Materials and Structures DOI 10.1617/s11527-015-0647-x
ORIGINAL ARTICLE
Evaluation of the mechanical behaviors of cement-stabilized cold recycled mixtures produced by vertical vibration compaction method Xiaoping Ji . Yingjun Jiang . Yanjin Liu
Received: 28 January 2015 / Accepted: 1 June 2015 Ó RILEM 2015
Abstract Cement-stabilized cold recycled mixtures (CCRMs) are widely used to rehabilitate and repair asphalt pavements. During laboratory production, the mechanical behaviors of these CCRMs are affected by the compaction method of the two types of compaction methods used in CCRM production, namely static pressing compaction method (SPCM) and vertical vibration compaction method (VVCM), and the latter has caught the attention of researchers owing to its advantages. However, current studies have only investigated the mechanical behaviors of CCRMs produced by SPCM but not through VVCM. The current research aimed to evaluate the mechanical behaviors of CCRM produced by VVCM. A total of 18 CCRMs were designed with different ratios of recycled cement base to recycled asphalt pavement, virgin aggregate contents and cement contents. The mechanical behaviors of CCRM produced by VVCM were evaluated, and the influencing factors were investigated. Scanning electron microscope was also used to analyze the strength formation mechanism of CCRM.
X. Ji (&) Y. Jiang Key Laboratory for Special Area Highway Engineering of Ministry of Education, Chang’an University, Xi’an 710064, Shaanxi, China e-mail:
[email protected] Y. Liu School of Civil Engineering, Qinghai University, Xining 810016, China
The study results are useful for engineering practice, particularly for identifying the properties of CCRM produced by VVCM. Keywords Cement-stabilized cold recycled mixtures Vertical vibration compaction method Mechanical behavior Strength formation mechanism
1 Introduction With mounting concerns related to the reduced use of virgin materials, fuel consumption, curing time, land usage and emissions, pavement recycling techniques are widely being used to rehabilitate and repair asphalt pavement [1, 2]. Pavement recycling techniques are classified into two: hot recycling and cold recycling. Compared with hot recycling, cold recycling can be achieved without any special equipment and is more suitable for the reconstruction and maintenance of low-grade asphalt pavements. Most pavement distresses, such as fatigue cracks, reflection cracks and rutting caused by unstable mix, can be successfully repaired by cold recycling [3, 4], making it a popular recycling method throughout the world. Extensive research has focused on investigating the mechanical behaviors of cement-stabilized cold recycled mixtures (CCRMs), including compaction properties, comprehensive strength, splitting strength, shrinkage performance, and failure performance. The
Materials and Structures
mechanical behaviors of CCRMs are dependent on cement content [5], recycled asphalt mixture content [6], curing time [7], and admixture type [8–11]. These mechanical behaviors are also affected by density, which is directly influenced by the compaction produced in the laboratory [12]. Thus, considering the effect of the compaction method is important in investigating the mechanical behaviors of CCRM. There are two types of compaction methods for inorganic binder-stabilized granular base specified in the Chinese Specification of JTG E51-2009. The static pressing compaction method (SPCM) is a widely used method for inorganic binder-stabilized granular base owing to its easy operation and economical testing equipment requirement [13]. However, the correlation of engineering properties between the indoor specimens performed by SPCM and the field cores is less than 46 % [14]. Such phenomenon is a result of the difference of the SPCM loading condition from the heavy roller in field, which is always associated with vibration and oscillation. Meanwhile, vertical vibration compaction method (VVCM) is mainly performed using the vertical vibration test equipment (VVTE) [15, 16]. The VVCM can better simulate the field roller compaction, because the VVTE can simulate the vibration and oscillation of a heavy roller [14, 17]. The previous study [14, 17] compared the compressive strength of laboratory produced cement stabilized granular respectively by VVCM and SPCM with that of cement stabilized granular cored from two highway sites in China. The results showed that the compressive strength of laboratory produced cement stabilized granular by SPCM were in a range of 20.2–46.1 % compared to the compressive strength of field drilled cores, and the compressive strength of laboratory produced cement stabilized granular by VVCM can research the range 90.1–96.6 % of that of field drilled cores. In other words, the correlation of engineering properties between the indoor specimens performed by VVCM and the field cores reached above 90 %. Therefore, VVCM has gained increasing research attention in recent years. Previous studies have mainly focused on the mechanical behaviors of CCRMs produced by the SPCM. To address the research gap, there is a need to evaluate the mechanical behaviors of CCRMs produced by VVCM. This paper presents an evaluation of the mechanical behaviors of CCRMs using VVCM. The unconfined comprehensive strength (UCS),
splitting tensile strength, and resilient modulus were evaluated.
