Materials and Structures DOI 10.1617/s11527-013-0127-0
ORIGINAL ARTICLE
Laboratory study of the effect of RAP conditioning on the mechanical properties of hot mix asphalt containing RAP Asmaa Basueny • Daniel Perraton • Alan Carter
Received: 10 July 2012 / Accepted: 14 June 2013 RILEM 2013
Abstract This paper evaluates the effect of reclaimed asphalt pavement (RAP) laboratory conditioning on the rheological properties of recycled hot-mix asphalt. Four different conditioning processes were used on a single RAP source before mixing: unheated RAP, RAP heated at 110 C in a microwave, RAP heated in a covered pan at 110 C in a draft oven, and RAP heated in a non-covered pan at 110 C in a draft oven. Dense graded 20 mm HMA was designed using a PG 64-28 binder and mixed with 25 % of the four different conditioned RAPs. Thermal stress restrained specimen test (TSRST) and complex modulus test were used to characterize RAP conditioning effect. Test results showed that the complex modulus of the four mixes has no different rheological behaviour, and did not affect TSRST results as much. Keywords Reclaimed asphalt pavement Recycling Binders Complex modulus TSRST RAP conditioning
1 Introduction Reclaimed asphalt pavement (RAP) has been used in hot-mix asphalt (HMA) since the 1930s. Unlike with A. Basueny D. Perraton A. Carter (&) De´partement du Ge´nie de la Construction, E´cole de Technologie Supe´rieure, 1100 Notre-Dame Ouest, Montreal, QC H3C 1K3, Canada e-mail:
[email protected]
crushed Portland cement concrete or recycled aggregates, the possibility of using the old asphalt binder in the newly blended mixtures and, therefore, reducing the required new asphalt content (virgin binder) make the use of RAP in HMA economically attractive [9]. The mid-90s saw the start of the implementation of the Superpave mix design method. The original specifications for Superpave did not include guidance on how to integrate RAP into the new mix design system. Interim recommendations were developed through the FHWA Asphalt Mixture Expert Task Group [7] based on experience and on the performance of Marshall’s mixes with RAP. The specifications were changed in 2002 after the results of an NCHRP research project, Incorporation of RAP in the Superpave System, became available [15]. AASHTO Standards MP2 (now M323), standard specification for Superpave volumetric mix design for hot mix asphalt, describe how to design HMA with RAP. The guiding principle of the AASHTO standard is that mixtures with and without RAP should satisfy the same requirements. The aggregates provided by RAP are included in the determination of the gradation of the mixture and in the consensus properties (except for the sand equivalent value, which is waived because of the inability to test). The bitumen contained in RAP is regarded as part of the total binder content of the mixture. Numerous research studies have been reported in the literature on the laboratory performance, field performance and pavement design of virgin asphalt
Materials and Structures
mix (i.e., mix containing virgin binder and new aggregates) [15, 21]. However, published studies on the effect of RAP conditioning on hot mix asphalt are rather scanty. When mixing materials in the laboratory, the goal is to obtain properties that are representative of the material produced in a batch plant. However, no clear recommendations are proposed on how it should be done, at least based on scientific evaluation. A review of the Europe standard method (EN 12697-35) on how to prepare specimens of recycled mixtures in the laboratory for testing provided that the RAP materials are preheated in an oven at 110 ± 5 C for 2.5 ? 0.5 h, however, this specification does not state whether or not the RAP materials should be covered during this short-term aging procedure. Since asphalt binders react with oxygen from the environment to create oxidation [11], and become more brittle (age hardening), it is not certain whether having the mixes covered (or not covered) when inserted an oven has an influence on the characteristics of the mixes. Moreover, the Europe standard which is considered the only existing guideline that specify how to add RAP in the laboratory, it does not state if the RAP can be heated also in a microwave oven or added cold as it done in some plants in the field. Since RAP contains a certain amount of binder, the RAP conditioning process must be controlled in order to ensure that the characteristics in the lab are similar to those observed in the field. It should be noted that in the field, most HMA with the addition of RAP are produced by different hot mix asphalt plants which dominate the market nowadays such as batch plants with a separate heating drum (hot addition), batch plants without a separate heating drum (cold addition), and drum mixers. RAP is usually added cold and becomes hot by contact with overheated aggregates.
2 Objective The main objective of this study is to evaluate, based on a rheological perspective, the laboratory performance of hot mix asphalt containing 25 % RAP mixed in a laboratory with four different RAP conditioning processes of the same source of RAP. Thus, to evaluate whether RAP conditioning does have an impact on the characteristics of HMA containing RAP, a decision was made to prepare samples in the
laboratory, with three different conditioning processes: (1) add RAP cold (unheated: UH); (2) add RAP heated in an oven, but covered (heated-covered: HC), and (3) add RAP heated in an oven and not covered (heated not covered: HNC). In addition, It has been demonstrated that using microwaves for heating asphalt mixtures is fast, deep, and uniform [1]. Therefore, a fourth conditioning process was investigated: RAP heated in Microwave (HM). Since thermal cracking is considered a common phenomenon in cold regions [20] and a recognized problem with pavements in Canada, it was decided that our mixes would be evaluated for low temperature cracking resistance. Complex modulus testing was used to characterize our mixes. Research described elsewhere has shown that the dynamic modulus of asphalt mixtures is associated with major distresses such as fatigue and low temperature cracking [12]. Moreover, findings in the literature have shown that complex modulus has been widely applied in investigating the effects of RAP content and RAP type on the mechanical properties of HMA [8]. Alvarez et al. [2] showed that the complex modulus varies with the mode of introduction of RAP into the mix plant. It is important to verify whether the RAP conditioning method does not significantly modify the recycled HMA properties. This verification was accomplished by the evaluating engineering properties, such as low temperature cracking resistance with the TSRST, and linear viscoelastic (LVE) properties with the complex modulus (stress–strain distribution effect on pavement).
3 Properties of hot mix asphalt with RAP Daniel and Lachance [4] found that, as expected, adding 15 % RAP increased the stiffness of the mixture, and that mixtures containing 25 and 40 % RAP did not follow the expected trends. Instead, the norm of the complex modulus curves was similar to that of the control mixture. On the other hand, other researchers have found a contrasting tendency for complex modulus test results. For example, Shah et al. [23] observed no increases in stiffness with the addition of 15 % RAP, compared with the control mix, while the addition of 25 and 40 % RAP resulted in an increase in the stiffness of the mix.
Materials and Structures
Huang et al. [9] conducted a study on the laboratory fatigue characteristics of asphalt surface mixtures containing screened RAP. They found, especially when the RAP content represented less than 30 % of the total mix, that adding RAP generally improves the fatigue performance of asphalt mixtures. This conclusion appears to contradict the common belief—the more RAP, the more brittle the mixture, thus lower the fatigue resistance. However, Similar results were reported by Sargious and Mushule [22], and results were even supported from the NCHRP 9–12 study by some of the results of fatigue tests [15]. Other studies have shown the lack of consistency of results seen in past research. Tam et al. [25] found that mixtures with RAP are less resistant to low-temperature cracking than are non-recycled mixtures, while Kandhal et al. [12] found no significant difference in cracking performance in both cases. Sargious and Mushule [22] found that recycled mixes perform better than virgin mixtures with respect to thermal cracking. In general, most studies on laboratory-produced mixtures conclude that recycled mixes exhibit greater resistance to rutting than virgin mixes [12, 13]. From field studies, the rutting performance of recycled mixes has been found to be better than for virgin mixes [24], while other studies show no significant differences between the rutting behaviour of recycled and virgin mixes [15]. It can be noted from the above discussions that the performance of recycled mixes in terms of stiffness, fatigue, thermal cracking or rutting could be better, worse, or similar to that of the corresponding virgin mix. For this reason, the authors of the present paper believe that the RAP conditioning method could lead to variations in results. A testing program was conducted to evaluate the impact of RAP heating conditioning process on the rheological behaviour of recycling asphalt mixtures.
