Sole of Concrete—Mix Proportion - The Issues and Discussion of Modern Concrete Science - page 15

engineer will determine the needed slump for construction according to the above
conditions.
What we should make clear is that slump is the demand for construction tech-
nology rather than the need of concrete or engineering. Under the condition of high
steel bar intensity, smaller volume, and higher

floor, especially adopting pumping
construction, our construction technology level is unable to fill the low-slump

concrete into the component and guarantee their compaction. Therefore, the prin-
ciple that increasing slump is the need of current construction technology level must
be established.
Before the 1970s, stiff concrete are encouraged particularly in

fields of airport,
wharf, and concrete precast component. By means of strongly vibrating formation
method, concrete cracks are dramatically reduced and the project quality is
improved. Principle of construction technology and mix design in our country at
that time is lowering the slump as much as possible and adopting stiff concrete. But
thanks to the invention of high-range water-reducing agent in the late 1970s, W/C
had been greatly decreased. People at that time thought lots of problems, such as
strength and cracking, were caused by oversized W/C. High-range water-reducing
agent has solved the problem of W/C, as stiff concrete construction is dif

cult
together with high labor intensity. As a result, stiff concrete should be avoided
utilizing as far as possible. Except in the industry of airport and few precast
component construction, stiff concrete has been abandoned gradually in other
industries especially in housing construction industry. Plastic, flowing, and high-

performance concrete are extensively used at construction sites; the character is
with a growing slump. Now, we need to re ect on the effect of principle changing

on concrete and project quality, especially durability. Is it bene cial or not?

firstly analyzed that what changes does high-range water-reducing
agent bring to concrete. The main change is great reduction of W/C. Concrete with
W/C above 0.5 is rarely used at construction site. If the mineral admixture was
added, the water-to-binder ratio could be lower. Unilateral water consumption of
concrete has basically been controlled under 160 kg; these changes are brought by
high-range water-reducing agent. These changes enhance the 28-day compressive
strength of concrete and lighten labor intensity of worker.
But we should

It is should be

find at the same time that abandoning the principle of stiff
concrete has resulted in big changes. That is, the unit weight of concrete is lowered,
coarse aggregate amount is reduced, volume stability becomes degraded, and
possibility of generating shrink crack is increased.
There is another serious problem which is also very controversial in current
scienti c academia, that is, the negative effects of utilizing high-range water-

reducing agent on quality and durability of concrete. Many researchers insist that
high-range water-reducing agent has increased shrinkage and cracking possibility of
concrete, especially accelerated drying shrinkage, which bring extremely adverse
impacts to durability [ 3 ]. The author agrees with this view basing on my own
engineering practice either (as shown in Chap. 10 of this topic).
From what has been discussed above, the author believes reducing slump as far
as possible and adopting high-

owing concrete as little as possible, and advocating