Laitance is a thin layer of cement paste with finer
fines of fine aggregates, which has no strength and is a pathway for corrosion
of the column.
No fresh concrete be laid, without laitance
removal.
It can be easily removed with a wire brush.
BOND or SHEAR KEY
Members like footing top, pedestals, and columns
should have a bond key.
The bond key guarantees an excellent connection
between an old and fresh concrete.
The bond key should have a size of 100 mm x 100 mm
and 150 mm depth.
Concrete structure can disintegrate if
a) concrete is porous � due to high water cement ratio
b) concrete has honeycombs � due to
inadequate vibration and segregation
c) Improper cold joint � if the joint is concreted after the concrete
exceeds the setting time.
d) Poor construction joint � if the end stopper
is not properly designed.
e) Inadequate concrete cover to the reinforcement.
Properties of good concrete.
a) Concrete should be cohesive � satisfy
'laddoo' test.
b) Concrete should be workable � easy for the
labours to work upon.
c) Concrete should satisfy strength parameters � as decided by
the Structural Engineer.
d) Concrete should be durable � depending on
the zone of exposure and the structure life expectancy.
CONCRETE MIX DESIGN (CMD) DEFINITION.
It is the technique of CHOOSING (No longer fixed)
The most ECONOMIC proportions (cost important)
Of available materials (No longer specified)
(Cement, Water, Fine aggregates, coarse aggregates,
and Admixtures)
To obtain
a
(1) COHESIVE
concrete FA: CA Ratio
Of the required
(2)
WORKABILITY A/C Ratio
And the specified (3)
STRENGTH
W/C Ratio
And the desired
(4)
DURABILITY
W/C
Ratio
