Shaopeng Wu, Wenfeng Yang & Yongjie Xue
Key Laboratory for Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of
Technology, Wuhan 430070,P.R China
ABSTRACT: With the rapid economy growth and continuously increased consumption, a large amount of waste materials is generated. Among them, waste glass material is an important part.
Glass material is nonmetallic and inorganic, it can neither be incinerated nor decomposed, so it may be difficult to reclaim.
This paper attends to study the performance of asphalt concrete where some of the fractional fine aggregate is substituted with crushed glass material. Glass materials are brittle and rich in silicon, so the key technical indexes of glass-asphalt concrete are strength and resistance against water damage. Materials used in the tests included AH-70 and SBS modified asphalt, limestone aggregate and crushed recycled glass. The Marshall test was used to examine the influence of optimal asphalt content, volume properties and strength of asphalt concrete when different percentages of crushed glass were added. The tests data from modified Lottman test, freeze-thaw pedestal test and wheel tracking test immerged in water showed that the resistance against water damage of glass-asphalt concrete is more feeblish than ordinary asphalt concrete. The properties can be improved by using liquid anti-stripping agents or hydrated lime admixture. The high-temperature stability and fatigue performance of glass-asphalt concrete was also tested and the results are satisfactory. The research has demonstrated that the recycling and use of waste glass in asphalt concrete is feasible.
KEY WORDS: Glass, asphalt concrete, water stability, anti-stripping agent, hydrated lime
1 PREFACE
How to effectively utilize the large amounts of waste glass from glass industry has been an urgent subject in national and global range. Glass is a non-metallic and inorganic material made by sintering selected raw materials, so it can neither be incinerated nor decomposed. Glass recycling can save energy and decrease environmental waste. Focus on glass recycling technology will also widen the application domain of waste glass and promote further development of glass techniques.
Nearly 10 million tons of waste glass has been generated in metropolises every year, that is about
3~5 wt % of the domestic waste. The recycle systems are commendably constructed in many countries and the recycle ratio is above 70%, whereas it is 0 at home.
The recycle ratio in Europe reached 77.8% in 2002, predicted to 80% in 2005, while the ratio in Japan was merely 20% before 1996.

The quality and performance of glass products made by recycled glass is not easy to control, few glass enterprises will manufacture glass products utilizing re-melted recycled glass. What is more, the process of manufacturing cheesy glass products is inherently second waste of resource.
There are two primary approaches of recycling waste glass, one is direct and the other is indirect. The first process is to manufacture tabulate glass products utilizing worked broken glass particles. This can save 1.1 ton raw materials (quartzite gravel, limestone, etc) and 140 liters heavy oil. The second process is to manufacture products with some class content, such as clay bricks, filling materials, building decorations, soundproof or adiabatic materials etc.
By crushing and sieving, waste glass can be used as fine aggregate in asphalt concrete. This is called glasphalt. Satisfactory performance of upper asphalt pavement layers can be received with a dosage of 10~15 wt%. Larger amounts may induce stripping problems and make the pavement sensitive to water damage. The ratio of 25% or more of glass additive can only be applied to middle or bottom layers.
The initial applications of “glasphalt” took place in late 1960s when United States and Canada constructed test roads with glasphalt to testify the water stability of pavements. Middle 1970s to middle 1980s, Baltimore city (Maryland, United States) paved many sidewalks with glasphalt. The sidewalks were shiny at night because of the reflecting performance of glasphalt, and later they intentionally constructed lots of glass recycling and disposing factories. From 1990 to 1995, the total amount of glasphalt application reached about 250 thousand tons in New York.
Up to now, in China, there have been few special investigations on the application of waste glass in the field of pavement, not to mention field applications.
2 RAW MATERIALS
2.1
Glass
Ordinary glass is rigid and brittle and easy to crush to form satisfactory particles for asphalt concrete applications. The broken glass used in asphalt concrete is characterized by:
1.
Numerous long and flat particles (especially for big broken glass particles). This may cause problems like stripping of the asphalt film from glass particles surfaces, infirm skid resistance, abrasion of tires, too high reflectance etc.
2.
The surface of broken glass particles is exceeding smooth and the silica content is relative high, making glass particles a hydrophilic acid aggregate. Pavements with glasphalt may then be sensible to water damage (especially when glass particle size is increased or vast dosage).
3.
The angularity and friction angle afford insufficient transverse stability (at braking or start-up).
4.
Low asphalt absorption ratio and density may cause bleeding problems.
5.
Excellent light reflection properties assure safe nighttime driving, but when glass particle size is increased there is a risk of dazzling.
6.
Volume stability is good because of the inflation coefficient when heated is small
(about 8.8×10-6cm/cm/°C 9.2×10-6cm/cm/°C when the temperature is below 700°C). This is beneficial to the resistance of low temperature cracking.
7.
The asphalt absorption ratio is near upon zero which is unfavorable to the adhesion of the asphalt film to the broken glass particles.

