Influence of reclaimed asphalt content on the mechanical ... · Road Materials and Pavement Design...

Road Materials and Pavement Design Volume 14, Issue 3, pages 666-678,2013 DOI: 10.1080/14680629.2013.794367

Published version available at Taylor and Francis Online 1

Influence of reclaimed asphalt content on the mechanical behaviour of cement-treated mixtures Grilli A.* - Bocci E.** - Graziani A.** *Università degli Studi della Repubblica di San Marino 44 Via Salita alla Rocca, 47890 San Marino, Republic of San Marino [email protected] **Università Politecnica delle Marche Via Brecce Bianche, 60100 Ancona, Italy [email protected] [email protected] ABSTRACT. For the rehabilitation of asphalt pavements the upper distressed layers are usually milled before overlaying to eliminate reflection cracking–related problems and to preserve the pavement geometry. This maintenance technique generates a large amount of reclaimed asphalt (RA) as a product of the milling operation. The recycling of RA in cement-treated base and subbase courses represents a valuable solution in terms of technical, economic and environmental benefits. However, the influence of RA on the mechanical properties of cement-treated materials (CTMs) is still not completely understood. As a consequence, CTMs using a high content of RA have not yet been widely applied. The present paper shows the findings of an experimental analysis on CTMs including 50 % and 80 % RA in comparison with the reference CTM consisting of 100 % mineral aggregates. In particular, indirect tensile tests and unconfined compressive tests were conducted to evaluate the resistance characteristics of the CTMs. In addition, complex modulus tests at and ultrasonic pulse velocity tests were performed to investigate the stiffness properties of CTMs. The investigation shows promising results as regards the use of high percentages of RA in CTMs and offers a substantial contribution for the understanding of the mechanical behaviour of CTMs.

KEYWORDS: Cement-treated materials, reclaimed asphalt, resistance, stiffness properties.



1. Introduction

The evolution of road transportation and the increase in heavy traffic loading require road pavements with high-structural performance. Generally the pavement structure and its bearing capacity can be improved by replacing untreated granular layers with hydraulically-treated layers (AASTHO, 1993). Typically, the stabilising agents for hydraulically bound mixtures are lime, fly ash, pozzolanas, granulated blast furnace slag and cement.

Cement treatment is specifically recommended and is effective in increasing the strength, cohesion and durability of coarse graded mixtures with a low plasticity index (De Beer, 1990,Xuan et al., 2012). Two main techniques for the production of cement-treated materials (CTMs) can be distinguished: in-situ cement stabilisation and in-plant cement treatment.

In-situ, CTMs are generally produced by means of a recycling train that is usually made up of a vehicle equipped with volumetric batchers for spreading cement, a recycler coupled to a tank truck for the addition of water, a vibrating smooth drum roller, a pneumatic tyre roller and a grader for shaping and levelling. The development of recyclers has allowed the stabilisation of a single layer of up to 40 cm thick in a single pass, thereby significantly increasing the daily production. A relatively low dosage of cement (from 1.5 % to 3.5 % by aggregate weight) is used to stabilise marginal soils (Lewis et al., 1999, Muhunthan et al., 2008), granular materials from unbound foundation layers, milled materials from cement-bound layers or a combination of them. In-situ cement stabilisation is the most cost-effective and environmentally friendly method. On the other hand, the quality of the constituent materials could be rather poor thereby affecting the mechanical properties of the final mixture. Moreover the homogeneity of the production could be difficult to control. For these reasons in-situ cement stabilisation is applied to deep layers (subgrade, or subbase courses), where the effect of tyre pressure is less critical, or to base courses for low-volume roads (Paige-Green et al., 2006).

