DOI: https://doi.org/10.62204/2336-498X-2023-4-15
COMPARISON OF DENSITY OF ASPHALT CONCRETE
MIXTURE WITH FLY ASH AND LIMESTONE FILLER
Ivan Kopynets,
Ph.D. in Technical Sciences,
SE «National Institute for Development Infrastructure», Ukraine,
viddbm@gmail.com; ORCID: 0000-0002-0908-4795
Oleksii Sokolov,
SE «National Institute for Development Infrastructure», Ukraine,
bitumen_lab@ukr.net; ORCID: 0000-0002-4694-9647
Annotation. The road construction industry is one of the strategic industries of Ukraine. Currently, the issue of quality and availability of basic building materials for road construction is particularly acute, which is directly related to its high material intensity. Known stocks of conditioned raw materials that could be used as components of asphalt concrete are constantly decreasing, so it is necessary to look for alternative sources of raw materials for construction materials and study the possibility of their use. In this regard, the most effective use of local raw materials is the use of industrial production waste, which can become one of the solutions to the problem of the lack of raw materials of inorganic origin.
In Ukraine, about 30% of all electricity is generated by burning solid fuels such as coal, oil shale, and peat. There are about 15 operating thermal power plants in Ukraine, which generate about 5-6 million tons of ash and slag waste per year. Thus, the fuel and energy complex waste generated at thermal power plants is a huge accumulation of ash in the form of dusty residues and lumpy sludge, as well as various ash and slag mixtures. These products of high-temperature treatment (1200-1700 °C) of the mineral part of the fuel are widely used in many countries and given the global trend towards an increase in the share of the secondary market for the use of secondary industrial products, an increase in the rate of their processing in Ukraine should be predicted.
Keywords: asphalt concrete mixture, fly ash, industrial waste, locking point.
Introduction. One of Ukraine’s top priorities is the development of its transport infrastructure. The operation of asphalt concrete roads under the aggressive influence of external factors and the continuous increase in the number of vehicles leads to the formation of various defects and, as a result, to premature deformation and destruction of roads [1].
Nowadays, scientists and road industry specialists pay special attention to the quality of road construction, search for new effective solutions to further improve the quality and increase the maintenance-free service life of pavements. At the same time, asphalt concrete remains the main material for road pavement. The length of roads with such pavement in Ukraine exceeds 169.5 thousand kilometers, as well as 250 thousand kilometers of city streets.
Asphalt concrete is an artificial construction material that is formed after compaction of an asphalt mixture prepared in a heated state in mixers and includes rationally selected mineral materials (crushed stone, sand, mineral powder) and bitumen. The cost of materials makes up the largest part of the cost of producing asphalt mixtures – 80%. The annual increase in the cost of materials (including crushed stone, mineral powder and bitumen) and energy resources leads to an average increase in the cost of producing asphalt mixtures by 10-15%.
This circumstance requires a detailed study of the market for asphalt concrete feedstock, identifying existing problems and methods to solve them.
Improving the quality and service life of asphalt pavement remains a top priority for the road sector. The solution to this problem is the production and use of asphalt concrete with improved performance and service life, as well as cheaper production of asphalt concrete mixtures without reducing the physical and mechanical properties of the material [2].
One of the prerequisites for designing the composition of the hot asphalt concrete mixture is that the density of the samples produced in the laboratory, which are used to determine the optimal bitumen content, should be close to the final density of the asphalt concrete layer of the road surface. If the density of asphalt concrete is too low, then the durability of such material will be reduced, and if the density of asphalt concrete is too high, then such material will have a tendency to sweat or form ruts. The density of asphalt concrete when compacted in the field is almost always 1.5% less than the density of samples made in the laboratory. This indicates that the laboratory compaction effort is too high [3].
The Superpave mixture composition design system takes into account different traffic and environmental conditions. One of the main pieces of equipment in the Superpave system is the Superpave gyratory compactor. The gyrator compactor is used to compact asphalt concrete mixtures of the selected composition at the design number of rotations in the laboratory in order to be able to evaluate the volumetric properties of the compacted sample. The bulk properties evaluated include porosity, mineral aggregate voids, bitumen-filled voids, and the ratio of mineral powder to effective bitumen content.
To check the compaction speed, two additional parameters are included: the density at the initial number of gyrations (Ninitial) and the density at the highest number of gyrations (Nmaximum). It is assumed that the laboratory design amount of residual pores is related to the final density of asphalt concrete. The general characteristics of asphalt concrete strongly depend on the design of the road surface and the quality of construction.
