LA HISTORIA A TRAVÉS DEL CINE 1.-Objetivos

The results of corrosion potentials (Ecorr) as a function of the exposure period obtained from both steel bars embedded in mortars containing various levels of chloride and mixed with and without 2 % BHD addition by weight of cement are presented in Figure 4-6 to 4-8. Each individual data point represents an average of three values obtained from replicate specimens.

Figure 4-6 demonstrates the influence of BHD addition on the corrosion behaviour of steel bar embedded in mortar made without addition of chloride. This figure indicates that the addition of BHD causes a fluctuation in the corrosion potentials compared to those in the control specimens from the beginning of the exposure, which implies that the addition of BHD does not help to stabilize the passive film. In addition, during the first 38 days of curing, the steel bar embedded in BHD mortar exhibited more negative Ecorr values, more negative than the corrosion potential threshold value of -270 mV (versus SCE) which suggested a 90 percent chance of corrosion (ASTM C876, 1999). However, after this period of exposure, the Ecorr values decreased gradually and crossed the corrosion potential threshold value toward the passive state region. The Ecorr values continued to move toward less negative values and exhibited less negative potentials for steel bar embedded in BHD mortar compared to Ecorr values of those steel bars embedded in control specimens and this potential

stayed stable between days 108 and 150 of curing. After that, the corrosion potentials in the steel bar embedded in BHD mortar started to diminish as exposure time increased and reached a value of -123 mV at day 171. In the case of the control specimens, the Ecorr values were less negative than the corrosion potential threshold value from the beginning and exhibited stable Ecorr values to the end of the experiment, the measured potential at day 171 was recorded as - 195 mV.

In the case of 0.4 per cent chloride addition, both mortars mixed with and without BHD initially exhibited more negative Ecorr values which then moved sharply in a positive direction (during a period of approx. 2 to 3 weeks) indicating a re-passivation of the steel bar. After this period of exposure, the corrosion potentials more or less stayed border-line, -270 mV (SCE), until day 157 as seen in Figure 4-7. Afterwards, both steel bars embedded in mortar mixed with and without BHD and both contaminated with 0.4 % chloride started to reveal more negative potentials (towards active corrosion region) where the Ecorr values became more negative gradually as exposure time increased indicating that active corrosion of the steel bar took place until the end of the experiment. In general, there were no significant differences between the electrochemical response of the steel bars embedded in control and BHD mortars both containing 0.4 % chloride, as shown in Figure 4-4 and Figure 4-7.

In Figure 4-8, the steel bar embedded in mortar containing 2 per cent chloride and mixed with and without BHD addition experienced corrosion potentials more negative than -270 mV (SCE) throughout the whole experiment span. At around day 80, however, the steel bar embedded in BHD mortar started to reveal less negative corrosion potentials compared to those bars embedded in mortars containing the same level of chloride and made without BHD

addition. A similar trend of corrosion resistance was noticed on the same steel bars, where Icorr values were less compared to those Icorr values obtained from steel bars embedded in mortar cast without BHD, see Figure 4-5. This could presumably be attributed to the higher level of [OH-] concentration found in the pore solution extracted from BHD mortar compared to that in mortar made without BHD addition, as seen in Figure 4-20. This could be supported by the finding of the work conducted by Gouda (1970) and Goñi and Andrade (1990) that showed an increase of Ecorr values (less negative) as pH values increases.

Chaudhary et al. (2003) studied the effect of 2 % BHD addition on the corrosion of steel bar embedded in concrete slabs that were mixed with and without 1 % chloride by weight of cement. The corrosion potentials were less negative in BHD concrete mixed with and without chloride compared to concrete slabs that were cast without BHD and containing the same level of chloride. Al-Sugair et al. (1996) carried out an investigation on the effect of 2 and 3 % BHD as cement replacement on the corrosion behaviour of the steel bar embedded in concrete. The concrete specimens were placed in a tank filled by 3.5 % sodium chloride solution. They found the corrosion potentials in BHD concrete to be less negative than that in the control specimens.

Figure ‎4-6 Corrosion potentials of steel bars embedded in the control and BHD mortar specimens without any addition of sodium chloride

Figure ‎4-7 Corrosion potentials of steel bars embedded in the control and BHD mortar specimens containing 0.4 per cent chloride by weight of cement

-380 -340 -300 -260 -220 -180 -140 -100 E c orr ( m V )

Exposure time (days)

0 % BHD 2 % BHD

Corrosion potential threshold

-600 -550 -500 -450 -400 -350 -300 -250 -200 E c orr ( m V )

Exposure time (days)

0 % BHD 2 % BHD

Figure ‎4-8 Corrosion potentials of steel bars embedded in the control and BHD mortar specimens containing 2.0 per cent chloride by weight of cement

The corrosion rate of steel bar embedded in concrete is controlled by several factors including diffusion of oxygen, resistivity of the concrete and the composition of the pore solution (Goñi and Andrade, 1990). The important implication is that the addition of BHD into mortar does not accelerate the corrosion rate of the steel bar in the presence of chloride, as shown in Figure 4-4 and Figure 4-5, despite BHD containing an additional amount of chlorides as can be seen from the BHD chemical composition in Table 3-2. This suggests that there are other factors that influence the propagation of the corrosion process on steel bars embedded in BHD mortars. One of the main reasons responsible for the corrosion process in steel bar embedded in concrete is the pore solution which is in contact with embedded steel. Therefore, the corrosion behaviour of steel bar embedded in BHD mortar is likely to be attributed partly to the pore solution composition and partly to the physical properties of concrete. The

-630 -580 -530 -480 -430 E c o rr ( m V )

Exposure time (days)

magnitude of the corrosion process is also mainly affected by the composition of pore solutions and/or by the physical properties of the concrete such as the porosity. Consequently, the changes in the pore solution chemistry over time were studied to investigate the aggressiveness of the pore solutions (as discussed in Part B of this Chapter) and the porosity of cement pastes which were responsible for providing access routes for the corrosive media/agents were also investigated in Chapter 5 part A.

Figure ‎4-9 Best-fit straight-line plots of corrosion potentials versus log corrosion current density for steel bar embedded in mortar incorporated with BHD additions of A) 0 and

B) 2 and both mixed with 0, 0.4 and 2 % chloride

Table ‎4-1 Statistical data from regression analysis

Figure No. B (mv) R2

Figure 4-9 (A) -133 0.80

In general, Figure 4-9 (A) and (B) show that, the Icorr values increase with increasingly negative values of Ecorr, which suggests that the process of corrosion is controlled by the anodic reaction e.g. iron dissolution from steel bars embedded in plain and in BHD-mortars.