EATA 2013, pages 1 to 14 Performance evaluation of cold recycled mixture containing high percentage of reclaimed asphalt A. Stimilli — G. Ferrotti — A. Graziani — F. Canestrari Università Politecnica delle Marche Via Brecce Bianche 60131 Ancona, Italy [email protected][email protected][email protected][email protected]ABSTRACT. Cold Recycling of Asphalt Pavements proved to be an effective maintenance and rehabilitation technology for both environmental and economic reasons. Nevertheless, the use of Cold Recycled asphalt mixtures requires a careful assessment of their mechanical properties, especially when they are designed to replace traditional Hot-Mix Asphalt Concrete mixtures. In this study the potential use of a Cold Recycled (CR) asphalt mixture as base course of an Italian motorway was evaluated. The studied mixture was produced in a central plant employing high RA content and used to construct two experimental sections along an in-service Italian motorway. In particular a special mixing procedure, involving the use of water vapour and bituminous emulsion, was tested. A third experimental section was constructed with the same layer thickness using the Asphalt Concrete (AC) mixture currently used in rehabilitation projects, incorporating 30 % of RA. Volumetric properties, stiffness, resistance to permanent deformation and fatigue behaviour of mixtures were investigated performing tests on samples cored from the three test sections and on laboratory compacted samples. Results of the mechanical tests showed that Cold Recycled mixtures provide lower stiffness modulus and lower resistance to repeated loading, but better resistance to permanent deformation respect to Asphalt Concrete. This behaviour can be explained with the presence of cementitious bonds that reduce thermal sensitivity and viscous response. KEYWORDS: cold recycling, mechanical performance, stiffness modulus, permanent deformation, fracture resistance.
Transcript
EATA 2013, pages 1 to 14
Performance evaluation of cold recycled mixture containing high percentage of reclaimed asphalt A. Stimilli — G. Ferrotti — A. Graziani — F. Canestrari Università Politecnica delle Marche Via Brecce Bianche 60131 Ancona, Italy [email protected][email protected][email protected][email protected]
ABSTRACT. Cold Recycling of Asphalt Pavements proved to be an effective maintenance and rehabilitation technology for both environmental and economic reasons. Nevertheless, the use of Cold Recycled asphalt mixtures requires a careful assessment of their mechanical properties, especially when they are designed to replace traditional Hot-Mix Asphalt Concrete mixtures. In this study the potential use of a Cold Recycled (CR) asphalt mixture as base course of an Italian motorway was evaluated. The studied mixture was produced in a central plant employing high RA content and used to construct two experimental sections along an in-service Italian motorway. In particular a special mixing procedure, involving the use of water vapour and bituminous emulsion, was tested. A third experimental section was constructed with the same layer thickness using the Asphalt Concrete (AC) mixture currently used in rehabilitation projects, incorporating 30 % of RA. Volumetric properties, stiffness, resistance to permanent deformation and fatigue behaviour of mixtures were investigated performing tests on samples cored from the three test sections and on laboratory compacted samples. Results of the mechanical tests showed that Cold Recycled mixtures provide lower stiffness modulus and lower resistance to repeated loading, but better resistance to permanent deformation respect to Asphalt Concrete. This behaviour can be explained with the presence of cementitious bonds that reduce thermal sensitivity and viscous response.
The growing availability of Reclaimed Asphalt (RA) imposes a reflection about technical solutions that can facilitate its large-scale reuse as part of pavement rehabilitation projects. In this context, Cold Recycling assumes fundamental importance both for economic and environmental reasons (reduction of costs, quarries exploitation, consumption of energy and dispersion of pollutant, etc.).
Two methods of cold recycling have been developed: Cold Plant Recycling (CPR) and Cold In-place Recycling (CIR). In both cases RA, generally recovered by cold milling, is mixed with a bituminous binding agent (emulsion or foam) and water. Virgin aggregates can be added to meet grading criteria, and active fillers, such as cement, are normally used to accelerate the curing process and improve mixture properties (Kandhal et al., 1997; Kearney, 1997; Grilli et al., 2012).