In plant, CTMs are generally produced mixing selected aggregates, cement, ranging from 2 % to 5 % by aggregate weight (Siripun et al., 2009, Guthrie et al., 2002), and a defined percentage of water, which plays an important role in both compaction and cement hydration (White et al., 2005). In-plant production is carried out in mixing facilities (concrete mixer) which do not require equipment for heating and reselecting aggregates or filtration and exhaust extraction systems. An accurate proportion of different input materials (selected aggregates, additives, cement and water) and a thorough quality control increase the degree of confidence that the product meets performance expectations. The selected gradation and the layer thickness make compaction easier and allow a high level of compaction efficiency. For these reasons CTMs, originally produced using natural or crushed aggregates, are preferred in the production of base courses rather than in-situ cement stabilised materials.



Currently, the term CTMs is increasingly associated with recycling. In order to preserve natural resources and to reduce the disposal of materials coming from the demolition of civil constructions, recycled aggregates such as crushed concrete (Molenaar et al., 2011, Marradi et. al, 2008), crushed masonry (Xuan et al., 2012, Molenaar et al., 2011), foundry sand (Gupta et al., 2009), reclaimed asphalt (Taha et a., 2002, Rupnow et. al., 2002, Gupta et al., 2009, Deren et al., 2011) and reclaimed aggregates (White et al., 2005, Paige-Green et al., 2006) are being used in CTMs. Moreover, considering the scarce availability of high quality virgin aggregates, the cement treatment of recycled materials proves to be one of the most viable recycling techniques for road construction and maintenance (Tam et al., 2006, Deren et al., 2011).

Reclaimed asphalt (RA) is currently reused in the production of hot mix asphalt (HMA), but technical difficulties or environmental issues limit the integration of HMA with RA (NCAT, 2009). Cold recycled mixtures can maximize the amount of RA to be recycled by using bituminous and hydraulic binders. If only cement is added as binding agent, conventional CTMs can be obtained (Paige-Green et al., 2006). However, the lack of well-established technical knowledge seems to hinder the use of RA in CTMs. Indeed, universally accepted specimen preparation, testing and field control procedures for CTMs are not available even if universities, research centres and road administration agencies have developed specific methods based on laboratory tests and field experiences. In this context, the task group 6 (TG6) of RILEM Technical Committee on Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems (TC-SIB) is currently working on specimen preparation procedure for cold recycled mixtures.

This paper presents the results of a comprehensive laboratory testing program on CTMs containing high amounts of RA with particular emphasis also on the specimen preparation and testing procedure.

2. Objective

The objective of this research study is to evaluate the influence of RA on the mechanical properties of CTMs. In particular, resistance properties were analyzed in terms of Indirect Tensile Strength (ITS) and Unconfined Compressive Strength (UCS). In order to highlight the effect of RA on time- and temperature-dependence of CTMs stiffness properties, the complex modulus at different frequencies and temperatures was measured through Cyclic Compression Tests (CCT) and Ultrasonic Pulse Velocity Tests (UPVT).

3. Materials

The RA and virgin aggregates were characterised in terms of gradation (EN 933-2), particle density and water absorption (EN 1097-6), and shape (EN 933-3, EN



933-4). Table 1 reports the designations and abovementioned physical properties of RA and virgin aggregates. In particular the RA was coded according to the EN 13108-8 and the virgin aggregates following the EN 13242.

RA lumps consist of mineral aggregate particles bonded by the bituminous mastic. Therefore, a single particle of RA contains intergranular air voids and bitumen causing lower particle density than virgin aggregates. Moreover, the old bitumen seals the aggregate surface porosity determining a low water absorption.

Aggregate Designation

Apparent particle density

Water absorption

Flakiness Index

Shape Index

[Mg/m3] [%] [%] [%] 25 RA 0/12 2.530 0.2 7 8 0/4 GF 90 2.720 1.4 - - 2/6 GC 85/15 2.720 1.1 - - 6/12 GC 85/15 2.710 1.3 9 15 10/20 GC85/15 2.710 1.3 9 3

Table 1.Designation and physical properties of the granular material used

The gradations of aggregate fractions used are shown in Figure 1. The gradation analysis of RA was carried out by washing the fines because of the dust content present in this material. No foreign matter was found in RA.

