Breeding and False Setting, Which Is Better? - The Issues and Discussion of Modern Concrete Science - page 74

particularly retarding agent, adopting aggregate with low water absorption or
temperature are more easily for breeding under circumstance of same W/C and
other construction condition. What

is more, breeding is more easily for plain con-
crete than reinforced concrete. An excessively high or thick structural dimension
will increase inner redundant water of concrete, so possibility of breeding is
increased in turn.
It is believed by the author that most breeding of concrete structures are com-
prehensive effects of several factors belong to the above nine factors. Therefore,
when studying breeding of anyone project, we should never just concern about one
or two causes and neglect the others, this may bring one-sidedness to our study and
dif

'

culty to prevention of breeding. Therefore, research and discussion must be
carried out on principles of breeding caused by the above factors, especially the
nine main factors, so as to

find a practical and operable method at construction site
and solve the breeding problem of any speci c project thoroughly.

6.2 Reasons for False Setting

There are many reasons for false setting, but they can be concluded in one, that is
temporary water shortage under certain condition. During certain period, there
exists a

for water in concrete, when the present overall water
content in concrete is less than

“ total demand ”

“ total demand ” , false setting may take place. There

are mainly three factors that affects “ total demand ” for water in concrete: water

①

demand of cement hydration during a period of time; water demand of water

②

evaporation inside concrete caused by climate; water emissions which can

compact concrete structure and lower void content after vibration.
When concrete meet strong wind after vibration or excessive evaporation caused
by high temperature, but cement hydration has already started, if free water stayed
within a few centimeters of concrete surface is less than total demand for water
caused by the above comprehensive factors, concrete will generate false setting and
cracking. In plastic stage, concrete must have proper amount of free water so as to
ful

③

ll the loss of water right now (also called water demand). If the demand is not
satis ed, false setting or cracking, or both of them will appear.

Reasons resulting in temporary water shortage inside concrete are listed as
follows.

1. W/C

A smaller W/C results in a greater possibility of temporary water shortage inside
concrete, meaning a greater possibility of false setting. As a result, when con-
structing under circumstance of high temperature in summer and strong wind, W/C
should be properly ampli

ed so as to prevent occurrence of false setting.