2 Materials and methods 2.1 Materials Portland cement graded 32.5 was utilized. The technical properties of cement were tested according to the Chinese Specification of JTG E30-2005 [18] and shown in Table 1. The virgin aggregate used in this study was limestone and its technical properties are presented in Table 2 in accordance with the Chinese specification of JTG E42-2005 [19]. Recycled pavement materials were obtained from provincial highway #325 located in Xuchang, Henan Province, China. The pavement consisted of cementstabilized crushed stone with a depth of 18 cm and asphalt surfacing with a depth of 7 cm. The recycled cement base (RCB) and recycled asphalt pavement (RAP) were obtained by milling machine. The technical properties of recycled pavement materials were tested in accordance with the Chinese Specification of JTG E42-2005. The experiment results are presented in Table 3. Table 4 shows the particle size distribution of the used materials. 2.2 Mixtures As mentioned earlier, the mechanical behaviors of CCRM is related to the curing time, cement content, virgin aggregate content, and the ratio of RCB to RAP. Therefore, all of the above factors should be considered. In consideration of the pavement used in provincial highway #325, three possible ratios of RCB to RAP were selected. The first involved milling the entire asphalt surfacing without the base layer, the second involved milling the entire asphalt surfacing and part of the base layer, and the third involved milling the entire surfacing and base layer. The ratios of RCB to RAP in these three conditions were 0:1, 1:1 and 7:3, respectively. To examine the influence of virgin aggregate on the performance of CCRMs, virgin aggregate contents were set at 0, 20 and 40 % respectively. For the cement content, the most important factor, 3 and 4 % were applied.
Materials and Structures Table 1 Technical properties of cement Surface area ratio (Blaine method) (m2/kg)
367
Fineness (%)
1.0
Setting time (min)
Flexural strength (MPa)
Compressive strength (MPa)
Initial time
Final time
3 days
28 days
3 days
28 days
210
300
4.7
8.9
22.4
47.1
Table 2 Technical properties of limestone Limestone size (mm)
Crushing value (%)
Flakiness index (%)
Apparent relative density
Water absorption (%)
19–37.5
10.4
10.5
2.774
1.14
9.5–19
11.3
2.821
1.18
4.75–9.5 ^4.75
11.9 –
2.772 2.696
1.36 1.78
Table 3 Technical properties of recycled materials Materials
Crushing value (%)
Flakiness index (%)
Apparent relative density
Water absorption (%)
RAP
12.3
4.7
2.535
7.73
RCB
14.7
5.3
2.437
9.55
Table 4 Particle size of materials
Materials
Passing ratio at different sieve size (%) 37.5
31.5
19
9.5
4.75
2.36
0.6
0.075
19–37.5 mm
100
84.6
1.3
0.1
0.1
0
0
0
9.5–19 mm 4.75–9.5 mm
100 100
100 100
83.3 100
2.87 95.7
0.2 7.5
0 0.6
0 0.4
0 0
Limestone
^4.75 mm
100
100
100
100
99.4
85.5
42.2
14.9
RAP
100
98.9
96.3
83.1
62
42.4
17.4
5.1
RCB
100
95.7
83.6
56.8
35.6
21.9
10.9
1.8
Therefore, a total of nine CCRM gradations and 18 kinds of CCRMs (two cement contents for each gradation) were used for the investigations. The particle sizes of the CCRM gradations are presented in Table 5.
