3.1 Properties of fresh concrete
The properties of fresh concrete include the flow, air content, and unit weight, as
listed in Table 5. From the experimental results, the PP fibers tended to cause a decrease in the flow
and increase in the air content during the mixing. This can be attributed to the higher
aspect ratio and large surface areas of PP fibers.
The superplasticizer promoted the increase in flowability. However, the superplasticizer
increased the air content of the composite during the mixing. Because it is a type
of surface active agent. Therefore, antifoamer is necessary for fiber reinforced composite.
However, to obtain the direct effect of superplasticizer in the matrix, antifoamer
is not introduced to any matrix in this study. Based on the experimental results,
the amount of superplasticizer used should be carefully controlled, and strong mixing
should be avoided. The PP fibers exhibited the lowest density of the all the components.
The addition of PP fibers into the cementitious composite led to a decrease in the
unit weight; whereas the increase in the unit weight of the component due to the addition
of steel fibers was not significant. Among all the mixtures, only the GS100P0 mixture
exhibited a higher unit weight than the control mixture (GS0P0).
3.2 Hardened concrete properties
3.2.1 Flexural and compressive strength
Table 5. Fresh properties of hybrid fiber reinforced RC-LFS based composite
|
ID
|
Flow (mm)
|
Air content
(%)
|
Unit weight
($kg$/$m^{3}$)
|
|
GS0P0
|
200
|
2.6
|
2,258
|
|
GS25P75
|
190
|
6.4
|
1,966
|
|
GS50P50
|
183
|
5.6
|
2,161
|
|
GS75P25
|
190
|
2.7
|
2,244
|
|
GS100P0
|
190
|
1.6
|
2,317
|
The flexural and compressive strengths are the most critical parameters with respect
to the evaluation of the mechanical properties. Using samples with dimensions of 4
cm×4 cm×16 cm, the values of the compressive and flexural strengths were measured
in accordance with the ASTM C348 (2018) and C349 (2018). The experimental results
are presented in Fig. 9 and Fig. 10. The results indicate that a high early strength was achieved. Compared with the
control mixture, the fiber reinforced mixtures exhibited a higher flexural strength
at the same curing age. The average flexural strength of GS100P0 (the sample with
1 % steel fiber), obtained over 28days, was 12.1 MPa, which was a 53.2 % improvement
when compared with the control mixture. Moreover, GS75P25, which had 0.75 % of steel
fibers combined with 0.25 % of PP fibers, exhibited a 49.4 % improvement. The flexural
strength increased in accordance with an increase in the amount of steel fibers. There
are two reasons for this. First, the steel fibers have a higher tensile strength,
particularly with the hooked- shaped ends. Hence, the steel fibers can provide a better
connection with the matrix and bridge the cracked region after the generation of the
first crack. Second, the mixture between the PP fibers and superplasticizer led to
a general increase in the air content. Third, for the same volume, the PP fibers exhibited
a higher density than the steel fibers, which increased the inner flow of the total
matrix.
Fig. 9. Flexural strength
Fig. 10. Compressive strength
The same trends were observed in the compressive strength data. The GS25P75 mixture,
which was 0.25 % of steel fiber with 0.75 % PP fiber, exhibited the minimum compressive
strength value, which was 49.52 % of that of the control mixture. The composite between
the steel and PP fibers improved the compressive strength to a lesser extent. The
strength depression of GS25P75 may be caused by the air entrapped by PP fiber during
the mixing process. Because GS25P75 exhibited the highest air content among all mixtures
(see Table 5). Therefore, it is necessary to introduce antifoamer to prevent unintentional air
in fiber reinforced composite. On the other hand, the GS100P0 mixture, which was 1.0
% of steel fiber without PP fiber, resulted in a compressive strength that was 11.43
% higher than that of the control mixture.
3.2.2 Deformation of samples under the flexural load
Generally, the embedded fibers do not have an influence on the cracking strength;
however, they have an influence on the post-cracking behavior owing to the remarkable
residual post- cracking strength resulting from the enhanced material properties.
Hence, in addition to the compressive and flexural strength tests, the deformation
of samples under the flexural stress was evaluated. The data recorded by LVDT are
presented in Fig. 11. The control mixture (GS0P0) exhibited high brittleness. The other mixtures with
additional fibers exhibited improved deformation properties after reaching the heaviest
load. The GS75P25 mixture exhibited the most superior properties. The GS25P75 mixture,
which contained the minimum volume of steel fibers, exhibited different trends with
other reinforced mixtures. After the heaviest load was reached, there was a significant
decrease in the stress, followed by a slight increase. This can be attributed to the
decrease in the steel fiber volume after the crack generation thus, the sample trends
to collapse. The bumps of the curves can be considered as the partial removal of the
steel fibers from the matrix. The LVDT results were in good agreement with the abovementioned
trends.
Fig. 11. Deformation of samples under the flexural load
3.2.3 Distribution of embedded fibers
The properties of the embedded fibers were tested using SEM, as presented in Fig. 12. In particular, Fig. 12 (a)~(c) present the PP fiber properties in the mixture. Fig. 12 (d) presents the steel fiber condition in the mixture. The SEM images shows that the
gaps between the PP fibers and the matrix are presented in the Fig. 12. During the loading process, the cracks were generated from the surface between the
PP fibers and matrix. With the crack propagation, the fibers were eventually pulled
out of the matrix. There were hollows where the PP fibers were pulled out, as shown
in Fig. 12 (c). The micro cracks around the steel fiber were as presented in Fig. 12 (d).