In CPR (also referred as Cold Central-Plant Recycling), RA material is hauled to a central plant where it is selected, pre-treated (crushed and screened) and tested before it is fed in the mixing process. Therefore, respect to CIR, this process allows a better control of RA variability and allows to obtain higher quality of the cold recycled mixes (Kandhal et al., 1997; Kearney, 1997; Wirtgen, 2006).
Besides the reduced environmental impact, a major advantage of Cold over Hot Recycling techniques is the possibility to reuse higher percentages of RA. In Hot recycled asphalt mixtures a maximum of 40 % RA is generally accepted in the base layers, and this amount is reduced to 15 % or even prohibited in the surface layers. In Cold Recycled asphalt mixtures the reuse of RA can be as high as 100 %, but this generally results in a loss of mechanical properties and durability.
Although over the last decade the performance gap between Hot and Cold recycled mixtures has certainly been reduced, a too conservative approach still exists for the use of cold recycled mixtures. This is generally due to an outdated knowledge and lack of practical experience. Consequently, one of the main challenges of pavement research is the optimization of cold recycled mix design and production process, in order to achieve the performance level of hot mixes, without increasing pavement layers thickness.
This paper presents the results of an experimental study on Cold Plant Recycled mixtures that involved the construction of three experimental sections along an in-service Italian motorway. This experience about cold recycling is not the first one carried out in the Italian road network. Previous studies, mainly focused on Cold In-Place Recycling have already achieved good results, with excellent data both for mechanical performance (Bocci et al., 2011; Santagata et al., 2009) and environmental benefits (Bocci et al., 2010).
2. Objectives
The overall objective of this study was to evaluate the potential use of a Cold Recycled (CR) asphalt mix as base course of an Italian motorway. The CR mix,
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produced in a central plant employing an high RA content, was designed to replace the existing Asphalt Concrete (AC) base.
Stiffness, resistance to permanent deformation and fatigue behaviour of both mixtures were investigated performing tests on laboratory compacted mixes and on cores obtained from three full-scale test sections realized along the in-service motorway.
Together with the traditional production process, during the construction phase a special mixing procedure was also applied. Specifically, water vapour was used at the mixing plant to add moisture to the CR mix. The effects of this half-warm procedure on mix workability and mechanical performance were evaluated.
3. Experimental
3.1. Full-scale test sections
Three full scale experimental sections (figure 1a) were realized near Rome as part of the A1 Italian motorway rehabilitation project. The maintenance activities consisted of milling the existing asphalt layers to a depth of 32 cm and reconstruction of a new asphalt pavement. In the three sections, a 20 cm base course was realized using different mixtures. Above this layer, an 8 cm binder course and a 4 cm porous wearing course were laid (figure 1b).
a)
CR base layerkm 537+772 to 540+356
Section 1
Section 2
CR base layer – vapourtreatment
km 541+338 to 547+355
Section 3
AC base layerkm 547+355 to 547+673 b)
POROUS ASPHALT: 4 cm
BINDER COURSE: 8 cm
BASE LAYER: 20 cm
EXISTING PAVEMENT
Figure 1. Full-scale test section: a) schematic view of the analyzed trial sections; b) Pavement structure scheme
In the first section, the base course was constructed using the CR mix produced with a traditional mixing procedure, consisting in the mixing, at ambient temperature, of milled materials, additional virgin aggregate, bitumen emulsion, active filler (cement) and water. To this aim, a fixed Hot Mix Asphalt plant has been employed performing minor modifications essentially related to the provision of a new cold mixer for water, emulsion and cement, excluding the hot-mixer system. The milled material was previously separated in particle size fractions and re-composed in accordance with the chosen mixture grading. For the construction of the second section, the same CR mix was used but a special mixing procedure was applied. In fact, since this section was built during a particularly cold period, moisture was added to the recycled mixture by means of water vapour. This allowed the aggregates in the drum to reach a temperature around 50-60°C before the addition of the binding agents (cement and bitumen emulsion). In the third section an AC20 mixture with SBS modified binder was employed in the base course. This was used as control section as the mix was produced following the current Italian technical specifications for motorway pavements.
