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

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OBJECTIVE: Calculate alkalinity with help of spray of 0.1% solution of 

phenolphthalein as pH indicator of concrete.

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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.

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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.

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               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

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).

R varies depending on the surface coating on the concrete (β) and 

whether the concrete has been in external or internal service (α).

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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.

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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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CONCLUSION: The depth of carbonation is estimated based on change in colour profile.