Literature review. The Illinois Department of Transportation has developed an alternative to Ndesign called the “lock point” concept to prevent over compaction and subsequent failure of the aggregate in the asphalt mixture [4]. The lock point, defined as the rotation at which the aggregate “locks” together and further compaction results in aggregate failure and very little additional compaction, was compared to a growth curve constructed to determine the highest number of roller passes along the road before density increases is leveled or reduced on the spot.
It was noted that mixes do not compact with the same number of passes because each mix is different. Compaction was stopped at peak density before excessive aggregate failure occurred. The concept of the locking point was developed based on a comparison of three years of Marshall and Superpave data and the density growth curves during pavement placement [4]. Originally, the Illinois lock point was defined as the first rotation in a set of three rotations at the same height, preceded by one set of two rotations at the same height (each 0.1 mm greater than the set of three rotations). The locking point was believed to indicate the development of some degree of coarse aggregate cohesion and was related to the density reached on the field growth curves. The standard deviation of the number of gyrations equal to the locking point was less than the standard deviation of the number of gyrations to obtain 4% air voids.
Vavryk and Carpenter [5] refined the definition of the locking point as the first rotation in the first occurrence of three rotations at the same height, preceded by two sets of two rotations at the same height (each 0.1 mm higher than the set of three rotations), as shown in Figure 1.
The objective of the study. In previous studies, [6-8], it was found that the physical and mechanical characteristics of asphalt concrete with fly ash practically do not differ from those of asphalt concrete with standard aggregate. Moreover, water resistance and frost resistance in some cases are even higher in samples using fly ash, while the optimal bitumen content in asphalt concrete with fly ash is 0.5% less than in standard asphalt concrete.
According to the data obtained, it was hypothesized that the compaction of asphalt concrete mixtures with fly ash may be faster than that of mixtures with limestone aggregate. However, national standards do not establish a method for determining the compaction of such a mixture. Therefore, it was decided to conduct additional research and establish an effective method for determining the compatibility of an asphalt concrete mixture with fly ash.
Test methods. Bulk density of asphalt concrete.
The bulk density of asphalt concrete was determined by its weight and volume in accordance with Method B of DSTU EN 12697-6 [9] (water-saturated state with a dried surface). The mass of the sample was determined by weighing the dry sample in an air environment (in air). The volume of the sample was determined by its weight in air and water.
The determination of the bulk density of asphalt concrete was performed in the following sequence:
- the mass of the dry sample (m1) was determined;
- the density of water (ρw) was determined at the test temperature with an accuracy of 0.0001 Mg/m3 in accordance with formula (1):
7,59 × t – 5,32 × t2 ρw = 1,00025205( 106 ), (1)
where — t is the water temperature, in degrees Celsius (°C);
РW is the density of water at the test temperature, in megagrams per cubic meter (Mg/m3);
- the sample was immersed in a water bath in which the test temperature was
maintained and kept for at least 30 minutes;
- determine the mass of the saturated sample in water (m2), making sure that during
weighing no air bubbles adhere to the surface of the sample or escape from it;
- remove the sample from the water and dry its surface from residual water by
wiping it with a damp cloth;
- immediately after drying the surface of the sample, the mass of the saturated
sample with the dried surface in the air was determined (m3);
- f) calculate the bulk density of the sample (Ρbssd) to the nearest 0.001 mg/m3 in accordance with formula (2):
m1
ρbssd=m3 – m2 × ρw, (2)
where — Ρbssd is the bulk density, in megagrams per cubic meter (Mg/m3); m1 is the mass of the dry sample, in grams (g); m2 – mass of saturated sample in water, in grams (g);
m3 is the mass of the saturated sample with a dried surface, in grams (g); Pw is the density of water at the test temperature, in megagrams per cubic meter (Mg/m3).
Air pore content (air void)
Air pores are the voids between the bituminous aggregate grains in a compacted asphalt sample.
Air pore content is the volume of air pores in an asphalt specimen expressed as a percentage of the total volume of the specimen.
The air pore content of the asphalt concrete sample was calculated using the maximum density of the asphalt mixture and the bulk density of the asphalt concrete with an accuracy of 0.1 % according to formula (3):
𝜌𝜌m – 𝜌𝜌b
Vm = 𝜌𝜌m × 100, (3)
where — Vm is the air pore content, % by volume; pm is the maximum density of the mixture, in megagrams per cubic meter (Mg/m3); pb is the bulk density of the sample, in megagrams per cubic meter (Mg/m3).
Binder-filled pores
Binder-filled pores are the percentage of pores in the mineral aggregate that are filled with binder.
