Assessing the Flexibility of Recycled Asphalt Concrete Pavement

Utilizing reclaimed asphalt pavement (RAP) in new asphalt mixtures has increased in recent years because of its economic and environmental benefits. The flexibility of the recycled asphalt concrete (with cutback and emulsion) in terms of resilient modulus (Mr), rutting resistance, and permanent microstrain have been investigated in this work. Cylindrical specimens of 102 mm in diameter and 102 mm in height have been prepared from the recycled mixture after the short-term aging process. Specimens were subjected to 1200 repeated compressive stresses at (25) oC. The vertical permanent microstrain was monitored through video capture. It was concluded that RAP mixture can hold the applied loading with minimal permanent deformation as compared to the recycled mixtures. The resilient modulus is lower by (24 and 39) % for mixes recycled with cutback and emulsion respectively as compared to that of RAP. The rate of strain (slope) increases by 11 % and 4 % when cutback and emulsion were implemented as recycling agent respectively as compared to that for RAP mixture.


Introduction
The challenge facing road engineers is to develop a sustainable asphalt mixture that reduces the pavement system failure by developing new road materials and new methods of road construction and maintenance [1]. Progress in the road material science has focused on aged and waste materials, such as RAP, likely because of the continued availability of low-cost materials and due to the functional design of asphalt pavements. Recycling can provide the RAP, which is an important economical savings, RAP is usually considered to be a cost-effective pavement construction material which is placed in pavement at increasing percentages [2]. Many researches had indicated the economic benefits of recycling [3]. construction and reconstruction of road pavements imply a considerable consumption of valuable and non-renewable natural resources and the component materials of asphalt mixtures [4]. The properties of the recycled mixture are believed to be mainly influenced by the aged reclaimed asphalt pavement (RAP) binder properties, and the amount of RAP in the mixture [5]. Asphalt binder loses many of its oil components during construction and service resulting in a high proportion of asphaltenes in the blend, which leads to increased stiffness and viscosity of the binder and decreased ductility, making the binder hard and brittle [6]. To recycle this hard and brittle aged pavement, the asphalt must be returned or changed to have the rheological properties of original asphalt. This transformation is completed by adding liquid additives to the mixture being recycled, these additives have been called recycling agents or softening agents [7]. Rejuvenating emulsions are normally used, containing oils that reduce the viscosity of aged asphalt cement, thus improving the adhesion and cohesion properties, as well as the flexibility of the binder. In addition, rejuvenators can penetrate the voids of the pavement, filling them and minimizing the binder oxidation [8]. The rutting resistance of recycled mixtures was studied by [9]. Four mixtures with RAP percentages of 0%, 15%, 30% and 50%, were tested. Results obtained from the wheel tracking test indicated that RAP mixtures have very similar rut depth values at the end of the test when calculated between cycles 5000 and 10,000 which means that the presence of RAP in mixtures provides greater resistance to rutting. laboratory investigation of permanent deformation characteristics of asphalt concrete mixes containing reclaimed materials was presented by [10]. The permanent deformation characteristics of asphalt concrete with and without reclaimed materials were evaluated in laboratory using Repeated Load Axial Test and Wheel Tracking Test at a range of test temperatures. Test results showed that the asphalt concrete prepared using reclaimed materials such as waste plastic and Reclaimed Asphalt Pavement (RAP) were more resistance on permanent deformation over a range temperature. The use of resilient modulus testing to compare mixtures compacted with only virgin materials to those compacted with varying amounts of RAP was conducted by [11]. Resilient modulus testing was conducted in accordance with ASTM D 4123-82. The test was performed at 0.33, 0.5, and 1 Hz. In a 1-Hz test the applied cycles consisted of a 0.1-second load followed by a 0.9-second rest period.
It was concluded that the resilient modulus rapidly decreases with increasing temperature. This is due to the softening of the asphalt binder as the temperature is increased.
In this investigation, the variation in the flexibility of the RAP and recycled asphalt concrete pavement (with cutback and emulsion) in terms of resilient modulus (Mr), rutting resistance, and permanent microstrain have been investigated.

