Civil Engineering Reference
In-Depth Information
12. Concrete mix proportion
Just for strength, there are thousands of mix proportions for a given strength, yet
improper choosing of mix proportion can also lead to crack. These improper
choosing may include the following:
high content of
fine material (including
①
cement);
improper W/C; and
unreasonable grading constituted by various
②
③
material particles in concrete.
13. Fine mineral admixture in mix proportion
technology, yet
improper mixing will also readily result in crack, especially for silica fume.
Some modern high-grade concrete adopts
“
double-mixing
”
14. Chemical admixture
Compatibility between cement and chemical has become poorer since new
cement standard was implemented. False setting and crack phenomenon take place
occasionally for concrete which employs chemical admixture. This problem is
relatively more frequent for sulfonate-based chemical admixture. According to our
multiple investigations, questions of this category are ordinarily resulted in by
cement and associated with cement calcination temperature, cooling speed, grinding
temperature, gypsum type, alkali content, C
3
A content, etc.
15. Relative air humidity at construction site
In northwest arid areas, as relative air humidity is low, evaporation tare at
concrete surface is too fast, which easily causes dehydration of the crack.
16. Concrete vibration technology
Both inadequate vibration and excessive vibration are causes for crack genera-
tion of concrete.
17. Concrete curing
Delayed and inadequate curing may also cause dehydration crack.
18. Cement paste and mortar thickness on concrete surface
If thickness of cement paste or mortar on concrete surface was too thick, drying
speed in thick place would be slower than that in thin place during construction. As
a result, compared with outer places, places where cement paste or mortars are
thicker would more easily create dehydration crack. Besides, it is much easier to
generate crack in places where cement paste is thick than places where it is thin
after curing.
The above 18 causes for crack generation are summarized by us at construction
frontline, and it might not be comprehensive. Dehydration cracks that can be seen at
construction
field are basically generated by the above 18 causes. For a speci
c
construction
field, generation of a crack is the compressive effect of several or more
than ten causes in the above.
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