Key Engineering Materials Vols. 334-335 (2007) pp. 101-104 online at http://www.scientific.net at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland
Monitoring method of steel reinforcement corrosion rate in concrete Hiroyuki Saito1 1
Osaka University of Arts, Higashiyama, Higashiyama, Kanan, Osaka, Japan 1
[email protected]
Keywords: Reinforcement steel in concrete, Corrosion, Electrochemical method, Amount of water, Oxygen supply
Abstract. Steel reinforcement concrete is one of the most widely used composite materials in architectural and civil engineering. The alkaline of concrete makes steel passive state, so the steel bars in conrete are kept in anti-corroded phase. Carbon dioxide in atmosphere, however, chemically reacts to the alkaline components in long term, and the passive state of steel is missing. Steel corrosion in concrete is not visible but it may cause decaying buildings or civil construction. The author developed the electrochemical measurement method method of monitoring monitoring steel corrosion rate in concrete. With the measurement method, the author measured the variety of corrosion rates of steel reinforcement in several atmospheres, such as indoors, outdoors, in-water, underground, and so on. In this paper, found and analyzed mechanisms of steel corrosion in those atmospheres are mainly shown. For an interesting result, the most corroded sample is not the sample exposed outdoor, nor exposed in water, but the sample exposed in wet room. This may cause the diffusion balance of oxygen and water in concrete. Introduction
Steel reinforced concrete is one of the most popular materials for architects and civil engineers. With this material, Le Corbusier, one of the most famous architects in the 20th century, designed many grate works including National museum of western art built in Tokyo Japan [1]. His pupil Maekawa Kunio, Maekawa’s pupil Tange Kenzo and other Japanese architects have desiged a lot of concrete building works in world wide. The reinforcement steel bars, however, sometimes be corroded by some sorts of environmental factors. The corrosion of steel bars in concrete is one of the main causes of damage and early failure of reinforced concrete structures, buildings and architecture. Therefore measurement of corrosion rate of steel bars in concrete is important[2]. Since the equivalent circuit of wet-corroded metal generally consists of the polarization resistance and electrical double-layer capacity, corrosion rate of metal is measured by the electrochemical method which is called AC impedance analysis. The electrical double layer capacity of steel in concrete is, however, absolutely so large that measuring polarization resistance of that is less easy than that of other metals in ordinary conditions. The author used potential step method technique to solve the problem, and the author measured the corrosion rate of steel bars in concrete under several conditions[3]. Since finding out the determinative factor of steel corrosion in concrete is the essential first step of corrosion protection, measuring corrosion rate of steel in laboratory and measuring that in exposure test are both important. The author electrochemically and physically measured the corrosion rate in the outdoor, indoor, wet room, underground, and in-water conditions, and find out the two corrosion-control factors; total amount of water in concrete and oxygen supply to concrete. Experimental Electrochemical measurement. The equivalent circuit of metal corrosion consists of the polarization resistance and the electrical double-layer capacity. The schematic model is shown as Fig.1. To solve the problem of lengthy process of measuring the polarization resistance of steel in All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net www.ttp.net.. (ID: 64.76.110.6-28/03/07,18 64.76.110.6-28/03/07,18:18:23) :18:23)
102
Advances in Composite Materials and Structures
concrete, we used the potential step method, in which the capacity of the electric double-layer is electrically charged. Since, the value of the current after charging is proportional to the value of its derivative with respect to time, we can obtain the experimental current equation by measuring the current values and their derivative values for several charges [3]. The corrosion current of the corroded system is the value of this equation substituted for the derivative value 0. Then, the polarization resistance of steel in concrete can be calculated. This electrochemical meas urement was performed on the samples at least once a week.
Fig.1 The equivalent circuit of reaction Specimens. Steel bars, 9mm-diameter, and 250mm-length, were used. An electrical lead was connected to one end of the bar and covered with epoxy resin (Fig.2). A pair of bars was imbedded in a concrete block (25cm x 22cm x 12cm). The thickness of covering concrete was 5, 10, 20, and 30mm, the water/cement ratio was 55%, and no Chloride ions were included. The concrete blocks were kept in 90% carbon dioxide gas at 60 degree in centigrade for one month. Those blocks were then dipped in water one night and kept in one of several environments (outdoors, indoors, underground, in wet room or in water). Those treatments were carried out in Japan. Mass loss measurement. The steel bars were removed from the concrete blocks after one year exposure. We measured the mass of each exposed bar after being taken rust oxides away and substitute from that before exposed. The mass loss is determined as the difference of those two values.
Fig.2 Test sample
Key Engineering Materials Vols. 334-335
103
Results and discussion Current and its derivative . The measured relation between the value of the current and the value of its derivative with respect to time was obviously linear. Polarization resistance and concrete resistance . The polarization and concrete resistance for outdoor and indoor environments, as measured by the potential step method, in the indoor environment, concrete resistance increased and polarization resistance decreased monotonically, probably due to the concrete drying. In the outdoor environment, both the concrete resistance and polarization resistance were variable. Concrete resistance increased on non-rainy days, and decreased on rainy days. In the case of outdoor-exposed samples, concrete become wet condition at rainy day, and the resistance may be decreased. For the 5mm covered concrete, the corrosion rate was 10-25 micrometers par year. This is about 0.1 times as that of general corrosion cases, such as for bare steel bars in water or bare steel bars in outdoors . Corrosion rate, polarization resistance, and environment . The corrosion rates were ordered as indoors, outdoors, and in wet room, while the underground and in-water corrosion rates were not ordered. The relation of mass loss and the inverse of the mean value of the polarization resistance as measured by potential step method for indoors, outdoors, and wet room is shown in Fig. 3. The inverse is almost proportional to the actual mass loss. However, for underground and in water, a linear relation was not found. This is because in the former case, water trapped in the concrete block determines the corrosion rate, while in the latter case, the oxygen supply determines of the corrosion rate. Figure 4 schematically shows the relation of corrosion rate and amount of water in concrete.
Fig.3 Relation of actual mass loss and electrochemical value Summary
This work demonstrates that the potential step method can be used to measure the corrosion rate of steel and that the corrosion rate of steel in concrete is controlled by two factors of total amount of water in concrete and oxygen supply. The conclusions are as below: 1) Electrochemical method can be used to measure the corrosion rate of steel in concrete. 2) In the Indoor environment, concrete resistance increased and polarization resistance decreassed monotonically, due to the concrete drying. 3) In the outdoor environment, both the concrete resistance and polarization resistance are
104
Advances in Composite Materials and Structures
variable. Concrete resistance increased on non-rainy days, and decreased on rainy days. In the case of outdoor-exposed samples, concrete become wet condition at rainy day, and the resistance is decreased. 4) The corrosion rates are ordered as indoors, outdoors, and in wet room, while the underground and in-water corrosion rates are not ordered. 5) The reason of above phenomena is that water trapped in the concrete block determines the corrosion rate in former cases, while the oxygen supply determines of the corrosion rate in latter cases.
Fig.4 Corrosion mechanism model Acknowledgment
The author thanks Dr. Y. Miyata for his guidance of electrochemical methods. References
[1] H. Saito, Proc. 2006 Japan Institute of Architecture Shikoku-Chapter Conference. #29,AIJ (2006) (in Japanese) [2] H. Saito, Proc. 2006 Japan Institute of Architecture Shikoku-Chapter Conference.#1,AIJ (2006) (in Japanese) [3] H. Saito, Proc. 2003 Asia-Pacific Corrosion Control Conference,p.5,APCCC (2003)
