3197
Technical Committee 307 /
Comité technique 307
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
3
percent moisture absorption and MDU correlates with OMC for
RCA (Bozyurt et al. 2012)
RAP RCA
Figure 1. Recycled asphalt pavement (RAP) and recycled concrete
aggregate (RCA)
Summary resilient modulus (SRM calculated at a bulk stress
of 208 kPA, typical of base course layer) of the 7 RCA samples
measured at OMC and 95% modified Proctor MDU, indicated
that RCA has higher SRM (163 to 208 MPA) than natural
crushed aggregate (152 MPa) and lower than RAP/RPM. SRM
is significantly correlated with D
30
and OMC (Bozyurt et al.
2012).
Application of freeze
-
thaw cycles indicated that SRM
decreased 10
-
18% during the first five freeze
-
thaw cycles, but
then an increased 30
-
38% above the initial SRM after 20 freeze
-
thaw cycles. The self
-
cementing properties of RCA and fines
content generation over time could explain why an increase in
stiffness after five freeze
-
thaw cycles occurred. (Bozyurt et al.
2011). Micro
-
Deval and particle size distribution tests were
conducted on RCA after 5, 10, and 30 wet/dry cycles and no
apparent trend was found between particle degradation and
wet/dry cycling of the material
RCA has high drainage capacity but retains moisture more
than RAP and natural aggregate base because of its hydrophilic
cement mortar (Nokkaew et al. 2012). Laboratory batch and
column leach tests and field leachate samples collected
indicated that RCA base course has high alkalinity (pH = 10.8
to 12.9). As, Cr, Pb, and Se exceeded the maximum
contaminant levels (MCLs) for the USEPA drinking water
standard both at the field sites and in the laboratory column
leaching tests. The concentrations of As, Pb, and Se for RCA
exceeded the corresponding MCL only once or twice and the
leaching behaviors were similar to that of the control natural
crushed aggregate base course. As and Cr appear to be mainly
sourced from the cement mortar based on the acid digestion
results (Edil et al. 2012). Falling Weight Deflectometer (FWD)
tests that were conducted at the MnRoad test facility on
pavement with base course material of RCA indicated relatively
small seasonal variation in modulus and no deterioration over 4
years.
The investigation undertaken on RCA indicate that it is a
suitable material for unbound base course applications and
shows equal or superior performance characteristics compared
to natural aggregates in terms of stiffness, freeze
-
thaw and wet
-
dry durability, and toughness. Their compositional and
mechanical properties vary in relatively small range. The
relative difference of RCA from RAP and natural aggregate is
its water absorption and retention characteristics. RCA displays
high alkalinity thus oxyanions (As, Se, and Cr) should be given
more attention to as they demonstrate enhanced leaching in a
highly alkaline environment.
To determine the various properties of RCA (e.g.,
compositional characteristics, grain size distribution,
compaction, resilient modulus), existing standard test methods
employed for natural crushed aggregate can be used. There are
no established standards for freeze
-
thaw and wet
-
dry cycling
but published research methods can be adopted (Edil et al.
2012).
3.3 Coal Combustion Products (CCP)
CCPs of interest to highway construction include fly ash and
bottom ash. When pulverized coal is burned in a dry bottom
boiler, about 80 percent of the unburned material or ash is
entrained in the flue gas and is captured and recovered as fly
ash (Figure 2). The remaining 20 percent of the unburned
material is dry bottom ash, a porous, glassy, dark gray material
with a grain size similar to that of sand or gravelly sand (Figure
3).
Although similar to natural fine aggregate, bottom ash is
lighter and more brittle and has a greater resemblance to cement
clinker. Beneficial use of bottom ash in highway applications,
which is less than 50% of the material produced in the U.S.A.,
include structural fill (nearly half of all use), road base material,
working platform material for construction of pavements over
soft subgrade, fine aggregate in wearing surface in pavements
and flowable fills, and as snow and ice control products. Bottom
ash is predominantly well
-
graded sand
-
sized, usually with 50 to
90 percent passing a 4.75 mm (No. 4) sieve and 0 to 10 percent
passing a 0.075 mm (No. 200) sieve (
).
Bottom ash has MDU of 11.8 to 15.7 kN/m
3
and OMC of
12
-
24%. Its internal friction angle varies from 32
to 45
.
California bearing ratio (CBR) is typically 20 to 40. Summary
resilient modulus (SRM calculated at a bulk stress of 208 kPA,
typical of base course layer) for properly compacted bottom ash
can be taken as 100 MPa. Bottom ash has similar drainage
characteristic as sand with a hydraulic conductivity of 1x10
-
2
mm/s. All standard tests used to characterize natural granular
materials like sand can be directly used for bottom ash.
The fly ash is a fine
-
grained, powdery particulate material
that is carried off in the flue gas and usually collected by means
of electrostatic precipitators, baghouses, or mechanical
collection devices such as cyclones. Beneficial use of fly ash in
highway applications, which is less than 50% of the material
produced in the U.S.A., include cement replacement /additive in
concrete (nearly half of all use), structural fill, stabilization
agent for road subgrade and base material, working platform
material for construction of pavements over soft subgrade, flow
agent in flowable fills, and mineral filler in asphalt layers
). Self
-
cementing coal fly ashes are
suitable materials for the stabilization of subgrade soils,
recycled pavement materials, and road surface gravel. Fly ash
stabilization can result in improved properties, including
increased stiffness, strength and freeze
-
thaw durability; reduced
hydraulic conductivity, plasticity, and swelling; and increased
control of soil compressibility and moisture. Fly ash stabilized
materials may be used in roadway construction, such as
working platforms during construction, stabilized subgrade,
subbase, and base layers (Edil et al. 2006). Recently published
ASTM standard practice provides guidance for testing and
designing
of stabilization of soil and soil
-
like materials with
self
-
cementing fly
(ASTM D7762 2011).
The possibility of groundwater contamination by trace
elements that are commonly associated with coal combustion
by
-
products is a concern. Areas with sandy soils possessing
high hydraulic conductivities and areas near shallow
groundwater should be given careful consideration especially
Figure 2. Fly Ash (different colors)
