Title: The Suitability of Coal Bottom Ash in Hot Mix Asphalt
By: Duncan Rioba Oteki-1386142
TERM PAPER
PAVEMENT ANALYSIS AND DESIGN
Fall Semester, 2021
Abstract
This paper discusses the suitability of using coal bottom ash (CBA) in hot mix asphalt (HMA)
mixes used in the paving of flexible pavements. Fly ash and bottom ash are the two types of ashes
formed as waste products of thermal power plants in the process of electricity generation. Due to
the significant role that coal plays in power generation, the volume of these waste ashes continues
to increase. There would be a significant benefit to both the road construction industry and the
environment if coal bottom ash could be used as an aggregate in HMA mixes. This paper
investigated three research papers that investigated the effects of using CBA in HMA. In all the
papers, coal bottom ash was used as a partial replacement of the aggregates in the HMA mixes.
This was attributed to the fact that CBA has inferior mechanical properties when compared to
traditional aggregates. The first study showed that the inclusion of CBA into HMA mixes did not
increase the moisture susceptibility of the asphalt mixes. It was also found that HMA mixes with
CBA had higher fatigue cracking resistance when compared to control mixes. The second study
showed that HMA mixes containing limestone aggregates and CBA performed similarly to control
mixes in terms of resistance to low temperature cracking and stripping. In the final study, results
showed that the dynamic moduli of HMA mixes containing CBA were lower compared to control
mixes implying that there were less stiff. It was concluded that the determination of mechanical
properties of HMA mixes containing CBA could enable the implementation of mechanisticempirical design procedures.
1.0
Introduction
Hot mix asphalt or HMA designation is given to asphalt mixes that are mixed and placed at
temperatures between 300 and 350 0F. HMA is the most common type of asphalt mix used as
surfacing for flexible pavements in the United States due to its cost-effectiveness, durability, and
ease of construction (Ksaibati & Conner, 2004). Its ingredients consist of asphalt cement (binder),
coarse aggregate, fine aggregate, and mineral filler at varying proportions depending on the mix
design. Aggregates comprise 90-95% of asphalt mixes and have a direct impact on the strength,
workability, and durability of pavements (Ksaibati & Conner, 2004). However, the decreasing
sources of traditional aggregates coupled with the increasing haulage distances have resulted in a
call for alternative sources of aggregates.
Recently, researchers have investigated the possibility of using coal bottom ash (CBA) as an
aggregate source for HMA mixes (Goh & You, 2008; Ksaibati & Conner, 2004; Yoo et al., 2016,
and others). The utilization of CBA in HMA could reduce the overreliance on the costlier
traditional aggregates and thus decrease the costs incurred in the production of HMA mixes. CBA
is commonly disposed of in landfills as waste where it poses potential environmental risks.
Therefore, using CBA as aggregates in HMA could be environmentally beneficial. The
performance of a pavement relies primarily on the selection of appropriate aggregates.
Consequently, it is necessary to conduct a prior investigation into the suitability of using coal
bottom ash as aggregates in HMA. This literature review will lookinvestigate research that has
already been carried out on the effect of using CBA on HMA mixes, what methods were used,
their findings, and potential future work that could be done in this area.
2.0
Background
2.1
Coal Bottom Ash (CBA)
Bottom ash and fly ash are the two types of ashes formed as waste products of thermal power
plants in the process of electricity generation (Fig. 1). Coal bottom ash, being heavier, falls through
the bottom of the furnace where it is collected by a hopper, while fly ash, being very fine, is carried
through the furnace with the exhaust gases and is collected by ash precipitators (Huang, 1990).
Fig 1: Diagram of Pulverized Coal-Fueled Power. (Source:(Ramme & Tharaniyil, 2013))
Due to the negative climatic effects of relying on fossil fuels as the main energy source, the United
States has made efforts to shift to renewable energy sources. Despite these efforts, 61% of utilityscale electricity generation is still obtained from fossil fuels. As of 2020, coal was the third largest
energy source at about 19% (Energy Information Administration, n.d.). Indeed, at near130lion
tons, coal ash is one of the largest types of industrial waste generated in the U.S (American Coal
Ash Association, 2014). Efforts have been made to reuse fly ash in concrete products with positive
results. However, coal bottom ash has mostly been disposed of in landfills which has resulted in
growing environmental concerns due to the contamination of groundwater sources coupled with
the increasing lack of land space(Goh & You, 2008).
