Scientific Foundation of Modern Concrete - The Issues and Discussion of Modern Concrete Science - page 180

In important structure of architecture, such as slab, beam, and column, concrete that
is below C30 is basically no longer employed. As previously mentioned, the method
of boosting strength by increasing cement consumption is unsuitable for concrete
that is above C40. That is to say, the method, which has been used for hundreds of
years, does not conform to practical situation of modern concrete remarkably.

4. Cement neness

Before 1990s, as limited by backward of cement producing technology, neness

cult to be grinded to 300 m 2 /kg or more. However, with continuous
development of machine industrious technology,

of cement is dif

fineness of 42.5-grade cement in

350 m 2 /kg, and China is always 330 fineness of 52.5-grade cement is generally

above 380 m 2 /kg, some even exceed 400 m 2 /kg. Increasing cement grade by means
of boosting cement

-

fineness is a basic principle of the past cement subject. How-
ever, nowadays, when cement neness reaches above 420 m 2 /kg, cement strength

would not be enhanced further but instead had a tendency of decreasing. Therefore,
this method is also unsuitable for practical situation of modern concrete remarkably
[ 22 ].
In short, compared with past concrete, modern concrete is strength is higher and

higher, cracking is becoming so seriously that cannot be cured [ 23 ], creep is smaller
and smaller, and self-healing ability becomes weaker and weaker, etc., which differ
a lot from concrete 30 years ago.

'

5. Others

In aspect of concrete, wide application of high performance and pumping,
adoption of high-dosage fly ash, utilization of water-reducing agent; and in aspect

of cement industry, application of closed-circuit grinding, high-effective separator
and grinding aid, signi

cantly increase of neness, etc., have made concrete

technology generate radical changes.
We can find out that main reason for why the above chaos and different statements

are emerged in current concrete theory is we have not divided concrete that is before
1990s from modern concrete, while the former and latter vary a lot in performance
and principle. Current situation of concrete scienti

c community is we sometimes
apply theory before 1990s in modern concrete mechanically. For example, our
current speci cations still continue to use old parlance, namely increasing sand ratio

will decrease concrete strength. But actually increasing sand ratio properly does not
decrease strength of current high-performance concrete; our current concrete mix
proportion design speci

c area method.
Emerged deviation is already quite serious when this kind of theory is used to prepare
modern high-performance concrete; other questions are similar. A college graduate,
who is majoring in material,

cation is still continuing to use old speci

finds out concrete theories he learned in the class are
completely different from practice when going into the workplace; a postgraduate

finds conclusion error that is big enough when applying old theory to guide his/her
research work, which really make many worker fell at loose ends.
It is just because the above reasons that the author proposed conception of

“ second-stage concrete. ”