Figure 1. Gradation of granular material used

Each mixture was characterised by a similar gradation through an accurate proportion of the granular blend composition (Figure 2). In this way the aggregate gradation does not influence the relevant comparison among mixtures.

0

20

40

60

80

100

0,01 0,1 1 10

Passing [%]

Sieve size [mm]

25 RA 0/12

10/20 Gc 85/15

6/12 Gc 85/15

2/6 Gc 85/15

0/4 GF 90

Filler



Figure 2. Composition and gradation of aggregate blends

The California bearing ratio (CBR) and linear swelling were used to characterise the bearing capacity and water susceptibility of the granular mixtures. In accordance with EN 13286-47, the specimens were soaked in water for 96 hours. The CBR decreased with the increase in RA content. In detail, the CBR values of mixtures containing 0 %, 50 % and 80 % RA were 152, 36 and 31, respectively. Therefore the RA generated an overall weakening of the granular mixture although all the mixtures proved to be adequate as a constituent material for a subbase course. No expansion was measured for any of the granular mixtures, thereby ensuring good performance even in wet conditions.

The aggregate blends were treated with a Portland limestone cement (type II/B-LL), strength class 32.5 R (EN 197-1).

4. Experimental program

Three kinds of aggregate blend, produced respecting the same gradation (Figure 2), were treated with 3 % of cement by aggregate weight, that represents the minimum dosage for CTMs (EN 14227-1) and, based on previous experiences, is a reference value for mixture design. The three aggregate blends differ from each other in RA content: the first one containing 80 % of RA, the second one containing 50 % of RA and the third one, used as a reference mixture, with no RA (100 % limestone crushed aggregates).

The mixtures were produced using their optimum (total) water content which was established as 5.5 % in a preliminary compaction study. Since RA has a lower water absorption respect to virgin aggregates (Table 1), recycled mixtures had a higher amount of free water respect to the reference mixture. The additional free water, required to obtain an effective lubrication effect in the compaction phase, can

0

20

40

60

80

100

0,01 0,1 1 10

Passing [%]

Sieve size [mm]

50RA

80RA

00RA

00RA 50RA 80RA25 RA 0/12 0 50 80

10/20 GC 85/15 36 17 46/12 GC 85/15 13 6 02/6 GC 85/15 12 0 0

0/4 GF 90 35 25 15Filler 4 2 1

MaterialComposition [%]



also influence the cement hydration process and thus the mechanical properties of the cured mixtures.

From a technical point of view, CTMs are characterized by their failure properties. In the first phase of the experimental program, the CTMs were studied by means of traditional strength tests which define the parameters used for design or quality control. To this end, indirect tensile tests (ITT) and unconfined compression tests (UCT) were carried out as requested by the European specifications for cement bound granular mixtures (EN 14227-1). According to these specifications, specimens were cured for 7 days at 25 °C and tested at 25 °C.

The second phase of the experimental program focused on the measurement of mixture stiffness for CTMs containing RA by means of Cyclic Compression Tests (CCT) and Ultrasonic Pulse Velocity Tests (UPVT). CCT were carried out on cylindrical specimens at different frequencies (0.1, 0.3, 1, 3, 10, 20 Hz) and temperatures (10, 20, 30 °C). Though normally CTMs are not considered temperature-dependent materials, these ranges of test conditions, commonly employed for the study of bituminous materials, was selected because of the use of RA. UPVT were carried out at 20 °C which was selected as a reference temperature. The same specimens were tested by CCT and UPVT after a 28-day curing period at 25 °C.

For each mixture type, and for each test (ITT, UCT, UPVT, CCT) three replicate specimens were prepared and tested.

5. Specimen preparation

Before compaction, each aggregate blend was preliminary mixed with a water content equal to the absorption of the constituent aggregates. The blend was then kept in a sealed plastic bag for 12 hours in order to ensure a homogeneous spread of humidity. Before binder addition, aggregates were thoroughly mixed with additional water (free water) to reach the optimum water content. Finally, the selected amount of cement (3 % by aggregate weight) was added and the mixtures were hand-mixed at room temperature for at least two minutes.

