Chemical Analysis of Samples
Chemical analysis of concrete can provide extremely useful information regarding the cause or causes of failure of concrete. The chemical laboratory is equipped with state-of-the-art apparatus to carry out analysis of samples. This includes titrators, atomic absorption spectroscopy, wet chemical analysis and Ionic Selective Electrodes.
- Complete Chemical Analysis
Determination of contents of these elements: Ca, Al, Fe, Ti, Mg, Si, Na, K and anions: Cl-, SO42-. Analysis of ground water from the view of presence of aggressive media – such as sulphates, chlorides, nitrates.
- Cement Content of Concrete and Mortar
The cement content of concrete is important from the aspect of durability, impermeability and strength. The test involves crushing a representative sample of the concrete (usually a core), to a fine powder and chemically analysing for insoluble residue, soluble silica and lime content. The prepared sample is extracted with dilute acid and dilute alkali solution to remove the cement. The extract is then analysed for soluble silica and calcium oxide, being the two major components (expressed as oxides) of Portland cement.
- Water/cement ratio.
The original water/cement (v/c) ratio of a concrete mix affects a number of properties in the hardened concrete such as drying shrinkage, permeability, frost resistance, compressive strength. In essence this method involves determining the volume of capillary pores and the weight of cement and combined water. The sum of the combined water and the pore water gives the original mixing water and the cement content is determined as previously described. Hence, the water/cement ratio can be calculated. The test cannot be carried out on a poorly compacted concrete as this artificially inflates the value determined for capillary water.
- Chloride Content and Depth of Penetration
Chloride salts may be present in concrete from a number of sources:- an accelerating admixture, ingress of de-icing salt from an external source, impurities in the aggregates and/or mixing water. It is generally accepted that the likelihood of corrosion of steel reinforcement caused by the post hydration ingress of chloride salts varies with the level of salt present as follows: chloride < 0.2 % by mass of cement – corrosion caused by chloride salt unlikely; chloride between 0.2 and 1.0 % by mass of cement – corrosion of steel in interior conditions unlikely but possible in the presence of sufficient moisture or in exterior conditions; chloride > 1.0 % by mass of cement – high risk of corrosion whether concrete is in a protected environment or not.
The chloride content of the concrete can be assessed either on the crushed concrete or more often on dust drillings taken with a rotohammer drill. Using the later method samples may be taken at successive depths to assess the penetration of chlorides into a structure from an external source. The chloride content is determined by acid extraction of the crushed concrete, followed by a chemical determination of the chloride content.
- Sulfate Content
Sulphates can be present in concrete either from an external source, usually soil in the case of buried concrete or, rarely, from contamination of the aggregates. Sulphates can attack concrete causing an expansive disruptive effect resulting in gradual deterioration or eating away of the cement matrix.
- Alkali Content (where risk of alkali silica reaction is suspected).
In recent years, some deleterious reactions involving the aggregate and the surrounding cement paste have been observed. The most common reaction is that between certain forms of reactive silica in the aggregates and alkalis in the concrete, normally derived from the cement. The alkali content of hardened concrete is determined by flame emission or atomic absorption spectrophotometric methods on an acid extract of the crushed concrete.
- Test for HAC (High Alumina Cement) (Presence and condition of High Alumina Cement).
It essentially tests for a significant content of soluble aluminium in solution, following extraction with dilute sodium hydroxide solution. The presence of the carbonate minerals renders any determination of the degree of conversion of the concrete potentially inaccurate. The best procedures for examination of HAC are chemical and X-Ray diffraction analyses.
- Determination of Degree of Conversion
The C4AH10 form of HAC gradually converts to the C3AH6. These two hydrated forms can be detected and quantified by the method of DTA. The water of crystallisation of the unconverted and converted forms of HAC are lost at different temperatures and therefore show two distinct troughs in the DTA The ratio of the height of one to the other gives the degree of conversion expressed as a percentage. i.e. x 100 = degree of Conversion %. The degree of conversion test is considered accurate to plus or minus 5%. The cement content determination for HAC is estimated to be accurate to plus or minus 15%. Soil and aggregate analysis for contamination of aggressive media such as humine acids, chlorides, sulphates.
- Depth of Carbonation
The depth of carbonation can be measured on a freshly exposed section of the concrete, such as a core, by spraying with an indicator spray such as phenolphthalein. This turns to a pink colour when the concrete is alkaline (above pH 9.2) but remains colourless where the concrete is carbonated, usually as a more or less even zone extending to some depth from the surface. It should be noted that the pH at which the colour of phenolphthalein changes is lower than that at which passivity is lost (which occurs progressively below about pH 11). It should be noted that carbonation along microcracks and along diffusion paths in poorly compacted concrete, may not be readily revealed by the phenolphthalein spray method.