Fiber, When Is Useful? - The Issues and Discussion of Modern Concrete Science - page 87

On July 7, we reverse the sequence. Experiment with no ber was arranged at

1:00 am, and experiment with fiber addition was settled at 3:00 am before dawn.

The experiment had adequately demonstrated that conclusion drawn from experi-
ment on July 6 was correct. Concrete prepared at 1:00 am had ability to resist
emerging of plastic cracking by itself, so there was no plastic crack, while concrete
at 3:00 am also had no plastic crack with the help of

ber.

fiber is only effective if there is possibility of generating
plastic crack for concrete. Contrarily, it is ineffective.
The experiment has cleared out many enigmas in the past and made a reasonable
explanation to why absolutely different conclusions are gained by different people
when dong experiment. When dong experiment in Ningbo, as local air humidity is
high and diurnal temperature difference is small, there is no possibility of gener-
ating plastic crack for concrete. Therefore, the drawn conclusion is that

It is thus clear that

fiber is
ineffective; when colleague of the author carries out experiment in northwest arid
region, as strong wind, high temperature, and low air humidity are quire common,
possibility of generating plastic crack has been intensi ed, so a conclusion that ber

is effective has been drawn.
Many university professors have drawn conclusions that

cant
effect on prevention of plastic crack [ 3 , 4 ]. It can be easily explained by the
experiment.

fiber has signi

As can be seen in their papers, cements employed in experiment are
mostly ordinary Portland cement or even Type R early strength cement, which have
great

①

Effect of trowel on prevention of
plastic crack has been paid less attention in laboratory. These two points have
already made concrete has possibility of generating plastic crack.
According to the above analysis, multiple experiment results and construction
experiences, and undercurrent concrete producing technical condition in China, the
author has classi

neness and short final setting time.

②

ed an approximate using range focusing on effect of ber on

prevention of plastic crack.

(1) In southern moist region, when temperature is below 30 C, there is no strong

wind that exceeds four grade, adopted cement is highway Portland cement, or
its

°

final setting time is over 3.5 h, and W/C is less than 0.45, concrete right
now has no possibility of generating plastic crack, so it is unnecessary to add

nal
setting time of adopted cement is less than 3.5 h, and concrete has possibility
of generating plastic crack, so it is necessary to add

fiber; when temperature is above 30 C, the wind is more than 3 grade,

°

ber.

(2) In northwest arid region, when temperature is below 28 C, there is no strong

wind that exceeds three grade; adopted cement is highway Portland cement, or
its

°

final setting time is over 3.5 h, and W/C is less than 0.45, and concrete right
now has no possibility of generating plastic crack, so it is unnecessary to add

nal
setting time of adopted cement is less than 3.5 h, and concrete has possibility
of generating plastic crack, so it is necessary to add

fiber; when temperature is above 28 ° C, the wind is more than 3 grade,

ber.

(3) In northern and other regions, when temperature is below 28 ° C, there is no

strong wind that exceeds three grade; adopted cement is Highway Portland