the specimen. The core of VVCM is VVTE. In this study, a vertical sinusoidal force with a frequency of 30 Hz and an amplitude of 7.6 kN was applied using the VVTE. 3.1.1 Determining the maximum dry density and optimal water content
3 Testing method 3.1 Vertical vibration compaction method VVCM includes two processes. The first process determines the maximum dry density (MDD) and optimal water content (OWC). The second produces
The materials were dried to constant weights in the oven with temperatures ranging from 105 ± 5 °C for 4–6 h. Subsequently, cement was added into the dried materials and mixed to uniformity, to which water was added and mixed again. Then, the mixtures were poured into molds with an inner diameter of 150 mm
Materials and Structures Table 5 Gradation of CCRM Materials composition
Passing ratio through different sieve size (%)
RCB:RAP
37.5
0:1
1:1
7:3
Virgin aggregate content (%)
31.5
19
9.5
4.75
2.36
0.6
0.075
0
100
95.8
83.7
56.9
35.6
22.0
11.0
1.8
20
100
94.3
72.7
50.5
33.5
21.8
10.9
2.2
40
100
93.0
61.6
45.1
32.3
22.6
11.2
2.7
0
100
97.3
90.0
70.0
48.8
32.2
14.2
3.4
20
100
94.8
72.2
56.0
39.1
25.7
11.4
2.8
40
100
94.5
67.5
46.3
33.3
22.7
10.2
2.7
0
100
97.9
92.5
75.2
54.1
36.3
15.5
4.1
20
100
95.3
74.3
60.2
43.3
29.0
12.4
3.3
40
100
94.1
65.1
50.3
37.5
26.1
11.4
3.2
and height of 230 mm. The molds were placed at the bottom plate of the VVTE and vibrated for 100–120 s. The specimens were removed from the mold and their heights, which should be 120 ± 5 mm, were measured. The mass of specimens were measured and the dry densities of the specimens were calculated. Finally, water content was changed, and the dry densities of materials with five different water contents were determined in accordance with above processes. The dry density-water content curves were drawn in the X–Y coordinate system to determine the MDD and OWC according to the peak of the curve. 3.1.2 Preparation of specimen The materials were dried to constant weights in the oven with temperatures ranging from 105 ± 5 °C for 4–6 h. Cylinder specimen with a size of 150 mm 9 150 mm was performed by VVCM, and the weight of one specimen was calculated using Eq. (1), in accordance with the MDD, OWC and the compaction degree. The mixtures were poured into molds (inner diameter: 150 mm, height: 230 mm) in three layers, and then stirred for 15–25 times from the edge to the center for each layer. The molds were placed at the bottom plate of the VVTE and vibrated for 60–70 s when the demanded compaction degree was K = 97–98 %, for 70–80 s when K = 98–99 %, and for 80–90 s when K = 99– 100 %. Finally, the specimens were removed from the molds, and their heights were measured. Next, the specimens were packed in plastic bags and placed into
the curing room with an air temperature of 20 ± 2 °C and relative humidity of 95 %. The mass of one specimen can be calculated by Eq. (1). m0 ¼ V qmax ð1 þ wopt Þ K;
ð1Þ
