Performance of rubber asphalt in split mastic asphalt mixture

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Introduction
Material technology in the field of asphalt pavement is improving yearly, so many types of modified asphalt are found, including polymer asphalt [1][2].With the addition of polymer, asphalt becomes more elastic with a relatively high elastic recovery value, making the asphalt more resistant to deformation [3].Therefore, research was carried out using an asphalt mixture with additional ingredients in the form of latex or what is usually called rubber latex.
Latex is a white liquid/solution obtained by cutting the stems of sap-producing plants.Latex can stabilise asphalt mixtures [1].Rubber asphalt is polymer-modified asphalt to which additives are added as natural rubber in the form of a liquid, namely latex.
Rubber asphalt has been modified by adding ingredients in natural rubber, both in liquid (latex) and solid (crumb rubber) forms.This rubber component makes rubber asphalt superior to pure asphalt because it can increase the softening point, elasticity and adhesiveness, making it more durable and long-lasting [4][5][6].
Using rubber binders can enhance aggregates and asphalt's adhesion in forming a pavement.Using crumb rubber derived from recycled tires is a valuable resource in road pavement applications, potentially enhancing adhesion performance significantly.Moreover, it provides an effective means of waste tire management while concurrently mitigating the adverse environmental impacts of tire disposal [2,7].Testing the use of latex in pavement can also enhance resistance to deformation, as seen from the indirect tensile stress test results.The M R value increased by 20% with the addition of 6% latex.This increase can be attributed to the improved interfacial adhesion between aggregates and modified asphalt binder due to the presence of latex, thereby enhancing the bond and adhesion within composites to resist rutting [8].
Split mastic asphalt (SMA) entails blending an opengraded asphalt concrete mix with a substantial asphalt cover, necessitating the inclusion of cellulose fibres to maintain the asphalt's height consistency [9].In this study, latex is employed as the adhesion component in the SMA mixture.
This study aimed to determine the effect of using 7% rubber asphalt on the performance of Split Mastic Asphalt (SMA) mixtures compared to SMA without rubber, namely no rubber asphalt.The results of this research can also be used as a reference for researchers in the field of road pavement, who use latex as an asphalt mixture and can be a trigger for other research regarding the use of latex.

Material and methods
The Marshall test is utilised to assess how well an asphalt mixture can resist deformation under pressure.Stability refers to the asphalt mixture's capacity to endure various loads before it undergoes plastic deformation.This stability is typically measured in kilograms or pounds.Plastic deformation, often referred to as flow, is the alteration in the shape of the asphalt mixture due to applied stress until it reaches the point of failure, measured in millimetres or hundredths of an inch [7][8].
The Marshall method is used to design the mixture and determine the Optimal Asphalt Content (OAC) for each mixture variant.The parameters used to determine the OAC are Marshall Stability, Flow, Void in Mixture (VIM), Void in Mineral Aggregate (VMA), Void Filled with Asphalt (VFA), and Marshall Quotient (MQ).
Marshall parameter values that meet the criteria according to Pd 08-2019-B concerning Specifications Guidelines for hot asphalt mixtures with natural rubbercontaining asphalt will be determined as the median value to obtain Optimal Asphalt Content (OAC) [10][11].
The durability or durability of asphalt is the ability of asphalt to maintain its original properties due to the influence of weather or temperature changes during the road's service life.Good asphalt is not easily brittle and loses its plastic properties due to temperature changes [12].
This research was conducted at the Transportation and Road Pavement Laboratory of Andalas University.The aggregate originates from CV Bakri Mandiri, Batang Anai sub-district, West Sumatera.Aggregates play a critical role in determining the performance, durability, and overall quality of the asphalt mixture.In addition, as seen in Table 1, it can be concluded that the results of the aggregate inspection for all tests meet the standards in accordance with the General Specifications for Bina Marga 2018 Revision 2 [13].
The asphalt used is rubber asphalt with a latex content of 7% of the total weight of asphalt from PT Bumi Mulia, North Sumatera.The type of mixture used is Split Mastic Asphalt (SMA).The method used to determine the aggregate content is the mid-sec point method, where the weight of the aggregate is obtained by taking the middle value of the SMA mixture specifications, which are in accordance with the 2019 SE Minister of PUPR standards (Pd 07-2019-B) for specifications hot asphalt mixture with asphalt containing natural rubber.Determination of aggregate content can be seen in Table 3.As can be seen in Table 3, the percentage of aggregate content in each sieve size for the SMA mixture is obtained in accordance with the 2019 PUPR Ministerial Decree (Pd 07-2019-B) for specifications for hot asphalt mixtures with asphalt containing rubber.
The method used to determine the theoretical asphalt content is the Depkimpraswil 2002 method.Hence, the theoretical asphalt content obtained in this test is 5.35%.