3.2. Materials
The RA employed for the CR mixture came from milling of bituminous layers of the same motorway. Both base and binder layers, made with SBS modified bitumen, were milled off together and the resulting material was accumulated over time at the central plant. Before being used to produce the CR mixture, the RA was crushed and screened to discard foreign matter and clumps larger than 30 mm. The average particle size distribution of both RA particles and RA aggregate obtained from the Factory Production Control are reported in figure 2. According to EN 13108-8 the material was classified as 30 RA 0/20. The RA binder type was an SBS modified bitumen and its average content was 4.5 % by aggregate weight.
Due to the small amount of fines in the RA particle distribution, the mix design of the cold recycled mixture required the addition of 10 % virgin fine aggregate (by dry RA weight). This ensured the presence of an acceptable quantity of fines and an improved degree of densification during the laydown process. The aggregate size distribution of the CR mixture measured on cores extracted after construction is reported in figure 2.
An over-stabilized cationic asphalt emulsion (C 60 BP 7) with addition of natural latex (2.5 %) was used in the cold recycled mix. During the mixing procedure, the percentage of emulsion was 4 % (by dry aggregate weight) which resulted in an average residual binder addition of 2.4 %. Portland cement was also included in the mix design in a content of 2 % (by dry aggregate weight).
The AC20 mix used in the control section was a typical dense graded mixture produced following the current Italian technical specifications (figure 2). A 50/70 SBS modified binder was used (pen = 60 dmm, TR&B = 90 °C, RE = 95 %).
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RA (U = 30)
RA Aggregates (D = 20)
Cold recycled mix
AC20
Fine Aggregate 0/2
Figure 2. Particle size distribution of tested mixtures
3.3. Test program
Volumetric and mechanical properties of the mixtures were measured both on cores and laboratory compacted specimens. Cores (D = 98 mm, H = 200 mm) were taken from the three experimental sections. In particular, samples from the first section were cored both one month and six months after construction (CR-C1 and CR-C6) whereas cores from the second section were drilled 18 months after construction. Moreover, during the mixing procedure, samples of the loose CR mix were taken and immediately compacted at the central plant laboratory. A gyratory compactor was used to produce 150 mm specimens that were then cored to 95 mm.
A servo-pneumatic testing machine was used to characterize the mixtures in terms of complex modulus (E*), resistance to permanent deformation, ITSM and crack initiation (table 1).
Cyclic uniaxial compression tests (figure 3a) were performed to measure complex modulus (E*) in accordance with AASHTO TP 79-09. Tests were run at 5 temperatures (0, 10, 20, 30, 40 °C) by application of sinusoidal load waves at 5 frequencies (0.3, 1, 3, 10, 20 Hz). Tests were carried out in controlled stress mode maintaining the axial deformation level below 50 µε.
Cyclic triaxial compression tests (figure 3b) were performed for the assessment of permanent deformation resistance, according to EN 12697-25. Tests were run at 2 temperatures (30 and 40 °C) with a confining pressure of 50 kPa.
Figure 3. Test configurations: a) cyclic uniaxial compression; b) cyclic triaxial compression
Cyclic indirect tension tests were performed for the assessment of the Indirect Tensile Stiffness Modulus (ITSM) according to EN 12697-26, and subsequent estimation of the resistance to crack initiation (Read et al., 1996). Tests were carried out at a temperature of 20 °C in controlled stress mode, with a rise time of 124 ms. Failure was established when the complete fracture of specimens was obtained (BS DD ABF, 1997).
The test program is summarized in table 1. A total of 16 specimen were tested for complex modulus, 9 specimen for permanent deformation and 24 specimens for ITSM and resistance to repeated loading and crack initiation.