Glass is a non-metallic inorganic made by sintering selected raw materials comprising silicate and other minor oxides. The ratio of main oxides SiO
2
, Na
2
O, CaO are: 77%, 9.4% and 6.7% respectively.
The waste broken glass used in the study is reclaimed beer bottles that were factitious crushed into required particles. The maximum glass particles size is 4.75mm and is adaptive to AC type gradation. Figure 1 shows the gradation curve of the broken glass particles.
Figure 1: Sieving Curve of Broken Glass Particles
2.2
Asphalt
Asphalt is the most commonly used material in pavement construction today because of its high engineering performance capabilities such as elasticity, adhesion and water resistance. Asphalt is known to be a complicated colloidal system of hydrocarbon materials which are composed of asphaltenes, resins and oils. Today’s asphalt is produced mainly by the refining of crude oil and the physical and chemical properties can be altered or improved by blending, air blowing, additives etc. The interface between the asphalt and aggregate has been much focused in order to determine the chemical factors that influence bonding between the two materials.
AH-70 asphalt from Korea is used in the study, the relative performance indexes were tested according to “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway
Engineering” (JTJ 052-2000) and are listed below:
• penetration (25°C, 100g, 5s, 0.1mm): 73.4
• softening point (°C):
• ductility (15°C, cm):
46
120 cm
• flash point:
• olefin content:
• density (25°C):
287°C
1.9%
1.028g/cm
3
The performance indexes after RFOT (Rotation Film Oven Test) are listed below:
• ductility (25°C, cm): 150 cm
• ductility (15°C, cm): 55

• weight loss:
• penetration ratio:
2.3
Mineral Aggregate
0.03%
75%
The physical and chemical properties have intense effect on asphalt mixtures because of the effect of aggregate properties on the adhesive bond between asphalt and aggregate. Many factors have an influence on bonding, such as size and shape of aggregate, pore volume and size, surface area, chemical constituents at the surface, acidity and alkalinity, adsorption size surface density, and surface charge or polarity.
The mineral aggregate used in the study is limestone. The CaO and SiO2 contents are 48.6% and
25.1%, respectively. The crushed and weared stone values are 12.5% and 21.6% respectively. The limestone surface was coated with montmorillonite and ferrous hydroxid which result in proper coating and bonding between the aggregate and asphalt, the boiling test result is grudging 4 grades.
The mineral filler was milled with limestone with CaO content of 58%.
2.4
Other Additives
Hydrated lime belongs to I grade with the CaO content of 67%, the sieving result is similar to mineral filler’s.
Liquid anti-stripping agent is coconut oil ethanolamine, the TG-DSC result indicate that the heat stability is excellent.
3 TEST PROJECTS
The broken glass is classified into acid aggregate according to the SiO2 content of more than 70%.
The surface is so smooth that the adhesive bond between glass particles and asphalt is unsatisfactory. Biggish particles size and dosage of glass have negative effects on the water stability of asphalt mixture prepared with partial waste broken glass. It is well known that increasing size of aggregate and thus smaller specific surface causes weaker adhesive bonding.
The smaller glass particles have more fragmentized and coarse surface areas, which is more beneficial regarding adhesion between glass particles and asphalt.
The biggish particles may abrade the tires seriously and also decrease the skid resistance when brittle glass particles are crushed by traffic loads.
Large glass particles should thus be avoided, reference
[1]
suggest 4.75mm as the maximal size of broken glass particles.
The results of rutting tests immersed in water indicated that dynamic stability of samples with maximal glass particle size of 9.5 mm is lower than the samples with maximal size of 4.75 mm.
Further, there were much more uncovered surface areas in the 9.5mm mix, as shown in figure 2. So the maximal size of 9.5mm is improper to use in asphalt mixtures, and the study adopted the maximal size of 4.75mm.

