Admixtures: All Medicines Have Their Own Side Effects - The Issues and Discussion of Modern Concrete Science - page 121

such as airport runway, pavement and wharf, and so on. It is specially discussed in
Chap. 5 and will not be repeatedly discussed here. Additionally, a small dosage of
air-entraining agent (entrained air content is below 2 %) can properly increase
concrete slump, compactness, and strength; high dosage of air-entraining agent
(entrained air content is above 4 %) will reduce compactness and strength, which
should be paid attention to when employing it.

10.1.3 Expansive Agent

The role and effect of expansive agent are always controversial in whether academia
or engineering. The purpose of employing expansive agent for us is to reduce
shrinkage and possibility of cracking. Yet one recognized point is that expansive
agent cannot completely solve crack problem. That is to say everything isn

' t just

fine after employing expansive agent, crack still generates in concrete. Thus, three
are few people can make it clear how big a role expansive agent has played in crack
prevention so far.
At

first, chemical reaction mechanism of expansive agent is that expansive effect
can only generate when concrete itself is fully wetted. However, another important
question is concrete does not generate shrinkage in condition that concrete itself is
fully wetted. It has expansive character and does not generate crack. Concrete will
only generate shrink crack in the context of drying, while expansive agent does not
react and generate expansion under such condition.
Having repeatedly employed expansive agent at construction site for more than
20 years, the author is always not in favor of employing expansive agent because it
fails to have remarkable effect on prevention of shrinkage crack.
There is another question few people are studying, yet it is the question we
should pay attention to. What to do if expansive agent generates delay?
Optimum reaction time of expansive agent ought to be within the 48 h after
concrete was molded. If expansive reaction did not generate in this period due to a
lack of water content and generate reaction in later period owning to suf cient water

content, namely it does not react when the reaction is needed while reacts when the
reaction is not welcomed, which is extremely dangerous and harmful to security
and durability of engineering structure if imagining.

10.1.4 Early-Strength Agent

Most salts have early-strength effect [ 3 ]. Similar to water reducer, its negative effect
is increasing shrinkage and possibility of cracking, making late period strength
grow a little or stop growing due to an excessive high strength at early stage, all of
which result in concrete overall strength, which is not high enough.