Page 1ONLINE CIVIL ENGIEERING: September 2010

08-11-2013 12:22:04http://civil-online2010.blogspot.no/2010/09/

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► Concrete ► Cement ► B E Test ► Test Test

CARBONATION DEPTH MEASUREMENTTEST

Carbonation of concrete occurs when the carbon dioxide, in the atmosphere in

the presence of moisture, reacts with hydrated cement minerals to produce

carbonates, e.g. calcium carbonate. The carbonation process is also called

depassivation. Carbonation penetrates below the exposed surface of concrete

extremely slowly. The time required for carbonation can be estimated knowing

the concrete grade and using the following equation:

t=(d/k)2

where,

t is the time for carbonation,

d is the concrete cover,

k is the permeability

Typical permeability values are shown in Table 1.

The significance of carbonation is that the usual protection of the reinforcing

steel generally present in concrete due to the alkaline conditions caused by

hydrated cement paste is neutralized by carbonation. Thus, if the entire concrete

cover over the reinforcing steel is carbonated, corrosion of the steel would occur

if moisture and oxygen could reach the steel.

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% phenolthalein solution is made by dissolving 1gm of phenolthalein 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 uncoloured 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 uncoloured layer measured at 4 or 8

positions and the average taken. If the concrete still retains its alkaline

characteristic the colour 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.

Another formula, which can be used to estimate the depth of carbonation,

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Page 2ONLINE CIVIL ENGIEERING: September 2010

08-11-2013 12:22:04http://civil-online2010.blogspot.no/2010/09/

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/(R

2(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). This formula is contained in

the Japanese Construction Ministry publication “Engineering for improving the

durability of reinforced concrete structures.” α is 1.7 for indoor concrete and 1.0

for outdoor concrete. β values are shown in Table 2.

RANGE AND LIMITATIONS

The phenolphthalein test is a simple and cheap method of determining the

depth of carbonation in concrete and provides information on the risk of

reinforcement corrosion taking place. The only limitation is the minor amount of

damage done to the concrete surface by drilling or coring.

Modified Slump Test

The modified slump test (Ferraris and de Larrard 1998; Ferraris 1999; Ferraris

and Brower 2001) is intended for use as a field test to measure both the plastic

viscosity and yield stress of concrete mixtures. The test adds the parameter of

time to the standard slump test in order to measure plastic viscosity.

The apparatus for the modified slump test consists of a vertical rod that

extends from a horizontal base plate through the center of the standard slump

cone. The slump cone is filled in accordance with ASTM C143 and a sliding disk is

placed atop the fresh concrete. Once the slump cone is removed, the time for

the disk to slide a distance of 100 mm is measured. The sliding disk comes to

rest on a stop located on the vertical rod. After the disk comes to rest, the

concrete continues to subside to its final position. The final slump measurement

is recorded no later than 60 seconds after the slump cone is removed. A

schematic of the test procedure is shown in Figure below.

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