A.Hussain，Qiu.Yanjun

(School of Civil Engineering，Southwest Jiaotong University，Chengdu 610031，Sichuan，China)

0 Introduction

The use of reclaimed asphalt pavement(RAP)in the construction of new Hot Mix Asphalt(HMA)pavements has increased in recent years.RAP is an old asphalt pavement that is milled up or ripped off the roadway［1］.This material can be reused in new asphalt mixtures because the components of the mix(the asphalt binder and aggregates)still have value.Using RAP in new mixtures can reduce the amount of new material that has to be added，save money and natural resources.In addition，asphalt mixtures with RAP can perform as well as mixtures made with all new materials.Due to these advantages of using RAP，many state highway agencies are moving toward rising the percentages of RAP in their hot-mix asphalt pavements(EPA and FHWA，1993).RAP has been used in hot mix asphalt pavements in various percentages that reached in some cases up to 80%［2］.Most studies have been used a range form 20%～50%［1，3－4］.

When RAP is reused in a new mixture，it is necessary to properly account for the old material in the new design.The aggregate from the RAP has to be included with the new aggregate，and that blend of aggregate has to meet certain properties.Past experience with RAP in Marshall and Hveem mixtures has shown that properly designed and constructed RAP mixes can perform as well as，or even better than，mixtures made with all new materials.In the current study only the Marshall method was used for the evaluation of asphalt mixtures containing various RAP ratios.

1 Materials Consideration

1.1 Virgin Aggregates

In Pakistan，various aggregate sources are available for road constructions.For this research，virgin aggregates have been collected from two dominant aggregate sources Margalla and Dina，which are considered to be the largest live aggregate quarries in the country.The type of aggregate from Margalla and Dina is crushed limestone and quartzite respectively.The limestone from Margalla is one of the best mechanically fractured aggregate in the country free from rounded particles or river bed gravel.On the other hand，Dina quarry aggregates contain riverbed and rounded gravels.The pavement construction and maintenance consume large amount of aggregates from these two quarries which diminishes the natural resources.

1.2 Virgin Binder

Asphalt binder 60/70 penetration grade which is being in use in most highways in the country has been acquired from Attock Refinery Limited(ARL)at Rawalpindi.ARL is using heavy crude oil for the production of 60/70 and 80/100 grade asphalt.Most of local heavy crude oil are produced from northern part of the country.The asphalt binder price is increasing day by day due to increase in the cost of the crude oil.

1.3 RAP Material

RAP is the old asphalt pavement produced by milling existing asphalt pavement or crushing old removal from asphalt pavements.For this study，the RAP material has been obtained in forms of chunks from two sites(Mandra＆ Nowshera)along national highway N-5，Pakistan.The RAP materials were subjected to severe aging and weathering on the site.The RAP chunks were crushed and screened into different sizes to meet the specified grading requirements.The two RAPs，two virgin aggregates and virgin binder ARL 60/70 were used in this research to investigate the effects of RAP on the resulting mixture properties.RAP contents of 0%，10%，20%，30%，45%，60%and 100%were evaluated in this study to see the effect of various RAP contents and sources on the mixture properties.

2 Laboratory Evaluation

In order to access the percentage of the asphalt present in the RAP materials，asphalt extraction was done using AASTHO T 160，“Quantitative Extraction of Bitumen from Paving Mixtures”and AASTHO T 170，“Recovery of Asphalt from Solution by Abson Method”.The gradations of virgin and RAP materials after extraction are shown in Table 1.The asphalt content of RAP was found to be 5.05%and 5.57%for the Mandra and Nowshera RAP respectively.

Table 1 Virgin and RAP gradation表1 篩選后新集料及RAP回收集料的級配 /%

The laboratory testing for this research was limited to 25 mm nominal maximum aggregate size(NMAS)mixture.Four different mixtures combinations of two virgin and two RAP sources were designed and tested as shown below:

1)Margalla Base+Nowshera Rap denoted in this paper by(MB+N RAP);

2)Margalla Base+Mandra Rap denoted by(MB+M RAP);

3)Dina Base+Nowshera RAP(DB+N RAP);

4)Dina Base+Mandra Rap(DB+M RAP).

Prior to use RAP in recycling asphalt，it is essential to evaluate the extracted(old)binder properties and their blends with virgin(new)binder.The recovered binder and its blends with virgin binder were tested for penetration and ductility and the results are presented in Table 2.