where m0 is the mass of one specimen, g; V is the volume of one specimen, cm3; qmax is the MDD, g/cm3; wopt is the OWC, %; and K is the compaction degree, %. 3.2 Laboratory evaluation of mechanical behavior The unconfined compressive strength (UCS) of CCRM cured for 7, 28, 60 and 90 days and then immersed to water for 24 h were tested according to T0805-1994 of Chinese specification of JTG E512009. The UCS is defined as the maximum unit stress obtained from monotonic load testing. The UCS tests were conducted using strain-control conditions, with an axial strain rate of 1 mm/min. The splitting strength cured for 7, 28, 60 and 90 days were tested referring to T0806-1994. The splitting strength tests were conducted using strain-control conditions, with an axial strain rate of 1 mm/min. The splitting strength is calculated out by Eq. (2). P Ri ¼ 0:004178 ; h
ð2Þ
where Ri is the splitting strength, MPa; P is the maximum loading leading to specimen failure, N; h is the height of specimen, mm. The compressive resilient modulus cured for 7, 28, 60 and 90 days were tested
Materials and Structures
referring to T0808-1994. The testing process of resilient modulus is described as follows. The maximum applied loading is seventy percent of UCS and divided into six progressively loading. The first stage loading p1 (1/6 of the maximum loading) is applied with the rate of 1 mm/min and continued to 1 min, and then the deformation of the specimen is recorded. Remove the loading so that the elastic deformation restores, and the deformation of the specimen is recorded again when unloading 0.5 min. The resilient deformation l1 is the difference between the deformations in loading and unloading process. According to the above method, the last process of loading and unloading is completed step by step. The compressive resilient modulus is finally given by Eq. (3). P h pi Mr ¼ P ; ð3Þ li
optical system, signal collection and display system, vacuum system and power system. The imaging principle of SEM was described as followings. First, the electron beam emitted by the electron gun converged to a small electron probe under the action of accelerating voltage. And then, the electron probe was forced to scan the surface of sample under the action of a scanning coil of the final lens. Finally, because of the interaction of high-energy electrons with matter, the secondary electron signal and backscattered electron signal, etc., were stimulated from the sample. The secondary signals were received and treated by the secondary electron or backscattered electron detectors, as a result the images were formed on the picture tube, which can help to observe the surface morphology of sample.
where Mr is the compressive resilient modulus, MPa; pi is the progressively load, MPa; li is the resilient deformation, mm; h is the height of specimen, mm. All specimens were cured with the air temperature was 20 ± 2 °C and relative humidity was 95 %. All samples were subjected to 24 h of immersing in water prior to the above tests. The specimens were prepared by VVTE with a size of 150 mm 9 150 mm.
4 Results and discussion
3.3 Scanning electron microscope for observing strength formation mechanism Scanning electron microscope (SEM), produced by Hitachi Ltd., was used to study the strengthening mechanism of CCRM. This SEM consists of electro-
