Quality
of Coarse Aggregate
Since characteristics of concrete are directly related to those of its constituent aggregates, aggregates for load bearing concrete should be hard ,strong, non-porous ,free from friable ,elongated and laminated particles, and should be suitable for the purposes required. Stones absorbing more than 10 per cent of their weight of water after 24 Hours immersion in water are considered porous. Porous materials corrode reinforcement.
A friable aggregate will produce a
concrete of similar nature. Elongated or laminated particles are weak in shear.
Stones having mica inclusion should be avoided. Stones of the varieties of
granite, quartzite , trap and basalt, and those with rough non-glossy surface
are considered best.
All sand-stones tend to be porous.
Soft varieties of sand stones make poor concrete and also produce shrinkage
cracks. Limestone is quite good provided it is hard ,crystalline and entirely
free from dust. Limestone should not be used in works subject to excessive
heat. Both lime and sand-stones and other porous stones are not suitable for
structures retaining water, or existing in the atmosphere near to sea sides.
Size
of Aggregates
The material retained on a 4.75 mm
IS sieve is classified as coarse aggregate and below that size as fine
aggregate or sand. The material passing a 75-micron IS sieve is generally
considered to be clay, fine silt or fine dust in an aggregate.
Coarse aggregates should be ordered
in separate sizes and recommended in proper proportions while batching.
40 mm to 20 mm , 20 mm to 10 mm and 10 mm to 4.75 mm. A 20 mm nominal
maximum size will be ordered in two sizes--- 20 mm to 10 mm , and 10 to 4.75
mm. Separate stockpiles should be maintained for the different sizes.
Angular and roughly cubical
particles are ideal. River gravel makes the best coarse aggregate.
Storing
or Stockpiling of Aggregate
During storing or handling of
aggregate it is of utmost importance to see that there is no segregation i.e.,
separation of the various sizes of particles. Stockpiling segregation does take
place if successive consign- me it's are dropped on the same place each time
and it forms a pyramid like heap, as the coarser materials roll down the sides
of the pile while the finer particles stay on in the centre of the pile on
concentration at the top. Therefore, all the material should not be piled at
the same place but should be placed in individual units side by side not larger
than a truck load and should not be thrown from a height as this will also
result in segregation by the winds. Each separate size of the coarse aggregate
and that of sand should be stacked separately ,large in area and low in height
1 to 1.5 meters. All washed aggregate should be stacked for draining at least
12 hours before being batched.
Maximum
Aggregate Size.
The maximum size of aggregate is
governed by the type of the work to be built. The bigger the maximum size of
the aggregate, less than voids when the aggregate is graded. A coarse aggregate
which has less void content is economical and will give higher strength for the
same amount of cement in the concrete. The maximum size of aggregate may be up
to 160 mm for mass concrete, but size up to 225 mm has been used in dams.
Aggregates of this size require careful mix design to avoid segregation and it
is probably wise to limit maximum size to 80 mm. Large stones which are
embedded in mass concrete works are called "plumbs". Plumbs should be
sound and hard and should not be placed nearer than 160 mm to one another or to
an exposed surface.
Testing
of Aggregates
In
order to decide the suitability of the aggregate for use in pavement
construction, following tests are carried out:
1-Crushing
test.
2-Abrasion
test.
3-Impact
test.
4-Soundness
test.
5-Shape
test.
6-Specific
gravity and water absorption test.
7-Bitumen
adhesion test.