The percentage of pores in the mineral aggregate filled with binder was calculated from the content of binder and pores in the mineral aggregate, as well as the bulk density of asphalt concrete and the density of binder to the nearest 0.1% using formula (4):
ρb
VFB = ((B × ) × VMA) × 100 %, (4)
ρB
where — VFB is the percentage of pores in the mineral aggregate filled with binder, %
by volume;
B – percentage of binder in the sample (in 100% of the mixture), % by volume; ρb is the bulk density of the sample, in megagrams per cubic meter (Mg/m3); ρB is the density of the binder, in megagrams per cubic meter (Mg/m3); VMA – pore content in mineral aggregate, % by volume.
Pore content in mineral aggregate
The pore content in mineral aggregate is the volume of intergranular voids between the aggregate grains of the compacted asphalt mixture, consisting of the volume of air pores and the volume of bituminous binder in the asphalt concrete sample, determined as a percentage of the total sample volume. The pore content in the mineral aggregate was calculated to an accuracy of 0.1 % using formula (5):
VMA = Vm + B × 𝜌𝜌b / ρB (5)
where —VMA is the pore content in the mineral aggregate, determined to within
0.1% (by volume);
Vm – air pore content in the sample, % by volume;
B – binder content in the sample (in 100% of the mixture), % by weight; ρb is the bulk density of the sample, in megagrams per cubic meter (mg/m3); ρB is the density of the binder, in megagrams per cubic meter (Mg/m3).
Maximum density
Maximum density is the mass of asphalt mixture per unit volume without air voids at a given test temperature. The maximum density in combination with the bulk density is used to calculate the air pore content of the compacted specimen and other bulk properties of the compacted asphalt mixture.
The maximum density was calculated to the nearest 0.001 mg/m3 using formula (6):
100
ρmc= (pa1 / ρa1) + (pa2 / ρa2) + … + (pb / ρb1), (6)
where ρmc is the maximum density of the material, in megagrams per cubic meter
(Mg/m3); pa1 – content of mineral aggregate 1 in the mixture, % by weight;
ρa1 is the bulk density of mineral aggregate 1, in megagrams per cubic meter (Mg/
m3); pa2 – content of mineral aggregate 2 in the mixture, % by weight;
ρa2 is the bulk density of mineral aggregate 2, in megagrams per cubic meter (Mg/
m3); pb – content of binder in the mixture, % by weight; ρb – density of the binder, in megagrams per cubic meter (Mg/m3); pa1 + pa2 + … + pb = 100.0 % (by weight).
Presentation of the main material. The concept of the locking point was taken as
a basis for developing a method for comparing the technological properties of asphalt concrete mixtures with different types of fillers. Thanks to this method, it is possible to evaluate the compaction of asphalt concrete mixtures with obtaining the optimal residual porosity of the material.
The essence of this method is to evaluate the technological properties of the asphalt concrete mixture by the locking point as the first rotation in a block of three consecutive rotations during which the thickness of the sample decreases by less than 0.1 mm, which is preceded by two blocks of two rotations during which the thickness of the sample decreases by less than 0.1 mm, as shown in Figure 1 and Table 1.
Fig. 1. Locking point definition
Table 1
Determination of the locking point by the number of gyrations
| Number of gyrations | Height | Density | Blocks |
| 70 | 105.258 | 2341 | 1 |
| 71 | 105.215 | 2342 | 2 |
| 72 | 105.179 | 2342 | 1 |
| 73 | 105.139 | 2343 | 2 |
| 74 | 105.098 | 2344 | 1 |
| 75 | 105.054 | 2345 | 2 |
| 76 | 105.007 | 2346 | 3 |
| 77 | 104.973 | 2347 | |
| 78 | 104.940 | 2348 | |
| 79 | 104.897 | 2349 | |
| 80 | 104.867 | 2349 |
For testing, the grain composition of the asphalt concrete mixture was selected according to DSTU EN 13108-1 [4].