Result and discussion
Marshall testing is conducted to determine the stability and flow characteristics, as well as the analysis of the density and porosity of the formed solid mixture.Five variations of filler content were prepared to determine and analyse the influence of the 7% rubber: 4.6%, 5.1%, 5.6%, 6.1%, and 6.6%.
Stability in the pavement layer should be able to withstand traffic loads without undergoing shape changes such as waves, ruts, or bleeding.The examination of Marshall Stability aims to determine the resistance (stability) to the flow of the asphalt mixture.

Stability
A stability assessment was conducted to evaluate the applicability of latex utilisation as a binder in asphalt for Stone Matrix Asphalt (SMA) mixture.Marshall Stability represents the capability of an asphalt mixture to endure a load until reaching a state of flow, quantified in kilograms.Conversely, flow denotes the deformation of an asphalt mixture due to an applied load, measured in mm.Based on Figure 2, the graph shows the relationship between stability and asphalt in the mixture.The required Stability value is ≥ 600 kg, so the stability values of rubber asphalt and no rubber asphalt for the asphalt content range of 4.6% -6.6% have met the standard values.The stability value of rubber asphalt at the Optimum Asphalt Content (OAC) is 2791.712,which is higher than the stability value of no rubberised (NR) asphalt, i.e. 1943.228.This comparison shows that the stability of the rubberised (RA) asphalt is significantly higher than that of the NRA asphalt within this asphalt content range.The decrease in stability value is caused by the asphalt, which initially acts as a binder for the aggregate, changing its function to a lubricant after reaching its optimum value, resulting in a decrease in adhesion and friction between the aggregates, leading to a decrease in the stability value of the mixture.

Flow
Flow is the amount of settlement or deformation in the asphalt concrete layer due to its applied load.Figure 2 shows the graph showing the relationship between flow and asphalt content in the mixture.Even though the Rb asphalt mixtures have high flow values, they are more likely to undergo plastic deformation.The required flow value is 2.0 mm -4.5 mm, so the flow values for Rb asphalt and NRb asphalt meet the standard range of 4.6% -6.1% and 4.6% -6.6%, respectively.The flow value of Rb asphalt tends to be higher than that of NRb asphalt.The high stability of Rb asphalt indicates that the mixture has a strong structure and is less likely to experience significant deformation than NRb Asphalt.
The appropriate flow value according to the standard indicates that the mixture has controlled deformation capabilities, which help adapt to movements and traffic loads without experiencing excessive deformation or plasticity excess.

Void in mixture
VIM refers to the percentage of air voids within an asphalt mixture to the total volume of the mixture after compaction.A high volume of air voids and a low asphalt content in the mixture can lead to faster fatigue.Indeed, pavement with high VIM will encounter some challenges, but it can still be used with specific treatments.From Figure 3, it can be seen that there is a high VIM for both types of binders.This indicates many air voids in the asphalt mixture after compaction.Although a low VIM is preferred for optimal road quality, pavement with high VIM can still provide certain functionality.
However, it is essential to note that pavement with high VIM tends to require more frequent maintenance due to its susceptibility to deformation, cracking, and faster damage.

Void in mineral aggregate (%)
VMA (Void in Mineral Aggregate) is the air voids between aggregate particles in a compacted asphaltaggregate mixture, including spaces filled with asphalt.VMA is the space to accommodate asphalt and the necessary air voids within the asphalt-aggregate mixture.The VMA value results can be seen in Figure 4. From the research results, the VMA values increased, consistent with the tendency of VMA values to increase with the percentage of asphalt content.The increase in VMA is due to the thicker asphalt film coating the aggregate as the asphalt content increases, resulting in larger void spaces between them.According to the Ministry of Public Works (Department of Public Works and Spatial Planning) specifications in Highway Construction 2002, the VMA value requirement is a minimum of 18%.The research results show that the VMA values meet the requirement starting from all asphalt content percentages, with a minimum VMA value of 18.0%.However, in the 1987 Bina Marga specifications, the VMA value is not listed as a requirement.High VMA and high asphalt content indicate a flexible pavement layer.

