Chloride Testing & Chemical Tests
Chloride testing (and other chemical tests) of concrete can provide extremely useful information regarding the cause of defects or damage to concrete.
The chemical concrete tests most frequently carried out are Chloride Test, Cement Content Test, Concrete Carbonation Test, Sulphate Test, Type of Cement Test and the Alkali Test.
We provide ALL of these services.
Call us today on 0800 1588 318
The test for chloride content in concrete is very significant as when chloride is present in reinforced concrete it can cause very severe corrosion of the steel reinforcement. Chlorides can originate from two main sources: a) “Internal” Chloride, i.e. chloride added to the concrete at the time of mixing. This includes calcium chloride accelerating admixtures, contamination of aggregates and the use of sea water or other saline contaminated water. b) “External” chloride, i.e. chloride ingressing into the concrete post-hardening. In this category, we find both rock-salt (used on roads) which gets into concrete structures such as flyovers and sea salt, either directly from sea water in structures such as bridges, or in the form of air-borne salt spray in structures adjacent to the coast.
The test involves crushing a sample of the concrete to a fine dust, extracting the chloride with hot dilute nitric acid and then adding silver nitrate solution to precipitate any chloride present.
Exposure of concrete made with Portland cement to sulphate salts can cause damage due to an expansive reaction between the cement and the sulphate salt to form crystals of ettringite. Given adequate space to form, the ettringite forms needle like crystals, but in confined space causes an expansive reaction.
Sulphate Testing involves an acid extraction and precipitation of the sulphate as barium sulphate with barium chloride solution. The resulting barium sulphate is filtered and weighed to determine sulphate gravimetrically.
It is a fundamental requirement of good quality concrete that it contains an adequate cement content, or more precisely, a sufficiently low water to cement ratio, to provide adequate durability for the intended exposure conditions. In the absence of chemical admixtures, a certain amount of water is required to provide ‘workability’; essentially to simply lubricate the aggregate particles and the cement. If the cement content is too low, the concrete will be weak against the weather and be liable to frost attack and the effects of carbonation. If the cement content is too high, heat of hydration can cause thermal cracking in large pores, and the risk of shrinkage increases.
Testing the concrete requires the crushed concrete to be 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.
Type of Cement
A type of cement called High Alumina Cement (HAC) achieved some notoriety during the 1970’s following the collapse of several buildings in which it had been used. This was due to a conversion of the cement from one crystalline form into another, weaker, form. This phenomenon is simply known as “conversion”, and the amount of the change occurring, “the degree of conversion”. At normal temperatures, conversion may take many years but at temperatures in excess of 40°C a considerable amount of conversion can occur within a few months. Following conversion, the increased porosity may permit rapid carbonation of the concrete, removing alkaline protection to the steel reinforcement, which may then suffer from corrosion.
A test was devised which essentially tests for a significant content of soluble aluminium in solution, following extraction with dilute sodium hydroxide solution. However, the presence of the carbonate minerals render any determination of the degree of conversion of the concrete potentially inaccurate. The best procedures for examination of HAC are petrography and X-Ray diffraction analyses.