4.1 Maximum dry density and optimal water content Table 6 shows the MDD and OWC of CCRMs produced by VVCM. Several observations can be seen in Table 6. First, the mean OWC values were 4.3, 5.8 and 6.9 %, whereas the mean MDD values were 2.187, 2.241 and 2.258, when the ratios of RCB to RAP were 0:1, 1:1 and 7:3, respectively. OWC increased with the increase in the ratio of RCB to RAP, which was caused by the higher water absorption of RCB compared with RAP. MDD also increased gradually with the increase in the ratio of RCB to RAP, which can be attributed to
Table 6 OWC and MDD of CCRM produced by VVCM RCB:RAP
0:1
1:1
7:3
Virgin aggregate content (%)
Cement content was 3.0 %
Cement content was 4.0 % 3
OWC (%)
MDD (g/cm )
OWC (%)
MDD (g/cm3)
0
4.6
2.142
4.6
2.147
20
4.2
2.153
4.2
2.157
40
4.1
2.257
4.2
2.263
0
6.2
2.144
6.2
2.151
20
5.8
2.251
6.0
2.260
40
5.2
2.307
5.4
2.332
0
7.6
2.135
7.8
2.151
20
6.8
2.283
7.0
2.289
40
6.2
2.336
6.2
2.352
Materials and Structures Table 7 UCS of CCRM produced by VVCM RCB:RAP
Cement content (%)
Virgin aggregate content (%)
UCS (MPa) at curing time of (day) 7
0:1
3.0
4.0
1:1
3.0
4.0
7:3
3.0
4.0
60
90
0
2.5
4.7
5.7
6.1
20
2.6
5.3
6.2
6.4
40
3.4
5.8
6.5
6.9
0
3.1
5.5
6.3
6.8
20
3.3
6.0
6.9
7.2
40
3.8
6.5
7.3
7.8
0
2.7
5.2
6.1
6.3
20
3.3
6.1
6.6
6.7
40
3.7
6.3
6.9
7.3
0
3.6
6.0
6.6
6.9
20
4.2
6.5
7.4
7.4
40
4.5
6.9
7.9
8.2
0
3.1
5.7
6.5
7.4
20
3.6
6.2
7.0
7.3
40 0
4.1 4.0
6.7 6.4
7.4 7.2
7.7 7.7
20
4.5
6.8
7.7
8.0
40
4.8
7.4
8.3
8.6
(a) 10.0
(b) 12.0 7d 60d
28d 90d
7d 60d
10.0
UCS/MPa
8.0
UCS/MPa
28
6.0 4.0
28d 90d
8.0 6.0 4.0
2.0
2.0 0.0
0.0 0:1
1:1
7:3
RCB:RAP
0:1
1:1
7:3
RCB:RAP
Fig. 1 UCS of CCRM varied with the ratio of RCB to RAP. a Cement content = 3 % and virgin aggregate content = 0 %. b Cement content = 4 % and virgin aggregate content = 40 %
the fact that the RAP surface was covered by asphalt and its density was lower than RCB. Second, the mean OWC values were 6.2, 5.7 and 5.2 %, whereas the mean MDD values were 2.145, 2.232 and 2.308, when the virgin aggregate contents were 0, 20 and 40 %, respectively. The results indicated that OWC
decreased gradually with the increase in virgin aggregate content, which was due to the lower water absorption rate of virgin aggregate compared with RAP and RCB. However, MDD increased with the increase in virgin aggregate content because with such increase, the density became greater than those of RAP
Materials and Structures
(a) 10.0
(b) 12.0 7d
8.0
28d
60d
10.0
90d
7d
28d
60d
90d
UCS/MPa
UCS/MPa
8.0 6.0
4.0
6.0 4.0
2.0
2.0 0.0
0.0 0
20
0
40
20
VGA content/%
40
VGA content/%
Fig. 2 Variations of UCS of CCRMs based on virgin aggregate content. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
(a)
(b) 12.0
10.0 virgin aggregate content=0 virgin aggregate content=20% virgin aggregate content=40%
8.0
virgin aggregate content=0 virgin aggregate content=20% virgin aggregate content=40%
10.0
UCS/MPa
UCS/MPa
8.0 6.0
4.0
6.0 4.0
2.0
0.0
2.0
0
20
40
60
80
100
Curing times/day
0.0
0
20
40
60
80
100
Curing times/day
Fig. 3 Variations of unconfined compressive strength of CCRMs based on curing time. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
and RCB. Third, the mean OWC values were 5.6 and 5.7 %, whereas the mean MDD values were 2.223 and 2.234, when the cement contents were 3 and 4 %, respectively, indicating that these variables increased with the increase in cement content. 4.2 Unconfined compressive strength Table 7 shows the UCS of CCRMs produced by VVCM. The curing times for CCRMs were 7, 28, 60, and 90 days.
