Effects of nano-particles on cold recycled asphalt properties

Cold recycling of asphalt is an effective solution to reduce the environmental impacts of pavement construction and operation. This approach reduces the use of scarce resources and fossil fuels, and prevents the spread of greenhouse pollutants in the air. Despite its benefits, recycled asphalt does have drawbacks, some of which include nibbling, stripping, and less resistance to fatigue. The use of additives can improve the mechanical properties of recycled asphalt. In this research, the addition of Nano-lime, Nano-Alumina, and Nano-clay instead of cement, to bitumen emulsion in recycled asphalt was used to improve the properties of asphalt resilient modulus and fatigue. A comparative method was used to analyze the obtained experimental results. In this study the resilient modulus and fatigue properties of asphaltic concrete were measured. The results show that asphalt fatigue properties and resilient modulus improve significantly with the addition of different Nano-particle percentages.


Introduction and background
The use of cold recycled asphalt helps reduce the use of aggregates, construction cost and time, energy consumption, and air pollution. Failures in flexible pavement, are mainly due to fatigue cracking and rutting [1][2][3]. Rutting generally occurs from load passages that exceed the pavement strength, but cracks usually occur from the high passage of vehicles with normal loads and the contraction and expansion of pavements caused by temperature changes. Among these two mechanisms, fatigue cracks are seen in a large number of asphalt pavement. Fatigue occurs as a result of repeated loads, dynamic variations and load dynamics, and load is usually less than the final static resistance of the asphalt. Fatigue resistance of an asphaltic mixture is the ability of the mixture to reciprocate bending loads without failure [4][5][6][7].
In recent years, new additives such as Nano-particles and fibers have been developed to improve pavement performance and reduce its damages. Fiber modifiers are used to improve the fatigue properties. Qunshan et al. conducted fatigue testing and concluded that modified fiber asphalt mixtures are more resistant to wear than the control mixture [5,8,9].
With the advent of Nano-technology and the recognition of Nano-scale materials, the use of Nano-technology in the road construction industry and in particular asphalt pavements has become evermore important. Researchers have found that the use of different Nano-materials such as Nano-clay, Nano-silica, Nano-zinc and Nano-lime in cold recycled mixtures can increase durability, Marshall Stability, rutting resistance, and fatigue resistance, and provide a robust and durable mixture [10].
Kavussi and Modarres studied the role of cement in refined bituminous emulsion mixtures and found that cement significantly increases the stiffness and reduces moisture sensitivity, thermal sensitivity and permanent deformation [11].
In a study similar to the one, Height Float polymer slack emulsion was used instead of SS-1 and CMS-2 s to eliminate rutting problems, reflective cracks and moisture damage. It was also found that US states often use some kind of high float emulsion bitumen, most of which are slow curing; although, only a few states prefer cationic slow curing or medium curing [12].
Through a series of mechanical experiments on Nanoclay modified asphalt mixtures, Ghile et al. saw improvements in the mechanical properties of asphalt mixture, such as the indirect tensile strength, creep and fatigue [13].
Sureshkumar et al. studied the effects of clay admixtures on polymer asphalt mixtures and found that clay had a consistent effect on asphalt and polymer. The high compatibility of clay and polymer can not only lead to better dispersion of polymer in asphalt, but can also affect the final rheological properties of the studied systems [14].
Ziari et al. evaluated the effect of bentonite additives on the fatigue behavior of hot asphalt mixtures and mixing bitumen with different percentages of bentonite. The results show that fatigue life of asphalt mixtures prepared with bentonite modified bitumen is longer than conventional HMAs. Also, bentonite leads to relative increase in indirect tensile strength and resilient modulus of asphalt mixtures. Finally, based on experimental results, a model is proposed to describe the fatigue behavior of asphalt mixtures containing bentonite modified bitumen [15]. In another study, they evaluated the impact of bentonite on the rutting behavior of asphalt mixtures. The results showed that the bituminous modification of hot asphaltic mixtures with bentonite significantly increases rutting resistance and decreases permanent deformation at high temperatures [16].

Research approach
The aim of the current research study was to investigate the effect of Nano-clay, Nano-Lime, and Nano-Alumina on the mechanical properties of cold recycled asphalt, such as resilience modulus and fatigue properties, using bitumen emulsions. The experiments were carried out dynamic creep, modulus of resilient, and four point bending test.

