4. RESULTS AND DISCUSSION
4.1 Salt Fog Test
The extent of rebar corrosion in concrete specimens subjected to salt fog test was
monitored visually by sectioning them longitudinally into two parts. Visual observations
on the condition of rebar after 3, 9, 18 and 24 months of exposure are recorded in Table
5. These results indicate that the OPC and SRC specimens fail to protect the rebar from
corrosion after 18 months. However, addition of densified and undensified microsilica
appears to be effective in reducing the corrosion of rebars in both the type of cements.
But, it is more effective in the case of OPC cement, where no rebar corrosion was
detected even after 2 years of exposure, while in few specimens of densified microsilica
added SRC corrosion was observed after 2 years. During this test OPC blended with
densified and undensified microsilica were found equally protective in nature.
4.2 Electrochemical Test
The open circuit potential (OCP) Vs time data can be used with reasonable success to
interpret the susceptibility of the rebar to corrosion particularly localized corrosion
[Slater 1983]. A noble value OCP is ascribed to the reduction in the number of anodic
and cathodic sites which are formed by pores, defects, crevices etc. causing the surface
to be less reactive. On the contrary a shift of OCP in the negative direction generally
indicates a more reactive surface. Corrosion reaction taking place at the rebar-concrete
interface in a concrete specimen immersed in 5% NaCl solution and seawater was
monitored by OCP measurement of the rebars with time. The potential Vs time plots
shown in Figure 2 A and B indicate that the corrosion protection behavior of OPC and
SRC cement in 5% NaCl solution is very different from that found in seawater, which
has SO4
2+ and Mg2+ ions along with the Cl- ions. In 5% NaCl solution, rebar shows too
much rise in negative potential in OPC and SRC cement specimens as compared to the
densified microsilica (DMS) added concrete specimens. This confirms the effectiveness
of DMS in both the type of cement. However, undensified microsilica (UDMS) does not
appear to be effective in controlling the corrosion of rebar in OPC. In all the five types
of concrete mixes there is a linear rise in the negative potential up to 100 days. The