4 Test materials and methods In this experimental program, a dense graded 20 mm HMA commonly used as a base course in Quebec (GB20) was designed with a PG 64-28 binder and 25 % of conditioned RAP. The design asphalt binder grade (PG 64–28) was used in this study with no decrease by one increment on both the high- and lowtemperature grades as it is suggested by Kandhal and
Foo (2008) when using RAP percentage between 15 and 25. Experimental results of a study conducting by Shah et al. [23] indicated that the binder grade PG 64-22 can be used for RAP contents up to 40 %. Additional research concluded that a significant increase in the stiffness of the mix was observed at high, intermediate and low temperatures when no change to the virgin binder was made for only the higher RAP contents ([40 %) [14, 15]. Also, TSRST is affected by the type of binder. It well known that the low temperature of the bitumen grade (PG-L) is related to the fracture temperature of asphalt mixtures. Hence, our mixtures with binder grade PG 64-28 will be evaluated for thermal cracking. The choice of the amount of RAP to add is based on the fact that 25 % RAP is routinely added in HMA in Canada. A single source of RAP materials was used. The RAP was tested for specific gravity and asphalt content as well as gradation (LC 21-040 standard). The RAP aggregates were recovered using two methods: solvent extraction and the ignition oven method. The RAP source had a nominal maximum aggregate size of 10 mm and an asphalt content of 3.9 % [percent by the total mass of mixture (m/m)]. The selected virgin binder (PG 64-28) is a medium grade asphalt binder that can be used in warm climates. The mixing temperature for the virgin binder is 155 ? 2 C, and the corresponding compaction temperature is 145 ? 2 C (LC 26-003). It is to be noted that the LC method is a standard method used by MTQ, Canada. Five different classes of virgin aggregates were selected to produce the GB20 asphalt mixtures. Gradations and other properties of the virgin aggregates are shown in Table 1. The particular aggregates were selected based on past experience. The main purpose of this study was to show whether the initial RAP temperature and virgin aggregate temperature (with the same final temperature of the mixture after production at 160 C) have an influence on the rheological properties of the final mix. Four conditioning were carried out with adding RAP material at two different temperatures: 25 and 110 C. The first temperature corresponds to the usual cold introduction of RAP in the plant mixer. The second temperature corresponds to a temperature specified in the standard EN 12697-35. Table 2 presented initial temperatures of the RAP, virgin aggregate and virgin binder during the production of HMA in this laboratory investigation.
Materials and Structures Table 1 Properties of virgin aggregates (reported by supplier)
d/D Mineralogy Sieve size
10/20 5/10 Granitic Granitic Percent passing
0/5 Limestone
2.5/5 Granitic
Filler Limestone
28 mm
100
100
100
100
100
20 mm
96
100
100
100
100
14 mm
49
100
100
100
100
10 mm
12
92
100
100
100
5 mm
2.0
9.0
94
90
100
2.5 mm
1.0
3.0
58
2.0
100
1.25 mm
0.0
2.0
29
1.0
100
630 lm
0.0
2.0
14
1.0
100
315 lm
0.0
2.0
10
1.0
100
160 lm
0.0
1.0
8.0
1.0
100
80 lm
0.8
0.7
7.0
0.4
98
% Absorption (m/m)
1.0
1.4
0.6
1.4
1.0
Bulk specific gravity
2.867
2.842
2.766
2.762
2.700
For the microwave heating process, a 700 W microwave oven was used. The following steps were done to heat our sample from room temperature to 110 C in the microwave oven: • •
•
• •
Place the sample in the container. Place the container in the microwave oven and heat for a specific time. For our case, we use 5 min at the beginning. When the microwave oven stops, remove RAP from the oven and stir it for few seconds. After that, measure the temperature. Return the container to the oven and reheat for five more minutes. Repeat the process until to reach to 110 C.
It was found that the required time to heat our design mass is 20 min. When cold RAP is added, the new aggregate is superheated up to 300 C in the oven prior to mixing
with the RAP (Table 2). The cold RAP materials is added to the superheated aggregates and mixed before adding the virgin binder, and finally, the components are blended until the aggregate is thoroughly coated by the binder. When hot RAPs are incorporated (110 C), the virgin aggregates are heated to 25 C above the virgin binder mixing temperature prior to the mixing with RAP and virgin binder. 4.1 Characterization of the RAP material used 4.1.1 RAP material preparation For this research project 200 kg of RAP was sampled at an asphalt plant near Montre´al. The sampling was done in accordance with the testing method LC 21-010: Sample rate of The Ministe`re des Transport du Que´bec (MTQ). Afterwards, the RAP was stored in 30 kg buckets in the laboratory.
Table 2 Initial temperatures of RAP, virgin materials, and final mixture during HMA production in laboratory Unheated (UH)
Microwave (HM)
Heated cover (HC)
Heated non-cover (HNC)
T virgin aggregates (C)
300
T RAP (C)
25a (24 h)
180
180
180
110a (20 min)
110a (3 h)
110a (3 h)
T virgin binder (C)
155
155
155
155
T in a mixer (C)
160
160
160
160
a
The mass of RAP required for preparing a slab is about 6,300 g
Materials and Structures
In the laboratory, two bucket of 30 kg each was homogenized by mixing with the aid of a concrete mixer (Fig. 1). The material was then deposited on a clean, non-absorbent surface. The material was homogenized manually with a square head shovel before being divided by quartering in sample mass reduced (see Fig. 1). The separated samples were placed in sealed plastic bucket until when they are needed for different laboratory tests.
Table 3 RAP binder content by ignition and solvent method
4.1.2 Extraction of RAP binder and aggregates
oven-dried at 50 C before the sieve analysis. The RAP material gradation is shown in Table 4. It can be seen that the RAP contains 43 % coarse aggregates, retained on the 5 mm sieve, less than 1 % of particles passing on the 80 lm sieve before extraction and a nominal maximum size aggregate (NMSA) of 10 mm. The gradation of the aggregate part of the RAP was also measured after bitumen extraction, ignition and solvent, and the results are presented in Table 4, and graphically in Fig. 2a. The results support two tendencies in the gradation: (a) the intermediate sieve sizes (5, 2.5, 1.25, 0.630 mm) have more variability than other sieves, and (b) there is a higher amount of fines particles after extraction than before extraction. The purpose of the comparison shown in Table 4 and Fig. 2a is to identify how the asphalt cement
The asphalt binder content of the RAP was determined using two methods: ignition oven method (LC 26-006) and solvent extraction (LC 26-100). Table 3 illustrates the asphalt contents obtained from each of the extraction methods. The mean values for the asphalt binder content resulting from both the ignition and solvent extraction methods were close to the technical data provided by the supplier. Since the supplier’s binder content falls in between our lab results, their binder content was used in the mix design. The as-received gradation (before extraction) of the RAP material was determined using the LC 21-040 method. Homogenized RAP material was split to obtain eight 1,000-g samples, and each sample was
Fig. 1 RAP material preparation
From ignition (repetition: n = 3) % Asphalt binder (m/m)
From solvent (repetition: n = 2) SD
% Asphalt binder (m/m)
SD
0.006
3.66
N.A.
