The Usage of Additives on Porous Asphalt

Roads are an important infrastructure component, but often face problems such as changes in temperature, humidity, and standing water. To overcome this problem, a layer of porous asphalt which has air voids to reduce standing water on the road surface can be a solution. However, although porous asphalt has water-related benefits, its structure is less durable and requires increased resistance. Therefore, this study aims to analyze the effect of adding additives on the strength, permeability, and porosity of porous asphalt. Three types of additives studied were crumb rubber (CR), nano silica (NS), and Wetbond-SP. Several laboratory tests were conducted on the prepared specimens, namely the Marshall, Cantabro Loss, and permeability tests. The results showed that adding NS onto the specimen was able to increase the stability value of porous asphalt, while the addition of CR and Wetbond-SP had no significant effect on the stability parameter. However, the addition of Wetbond-SP improved the adhesion between the aggregate and asphalt, resulting in a denser mixture. The three additives also increased moisture resistance and cohesion loss in the mixture, as indicated by a decrease in Cantabro Loss (CL) percentage. Additives also impact the permeability and porosity values, where adding more additives in the mixture will reduce the permeability and porosity values. Based on the results of the study, the optimum concentration of CR added was 1% of the total weight of the mixture, NS was 4% of the weight of asphalt content, and Wetbond-SP was 0.4% of the weight of asphalt content.


Introduction
Water can reduce the quality of the road structure because it can remove the cohesive bonds in the binder and also the adhesive bonds between the aggregate and the binder [1].In addition, the presence of water on the road surface can cause hydroplaning, splash and spray.As a result, road conditions will decrease due to these factors.To overcome these problems, an open graded asphalt or porous asphalt layer can be a solution.Porous asphalt is one of the pavement layers that has many air voids which will allow water to quickly penetrate the subgrade and filter into the natural soil beneath.However, porous asphalt is substantially less durable due to its open structure and has a lower modulus of tensile strength and flexibility than hot mix asphalt.Consequently, it is necessary to increase the resistance of porous asphalt, one of which is by adding additional materials.Over the past decades, a number of research initiatives have concentrated on the manufacture of modified asphalt mixes to improve overall pavement IOP Publishing doi:10.1088/1755-1315/1324/1/012002 2 performance [2]- [4].Some of these studies concentrate on the use of additives to improve the quality of porous asphalt mixtures.
This study seeks to conduct a thorough examination to increase knowledge about the effects of adding crumb rubber (CR), nano silica (NS), and Wetbond-SP additives to porous asphalt mixtures.Laboratory tests will be used in this study to investigate how the addition of these additives changes the mechanical properties, resistance to deformation, and drainage characteristics of porous asphalt mixtures.

Experimental Design 2.1 Materials 2.1.1 Aggregates
In this study, the aggregates were divided into three categories based on their size, as shown in Table 1.While aggregate testing was divided into two types, namely coarse aggregates (Aggregate I and Aggregate II) and fine aggregates (Aggregate III and Aggregate IV).Table 2 and Table 3 show the results of aggregate testing which includes specific gravity, absorption, abrasion, and sieve analysis tests that have been carried out in the laboratory.It was found that all the results of the aggregate testing met the specifications.

Asphalt
The asphalt used as a binder in the mixture in this study had a penetration value of 60/70.The results of asphalt testing that has been carried out in the laboratory are shown in Table 4. Based on the results obtained, it can be seen that the asphalt satisfies all the requirements, and hence, is suitable to be used in the asphalt mixtures.Additives are a material added to asphalt mixtures to improve the properties and performance of asphalt mixtures.Crumb rubber (CR), nano silica (NS), and Wetbond-SP are the three additives used in this study that have been proven to increase moisture resistance so as to improve the performance of porous asphalt.
Crumb rubber (CR) is used tires that have been ground into small particles ranging in size from 4.75mm to 75mm.CR has hydrophobic and non-polar properties which cause it to repel water and trap air on its surface [5].Mixtures with added CR were found to have longer fatigue life, reduced potential for permanent deformation, stronger resistance to crack initiation, lower temperature susceptibility, and greater resistance to moisture damage [6].The CR content added to the modified porous asphalt mixture in this study was 0.5%; 1%; and 1.5% of the total weight of the mixture.Nano silica (NS) is a relatively new inorganic substance used due to its high chemical purity, large surface area, strong adsorption, excellent dispersing capacity, and excellent durability [7].NS has a significant affinity for water because the NS surface is evenly distributed from the silanol and siloxane groups with high water absorption [8].NS can make coarse aggregate more polar, increase the adhesion of aggregates to the binder and reduce the loss of particles due to abrasion [9].According to several studies, the ideal concentration of NS is 2 -6% by weight of the binder [10].In this study, the NS content added was 2%, 4%, and 6% by weight of asphalt content.
Wetbond-SP is a type of anti-stripping agent that can help strengthen the bond between aggregate and asphalt [11].When Wetbond-SP is combined with aggregate, the aggregate will become lipophilic which makes it insoluble in water and makes the aggregate surface hydrophobic so that it is water resistant [12].The Wetbond-SP content used in this study was 0.3%, 0.4% and 0.5%.

