Influence of reinforcement mesh configuration for improvement of concrete durability

Steel bar in concrete structures under harsh environmental conditions, such as chlorine corrosion, seriously affects its service life. Bidirectional electromigration rehabilitation (BIEM) is a new method of repair technology for reinforced concrete structures in such chloride corrosion environments. By applying the BIEM, chloride ions can be removed from the concrete and the migrating corrosion inhibit can be moved to the steel surface. In conventional engineering, the concrete structure is often configured with a multi-layer steel mesh. However, the effect of the BIEM in such structures has not yet been investigated. In this paper, the relevant simulation test is carried out to study the migration law of chloride ions and the migrating corrosion inhibitor in a concrete specimen with complex steel mesh under different energizing modes. The results show that the efficiency of the BIEM increases 50% in both the monolayer steel mesh and the double-layer steel mesh. By using the single-sided BIEM, 87% of the chloride ions are removed from the steel surface. The different step modes can affect the chloride ion removal. The chloride ions within the range of the reinforcement protective cover are easier to be removed than those in the concrete between the two layers of steel mesh. However, the amount of migrating corrosion inhibitor is larger in the latter circumstances.


Introduction
Corrosion of embedded steel in concrete closely relates to the durability of reinforced structures. Real et al. (2017) and Castro and Ferreira (2016) proposed that the steel in hardened concrete is protected by a passivating film. Researches of Vedalakshmi et al. (2011) and Marcotte et al. (1999) illustrate the importance of concrete durability with "five times the law". The reinforcing steel in concrete structures exposed in a marine environment or suffering de-icing salt may corrode when the chloride concentration around the steel bar reaches a threshold value, resulting in degradation or even early structural failure. Therefore, the steel of chloride contaminated reinforced concrete structures must be protected from corrosion in the service life.
Owing to the long period in harsh environments, reinforced concrete structures bearing durability becomes a critical issue. The corrosion of steel bars caused by the environment has become the main reason for the failure of reinforced concrete structures, for example, Carvajal et al.  Yu et al. (2017) studied that chloride erosion and concrete carbonation are the main factors causing corrosion to steel bars. When reaches the steel surface, it damages the concrete inside the surface of the passive film. Muthulingam and Rao (2015) and Michel et al. (2013) reported the similar effect for the formation of corrosive batteries, under the action of depolarization caused by the formation of the reinforced concrete corrosion. On the other hand, when the concrete is not fully saturated in water, part of the metastable material (C 3 A) undergoes a chemical reaction leading to volume expansion of the complex salt. As a result, the internal stress of the concrete changes and destruction of the original structure occurs (Wang et al., 2015;Muthulingam and Rao, 2015;Zhao et al., 2016;Val and Chernin, 2012). The failure of the reinforced concrete structure, which is affected by the chloride salt, is mainly manifested as surface rust of the concrete, the exposed bar, the violent tendon, the concrete caking crack, hollowing, and peeling (Michel et al., 2013;Miranda et al., 2006). Electrochemical chloride extraction (ECE) is an effective method for the electrochemical repair of chloride-containing concrete structures. However, it can only remove chloride ions penetrating into the concrete and cannot re-protect the bars (Jin et al., 2016;Ormellese et al., 2011). Ormellese et al. (2011), Huang et al. (2012, and Amareanu and Melita (2016) tried to use inhibitor to prevent the corrosion of the reinforcement. Under an electric field, the migration of the migrating corrosion inhibitor action to the steel bar can protect the steel, but unfortunately there is little study on the removal of the chloride ions (Hu et al., 2013). Jin et al. (2016) proposed a two-way electroosmosis technology based on the electrochemistry principle. This method combines the electrochemical dechlorination method and the electromigration inhibitor method. By applying this method, the introduction of external induced corrosion inhibitors on the reinforced active protection is eliminated (Yan et al., 2012). Research on electrified time, current density, water binder ratio, initial chloride content and the thickness of the protective layer of the two-way electroosmotic short-term effect of single steel specimens has been conducted for bidirectional electroosmosis technology (Xu et al., 2016b).
Chloride ion corrosion of concrete structures occurs mainly in the chloride layer. With the electrochemical method, the chloride ions of this part can be easily removed. For structure with desalinated sea sand buildings, chloride ions are distributed uniformly in the whole structure. The configuration of multi-storey reinforced net parts should be reinforced between parts of the chloride ion discharge. The double layer reinforced concrete dechlorination effect has been investigated by our research group (Xu et al., 2016b). The result reveals that the electrochemical dechlorination of the reinforced protective layer and between the two layers of the steel mesh parts have a certain effect on chlorine removal. Based on this preliminary research, we conduct experimental study on the single-sided and double-sided BIEM with reinforced concrete. Based on the study of different BIEMs on the electroosmotic effect of the specimen, the result is further applied for investigation of concrete durability.