Specimens were compacted by means of a Shear Gyratory Compactor (SGC). This is a rather innovative approach for CTMs, that are usually compacted by means of impact or vibratory equipment (EN14227-5). SGC was recently considered by some research studies also for unbound materials (Cerni et al, 2011).

In this experimental study the compaction protocol provided a similar procedure to that for bituminous mixture: a constant pressure of 600 kPa, a speed of 30 rpm and a constant angle of inclination of 1.25°. The number of gyrations was adjusted to achieve a bulk density of 2100 kg/m3, that represents a typical field value for CTM courses.

For ITT 2.7 kg specimens were compacted in a 150 mm diameter mould with 100 gyrations, while for UCT 4.5 kg specimens were compacted in a 150 mm



diameter mould with 180 gyrations in order to obtain a specimen height suitable for each test type and a similar compaction energy per volume unit.

For CCT specimens a preliminary phase was aimed at defining the compaction procedure in order to maximize the height-to-diameter ratio using the available SGC. 2.4 kg specimens were compacted in a 100 mm diameter mould with a variable number of gyrations, in order to reach the same bulk density obtained for ITT and UCT specimens.

All the cement-treated specimens were sufficiently stable to allow extrusion immediately after compaction. Before testing, the specimens were kept in a moisture-saturated chamber at 25°C throughout the curing period.

Table 2 summarises the characteristics of the specimens, the type of compaction and the curing process.

Test Compaction method Mould diameter [mm]

Nominal height [mm]

Curing conditions

ITT SGC@100 gyrations 150 70 7 days at 20°C

UCT SGC@180 gyrations 150 120 7 days at 20°C

CCT SGC@180/250 gyrations 100 135 35 days at 20°C

Table 2. Specimen preparation procedure and curing conditions

6. Testing procedures

6.1 Indirect tensile test and Unconfined compressive test

A servo-hydraulic automatically controlled testing machine was used to measure the ITS and the UCS of the mixtures.

For ITT, the load was applied along the two specimen generatrices till specimen failure. The tests were performed at 25 °C applying a constant rate of deformation of 50 ± 2 mm/min (EN 12697-23). The ITS is the maximum tensile stress that causes the failure of the specimen.

For UCT, the equipment applied a compression force on the two basis of the specimen until it reaches the failure. The tests were performed at 25 °C applying a constant rate of stress of 0.5 kg/cm2/s (EN 13286-41). The UCS is the maximum compressive uniaxial stress that can be applied to the cylindrical specimen in the absence of a lateral confinement.



6.2 Cyclic testing machine

A servo-pneumatic testing machine was used to measure mixture stiffness in terms of complex modulus E*. The norm |E*| and the phase angle φ were determined through Cyclic Compression Tests (AASHTO TP62) performed in control strain mode, applying a sinusoidal wave with a peak-to-peak of 25 µε. Though higher values (up to 100 µε) are currently used to test HMA in the linear viscoelastic domain, this low strain level was used to prevent brittle failure of the specimen during the test.

The deformation was measured by mean of two LVDT transducers, clung to the lateral surface of the specimen in a opposite position with a 70 mm measurement base.

In order to highlight time- and temperature-dependence of stiffness, due to the presence of RA, tests were carried out at three temperatures (10, 20 and 30 °C) and six frequencies (0.1, 0.3, 1.0, 3.0, 10 and 20 Hz). Since CTMs usually are not considered time- or temperature-dependent materials test temperatures and frequencies typically used for HMA (EN 12697-26) were selected. Higher temperatures were not investigated to avoid excessive permanent (visco-plastic) deformations of the mixture caused by RA. Moreover in typical European climatic conditions the temperature of CTM layers usually does not raise above these values.