2.2
Properties of Coal Bottom Ash
It is important to establish the physical, chemical, and mechanical properties of CBA in order to
understand its interaction with the other ingredients that make up an HMA mix. This exercise
forms a basis for determining which ingredients can be partially replaced with CBA, and the
possible outcomes of doing so.
A. Physical Properties
Coal bottom ash is gray to black in color, angular in shape, and has a porous structure with a rough
surface texture (Huang, 1990). The level of angularity of an aggregate can indicate the rutting
performance of an asphalt mixture (Kim & Souza, 2009). Therefore, the angular shape of CBA
gives it a favorable rating in this regard.
The specific gravity of CBA is dependent on its chemical composition and the porosity of its
particles. Ashes containing higher amounts of iron will naturally have higher specific gravities.
Studies have shown that the specific gravities for dry bottom ash range from 2.0 to 2.6 with an
average of 2.35 (Huang, 1990). The specific gravity of a CBA can be used as an indicator of its
quality since a higher value would be an indication of a denser material as opposed to a lower
value which would indicate a porous material.
Aggregate gradation is a primary determinant of the stability and durability of HMA mixes. As
such, it is important to establish the particle size distribution of CBA in order to determine its
suitability as aggregate in HMA mixes. In general, CBA is classified as a well-graded material
with a gradation ranging from 1 inch (25.4mm) to the No. 200 sieve (0.075mm). With
approximately, 50 to 90 percent of the bottom ash passing the 4.75 mm sieve and 0 to 10 percent
passing the No. 200 sieve (Huang, 1990). Figure 2 illustrates the range of particle size distribution
of 6 bottom ashes found from different studies. It should be noted that it is possible to obtain
varying grain size distribution even from the same source.
Fig 2: Grain size distribution curves of several bottom ash samples. (Source: (Vasudevan, 2017))
B. Chemical Properties
The chemical composition of coal is dictated primarily by the source of the coal and not the type
of furnace. An investigation carried out on over 600 coal ashes showed that the three main
constituents of bottom ash are silica (SiO2), ferric oxide (Fe2O3), and alumina (Al2O3) at an average
composition of 45.7, 26.0, and 18.1 percent respectively (Huang, 1990). These three are the main
chemical components in calcined natural pozzolans and make CBA a suitable mineral filler when
pulverized (Gooi et al., 2020).
Heavy metal compounds present in the earth’s crust could also be found in coal. The amount
present in any coal is dependent on its source. Consequently, there is a growing concern about
disposing of CBA in landfills due to the potential of exposing groundwater reservoirs to these
heavy metal compounds. Incorporating CBA in HMA mixes has the potential of preventing
groundwater contamination with heavy metal compounds (Kadir et al., 2016).
C. Mechanical properties
Aggregates comprise 90-95% of asphalt mixes and have a direct impact on the strength,
workability, and durability of pavements. The mechanical properties of CBA are inferior to those
of traditional aggregates. However, for pavements that support low volume traffic, a blend of
traditional aggregates with CBA could result in a more economic HMA mix with acceptable
performance. Therefore, it is still important to determine the mechanical properties of CBA in
order to understand the performance of the HMA mix blended with CBA.
Table 1: Typical mechanical properties of bottom ash. (Source:(Huang, 1990))
Table 1 shows that coal bottom ash has a Los Angeles abrasion loss percentage ranging from 3050%, and a sodium sulfate soundness loss percentage ranging from 1.5-10%. This indicates that
CBA compares fairly to traditional aggregates in regards to their performance under the impact of
traffic loads and the weathering processes of the environment. Ramme and Tharaniyil (2004)
found comparable results in their study of the mechanical properties of bottom ash using the
AASHTO standards. They concluded that bottom ash needs to be blended with other aggregates
to meet the gradation requirements of most transportation agencies.