Asphalt binder content 3.90
Materials and Structures Table 4 Comparison of gradation before and after extraction with ignition oven and solvent method
Sieve size
RAP aggregates RAP-material ‘‘As Received’’ (repetition: n = 8)
Recovered from ignition (repetition: n = 4)
Recovered from solvent (repetition: n = 3)
% passing
SD
% passing
SD
% passing
SD
14 mm
100
0.00
100
0.00
100
0.00
10 mm
98
0.93
98
0.50
99
0.58
5 mm
57
5.21
71
2.89
69
4.00
2.5 mm
34
4.60
52
2.87
51
4.51
1.25 mm
20
2.70
41
1.73
40
3.51
630 lm
11
1.64
32
0.82
31
2.08
315 lm
4.1
0.82
23
0.96
23
1.53
160 lm
1.3
0.16
16
0.58
15
0.58
80 lm
0.4
0.05
10
0.21
10
0.61
needed for the mixtures. As the extracted asphalt binder leaves the RAP aggregates, the gradation curve tends to shift to a smaller size (to the left). The shape of the gradation curve remains the same since the asphalt binder itself has no gradation, and the components of the RAP which could constitute a gradation are left behind after extraction. The only difference is that the
present in the RAP material could affect the gradation of recycled asphalt mixtures. It is assumed that as RAP is added to a hot mix asphalt mixture, the asphalt binder of the RAP will tend to separate and disperse in the mix because of heat. Consequently, the added virgin aggregate will receive some coating, and basically, the added RAP reduces the virgin asphalt
(A)
(B)
500 mm
Microwave
Covered
Non-Covered
180 mm
Cold
100 mm
(C) Fig. 2 Representation of our mix design results and its production, a gradation of average samples of RAP material and RAP-aggregate, b 20 mm RAP mixture gradation,
(D) c compacted slabs with different RAP conditioning before coring, d cylindrical cored sample
Materials and Structures
particles are no longer coated, and are thus smaller by approximately one sieve size [19]. In addition, it can be stated that this before and after extraction curves represent the extremes that will occur when the RAP material is added to a hot mix asphalt. For this study, the after extraction gradation was used for the mix design. That is because, as explained previously, the RAP particles are separated to smaller sizes and produce more fine particles because of heating, and that should be considered in the gradation curve of the final mix. The produced fine aggregates will dramatically affect the volumetric characteristics of the resulting mix such as increasing the air void content in the case of it did not take into account in the mix design. Problem of compacting using gyratory compacted will be appeared, the required air void at the number of gyrations 10, 120, and 200 will never be achieved to be in the limit of the specification. Briefly, it is improbable that using the gradation of RAP before extraction would be able to meet gradation and volumetric requirements of the mix design. 4.1.3 RAP aggregate specific gravity It is important to obtain the bulk specific gravity (Gsb) of the combined aggregates because it is one of the inputs for calculating voids in mineral aggregate (VMA). The Gsb of the combined aggregates is determined from specific gravities tests conducted on samples from each component in the mixture. The Gsb of the RAP aggregate was estimated using the recommended methodology in NCHRP Report 452 [16], Recommended Use of Reclaimed Asphalt Pavement in Superpave Mix Design Method: Technical manual. The estimated Gsb values were calculated from a maximum specific gravity (Gmm) tests on the RAP samples (the Gmm method). In the Gmm method, the RAP Gmm and the asphalt content of the RAP were used to estimate Gsb. The effective specific gravity of the RAP aggregate (Gse) can be calculated based on the Gmm and asphalt content values as follow: Gse ¼
100 Pb Pb 100 Gmm Gb
ð1Þ
where, Pb is the RAP binder content, percent by total mass of mixture; and Gb is the specific gravity of RAP binder (assumed to be 1,020 in this study)
This Gse is used to calculate Gsb as follows: Gsb ¼
Gse
Pba Gse 100Gb
ð2Þ þ1
where, Pba is the absorbed binder, percent by Gsb weight of aggregate (assumed to be 1.4 % in this study this percent presents about 63 % of the typical water absorption value of the aggregate). The RAP materials maximum specific gravity (Gmm) was measured by performing the Gmm test (LC 26-045) and it was found to be 2.602. Then, the estimated Gsb of the RAP aggregate can be calculated to be 2.679. 4.1.4 Properties of recovered asphalt binder The binder was extracted from ‘‘as-received’’ RAP using the procedure recommended in Quebec’s standard LC 25-001. 200 grams was recovered using the binder recovery process. The properties of the RAP binder were evaluated using selected Superpave binder test procedures including the penetration test (ASTM test methods D5-06e1), the viscosity test (ASTM test methods D4402-06), the dynamic shear rheometer (DSR) test (AASHTO TP5-98), and the bending beam rheometer (BBR) test (AASHTO test method T 313). The penetration test was conducted to measure the consistency of RAP binder. The viscosity test results were performed to establish the mixing and compaction temperatures and for grading of hot mix asphalt mixtures. The DSR test was used to characterize asphalt binder properties at high and intermediate service temperatures, while he bending beam rheometer (BBR) test was used to characterize it at low service temperatures. In this study, the DSR test was performed at 76 and 82 C, in an attempt to estimate the SUPERPAVE performance grade of the RAP binder. The actual strain and torque were measured and used to calculate various LVE parameters, including the norm of the complex modulus, jG j, and phase angle, d. These test values are commonly used to calculate two measures: the rutting factor jG j= sin d, and the fatigue cracking factor, jG j sin d. Test temperatures used for the bending beam rheometer (BBR) test were -18 C, and -24 C. Superpave binder specification includes a maximum of 300 MPa for creep stiffness, and the decrease in
Materials and Structures
stiffness leads to smaller tensile stresses in the asphalt binder and less chance for low temperature cracking [26]. The results of the penetration test show that the penetration value of the RAP binder is 21 (0.1 mm). The rational viscometer test results presented in Tables 5 shows that the RAP binder has a viscosity of 2.586 and 0.471 Pa.s at 135 and 165 C, respectively. The results of the DSR test are shown in Table 6. The G*/sin d value measured using DSR at 76 C was found to be 4.0 kPa. The RAP binder would have an estimated high temperature grade of PG82 as the binder meet the requirements of G*/sin d [ 2.2 kPa at the temperature 76 C. Table 7 shows the results of the BBR test. From this table, it can be seen that the stiffness value for the RAP binder is 141 MPa. The measured creep stiffness value meet the 300 MPa maximum requirement of MP1 for binders to satisfy a low temperature grade of -18 C. Also, the measured
Table 5 Viscosity of recovery bitumen binder of RAP as it is (UH) Test temperature (C)
Speed (RPM)
Torque (%)
Viscosity (mPa.s)
Average viscosity (mPa.s)
135
12
64.3
2,679
2,586.0
135
12
61.8
2,579
135
12
60.0
2,500
140
12
45.8
1,908
140
12
43.4
1,808
140 145
12 12
43.2 35.0
1,800 1,475
145
12
32.3
1,345
145
12
31.8
1,325
150
12
24.0
1,000
150
12
23.5
979
150
12
23.5
979
155
12
18.0
750
155
12
18.0
750
155
12
18.0
750
160
12
14.1
588
160
12
14.1
588
160
12
14.1
588
165
12
11.3
471
165
12
11.3
471
165
12
11.3
471
m values are lower than the 0.300 minimum established to fulfill the same grade requirement.
