Cancer of Modern Concrete—Cracks - The Issues and Discussion of Modern Concrete Science - page 94

At present, beam that has a length of over twenty meters is quite common in
house building design. The longest beam constructed by the author has reached 61
meters, and they are all statically indeterminate structures. Concrete has charac-
teristic of thermal expansion and contraction, so how many methods can be adopted
by

field engineer to ensure than such long structure does not generate cracking
under effect of temperature stress? Areas of some situ board can be as large as
several thousands m 2 , and it is required by design that the board should be placed
successfully in one time. Sometimes, it costs several continuous days to accomplish
of a situ board, how can emerging of crack be avoided in such long time?

2. Designed steel bar consumption is getting higher and higher; the arrangement is
becoming denser and denser

Steel bar consumption of concrete board, beam, and column has approximately
increased by times after 1990s. Especially for some load-bearing beams that afford
weight, rebar with diameter above 20 mm are arranged closely, gaps of 1.5 cm,
2 cm, and 3 cm are most common. Thus, construction of concrete is extremely
dif cult and it is unable to insert and vibrate even use regular vibrate bar. As is

known to all, in order to prepare concrete with larger slump, beside adopting water
reducer, it also has to reduce coarse aggregate content and increase content of

ne

aggregate, cement, fly ash and water, all of which will deteriorate concrete volume

stability, reduce flexural strength, and amplify possibility of crack generation.

3. Common application of high-strength concrete

Concrete from C40 to C80 or higher grade is widely applied in engineering of
bridge, house architecture, etc. Correlation between high strength and big shrinkage
can be found in nearly any engineering structure. Yet shrinkage is a very important
reason for generation of crack yet.

4. Cement fineness is getting ner and finer, especially strength at 3d is getting

higher and higher

fineness has increased drastically after implement of mew standard,
especially strength at 3d has been excessively increased. Besides, improper use at

Cement

field sometimes, abuse of Type R cement, even taking it for granted that high-
strength cement flawless, all of which has made concentration of concrete

hydration heat and increased possibility of generating crack.

is

5. Pumped concrete is more and more common in construction

Pumped concrete is a new technology for concrete construction, which supplies
very bene s labor intensity, lessening project

duration, and increasing height of city skyscrapers. However, it has negative effect
on project quality and possibility of crack generation. In order to boost pumpability,
construction unit has to reduce coarse aggregate content and increase content of

cial precondition for relieving worker '

ne
aggregate and cement when making concrete mix design, but chance of crack
generation has also been increased.