The ICCP-SS technique for retrofitting reinforced concrete compressive members subjected to corrosion
Introduction
A variety of structures can be supported by reinforced concrete (RC) columns, including bridge decks and floor slabs. Columns may function as piers or piles, either above or below water level. While columns may vary in shape depending on their usage, circular-sectioned columns are typically used for ease of construction. Such columns may be subjected to corrosion issues due to a few reasons: some of the bridges are built by using sea-sand sea-water concrete; some are located in marine environments; some are exposed to winter deicing salts. Structural deterioration and a shortened lifespan of RC structures can be caused by the corrosion of embedded re-bars [2], [3], [4], [5].
Over the past 50 years, a number of technologies have been developed to tackle the corrosion of steel reinforcement in concrete. The most effective method of reducing or eliminating ongoing re-bar corrosion in RC structures is through cathodic protection (CP), especially in cases caused by chlorides [6], [7], [8]. The selection of a suitable anode for the system is an important consideration in impressed current cathodic protection (ICCP) design, especially when it is to be used in reinforced concrete structures with their high resistivity [9]. Conductive carbon loaded paints, coated titanium expanded mesh or mesh ribbon in concrete overlays, and internal conductive ceramic Titania or coated titanium ‘discrete’ anodes are among some of the anodes currently used for CP systems. While some anodes are impractical due to their high cost, carbon fiber mesh is found to be a promising cathodic anode from a recent feasibility study [10], [11]. However, it should be noted that the adoption of ICCP cannot recover the strength loss due to the existing corrosion of re-bars.
One of the most widely used techniques to improve the loading resistance of RC columns is to provide additional confinement to them. Rehabilitation of RC columns by external strengthening material has been extensively studied in the past few decades [12], [13]. One of the most popular strengthening materials is fiber reinforced polymer (FRP). A great number of studies on FRP strengthening have been carried out [14], [15], [16], [17] because of its ease of formation, light weight, high strength, and relatively low cost. Previous researchers found that strengthened RC columns could have their loading capacities enhanced significantly. These beneficial effects are achieved because the confinement adds to the rigidity of the concrete column by preventing lateral expansion under axial load [18].
There has been only limited research into the combination of these two remedial methods, which provide both strengthening and cathodic protection by using the electrically conductive properties and excellent durability of carbon fiber mesh [9], [19]. Typical applications and most of the studies deal with either the ICCP system [6], [7], [8] or the strengthening of RC members [12], [13], [14], [15], [16], [17]. Little data [9] exists for cases when ICCP is combined with the structural strengthening (SS) provided by anodes of carbon fiber mesh, while experimentation is required to study structural behavior and to optimize design procedures. The ICCP-SS dual-functional retrofitting technique for RC columns is a subject of ongoing research and development.
In the ICCP-SS system, both carbon fiber mesh and adhesive material have great impacts. Feasibility studies on both materials have been conducted by the authors [19], [20], [21], [22]. In our previous studies, the behavior of carbon fiber mesh in the following three environments has been investigated: (1) the actual concentrations of the pore water components [19]; (2) the chlorine evolution environment [23]; and (3) the oxygen evolution environment [20]. It was found that the degradation of carbon fiber mesh in the chlorine evolution environment is much more serious compared to the actual concentrations of the pore water components and the oxygen evolution environment. Furthermore, in order to simulate real cases, the performance of carbon fiber mesh in varying chloride concentrations has also been investigated [21], [22]. The test results indicated that the degradation of carbon fiber was more significant in the lower chloride content solution with higher current density. In light of the test results and micro-structural mechanism analysis, promising conclusions on the residual strength and the service life of carbon fiber mesh after polarization were drawn [14], [15], [16], [17]. The service life of carbon fiber mesh can be more than 40 years even in serious polarization condition. As for the adhesive material used for bonding carbon fiber mesh, literature can be found on both organic and inorganic material. In the study on the ICCP-SS technique conducted by Revie and Uhlig [6], epoxy resin was initially employed as the adhesive material; later, in order to improve the bonding behavior, a combination of geopolymer and epoxy was proposed and used in the experimental program [24]. Meanwhile, in recent years, some investigations have been focused on a cement-based composite system [25], [26], [27]. Cementitious material provides excellent resistance to fire and high temperature, as well as good mechanical performance. Zhu et al. [21], [22] also presented the ingredients of a cementitious material, which were found to have great average bonding strength and a favorable failure mode in single shear tests.
This study proposes a dual-functional retrofitting technique for RC columns subjected to corrosion, which is termed as impressed current cathodic protection – structural strengthening (ICCP-SS). Both the anodic material and the strengthening material in the ICCP-SS system are carbon fiber mesh. A novel modified cement-based inorganic cementitious material is adopted as the adhesive material. In this paper, an experimental program of circular RC columns is studied in order to examine the effectiveness of the ICCP-SS technique on RC columns. The columns were constructed with typical internal steel reinforcement in order to simulate the practice of retrofitting in-situ damaged columns. In addition, the beneficial effects of the ICCP-SS technique are shown by comparison with reference columns, i.e., columns without any repairing treatment. Finally, different design codes for confined concrete columns are used to predict the design capacities of the experimental specimens.
Section snippets
Experimental program
An experimental program that included the testing of 10 reinforced concrete stub columns was carried out in the structural laboratory of Shenzhen University. A certain amount of NaCl was contained in the concrete mix to introduce accelerated corrosion on the test specimens. The main target of the experimental program was to establish a database compiled from the experimental results.
Results of ICCP
During the 100-day operation of the ICCP, the open circuit potential values of the re-bars of all the specimens were recorded during the wet cycles and plotted in Fig. 6. In accordance with the recommendations of Concrete Society Technical Report number 73 [31], if the open circuit potential value is greater than −126 mV, it demonstrates that the embedded steel has only 10% chance of being corroded; if the open circuit potential value is less than −275 mV, it demonstrates that the embedded
Discussion on loading capacities
As summarized in Table 5, the corroded columns not subjected to any repair methods (CO-C and CO-C-R) have the lowest compression capacities. It can be seen that the accelerated corrosion process is effective, and the corrosion of re-bars has caused the deterioration of the RC columns [37], [38]. For all repaired columns, an increase in the load-carrying capacity was observed compared to the unrepaired corroded ones. However, the effects of different current densities are not distinct in the
Conclusions
This study tested 10 axially loaded RC columns with/without carbon fiber mesh external confinement in order to investigate the performance of columns repaired by ICCP, SS or ICCP-SS techniques. The experimental program included the accelerated corrosion procedure, the ICCP operation, and the compression tests. Carbon fiber mesh acted as both the anode and strengthening material in the impressed current cathodic protection – structural strengthening (ICCP-SS) system. Experimental results showed
Acknowledgements
The research work described in this paper was supported by the National Natural Science Foundation China (Project No.: 51538007, 51478269, 51508336) and the Natural Science Foundation of SZU (Grant no. 2016068).
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