Aged materials
The reclaimed asphalt mixture was obtained by the rubblization of binder course layer of asphalt concrete of the highway in the province of Babylon. This highway was heavily deteriorated with various cracks and ruts existed on the surface. Reclaimed asphalt mixture obtained was assured to be free from deleterious substances and loam that gathered on the top surface. The reclaimed mixture was heated, combined and reduced to testing size as per AASHTO [12]; a representative sample was subjected to Ignition test according to AASHTO T 308, [12] procedure to obtain binder and filler content, gradation and properties of aggregate. Table 1. Presents the properties of aged materials after Ignition test.
Gradation for the RAP obtained from reclaimed mixture was determined; six samples have been selected randomly from the rubblization process of material stack. These samples were subjected to Ignition test to isolate binder from aggregate and then aggregate was sieved and separated to various sizes to calculate gradation for each sample. The differences between samples were in a minor extent, and the average gradation of the six samples obtained to be the old aggregate gradation is shown in Figure 1 which illustrates that the gradation of old (reclaimed) aggregate for binder layer has slimly deviation with Specification limits of Roads and Bridge SCRB [13].

Cutback asphalt
Medium curing cutback (MC-30) obtained from Al-Dura refinery was adopted for recycling in this work. The properties are listed in Table 2.

Recycling agents
Two types of liquid asphalt have been implemented as recycling agent based on available literature [1,[4][5][6]14]. They are medium curing cutback, and cationic emulsion.

Emulsified asphalt
Cationic emulsion obtained from ministry of industry and minerals was adopted for recycling in this work, the properties are listed in Table 3.

Recycling of RAP mixture
Recycled mixture consists of 100 % reclaimed pavement RAP and recycling agent mixed together at specified percentages according to the mixing ratio. First, RAP was heated to approximately 160º C and liquid asphalt was added to the heated RAP at the desired amount of 0.5% by weight of mixture and mixed for two minutes until all mixture was visually coated with recycling agent as addressed by [1]. The recycled mixture was prepared using two types of liquid asphalt: medium curing cutback and cationic emulsion.

Preparation of accelerated short term aged recycled mixture
Recycled mixture was heated to 130°C to become loose and then diffuses in shallow trays with 3cm thickness and subjected to accelerated aging by laying inside an oven at 135°C for 4 hours as per Superpave procedure [12,16]. The mix was stirred every 30 minutes during the short-term aging to prevent the outside of the mixture from aging more than the inner side because of increased air exposure.

Preparation of asphalt concrete specimens
Cylindrical specimen of 102 mm in diameter and 102 mm in height have been prepared from the recycled mixture after the short-term aging process. The mold, spatula, and compaction hammer were heated on a hot plate to a temperature of 150º C. A piece of non-absorbent paper, cut to size, was placed in the bottom of the mold before the mixture was introduced. The asphalt mixture was placed in the preheated mold, and then it was spaded drastically with a heated spatula 15 times around the perimeter and 10 times around the interior. Another piece of non-absorbent paper cut to size was placed on the top of the mix. The temperature of mixture immediately prior to compaction temperature was 150ºC. The mold assembly was placed on the compaction pedestal and subjected to static compaction. The mixture was compressed at top and bottom at temperature 150 °C under an initial load of 1Mpa to set the mixture versus the sides of the mold, after that the required load to achieve the target density of 2.372 gm/cm 3 was applied for two minutes and the specimen was left to cool at room temperature for 24 hours and then it was removed from the mold using mechanical jack. Specimens were implemented for the Repeated compressive stresses test. Details of obtaining the target density were published elsewhere [17]. Figure 2 exhibit part of the prepared cylindrical specimens.

