Errors introduced by the guard ring device in the on-site measurement of rebar corrosion rates.

In 1987 the authors published their first paper on the possibilities of on-site measurement of rebar corrosion rate. In that paper the transmission line model was applied in order to calculate the critical length reached by an electrical signal along a rebar and therfore, the corrosion rate by unit of metal and concrete surface. Other papers have been recently published based in the same transmission line model. In the present paper the experimental errors which the guard ring induces, are commented upon. For this purpose beams and slabs were fabricated. The corrosion rate was measured by different methods: apparent polarization resistance, attentuation of potential and with the guard ring. The results allow comparison of the reliability of the different methods of measuring the real Rp value and therefore, of the corrosion rates. Particularly, the guard ring cannot completely confine the electrical signal when the steel is passive and the confinement is only partial in the case of active corrosion, giving therefore higher corrosion intensity values than the real ones, especially in the case of the slabs (two dimensions). (A)