Materials
The samples used in this research were made from, Nanoclay, Nano-lime, Nano-Alumina, and anionic emulsified bitumen SS-1 to investigate the effect of Nano-particles on the mechanical properties of cold recycled asphalt.
ASTM C136 (Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates) were used to initiate aggregate grading once the aggregates are separated from the bitumen. The results of this sieve analysis are shown in Table 1. A 6.3% bitumen content was determined for recycled materials according to the AASHTO TP53-95 (Determination of Binder Content by the Ignition Method). Table 2 illustrates the characteristics of the anionic emulsified bitumen SS-1 used in this research, which is based on an aggregate separate in mixtures with calcareous materials.
Nano-particles' properties that used in this study shown in Table 3.

Preparation of the test sample
A total of three Marshall Samples for aggregate blend combinations were prepared according to the grading, specification and percentage of used emulsified asphalt.
One un-crushed sample was prepared to determine the maximum weight. Additionally, samples were built to determine the free space in the recycled mixture of the optimized bitumen and the optimal water percentage. The optimum bitumen is obtained when the bitumen percentage has the highest stability, the highest specific weight, the most suitable percentage of air voids, and has the empty space of aggregates within the regulation limits.
In this study, four different amounts of Nano-materials were added to the samples to determine the resilience modulus. Three types of Nano-materials were investigated and 3 test samples were prepared for each nanomaterial.
To determine the fatigue properties, three types of Nanomaterials were added to the asphalt mixture and three stress ratios were calculated for each nanomaterial. A total of 117 samples were tested.
The emulsified asphalt in this design is used in mixtures that are prepared in accordance with ASTM-D5505; RAP materials and if necessary, new aggregate materials are used as well. The amount of used emulsion in the Marshall Mix method ranges usually from 1 to 2 percent, and young emulsions vary from 5.0 to 1.25% compared to the weight of the asphalt mixes. The more the mixtures, the higher their percent of broken aggregates, and a lesser amount of available bitumen is used in RAP.
RAP samples from crushed materials are removed by the recycling machine to represent the used mixture in the recycling process. Cement consumption in the mixture is effective in obtaining the initial resistance, but its amount should not be more than 2% of the mixture weight, which should be clear during the designing process.
When using the new aggregate, both size and weight ratio must be determined in order to mix with RAP. This is so that the resulting RAP mixture and the material with the desired particle sizes have corresponding characteristics. Temperature of the aggregate mixture with emulsified asphalt, along with its compacting Marshall temperature, and the experimental temperatures of the of Marshall samples are generally 25 °C. Another option is to measure the maximum temperature of the project's environment during operations.
The following formula is used to estimate the amount of needed bituminous emulsion for the recycled mixture: where E, the percentage of required bitumen; 1.2, the fixed percentage of bitumen; A G , correction factor for the size in terms of percent; A AC , correction factor for the percentage of existing bitumen in recycled mixture in terms of percent; A PV , correction factor for penetration grade.
In cases where E comes to have different results for a recycled material sample, the bitumen percentage is shown to be less. The percentage of water is equal to 3% minus the sum of the moisture percent of aggregate mixture and the percentage of the water in bitumen mixture. Nanomaterial has been added to the water and mixed then the blended materials have been added to the emulsified asphalt.
Emulsified asphalt, which has been heated to 60 °C is added to the mixture, in accordance with the project's needs for each specific bitumen percentage and 0.5 difference. The resulting mixture is stirred until the emulsified asphalt or rejuvenating emulsion is evenly distributed in the material.
To beat the samples, the Marshall empty models are warmed at a 60 °C temperature for 1 h. In any mixture, three models are prepared for certain amounts of bitumen or rejuvenating emulsions, and each side is compressed with 50 Marshall Hammer lashes. The sample models for treatment are placed in an electronic furnace for 6 h at a temperature of 60 °C, where they will then be removed from the furnace. The models are kept at room temperature overnight. Table 4. Summarized the characteristics of samples.

Resilience modulus test
Indirect resilience modulus test is performed on samples made in the laboratory or samples taken from existing paths. The temperature, load, loading frequency and durability of the loading time can be changed. The samples are loaded at different frequencies under the recommended

Fatigue test of four-point bending beam
The fatigue test was performed by placing the asphalt beam under a four-point repeat loading at a given stress level. For this study, a constant stress and loading time was considered, and the number of loading times was obtained. A four-point bending test was used in this study to measure the maximum tensile stresses, the maximum strain, the resilient modulus of asphalt samples without additives, and various percentages of additives. Asphalt beams (380 mm length, 50 mm wide and 50 mm tall) were constructed. The following relationships were used in the test.