5 Recycled asphalt concrete mixes RAP material, virgin asphalt, and virgin aggregate were proportioned to produce dense graded 20 mm HMA (GB20) mix design. In the present study, the procedures described in LC 26-045 (Determination of the maximum density) and LC 26-003 (Determination of the ability of compaction of hot mix asphalt by means of the gyratory compactor) regarding the preparation of HMA specimens were followed. Table 8 gives all procedures used to introduce the RAP material in the recycled HMA mixes. The properties of the produced mixtures are summarized in Table 9. The voids in mineral aggregates (VMA) and air voids of mixes calculated from the Superpave gyratory compactor results at 200 gyrations ranged from 12.3 to 14.5 % and from 2.2 to 4.8 % (v/v), respectively, and the gradations were consistent. The gradations of all the 20 mm recycled asphalt mixtures are illustrated in Fig. 2b; as can be seen in the figure, the design gradation for the produced recycling asphalt mixtures did not violate the Superpave control points recommended for use in the LC method. All mixes meet LC requirements for the mix design.
1,838.7
6 Sample preparation 1,382.0
986.0
750.0
588.0
471.0
Four parallelepiped slabs, 500 mm wide by 180 mm long by 100 mm in height were prepared for each mix, and compacted in the laboratory using an MLPC slab compactor with 5 % target air voids (Fig. 2c). These slabs were then cut and cored to prepare specimens for TSRST and complex modulus testing. For the TSRST, the specimens were 60 mm in diameter, and were sawcut to a final height of 250 mm according to AASHTO designation TP10. For complex modulus tests, the specimens were 75 mm in diameter and saw-cut to a final height of 120 mm (ASSHTO TP 62-03). Two specimens having a diameter of 75 mm and two of 60 mm were extracted from each slab, as shown in Fig. 2d. The cored specimens were glued to two aluminum plates for mechanical connection before the TSRST and complex modulus tests.
Materials and Structures Table 6 DSR test results of recovery bitumen binder of RAP as it is (UH) Replicate number
Temperature (C)
G*/sin d (kPa)
Phase angle, d, ()
Min allow G*/sin d
Test status
1
76
4.0915
71.3
2.2
Passed
2
82
2.1087
73.8
2.2
Failed
Table 7 BBR test results of recovery bitumen binder of RAP as it is (UH) Replicate number
Temperaturea (C)
Creep stiffness, S (60)
m value m (60)
Average S (60) (SD)
Average m (60) (SD)
1
-18
133
0.320
141
0.326
2
-18
149
0.331
1
-24
313
0.292
0.300
Failed
2
-24
259
0.291
0.300
Passed
3
-24
260
0.287
0.300
Passed
a
259.5 (30.892)
0.290 (0.003)
Min allow
m (60) test results
0.300
Passed
0.300
Passed
Fluid bath temperature at 60 s; target test temperature = -18 C and -24 C
Table 8 Recycled asphalt mixtures [GB20 HMA; PG 64-28; 25 % of RAP; asphalt content of 4.5 % (m/m); virgin binder of 3.5 % (m/m)] RAP conditioning
Designation
Mixing
Methoda
Unheated
UH
Cold RAP ? superheated virgin aggregates 300 C ? asphalt binder, 155 C
Cold RAP was mixed with virgin aggregates superheated to 300 C. The asphalt binder, heated at 155 C, was added when the blended temperature of RAP material and virgin aggregates reach the mixing temperature of 180 C
Heated in microwave
HM
Heated RAP in a microwave 110 C ? heated virgin aggregates, 180 C ? asphalt binder, 155 C
Heated RAP in a microwave up to 110 C was mixed with virgin aggregates heated to 180 C. After mixing, the asphalt binder heated at 155 C was added and then mixed
Heated in a covered pan in a forced draft oven
HC
Heated RAP in a non-covered pan in a forced draft oven, 110 C ? heated virgin aggregates, 180 C ? asphalt binder, 155 C
Heated RAP to 110 C in a covered pan in the oven and heated virgin aggregates to 180 C were mixed before mixing with the heated asphalt binder at 155 C
Heated in a noncovered pan in a forced draft oven
HNC
Heated RAP in a covered pan in a forced draft oven, 110 C ? heated virgin aggregates, 180 C ? asphalt binder, 155 C
Heated RAP to 110 C in a non-covered pan in the oven and heated virgin aggregates to 180 C were mixed and then the asphalt binder heated at 155 C was added and then mixed
a
The mixture of RAP, virgin aggregates, and virgin binder were mixed for 2 min. Finally the mixture was heated in a pan noncovered for 2 h in order to maintain the target temperature for compacting
For this experimental program, two other replicates for only the mix of heated microwave (HM) were produced and labeled as mix No. 2 and mix No. 3, respectively. Samples from the second mix (No. 2) and the third mix (No. 3) have very high air content as compared to samples from the first mix (No. 1). The difference in air voids is being
probably due to the variability of the RAP material, as discussed in this paper. Two samples of each mix were prepared and also exhibited higher air void ratios [5.7 % (v/v) B air voids B 7.3 % (v/v)]. Each sample was subjected to the TSRST in order to investigate the effect of air voids on the TSRST results.
Materials and Structures Table 9 Mixture properties Mix No. 1 (UH)
Mix No. 1 (HM)
Mix No. 1 (HC)
Mix No. 1 (HNC)
LC standard GB20
28 mm
100
100
100
100
100
20 mm
99
99
99
99
95–100
14 mm
82
82
82
82
67–90
10 mm
66
66
66
66
52–75
5 mm
40
40
40
40
35–50
2.5 mm
25
25
25
25
–
1.25 mm
18
18
18
18
–
630 lm
13
13
13
13
–
315 lm
11
11
11
11
–
160 lm 80 lm
8.3 6.8
8.3 6.8
8.3 6.8
8.3 6.8
– 4.0–8.0
RAP conditioning
Unheated
Heated in microwave, 110 C
Heated in a covered pan in the oven, 110 C
Heated in a non-covered pan in the oven, 110 C
N.A.
% Total binder content (m/m)
4.5
4.5
4.5
4.5
4.5b
3.5
Grading–percent passing
% Virgin binder (m/m)
3.5
3.5
3.5
Binder grade
PG 64-28
PG 64-28
PG 64-28
PG 64-28
% RAP binder (m/m)
1.0
1.0
1.0
1.0
% RAP binder content (m/m) Maximum specific gravity (Gmm)
3.9 2.622
3.9 2.617
3.9 2.612
3.9 2.613
Bulk specific gravity (Gmb)
2.517
2.492
2.521
2.550
Effective bitumen content: Vbe (%)a
10.2
10.2
10.2
10.2
Voids in mineral aggregate: VMA (% of bulk volume)
13.9
14.5
13.4
12.3
% Voids filled with asphalt: VFA (% of VMA)
70.5
66.9
73.6
80.7
N.A.
10.2
Voids at Superpave gyratory compactor (% of total volume) 10 gyrations
16.7
16.7
14.9
14.4
120 gyrations
5.6
6.4
5.0
4.0
4–7
200 gyrations
4.0
4.8
3.5
2.2
C2
a
Expressed by part of volume of the HMA without void
b
The main target for the MTQ regulation is to reach the prober Vbe
7 Test equipment and procedure 7.1 Tensile stress specimen test (TSRST) In this project, the thermal stress restrained specimen test (TSRST) was used to evaluate the lowtemperature-cracking resistance of asphalt mixtures.
