MIL-HDBK-1110
cylinder containing 50 ml of xylene, and the resultant increase
in volume is noted. The specific gravity is then determined by
simple division:
Specific Gravity
=
Weight of Sample (about 60 g)
New Total Volume - 50 ml
g) Procedure 7: Index of Refraction of Beads. The
index of refraction of glass beads can be determined by immersing
them in standard liquids with different refractive indexes and
observing whether the beads blend into the liquid. Blending
occurs when the liquid has a higher refractive index than do the
beads. Indentations of a ceramic spot plate can be conveniently
used for holding the beads and liquid. Run this test when
substitution of TT-B-1325, Type I beads for Type III beads is
suspected. This is normally suspected from a low specific
gravity value in Procedure 6.
h) Procedure 8: Preparation of Drawdowns for
Determining Retroreflectivity. Drawdown specimens are prepared
using a metal drawdown bar as described in ASTM D 823, Producing
Films of Uniform Thickness of Paint, Varnish, and Related
Products on Test Panels. A paint film of 16 mil or other wet
film thickness is screeded onto a the white surface of a black
and white chart by drawing it across the paper in front of a bar
of proper clearance. Immediately after this action, beads are
manually sprinkled into the wet paint. After drying, the
drawdowns are measured with a retroreflectometer. The instrument
can be held in either direction, since the application procedure
does not have a directional effect.
i) Procedure 9: Surface Preparation. Inspection of
pavement surfaces prepared for marking with paint is basically
determining whether the surface is clean enough and sound enough
to permit tight bonding of the paint. Cleaning of concrete for
painting is described in ASTM D 4258, Surface Cleaning Concrete
for Coating. Of the several procedures described for cleaning,
high-pressure water blasting with truck-mounted equipment is
almost always the procedure selected for rubber and paint
removal. Washing with an aqueous detergent solution may be
necessary to remove oil, grease, and tightly bonded dirt. The
extent of paint removal should be verified visually. If only
loose paint is to be removed prior to restriping, the remaining
paint can be checked with a dull putty knife to determine whether
only sound paint remains. After a build-up of five coats (a dry
film thickness of 37 mils), the film becomes rather inflexible
and subject to cracking, and the skid resistance significantly
reduced. Thus, complete removal of the old marking is
recommended at this time. From a standpoint of eliminating the
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