Laboratory Tests
Asphalt mixture testing is used to determine the physical and mechanical qualities of asphalt mixtures.Physical testing includes volumetric testing of asphalt mixtures: air voids (VIM), mineral aggregate voids (VMA), voids filled with bitumen (VFA), and porosity.Meanwhile, mechanical testing was carried out by Marshall, Cantabro Loss, and permeability testing.The results of the sample testing were then compared with several specifications from the four different institutions as listed in Table 5.

Marshall Test
Marshall test is the standard used to determine whether a mix with a maximum aggregate size of 25 mm is suitable for use [17].By conducting the Marshall test, Marshall characteristics will be obtained: stability, flow, and Marshall Quotient (MQ).In addition, other parameters that can be calculated from the Marshall test results are voids in the mixture (VIM), voids in mineral aggregate (VMA), and voids filled with asphalt (VFA).

Cantabro Loss Test
The Cantabro Loss test is used to assess how well the mixture resists abrasion under impact loads [18].The purpose of this test is to find out how much sample weight is lost after an abrasion test is carried out using a Los Angeles machine [19].The lost sample weight illustrates the resistance of the porous surface layer to loss caused by tire wear and environmental damage [20].The Cantabro Loss test was carried out by placing the compacted sample into the Los Angeles abrasion machine and rotated 300 times at a speed of 30-33 revolutions per minute.

Permeability and Porosity Test
Permeability is the capacity of a material to allow fluids to flow vertically and horizontally.The best method for measuring permeability is to use falling head permeability (FHP), where water in a pipe (or standpipe) falls freely at a predetermined height until it passes through the cavity in the porous mixture [21].Meanwhile, porosity is a measurement of pavement porosity, which includes air gaps on the surface and connected voids in the mix [22].

Optimum Asphalt Content
The optimum asphalt content (OAC) was determined using the average stability, flow, Marshall Quotient (MQ) VIM, VMA, and VFA values that satisfied the requirements listed in Table 5.Table 6 displays the results of sample testing for OAC determination, which revealed that all of them satisfied the requirements.The Marshall test results for determining OAC are shown in Figures 1 to 7, and it was discovered that all samples with various percentages of asphalt content met the requirements.This is because several specifications were employed in this study, and each of those specifications had different design requirements for porous asphalt, including application objectives, material testing, the binder used, and the mix test technique.As a result, the OAC is determined using the average value of the five asphalt contents, resulting in the OAC content is 5% for the mixture with additives.

Marshall Test Result
Table 7 shows the average values of the three samples tested using the Marshall test on samples containing crumb rubber (CR).It can be seen the samples containing 0.5% and 1% CR met the requirements, however the samples containing 1.5% CR did not satisfy the requirements for stability and VIM.Increasing the level of CR in the mixture reduces the stability value while increasing the flow value.This is owing to the elastic nature of CR, which makes the sample more flexible.A comparison of the Marshall parameters between samples with crumb rubber (CR), nano silica (NS), and Wetbond-SP and the control sample is shown in Figure 8 to Figure 14.According to Figure 8, the sample with the addition of nano silica (NS) has the greatest stability value and shows an increase in all samples.Samples containing NS were shown to have greater stability values than the control sample.In contrast, compared to the control sample, the sample with the addition of CR and Wetbond-SP did not show an increase in stability.
Figure 9 shows the flow parameters for each sample variation.It can be shown that samples with CR content have varying values.On the other hand, samples containing NS showed flow results that tended to decrease as the amounts of NS in the mixture increased, whereas samples containing Wetbond-SP showed flow values that increased as the amount of Wetbond-SP in the mixture increased.The increase in flow value may be the result of an increase in the mechanical performance of the mixture.
Figure 10 shows the results of the comparison of the Marshall Quotient (MQ) parameters for each sample variation.Various results were obtained for each addition of additives to the mixture.where samples with CR were found to decrease, samples with NS experienced an increase, and samples with Wetbond-SP experienced an increase and decrease with increasing levels of additives in the mixture.
The comparison of the VIM, VMA, and VFA parameters for each sample variation is shown in Figures 11, 12, and 13 accordingly.As can be observed, samples containing CR have a significant effect on the VIM value.Meanwhile, samples with NS and Wetbond-SP had VIM values that tended to decrease compared to the control samples.As for the VMA parameter, the addition of additives tends to increase the VMA value.This happens because the addition of additives to the mixture increases the viscosity of the asphalt, causing the asphalt covering the aggregate to become thick.In addition, the results of the VFA parameter in samples with CR content decreased with increasing CR levels.While samples with NS and Wetbond-SP have VFA values that tend to increase.Figure 14 shows the density values in the samples with the addition of different additives, the results were varied.Whereas samples containing 0.5% CR have the greatest density among samples with the other additives.On the other hand, the density value tends to decrease as the CR content in the mixture increases.While samples containing Wetbond-SP and NS showed fluctuating results.

