IJCRR - 8(12), June, 2016
Pages: 14-20
Date of Publication: 20-Jun-2016
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EFFECT OF CORROSION ON THE MECHANICAL PROPERTIES OF STEEL REINFORCEMENT
Author: Ponjayanthi D., Vinodh K.R.
Category: Technology
Abstract:Reinforced concrete structures in coastal areas under goes severe damage due to corrosion of reinforcement during its design life. The gradual degradation of the structural integrity due to corrosion results in considerable financial burden. Such structure when subjected to seismic action undergoes severe damage. Under the action of seismic loads, the reinforcement bars are subjected to tension and compression. Hence in present study, the effect of corrosion on the mechanical properties of reinforcement is studied. In compression test there is no significant reduction in average ultimate stress and average ultimate load for corrosion level of 5% for specimens with L/D ratio of 10 and 15. The reduction in average ultimate load is not proportional to the level of corrosion. It is also observed that the percentage decrease in average ultimate load is higher than the level of corrosion. It could be noted that the average ultimate stress decreases slightly lesser than the level of corrosion. The effect of L/D ratio affects both average ultimate load and ultimate stress significantly. In cyclic test the maximum load decreases slightly lesser than the level of corrosion. It is also observed that the percentage decrease in maximum stress in cyclic test is lesser than the level of corrosion in tension and it is higher than the level of corrosion in compression. A huge reduction in both mean load and mean stress beyond 0.5% of strain in compression is observed in cyclic test.
Keywords: Reinforced concrete, Corrosion, Severe damage, Ultimate load, Ultimate stress
Full Text:
INTRODUCTION
Reinforced concrete structures are durable and strong, hence are versatile. However, reinforced concrete exhibit structural distress due to inadequate design, substandard construction practice and environmental effects or a combination of above factors. Corrosion of reinforcing bars is the most common environmental effect that results in premature deterioration of reinforced concrete structures. The diameter of the steel bar diminishes causing reduction in cross section and thereby affecting the load carrying capacity of member. Corrosion reduces the ductility of the steel and energy absorption capacity of steel significantly (Du et al. (2005), Almusallam (2001), Palsson and Mirza (2002). It is well known reinforced concrete structures in coastal areas; under goes severe damage due to corrosion of reinforcement during its design life. The behavior of corroded rebar under tension is well understood. Hence in present study, the effect of corrosion on the mechanical properties of reinforcement is studied.
METHODOLOGY
Corrosion of steel in concrete is a slow process. Due to protective nature of concrete, it takes reasonably a long time for initiation and progress of reinforcement corrosion even in the case of severe exposure conditions which is too long for laboratory studies. It is difficult to achieve a significant degree of corrosion in reinforcement within a limited duration (Shamsad Ahmad, 2009). Thus for laboratory studies, the corrosion is accelerated. Acceleration corrosion process is a technique based on the fact that corrosion process is activated by Chloride ions and accelerated by electrical polarization of the reinforcement steel. For present study the rebars are subjected to three different levels of corrosion viz, 5%, 10% and 20% of weight loss over the gauge length and also the L/D ratio of the specimen are varied as 5, 10 and 15. Three specimens are tested for each case hence totally 36 specimens of 8mm diameter are tested in compression. The details of test specimens are given in table I
ACCELERATED CORROSION PROCEDURE
Accelerated corrosion is carried out by connecting reinforcement to the positive terminal (Anode) of the DC pack and the stainless steel plates to the negative terminal (Cathode) of the DC pack of 32V and immersing reinforcement rod in 3.5% NaCl solution. The time taken for inducing particular level of corrosion is estimated based on Faraday’s law of electrolysis and the typical accelerated corrosion test setup is shown in figure 1.
