75 percentage of crumb rubber, asphalt concrete with 3 CR showed the most decent result
since the UCS value of 3 CR was more close to the UCS value of 0 CR, except 4 crumb rubber at 40
o
C, even after softening temperature at 40
o
C compared to two others. Moreover, the UCS value of two others percentage of crumb rubber was declined
dramatically, except 4 crumb rubber at 40
o
C, with the increasing of temperature. The inconsistency of 4 CR might be happed due to the influence of softening point on
deformation resistant properties of asphalt concrete at optimum crumb rubber percentage.
4.2.9 Indirect Tensile Stiffness Modulus Test ITSM
The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement
required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early
tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension. However, if the strength of reinforcement is lower than immature
concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the
formation of such wide crack widths, the concrete mix is specified to have a tensile strength normally measured by Brazilian test at 7 days not exceeding a certain value e.g. 2.8Nmm
2
for potable water. The result of ITSM for 0, 3, 4 and 5 crumb rubber is presented in Table 4.16.
Table 4.16.ITSM
CR OBC
20
o
C 40
o
C 50
o
C 5.60
3905,67 717,67
266.67
3 5.485
2438 322,33
132,33
4
5.55 1437
580,33 124,33
5 5.425
1086,67 220,00
115,33 Based on the result of ITSM in Table 4.16, it can be seen that the added of crumb rubber
influenced the ITSM score. In the mixture of 20
o
C without crumb rubber, the score of ITSM was 3905,67, then when the crumb rubber was added into the mixture, the score of ITSM was
decreased 1467,67 to 2438. As we can see on formula 2.12 perpustakaan.uns.ac.id
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76 Then, the added of crumb rubber decreased the ITSM score again to 1437 for 4 crumb
rubber and 1086,67 for 5 crumb rubber.Based on ITSM 20
o
C, it can be concluded that the added of crumb rubber influenced the decreased of ITSM score.The next test was conducted
for ITSM 40
o
C. The result of the test is presented in Table 4.18. Based on Table 4.16, the pattern of strength was same as ITSM test 40
o
C. When the crumb rubber added into the mixture, the score of ITSM was decreased. It also can be concluded that
the existence of crumb rubber in the mixture influenced the score of ITSM. The description of ITSM is presented in Table 4.37.
Figure 4.37. ITSM
From the figure 4.37 it can be seen that asphalt concrete without crumb rubber showed the far less sensitive to crack formation in compared with asphalt concrete with crumb rubber in this
study. As the ITSM value of asphalt concrete without crumb rubber is larger than asphalt concrete with crumb rubber at different temperature. However, among three different
percentage of crumb rubber, asphalt concrete with 3 CR showed the most optimum result since the ITSM value of 3 CR was more close to the ITSM value of 0 CR even after
softening temperature at 40
o
C compared to two others. Moreover, the ITSM value of two others percentage of crumb rubber was dropped rampantly with the increasing of
temperature.
500 1000
1500 2000
2500 3000
3500 4000
4500
20oC 40oC
50oC
ITSM
3 4
5
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77
4.3 Durability