C11
The principle underlying the test system is to keep the length of the specimen constant during cooling. A cylindrical specimen is mounted in the load frame enclosed by the cooling cabinet. As the temperature decreases, the thermal stress inside the sample increases, until the specimen ultimately breaks.
Materials and Structures
7.2 TSRST results
ru ¼ r0 ð1 PÞ3
TSRST test results include the fracture temperature, fracture stress, slope of the thermally induced stress, and the transition temperature as shown in Fig. 3a. At the beginning of the test, there is a relatively slow increase in thermal stress due to a relaxation of the asphalt mixture. However, beyond a given temperature, known as the transition temperature, the relationship between the thermally induced stresses and the temperature is approximately linear. The transition temperature is defined as the temperature where the material changes from elastic to visco-elastic behaviour, or vice versa [10]. The transition temperature and the slope of the stress–temperature curve below the transition temperature (slope 2) may play a major role in characterising the rheological behaviour of the asphalt mixtures at low temperatures. Only the fracture stress, the fracture temperature and the slope are discussed below. The tensile stress and the temperature at the breaking point represent the fracture stress and the fracture temperature, respectively. Stress–temperature curves for all samples tested under TSRST are presented in Fig. 3b, c, and a summary of the results are reported in Table 10. In general, the fracture temperature (Tf) observed for the mixture tested exhibited minor variability between replicates. Generally, the following results are observed for the first mix (No. 1): (1) the average fracture strength and fracture temperature for UH specimens are 5.0 MPa and -32.8 C, respectively; (2) for the HM specimen, the fracture strength and the fracture temperature are 5.9 MPa and -35.3 C, respectively; (3) for the HC mix, the fracture strength and the fracture temperature are 5.0 MPa and -32.3 C, respectively; (4) for the HNC mix, the fracture strength and the fracture temperature are 5.5 MPa and -35.3 C, respectively. The fracture temperature is a good indicator of the low temperature performance of laboratory-produced mixes. No significant variation was found in the results, indicating that RAP conditioning has a limited effect on low temperature behaviour. To compare samples of different air voids, the parameter r0, which is the strength at zero porosity, was used. The r0 will give an indication if the ranking between mixes will change or not. To adjust the strength according to the porosity (air voids) of the mixes, the following equation was used [27]:
where ru and P represent strength and porosity. The r0 calculated for each replicate is illustrated in Fig. 4a and reported in Table 10. In Fig. 4, the values between parentheses indicate the air voids content (v/v) in each sample. As shown in Fig. 4a, by considering the calculated (r0) values, we found that there is no difference between conditioning processes. For analyzing the results statistically, STATGRAPHICS Centurion XVI program was used. The two sample comparing procedure test was used to determine whether or not there is a significant difference between the fracture temperatures (t test). The p value is the probability of obtaining a test static (or data point) that is significantly different from the null hypothesis. When the computed p value is less than 0.05, we can reject the null hypothesis because the differences observed in the mean values are true errors or differences and not due to random sampling errors. Results of the two sample t-tests on the fracture temperatures of the RAP mixes samples showed that no significant differences between the four RAP conditioning mixes: UH, HM, HC, HNC, as shown in Table 11. The fracture temperatures of the RAP mixes produced with different conditioning appeared to be similar.
ð3Þ
7.2.1 Slopes of stress–temperature curves In order to determine, as precisely as possible, the value of the slope 2, we will truncate the experimental values by keeping only the data better representing the linearity of the stress–temperature curve at the end of the test associated to the failure point. As a first step, we subtract the experimental data associated with the beginning of the test in the range of temperatures from 5 to -10 C. Subsequently, we determine the evolution of the coefficient of regression (R2) by reducing progressively the data from -10 C, until the calculated value of R2 reach a maximum. At the end of the process, we calculate the value of the slope 2 and report it in Table 10. In the same manner, to fix the value of the slope 1, we subtract the experimental data associated with the end of the test and keep only the data that are representative of the linearity at the beginning of the test (in the range of temperatures from 5 to -20 C). Then, we still determine the R2 of
Materials and Structures Fig. 3 TSRST test results, a typical results, b TSRST thermal stresses for all tested samples of the first mix (No. 1), c TSRST thermal stresses for tested samples of HM mix produced by mix No. 1, mix No. 2, and mix No. 3
the linear approximation after reducing progressively the filtered sampling data from -20 C, until the calculated R2 reaches a to the maximum. Then, the value of the slope 1 is calculated and reported in Table 10. Figure 4b, c shows slope 1 and slope 2 for all samples tested in this study, respectively. Slope 1
values range from -0.011 to -0.015 (MPa/C), which reveals no clear trend between fracture stress and fracture temperature regarding RAP conditioning method. Nevertheless, slope 1 is an indicator of the stress relaxation potential of the mix: lower is their absolute value, higher is their relaxation potential. Curiously, an asphalt mixture prepared with RAP
-33.8 -32.8 190 from top
Mean fracture stress (MPa)
Fracture temperature (C) Mean fracture temperature (C)
Fracture position (mm)
-0.295 -24.0 -16.1
Slope 1 (MPa/C)
Slope 2 (MPa/C)
Transition temperature (C)
Cross temperature (C)
a
-17.2
-25.0
-0.302
-0.015
5.1
1.9
30 from top
-31.9
4.8
1.9
2
Sample No. 2 (mix No. 1) was broken during test
P porosity, r0 strength at zero porosity
5.35 -0.013
Average r0 (MPa)
1.9
5.0
Fracture stress (MPa)
5.6
5.3
Sample air voids (%)
r0 (MPa)
1.9
Sample number
Sample P (%)
3.9 1
Slab air voids (%)
-17.7
-25.8
-0.317
-0.012
–
6.1
1.1
50 from top
-35.3 -35.3
–
5.9
1.1
1
3.4
Mix No. 1
Mix No. 1
a
HM
UH
Mixture variable ID
-17.9
-25.4
-0.236
-0.011
4.5
4.7
6.6
20 from edge
-33.2 -32.6
3.6
3.8
6.6
1
6.8
Mix No. 2
Table 10 Summary of all conditioning process RAP mixtures TSRST results
-17.2
-24.4
-0.211
-0.012
4.3
7.3
175 from top
-32.1
3.4
7.3
2
-17.1
-25.1
-0.241
-0.012
4.45
4.8
5.7
60 from edge
-32.6 -32.5
3.7
4.0
5.7
1
6.3
Mix No. 3
-17.4
-24.3
-0.212
-0.013
4.1
6.1
50 from edge
-32.4
3.4
6.1
2
-17.1
-24.2
-0.304
-0.014
5.35
5.4
1.4
125 from top
-33.0 -32.3
5.0
5.2
1.4
1
3.8
Mix No. 1
HC
-17.6
-26.1
-0.326
-0.014
5.3
2.4
120 from top
-31.5
4.9
2.4
2
-19.3
-27.7
-0.313
-0.013
5.75
5.8
0.9
Is not clear
-36.2 -35.3
5.5
5.6
0.9
1
3.5
Mix No. 1
HNC
-18.6
-27.6
-0.324
-0.014
5.7
1.7
25 from top
-34.4
5.4
1.7
2
Materials and Structures
Materials and Structures Fig. 4 Other TSRST test results a fracture strength at zero porosity, b slope 1, c slope 2, and d TSRST average fracture temperature for all conditioning RAP mixtures
