0. Introduction

Large stone porous asphalt mixture (LSPM) has the largest nominal diameter greater than 26.5 mm and larger porosity to discharge water freely from pavement structure, and it is usually used as the base course of pavement^[1]. Domestic and foreign scholars have conducted much research on LSPM^[2-8], and it has become the principal methods to improve asphalt pavement performance by adding modified asphalt in asphalt mixture^[9-13]. MAC is the acronym of multigrade asphalt cement, and is invented by a asphalt material company in the United States.In this paper, the performances of matrix asphalt, MAC modified asphalt and SBS modified asphalt are compared, then MAC modified asphalt is selected as the binder of LSPM. The road performances, material component design and construction technology of MAC modified LSPM are studied.

1. Performance of MAC modified asphalt

1.1 Viscosity

At room temperature, matrix asphalt flows as the passage of time, but MAC modified asphalt does not flow.

1.2 Temperature Sensibility

Fig. 1 shows the relationships between the viscosities and temperatures of MAC modified asphalt and matrix asphalt. Compared with matrix asphalt, MAC modified asphalt has higher viscosity at high temperature and lower stiffness at low temperature.

Figure  1.  Temperature sensibilities of asphalts

DownLoad: Full-Size Img PowerPoint

1.3 Aging resistance

Fig. 2 shows the relationship curves between the aging times and the viscosity changes of the asphalts under thin film oven test. With the increase of heating time, the viscosity growth of MAC modified asphalt is significantly lower than that of matrix asphalt. It shows that the short-term anti-aging perfor-mance of MAC modified asphalt is better than that of matrix asphalt.

Figure  2.  Anti-aging performances of asphalts

DownLoad: Full-Size Img PowerPoint

Experiment shows that the residual penetration ratio of matrix asphalt decreases rapidly in a few years, while that of MAC modified asphalt decreases slowly as time. The result means that MAC modified asphalt has better long-term anti-aging ability.

1.4 Storage stability

MAC modified asphalt has been chemically modified with better stability. It won't isolate in six months without any stabilizer when stored at room temperature, doesn't contain sulfur, and is not harmful to human body.

1.5 Leakage test

At the same gradation, LSPMs are made of ordinary heavy traffic asphalt(AH), SBS modified asphalt and MAC modified asphalt respectively, then the leakage test contrast study of these LSPMs are conducted, and the results are shown in Tab. 1. Because the gradations of LSPMs are too rough, the asphalt content of each LSPM is influenced by its amount of leakage. In the premise of small amount of leakage, greater content of asphalt is needed. When the amounts of lea-kage are same, the contents of SBS modified asphalt and MAC modified asphalt are same, while that of matrix asphalt is lower. In order to ensure the thickness of asphalt film of LSPM, the content of asphalt should be greater than 2.9%, and the leakage amount should be less than 0.3%. When the content of matrix asphalt reaches 2.9%, the amount of leakage is more than 0.3%, which illustrates that ordinary heavy traffic asphalt is not very appropriate to LSPM. Considering of economy, for LSPM, MAC modified asphalt is better than SBS modified asphalt.

Table  1.  Leakage test results of asphalts

 | Show Table

DownLoad: CSV

2. Road performances' requirements of MAC modified LSPM

2.1 High-temperature stability

LSPM is one mixture with a skeleton formed by the single-size particles embedded with each other, the proportion of coarse particles bigger than 9.5 mm is about 70%, so LSPM has good high temperature stability.

When the high-temperature stability of LSPM is evaluated by rut test, according to the principle that the compacted thickness of mixture should be greater than 3 to 4 times of the largest nominal size, the traditional 5 cm thickness of rut specimen can not be adopted. The minimum thickness of LSPM rut specimen should be 8 cm, and the test temperature is 60 ℃, which are stipulated in the existed specification.

