2. INTRODUCTION
• The base course is immediately beneath the surface course.
• It provides additional load distribution and contributes to
drainage and frost resistance. Base courses are usually
constructed out of:
– Aggregate. Base courses are most typically constructed from
durable aggregates that will not be damaged by moisture or frost
action. Aggregates can be either stabilized or unstabilized.
– HMA. In certain situations where high base stiffness is desired,
base courses can be constructed using a variety of HMA mixes. In
relation to surface course HMA mixes, base course mixes usually
contain larger maximum aggregate sizes, are more open graded
and are subject to more lenient specifications
Eng. Suneth Thushara Salawavidana 2
3. General Requirements for the Aggregate for the Construction of
Bases
• The coarse aggregate (aggregate substantially
retained on 4.75mm sieve) used for road bases and
surfacing must be crushed rock from an approved
quarry.
• It must consist of clean hard sound durable
particles of angular shape and rough surface
texture.
• They must be free from weathered, soft, laminated
or elongated pieces, deleterious matter. It must
also be free from clay and excess dust.
Eng. Suneth Thushara Salawavidana 3
4. General Requirements for the Aggregate for the Construction of
Bases
• The coarse aggregate must also conform to the
following requirements.
– The Aggregate Impact Value (AIV) must not be
greater than 30%.
– The Flakiness Index must not be greater than
35%.
• The lower layers of the base and for bases of roads
carrying low traffic (total of less 300 vehicles per
day), these requirements may be relaxed at the
discretion of the Engineer.
Eng. Suneth Thushara Salawavidana 4
5. General Requirements for the Aggregate for the Construction of
Bases
• However, aggregates of AIV greater than 35%
must normally not be used in road bases.
• Fine aggregates (aggregates substantially passing
the 4.75mm sieve) used for road bases and
surfaces must either be crusher fines or river
sand.
• Where crusher fines are used, they must be
derived from rock meeting the requirements of
coarse aggregate for road bases and surfacing.
Eng. Suneth Thushara Salawavidana 5
6. SINGLE SIZED AGGREGATE BASES
• The aggregates used must be of nominal sizes 50mm or
37.5mm and 19mm or 12.5mm respectively and nominal
single size 100mm.
• All must conform to the requirements given above. The size
requirements of 100mm aggregate must be such that the
maximum dimensions of the pieces of aggregate must not
exceed 150mm and their minimum dimensions must not
be less than 75mm.
• In addition, the aggregates must be well graded between
these two limiting dimensions.
• The grading requirements of the other aggregates must be
according to the following table.
Eng. Suneth Thushara Salawavidana 6
8. Thickness of base course
• The nominal compacted thickness of the base course
must be 75mm or 150mm depending on whether only
two spreads of 50mm aggregate or whether one spread
of 100mm aggregate and two spreads of 50mm
aggregate are used in the construction respectively.
• Where 37.5mm aggregate is used, instead of the 50mm
aggregate, the nominal compacted thickness must be
reduced to 55mm and 130mm respectively.
• For higher thicknesses of bases, the composition of the
aggregate courses must be specified.
Eng. Suneth Thushara Salawavidana 8
9. Preparation of existing surface
• Where the base course is laid over a prepared subgrade or
subbase, the surface must be brushed free from dust and
extraneous matter and if required wetted before laying the
base course.
• Where the base course is laid over an existing gravel road any
unevenness of the surface and places of failure must be
corrected and surface must be well prepared and wetted as
above, prior to laying operations.
• Where the base course is laid over an existing bituminous
pavement, the surface must be scarified lightly and cleaned
of any scarified material or in the alternative, a tack coat
must be applied prior to laying operations.
Eng. Suneth Thushara Salawavidana 9
10. Base Construction
• Before spreading the aggregate base,
construct a partial width of the shoulder
(not less than 0.6m wide) to the elevation
of the top of each compacted layer.
• Trim the inside edge to near vertical face.
Eng. Suneth Thushara Salawavidana 10
11. With 50mm and 37.5mm aggregate
• Spread aggregate evenly on the prepared
surface at the specified rate to form a
uniform layer of two stones thick.
• Eg: 75mm (3”) nominal thickness 50mm
aggregate 8.5-10m3/100m2
Eng. Suneth Thushara Salawavidana 11
12. With 100mm aggregate
• Spread aggregate (hand pack where
necessary) to one stone thickness at the
given rate.
• Use a 10 tonne roller to achieve sufficient
embedment (avoid excessive rounding off).
• Fill surface voids with crusher dust if
directed.
• Then spread 50mm or 37.5mm aggregates to
two stones thickness.
Eng. Suneth Thushara Salawavidana 12
13. Compaction
• Compact using 8-10 tonne roller. Rolling should be
done by a series of overlapping longitudinal passes.
• Start from the edge and proceed towards the centre.
• While rolling, add or remove aggregates where
necessary.
• Continue rolling till the aggregate does not shift or
wave in front of the roller. (Avoid over rolling).
• The surface can be watered to help the bedding in of
the aggregate except where the base is laid over an
existing bituminous pavement.
Eng. Suneth Thushara Salawavidana 13
14. Compaction
• Then spread 19mm or 12.5mm choker stone evenly
at the given rate.
• Roll to ensure that the stones are well embedded in
the surface interstices.
• While rolling, wet the surface and brush lightly to
ensure that fine aggregate is worked into the
crevices.
• The surface of the base should be maintained in its
finished condition until a surfacing is placed.
Eng. Suneth Thushara Salawavidana 14
15. Quality Control
• Aggregate Impact Value
and Sieve Analysis one test
per 400m3.
• Rate of spread-one test
per day.
Eng. Suneth Thushara Salawavidana 15