Void filled with asphalt
Void Filled with Asphalt (VFA) is the percentage of voids in the mixture filled with asphalt after compaction.In this study, the VFA results can be observed in Figure 5.
Figure 5 represents the percentage of voids in the mixture filled with asphalt after the compaction process.Various factors, including the asphalt content, aggregate gradation, compaction energy, compaction temperature, and water absorption by the aggregates, influence VFA (Voids Filled with Asphalt).
When VFA is excessively high, it can lead to asphalt rising to the surface at elevated temperatures.Conversely, if VFA is too low, the mixture may become porous, making it more susceptible to oxidation.In Figure 5, Rb Asphalt and NRb Asphalt exhibit small VFA values.This raises concerns about the potential for oxidation in this Split Mastic Asphalt (SMA) pavement.To mitigate this, the use of a strongly adhesive binder, such as rubberised asphalt, is anticipated to reduce the occurrence of oxidation, even with a low VFA.

Marshall quotient
A higher Marshall Quotient (MQ) value typically suggests that the mixture is rigid and less flexible when under a load.Conversely, a mixture with a lower MQ value tends to be more flexible in accommodating the applied load.The outcomes obtained from the Marshall Quotient analysis are illustrated in Figure 7.The Marshall Quotient (MQ) is the ratio of stability (measured in kN) to flow (measured in mm), serving as an indicator of the mixtures' stiffness.This metric signifies the material's ability to withstand shear stresses and permanent deformation, specifically rutting.
The Marshall Quotient (MQ) is determined by comparing the corrected stability value to the flow value, providing insights into the stiffness of the pavement mix.The trend of MQ shows an initial increase up to a certain asphalt content, followed by a decrease with subsequent increments in asphalt content.
Analysing Figure 6, it is evident that the MQ value for NRb is higher than that of Rb.Consequently, it can be inferred that the flow value for Rb is lower than that of NRb, a trend depicted in the flow vs. asphalt content graph.Despite its relatively lower value, it still satisfies the necessary criteria.Hence, using Rb Asphalt in the SMA pavement remains viable and robust, ensuring the pavement's structural integrity.

Marshall parameter evaluation
An evaluation is undertaken to ascertain whether the planned asphalt mixture aligns with predetermined criteria based on the results obtained from Marshall testing.This evaluation encompasses an analysis of stability and flow values and volumetric calculations.Apart from structural strength (stability and flow), the performance of the asphalt mixture is substantially influenced by the volumetric attributes of the compacted mixture [14,11,15].
The stability of Rubber Asphalt is observed to surpass that of No Rubber Asphalt across all generated samples.Furthermore, regarding the flow values for both mixtures, Rb Asphalt demonstrates a superior flow value compared to NRb Asphalt.However, all fall within the accepted standard flow value range of 2-5 mm.The heightened flow value of Rb Asphalt results in a diminished Marshall Quotient (MQ) for Rb Asphalt in comparison to NRb Asphalt, as MQ is defined as the ratio of stability to flow.
In the realm of volumetric characteristics of the mixture, the VMA value for Rb Asphalt eclipses that of NRb Asphalt.This VMA value serves as an indicator of the mixture's flexibility.A higher VMA value corresponds to heightened flexibility within the mixture.

Conclusion
Natural rubber or latex was employed in this research to ameliorate the Marshall parameter of Stone Mastic Asphalt (SMA) pavement while also evaluating the appropriateness of incorporating 7% rubber into 60/70 pen bitumen as a binder for the SMA pavement mixture.The 7% rubber was sourced from PT. Bumi Mulia.
This investigation ascertained that the integration of 7% rubber in the asphalt exhibited improved stability and flow values within the standard range.Consequently, the SMA incorporating rubber in its asphalt manifested elevated VIM and VMA values alongside a diminished VFA.Nonetheless, Rb Asphalt for this SMA pavement exhibited heightened stability compared to SMA that did not utilise rubber in its asphalt composition.

Table 1 .
Physical characteristics of the aggregate.

Table 2 .
Properties of asphalt used.

Table 3 .
Sieve analysis of the aggregate.

Table 4 .
The required value for Split Mastic Asphalt.