Table 1. Test program summary and tested specimens
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4. Results and analysis
4.1. Volumetric properties
The volumetric properties of the studied mixtures are summarized in Table 2. Results show the importance of mixing and compaction procedures on the compactability of CR mixtures. In the first test section, where the CR mix was produced with the traditional mixing procedure (CR-C1), the average air void level was as high as 12.1 %. In the second test section, where water vapour was used in the mixing phase (CR-CV), the average air void level was 4.9 %, even though the mixtures had the same composition. The positive impact of the new mixing procedure on the CR workability can be related to its increased temperature, better homogeneity and less segregation. Moreover, when compacted in the laboratory with the gyratory compactor, the CR mix produced with the traditional mixing procedure reached an air void content of 7.1 % (CR-CL). This indicates that the mix composition should be optimized for field compaction procedures, in particular respect to water content and compactability.
In this context it is also important to remark the different void level between asphalt concrete cores (AC-C) and cold recycled cores (CR-C1) as this difference will potentially affect mixture stiffness and resistance.
Table 2. Average volumetric properties of mixtures
4.2. Stiffness
The stiffness properties of the studied mixtures for all tested temperatures and frequencies are reported in the Cole-Cole diagram (figure 4) in terms of storage (E1) and loss modulus (E2); results of all mixtures are also summarized in the Black Space (figure 5) in terms of complex modulus norm (|E*|) and phase angle.
Figure 4. Storage (E1) and loss modulus (E2) of the studied mixtures in the Cole-Cole diagram.
1000
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0 5 10 15 20 25 30 35
Nor
m o
f the
com
plex
mod
ulus
|E*|
[MPa
]
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CR-C1 CR-C6 CR-L AC-C
Figure 5. Complex modulus (E*) of the studied mixtures in the Black space.
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For all the tested samples (CR-C1, CR-C6, CR-L, AC-C) the Cole-Cole curves and the Black curves are not unique and continuous, revealing that their behaviour is not thermo-rheologically simple (Christensen, 2003). This was an expected result since a relevant content of RA is present in both AC and CR mixes (30 % and 90 %, respectively). Evidently, the rheological response and the thermal sensitivity of the aged binder contained in the RA was different from that of the virgin binders. Moreover, modified bitumens were used both for the CR emulsion and the AC mix and it is known that these type of binders do not follow the Time-Temperature Superimposition Principle (Di Benedetto et al., 2011).
As long as only the complex modulus norm is considered, the master curves reported in figure 6 are obtained. A four-parameter sigmoidal function was adopted as analytical model for |E*| (Pellinen et al., 2004):
[1]
where α, β, δ and γ are experimental parameters and fr is the reduced frequency (fr = αTf). The model optimization procedure was carried out allowing free variation of the shift factors αT , results al reported in table 3.
Mixture type α δ β γ
CR-C1 3,59 0,40 -1,86 -0,22
CR-C6 3,01 0,94 -1,74 -0,21
CR-L 3,73 0,67 -1,69 -0,28
AC-C 1,39 2,80 -0,79 -0,62
Table 3. Parameters of the sigmoidal function for the mixtures studied
Figure 6. Master curves and shift factors of complex modulus norm @ 20°C
The shape of the |E*| master curves confirm that the Partial Time-Temperature Superimposition Principle can be considered valid for all the tested samples (Di Benedetto et al., 2011).
At medium and high reduced frequencies (medium and low temperatures), stiffness of CR mix laid in the first section was considerably lower respect to the AC mix laid in the control section (figures 4-6). The difference can be mainly related to the higher voids content of the cold recycled course. In fact, when compacted in the laboratory, at lower voids content, the CR mix showed stiffness values similar to the AC mix, and a comparable glassy modulus (Eg ≈ 2.0 GPa). These different
Cold recycled performance evaluation 11
behaviours can also be due to differences in the distribution of air voids inside field cores and shear gyratory compacted specimens (Tashman et al., 2002) and to the strong dependence of aggregate orientation on the compaction method used (Masad et al., 1999; Hunter et al., 2004)
At lower reduced frequencies (higher temperatures), although the experimental data do not allow a reliable estimation of the equilibrium modulus, the stiffness of CR mixes appears to be less influenced by the voids level (figure 6), as the master curves get closer. Moreover, the CR mixes showed considerably lower phase angles respect to AC mixes (figure 5). This behaviour can be explained with the presence of cementitious bonds inside the CR mixes; these bonds create a solids skeleton that is less thermo-dependent and exhibits a reduced viscous response. The same considerations arise observing the response of cores with different curing times. In particular, samples cored from the first section after 6 months (CR-C6), show lower phase angles (figure 5) respect to samples cored after one month (CR-C1).