2.2 Water stability

In order to ensure the water stability of the mixture, the binder of LSPM should be the modified asphalt with higher viscosity. MAC modified asphalt has greater viscosity, and it can make LSPM have asphalt membrane thicker than 12 μm.

At present, the research methods of water stability for LSPM are different in different countries, and there is no uniform standard. Test methods adopted in China are residual stability and freeze-thaw splitting strength ratio. But for LSPM, because the contact point between particles decreases and the structure density is low, the Marshall stability is lower, and its split strength is significantly lower than that of dense asphalt mixture. At present, the Marshall test of adopting large specimen has some shortcomings, and it is difficult to ensure the accuracy of the test. So the water stability of LSPM is ensured by controlling the thickness of asphalt membrane.

2.3 Fatigue performance

The fatigue performance of LSPM is lower than that of dense graded asphalt concrete, and approaches that of dense graded asphalt macadam. When greater tensile stress occurs in LSPM layer, there are two methods to improve the anti-fatigue performance of the structure: changing the combination design of pavement structure, so that LSPM layer is in the region of compressive pressure; adding anti-fatigue layer made of fine-graded asphalt mixture under LSPM layer.

2.4 Permeability

According to study result, when the porosity of LSPM is up to 13%, the permeable coefficient of the mixture will mutate. When the porosity is more than 18%, the change of the permeable coefficient is not obvious. Generally speaking, when the permeable coefficient is between 0.01 and 1.00 cm·s^-1, the drainage performance of the mixture can be ensured^[14]. For these reasons, the design porosity of LSPM is between 13% and 18%, and the permeable coefficient should be greater than 0.01 cm·s^-1.

2.5 Anti-reflective crack performance

Under traffic load, the extension to the surface course of the cracks produced in the base course or the old pavement mainly depends on the singularity of the shear stress at the end of the cracks. The intensity factor of the shear stress will increase as the modulus of the resurfacing layer increases. The analysis with fracture mechanics shows that the singularity of the shear stress will be great when the mixture has no pore or the pore is very small. On the contrary, the singularity will be small when the pore is great.

MAC modified LSPM has lower modulus, which is between 400 and 600 MPa, and has big porosity, so it has more resistance against reflective cracks.

3. Material cmposition design of MAC modified LSPM

3.1 Coarse aggregate

Coarse aggregate should be hard crushed stone and has good shape. The content of slender and flat particles should not exceed 15%, and the crushing value of coarse aggregate should not be more than 20%. The adhesive force between coarse aggregate and asphalt should get to fifth class. When the class is less than 5, the anti-off measures should be taken to ensure the water stability. Other indices should meet the requirement of the references [15] and [16].

3.2 Fine aggregate

Fine aggregate includes artificial sand, stone chip and natural sand. Hard rock aggregate less than 2.36 mm produced by counter or hammer crusher can be used as artificial sand. Artificial sand and stone chip are used as the fine aggregate of LSPM, but natural sand isn't. The edges and corners of fine aggregate must be greater than 42%, and sand equivalent value is not less than 65%.

3.3 Filling material

Dry white and quick lime powders are suitable for filling material, and the technical requirements for filling material can be decided according to local conditions, but they must meet the requirements of grade Ⅲ.

3.4 Asphalt binder

The MAC-70^# modified asphalt is used and the relevant technical requirements are listed in Tab. 2.

Table  2.  Technical requirements of modified asphalt

 | Show Table

DownLoad: CSV

3.5 Grading design

The aggregate of LSPM does not have a fixed grading curve, and its gradation is related to rawmaterials. According to the research result in Shandong Province, the recommended gradation ranges are listed in Tab. 3.

Table  3.  Recommend gradations for LSPM

 | Show Table

DownLoad: CSV

The ordinary limestone aggregate suits the ranges of the gradations in Tab. 3. When the aggregate property changes greatly, the grading design is necessary. The grading design can adopt the methods provided in NCHRP R386 or the method to ensure the skeleton formed by coarse aggregates embedded with each other.