Table 2 Penetration and ductility for all the blends表2 瀝青針入度及黏度

Literature researches reveal that normally low RAP contents up to 20% can be designed without changing the binder grade.From Table 2，it can be seen that even with 40%RAP，the penetration and ductility show that the aged binders still have enough life.Since the virgin binder ARL 60/70 selected is soft binder(graded as PG58-16 according to superpave system)，so this will serve as rejuvenating agent in the mixture.As NMAS for base course mix is 1 inch，therefore 6-inch diameter specimens were prepared for Marshall testing following ASTM D 5581.Table 3 shows the criteria for modified Marshall Mix design for heavy traffic［5］.As heavy traffic and overloading has been one of the main reasons behind early deterioration of our pavements，therefore mixes have been designed for heavy traffic.The number of blows on each end of the specimen was 112 instead of 75 for heavy traffic.It was assumed that the total asphalt content in the mixture is equal to the 100%virgin base course control mixtures.The optimum asphalt content for the control mixes were 3.69%and 4.75%for Margalla and Dina aggregates respectively based on 4% air voids.The percentage of new binder was calculated by using the following equation［5］for the 10% ～ 60% RAP mixtures.The 100%RAP mixtures were compacted without adding any virgin binder since the RAP binder present is higher than the optimum binder.

Where，Pnbis percent of new asphalt binder in recycled mix expressed as whole number，%;r is new aggregate expressed as a percent of the total aggregate in the recycled mix expressed as a whole number;Pbis percent of estimated asphalt content of recycled mix assumed to be the same as that of 100 percent virgin HMA mix，%;Psbis percent of asphalt content of RAP，%.

Table 3 Modified Marshall mix design criteria表3 修正的馬歇爾配合比設(shè)計標(biāo)準(zhǔn)

3 Results and Discussions

The results of Marshall mix design of all the four mixtures with various RAP contents are summarized in Table 4.With the increasing of the RAP contents，the mixture stability increases for the first three mixture groups，but for the last one there is no significant change in the stability values.The entire mixtures satisfy the minimum stability of 18kN.But for Dina source with high RAP contents，the mixture exceeds the maximum flow limit of 5.25mm for modified Marshall.Marshall high flow values indicates excess binder content.Generally，the variability in the volumetric parameters increases with increase RAP contents.

The RAP sources effect on the first two mixtures which consist of crushed limestone is shown in Fig.1.The stability increases linearly with increase in RAP.As the first mixture consist of crushed limestone and also the Nowshera RAP aggregate is also crushed so result in good combination and give high stability.Also the N RAP binder is less aged than the M RAP binder as indicated by the penetration and ductility values.Similarly for the third mixture the stability increases with increase RAP content with good linearity(R2=0.939)but for the fourth mixture the increase is not significant(R2=0.795).

Table 4 Marshall mix design for all mixtures表4 馬歇爾配比設(shè)計

(續(xù)表4)

Fig.1 Effect of RAP sources on Marshall stability圖1 不同類型RAP對馬歇爾穩(wěn)定度的影響

The fourth mixture is combination of rounded riverbed aggregate and also the M RAP is rounded so no prominent change in stability with increase in RAP.Similarly，the effect of the virgin aggregate sources on the stability of the N RAP contained mixtures is shown in Fig.2.

Fig.2 Effect of virgin aggregates sources on Marshall stability圖2 新集料類型對馬歇爾穩(wěn)定度的影響

The Marshall stability increases for Margalla and Dina aggregates with increase in RAP with good linearity having R2value of 0.916 7 and 0.928 6 respectively.The flow values for all the mixtures are presented in Fig.3 which indicated that most of the values are within the specifications limits of 3 ～ 5.25 mm for modified Marshall criteria for heavy traffic.These analysis shows that RAP blending is a viable option for base course construction in the country.

Fig.3 Marshall flow values for all the mixtures圖3 馬歇爾流值

The laboratory testing confirmed that good quality mixtures can be design using RAP material.The quality of the virgin aggregates and the quality of RAP material greatly affect the mixture volumetric properties.E-ven the 100%RAP mixtures of both the sources give good results in laboratory compaction.Few mixtures exceed the maximum flow limits but most RAP blends fall within the specified limits for the volumetric properties.The low RAP contents up to 30%can be used for heavy traffic and higher RAP contents can be used for medium and low traffic.

4 Conclusions

Based on extensive laboratory evaluation of different Marshall mixtures containing RAP concludes that the blending of virgin and RAP materials overall improve the mixture properties.The main conclusions drawn from this research are as following:

1)The RAP mixtures designed using Marshall method in laboratory perform the same as virgin mixtures.

2)Generally，the Marshall stability increases with increase in RAP contents with good linearity.The stability of the 100%RAP mixtures is two times that of the virgin mixtures.

3)The crushing limestone gives better performance with both the RAP sources as compared to the natural quartzite.

4)When mostly riverbed and rounded particles are used，the stability does not change significantly and the flow exceeds the maximum limit.

5)Using RAP in design even up to 30%will help in conserving the natural resources，reducing the HMA price and improving the performance.

6)It is suggested that to construct a trial section using virgin and RAP blends to verify the suitability of RAP mixtures to the country climate condition and traffic loadings.

7)It is recommended that for future study to use modified binder and 19mm NMAS to see the RAP mixture performance as wearing course.

Acknowledgement:Thanks are extended to National Institute of Transportation(NIT)of NUST，Islamabad for providing the testing facilities.Thanks are also extended to ARL for providing the asphalt binders for testing.

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