Chemical analysis of concrete can provide extremely useful information regarding the causes of failure of concrete. The tests most frequently carried out are listed below:
- Depth of carbonation
- Chloride content
- Cement content
- Sulphate content
- Type of cement
- Alkali content
This test is carried out to determine the depth of concrete affected due to combined attact of atmoshpheric carbon dioxide and moisture causing a reduction in level of alkalinity of concrete. A spry of 0.2% solution of phenolphthalein is used as pH indicator of concrete. The change of colour of concrete to pink indicates that the concrete is in the good health, where no changes in colour takes place, it is suggestive of carbonation-affected concrete. The test is conducted by drilling a hole on the concrete surface to different depths upto cover concrete thickness, removing dust by air blowing, spraying phenolphthalein with physician’s injection syringe and needle on such freshly drilled broken concrete and observing change in colour. The depth of carbonation is estimated based on the change in colour profile. The pH value can also be determined by analysing samples of mortar collected by drilling from the site, dissolving the same in distilled water and thereby titrating in laboratory.
Chloride content can be determined from broken samples or core samples of concrete. Primarily the level of chloride near the steel-concrete interface is of prime importance. Chloride present in concrete are fixed (water insoluble) as well as free (water soluble). Though it is the water soluble chloride ions, which are importance from corrosion risk point of view, yet total acid soluble (fixed as well as free) chloride contents are determined and compared with limiting values specified for the concrete to assess the risk of corrosion in concrete. The total acid soluble chloride are determined in accordance with IS:14959 Part – III – 2001, whereas for assessment of water soluble chlorides the test consists of obtaining the water extracts, and conducting standard titration experiment for determining the water soluble chloride content and is expressed by water soluble chloride expressed by weight of concrete or cement. The method gives the average chloride content in the cover region. Further a chloride profile across the cover thickness will be a more useful measurements as this can help to make a rough estimate on chloride content diffusion rate. One recent development for field testing of chloride content includes the use of chloride ion sensitive electrode. This is commercially known as “Rapid Chloride test kit-4”. The test consists of obtaining powdered samples by drilling and collecting them from different depths (every 5mm), mixing the sample (of about 15.g weight) with a special chloride extraction liquid, and measuring the electrical potential of the liquid by chloride-ion selective electrodes. With the help of a calibration graph relating electrical potential and chloride content, the chloride content of the samples can be directly determined.
Based on the chemical analysis, corrosion-prone locations can be identified as per the guidelines given in table-1.
Table-1: Guidelines for Identification for Corrosion Prone Locations based on Chemical Analysis
High pH values greater than 11.5 and very low chloride content
High pH values and high chloride content greater than threshold values (0.15 % by weight of cement)
Low pH values and high chloride content (greater corrosion prone than threshold values of chloride 0.15% by weight of cement.
Increased risk of corrosion