The fiber orientation and distribution are the main influencing factors of the engineering
performance (Lee et al. 2002; Song et al. 2018); hence, they should be investigated.
In fiber-reinforced cementitious composites, the fibers are typically randomly distributed
and oriented, depending on the fiber conditions of the casting method and matrix properties.
Form the reinforced mechanization, the fibers should bridge the crack sections to
delay and retard crack generation and propagation.
The orientations of the steel fibers were determined based on their sizes and cross-sectional
shapes. The sampling method employed is presented in Fig. 5. The cross-section after dying and polishing is presented in Fig. 13.
Fig. 12. (a)~(c) Embedded PP fiber and (d) steel fiber
Fig. 13. Cross-sections after dying and polishing
Fig. 14. Fiber distribution condition
The black-colored part indicates the hardened matrix, and the white dots indicate
the section surfaces of the steel fibers. After the application of the IA software,
the steel fibers exhibited black-colored dots, as presented in Fig. 14.
By observing the morphological properties of the steel fiber sections, several typical
conditions were observed, as provided in Table 6. The conditions can be classified as “positive” and “negative,” depending on the
cross-sectional shapes of the steel fibers. As presented in Fig. 14, several cross-sections exhibited ellipse-like shapes, which indicated the presence
of the out-of- plane angle ($\theta$). The angle ($\theta$) can be calculated using
the value of sine ($\theta$) as has been illustrated in Fig. 5.
Table 6. Typical conditions of steel fibers
The images obtained from the application of the computer software are presented in
Fig. 14. As can be seen, the number of steel fibers on the friction surface increasing with
the amount of steel fibers. Moreover, with an increase in the fiber amount, the orientations
of the fibers exhibited differences.
The GS75P25 mixture exhibited a good distribution and orientation. Another critical
result is the variation in the distribution in accordance with an increase in the
amount of steel fibers. For the GS100P0 mixture, several fibers were found to be clustered.
This case was observed more frequently when the fiber amount was increased.
This indicates that 80 % of the steel fibers were oriented nearly vertical to the
fraction surface and vertical to the load direction. Furthermore, several fibers were
nearly parallel to the friction surface. Those paralleled fibers contribute nothing
to flexural strength improvement. While in this experiment, only 0.3 % to 0.8 % of
fibers were parallel to the fraction surface, which is very low for the composite.
The average value of the angle was approximately 60°. The experimental and analysis
results are summarized in Table 7.
Another phenomenon that can be observed form Fig. 14 is that RC-LFS based matrix can hold steel fibers well. Steel fiber has the highest
density among all others materials. During the casting process, vibration is common
procedure for fiber reinforced composite. However, if the vibration is too strong
or the matrix has low viscosity, high density fibers sediment toward the bottom of
a sample. However, as shown in Fig. 14, there is no significant difference in the amount of fiberbetween the bottom parts
and the top parts. It is because RC-LFS composite is a rapid setting and hardening
material. After the final set was achieved, the embedded fibers cannot be moved.
Table 7. Analysis results of $bold d/l_{1}$ values
|
Mix
|
d/l1
|
Aggregate
|
Average $d/l_{1}$
|
Present of total num. (%)
|
Counting numbers
|
Average $\theta$
|
|
GS25P75
|
0.8-1
|
0.8< $d/l_{1}$ ≤1.0
|
0.849
|
80.8
|
413
|
58.1
|
|
0.6-0.8
|
0.6< $d/l_{1}$ ≤0.8
|
14.3
|
73
|
|
0.4-0.6
|
0.4< $d/l_{1}$ ≤0.6
|
3.5
|
18
|
|
0.2-0.4
|
0.2< $d/l_{1}$ ≤0.4
|
1.2
|
6
|
|
0-0.2
|
0≤ $d/l_{1}$ ≤0.2
|
0.2
|
1
|
|
GS50P50
|
0.8-1
|
0.8< $d/l_{1}$ ≤1.0
|
0.867
|
82.8
|
961
|
60.1
|
|
0.6-0.8
|
0.6< $d/l_{1}$ ≤0.8
|
10.4
|
121
|
|
0.4-0.6
|
0.4< $d/l_{1}$ ≤0.6
|
4.7
|
54
|
|
0.2-0.4
|
0.2< $d/l_{1}$ ≤0.4
|
1.8
|
21
|
|
0-0.2
|
0≤ $d/l_{1}$ ≤0.2
|
0.3
|
4
|
|
GS75P25
|
0.8-1
|
0.8< $d/l_{1}$ ≤1.0
|
0.862
|
78.1
|
1,317
|
59.5
|
|
0.6-0.8
|
0.6< $d/l_{1}$ ≤0.8
|
13.8
|
232
|
|
0.4-0.6
|
0.4< $d/l_{1}$ ≤0.6
|
4.7
|
79
|
|
0.2-0.4
|
0.2< $d/l_{1}$ ≤0.4
|
3
|
50
|
|
0-0.2
|
0≤ $d/l_{1}$ ≤0.2
|
0.5
|
8
|
|
GS100P0
|
0.8-1
|
0.8< $d/l_{1}$ ≤1.0
|
0.852
|
81.9
|
1,556
|
58.4
|
|
0.6-0.8
|
0.6< $d/l_{1}$ ≤0.8
|
11.2
|
213
|
|
0.4-0.6
|
0.4< $d/l_{1}$ ≤0.6
|
4.5
|
86
|
|
0.2-0.4
|
0.2< $d/l_{1}$ ≤0.4
|
1.6
|
31
|
|
0-0.2
|
0≤ $d/l_{1}$ ≤0.2
|
0.8
|
15
|