Here are some details on the concrete carbonation test for the analysis of the structures.

Concrete Carbonation occurs when the atmospheric carbon dioxide (CO2) reacts with hydrated cement minerals (CaOH2). CO2 react in the presence of moisture with CaOH2 and produce carbonates (CaCO3). Carbonates slowly penetrate below the exposed surface of the concrete. Thus the carbonation affects the concrete cover over the reinforcing steel. Hence steel corrosion occurs due to the process of concrete carbonation. The concrete loses its durability, and finally, it cracks due to the expansion of film over the bar. At the same time, the rate of steel corrosion is at the highest level. The concrete carbonation process is also called Concrete passivization.

Factor Affecting Concrete Carbonation Process:

The concrete carbonation is a slow process. The concrete carbonation highly depends on humidity, temperature and atmospheric carbon dioxide CO2.

1. Relative Humidity: The Carbonation of concrete occurs when the level of humidity in the atmosphere is ideally between 50 to 70%.

2. Temperature: The carbonation is worse in hot environments.

3. Atmospheric CO2: The concentration of CO2 gas in the atmosphere is about 0.04% (400 ppm) by volume of the atmosphere, but it is increasing annually due to motor vehicles and fossil fuel burning in cities. Which is also known as “Green House Effect”.

The Procedure of Concrete Carbonation Test:

When the steel is surrounded by concrete layer, Hydrated cement provides an alkaline environment at which concrete pH is around 12.6. It helps steel to form the passive film on the surface of the steel. The passive film protects the reinforcement steel from the corrosion. Thus concrete cover provides protection to the steel.

The attack of the atmospheric CO2 reduces the alkalinity in the concrete. The concrete porosity allows the CO2 to react with the alkali like calcium, sodium and potassium hydroxides which are formed due to cement hydration in concrete. The reaction produces the carbonates which reduce the level of concrete pH. When carbonates ingress deeply in concrete, they also break the protective passive film of the steel. So the oxygen and moisture are easily available to steel, and it gets corroded. Hence we need to perform the carbonation test to determine the level or degree of carbonation to evaluate the rate of corrosion.

Two Method Measure the Level of Concrete Carbonation:

1. IR Spectrum Analysis for Concrete Carbonation:

IR spectrum method measures the concentration of CO2 absorbed by the concrete specimen. This method of testing is not much attractive for chemical testing of concrete.

The IR spectrum method set up consists of a closed loop in which a mixture of air and carbon dioxide could be introduced at certain relative humidity. The pump circulates the mixture of air and CO2. The concentration of CO2 is reduced in gas mixture due to the carbonation reaction. The concentration of Co2 in gas is measured by using IR absorption device. The relative humidity and temperature is also recorded.

2. pH Indicator Analysis for Concrete Carbonation:

The pH indicator method is a very popular method for chemical testing of concrete. A spray of 0.2% solution of phenolphthalein chemical is done on the surface of the concrete to find the area affected by the carbonation. Phenolphthalein solution indicates the change of pH level in the concrete. If the concrete changes its gray color to pink, it means that the concrete is in good condition. When there is no change in color of concrete, it means that the area is affected by carbonation.

The carbonation test is performed by extracting the cores of in-situ concrete.The carbonation test is also performed by drilling a hole on the concrete surface to the different depth up to concrete cover. Remove the dust by brush and Blow the air to clean the hole.

Spray 0.2% solution of the phenolphthalein with physician’s injection syringe or needle on such freshly drilled/broken concrete and observe the change in color. Measure the depth of uncolored layer (carbonated layer) in millimeters from the external surface at 4 to 8 positions. Take the average of measurement.

The Depth of Concrete Carbonation:

The Depth of concrete carbonation is estimated by the change in color profile, and the degree of carbonation that can be measured in millimetre. The depth of carbonation is proportional to the square root of time. I.e, if the depth of carbonation is 1 millimetre in one-year-old concrete, it will be 3mm after 9 years, 5mm after 25 years and 10mm after 100 years.

We can also determine the pH value by analyzing the sample of core powder collected by drilling from the site. It is determined by dissolving the powder sample in distilled water and thereafter tit-ration in the laboratory.

The phenolphthalein method for concrete carbonation test is a simple and cheap method. It easily determines the depth of carbonation in concrete. It also helps to decide about the information on the risk of reinforcement corrosion. The only limitation is the minor amount of damage done to the concrete surface by drilling a core.

In concrete carbonation test, the depth of carbonation is also confirmed with microscopy either optical microscopy using the thin concrete section or scanning electron microscopy using polished concrete section. The carbonation test is very useful to make an initial assessment of reinforced concrete structures. Carbonation test is quick, easy and used in the current scenario for the carbonation affected in-situ concrete.