Cancer of Modern Concrete—Cracks - The Issues and Discussion of Modern Concrete Science - page 98

8.3 Category of Cracks

Causes for different cracks may be various. They can only be settled effectively if
different causes are found out. There is no uniform opinion about classi cation of

crack academically. As causes for crack which generates at different time is vari-
able, cracks are classi

ed into four categories according to different generation time
of crack. They are described, respectively, in the following.

8.3.1 Dehydration Cracks

During plastic stage of concrete, including a period around final setting, hydration of

most cement particles has just started, and majority of water in concrete exists as free
water at this moment. If affected by factors of high temperature, strong wind, rapid
cement hydrating speed, etc., free water would evaporate massively or disappear,
which results in voids in surface and then gives rise to generation of crack [ 3 ]. Small
dehydration crack is only several millimeters or centimeters long, and its width is
nearly the same; large dehydration crack may penetrate the whole component or even
form rupture, and this is the

first crack the author would like to say. The occurrence
time mainly centralizes in plastic stage of concrete; generally, it is within 48 h after
concrete mixing. Thus, it is called plastic crack or plastic cracking by someone.
Cement hydration has changed flowing or plastic concrete into structure that has

strength. This process is a process of dehydration from appearance, which is mainly
the transformation of free water in concrete to bound water in cement-hydrated
particle. Dehydration creates shrinkage stress in concrete. If dehydration rate is so
rapid that flexural strength generated by strength enhancement would be less than

the shrinkage stress, thereby crack is generated. Therefore, this kind of crack is
created by rapid dehydration.
Causes for rapid dehydration can be divided into internal causes and external
causes. External causes are mainly rapid dehydration created by high temperature
and strong wind in environment; internal causes are mainly sectional high tem-
perature inside concrete created by rapid cement hydration rate. (It will be discussed
in detail below). The author names it as dehydration crack so as to differentiate with
drying shrinkage crack. Drying shrinkage crack is also created by dehydration, yet
the lost water is bound water in cement hydration products. However, the lost water
for dehydration crack is free water in concrete. It can be further stated that the
former is physically dehydration, while the latter is chemically dehydration.

8.3.2 Temperature Crack

Liking other material, thermal expansion and contraction is a basic characteristic of
concrete. When temperature drops, shrinkage stress will be created in concrete.