Figure 1 shows the variation of UCS with the ratio of RCB to RAP. As can be seen, at different curing times, UCS increased with the increased ratios of RCB to RAP. This phenomenon resulted from (1) the strength of asphalt particles in RAP being lowing than that of the RCB; and (2) the presence of gaps between asphalt and cement or aggregate, which destroyed the three-dimensional network structure. Figure 2 shows the variation of UCS with the virgin aggregate content. As can be seen, at different curing times, UCS increased with the increase in virgin aggregate content
Materials and Structures Table 8 Splitting strength of CCRM produced by VVCM RCB:RAP
Cement content (%)
Virgin aggregate content (%)
Splitting strength (MPa) at curing time of (day) 7
0:1
3.0
0.25
0.47
0.57
0.60
0.29
0.52
0.61
0.65
40
0.35
0.6
0.65
0.71
0
0.32
0.57
0.64
0.69
20
0.34
0.61
0.7
0.73
40
0.38
0.66
0.77
0.82
0
0.28
0.53
0.6
0.65
20
0.36
0.6
0.67
0.68
40
0.39
0.64
0.72
0.74
0
0.39
0.62
0.73
0.74
20
0.43
0.67
0.76
0.77
40
0.45
0.70
0.80
0.84
0
0.35
0.60
0.67
0.72
20
0.37
0.63
0.72
0.75
40 0
0.40 0.42
0.68 0.66
0.76 0.75
0.78 0.77
20
0.45
0.71
0.78
0.81
40
0.48
0.74
0.84
0.86
7d 60d
28d 90d
4.0
3.0
4.0
(b)
1.0
7d 60d
0.8
28d 90d
1.2 1.0
Splitting strength/MPa
Splitting strength/MPa
(a)
90
0
3.0
7:3
60
20 4.0
1:1
28
0.6
0.4
0.2
0.8 0.6 0.4 0.2 0.0
0.0 0:1
1:1
7:3
0:1
RCB:RAP
1:1
7:3
RCB:RAP
Fig. 4 Variations of splitting strength of CCRMs based on different RCB to RAP ratios. a Cement content = 3 % and virgin aggregate content = 0 %. b Cement content = 4 % and virgin aggregate content = 40 %
because the strength of virgin aggregate was greater than those of RAP and RCB. Figure 3 shows the variation of UCS with the curing time. As can be seen, UCS increased sharply at the initial time (e.g., curing time of 14–28 days) and then slowed down after 28 days.
4.3 Splitting strength Table 8 shows the splitting strength of CCRMs produced by VVCM. The curing times for the CCRMs were 7, 28, 60 and 90 days, respectively. Meanwhile, Fig. 4 shows the variations of the
Materials and Structures
(b)
1.0
Splitting strength/MPa
0.8
1.2
28d
7d 60d
1.0
90d
Splitting strength/MPa
(a)
0.6
0.4
0.2
7d
28d
60d
90d
0.8 0.6 0.4 0.2
0.0
0
20
0.0
40
0
20
40
VGA content/%
VGA content/%
Fig. 5 Variations of splitting strength of CCRMs based on virgin aggregate content. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
(b)
1.0
virgin aggregate content=0 virgin aggregate content=20% virgin aggregate content=40%
0.8
0.6
0.4
0.2
0.0
1.2
virgin aggregate content=0 virgin aggregate content=20% virgin aggregate content=40%
1.0
Splitting strength/MPa
Splitting strength/MPa
(a)
0.8 0.6 0.4 0.2
0
20
40
60
80
100
Curing times/day
0.0
0
20
40
60
80
100
Curing times/day
Fig. 6 Variations of splitting strength of CCRMs based on curing time. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
splitting strengths of CCRMs with varied RCB to RAP ratios. Similar with the results of UCS in Fig. 1, the splitting strength of CCRMs increased as the RCB to RAP ratios increased. Figure 5 shows the variation of the splitting strengths of CCRM with the virgin aggregate content. Similar with the results of UCS in Fig. 2, the splitting strength of CCRMs increased with the increase in virgin aggregate content, because the strength of virgin aggregate was greater than those of RAP and RCB. Figure 6 shows the variations of the splitting strengths