The different rheological behaviour between AC and CR mixes suggests that, in the pavement design process, the stiffness modulus of cold recycled mixtures should not be estimated through the prediction models available in literature for asphalt concrete mixes. Instead, a thorough laboratory investigation is necessary for CR mixes, considering also the effects of compaction and curing time.
The average values of the norm of the complex modulus measured at 20 °C and 2 Hz, were also compared with ITSM values measured at the same temperature (table 4). The comparison shows that the two tests provide similar results, validating laboratory measurements. However, it is worth noting that during ITSM test, some CR specimens broke during the preload phase, demonstrating a fragile behavior, which has been subsequently confirmed during the investigation of the resistance to repeated loading.
Table 4. Norm of average complex modulus and ITSM @ 20 °C and 2 Hz
4.3. Resistance to permanent deformation
The resistance to permanent deformation of the studied mixtures was evaluated through cyclic triaxial compression tests. The average creep curves at 30 and 40°C
are shown in figure 7 while the corresponding creep rates and maximum axial permanent deformations εp,max are summarized in table 4.
As expected, at higher temperatures all the tested samples (CR-C1, CR-C6, CR-L, AC-C), showed higher permanent deformation susceptibility and higher creep rates. CR mixes proved to be clearly more resistant to permanent deformation and less susceptible to temperature respect to AC mixes. This confirms the results of other studies (He et al., 2007; He et al., 2008) and is in accordance with the results of complex modulus tests at low reduced frequencies (high temperatures).
In fact, from figure 5 and figure 7, it can be noticed that mixtures showing higher phase angles were also more susceptible to permanent deformation. Specifically, mixtures preserve the same rank: from AC-C (higher phase angles, lower resistance to permanent deformation), to CR-C6 (lower phase angles, higher resistance to permanent deformation). Considering only the CR mixes, it can be noticed that air voids and compaction type did not have an evident effect on permanent deformation susceptibility. This can be explained with the presence of lateral confinement in the triaxial compression test.
The overall behaviour of the studied mixtures confirmed the importance of the cementitious bonds that develop in the CR mixes creating a solids skeleton which is able to effectively withstand the accumulation of permanent deformation.
Table 4. Creep rate and maximum axial permanent deformations.
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Tota
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T = 40 °C
CR-C1CR-C6CR-LAC-C
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CR-C1CR-C6CR-LAC-C
Figure 7. Creep curves @ 30°C and 40°C.
4.4. Resistance to repeated loading
The resistance to repeated loading was evaluated through the cyclic indirect tension test. Results are plotted in figure 8 where the number of cycles to failure is reported as a function of the initial maximum horizontal deformation (εinit-horiz-max). It is worth noting that the regression lines drawn in figure 8 do not define the fatigue life but can only be considered as an indicator of the crack initiation characteristics of the studied mixtures (resistance to early cracking).
Results show that CR mixtures are characterized by lower resistance to repeated loading respect to AC mixtures. This behaviour can be partially explained in terms of air voids content, and shows that the cementitious bonds which give the CR mixture high resistance to the accumulation of permanent deformation, also make it more fragile and susceptible to early cracking.
It was also observed that all cyclic indirect tension tests resulted in a complete splitting of the specimens along the vertical diameter. However, in the case of CR mixes, this fracture was often combined with a sample collapse, a clear symptom of the brittle behaviour of the material. This behaviour could be attributed to the fact that without lateral confinement the bituminous phase of CR mixtures appears not able to give its full contribute, making the material too brittle. This tendency suggests that this kind of test, developed for asphalt concrete mixture, is not suitable to characterize cold recycled mixes that, at low binder/cement ratio values, are more similar to weakly-cemented materials (Grilli et al., 2012).