6.3 Ultrasonic pulse velocity test

The propagation of stress wave pulses is a widely used non-destructive technique to determine uniformity, presence of voids, integrity (presence of cracks) and dynamic modulus of elasticity of civil engineering materials. UPVT are routinely used to measure stiffness of cementitious and bituminous mixtures (EN 12504-4; Di Benedetto et al., 2009, Arabani et al., 2009; Biligiri et al., 2009). This technique was also successfully applied to recycled mixtures containing hydraulic and bituminous stabilizing agents (Bocci et al., 2011).

UPVT have been carried out at 20°C on the same cylindrical specimens used for the cyclic modulus tests. Pulses of stress waves with a frequency of 39 kHz were generated by a transmitting transducer, held in contact with one specimen base, and received by another transducer on the opposite side. Silicone grease was used to couple specimen and transducers surfaces. The measurement of the time interval to cover the distance between the transducers (135 mm) allowed to calculate the ultrasonic pulse velocity. According to ASTM C 597 – 02, the dynamic stiffness modulus of the material was estimated using the following relation:

! =! 1 − !

! 1 + ! 1 − 2! [1]



where V is the pulse velocity (m/s), E is the dynamic stiffness modulus (Pa), µ is the dynamic Poisson’s ratio (assumed equal to 0.30)and ρ is the density (kg/m3).

7. Analysis of results

7.1 Volumetric properties

The volumetric properties of the specimens prepared for ITT and UCT (compacted under constant energy) were evaluated in terms of dry bulk density and residual intergranular air void content (AVC) calculated immediately after compaction.

For the dry bulk density, the volume of the specimen was obtained by measuring its dimensions (EN 12697-6, procedure D). Each specimen was accurately weighed before and after compaction. Taking into account the water content added in the mixing phase (design water content) and the incidental leak of water during the compaction process, the effective water content after compaction of each specimen was estimated and used to calculate the dry bulk density.

Table 3 shows the average dry bulk densities of different mixtures and the relative coefficient of variation (CV). Moreover two compaction procedures can be compared: sample mass of 2.7 kg and compaction energy of 100 gyrations for ITT specimens, sample mass of 4.5 kg and compaction energy of 180 gyrations for UCT specimens.

The dry bulk density is closely related to the particle density of the constituent materials and thus decreases with the increase in RA content. In addition, results of the two compaction procedures differ by less than 1%, confirming that the used combinations of sample mass and compaction energy had the same effects on the densification of samples.

However, when comparing mixtures containing a combination of different aggregates (recycled and virgin aggregates) which are characterised by different particle densities, the dry bulk density could be a misleading indicator for the compaction level (Grilli et al., 2012). The volumetric approach used for the HMA seems to be more suitable for analysing the compactability of CTMs using RA and, for this reason, the AVC was evaluated.

The maximum density (EN 12697-5) of each loose mixture (2680, 2590 and 2550 kg/m3 for the mixture using no RA, 50 % of RA and 80 % of RA, respectively) was preliminarily determined and used, along with the dry bulk density, to calculate the AVC of each specimen. Table 3 shows the average AVC values, and the relative CV, for each mixture and compaction method. It can be observed that the AVC increased as the amount of RA increased. Probably the considerably rough RA surface texture affected the compactability of CTM.



7.2 Indirect tensile strength

Table 3 shows the average ITS values, and the relative CV, of each mixture after a 7-day curing period at 25 °C. Even if the cement dosage and the gradation are the same for all the mixtures, the results indicate that the ITS decreased as the RA content increased.

This finding could be related to several factors. The higher AVC and water/cement ratio (recycled mixtures needed a higher amount of free water in the mixture) surely affected the failure resistance of recycled mixtures (Isola et al., 2012). In addition, the cement could have less effectiveness to bind bitumen-coated aggregates than virgin one. Eventually, RA had an high amount of fines (P0.063mm = 3.9 %) in form of bitumen powder or dust (“black filler”) and, for this reason, the mixtures containing RA required the addition of less limestone filler to meet the design gradation. Therefore, the amount of limestone filler added to the mixture was reduced as the RA content increased. It is likely that the black filler contained in RA did not have the same affinity to cement and aggregates as the limestone filler.

