Construction and demolition waste contain large amount of concrete and bricks. To identify its feasibility of applying in asphalt stabilized gravel mixtures as pavement base, this paper carried out researches on properties of recycled asphalt mixture (RAM) by laboratory testing, under different contents of recycled aggregates (RA) and proportions of bricks to concrete. First, the basic physical properties of each component of the RAM were determined. According to the actual screening passing rate, the Bailey method was used to design and verify the grading of RAM, and then initial asphalt content was estimated. The stability, flow value, voidage, and bulk density were obtained by Marshall Test, and the optimal asphalt-aggregate ratios were determined under different RA contents and proportion of bricks to concrete. Subsequently, the uniaxial compression test, splitting tensile test, immersion Marshall test, freeze-thaw splitting test, and Hamburg rutting test were performed to investigate mechanical properties, water stability, and high-temperature stability of RAM. The results shows that as the content of RA increases from 0% to 100%, the corresponding compressive strength, resilient modulus, and splitting tensile strength all decrease; the same change trend is presented in the residual stability and freeze-thaw splitting test strength ratio and rutting depth. According to the requirement of pavement base of high-grade roads in China’s standards, the recommended RA content and proportion of bricks to concrete were proposed for the design RAM.
Extensive urbanization in China has led to the generation of multitudinous construction and demolition wastes (CDW), about 1.7 billion tons in 2019, resulting in many imminent problems to be solved urgently, e.g., occupation of land, overconsumption of resources, and environmental pollution [
The composition of CDW is complicated with many sources; waste concrete and bricks are two main parts occupied for 60% and 20% of the whole wastes [
In this paper, the recycled coarse aggregates (4.75∼26.5 mm), taken from RCA and RBA, were used for replacing natural ones, for application of RAM as pavement base. The RA content was set in five ratios: 0%, 25%, 50%, 75%, and 100%, and four proportions of bricks to concrete (0 : 10, 2 : 8, 4 : 6, 6 : 4) were considered. First, the adhesions between asphalt and three types of coarse aggregates (NA, RCA, RBA) were investigated from the view of microstructure by scanning electron microscope (SEM). Then the properties of RAM were determined by the Standard Marshall Test, and the optimal asphalt dosage was obtained under different RA contents and brick-concrete proportions. On this basis, the performance tests of RAM applicated as the pavement base, including mechanical properties, temperature stability, and water stability, were carried out to find a suitable design scheme of RAM.
Three types of aggregates were used: RCA, RBA, and NA. RA were supplied by a CDW recycling plant in Changsha, China. After removing impurities (ceramics, wood, rubber, etc), the CDW were screened into RA in four gradings: 0–4.75 mm, 4.75–9.5 mm, 9.5–19 mm, and 19–26.5 mm. The particles of size larger than 26.5 mm were eliminated, and the RCA and RBA were separated manually (Figure
Source materials. (a) RCA. (b) RBA.
Since it was difficult to identify the ingredients of finer RA, the used aggregates of grading less than 4.75 mm were all selected from NA. The remaining three gradings’ aggregates were composed of NA and RA in proportion. The properties of RA and NA are evaluated in Table
Characterisation of aggregates.
Detection indexes | RCA (mm) | RBA (mm) | Limestone natural aggregates | Code value | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
4.75–9.5 | 9.5–19 | 19–26.5 | 4.75–9.5 | 9.5–19 | 19–26.5 | 0–4.75 | 4.75–9.5 | 9.5–19 | 19–26.5 | ||
Apparent density (kg/m3) | 2.50 | 2.55 | 2.53 | 2.38 | 2.41 | 2.44 | 2.723 | 2.721 | 2.713 | 2.711 | |
Water absorption (%) | 5.71 | 5.44 | 5.50 | 17.08 | 16.55 | 14.61 | 0.78 | 0.61 | 0.52 | 0.34 | |
Needle flake content (%) | 7.8 | 7.1 | 3.2 | 7.3 | 6.9 | 3.5 | 12.6 | 8.3 | 9.8 | 6.9 | |
Crushed value (%) | 26.1 | 26.6 | 27.3 | 28.6 | 29.3 | 29.8 | — | 21.6 | 22.3 | 22.5 |
The engineering properties of asphalt binder are presented in Table
Asphalt property test results and standard value.