3.5.1   NCHRP R386 method

NCHRP R386 designs the grading composition of the mixture by linear programming and volume filling, design process is as follow.

(1) According to the loose embedded squeeze density and bulk density of the most coarse aggregate to determine the content of the most coarse aggregate.

(2) According to the maximum aggregate size and the porosity requirements to calculate the effective asphalt content.

(3) According to water absorption to estimate the absorbed asphalt content.

(4) Under the restraints of the ratio of power to binder and total mineral aggregate which equal to 100%, mineral aggregate gradation is adjusted through the linear programming until satisfied result can be got.

(5) Using the sum of effective and absorbed asphalt content as design asphalt content, and carrying out performance tests to LSPM.

3.5.2   Coarse aggregate skeleton embedded squeezemethod

During the process of the grading design, the formation of skeleton by coarse aggregates embedded with each other is one premise. It is not necessary to consider the secondary and tertiary fillings in Bailey's design method, and fine aggregate in the mixture is loose. The embedded squeeze becomes more closely in the process of compaction, so coarse aggregate should undergo vibration compaction at least.

3.6 Determining of volume indexes

Volume indexes include density and porosity. Density can be determined by test, calculation method and CoreLok method. CoreLok method to determine density includes following steps.

(1) Putting a compacted specimen into plastic bags with known density.

(2) Putting plastic bags with the specimen into CoreLok vacuum chamber.

(3) Closing the door of the chamber, then vacuum pump starts working. 2 min later, the door will open automatically, the bags are sealed completely.

(4) Measuring the density of the specimen with dry surface method, and the result need be emended according to the density and volume of plastic bags.

CoreLok method can measure the density of the mixture with great porosity accurately, but restricted by the apparatus it is not wide used in China. So the density of LSPM can be determined through test or calculation. The difference between the results from test or calculation and CoreLok method need be determined.

3.7 Determining optimum asphalt content

The optimum asphalt content should be determined based on the calculation of asphalt membrane thickness, leakage test and dispersion test. The thickness of asphalt membrane can be calculated according to asphalt content and the surface area of aggregates, and asphalt content should be effective asphalt content. Considering the complexity of the calculation for effective asphalt content, when aggregate water absorptive capacity is littile, total asphalt content can be adopted. Aggregate surface area, denoted as A_rea, can be estimated according to the empirical formula given by the Asphalt Institute in the United States

A_rea=0.410 0+0.004 1P_4.75+0.008 2P_2.36+0.016 4P_1.18+0.028 7P_0.6+0.061 4P_0.3+0.122 9P_0.15+0.327 7P_0.075

3.8 Technical requirements of LSPM

The technical requirements of open-grading LSPM are shown in Tab. 4.

Table  4.  Design technical standards

 | Show Table

DownLoad: CSV

4. Construction technology of MAC modified LSPM

4.1 Technical requirements for transportation of MAC modified asphalt

The temperature of MAC modified asphalt when being loaded in manufacturing plant is about 170 ℃, the temperature of the mixture when arriving in the construction field should not be less than 150 ℃, and the temperature of empty tank should be maintained at more than 160 ℃ until the asphalt is loaded.

4.2 Technical requirements for storage of MAC modified asphalt

MAC modified asphalt storage temperature should be maintained between 170 ℃ and 190 ℃. Even if it is not used for several days, the temperature should not be lower than 160 ℃. MAC modified asphalt in storage tank should not be too full, some room should be left to prevent the asphalt overflowing because of its expansion due to temperature change. However, too little asphalt is not allowed, and the level of the asphalt must be higher than the blender blades.

