In the reference method for the determination of chlorides used in the cross-testing experiments reported here, a sub-sample of the aggregate is dried to constant mass, and sieved through a 16.0mm sieve. Any oversized particles are crushed to pass this sieve. A weighed test portion (2.0kg +/- 0.3kg of a coarse aggregate, or 500g +/- 75g of a fine aggregate) of this material is then extracted with an equal mass of water by agitating a bottle containing the aggregate and water continuously for 60 minutes (using a shaker or roller). With a lightweight aggregate, a test portion of about 1L is used, and extracted with 1L of water.
A 100mL aliquot of the extract is taken and analysed by Volhard titration, in which an excess of silver nitrate solution (0.100mol/L) is added to the aliquot, and the unreacted portion is back-titrated using a standardised solution of thiocyanate (approximately 0.1mol/L), using ammonium iron (III) sulfate solution as an indicator.
The result of the test is reported in terms of the chloride ion content, as a percentage by mass of the aggregate. The method requires the extraction and analysis to be repeated on a second aggregate test portion.
The method of Volhard titration has been widely used for many years. The CEN version of the test method has been developed by the committee responsible for drafting test methods for determining chemical properties of aggregates (CEN/TC 154/TG 8), and is similar to the current British Standard method. The most significant difference between the CEN and BS methods is that in the CEN method the extraction process takes 60 minutes and uses continuous mechanical agitation, whereas in the BS method it takes 24 hours and requires only occasional shaking or rolling.
Participants in the cross-testing experiments were sent copies of the method taken from the draft document giving tests for chemical properties of aggregates that it was agreed should be sent to CEN members for CEN enquiry in 1993 (Proposed prEN Tests for chemical properties of aggregates. Part 1 Chemical analysis. CEN/TC 154/TG 8 committee paper.). According to this draft, the method outlined above is to be the reference method, in European Standards, for determining water-soluble chloride salts in aggregates.