CARBONATION TEST USING PHENOLPHTHALEIN
AIM: To determine the depth of concrete affected due to combined attack of atmospheric
carbon dioxide and moisture causing a reduction in level of
alkalinity of concrete
OBJECTIVE: Calculate alkalinity with help of spray of 0.1% solution of
phenolphthalein as pH indicator of concrete.
PRINCIPLE:
The change of colour of concrete to pink indicates that the concrete
is
in the good health, where no change in colour takes place, it is
suggestive of
carbonation-affected concrete. of phenolphthalein is used
as pH indicator.
THEORY:
Carbonation of concrete is associated with the corrosion of steel
reinforcement
and with shrinkage. However, it also increases both the
compressive and tensile strength of
concrete, so not all of its effects
on concrete are bad. The carbonation process is also called
depassivation It has been found that the depth of carbonation is
approximately proportional
to the square root of the time.
DEPTH = K x √TIME (Where K is a constant).
where,
t is the time for carbonation,
d is the concrete cover,
k is the permeability
t is the time for carbonation,
d is the concrete cover,
k is the permeability
Another
formula, which can be used to estimate the depth of carbonation,
utilizes the age
of the building, the water-to-cement ratio and a
constant, which varies depending on the
surface coating on the concrete.
y=7.2XC2/(R2(4.6x-1.76)2)
where,
y is age of building in years,
x is water-to-cement ratio,
C is carbonation depth,
R is a constant (R= αβ).
where,
y is age of building in years,
x is water-to-cement ratio,
C is carbonation depth,
R is a constant (R= αβ).
R
varies depending on the surface coating on the concrete (β) and
whether
the concrete has been in external or internal service (α).
EQUIPMENTS:
If there is a need to physically measure the extent of carbonation it
can
be determined easily by spraying a freshly exposed surface of the
concrete
with a 1% phenolphthalein solution. The calcium hydroxide is
coloured pink while
the carbonated portion is uncoloured.
PROCEDURE:
The
1% phenolphthalein solution is made by dissolving 1gm of
phenolphthalein
in 90 cc of ethanol. The solution is then made up to
100 cc by adding distilled water.
On
freshly extracted cores the core is sprayed with phenolphthalein
solution, the depth
of the uncolored layer (the carbonated layer) from
the external surface is measured to
the nearest mm at 4 or 8 positions,
and the average taken.
If
the test is to be done in a drilled hole, the dust is first removed from
the hole using an
air brush and again the depth of the uncolored layer
measured at 4 or 8 positions and the
average taken.
If the concrete still retains its alkaline characteristic the color of the concrete will change
to purple.
If carbonation has taken place the pH will have changed to 7 (i.e. neutral condition) and
there will be no colour change.
LIMITATIONS : The minor amount of damage done to the concrete surface by
drilling or coring.
RESULT:
The depth of concrete affected due to combined attack of atmospheric
carbon dioxide and moisture causing a reduction in level of alkalinity
of concrete
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