Materials and Structures Table 11 Two sample t test comparisons of fracture temperatures of 25 % RAP mixtures with different RAP conditioning Pairs tested
p value
Conclusion
UH vs. HM
0.8132
NSD
UH vs. HC
0.6692
NSD
UH vs. HNC
0.2021
NSD
HM vs. HC
0.4406
NSD
HM vs. HNC
0.0977
NSD
HC vs. HNC
0.1213
NSD
NSD No significant difference
conditioned in the microwave (HM) showed a decrease in its ability to relax stresses, which implies that the properties of the bitumen were much more affected. Figure 4c also shows that slope 2 has a relatively wide range (from -0.211 to -0.326 MPa/ C). As expected, the decrease in slope 2 is more related to air void content than to the RAP conditioning process. 7.2.2 Fracture temperatures The relationship between the fracture temperature and RAP conditioning is presented in Fig. 4d. Preliminary TSRST results indicate that there is no significant difference in the fracture temperatures of the four short-term aged conditions. The maximum difference in fracture temperature was observed between the unheated mixture (UH) and the heated non-covered (HNC) mixture, and came in at about 2.5 C. The difference between the unheated and HM (mix No. 1) was also about 2.5 C. The mixture prepared with a RAP material heated in the oven with no cover (HNC) reached a lower fracture temperature than other asphalt mixture tested. The higher fracture temperature recorded in the testing program was associated with the asphalt mixture prepared using the covered RAP material process (HC). Lower fracture temperatures values mean better low temperature behaviour, as shown for the HNC and microwave (HM) mixes. Based on our results, the RAP conditioning could slightly affect the low temperature behaviour of asphalt mixture. Finally, as reported in Table 10, by increasing the air void content, the fracture stress decreased, but it still in the magnitude of the fracture stresses resulting from other samples of low air void content. Moreover,
the effect of fracture temperature thought to be a much lesser extent. 7.3 Complex modulus test The complex modulus test is performed to determine the LVE behaviour of asphalt mixtures at various temperatures and different loading speeds. For viscoelastic materials, such as asphalt mixes, the stress– strain relationship under a continuous sinusoidal loading can be defined by the complex modulus E . The complex modulus is defined as the ratio of the amplitude of the sinusoidal stress of pulsation x applied to the material r ¼ r0 sinðxtÞ and the amplitude of the sinusoidal strain eðtÞ ¼ e0 sinðxt /Þ that results in a steady state [6, 18]: E ¼
r r0 eixt ¼ E1 þ iE2 ¼ e e0 eiðxt/Þ
ð4Þ
where, E1 is the storage modulus and E2 is the loss modulus. The modulus (length of vector) of the complex number is defined as the dynamic modulus jE j, the norm of the complex modulus, where r0 is the maximum stress amplitude and e0 is the peak recoverable strain amplitude: r0 ð5Þ jE j ¼ e0 The complex modulus was evaluated using unconfined uniaxial tension–compression tests, with a servo-hydraulic testing system (MTS 810, TestStar II) used, The specimens were subjected to sinusoidal oscillating axial loading in both tension and compression at constant amplitude (50 9 10-6 m/m), Three extensometers placed 120 apart were used during E testing to improve the accuracy of the results, The tests were performed at different temperatures (-35, -25, -15, -5, 5, 15, 25, 35 C), and at each temperature, a frequency sweep of eight frequencies (20, 10, 3, 1, 0.3, 0.1, 0.03, 0.01 Hz) was done. In our testing program, the norm of complex modulus (jE j) of the produced RAP mixes was determined on two specimens of each mix, with air voids ranging from 1.5 to 3.5 % (2.5 ± 1.0 %), Authors believe that only 1.0 % variation could have a minor effect on the results and it was not far from the current 0.5 % accepted air void variation as mention by [3].
Materials and Structures
7.3.1 Data acquisition and treatment of the signal
fe ¼ aT ðT; Tref Þ fr
At each data acquisition, data is collected for two consecutive cycles. The time interval is adapted to have 100 points per cycle. These experimental points are not exactly on a sine curve due to experimental variations and non-linear behaviour of bituminous mixtures. Experimental data related to the force and the displacement measured by the load cell and the three extensometers, respectively are then treated as approximate sinusoidal curves defined by the following equation:
where aT is the shift factor at temperature T, Tref is the reference temperature: aT ðTref ; Tref Þ ¼ 1, fr is the real frequency of solicitation. For example, the construction of the master curve of the mix UH by the shifting procedure at Tref ¼ 5:8 C is illustrated in Fig. 6a. The shift factor at a temperature T, named aT ðTÞ, used for the construction of the master curve can be determined by means of Eq. (8). For a given reference temperature Tref , the Williams, Landel and Ferry (WLF) Eq. (8) gives log aT as a function of three constants C1 , C2 , T, and Tref ð CÞ [17]:
y ¼ y0 þ yA sinðxt þ /Þ
ð6Þ
Thus, the part of frequency is constant, and the other parameters of the equation are calculated by the least squares method. In the treatment file, the signal of the stress is obtained from the recorded force by dividing this value by the cross section area of the sample. This is regarding to the fact that the test is conducted in homogenous conditions. Moreover, the deformation signal is considered as the average value of the three values obtained from the three extensometers. The data acquisition is stored in blocks of rows in sequence for each pair of cycles of solicitation and in column dedicated to different variables: time, stress, strain, and temperature in an excel file. Using a macro calculation developed with Microsoft excel, the data will be processed by the least squares method to calculate the approximate sinusoidal signals of stress and deformation as the average of the three values obtained from the three extensometers. Then, each treatment gives a point of the result in the treatment file that defines the parameters of approximate signals and the mechanical characteristics of the material, for each pair of cycles analyzed. 7.4 Complex modulus results The results can be plotted on a master curve. To that end, each isothermal curve can be shifted in frequency in order to obtain a single master curve at a reference temperature, Tref . The master curve was plotted as a function of the equivalent frequency fe based on the assumption that the asphalt mixtures exhibit a thermorheologically simple behaviour, which means that the Time–Temperature Superposition Principle (TTSP) is applicable. The expression of fe is given by the following equation [5]:
logðaT Þ ¼
ð7Þ
C1 ðT Tref Þ C2 þ ðT Tref Þ
ð8Þ
7.5 Modulus in the Cole–Cole plane, in the black space and master curve The complex modulus and other parameters, determined at eight temperatures and eight frequencies, can be used to determine nine parameters included in the 2S2P1D rheological model that characterize the asphalt concrete response in the linear visco-elastic domain. The nine parameters are Einf , Ezero , d, k, h, b, s0, C1, and C2, and Table 12 shows the nine of them obtained using the 2S2P1D model for the tests performed on two samples from the four hot recycling mixtures prepared with different RAP processes. The model’s parameters were determined by obtaining the best fit curve for the measured complex modulus values plotted in the Cole–Cole and Black diagrams.
