Concrete Durability Investigations
The most widespread type of deterioration in concrete encountered in the UK is cracking and spalling due to the corrosion of steel reinforcement.
Attack by oxygen, in the presence of water or water vapour, leads to the formation of iron oxides on the surface of the steel. The resulting expansion cracks the concrete covering the steel. The process is greatly accelerated by a lowering of alkalinity in the concrete. This is usually due to carbonation and/or the presence of chloride. Consequently, in investigating the causes and/or extent of attack, it is necessary to measure:
- The depth of the concrete cover to the reinforcement
- The extent to which any loss of alkalinity has penetrated the concrete
- The amount of chloride present at the depth of the reinforcement, whether it has arisen by incorporation in the original mix or by penetration e.g. by de-icing salts
Depth of cover
This is determined using a covermeter. The results can be presented in the form of a contour map.
Loss of Alkalinity
This is usually determined on site by spraying an indicator such as phenolphthalein on a freshly exposed surface. This method is usually referred to as determining the depth of carbonation since carbonation of the concrete is the most common process leading to lowering of the alkalinity. The resulting depth of the ‘carbonation front’ is an over simplification, since carbonation is a diffusion process and there is actually a ‘carbonation profile’. This can be determined in the laboratory by X-ray diffraction and gives information not only on the present condition of the concrete, but also enables prediction of the future rate of progress. Sulphates (a common source is de-icing salts) also lower the alkalinity of the concrete which results in reinforcement corrosion. Total sulphur is determined very easily in the laboratory by X-ray spectrometry.
We also determine the permeability and porosity of concrete, which is useful is assessing the quality of the concrete and hence the susceptibility of the reinforcement to corrosion.
If it is suspected that Chloride has penetrated into the concrete from an external source, samples can be taken from different depths and a ‘chloride profile’ obtained. For carbonation, a prediction can be made of the future rate of ingress of chloride. The chloride method can be measured either by the method in BS1881, or by the BRE approved X-ray spectrometry method. Our in house X-ray method is very fast and we can provide a fast turnaround for large numbers of samples.
Corrosion is an electro chemical process. Where corrosion is occurring, an anode is formed relative to the rest of the steel. The ‘half cell potential’ can be measured on site relatively simply. The voltage is measured in a grid pattern, and contour maps prepared, which indicate the areas where corrosion is most likely to occur. In addition, it is often useful to measure the resistivity of the concrete, using a similar grid pattern. Low resistivity is found in areas where the concrete contains ions such as chloride, meaning there is the potential for reinforcement corrosion.