Basic principle of BIEM
The principle of the BIEM is shown in Fig. 1. It includes power, cathode and anode systems, as well as a DC stabilized power supply. The cathode is the concrete inside the steel. The anode system includes rust-proof fluid in place, an anode metal plate, and rust. An electric field is formed between the anode metal plate and the cathode steel bar. Under the electric field, the chloride ions in the concrete migrate to the concrete surface and dissolve in the an-ode rust. The cation in the anode resistive solution migrates into the concrete. It crosses the concrete protective layer, and attaches itself to the surface of the steel by physical and chemical adsorption (Yan et al., 2012). The concentration of migrating corrosion inhibitor in the steel surface reaches a certain value. The steel surface then forms a layer of dense protective film, which can protect the rebar from chloride ions, oxygen, and other corrosive media and steel isolation, essentially preventing rust. In addition to the electromigration of ions, the electrolytic reaction occurs on the steel surface simultaneously, which can produce OH -, causing an increase in the alkalinity near the reinforcement. It is conducive to the re-passivation of steel.

Design of concrete specimen
The aim of the experiment is to test and study the concrete samples with both a monolayer steel mesh (S O ) and a double-layer steel mesh (S T ). The length and the width of the specimen were both 800 mm, the thickness of S O was 120 mm (the thickness of the protective layer was 55 mm), and the thickness of S T was 180 mm (the thickness of the protective layer was 40 mm). P·O 42.5 cement of the three lion brand, rubble with a particle size of 5.0-18.0 mm, wellgrained thick river sand, and tap water for mixing the materials were employed in this experiment. The specimen was poured and added to the chlorine salt, which accounted for 5% of the cement quality. The initial chloride ion concentration obtained was 2.7204% (percentage of the cement mass). Reinforcement was completed using HRB400 rebar with a longitudinal reinforcement diameter of 14 mm, a distributed steel diameter of 12 mm, and a reinforced spacing of 160 mm. The mixture proportions of the concrete and steel mesh configuration are shown in Table 1 and Fig. 2.

Process design
In order to comprehensively investigate the migration  S T-II ). Triethylene tetramine (TETA) inhibitor solution acted as the electrolyte solution and the plexiglass frame in full bloom guaranteed its long-term storage (Fouda et al., 2015). The reinforced concrete was used as the cathode with the stainless steel plate as the anode. The concrete was then energized and the current density was set to 3 A/m 2 , as shown in Fig. 3.
The energization duration was 15 days. During the test, the two-way infiltration device voltage, current, resistance, and other parameters were recorded in real time. The setup is shown in Fig. 4.
After power off, the chloride ions in the rust-resistant solution and the device on the specimen surface were removed. After being dried under natural conditions, the powder layer with the thickness of 5 mm of the specimens is drilled using a 12-mm drill. For the specimens S O-I and S O-II , the powder was taken from the surface to the steel. For the test pieces S T-I and S T-II , the powder was taken from the surface to the middle of the specimen. The powder was separated by a mesh screen. The powder with the particle size   PAN Chong-gen et al. China Ocean Eng., 2017, Vol. 31, No. 5, P. 631-638 633 of 0.075 mm or less was tested with a FLASH EA1112 type element analyzer. The nitrogen element (N element) was measured and the concentration of the migrating corrosion inhibitor in the concrete was calculated. After leaving the powder with a particle size distribution of 0.075-0.3 mm for 24 hours in deionized water, the chloride ion concentration in the powder was measured by using a TR-ClA2501B type chloride ion rapid tometer (Choi et al., 2017).

Variation of electrical resistance
The electric current is constant in the electricity process. The resistance of the concrete between the steel bar and the anode increases with the increase of the power supply time, as shown in Fig. 5.
The results show that the porosity of the concrete protective layer decreases as the density increases. Because the two sides of the specimen on both sides of the electrification device are connected in parallel, the resistance of doublesided electroosmotic is half of the one-sided electro-osmotic resistance. Thus, the resistance of the steel, the anode, and the thickness of the protective layer must be considered. The larger thickness of the protective layer leads to the larger resistance. During the energization process, the initial phase of the resistance increases gradually. On the fifth day, the resistance experiences a quick rise. But the increasing trend of the resistance appears to lessen at the 12th day. This indicates that the stabilization of resistance increases. Therefore, the 15-day dechlorination time is reasonable.