Finally, the comparison in terms of CR mixing procedure shows that the water vapour method employed for the second test section allowed to realize a base course with higher resistance to repeated loading respect to the traditional production process. As already underlined (see Section 4.1), these results confirm that the water vapour mixing process allows a higher workability of the cold recycled mixture guaranteeing a better in situ compaction.
y = 223.4x-0.075R² = 0.65
y = 215.72x-0.096R² = 0.75
y = 282.7x-0.095R² = 0.85
y = 617.65x-0.151R² = 0.67
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t-hor
iz-m
ax[µε]
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T = 20 °CCR-C6
AC-CCR-L
CR-CV
Figure 9. Fatigue relationships.
Cold recycled performance evaluation 15
5. Conclusions
This paper summarizes the results of an experimental study on the volumetric and mechanical properties of a Cold Plant Recycled asphalt mixture. Three experimental sections were realized as part of an Italian motorway rehabilitation project. Tests performed both on samples cored from the three experimental sections and on laboratory compacted specimens allowed the following conclusions to be drawn.
– The CR mix produced with a traditional mixing procedure (water and bitumen emulsion) was not able to guarantee a proper in situ compactability, indicating that the cold mix composition should be optimized for field compaction procedures, in particular respect to water content. An improved CR mix workability was obtained with a special mixing procedure (water vapour and bitumen emulsion), allowing a suitable air void content to be reached in the field.
– Results of the complex modulus tests showed that AC and CR mixes behaviour are not thermo-rheologically simple but, as long as only the complex modulus norm is considered, the Partial Time-Temperature Superimposition Principle can be considered valid for all the tested samples. This result is a consequence of the use of binders with different rheological behaviour (aged binders, modified binders).
– The values of complex modulus norm measured at medium and high reduced frequencies (medium and low temperatures) showed that CR cores stiffness was considerably lower respect to the AC cores, reflecting their higher voids content. At lower reduced frequencies (higher temperatures), the stiffness of CR cores was less influenced by the air voids level. At higher temperatures the viscoelastic behaviour of CR mixtures was also characterized by considerably lower phase angles respect to AC mixes.
– The different behaviour between the two materials suggests that, during pavement design process, the stiffness modulus of cold recycled mixtures cannot be estimated through the predictive models available in literature for asphalt concrete mixes but an in depth laboratory investigation is necessary.
– The CR mixes provided higher resistance to permanent deformation and less susceptibility to temperature respect to the AC mixes. This is in accordance with the results of complex modulus tests at higher temperatures where CR mixes behaviour showed a lower viscous component. Air voids level and compaction type did not have an evident effect on permanent deformation susceptibility of CR mixes.
– The CR mixes were characterized by lower resistance to repeated loading and early cracking respect to the AC mixtures. Moreover, failure of CR cores was often combined with a sample collapse (brittle behaviour). This behaviour can be related to higher air void content and to the absence of lateral confinement suggesting that this kind of test, developed for asphalt concrete mixture, is not suitable to characterize cold recycled mixes.
The mechanical behaviour of the studied CR mixtures revealed the importance of the solids skeleton that develops thanks to cementitious bond. The influence of this skeleton is particularly evident at high temperatures where it reduces the thermo-dependency and the viscous response respect to a hot mix asphalt concrete. Nevertheless, a low-voids structure produced with mixture compaction is essential, especially if tests without lateral confinements are carried out.
The overall experimental results suggest that the studied cold recycled mixture, thanks to its low susceptibility to permanent deformation and reduced thermal sensitivity, can be considered an optimum material for application as sub-base course. On the other hand, the use of CR mixture as base course must be carefully evaluated as alternative of hot mix asphalt concrete (depending on traffic loading, weather, subgrade conditions, etc.) because of its tendency to brittle behaviour.
Acknowledgements
This research was sponsored by Pavimental Spa. This support is greatly acknowledged. Test results and opinions are those of the authors and do not necessarily reflect those of the sponsoring agency.
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