Testing of the specimens under repeated compressive stresses
Asphalt concrete specimens were subjected to repeated compressive stresses in the pneumatic repeated load system PRLS. The axial repeated load was applied to the specimen and the axial permanent deformation was measured. Compressive loading was applied in the form of rectangular wave with a constant loading frequency of 60 cycles per minute and loading sequence for each cycle is 0.1 sec load duration and 0.9 sec. Load repetitions was applied under constant three stress levels of (0.069, 0.138, and 0.207) MPa, while the testing temperatures of (25) ºC was implemented in the test. Figure 3 exhibit the repeated compressive stress setup. The permanent vertical strain is measured as a function of the number of load applications recognizing the fact that the lower permanent strain is related to the lower sensitivity for rutting and corrugation. The accumulation of permanent and resilient strains (εp and εr) were monitored directly through a continuous video capture, while the resilient modulus (Mr) was calculated using equations 1 and 2 [16,18]. Specimens have been tested under three stress levels.
where Mr is resilient modulus (N/mm²) and εr is vertical resilient strain (mm/mm).  Figure 4 exhibit the influence of the stresses level on resilient modules for aged and recycled mixture (cutback and emulsion) under compressive stress at 25 °C after 1200 load repetitions. The resilient modulus increases up to stress level of 0.138 KPa, then decreases when the stress level increases to 0.207 MPa. The highest resilient modulus could be achieved at 0.138 MPa level of stress for all mixtures. This may be attributed to that the recycled mixture requirement of strength is suitable under the moderate traffic loading condition. Higher stress level of 0.207 MPa will possess extra tensile stresses which the mixture is unable to accommodate, then the resilient modulus is decreased. On the other hand, lower stress level of 0.069 MPa will not exhibit high impact on the resilient modulus. It can also be observed that the RAP mixture exhibits resilient modulus of 171 MPa, while the recycled mixtures shows lower resilient modulus by (24 and 39) % for mixes recycled with cutback and emulsion respectively as compared to that of aged mixture. Such results agree with [16] work.  For recycled mixture with (emulsion), the rate of increases is (12, and 17) % as compared to that at 0.069 MPa. The rate of change in slope value is different for different stress level and different mixture. At moderate stress level of 0.138 MPa, recycled mixture with cutback asphalt shows lower intercept value by 2 % as compared to that of RAP mixture, while the recycled mixture with emulsion exhibit higher intercept value by 14 % as compared to that of RAP mixture. On the other hand, the rate of strain (slope) increases by 11 % and 4 % when cutback and emulsion were used as recycling agent respectively as compared to that for RAP mixture. this may be attributed to more flexible nature of recycled asphalt concrete as compared to RAP. Such test results agree with [16,18] work.  Figure 6 exhibit the rutting performance of various asphalt concrete mixtures; it can be observed that the stiffer RAP mixture can hold the applied loading with minimal permanent deformation as compared to the recycled mixtures. At high stress level of 207 kPa, the permanent strain increases by (20 and 28) % for recycled mixtures with cutback and emulsion respectively as compared to RAP mixture. Table 4 demonstrates the mathematical models regarding the resistance to rutting of asphalt concrete where (Y) represent the permanent deformation (microstrain) and (X) denotes the load repetitions. Similar rutting trend was reported by [14].

Conclusions
Based on the testing program, the following conclusions could be drawn: 1) The resilient modulus is lower by (24 and 39) % for mixes recycled with cutback and emulsion respectively as compared to that of RAP (aged) mixture at moderate stress level of 0.138 MPa.
2) The resilient modulus increases up to stress level of 0.138 KPa, then decreases when the stress level increases to 0.207 MPa. The highest resilient modulus could be achieved at 0.138 MPa level of stress for all mixtures.
3) At moderate stress level of 0.138 MPa, recycled mixture with cutback asphalt shows lower intercept value by 2 % as compared to that of RAP mixture, while the recycled mixture with emulsion exhibit higher intercept value by 14 % as compared to that of RAP mixture.
4) The rate of strain (slope) increases by 11 % and 4 % when cutback and emulsion were implemented as recycling agent respectively as compared to that for RAP mixture.