Test item | Test value | Test methods | Specification of JTG F40-2004 | |
---|---|---|---|---|
Penetration (%) | 68.5 | T0604-2011 | 60∼80 | |
Ductility | (10°C, 5 cm/min), cm | 19 | T0605-2011 | |
(15°C, 5 cm/min), cm | 101 | T0605-2011 | ||
Softening point, °C | 48.3 | T0606-2011 | ||
Flash points, °C | 280 | T0611-2011 | ||
Paraffin content, % | 2.1 | T0615-2011 |
Test methods referenced from China’s Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011).
The aggregate grading design has a great impaction on the mechanical properties, an excellent grade is a necessary precondition to ensure the service life of the asphalt pavement layer [
Design grading curve of processed RA.
Marshall testing was used to obtain the optimal asphalt-aggregate ratio. First, the synthetical bulk specific gravity
The
The RAM of different proportions of bricks to concrete. (a) RBA : RCA = 0 : 10. (b) RBA : RCA = 2 : 8. (c) RBA : RCA = 4 : 6. (d) RBA : RCA = 6 : 4.
In the forming process of tested samples of RAM, the specimens were set as the standard size: 101.6 × 63.5 mm (diameter × height), after compacting for 75 cycles both side, they were cooled for 12 hours at room temperature. Finally, all specimens were stripped ready to undergo Marshell testing. The mass of specimens in water and air and surface dry mass were tested, respectively, then the stability and flow value were tested by Marshall Stability Meter. According to the requirements of the specification (JTG F40-2004), the optimum asphalt-aggregate ratio OACopt can be calculated as equations (
Technical requirements for dense gradation base (ATB-25) asphalt mixture.
Iterm | Technical requirements |
---|---|
Nominal maximum size of aggregate | 26.5 mm |
Marshall specimen size | 101.6 mm |
Number of Marshall specimen compaction | 75 cycles double sided |
Voidage | 2–6% |
Asphalt saturation | 55–70% |
Marshall stability | |
Flow value | 1.5–4 mm |
Minimum voids of mineral aggregate | |
Dynamic stability | |
Freeze-thaw split intensity ratio | |
Residual stability |
Figure
Marshall test results of asphalt mixtures with 25% RCA.
Identically, the OACopt of RAM in different RA contents is shown in Figure
Optimum asphalt-aggregate ratio in different RCA content.
To compare the adhesion mechanism between NA, RCA, RBA, and asphalt, it is necessary to analyze their microstructures. The tested aggregates of appropriate particle size were taken out from the internal structure of the specimen artificially, and they were cut into samples of nail size by a precision cutting machine; thus, the microstructure can be observed by SEM. Marshall specimens were prepared with 50% RA content and proportion of bricks to concrete 4 : 6, the OACopt was 4.14%.
Mechanical strength of RAM mainly depends on the particle friction, aggregate lithology, and performances of the asphalt. Thus, different gradation design and composition of aggregates affect mechanical properties of RAM obviously [
When the compressive strength and resilient modulus were measured, the specimens were formed by static pressing with a load of 2000 kN. The height and diameter of the specimens were both 100 ± 2 mm, and their densities met the requirements of Marshall Standard Compaction. After curing, specimens were subjected to uniaxial compression test with Mechanical Testing System (MTS) at room temperature with a loading rate of 2 mm/min.
According to the failure load
In the splitting tensile strength test, the Standard Marshall Compaction specimens were used, and both sides of them were compacted for 75 cycles. After demoulding, they were placed in a constant temperature tank and immersed in water for 1.5 h. The distance of each specimen was 15 mm, the Marshall Stability Meter was used to measure the splitting tensile strength with the loading rate of 50 mm/min.
