FREE AND WATER-SOLUBLE CHLORIDE IN CONCRETE

Two methods were used to determine the 'free' and 'water soluble' chlo ride ions concentration in four differing strength grade concretes. Ea ch grade of concrete had varying concentration of Cl- admired as NaCl. In all 24 different batches of concretes were made with admired Cl- c oncentration varying from 0.2 to 2%, by weight of cement. The first me thod was the analysis of the pore solution expressed from specimens an d the second by the analysis of the decanted solution of pulverised sp ecimens. The results suggest that the concentration of Cl- and OH- ion s as measured in the expressed pore solution represent their true free ion concentrations. Furthermore, the Cl- concentration of both the ex pressed and decanted solutions were found to depend on the (i) concent ration of the admired Cl- ions; (ii) strength of the concrete; (iii) p resence or absence of fly ash and (iv) presence or absence of a superp lasticiser. Accordingly, there is no simple relationship between the C l- concentration as found in decanted and pore solutions. Whereas the presence of fly ash improves the chloride binding capacity of a concre te, the presence of superplasticisers tends to lower it. The paper con cludes that in order to limit the damage caused by Cl- in the concrete , an upper limit on the total Cl- contents of the admired chloride rat her than the percentage of Cl- by the weight of cement should be presc ribed.