Carbonation Test of Concrete

Carbonation Test of Concrete

Carbonation Test of Concrete: Carbonation of concrete occurs when carbon dioxide in the air reacts with calcium hydroxide and hydrated calcium silicate in the concrete, resulting in the formation of mostly carbonates (CaCO3).

Carbonates are formed during these chemical reactions, resulting in the permanent retention of this CO2. Carbonation happens when CO2 is absorbed by the exposed surfaces of concrete buildings, as seen in the image below.

Carbonation of Concrete
CO2 from air diffuses into concrete (source)
This process gradually reduces the pH of the concrete from around 13 to between 8-9, beginning with the outer layers. In reinforced concrete structures, this shift in pH may break the steel reinforcement’s corrosion passivation, and if moisture and oxygen are present, the steel reinforcement can begin to corrode. This may result in concrete surface spalling and a loss of mechanical strength. The figure below illustrates such a scenario.

Carbonation Test of Concrete

Differential scanning calorimetry (DSC)

DSC is a thermal analysis technique that determines the relationship between the physical qualities of a sample and its temperature over time. In other words, the device is a thermal analysis tool that determines the temperature and heat flow associated with phase transitions in materials as a function of time and temperature.

DSC can be used to calculate the amount of calcium hydroxide and calcium carbonates. Through these results, the degree of concrete carbonation can be determined.

DSC Device (source)

pH Indicator Indicator (phenolphthalein)

The pH indicator method is a widely used technique for assessing the carbonation of concrete. A 0.2 percent solution of phenolphthalein solution is sprayed on the concrete’s surface to determine the area impacted by carbonation. The presence of phenolphthalein in the solution indicates a change in the pH level of the concrete. If the grey color of the concrete transforms to pink, it indicates that the concrete is not carbonated. When the color of concrete does not change, it indicates that the region is impacted by carbonation.

Carbonation Test of Concrete
Carbonation Test of Concrete

The carbonation test is conducted by collecting in-situ concrete cores. Carbonation tests are also conducted by drilling holes in the concrete surface to varying depths up to the depth of the concrete cover. Brush away the dust and blow air into the opening to clean it.

Spray a 0.2 percent solution of phenolphthalein over such newly drilled/broken concrete using a physician’s injection syringe or needle and notice the color change. At four to eight locations, determine the depth of the uncolored layer (carbonated layer) in millimeters from the exterior surface. Calculate the mean of the measurements.

Carbonation Test of Concrete
Carbonation Depth (source)


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