Figure 2: Internal states of broken glass asphalt mixtures after water immersed rutting test
A test program was set up to determine the optimal dosage of waste broken glass particles. A maximum glass particle size of 4.75mm and four glass contents were chosen: 0, 5, 10, 15 and 20% in terms of total aggregate weight. A suspending and close-grained gradation of AC-16I was used as showed in figure 3. These curves were similar for all the glass dosages.
The investigation was conducted on Marshall specimens (diameter 10 cm, height 6.35 cm). To enable comparison, the same binder content of 4.8% is used for all different mixtures. The additive of hydrated lime and liquid anti-stripping agent were used to improve the resistance to water damage of the asphalt mixtures.
The following properties were tested in the study:
•
Strength and volume properties: Marshall Test
•
Stability: Rutting test
•
Water stability: Residual stability test immersed in water, freezing and thawing splitting test, water immersed rutting test
Figure 3: Aggregate (limestone and glass particles) sieve analysis data

4 RESULTS AND DISCUSSION
4.1
Marshall Test
The Marshall test results of mixtures with different glass replacement are shown in Table 1.
The apparent specific gravity of glass is 2.506 g/cm
3
and 2.736 g/cm
3
for limestone aggregate.
From table 1, the maximal theoretical density of mixtures drops evidently with the increase of glass replacement ratio. But the air voids ratios of the Marshall specimens have a relatively lesser increase. Figure 4 shows that the strength indexes of Marshall stability and indirect tensile strength are decreasing substantially with the increase of glass replacement.
Table 1: Marshall test results of different glass replacement
Amount of glass added
(%)
Maximal theory density
(g/cm
3
)
Air void ratio
(%)
Marshall stability
(kN)
Indirect tensile strength
(25 , MPa)
0 2.530 4.13 13.76 0.846
5 2.521 4.19 13.32 0.775
10 2.514 4.16 12.72 0.733
15 2.503 4.22 11.69 0.637
20 2.496 4.25 11.13 0.594
Figure 4: Marshall stability and indirect tensile strength as functions of different glass contents
4.2
Rutting Test
The rutting test intended to test the dynamic stability and evaluate the high temperature stability of mixtures. According to regulations
[5]
T0703-1993, specimens of 30cmx30cmx5cm were prepared and laid down for 48h, then tested according to the regulation T0719-1993 (temperature 60°C,