of CCRMs based on curing time. The changing tendency of the splitting strengths versus the curing time was the same for the UCS of the CCRMs, as shown in Fig. 3. The splitting strength increased sharply with the curing time before 28 days; further, the increase became smaller after 60 days (Fig. 6). 4.4 Resilient modulus Table 9 shows the resilient modulus of CCRMs produced by VVCM. The curing times for CCRMs were 7, 28, 60,
Materials and Structures Table 9 Resilient modulus of CCRM produced by VVCM RCB:RAP
Cement content (%)
Virgin aggregate content (%)
Resilient modulus (MPa) at curing time of (day) 7
0:1
3.0
476
1013
1175
1209
663
1198
1287
1344
40
808
1312
1414
1497
0
678
1210
1375
1491
20
739
1311
1519
1576
40
816
1405
1601
1745
0
665
1123
1255
1339
20
699
1232
1339
1420
40
847
1358
1477
1575
0
735
1273
1405
1471
20
891
1434
1586
1624
40
1006
1573
1699
1842
0
702
1188
1298
1371
20
775
1285
1409
1491
40 0
913 780
1450 1352
1556 1469
1640 1521
20
954
1527
1674
1765
40
1071
1667
1890
1953
4.0
3.0
4.0
(b) 7d 60d
28d 90d
Resilient modulus/MPa
Resilient modulus/MPa
(a) 2400 2000 1600 1200 800 400 0
0:1
90
0
3.0
7:3
60
20 4.0
1:1
28
1:1
7:3
RCB:RAP
2400
2000
7d 60d
28d 90d
1600
1200
800
400
0:1
1:1
7:3
RCB:RAP
Fig. 7 Variations of the resilient modulus of CCRMs based on different RCB to RAP ratios. a Cement content = 3 % and virgin aggregate content = 0 %. b Cement content = 4 % and virgin aggregate content = 40 %
and 90 days. Figure 7 shows the variation of the resilient modulus of CCRM with different RCB to RAP ratios. Figure 8 shows the variations of the resilient modulus of CCRM based on virgin aggregate content. Similar with the results of UCS and splitting strength, Figs. 7 and 8
show that the resilient modulus of CCRM increased as the ratios of RCB to RAP and virgin aggregate content increased, respectively. Meanwhile, Fig. 9 shows the variations of the resilient modulus of CCRM based on curing time. As
Materials and Structures
(b)
2400
7d
Resilient modulus/MPa
2000
60d
2400
28d
7d
28d
90d
60d
90d
Resilient modulus/MPa
(a)
1600 1200 800 400 0
2000
1600
1200
800
400 0
20
40
0
20
40
VGA content/%
VGA content/%
(a) 2400
virgin aggregate content=0
Resilient modulus/MPa
2400
2000
virgin aggregate content=20%
2000
virgin aggregate content=40% 1600 1200 800 400 0
(b) Resilient modulus/MPa
Fig. 8 Variations of the resilient modulus of CCRMs based on virgin aggregate content. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
0
20
40
60
80
100
Curing times/day
virgin aggregate content=0 virgin aggregate content=20% virgin aggregate content=40%
1600
1200
800
400
0
20
40
60
80
100
Curing times/day
Fig. 9 Variations of the resilient modulus of CCRMs based on curing time. a RCB:RAP = 0:1 and cement content = 3 %. b RCB:RAP = 7:3 and cement content = 4 %
can be seen, the resilient modulus of CCRM increased sharply with curing time before 28 days; further, the increase became smaller after 60 days. 4.5 Relationship of mechanical behaviors Determining the relationship between UCS and splitting strength (Ri) is very important in understanding the mechanical behaviors of CCRM, because this link highly affects the failure mode of
cement-stabilized materials. The relationship of UCS and splitting strength is always used in the failure criteria. The UCS and splitting strength of cement-stabilized materials display a good linear relationship [20]. According to the UCS and splitting strengths obtained (see Tables 7 and 8), their linear relationship was identified [see Fig. 10 and Eq. (4)]. In accordance with the same method, the relationship of the resilient modulus and UCS can be obtained using Eq. (5).
