Specification Part 3 - Road & Bridge Works - Urban Administration ...

affinity for water than for bitumen, and when a coated stone comes into contact with water there
is tendency for the binder to be displaced from the surface of the stone. In a road mixture
containing fines susceptible to expansions when wet stripping is accompanied by swelling of the
road mass.
The adhesion of both bitumen is weakened by the presence of water and there is a
tendency for the binder to be displaced or stripper from the stone. The higher the viscosity to the
binder the grater the resistance to this effect.
The binder in a freshly laid surface dressing or surfacing is particularly sensitive to the
displacing action of water if prolonged rain falls before the binder has reached a sufficiently high
viscosity the adhesive joint may be broken and disintegration set in . Although failures of this type
are not frequent but when they do occur there is no remedial measure a new surfacing has to be
laid.
The properties of the aggregate, especially its surface texture, have an important bearing
on adhesion. Smooth a glassy surface and both fine textured and coarsely crystalline, generally
give poor adhesion. The rough textured stones generally give the best adhesion. There is an
indication that low silica content rocks have better resistance to stripping than high silica content
rocks of similar texture. Limestones have good adhesive properties except when crystalline.
Properties of hot application bitumen’s
30/40 Medium hard grade. Standard grade for grouting in uniform size aggregate work pre
mixing surface painting where a heavy application is considered necessary.
60/70 Sometime used in lieu of 80/100 penetration. in very hot localities where a slightly harder
grade is required.
80/100 Standard grade for surface painting per mixing grouting in graded aggregate dense
surface and seal coats.
180/20 Very soft grade for light surface dressing or in very cold climates pre mixed carpets.
(c) Penetration: - is a measure of hardness or consistency and therefore of primary importance
as an indication of suitability for any particular construction or condition. This test is probably the
most important of all tests on bitumen .penetration is measured by a standard penetrometer.
Penetration is the distance measured in unit of 1/10 mm that a standard blunt point needle will
penetration a sample at 25 degree centigrade. When the needle is loaded with 100 grams applied
for 5 seconds.
The higher the penetration numbers the softer the bitumen. The lower the penetration the
higher will be the melting point and lower the ductility in other word, the harder the bitumen the
higher will be the melting point but the ductility will be less for the bitumen becomes more brittle
the harder it is. The harder the bitumen the quicker it sets soft bitumen takes comparatively a
longer time to set.
Normally bitumen with greater penetration is used for surface painting and semi grouting
work and bitumen with lesser penetration are used for full grouting work.
Bitumen cut back Bitumen:-
Cutback are softened bitumen (solid thinned with a volatile distillate such as petrol, kerosene,
diesel oil or tar oil), which can be used without heating or require only light heating. Bitumen
cutbacks are classified into three types:
Rapid curing (RC) - Light type for use as primer.
Medium curing (MC)- heavy type for surface dressing.
Slow curing (SC) - heavy type for pre mixing
Each these three types of cutback are produced in six grades, numbered from 0 to 5 (designated
as R-0, RC-3, MC-3, or SC-5, etc.) according to their viscosities or fluidity the percentage of
bitumen increasing as the number rise. For example SC-0 contains about 40 percent bitumen
whilst CS-5 contains about 80 percent bitumen. (RC-1, MC-, SC-1 are all of the same viscosity).