Cancer of Modern Concrete—Cracks - The Issues and Discussion of Modern Concrete Science - page 97

6. Admixture

Compatibility between cement and admixture become poorer after implement of
new cement standard. For concrete with admixture addition, phenomenon of false
setting, crack happens frequently. According to our repeated investigation and
summarization, these questions occur due to cement under most conditions. Spe-
ci

cally, it relates with factors, such as gypsum type of cement, alkali content, C 3 A
content, grind aid, and adoption of high effective separator

7. Air relative humidity at construction eld

In northwest arid areas, as air relative humidity is low, evaporation tare at
concrete surface is too fast, which easily causes plastic shrinkage crack. The
question can be solved by means of adopting wood and iron trowel to smooth
concrete surface for many times generally

8. Wind force at construction eld

If wind force at construction site exceeds 4 grade, plastic crack will be easily
generated on concrete surface as extremely rapid water loss. Serious broad crack
will generate easily especially in some situ board structure with large area.

9. Environment relative temperature difference
If environment relative temperature difference is too big, concrete constructed at
high temperature would easily generate crack due to temperature stress at low
temperature. Particularly, concrete constructed in summer daytime would easily
generate crack at low temperature; concrete constructed in summer would easily
generate crack in winter either. These cracks are always bigger and more serious.

10. Vibrating technology

Both less vibration and over vibration are cause for cracking of concrete.

11. Curing

Delayed or insuf cient curing will also create drying shrinkage crack easily.

Generally speaking, the earlier and longer curing is, the less cracks will be

generated
The above are 23 causes for cracks summarized by the author. Some causes can
be solved or dif cult to be solved, while some causes are unable to be solved. That

is the reason why crack problem cannot be settled radically. It is recognized as the
biggest technical problem of modern concrete and needs to be studied by everyone.
The following are some crude research on this issue over these years. Different
kinds of cracks are classi

ed; causes and detriment of dehydration crack and drying
shrinkage crack are summarized, and several simple measures are proposed as
reference for others.