UFC 3-250-03
15 May 2001
the 6-days soak, it is preferred by some laboratories and user agencies for predicting the performance of
the system.
[2] The loaded wheel test (ISSA TM 109). This test is used to determine the
maximum asphalt content to avoid asphalt flushing in micro-surfacing systems. This is accomplished by
specifying and measuring fine sand that adheres to the sample subjected to simulated wheel loadings.
The ISSA recommends a maximum sand adhesion value of 0.54 kilogram/meter2 for heavy traffic
loadings. If the sand adhesion is below this maximum value mixture bleeding should not occur. In this
test a 50 millimeter wide x 375 millimeter long specimen of desired thickness (generally 25 percent
thicker than the coarsest particle) is fastened to the mounting plate and is compacted with 1000,
57 kilogram cycles at 25EC (77EF). At the end of compaction the specimen is washed, dried at 60EC
(140EF) to a constant weight. A measured quantity of sand is then placed on the sample, and the
loaded wheel test is repeated for a specified (usually 100) number of cycles. The specimen is then
removed and weighed. The increase in weight due to sand adhesion is noted.
(b) Marshall procedure (modified CRD-C 649 or ASTM D1559). The Marshall hot mix
asphalt mixture criteria can be used to determine the optimum asphalt content. Since these are cold
polymer-modified emulsion systems, the stability and flow test procedures have been modified to allow
for air and low temperature drying (at least 3 days of air curing, 18-20 hours of drying in an oven at 60EC
(140EF) before compaction at 135EC (275EF)). The mixes are usually compacted with 50 blows per
side. Under this procedure several test specimens are prepared for combinations of aggregate and
asphalt content. The asphalt contents are selected to provide voids in total mix (VTM) of about 4.5 to
5.5 percent. The compacted test specimens are tested for the bulk specific gravity (ASTM D 2726),
stability, and flow values. Finally, the optimum asphalt cement content is determined using results from
these tests. For thin micro-surfacing applications, the stability is not considered a primary factor in
determining the optimum asphalt cement content. The surface characteristics of some aggregates may
require adjustments in the VTM requirement to achieve the desired flow values.
(c) Design limitations.
[1] ISSA design. Torque values are measured in the laboratory under specific
conditions (there has been no correlation established with pavement performance in the field). The
mixing and wet cohesion test should be performed at various moisture contents, relative humidities, and
temperatures to simulate the expected field conditions. In addition, it has been reported that some
aggregates that met ISSA torque standards for 60 minutes have failed to meet the torque values for
30 minutes. Some laboratories also use a subjective analysis to determine torque. The sample is
examined after the torque is applied, and should it fail, the torque value is determined from a visual
examination of the condition of the sample. However, this analyses would appear to negate the
objectivity of the cohesion test. This indicates an area where the industry should reexamine their
procedures for cohesion test and consider the effect of various aggregates on test results. WTAT was
correlated to field performance for only 6 millimeter thickness and 0/4 gradations. Accordingly, values of
tests are needed to verify or establish new values. Also, some limestones meet the WTAT standard for
1-hour soak periods but fail to meet maximum abrasion loss when a sample with a 6-day soak is tested.
While WTAT on a 6-day soak specimen is generally used for information only, the industry may wish to
review and adjust their current design standards. The reproducibility of the loaded wheel test is
questionable. The arm that moves the wheel does not stay horizontal, but rather moves up and down
during the test. This changes the pressure on the sample. The arm should be modified to stay
horizontal. At the present time, the weights used to apply pressure are bags of lead shot. These bags
may shift during the test and can affect the applied pressure. The bags should be replaced by plates
that can be attached to the machine. Sample preparation has been shown to affect the LWT results by
a factor of as much as two. The test specimen can flush if water levels are not carefully controlled. This
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