7.3 Unconfined compressive strength

Table 3 shows the results obtained from UCS tests. The mixture containing only virgin aggregates showed the highest UCS, whereas the increase in RA content caused a significant decrease in UCS (Yuan et al, 2011, Isola et al. 2012). Once again, the higher AVC and water/cement ratio influenced the failure resistance of recycled mixtures. Moreover, as shown in CBR tests, the reduction of UCS can also be related to a lower bearing capacity of the aggregate skeleton when a higher RA content is used. Nonetheless, the recycled mixtures satisfy the Italian specifications (2.5 < UCS < 7.5 N/mm2) for CTMs and they could be compared with a C2 material used in the South African mechanistic pavement design method (Theyse, 1996).

Mixture Code

Dry bulk density

Air void content (AVC)

Indirect tensile strength (ITS)

Unconf. compr.strength (UCS)

[Mg/m3] [%] [MPa] [MPa]

3C00RA 2209 (0.8) 17.6 (3.3) 0.37 (10.6) - 2242 (0.5) 16.4 (2.5) - 12.4 (11.9)

3C50RA 2114 (0.6) 18.5 (2.6) 0.31 (6.8) - 2122 (1.2) 18.2 (5.1) - 6.2 (9.2)

3C80RA 2041 (0.7) 19.9 (2.7) 0.21 (12.5) - 2059 (0.1) 19.2 (0.2) - 4.2 (6.4)

Table 3. Volumetric properties and resistance of the studied mixtures: average values (Coefficient of variation)



7.4 Complex modulus

The results obtained by Cyclic Compression and Ultrasonic Pulse Velocity Tests are summarised in Figure 3, where the norm of the complex modulus |E*| and the dynamic modulus E are plotted as a function of test frequency and temperature. In both cases the average value of three tested specimens is reported for each mixture.

The stiffness of CTMs with no RA did not show any dependence on temperature and frequency, as commonly observed for cementitious materials. In particular, despite the different test frequency values, similar modulus values were measured by CCT (average value at all temperatures and frequencies is 18.3 GPa) and UPVT (19.5 GPa).

The introduction of RA in the mixtures caused a decrease in stiffness which can be related to the percentage of RA used and to the higher AVC of the recycled mixtures (Figure 3). Moreover, a dependence on temperature and frequency is observed, as occurs with HMA. In Figure 3, it can be also noticed that the dynamic modulus values determined through UPVT are higher than the |E*| measured with CCT. This could be an effect both of the different test frequency values and the strain levels applied during the tests (Di Benedetto et al., 2009).

Figure 3. |E*| values as a function of frequency and temperature

In order to investigate the combined effects of temperature and frequency, an attempt was made to verify the validity of the Time-Temperature Superposition Principle (TTSP).

3.0E+03

3.0E+04

1.0E-03 1.0E-01 1.0E+01 1.0E+03 1.0E+05

|E*|

[MPa

]

f requency[Hz]

T = 10°CT = 20°CT = 30°C

Cyclic compression tests UPV tests

3C-00RA

3C-50RA

3C-80RA



To that hand, measured values of complex modulus norm |E*| and phase angle φ of the studied mixtures are reported in Figure 4 (Black Diagram). As it can be noticed, low values of φ (0° ÷ 6°) were obtained for CTM with no RA, which can be related to the viscous properties of the cement paste (Sun et al., 2006).

As expected, CTMs containing RA showed higher phase angles (up to 10°) on increasing RA content, highlighting the influence of the bitumen component on time-dependent behaviour. However, since a unique curve was not obtained from measurements at different temperatures, the validity of the TTSP for the complex modulus E* was not proved.