rubber wheel load pressure 0.7±0.05 MPa, traversing distance 230mm, frequency 42 times/min).
The results are showed in Table 2.
Table 2: Dynamic stability of glass asphalt mixture with different glass contents glass replacement (%) dynamic stability (times/mm) 1436 1412 1357 1216 1058
We can conclude that the dynamic high temperature stability is descending with increasing glass replacement. No internal rushed glass particles could be observed when the samples were broken off with fingers and thumb after test. Quite thin binder film could be seen on the surface of the glass particles. The excellent angularity and friction angle of the glass particles were beneficial to enhance the high temperature rutting resistance, thus the decrease of dynamic stability is not remarkable.
4.3
Residual Stability Test Immersed in Water and Freezing-Thawing Splitting Test
The processes of residual stability test immersed in water and freezing-thawing splitting test are according to regulations
[5]
T0709-2000 and T0729-2000 respectively. In order to accelerate water damage of samples, the air voids of 6~8% and saturated ratio of 70~85% of splitting samples were strictly controlled in the study. Figure 5 showes the test results of AC-16I glass asphalt concrete.
Figure 5: Residual stability and splitting ratio as functions of different glass contents
Figure 5 shows that the residual stability declined with the increase of glass replacement. The residual stability at glass replacement of 15% fell short of the standard of 75%. The degressive trend of the splitting ratio was similar to the residual stability. However, the splitting ratio was

smaller than the residual stability with same glass replacement. The air voids of splitting samples were 6~8% which is higher than in the Marshall stability specimens (about 4%).
The vacuum saturation and freezing-thawing circulation remarkably accelerated the water damage of splitting samples relative to the Marshall stability samples. The splitting ratio was
73.4% on the glass replacement of 10%, 62.1% on the replacement of 15% and 51.3% when the replacement increased to 20%. The results indicated that with the replacement of glass, the water stability of asphalt mixtures declined. The weak adhesion between asphalt and glass surface was incapable of enduring the dual effects of high temperature and water.
It is necessary to improve the ability of resistance to water damage by introducing anti-stripping measures.
4.4
Rutting Test Immersed in Water
The rutting test immersed in water is traceable to Hamburg Wheel-Tracking Device which was developed by Esso A.G. in the 1970s in Hamburg, Germany. The test measures the combined effect of rutting and moisture damage by rolling a tire wheel back and forth across the surface of an asphalt concrete specimen that is immersed in hot water.
The apparatus used in this particular study was a modified version of the ordinary rutting test apparatus. In order to accelerate the damage, the following measures was taken; a) decreasing the molding times to augment air voids of rutting samples b) vacuum saturation c) freezing-thawing circulations
The rutting tests immersed in water at different temperatures indicated that if the testing temperature was too high, such as 70°C or 60°C, the samples would disperse because of the inferior stiffness. If the temperature was low, the anticipated effect could not be attained. So, 45°C was chosen as the testing temperature. The results were expressed by permanent rutting deformation and stripping weight loss from the samples as shown in Table 3 and Figure 6.
Table 3: Results of rutting test immersed in water glass replacement
(%) rutting deformation
(mm) stripping weight (g) remarks
10 13.6 49.5
(b) the maximal fixed deformation was 15mm

Figure 6: The curves of rutting test immersed in water
The data indicate that the permanent rutting deformation and stripping weight loss increase with increasing glass replacement ratio. According to Figure 6, the total loading times and rutting deformation varied with the glass replacement, the samples with higher glass contents broke or reached the upper deformation (15mm) just after a short period of time. Additionally, the ordinary rutting test curve resembles a transverse parabola, but the result curve of rutting test immersed in water is not the same according to literature
[3]
. The literature show an “S” shape of the testing result curve and the corresponding x-coordinate of the curve inflexion (the point of intersection of creep slope and stripping slope) denote the start of asphalt stripping. According to this theory, the starting points (corresponding to loading times) of mixtures with glass replacements from 0 to 25% are 1640, 1450, 1310, 920, 350 times respectively.
This testified the bad effect of glass replacement on the water stability, so measures to improve the water stability of glasphalt is highly necessary.
There are two qualifications to determine the optimal glass replacement ratio. How to furthest use the waste glass is the lower limit. The guarantee of mixture performance (especially for water stability) is another qualification, belonging to the upper limit.
By combining the two qualifications, 10% was determined as the optimal glass replacement ratio.
4.5
Measures to Improve the Water Stability of Glasphalt
Some additives such as hydrated lime and liquid anti-stripping agents are generously used to improve the water stability of asphalt mixtures. In general, water stability improvement may include a) pretreatment of the aggregate surface by chromeplating, organosilanes or saturating in
Ca(OH)2 solution b) modifying the asphalt binder using SBS, SBR, PE or phosphate c) addition of anti-stripping agents to the asphalt
AH-70 asphalt, liquid asphalt anti-stripping produced by a certain company and first-class hydrated lime were used in this study. The dosage of liquid anti-tripping agent followed the