Materials and Structures
10
UCS = 9.786Ri
UCS/MPa
8
R² = 0.991
6
4
2
0
0
0.2
0.4
0.6
0.8
1
Splitting strength/MPa Fig. 10 Relationship of unconfined compressive strength and splitting strength
Fig. 12 SEM image of CCRM
Resilient modulus/MPa
2400
Mr = 215.1UCS
2000
R² = 0.965
1600 1200 800 400 0
0
2
4
6
8
10
UCS/MPa Fig. 13 Asphalt particles embedded into the cement Fig. 11 Relationship of resilient modulus and unconfined compressive strength
UCS ¼ 9:786Ri
ð4Þ
Mr ¼ 215:1UCS
ð5Þ
As shown above, Ri is the splitting strength of CCRM, MPa, and Mr is resilient modulus of CCRM, MPa (Fig. 11). 4.6 Strength formation mechanism The strength of CCRM is mainly generated from the cement hydration reaction similar to the cementstabilized gravel. The hydration products, which
include needle-like Aft, fibrous C–S–H and so on, form a three-dimensional network structure that locks up the recycled materials and aggregates. However, the strength formation mechanism of CCRM is different from that of cement-stabilized gravel because of the aged asphalt contained in CCRM. Figure 12 shows an SEM image of CCRM (the dark area represents aged asphalt). Figure 13 shows that the asphalt particles are embedded into the cement. In the process of RAP milling, crushing and mixing, a small amount of aged asphalt is wrapped on the surface of the aggregate flakes and then mixed with cement, thus leading to the phenomenon of asphalt particles being covered by cement once the cement hydration reaction is
Materials and Structures
Fig. 14 Interface of cement and asphalt
completed. The comprehensive strength of the asphalt particles is lower than that of the aggregates or cement, thus the strength of CCRM is lower than that of the cement-stabilized gravel, and decreases with increasing RAP content. Figure 14 shows the interface of cement and asphalt film. As can be seen, there are gaps between them and a small amount of hydration products are embedded into the asphalt film. The phenomenon of asphalt film covering cement leads to the increases interface thickness and decreases water migration toward the aggregate surface. As a result, cement hydration reaction weakens, and the three-dimensional network structure forming the hydration products is destroyed. In turn, this phenomenon leads to the adhesion strength of aggregates in CCRM becoming lower than that of cement-stabilized gravel. When the CCRM is subjected to force, the aggregates easily slip from one another and generate micro-cracks, eventually making the tensile and compressive strengths of CCRM smaller than those of cement-stabilized gravel.
5 Conclusions An investigation of the mechanical behaviors of CCRMs produced by VVCM was conducted. Specifically, the influences of curing time, virgin aggregate content and different RCB to RAP ratios on the mechanical behaviors of CCRM were investigated. The strength formation mechanism of CCRMs was also observed through SEM.
The OWC and MDD of CCRMs produced by VVCM increased as the ratio of RCA to RAP and cement content increased. Meanwhile, the OWC decreased and the MDD increase as the virgin aggregate content increased. The UCS, splitting strength and resilient modulus of the CCRMs increased as the ratios of RCB to RAP, virgin aggregate contents and curing times increased. The UCS and splitting strength showed good linear relationship, and the same observation was made between UCS and resilient modulus. The strength of CCMR decreased with increasing RAP content and increased with increasing virgin aggregate content. This phenomenon resulted from (1) the embedding of asphalt particles into the cement and its comprehensive strength being lower than that of the aggregates or cement; and (2) the presence of gaps between asphalt and cement or aggregate, which destroyed the three-dimensional network structure. Acknowledgments This paper was based in part upon work supported by the National Natural Science Foundation of China under Project No. 51408044, the China Postdoctoral Science Foundation under Project No. 2013M532004 and the Special Fund for Basic Scientific Research of Central Colleges of China for Chang’an University under Project No. 0009-2014G1211007. Those financial supports are greatly appreciated.
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