Table 12 Parameter of different mixtures (mean value of two samples tested per mix) Conditioning
UH
HM
HC
HNC
35,950
36,300
34,500
34,800
Parameters 2S2P1d Einfini (MPa) Ezero (MPa)
115
82
120
105
d
1.80
1.85
1.70
1.83
k
0.177
0.177
0.177
0.177
h
0.500
0.544
0.520
0.530
b
500
500
500
500
s0 (s)
0.20
0.20
0.20
0.155
C1
27.88
26.24
21.97
24.16
C2
185.48
175.81
152.53
164.08
Materials and Structures
The fitting of laboratory data according to the Cole– Cole and Black diagrams are shown in Figs. 5b and Fig. 6, respectively. A detailed 2S2P1D model and its parameters are not discussed in this paper, as they have already been presented in the literature by Olard and Di Benedetto [17]. Statistical parameters such as, R2, the standard error of predicted values (se) divided by the standard
deviation of measured values (sy) were used in this study to perform the goodness-of-fit statistics for the 2S2P1D predicted model in arithmetic scale. The R2 parameter is a measure of correlation between the measured and the predicted values, and its function is to determine the level of accuracy of the fitting model, and thus a higher value of R2 is indicative of higher accuracy. Table 13 shows the goodness-of-fit statistics (R2) for the different RAP conditioning mixes. It was
Fig. 5 Representation of some complex modulus test results, a construction of the master curve of jE j at Tref ¼ 5:8 C with the shifting procedures for the mix UH, b Cole–Cole plane for all four mixes, experimental results and 2S2P1D
Materials and Structures Fig. 6 Black space diagram for all four mixes, experimental results and 2S2P1D model
Materials and Structures Table 13 Goodness-of-fit statistical analysis for the four recycled mixtures RAP conditioning mix
se/sy
R2
UH
0.0791
0.9937
HM
0.1134
0.9871
HC
0.0328
0.9989
HNC
0.0820
0.9933
observed R2 = 0.9871–0.9989, and therefore the 2S2P1D model showed a good correlation with the values of jE j of the mixes, This finding indicates that this linear visco-elastic rheological model can predict the bituminous mixes precisely. The test results presented according to the 2S2P1D rheological model effectively discriminate mixes according to their rheological properties. When the asphalt mixtures are tested within their linear viscoelastic domain of behaviour, the results will fit on a single curve in the Cole–Cole plane as well as in the Black space. As can be seen in Figs. 5b and Fig. 7, our results fit well on a single curve, which means the tests were properly performed. However, when the results are shown in a Black space (Fig. 6), a scattering of the results can be seen for high phase angle values, which represent results at high temperatures or low frequencies. This can be attributed to the bitumen type. The PG 64-28 used in this study contains polymer (SBS); it was shown by Olard and Di Benedetto [17] that mixes made with polymer-modified binder sometimes exhibit a behaviour that does not conform to time–temperature superposition principle (TTSP). Even if all results cannot be presented on a single line in a Black space domain, the results are still considered good since they fit on a single curve on a Cole–Cole plane (Fig. 5b) as well as on a single master curve plotted at a reference temperature using a shifting procedure. In this case, this property is called the ‘‘Partial Time–Temperature Superposition Principle’’ (PTTSP), as the shifting procedure gives a unique and continuous master curve only for the norm of the modulus. The master curves for the complex modulus for the four mixes UH, HM, HC, and HNC are shown in Fig. 8a. A visual inspection of the curves indicates that for most frequencies (or temperatures), the dynamic
modulus of the mixtures containing the same percentage of RAP (25 %), and prepared with different conditioning RAP addition processes are similar and do not vary greatly. The highest modulus values are observed for the mixture prepared with the addition of heated-microwave RAP and with the cold process. This trend is for equivalent frequencies higher than 0.1 Hz, and the HC mix is very slight lower in this range. The non-covered RAP condition mixture shows the slight lowest modulus.
7.6 Data analysis for complex modulus By comparing two samples procedure test on the modulus of the RAP conditioning samples, at 3 Hz, and at different three temperatures: 15, 25, and 35 C, results from these t tests showed that the differences in the mean jE j were found to be statistically not significant at 3 Hz and at all test temperatures. Figure 9 shows the jE j values at three test temperatures conducting at 3 Hz. The p values for these two sample t tests are shown in Table 14. Similar statistical analysis were conducted at 20 Hz and at the same three temperatures yielded to almost comparable conclusions, except the mixes (mix HM vs. mix HNC, and mix HC vs. mix HNC) at 25 C, were found to be statistically significant as shown in the low p values in the comparison between them. The p values for the other two sample t tests examined at 20 Hz are shown in Table 14. The average jE j and coefficients of variation (CV) of the replicate specimens for each mixture at each test temperature and at 3 and 20 Hz are shown in Table 15. As shown in this table the CV of the complex modulus data from different RAP conditioning range from 2.2 to 19.5 % and from 1.6 to 26.7 % at 3 and 20 Hz, respectively.
7.7 Conditioning effect coefficient CCEC
In this research, in order to objectively compare the results of complex modulus of mixes with RAP in numbers, a condition effect coefficient, CCEC was calculated. The calculation of the conditioning effect ) was developed by Di Benedetto [5], coefficient (CCEC and is defined as the ratio between the complex
Materials and Structures Fig. 7 Master curves for the four mixes, experimental results and 2S2P1D
modulus of a specific mix, in our case the recycled mixture made with a specific condition, ES:C: (i.e., HM, or HC, or HNC) at the equivalent frequency fe
[defined by Eq. (7)], and the complex modulus of a reference mix, herein the unheated process (UH) at the same frequency fe as written in Eq. (9):
Materials and Structures Fig. 8 Representation of other complex modulus test results for the recycled mixtures prepared by four different RAP processes, a Master curves of the 2S2P1D model (Tref ¼ 5:0 C), b C CEC
versus equivalent frequency (Tref = 5.0 C), c phase angle versus equivalent frequency (Tref ¼ 5:0 C)
ES:C: EUH i:e:; ES:C: can be EHM or EHC or EHNC
CCEC ðf e Þ ¼
Fig. 9 Average complex dynamic modulus of RAP mixes, f = 3 Hz
ð9Þ
is calculated at an equivalent frequency fe. It CCEC means that the complex modulus of the mixture prepared with a specific condition at frequency feSC and complex modulus of the UH at feU:H: must be considered at the same frequency fe ¼ feS:C: ¼ feU:H: for the calculation. CCEC is a complex number, as shown in Eq. (10). Its norm, calculated by Eq. (11), is the ratio of the norms of the complex modulus of the recycled mixture made with a specific condition to the jE j of the mix
Materials and Structures Table 14 Two sample t test results for the norm of the complex modulus Pairs tested
T (C)
UH vs. HM
f = 3 Hz
f = 20 Hz
p value
Conclusion
p value
Conclusion
15
0.6526
NSD
0.6864
NSD
25
0.5883
NSD
0.5675
NSD
35
0.2836
NSD
0.3160
NSD
15
0.7867
NSD
0.8148
NSD
25
0.3569
NSD
0.3435
NSD
35
0.8358
NSD
0.2244
NSD
15
0.3904
NSD
0.4112
NSD
25
0.1095
NSD
0.1329
NSD
35
0.5138
NSD
0.4114
NSD
HM vs. HC
15
0.7631
NSD
0.8448
NSD
25 35
0.4114 0.1294
NSD NSD
0.2250 0.7079
NSD NSD
HM vs. HNC
15
0.3075
NSD
0.3030
NSD
25
0.1862
NSD
0.0268
SD
35
0.4191
NSD
0.5003
NSD
HC vs. HNC
15
0.3050
NSD
0.3382
NSD
25
0.1826
NSD
0.0212
SD
35
0.1766
NSD
0.1856
NSD
UH vs. HC UH vs. HNC
prepared with the unheated RAP (UH) material. Its phase angle, determined by Eq. (12), is the difference between the phase angle of the recycled mixture made with a specific condition and the phase angle of the UH material. Table 15 Average complex dynamic modulus of mixtures
Mix
T (C)
HM
HC
HNC
ð10Þ ð11Þ
/CEC ¼ /ES:C: /EUH
ð12Þ
7.8 Relationship between CCEC and equivalent frequency (fe ¼ aT fr )
The norm of the complex conditioning effect coeffi cient CCEC is plotted for all mixes in Fig. 8b in accordance to the 2S2P1D model. This figure shows for UH (reference mix, so that the norm of CCEC C ¼ 1), HM, HC, and HNC mixtures are very CEC close at high frequencies and/or low temperatures. It value of one mean that should be noted that a CCEC no difference exists between the conditioning processes under comparison. These results are in accordance with the TSRST results presented in the previous section, which indicate no significant difference between the four investigated RAP conditioning situations at low temperatures. Nevertheless, at low frequency, from 1E-4 to 1 Hz, corresponding to high temperature condition, the RAP conditioning could no longer be considered as negligible. Results presented in Fig. 8 seem to show a different tendency of what we found for high frequencies. Noted that, results analysis of Fig. 8 are really tricky by the fact that the modelling process used to get that curves is very sensitive to errors due to the calibration procedures, especially for very low frequencies and high temperatures.
