University of Oklahoma, School of Civil Engineering and Environmental Science
CHAPTER IX
CONCLUSIONS AND RECOMMENDATIONS
9.1 CONCLUSIONS
A systemic study was conducted to examine the percentage of raw chat that can be used in
the design of hot mix asphalt for surface course and base course applications. The Superpave
mix design methodology was employed to prepare the mix designs. A total of six mix
designs were pursued with different percentages of raw chat: three for surface mixes and
three for base mixes. For the surface mixes, 40%, 60%, and 80% of raw chat was combined
with varying percentages of locally available non-chat aggregates in three mix designs,
SM40, SM60, and SM80. Similarly, for the base mixes, 40%, 50%, and 70% of raw chat
was combined with varying percentages of locally available non-chat aggregates. These mix
designs are referred to as BM40, BM50, and BM70. From the test results and their analyses
presented in Chapters VI through VIII, the following conclusions can be drawn:
(1) The use of raw chat in HMA showed a great potential for both surface course and
base course applications by meeting the Superpave mix design requirements. It is promising
to use raw chat in hot mix asphalt, as far as the engineering properties are concerned.
(2) All three surface mix designs (SM40, SM60, and SM80) satisfied the Superpave
volumetric requirements. These designs also met the performance tests requirements for a
surface mix, as specified by ODOT in its standards and specification.
(3) The optimum asphalt content (OAC) for SM40, SM60, and SM80 were found to
be 6.1%, 6.8%, and 7.0%, respectively. These asphalt contents are higher than what is
typically used by ODOT (4.7% to 5.4%) for a surface course. The increase in OAC with an
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University of Oklahoma, School of Civil Engineering and Environmental Science
increase in the amount of chat is due to the increase in the amount of finer particles. As a
result, more asphalt binder is required to coat these surfaces.
(4) Of the three base mix designs (BM40, BM50, and BM70), all three passed the
Superpave volumetric requirements.
(5) The OACs for BM40, BM50, and BM70 were 5.0%, 5.4%, and 6.8%,
respectively. Comparatively, a typical range of asphalt content for base mixes used by the
ODOT is 4.8% to 5.2%. The increase in OAC with an increase in the amount of chat is due
to the increase in the amount of finer particles as explained in case of surface mixes.
(6) Of the three base mixes BM70 failed to meet the ODOT rut criteria. Therefore,
this mix was not considered for any further performance tests.
(7) Twelve of the 20 samples collected from the Kenoyer North chat pile and
analyzed for total metals had Pb concentrations above the EPA action level. Additionally,
the TCLP regulatory levels for Pb are exceeded in the majority of bulk chat samples and the
TCLP regulatory levels for Cd are exceeded in 2 of the samples (KN-12 and KN-13). Since
Kenoyer North chat exceeds the EPA action level for Pb and the TCLP regulatory limits for
Pb and Cd, this indicates the need for providing a safe use for chat, such as encapsulating the
raw chat in HMA.
(8) Of the non-composited, size-fractionated raw chat, 7 of the 12 samples exceeded
both the Pb EPA soil action limit (greater than 400 mg/Kg) and the TCLP regulatory limit
(greater than 5 mg/L). Two of the 12 samples exceeded the Cd TCLP regulatory limit
(greater than 1 mg/L). The data obtained from the three analyses (total metals, TCLP, and
SPLP) of the non-composite, size-fractionated chat from KN-6 and KN-11 are in consensus
that the concentrations of Pb, Zn, and Cd in chat increase as the particle size decreases
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University of Oklahoma, School of Civil Engineering and Environmental Science
(becomes finer). In other words, the greatest concentrations of Pb, Zn, and Cd are in the
fines, which pose the greatest risk to human health and the environment. The chat fines pose
the greatest health and environmental risk because they are most readily available for
transport through the environment (by wind and water) and most easily up-taken through
ingestion.
(9) When total metals, TCLP, and SPLP for Pb in the composite, size-fractionated
chat, were plotted against each other, total metals consistently yield the highest average Pb
concentration. TCLP consistently yields the second greatest Pb concentrations, and SPLP
consistently yields the lowest Pb concentration. All composite, size-fractionated samples
have exceeded the TCLP regulatory limit for Pb except for sample Comp-4. Comp-200 and
Comp-PAN, which are the finer composited, size-fractionated chat, exceeded the TCLP
regulatory limit for Cd. Four out of the six composited, size-fractionated chat samples (the
smaller U.S. Standard sieve sizes, specifically, Comp-40, Comp-80, Comp-200, and CompPAN) all exceeded the EPA action limit for Pb.
(10) The size-fractionated non-chat aggregate (limestone and sand), used in the HMA
and obtained from the “APAC-Oklahoma” quarry, was analyzed for total metals, TCLP, and
SPLP of Pb, Zn, and Cd. It was determined that the concentrations of Pb are well below the
EPA action limit in the non-chat aggregates. Also, the concentrations of Pb and Cd are well
below the TCLP regulatory limits. It can be deduced that Pb and Cd contamination due to
the non-chat aggregate in the HMA is insignificant.
(11) The optimal HMA designs based on the engineering tests are BM50 and SM80.
Unweathered BM50 and unweathered SM80 had Pb and Cd concentrations below their
TCLP regulatory levels.
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(12) As long as the HMA samples remain unweathered, the asphalt binder
encapsulates the Pb and Zn, preventing then from leaching. Also, the asphalt binder does not
significantly contribute to Pb, Zn, or Cd contamination of the HMA samples.
(13) Even though the HMA samples were weathered (by milling), none of the milled
HMA has Pb concentrations exceeding the EPA action limit (400 mg/Kg). Additionally,
none of the milled HMA designs have concentrations of Pb or Cd above the TCLP regulatory
limits.
This bodes well for using BM50 and SM80, as both an engineering and
environmentally optimal HMA, for paving roads at the Tar Creek Superfund Site.
(14) None of the dry rutted or wet rutted HMA samples exceeded the EPA Pb action
limit (as determined by total metals analyses). It looks favorable for using the optimal HMA
designs (BM50 and SM80) in safely paving roads at the Tar Creek Superfund Site.
9.2 RECOMMENDATIONS
(1) This study used only two selected binders in all the mix designs, one for the
surface mixes (PG 70-28) and the other for the base mixes (PG 64-22). Binder properties can
vary significantly depending upon source and performance grade. Future studies should
consider other binders in developing mix designs with wider applications (climate, roadway
type).
(2) This study used a selected non-chat aggregate (limestone from Vinita) in all the
mix designs. Since a mix design is sensitive to aggregate types, it is recommended that other
non-chat aggregates that are commonly used in Oklahoma and neighboring states be
considered in future studies.
(3) Although bench scale laboratory studies are useful, field studies are essential for
real life implementations of the mix designs developed from laboratory studies. It is thus
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recommended that some field studies be conducted to demonstrate the responsible use of
chat-asphalt for paving.
(4) Raw chat is an excellent source of aggregate base using cementitious stabilization
such as fly ash and cement kiln dust (CKD). Preliminary studies have shown that fly ash or
CKD-treated chat base can be constructed economically for roadway pavements. Field
studies are needed to demonstrate the feasibility of using these materials.
(5) Environmentally, there are recommendations for future research, such as the
following: (i) construction of a test road using BM50 and SM80; (ii) soil sampling, over
time, along the aforementioned test road; (iii) air quality sampling, over time, along the
aforementioned test road, and (iv) milling of the aforementioned test road (one year after the
initial construction), and (v) soil and air quality sampling during and following milling of the
test road.
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