4.3 Technical requirements for pumping of MAC modified asphalt

The temperature of MAC modified asphalt is high in transportation or storage process, so the asphalt has a cleaning effect to asphalt transport and storage tanks. Carbonized asphalt and other material on asphalt tank wall can be cleaned down at high temperature, and the substances are often massive and can not be completely dissolved in asphalt. When asphalt mixture is produced, asphalt need to be send into asphalt mixer from asphalt tank. As asphalt pump has filters, the materials which are cleaned down are easy to plug filter mesh, then the pumping capacity of asphalt are seriously affected. If the problem appears, it needs to clean filters in time. The temperature of asphalt pipeline between asphalt storage tank and asphalt mixer should be maintained at about 180 ℃. When construction work is completed in one day, asphalt pump will be rotated reversely in order that the MAC modified asphalt in pipeline will be sent back to asphalt tank. On the other hand, people also can set a valve at the lowest point of pipeline, through which the MAC modified asphalt in pipeline is released. Both the two methods will prevent MAC modified asphalt from plugging pipeline when the temperature in pipeline is low.

4.4 Technical requirements for mix and transportation of asphalt mixture

Asphalt is heated by heat conduction oil, and the heating temperature is between 170 ℃ and 180 ℃, the heating temperature of aggregate should be higher 10 ℃ and 20 ℃ than that of asphalt. The temperature of asphalt mixture when leaving mixing station should be controlled between 170 ℃ and 185 ℃. If the temperature reaches 195 ℃, asphalt mixture should be abandoned.

4.5 Technical requirements for paving asphalt mixture

Because of thick paving layer, generally, paving speed should not be more than 2 m·min^-1, and should be slow, uniform and continuous. According to laboratory test research results, the loose laying coefficient of LSPM is between 1.18 and 1.20.

4.6 Technical requirements for compacting asphalt mixture

The temperature should be controlled between 165 ℃ and 175 ℃ during initial compaction. In order to prevent coarse aggregate from breaking down because of excessive vibration, intermediary compaction should be carried out with high frequency and low amplitude. Compaction speed should be between 1.5 and 2.0 km·h^-1. After vibration compaction, the mixture need be compacted by tire roller for 1 to 2 times before final compaction starts. During final compaction, the steel roller with 7 to 12 t of mass is adopted, and compaction speed should be controlled between 3 and 4 km·h^-1. As LSPM has large void and rough surface, the surface is prone to become loose subjected to heavy trucks. So non-construction vehicles should be prevented from driving on LSPM, or asphalt surface layer should be paved as soon as possible after construction completion.

4.7 Quality control and test methods

The compaction of LSPM should be controlled by using porosity and compactness at the same time. Porosity can be controlled from 13% to 18%, its maximum is 20%. Compactness should not be less than 98%. Sample frequency in site is in accordance with relative specifications.

5. Test road

In july 2001 and july 2002, LSPM test roads were constructed in the section from Yantai to Qingdao on national road 204 in Shandong Province. Daily average traffic volume was 15 000 veh·d^-1. For the test road constructed in july 2001, after opening to traffic for two years, its rut depthes were between 4 and 8 mm, and most of them were about 6 mm. For the test road constructed in july 2002, after ope-ning to traffic for one year, its rut depth were between 3 and 6 mm, and most of them were about 4 mm. The results show that LSPM has good resistance capacity to the permanent deformation at high temperature.

The survey conducted in july 2003 showed that there was not any crack on two test roads, but a large number of reflective cracks had taken place on the asphalt pavements with the base course made of cement stabilized macadam. The results show that LSPM can effectively prevent the production of reflective cracks.

6. Conclusion

(1) Compared with matrix asphalt, the performances of MAC modified asphalt, including temperature sensibility, anti-aging performance and anti-leakage performance, ect, have been improved greatly, its anti-leakage performance is close to that of SBS modified asphalt, and it has obvious economic advantage.

(2) MAC modified LSPM has good high-temperature stability, water stability, anti-fatigue performance, anti-reflective crack performance and other road-related performances, and also has good drainage capacity.

Nomenclatures :

P_i aggregate percent

i aggregate diameter(mm)