Chemical analysis of concrete is a valuable tool in determining the reasons behind concrete failure. Several commonly performed tests for chemical analysis of concrete structures include measuring the depth of carbonation, determining chloride content, evaluating cement content, assessing sulphate content, identifying the type of cement used, and analyzing alkali content. These tests provide crucial information that can help diagnose the causes of concrete failure and inform appropriate remedial measures.
Carbonation Test on Concrete Structures
The purpose of this test is to determine the depth of concrete affected by the combined attack of atmospheric carbon dioxide and moisture, which reduces the alkalinity of concrete. To conduct the test, a 0.2% solution of phenolphthalein is used as a pH indicator, and it is sprayed onto the concrete surface. If the concrete changes color to pink, it indicates that the concrete is in good health. However, if no color change occurs, it suggests that the concrete has been affected by carbonation.
The test is conducted by drilling holes at various depths up to the thickness of the concrete cover. Dust is removed from the freshly drilled holes by air blowing, and then phenolphthalein solution is sprayed onto the broken concrete using a physician’s injection syringe and needle. The change in color is observed to determine the depth of carbonation in the concrete.
Alternatively, the pH value can be determined by collecting samples of mortar from the site by drilling, dissolving the samples in distilled water, and titrating them in a laboratory. This method can also be used to estimate the depth of carbonation based on the change in color profile.
Chloride Content Test on Concrete Structures
Chloride content in concrete can be determined through broken samples or core samples. The concentration of chloride near the steel-concrete interface is particularly important in assessing the risk of corrosion in concrete. Chloride in concrete can be present in fixed (water insoluble) and free (water soluble) forms. Although water soluble chloride ions are the most relevant in terms of corrosion risk, the total acid soluble chloride content (including both fixed and free chloride) is typically determined and compared with specified limiting values to assess the corrosion risk in concrete, following the method outlined in IS:14959 Part – III – 2001.
To assess the water soluble chlorides, water extracts are obtained from the concrete samples and standard titration experiments are conducted to determine the water soluble chloride content, which is then expressed as a weight percentage of the concrete or cement. This method provides an average chloride content in the cover region of the concrete. However, a more useful measurement is the chloride profile across the cover thickness, as this can provide an estimate of the diffusion rate of chloride ions in concrete.
One recent development in field testing of chloride content is the use of chloride ion sensitive electrodes, commercially known as “Rapid Chloride Test Kit-4”. This test involves obtaining powdered samples by drilling and collecting them from different depths at regular intervals (typically 5mm). The samples are then mixed with a special chloride extraction liquid, and the electrical potential of the liquid is measured using chloride-ion selective electrodes. By establishing a calibration graph that relates electrical potential to chloride content, the chloride content of the samples can be directly determined.
Based on the chemical analysis results, corrosion-prone locations in concrete can be identified according to guidelines provided in Table-1. These guidelines serve as a reference for determining areas of concrete that may be at higher risk of corrosion based on their chloride content.
| Sl. No. | Chemical Test Results | Interpretations |
| 1 | High pH values greater than 11.5 and very low chloride content | No corrosion |
| 2 | High pH values and high chloride content greater than threshold values (0.15 % by weight of cement) | Corrosion prone |
| 3 | Low pH values and high chloride content (greater corrosion prone than threshold values of chloride 0.15% by weight of cement. |