Resilient modulus test
Under the ASTM D4123 standard, the asphalt mixture modulus is determined by an indirect tensile test. In this research, UTM5 is used to determine the resilient modulus. Initially, the asphalt specimen was adjusted with the help of a special frame to make it completely symmetric when loaded under the device and onto the sample center. The resilient modulus was determined when a 400 N semi-sinusoidal load was loaded in 0.5 s and 1.5 s at 25 °C (Fig. 1). The resilient modulus is one of the most important mechanical properties of asphalt mixtures when determining the thickness of pavement layers. Table 5 and Fig. 2. shows the resilient modulus variation in different percent of Nano-particles. Increasing the Nano-particles percentage increases the resilient modulus up to 2.5 percent. In 4 percent of Nano-particle the resilient modulus has been reduced. Nano-Alumina has the most resilient modulus and Nano-Lime has the least. This is probably due to the fact that increasing the percentage of nanomaterials improves the emulsion setting to some extent, resulting there is a better bond between the bitumen and the aggregate. To some extent, increasing the percentage of nanomaterials increases the setting speed of bitumen and eliminates the possibility of bonding between materials.

Fatigue test
An indirect tensile fatigue test was conducted in this research with the Universal Testing Machine. A roller compressor was used for a flexural bending test to make and compress the slab of samples of conventional and modified asphalt mixtures. The samples were cut with a power saw, and the asphalt beam dimensions were prepared according to the AASHTO T321 standard. The loading pattern used was a semi-sine signal. When the loading was repeated, the loading time was 0.1 s and was followed by a rest period of 0.4 s. The fatigue tests were conducted at a temperature of 20 °C. Three stress ratios (0.2, 0.3 and 0.5) were involved and were defined as the applied stress amplitude divided by the indirect tensile strength of asphalt mixtures. The fatigue test results for each stress ratio are shown in the Table 6.
Tables 5 illustrate that the addition of 2.5% of Nanoparticles to the recycled asphalt mixtures in all tensile ratios considerably increases the asphalt fatigue strength, whereas a 4% addition decreases its fatigue strength; although the strength of the asphalt mixture would still be more than zero percent additive's specimens. Figures 2, 3, and 4 illustrate the rate and number of loadings and allow for a better comparison of the loads and frequency of repetitions in various stress ratios and percentages of the test additive. Using Nano-clay increased the strength of the specimens against loading, especially of higher stress ratios. The specimens carry larger loads with more frequencies until they fail. As a result, the number of cycles and larger stresses are better tolerated up until they reach the failure point.

Conclusion
This study investigated the effect of Nano-clay, Nano-Lime, and Nano-Alumina on the mechanical properties of cold recycled asphalt using bitumen emulsions. The experiments were carried out resilient modulus and fatigue properties with four point bending test. Three different percent of Nano-particles used in this research. The experimental results are as follow:   Research Article SN Applied Sciences (2021) 3:632 | https://doi.org/10.1007/s42452-021-04463-1 • Increasing the percentage of nano-alumina increases the resilience modulus. This increase was noticeable in lower percentages of nano-alumina. Increases in the additive percentage however, had less rate. • Adding Nano-particles increases the resilient modulus.
The specimen resilience modulus with 2.5 percent of Nano-alumina increases more than 110 percent, and with Nano-clay and Nano-lime this value is 98 and 85 percent. • A 2 percent addition of Nano-clay improves the asphalt fatigue performance of a 0.2 stress by 4percent, 0.3 stress by 14percent, and 0.5 stress by 64 percent. • Fatigue strength for modified asphalt is more than nonadditive asphalt. Number of loading increase 22 percent with Nano-clay, 53 percent with Nano-lime, and 31 percent with Nano-alumina in 0.2 stress ratio. For 0.3 stress ratio, number of loading increase 57 percent with Nano-clay, 82 percent with Nano-lime, and 57 percent with Nano-alumina. Nano-particles has more than 100 percent number of loading in 0.5 stress ratio.

Conflict of interest
The authors declare that they have no conflict of interest.
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