Typical calculation to estimate time required for corrosion:
EXPERIMENTAL PROGRAMME COMPRESSION TEST ON REBARS
The compression tests are carried out on Compression Testing machine of 2000kN capacity in load control mode. The steel specimen are mounted between the grips and subjected to monotonic increase in loading till failure. The initial lengths between the grips are noted and displacements of the ram during the test are measured using LVDT. The applied loads are monitored using load cell. Figure 2 gives the typical test setup. The load data from load cell and LVDT are acquired using data logger (MGC+) at the sampling rate of 50Hz. The load is increased monotonically till the failure. Totally 36 numbers of specimens are tested under compression on 8mm diameter by varying L/D ratio and different level of corrosion. Three L/D ratios are chosen for study are 5, 10 and 15 and are subjected to three levels of corrosion (5%, 10% and 20%). It is also observed that with increase in level of corrosion the buckling load decreases. It is observed that for L/D ratio of 15 the influence of corrosion decreases the load carrying capacity drastically. Hence, cyclic tests are carried out on failure pattern of specimen with L/D ratio 15. 16mm diameter bars with L/D ratio of 15 and subjected to three levels of corrosion namely 5%, 10% and 20%.
CYCLIC TEST ON REBARS
A total of 12 specimens are subjected to reverse cyclic loading for L/D ratio of 15 with three different levels of corrosion. Cyclic test carried out on servo controlled Universal Testing machine of capacity 250kN with a grip length of 80mm as shown in figure 3. The displacement and load are recorded using data acquisition system (MGC+). The specimens are subjected to reverse cyclic loading in displacement control mode. The displacement histories are calculated as percentage strain shown in figure 4. Three cycles of each displacement are applied. All the specimens failed at the centre except one specimen with 20% of corrosion.
RESULTS
OBSERVED COMPRESSION TEST RESULTS
Average ultimate stress and load observed in compression tests are given in Table II. The ultimate stresses reported are calculated as load divided by reduced area due to corrosion. The reduced area is obtained by calculating diameter of the rebar at ten locations over the gauge length by digital vernier caliper are shown in Table III. The mean of ten readings are recorded as the diameter of the reduced area due to corrosion. Variation in ultimate stress and ultimate load with respect to different L/D ratio for each level of corrosion is shown in figure 5 to 10.
OBSERVED CYLIC LOAD–DISPLACEMENT RESPONSE
The mean load- displacement graphs observed from cyclic tests are shown in following figure 11 to figure 14.
OBSERVED CYCLIC STRESS-STRAIN RESPONSE
The mean stress verses strain observed from the cyclic tests are shown in following figure.15 to figure.18.
CONCLUSIONS FROM COMPRESSION TEST ON REBAR`
• It is observed that there is no signification reduction in average ultimate stress and average ultimate load for corrosion level 5% for specimens with L/D ratio of 10 and 15.
• It could be observed that reduction in average ultimate load is not proportional to the level of corrosion. It is also observed that the percentage decrease in average ultimate load is higher than the level of corrosion
• It could be noted that the average ultimate stress decreases slightly lesser than the level of corrosion. For example, for 20% level of corrosion the average ultimate stress is reduced by 9.1% and 9% for L/D ratio of 10 and 15.
• The effect of L/D ratio affects both average ultimate load and ultimate stress significantly.
FROM REVERSE CYCLIC TEST ON REBAR
• It could be noted that the maximum load decreases slightly lesser than the level of corrosion. It is also observed that the percentage decrease in maximum stress is lesser than the level of corrosion in tension and it is higher than the level of corrosion in compression.
• It is observed that there huge reduction in both mean load and mean stress beyond 0.5% of strain in compression.
ACKNOWLEDGEMENT
Apart from the efforts of me, the success of this project work depends on the encouragement and guidelines of many others. I take this opportunity to express my gratitude to the people who have been instrumental in the successful completion of this project. I am happy to express our proud reverence and deep sense of gratitude to my external guide Ms. A. Kanchanadevi, Scientist, ACTEL, CSIR-SERC, Chennai. I greatly benefited under the invaluable guidance, efficient encouragement, and informative suggestions. Authors acknowledge the immense help received from the scholars whose articles are cited and included in references of this manuscript. The authors are also grateful to authors / editors / publishers of all those articles, journals and books from where the literature for this article has been reviewed and discussed
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