f = 3 Hz jE j (MPa)
UH
iu e CEC ¼ CCEC CCEC ES:C: C ¼ CEC E UH
f = 20 Hz CV for the mean (%)
jE j (MPa)
CV for the mean (%)
15
5,925
6.2
9,833
5.1
25
2,307
4.8
4,769
3.4
35
743
17.0
1,715
12.5
15
6,173
9.1
10,070
5.1
25
2,535
19.5
5,058
12
35
574
18.5
1,327
26.7
15
6,023
4.3
9,962
4.6
25
2,165
2.9
4,547
1.9
35
764
2.2
1,793
7.6
15 25
5,361 1,759
11.9 14.7
9,137 4,041
8.9 1.6
35
664
10.1
1,587
3.3
Materials and Structures
results are shown in Fig. 10. For HM and HC mixes, shows that values slightly ‘‘true’’ values of CCEC = 1.0) like balance around the equality line ( CCEC what we obtained from modelling (Fig. 8b). For HNC values results from experimental data mix, CCEC (‘‘true’’ values) show that the tendency confirms the results obtained from modelling calculations, that the HNC gives always a lower stiffness values than the reference mix (UH). Nevertheless, we found a little disperse tendency between the variation of CCEC from ‘‘true’’ calculated values (Fig. 10) to that from modelling (Fig. 8b). values, obtained Figure 10 shows that the CCEC from experimental data and modelling, for the HM at low frequencies (less than 1 Hz) and high temperature (35 C) seem to be more effective and give us indication that the condition might be significant. Nevertheless, we need to keep in mind that for these condition the stiffness of mixes are very low, less than 300 MPa. At this level, the measurement capacity of the apparatus (load cell precision) and setting of the hydraulic press are not optimized. resulting In overall, our analysis using the CCEC from experimental and modelling data showed that the difference modulus between mixes is low even at low frequencies (high temperatures). 8 Conclusion
values versus frequency at different Fig. 10 The true CCEC high temperatures: 15, 25, and 35 C
As we noted previously, our mixes did not follow the concept of TTSP at high temperatures (low frequencies) and that part could be a sensitive part of our modelling. Therefore, in order to check the accuracy of our modelling results for the three mixtures (HM, HC, HNC), we compute the ‘‘true’’ according to the experimental data values of CCEC obtained for the three high testing temperatures (15, 25, and 35 C) and for all tested frequencies. To do this, we still use the UH mix data, as a baseline. The
This paper investigates the influence of warming RAP materials with different conditions on the mechanical properties of the manufacture recycled asphalt mixtures containing 25 % RAP. Four asphalt mixtures including one RAP source, one RAP content (25 %), one type of binder (PG 64-28), four RAP conditioning processes were investigated in this study. The TSRST has been carried out on all mixtures including two additional mixes with high air voids, and the complex modulus, the key parameter in the mechanistic-empirical design guide, was also conducted on the four different mixes at different temperature and frequencies. Based on the analysis of TSRST laboratory data, the following conclusions are made: •
The low-temperature behaviour of the recycled asphalt mixtures is not significantly affected by RAP conditioning processes. A maximum
Materials and Structures
•
•
•
•
difference of about 2.5 C in fracture temperature was observed between the HNC and the UH mixes. Fracture strength is highly influenced by air void content. Fracture strength was greater for mixes with lower air voids, as compared with those with higher air voids. Also, the air void affects the fracture temperature, albeit to a much smaller extent. The binder grade PG 64-28 can be used for 25 RAP content percent since the fracture temperatures for the four mixed were found to be ranged from -31.5 to -36.2 C, and these values are lower than the low temperature grade of the used virgin binder (-28 C). An analysis of the influence of RAP conditioning on the complex modulus of the recycled mixtures is carried out with the help of the two coefficients, CCEC . The main results are: The modulus differences between the four investigated recycled mixtures are very low over all the frequencies higher than 1.0 Hz whatever the RAP condition process, and were found to be not statistically significant. At low frequency (lower than 1.0 Hz) and/or high temperature, the influence of heated RAP in a pan covered can be considered as low from the results of the C CEC
•
calculated either from the model (13 %) or from the experimental data (5 %). In the same range of frequency (lower than difference between the UH 1.0 Hz), the CCEC mix and the HNC mix are close to 30 %. Nevertheless, its ‘‘true’’ value shows only less than 15 % variation between the two mixes at 35 C but it reach to 25 % at 25 C and it is in the same magnitude of what we get referred to the C
•
The repeatability of complex modulus is quite good, as the coefficient of variation for the jE j data ranged from 1.6 to 27 %.
The main finding of this study is the four proposed conditions of RAP adding to the virgin aggregates (cold, heated in a microwave, heated in an oven in a pan covered, and heated in an oven in a pan non covered) had little effect on the changes in the 25 % RAP added mixtures stiffness and its resistance to thermal cracking. Moreover, this study can answer the question concerning finding the best laboratory RAP heating methods by studying the four proposed RAP conditioning. Based on our results, we can not propose a specific method from the four methods to be used in the laboratory since each method has its advantage and disadvantages from the degree of handling and the required time saving. For example, for RAP added cold, we found that it is a terrible way for handling, since we have to superheat the virgin aggregate. Heating RAP in a microwave is a fast way, but sometimes the size of the specimen could be a problem for some type of microwaves. From our point of view, covered heating RAP in an oven could be the best. Nevertheless the specific time (3 h) for heating is important. Finally, additional research is needed to validate these results for other RAP sources and other RAP contents. Also, it is recommended for RAP source containing polymer to be studied in detail. Acknowledgments This work was funded by the Missions Sectors in Egypt and the E´cole de Technologie Supe´rieure. The authors would like to thank the companies in Quebec that provided us with the materials and with all the needed data for the project.
CEC
from the modelling. It is probably could be explained by the very sensitive accuracy of the model. For the materials made with HM RAP added, it seems to exhibit slightly different with a value equal to 11 % maximum modelling CCEC and its ‘‘true’’ CCEC value obtained from experimental data showed that the difference could be higher or lower this value. But by considering that the stiffness of all the mixes is very low at low frequencies (less than 1.0 Hz), less than 300 MPa, it means that the effect of heating RAP by microwave is also low.
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