15 May 2001
(1) Mix characteristics. Trial mixes are used to determine if emulsion and aggregate are
compatible, if a mineral filler or field control additive is needed, and if used, the concentration, and the
range of water that produces homogeneous mixtures. Trial mixes are also prepared to determine the
optimum filler content and the effects of mineral filler on wet cohesion. These mixes are prepared with
constant asphalt emulsion contents and incremental changes in the amount of mineral filler, usually
either hydrated lime or portland cement. Once the desirable mineral filler content has been determined,
trial mixes are again prepared, this time holding the mineral filler content constant and making
incremental changes in asphalt emulsion content.
(a) Cohesion test (ISSA TB-139). The cohesion test is the method used to classify the set
and traffic time of micro-surfacing systems. The cohesion tester is a power steering simulator that
measures the torque required to tear apart a 6 or 8 millimeter thick x 60 millimeter in diameter specimen
under the action of a 32 millimeter diameter rubber foot loaded to 200 kPa. A system is defined as
"quick-set" if it develops a torque value of 12-13 kg-cm within 20 to 30 minutes. A torque of 12-13 kg-cm
is considered the cohesion value at which the mixture is set, water resistant and cannot be remixed. A
system is defined as "quick traffic" if the mixture develops 20-21 kg-cm torque within 60 minutes. At
20-21 kg-cm, sufficient cohesion has developed to allow rolling traffic. Figure 4-1 provides a method to
classify various slurry seals and micro-surfacing systems. All micro-surfacing mixtures are designed as
quick set, quick traffic systems. Cohesion test results can also be used to optimize mineral filler by the
use of the "Benedict Curve" (see figure 4-2), in which the effect of an incremental addition of mineral
filler versus cohesion is plotted. The optimum filler content is the value that gives the highest cohesion
value. The shape of the curve will show the sensitivity of the system to changes in mineral filler. This
will help in determining the range of mineral filler that will give acceptable laboratory results.
(b) Stripping. Two tests can be used to evaluate the potential for stripping: Wet Stripping
Test (ISSA TM 114) and the Boiling Test (ISSA TM 149). The Wet Stripping Test is performed on 60EC
(140EF) cured cohesion specimens that are boiled in water for 3 minutes to determine the asphalt
adhesion to the aggregate. A coating retention of 90 percent or greater is considered satisfactory, with
75 to 90 percent being marginal and less than 75 percent unsatisfactory. The Boiling Test is similar to
the Wet Stripping Test. Both tests can also be used as an early compatibility indicator test.
(2) Determination of optimum asphalt content. There are several procedures used to
determine the optimum asphalt cement content. One way is to use ISSA test procedures, and another is
to use a modified Marshall procedure. A few states also specify requirements for Hveem stability.
(a) ISSA procedure. The optimum asphalt content is determined from ISSA procedures
by graphically combining the results of a wet track abrasion test (WTAT) and a loaded wheel test (LWT).
Figure 4-3 (a, b, and c) shows how the optimum asphalt content along with an acceptable range can be
determined by graphically combining WTAT and LWT data. The minimum and maximum asphalt
content should fall within the range usually provided in the specification. The ISSA recommends that
residual asphalt content be within a range of 5.5 to 9.5 percent.
 Wet track abrasion test (ASTM D 3910/ISSA TB 100). This test determines the
abrasion resistance of micro-surfacing mixture relative to asphalt content and is one of two ISSA tests
used for determining optimum asphalt content. This test simulates wet abrasive conditions such as
vehicle cornering and braking. A prepared and cured sample of mixture 6 millimeters thick x
280 millimeters in diameter that has been soaked for periods of either 1 hour or 6 days is immersed in a
25EC (77EF) water pan and is wet abraded by a rotating weighted (2.3 kilogram) rubber hose for
5 minutes. The abraded specimen is dried to 60EC (140EF) and weighed. Maximum allowed weight
losses for 1-hour and 6-day soaks are 0.54 kilogram/meter2 and 0.8 kilogram/meter2, respectively.
Asphalt contents that result in these weight losses are considered the minimum asphalt contents. The
WTAT on a 6-day soaked sample is generally not required. However, due to the increased severity of