Table 2 Asphalt concrete test results
| Indicator | Asphalt concrete with limestone | Asphalt concrete with fly ash | ||
| Number of gyrations | 74 | 80 | 69 | 80 |
|
Apparent density of coarse aggregate fraction 8/16 mm, Mg/m3 |
2,653 | 2,653 | 2,653 | 2,653 |
|
Apparent density of coarse aggregate fraction 4/8 mm, Mg/m3 |
2,689 | 2,689 | 2,689 | 2,689 |
|
Apparent density of coarse aggregate fraction 2/4 mm, Mg/m3 |
2,675 | 2,675 | 2,675 | 2,675 |
|
Apparent density of coarse aggregate fraction 0/2 mm, Mg/m3 |
2,676 | 2,676 | 2,676 | 2,676 |
| Apparent density of mineral powder, Mg/m3 | 2,84 | 2,84 | 2,61 | 2,61 |
| Apparent density of bitumen, Mg/m3 | 1,016 | 1,016 | 1,016 | 1,016 |
| Bulk density of asphalt concrete, Mg/m3 | 2,380 | 2,385 | 2,397 | 2,407 |
| The content of the coarse aggregate fraction 8/16 mm, % | 35,0 | 35,0 | 35,0 | 35,0 |
| The content of the coarse aggregate fraction 4/8 mm, % | 19,0 | 19,0 | 19,0 | 19,0 |
| The content of the coarse aggregate fraction 2/4 mm, % | 7,0 | 7,0 | 7,0 | 7,0 |
| The content of fine aggregate fraction 0/2 mm, % | 33,0 | 33,0 | 33,0 | 33,0 |
| Filler content, % | 6,0 | 6,0 | 6,0 | 6,0 |
| Bitumen content (over 100 %), % | 5,3 | 5,3 | 5,3 | 5,3 |
| Bitumen content (in 100 %), % | 5,03 | 5,03 | 5,03 | 5,03 |
| Calculated maximum density of the mixture of mineral materials, Mg/m3 | 2,680 | 2,680 | 2,666 | 2,666 |
| The average density of the aggregates, Mg/m3 | 2,260 | 2,265 | 2,276 | 2,286 |
| The maximum density of the asphalt concrete mixture, Mg/m3 | 2,476 | 2,476 | 2,465 | 2,465 |
| Void in mineral aggregate, % by volume | 15,65 | 15,47 | 14,62 | 14,27 |
| Air void, % by volume | 3,9 | 3,7 | 2,7 | 2,3 |
| Void filled with binder, % by volume | 11,8 | 11,8 | 11,9 | 11,9 |
| Void filled with binder, % | 75,3 | 76,4 | 81,2 | 83,6 |
According to the results of the test, it was established that the locking point for asphalt concrete mixture with fly ash corresponds to 69 gyrations of the gyratory compactor, and the locking point for asphalt concrete mixture with limestone filler corresponds to 74 gyrations. That is, it can be argued that the compaction of asphalt concrete mixture with fly ash occurs faster than asphalt concrete mixture with limestone filler. It was also established that the bulk density of asphalt concrete with fly ash at a lower number of gyrations is greater and is 2.397 g/cm3, and in asphalt concrete with limestone filler – 2.380 g/cm3. According to the results of the study, it was found that the air void of asphalt concrete using fly ash at a higher volume density is 1.1% lower than that of asphalt concrete with limestone aggregate. In addition, the percentage of pores filled with binder in the mixture with fly ash is 6.9% higher than in the case of limestone aggregate. These results indicate the possibility of reducing the amount of bitumen in asphalt concrete, which has potential economic and environmental benefits.
Conclusions. The residual porosity of asphalt concrete samples was determined by the calculation method and it was established that asphalt concrete with fly ash, obtained after 69 gyrations of the gyrator compactor, has a lower residual porosity than asphalt concrete with limestone filler. This indicates the possibility of bitumen reduction in the asphalt concrete mixture with fly ash.
The use of fly ash as part of the asphalt concrete mixture is a promising solution, as it allows reducing the negative environmental impact on the environment, reducing costs from ash waste storage, while ensuring proper quality.
Development and modernization of road infrastructure is of strategic importance for economic growth and increasing competitiveness of Ukraine.
The introduction of fly ash asphalt mixtures in Ukraine opens up new opportunities for infrastructure development, providing the opportunity to repair old and build new roads at more favorable prices.
References:
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- Brown, E. R., and M. S. Buchanan, NCHRP Research Results Digest 237: Superpave Gyratory Compaction Guidelines, Transportation Research Board, National Research Council, 1999 https://onlinepubs.trb.org/Onlinepubs/nchrp/nchrp_rrd_237.pdf
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- Vavrik, W. R., and S. H. Carpenter, “Calculating Air Voids at Specified Numbers of Gyrations in Superpave Gyratory Compactor,” Transportation Research Record 1630: Asphalt Mixtures: Stiffness Characterization, Variables, and Performance, Transportation
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- Volodymyr Kaskiv, Ivan Kopynets, Oleksii Sokolov. Study of fly ash from power generating enterprises to use it as an alternative to lime mineral filler for the production of asphalt mixtures. Roads and bridges. 2021. Iss. 24. P. 40–47 [in Ukrainian].
- DSTU EN 12697-6:2019 Bituminous mineral mixtures. Test methods for hot asphalt mixtures. Part 6: Determination of the bulk density of bituminous mineral samples. Kyiv, 2019. 13 p. (Information and documentation)
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