Figure 4. Black diagram of CTMs

Even if the tested CTMs were not strictly thermo-rheologically simple materials,

for practical uses it is possible to apply the partial time-temperature superposition (Olard et al., 2002) and represent only the complex modulus norm as a function of frequency or temperature. As shown in Figure 5, measured |E*| versus frequency can be shifted along the frequency axis to build a single master curve at the reference temperature of 20 °C. The master curve can be mathematically modelled by a 4-parameter sigmoidal function:

log |!∗| = ! +!

1 + !!!! !"#!! [2]

where !,!, ! and ! are material parameters representing the rheological behaviour and fr is the reduced frequency defined as follows:

!! = ! ∙ !! [3]

3.0E+03

3.0E+04

0 2 4 6 8 10 12

|E*|

[MPa

]

φ [°]

T = 10°CT = 20°CT = 30°C3C-00RA

3C-50RA

3C-80RA



where f is the testing frequency and aT is the time-temperature shift factor. The material parameters and the shift factors were obtained fitting the experimental data with a numerical optimization tcchnique.

As it can be observed in Figure 5, the main effect of increasing RA content on CTMs appears to be a downwad shift of the master curve and can be interpreted as the effect of a parallel arrangement between a stiffer phase (virgin aggregates and cementitious mortar) and the RA phase. This suggests that the stiffness properties of CTMs containing RA may be analaized using an approach based on the law of mixtures, similarly to what has been proposed for HMA (Christensen et al 2003). Further research is in progress to validate this kind of approach.

Figure 5. Application of the TTSP to CTMs experimental results (T = 20 °C) In Figure 6 |E*| values were shifted to the reference frequency of 20 Hz and a

linear regression was drawn in the semi-logarithmic plane. The slope of the regression is linked to the thermal sensitivity of the recycled mixtures and can be applied to correct stiffness values back-calculated from falling weight deflectometer tests. In particular it can be noticed that, between 10 °C and 20 °C, |E*| decreases by about 8 % and 10 % for 50 % RA and 80 % RA respectively.

3.0E+03

3.0E+04

1.0E-03 1.0E-01 1.0E+01 1.0E+03 1.0E+05

|E*|

[MPa

]

Fr [Hz]

3C-00RA 3C-50RA 3C-80RA

UPV tests



Figure 6.Complex modulus as a function of temperature

8. Conclusions

The technique of cement treatment is often adopted for the recycling of different materials including RA. An accurate selection of RA and correct integration with virgin aggregates so as to achieve a favourable grading can allow a great amount of RA to be recycled as a constituent material in CTMs, ensuring good mechanical characteristics.

CTMs using 50 % and 80 % of RA were compared with the reference CTM consisting in 100 % of virgin aggregates. Since universally accepted specimen preparation, testing and field control procedures are not available particular emphasis was placed on the specimen preparation and testing procedure.

The use of RA caused a general weakening of CTMs in terms of resistance (ITS and UCS) and complex modulus. However, both recycled mixtures showed appreciable mechanical properties.

Although the bitumen contained in RA appeared to be very stiff and the cement bond tended to inhibit any thermo-dependent or time-dependent response of the mixtures, both the temperature and the load frequency influenced the complex modulus of CTMs containing RA. In particular, the complex modulus of recycled mixtures decreased on increasing the temperature or decreasing the frequency.

The tested CTMs were not strictly thermo-rheologically simple materials, but the partial time-temperature superposition principle was applied to represent the

Log |E*| = -0.0037T + 4.01R² = 0.9436

Log |E*| = -0.0045T + 3.84R² = 0.9703

3.0E+03

3.0E+04

0 10 20 30 40 50 60 70 80

|E*|

[MPa

]

T [°C]

f requency= 20 Hz3C-50RA3C-80RA



complex modulus norm variation as a function of frequency or temperature. The relationship between |E*| and frequency appears amenable to be described by the law of mixtures, whereas the relationship between |E*| and temperature can be practically applied to correct stiffness values back-calculated from falling weight deflectometer tests on CTM courses containing RA.

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