3.0
Evaluating the Performance of CBA Enhanced HMA mixes
Several studies have been carried out to investigate the effect of using CBA as a replacement for
coarse aggregate, fine aggregate, or mineral filler in HMA mixes. The primary objective of these
studies was to predict the performance of the CBA modified HMA mixes when subjected to
operational traffic and environmental conditions. In addition, the long-term environmental impact
of using CBA was also a key concern.
3.1
Case Study 1:
Yoo et al., 2016, investigated the effects of replacing fine aggregate with coal bottom ash in asphalt
mixtures. The CBA used as fine aggregates were screened to particle size passing sieve #4
(4.75mm). The CBA was used to replace the fine aggregates in ratios of 10%, 20%, and 30% by
weight. Figure 3 shows the aggregate gradation with increase in CBA content.
Fig 3. BA gradation and Aggregate gradation with the combination of 10%, 20%, and 30% BA.
Using the Marshal mix design method, sample mixtures were prepared at different asphalt contents
of 4.5%, 5.5%, and 6.0%, with three replicates per asphalt content. They found that at 4% air voids,
about 6-9% more asphalt binder was required for HMA mixes containing CBA. However, there
was no significant increase in optimum asphalt content with change in CBA within 10 and 30
percent (Table 2).
Table 2: Marshall test result at the optimum asphalt content.
In this study, moisture susceptibility testing was carried out using the indirect tensile (IDT) test.
All specimens were fabricated using the gyratory compactor at an air void percentage of 7 ±0.5%.
Six specimens were separated into two subsets, one for the dry IDT test and the other for the wet
IDT test. The tensile strength ratio (TSR) was determined between the dry and wet IDT test results.
Figure 4 shows that although the TSR values of all the mixes were below the minimum value of
80% (AASHTO T 283-03, 2003), the presence of CBA did not affect the moisture susceptibility
of the HMA mixes when compared to the control mix.
Fig 4. IDT test results.
Fatigue cracking of the asphalt mixtures was tested using the repeated indirect tensile (IDT) test
on the Universal Testing Machine. During this test, the fatigue life of the mix was quantified by
the total number of cycles at failure, which is taken to occur when there is a dramatic increase in
vertical deformation of the test sample due to the initiation of near-central vertical cracks that lead
to rupture. Figure 5 shows a comparison between a mix B with 10% bottom ash content and C
(controlled) mix. It can be observed that the fatigue life of mix B is slightly better than mix C.
Fig 5. Repeated IDT test for mixes B and C.
Leaching tests were performed on the CBA and asphalt cement (AC) using the Synthetic
Precipitation Leaching Procedure (SPLP) which showed that the toxicity range in the bottom ash
was within permissible levels as shown in table 4.
Table 4. Leaching test result.
3.1.1 Summary
Yoo et al., (2016) showed that CBA can be used to partially replace fine aggregate in HMA without
compromising the moisture resistance of the HMA. In addition, samples with certain amounts of
CBA showed higher resistance to fatigue cracking than the control mix. The leaching test carried
out also showed that using CBA in HMA mixes does not pose significant detrimental effects on
the environment. However, the researchers did not test the samples for rutting and stripping which
could give more information on the performance of these mixes when subjected to repeated heavy
loading or excessive moisture.
3.2
Case Study II:
Ksaibati and Conner (2004) investigated the effect of adding Wyoming CBA in asphalt mixes. The
intention was to determine if mixes prepared with bottom ash resulted in degradation in desirable
performance measures when compared to control mixes. Limestone and granite aggregates were
used in this study and were replaced with 15% coal bottom ash from four sources around Wyoming
(Table 5 and Table 6).
Table 5: Limestone Aggregate Gradations for Control and with 15% Bottom Ash
Table 6: Granite Aggregate Gradations for Control and with 15% Bottom Ash
Asphalt cement with a performance grade of PG 64-22 was used to prepare the laboratory mixes
using the Superpave mix design method. Mixes were then tested for tensile strength, rutting
potential, and low-temperature performance.
3.2.1 Summary
The CBA mixes containing CBA displayed the quality of maintaining desirable tensile strength
properties when compared to control mixes. In addition, limestone mixes with or without CBA
had similar rut depth resistance. Therefore, it was recommended that limestone aggregates should
be used for preparing mixes in Wyoming because they displayed superior performance than granite
aggregates, especially when lime was not used as an additive.