The residual stabilities of the two groups of specimens formed by Marshall Compaction were used to evaluate their water stabilities. The size of specimens was 101.6 × 63.5 mm (diameter × height), satisfied with the requirements of China’s standard (JTG E20-2011): Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering. Before testing, two groups of specimens were immersed in a 60°C constant temperature tank for 30 min and 48 h, respectively, shown in equation (
In the freeze-thaw splitting test, the two groups of Marshall Specimens were processed with compaction for 50 cycles both side. The first group specimens were placed at room temperature, while the second were saturated in a vacuum for 15 min, and then placed in water for 30 min before taking out. Before sealing them with plastin warps, adding 10 ml water. Subsequently, they were put into a constant temperature and humidity box. One freeze-thaw cycle was completed at this moment. Finally, group 1 and group 2 were put into a 25°C constant temperature tank for 2 h, and the maximum failure load was measured to calculate the freeze-thaw splitting test strength ratio:
To study the impact of brick and concrete aggregates on the high-temperature performance of RAM, the Hamburg rutting test was carried out. The rutting specimens were made in size of 300 × 300 × 50 mm (length/width/thickness) according to China’s standard (JTG E20-2011). After being repeatedly rolled by a hydraulic molding machine for 12 cycles, they were placed in the temperature of 45°C for 48 h. This condition can simulate the dry state of pavement materials within 5∼12 h. Simultaneously, the comparative tests were conducted in the temperature of 60°C.
From the perspective of aggregate surface, RA, especially RCA is formed in cement hydration products, including ettringite, and calcium superoxide, these components have a certain activity to trigger chemical reactions with acid asphalt [
Scanning electron microscope results of three kinds of asphalt mixtures.
The interfaces of three aggregates and asphalt differed significantly. NA and asphalt were bonded with a clear dividing line in good adhesion shown in Figure
Figure
Mechanical test results of recycled asphalt mixtures. (a) Compressive strength. (b) Resilient modulus. (c) Splitting tensile strength.
For resilient modulus presented in Figure
Similarly, shown in Figure
In Figure
Water stability test results of recycled asphalt mixtures. (a) Residual stability. (b) Freeze-thaw splitting test strength ratio.
Figure
In general, with the increase of RA content and proportion of bricks to concrete, the water stability indexes all decrease. The porosity of RAM gets larger as RA content increases, and water molecules can move into internal part of the specimen more easily. Simultaneously, as the composition of RA is complex, the adhesion strengths between different recycled materials and asphalt are diverse, which directly affects the bonding effect. Large voidage and the shortage of adhesion make the water stability performance of RAM poorer.
Figure
Dynamic stability test results of recycled asphalt mixtures.
Figure
Rutting depth of recycled asphalt mixtures.
As a promising material in pavement base, RAM play an increasingly important role in recycling solid wastes and reducing engineering cost. Laboratory tests were performed to investigate the effect of recycled bricks and concrete on the property of RAM. The conclusions that can be drawn from the previous findings and observations are listed below: According to the design of Bailey method, the final proportion of each grading aggregate is 33.3%, 24%, 12.3%, 26%, and 4.4%, close to the recommended median curve; it is helpful to make tested aggregates form a dense skeleton structure. Through Marshall Test, it seems that the stability, density, and asphalt saturation decrease with the increase of RA content, but oppositely for the flow value and voidage. From the scanning electron microscopy, the adhesion interface morphology between three aggregates and asphalt, as well as their adhesion property, is significantly different; natural aggregates show the best bonding with asphalt, the recycled concretes the second, and the recycled bricks the worst. China’s technical specification (JTG F40-2004) and the test results of the road performances of RAM were taken as reference. The RA content and the corresponding proportion of bricks to concrete are recommended as follows to fill the requirement of pavement base in expressway: when the RA contents are 25% and 50%, the corresponding proportion of bricks to concrete should be controlled within 6 : 4 and 4 : 6, and it should be less than 2 : 8 if RA contents increase to more than 75%.
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors gratefully acknowledge the National Natural Science Foundation of China (51878078, 51911530215, and 51478054), Training Program for High-Level Technical Personnel in Transportation Industry (2018-025), Excellent Youth Foundation of Natural Science Foundation of Hunan Province (2018JJ1026), and Key Project of Education Department of Hunan Province (17A008). This work was also supported by the Key Project of Open Research Fund of National Engineering Laboratory of Highway Maintenance Technology (kfj150103) and Postgraduate Scientific Research Innovation Project of Hunan Province (CX20190644).