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Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2
3.1 Recycled Asphalt Pavement (RAP)
RAP (Figure 1) is produced by removing and reprocessing the
hot mix asphalt (HMA) layer of existing asphalt pavement
(Guthrie et al., 2007; FHWA, 2008). There is some ambiguity
regarding the nomenclature involved in the production of RAP.
Full depth reclamation (FDR) refers to the removal and reuse of
the HMA and the entire base course layer; and recycled
pavement material (RPM) refers to the removal and reuse of
either the HMA and part of the base course layer or the HMA,
the entire base course layer and part of the underlying subgrade
implying a mixture of pavement layer materials (Guthrie et al.
2007, Edil et al. 2012). Unless specified, these three distinct
recycled asphalt materials are collectively referred to as RAP.
RAP is typically produced through milling operations, which
involves the grinding and collection of the existing HMA, and
FDR and RPM are typically excavated using full-size
reclaimers or portable asphalt recycling machines (FHWA
2008, Guthrie et al. 2007). RAP can be stockpiled, but is most
frequently reused immediately after processing at the site.
Typical aggregate gradations of RAP are achieved through
pulverization of the material, which is typically performed with
a rubber-tired grinder.
RAP particles are coated with asphalt and its most value
added use is in production of hot mix asphalt (HMA) with the
benefit of reducing the fresh asphalt content. Seven RAP and 2
RPM samples collected from geographically diverse 7 states in
the U.S.A. indicated a range of 5-7% asphalt content. RAP and
RPM are widely used as unbound base material and the most
common test used for specification is Grain Size Analysis. The
most distinguishing physical characteristics are the grain size
with some samples coarser and others finer. D
50
of the 9
samples ranged 1.6 to 5.8 mm and the fines content was less
than 2%. These materials all classified as A-1-a or A-1-b
according to the AASHTO soil classification system. These
samples had an average impurity (geotextiles, pavement
markings, etc.) content of 0.2% for RAP, indicating that
recycling industry has developed sufficient controls.
The compaction characteristics using the modified Proctor
test indicated that the maximum dry unit weight (MDU) varies
within a narrow range (19.4 to 21.5 kN/m
3
) for RAP and the
optimum moisture contents (OMC) (5.2 to 8.8%). OMC
correlates significantly with the uniformity coefficient and
percent moisture absorption and MDU correlates with OMC for
RAP (Bozyurt et al. 2012).
Summary resilient modulus (SRM calculated at a bulk stress
of 208 kPA, typical of base course layer) of the 9 RAP and
RPM samples measured at OMC and 95% modified Proctor
MDU, indicated that RAP/RPM has higher SRM (168 to 266
MPA) than natural crushed aggregate (152 MPa) and is
significantly correlated with grain size characteristics (percent
fines, D
60
), asphalt content, specific gravity, and percent
absorption (Bozyurt et al. 2012).
Application of freeze-thaw cycles indicated that SRM
decreased in a range of 28 to 53% up to 20 cycles. However,
RAP still had a higher stiffness than natural crushed rock
aggregate regardless of the number of freeze-thaw cycles (Edil
et al. 2012). Because of its asphalt content RAP can be expected
to be sensitive to temperature changes. A decrease of
approximately 30% in SRM was observed in RAP between the
23 and 35 °C. These temperature effects were absent in control
specimens containing no asphalt. Micro-Deval and particle size
distribution tests conducted on RAP after 5, 10, and 30 wet/dry
cycles showed no apparent particle degradation (Edil et al.
2012).
RAP has excellent drainage capacity due to the
hydrophobic nature of the asphalt coating and does not retain
moisture (Nokkaew et al. 2012). Field leachate samples
collected indicated that the concentrations of As, Se and Sb for
RAP were slightly higher than the corresponding USEPA
groundwater maximum contaiminant level (MCL) but
decreased rapidly after the first flush (Edil et al. 2012). Falling
Weight Deflectometer (FWD) tests were conducted at a test
facility (MNROAD) on pavement with base course material of
RAP indicated relatively small variation in stiffness and
resilient modulus seasonally and indicated no deterioration over
4 years.
The investigations undertaken on RAP 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 differences of RAP from natural aggregate such as
temperature sensitivity, plastic deformations, and water
absorption and retention characteristics are also well
established. To determine the various properties of RAP (e.g.,
compositional characteristics, grain size distribution,
compaction, resilient modulus), existing standard test methods
employed for natural crushed aggregate can be used with added
consideration for temperature control. There are no established
standards for freeze-thaw and wet-dry cycling but published
research methods can be adopted (Edil et al. 2012).
3.2 Recycled Concrete Aggregate (RCA)
The production of RCA (Figure 1) involves crushing structural
or pavement concrete to a predetermined gradation. Fresh RCA
typically contains a high amount of debris and reinforcing steel,
and it must be processed to remove this debris prior to reuse
(FHWA 2008). One of the value
-
added applications is use of
RCA as a base course material although it can be used in
constructing working platforms over soft subgrade and drainage
medium. Depending on the crushing methods, the particle size
distribution of an RCA can have a wide variability; with a lower
particle density and greater angularity than would normally be
found in more traditional virgin base course aggregates.
Residual mortar and cement paste are typically found on the
surface of the RCA, as well as contaminants associated with
construction and demolition debris. The self
-
cementing
capabilities of RCA are an interesting secondary property. The
crushed material exposes un
-
hydrated concrete that can react
with water, potentially increasing the materials strength and
durability when used as unbound base course for new roadway
construction. It follows that service life could also be extended
as a result of these properties.
Seven RCA samples collected from geographically diverse 7
states in the U.S.A. indicated a range of 5-6.5% mortar content.
The most distinguishing physical characteristics are the grain
size with some samples coarser and others finer. D
50
of the 7
samples ranged 1 to 13.3 mm and the fines content was less
than 3-4% except two samples and higher than for RAPs. The
mortar content was about 50% with small variation for these
RCA samples. These materials classified mostly as A-1-a with
some as A-1-b according to the AASHTO soil classification
system. These samples had an average impurity (geotextiles,
pavement markings, etc.) content of 1% for RCA, indicating
that recycling industry has developed sufficient controls. The
most predominant impurities for RCA were asphalt aggregate,
aggregate with plastic fibers, brick, and wood chips. RCA
derived from structures tend to have brick content. The effect
of brick content up to 30% indicated no adverse effect on
resilient modulus of RCA (Shedivy 2012).
The compaction characteristics using the modified Proctor
test indicated that the maximum dry unit weight (MDU) varies
within a narrow range (19.4 to 20.9 kN/m
3
) for RCA and
optimum moisture contents (OMC) (8.7 to 11.8%). OMC is
greater than RAP’s due the higher absorption capacity due to
the porous nature of the mortar portion of RCA. OMC of RCA
correlates significantly with the uniformity coefficient and
