Study of Corrosion in Reinforced Concretes with Different Water to Cement Ratios at Splash zone

1. IntroductionConcrete structures in aggressive environments greatly suffer from corrosion which causes a premature failure in their life span. Persian Gulf region is one of the most aggressive environments in which concrete structures need further attention in order to increase their durability and serviceability as well. In the present investigation, the half-cell potential and corrosion resistance of rebar embedded in concrete with different water to cement ratios (0.35, 0.40, 0.45 and 0.50) exposed to splash zones in Qeshm Island for 18 month ware studied (Fig 1). The results demonstrated that concretes with lower water to cement ratio perform better in terms of corrosion protection and process retardation.2. Experimental Study2.1. MaterialsType II Portland cement was used throughout the experiment. Total aggregate mass of about 1850 kg/m3 with a largest size of 19 mm were used. Also, to achieve desired workability of 7-10cm in different mixture designs, the use of poly carbixilate-based superplasticiser was necessary.2.2. Mixture proportionsFour mix proportions with different w/c ratios were used for casting the concrete specimens. The details of mix proportion are shown in Table1.2.3. Concrete specimensReinforced concrete specimens of size 550X300X200 mm were prepared each having three rebars (anode) at top and six rebars (cathode) at bottom embedded at 30 mm cover from both sides (Fig. 2). Concrete prisms of size 150X150X600 mm were made for determination of chloride penetration after nine months of exposure. Prism samples were also moist cured for 2 days and were sealed at all sides with polyurethane-based coating and only the top side of prisms was left uncoated to be exposed to chloride ions.2.4. Methods of measurementThe macro cell and micro cell corrosion were investigated in this study. For this reason, various methods of corrosion measurements such as macro cell, half cell and corrosion rate methods were used throughout the investigation.3. Results and discussion3.1. MacrocellThe variation of macrocell current with time was almost negligible for specimen C1. In specimen C2, there was no indication of corrosion initiation while the current in specimen C3 reached the threshold value of 10μA after 13 months. The anodic current of specimen C4 passed the threshold value after about 6 months of exposure to chloride ion. The unusual behavior of cell 1 in specimen C3 was due to the fact that the steel rebars might be slightly rusted at the time they were embedded in concrete.3.2. Halfcell Specimens C1 and C2 did not reach the threshold value, while the specimens C3 and C4 attained a more negative potential of -230 after 14 and 5 months of exposure, respectively. Results obtained from macrocell and halfcell potential measurements indicate that the rebars in specimens C1 and C2 have steel remained unaffected by corrosion whilst specimen C3 and C4 have reached the threshold value after 14 and 6 months of exposure, respectively. 3.3. Corrosion rateAt splash zone, even after 18 months of exposure, the corrosion rate of rebar in specimen C1 did not pass the threshold value required for the initiation of corrosion. In specimen C2, although the corrosion rate passed the threshold value but it followed a continues trend and there was no sudden changes in the rate of corrosion concluding that the corrosion had not started yet. But in specimen C3, sudden changes in corrosion rate confirmed the initiation of corrosion after 13 month of exposure. Specimen C4 experienced the corrosion process in month 9-11. It is interesting to note that after 12 month of exposure, specimen C4 showed a marginal increase of corrosion rate and the measurement of corrosion rate was not possible after that. 3.4. Critical chloride measurementThe chloride amount penetrated into concrete specimens was measured after 9 months of exposure and thus the bases for comparison of chloride content and variations in macrocell current, halfcell potential and corrosion rate for critical chloride determination were taken as 9 month’s measurements. The results are presented in Table 2. According to these results, the macrocell current and the halfcell potential methods are environmentally dependant, and therefore, these two methods have more accuracy in corrosion activity estimation at splash zone. The data from these studies clearly indicate that macrocell current and halfcell potential methods are practical when the corrosion activity has passed the threshold value otherwise they are not more accurate for the estimation of corrosion initiation time. However, comparing the chloride content and corrosion rate of specimens with different w/c ratio, it can be inferred that critical chloride content in concrete specimens is higher at splash zone with the estimated value of about 0.11 % of concrete weight at splash zone.4. Conclusion[1] The critical chloride content required for corrosion to initiate was found to be 0.11% of concrete weight at splash zone under environmental condition of Persian Gulf region.[2] Considering the test results, splash zone is found to be the most sever zone in terms of chloride ion penetration and corrosion activity.[3] According to the test results, the macrocell current and halfcell potential have acceptable accuracy in the determination of corrosion initiation time.[4] The use of high w/c ratio concretes in reinforced concrete structures in Persian Gulf region causes an accelerated corrosion and failure in those structures; and as mentioned earlier, the corrosion in concrete specimens with w/c ratio of above 0.4 and the cover of 30 mm is evident. Therefore, the modification of concrete structures by using pozzolans as cement replacement is necessary.