增加而增多。在后面的第10章具體介紹收縮產(chǎn)生的變形和裂縫的計(jì)算。
圖2.2.1反應(yīng)了混凝土的收縮隨時(shí)間發(fā)展的規(guī)律，早期混凝土的收縮變形速率隨時(shí)間的增加變化較快，但是到了后期混凝土的收縮變形和初期相比有很大的不同，收縮的總量一般為（2～5）×10-4，有時(shí)達(dá)到1×10-3。