3.3
Case Study III:
Although there has been a significant amount of research conducted to evaluate the performance
and ability of bottom ash in replacing aggregates in asphalt mixtures, laboratory data is
significantly lacking in terms of mechanical properties. Goh & You (2008) sought to address this
gap by determining the dynamic modulus of an asphalt mixture containing an aggregate of bottom
ash. In this Michigan study, CBA with sieve size up to #4 was used to replace a portion of
aggregates in the asphalt mixes. Table 7 shows the gradation of the control mixture and bottom
ash. The blended mixture contained 9% of the bottom ash as mineral filler.
Table 7: Gradation of Control Mixture and Bottom Ash
3.3.1 Dynamic Modulus
The dynamic modulus (E*) test measures the response of a material to cyclic loading at different
frequencies in the undamaged state (Goh & You, 2008). Studies have shown that the E* obtained
from laboratory tests correlates well with in-situ permanent deformation and fatigue cracking
observed in field test sections (Pellinen, 2001). In this study, the E* was measured using the
Universal Testing Machine (UTM) 100 according to AASHTO TP62-03 specification. The tests
were conducted at different air void levels, which were 4% and 7%.
Figure 6 shows the comparison between the dynamic modulus for the control mixtures and ash
mixes at varying frequencies conducted at -5 0C and 21.3 0C. For both temperatures, the dynamic
modulus of the ash mixes was lower than that of the control mixes indicating that ash mixes were
less stiff due to the incorporation of CBA.
Fig 6. Comparison of Dynamic Modulus for Control mixtures and Ash Mixes
at -5°C and 21.3°C
Goh and You (2008) constructed a master curve in order to observe the entire results in a single
graph. As expected, figure 7 shows that the E* of the ash mixes is lower than that of the control
mixes. The master curve was used in the mechanistic-empirical design method to predict different
kinds of distresses such as rutting and fatigue cracking over time.
Fig 7. Comparison of dynamic modulus for control mixture and ash mix
using Master Curve
3.3.2 Summary
From this research the dynamic moduli of mixes containing CBA as mineral filler were obtained.
The values obtained indicated that mixes with CBA had lower stiffness when compared to the
control mix. The E* values can be used by engineers to carry out designs using the mechanisticempirical design methods thus showing the importance of determining the mechanical properties
of asphalt mixes.
4.0
Conclusion
This paper sought out to conduct a literature review to determine the suitability of using coal
bottom ash (CBA) in hot mix asphalt (HMA) mixes. From the information gathered the following
conclusions can be made:
1. Although CBA has inferior mechanical properties when compared to traditional
aggregates, it can be blended with traditional aggregates to create HMA mixes for
application in low-volume roads. However, it is important to carry out a thorough
investigation of the properties of CBA before using them because their properties have
been shown to vary even when they are obtained from the same source.
2. Partially replacing fine aggregates with CBA produced mixes that did not show significant
increase in moisture susceptibility. Therefore, CBA can be a suitable partial replacement
for traditional aggregates. However, testing should always be done before application in
HMA due to the inconsistent nature of CBA properties.
3. Leaching tests were performed on the CBA and asphalt cement (AC) which showed that
the toxicity range in the bottom ash was within permissible levels. However, toxicity tests
should be carried out to ascertain toxicity levels because the chemical composition of CBA
varies widely from one source to another.
4. In Wyoming, HMA mixes using limestone aggregates and CBA were found to be exhibit
no significant decrease in desirable performance when compared to control mixes.
Therefore, limestone aggregates should be selected when incorporating CBA into HMA.
This points to the importance of conducting material investigation to find out the most
suitable blending.
5. Pavement design is moving towards mechanistic-empirical design procedures. Therefore,
it is important to determine the mechanical properties of mixes which can be tied to
pavement performance. In the Michigan study, the dynamic moduli of ash mixes were
determined and a master curve was constructed which provided a powerful tool for
mechanistic-empirical design.
6. Implementation of the mechanistic-empirical method of pavement design requires the
determination of comprehensive material input information. Therefore, extensive
laboratory experiments need to be carried out on HMA containing CBA aggregates in order
to obtain these data. These will enable engineers to improve their prediction on the
performance of these mixes and hence provide suitable pavement designs
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