Free and water soluble chloride in concrete

Abstract Two methods were used to determine the ‘free’ and ‘water soluble’ chloride ions concentration in four differing strength grade concretes. Each grade of concrete had varying concentration of Cl− admixed as NaCl. In all 24 different batches of concretes were made with admixed Cl− concentration varying from 0.2 to 2%, by weight of cement. The first method was the analysis of the pore solution expressed from specimens and the second by the analysis of the decanted solution of pulverised specimens. The results suggest that the concentration of Cl− and OH− ions as measured in the expressed pore solution represent their true free ion concentrations. Furthermore, the Cl− concentration of both the expressed and decanted solutions were found to depend on the (i) concentration of the admixed) Cl− ions; (ii) strength of the concrete; (iii) presence or absence of fly ash and (iv) presence or absence of a superplasticiser. Accordingly, there is no simple relationship between the Cl− concentration as found in decanted and pore solutions. Whereas the presence of fly ash improves the chloride binding capacity of a concrete, the presence of superplasticisers tends to lower it. The paper concludes that in order to limit the damage caused by Cl− in the concrete, an upper limit on the total Cl− contents of the admixed chloride rather than the percentage of Cl− by the weight of cement should be prescribed.