Cantabro Loss Test Results
As can be seen in Figure 15, the porous asphalt samples containing CR and Wetbond-SP had Cantabro Loss (CL) results which showed a decrease with increasing levels of additives in the mixture.While the percentage of CL in the samples with NS decreased and increased.The addition of additives to the porous asphalt mixture resulted in a reduction percentage of CL, indicating that the presence of additives in porous asphalt may increase its moisture resistance and cohesiveness loss.16 and Figure 17 show the results of the permeability coefficients and porosity for each sample variation.As can be observed, samples containing CR and Wetbond-SP had a permeability coefficient that tended to decrease as the additive percentages in the mixture rise.Meanwhile, samples containing NS found variable results.The permeability coefficient tends to decrease in samples with the addition of additives because the additives will cover the flow cavities through which water will pass in the porous asphalt mixture.Additionally, it was discovered the porosity value of samples with the addition of CR showed a decrease as the increasing CR content in the mixture.While samples with the addition of NS showed an increase with increasing NS content.And samples with the addition of Wetbond-SP showed fluctuating values.
The difference in the porosity results for each sample variation might be attributed to the fact that each additive has a different composition, pore distribution, type, and material.

Summary and Condition
The addition of nano silica (NS) to the porous asphalt mixture increased stability, while crumb rubber (CR) and Wetbond-SP did not.However, the addition of Wetbond-SP can increase the adhesion between the aggregate and asphalt so that the mixture becomes denser.In addition, the three additives were found to decrease the percentage of Cantabro Loss (CL) which indicates that the addition of additives can increase the resistance of the mixture to moisture and loss of cohesion.The addition of additives to the mixture affects the permeability and porosity values, a greater number of additives in the mixture will reduce the permeability and porosity values.This results in reducing the size of the voids in the porous asphalt mixture.The optimum concentration for the addition of CR is 1% of the total weight of the mixture, for the addition of NS it is 4% by weight of the asphalt content, and for the addition of Wetbond-SP it is 0.4% by weight of the asphalt grade.

Figure 1 .
Figure 1.Comparison of Stability Values in Samples with Additives

Figure 2 .Figure 3 .
Figure 2. Comparison of Flow Values in Samples with Additives

Figure 8 .
Figure 8.Comparison of Cantabro Loss Values in Samples with Additives

Figure 9 .Figure 10 .
Figure 9.Comparison of Permeability Coefficients in Samples with Additives

Table 2 .
Aggregate Tests Results

Table 3 .
Aggregate Gradation Results

Table 4 .
Asphalt Tests Results

Table 5 .
Porous Asphalt Specifications from Various Institutions

Table 6 .
OAC Samples Marshall Test Results

Table 7 .
Marshall Test Results of Samples with Addition of Crumb Rubber

Table 8 displays
Marshall characteristic findings from the testing carried out in the laboratory on samples containing nano silica (NS).Where all of the findings satisfied the requirements.According to the test findings, the addition of NS increased the stability value of the mixture as the NS content increased.

Table 8 .
Marshall Test Results of Samples with Addition of Nano Silica

Table 9
shows the Marshall test results on samples containing Wetbond-SP, showing that all parameters satisfy the requirements.The addition of Wetbond-SP has been shown to have no effect on the mixture's stability but can decrease the VIM values, causing the mixture's voids to close and produce denser porous asphalt.Meanwhile, other parameters including flow, VMA, VFA, and density were not significantly impacted by the addition of Wetbond-SP to the mixture.

Table 9 .
Marshall Test Results of Samples with Addition of Wetbond-SP