technical suggestion of 0.4 wt% of asphalt. The dry hydrated lime of 2% dosage was combined with prewetting of the aggregate and airing. The study intended to testify the effect on water stability of glasphalt with 10% glass content.
Table 4 displays the results of Marshall residual stability and freezing-thawing splitting ratio after different types of additions.
Table 4: Effects of hydrated lime and anti-stripping agent on water stability indexes additives Residual stability (%) Splitting ratio (%)
2% hydrated lime
0.4% liquid anti- before aging stripping agent after aging
91.4
92.3
91.7
87.3
89.7
89.4
The ability to resist water damage was improved evidently. The liquid anti-stripping agent was more effective than hydrated lime. The performance indexes were more or less the same before and after a short aging of glasphalt with liquid antistripping agent.
TG-DSC analysis indicated that the liquid anti-stripping agent decompounds in atmosphere at the temperature of 230 °C.
These influences were also demonstrated by rutting tests immersed in water. This was more mimetic to the environmental conditions of a southern rainy area in our country. The results are dismayed in Figure 7.
Figure 7: The effect of additives on the rutting test immersed in water
The primordial samples cracked after 2500 loading cycles and rutting deformation depth reached 14.2mm. When additives were introduced, the situation changed remarkably. A substantial improvement in deformation resistance was recorded.
Figure 8 shows photos of the rutting samples before and after use of liquid anti-stripping agent.
Some aggregate surface was directly exposed to the air (without asphalt film coated on the aggregate surface) before the anti-stripping agent was used. The dissatisfactory phenomenon could

be improved by using anti-stripping agent, nearly all aggregates surface were coated by asphalt film and no exposed part was displayed.
Figure 8: Internal states of broken glass asphalt mixtures before and after use of anti-stripping agent
The test has illustrated that the application of hydrated lime and liquid anti-stripping agent in glasphalt are useful treatments to improve the water damage resistance. The glasphalt properties with liquid anti-stripping agent were slightly better than with hydrated lime.
5 CONCLUSIONS
From the studies on glasphalt described in this paper, the following conclusions can be attained:
1) Waste broken glass can be used in asphalt concrete with the maximal size of 4.75mm and the optimal replacement ratio of 10%.
2) The performances such as strength index, high temperature stability and water stability achieve the standards.
3) The water stability of glasphalt can be improved by introducing hydrated lime or liquid anti-stripping agent.
4) Liquid anti-stripping agent is more effective in improving the water stability of glasphalt than hydrated lime.
REFERENCES
Marti, M. M. and Mielke, P. E. A., 2002. Synthesis Of Asphalt Recycling In Minnesota [R],
Minnesota Local Road Research Board, Synthesis Report 2002-32;
Nansu, J S Chen, 2002. Engineering Propertioes of Concrete Made With Cycled Glass Resources
[J], Conservation and Recycling, June 2002,Volume35 Essue4, pages259-274;
Moisture Sensitivity of Asphalt Pavement (A NATIONAL SEMINAR) [R], Transportation
Research Board of the National Academies, February 4-6, 2003;
Jiang wenjiu, 2003. Recycle of waste glass, Shanghai Building materials [J], 2003.3;
JTJ052-2000, Standard Test Methods of Bitumen and Bituminous Mixtures for Highway
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