Changes of chloride concentration in inhibitor
Being continuously electrified, the chloride ions in the concrete continue to migrate to the external anode and finally move to the inhibitor solution in free form. The con-centration of chloride ions in the rust can reflect the exclusion of concrete within the chloride. After shutting down the power supply, the chloride ion concentration of the internal corrosion inhibitor in the external electroosmotic device is measured with the TR-ClA 2501B chloride ion instrument. The result is shown in Table 2.
It can be seen from Table 2 that the chloride ion concentration in the single-sided BIEM and the double-sided BIEM resisting fluid is high after being electrified for 15 days. This indicates that the chloride ion exclusion effect in the concrete is working. This can be verified by detecting the residual chloride ion concentration in the concrete.

Residual chloride distribution of concrete specimen
After the power supply ended, the specimen is static cooling down for one day. The powder samples are taken from the longitudinal reinforcement, the distributed rib and the blank space of the steel bar. The sampling positions are shown in Fig. 6.

Residual chloride concentration of monolayer
The residual chloride ion concentration at the sampling position of the monolayer steel mesh is shown in Figs. 7 and 8. And the chloride distribution of concrete were also performed by Fouda et al. (2015) and Xu et al. (2016a).
It can be seen from Figs. 7 and 8 that the chloride ions in the single-sided and double-sided specimens after BIEM are less than the initial chloride ion concentration of 2.7204%. Under the electric field, the chloride ions migrate towards the concrete surface and the chloride ion concentration in the protective layer decreases (Vořechovská et al., 2010). The chloride ion concentration at the longitudinal reinforcement is 1.3272% with the removal rate larger than 50%. However, the large residues of chloride ions are found in the middle of the protective layer (20-45 mm from the surface), which is mainly due to the thicker concrete cover (55 mm). The chloride ions in the thicker concrete are difficult to be moved, resulting in the accumulation of chloride ions in the intermediate protective layer portion.
Single-sided BIEM compared with the double-sided electroosmosis can also be inside the specimen and inside the steel chloride ions. However, the effect of chlorine removal on one side steel mesh and the effect of double-sided electroosmosis are slightly worse than those in single-sided electroosmosis. The main reason is that the chloride ion migration due to one side electric field is balanced by the other side electric field. For double-sided electroosmotic specimens, the chloride ion migration model is shown in Figs. 9 and 10.

Residual chloride concentration of double-layer steel mesh
The concentration of chloride ions in the test sampling position of the specimen with double-layer steel mesh is shown in Figs. 11 and 12.
From Figs. 11 and 12, it can be observed that the effect of chloride ion migration is decent in the concrete protective layer (40 mm) of single-sided or double-sided BIEM. The effect of chloride ion migration in the steel bar is better than that in the steel mesh. In the area between the two steel layers, due to the weak electric field strength, the chloride ion migration volume is relatively small. But it still shows a certain chlorine removal effect. In our experiment, the energization time can be increased to discharge the chloride ions in the middle of the double-layer steel mesh.
The surface of the steel mesh on the test piece is the same as that of the monolayer steel mesh. The remaining amount of chloride ions in the double-layer mesh specimen    after double-sided electroosmosis is slightly higher than that of the single-mesh specimen. For double-sided electroosmotic specimens, the two-layer reinforcement mesh can be used as the cathode. The chloride ion migration model can be represented by Fig. 11.

Analysis of chlorine removal efficiency
The chlorine removal efficiency of S O-I , S O-II , S T-I and S T-II steel bars is shown in Table 3.
Whether the monolayer or the double-layer steel mesh specimen is being investigated, the chlorine removal efficiency with BIEM exceeds 50%. Compared with doublesided electroosmosis, the dechlorination efficiency of single-sided electroosmotic is higher, up to 87%. The efficiency of chlorine removal on the surface of the monolayer steel mesh is lower than that of double-layer steel mesh. The main reason is that the thickness of the protective layer of single-layer steel mesh is too large for the chloride ions to be removed.

Analysis of immigration of rust inhibitor
Element concentration at different depths in the specimen is analyzed. The migrating corrosion inhibitor concentration converted is shown in Figs. 13 and 14.
As shown in Fig. 13, for the monolayer steel mesh specimen after the infiltration for 15 days, the concentrations of migrating corrosion inhibitor in single-sided and doublesided steel surfaces are almost identical. For the monolayer steel mesh test piece, the number of electroosmotic planes has little effect on the amount of inhibitor.
As can be seen in Fig. 14, the corrosion inhibitor migrates into the concrete cover to the cathode under electric field action. The chloride ions in the concrete migrate out of the concrete to the anode. The corrosion inhibitor forms a protective film around the embedded steel bars and isolates the corrosive substances, such as chloride and oxygen. However, in the steel protective layer, the migrating corrosion inhibitor concentration decreases as the depth increases. The peak occurs in the back of the steel position. This is mainly because the two layers of steel fabric and the parts between them can be regarded as the cathode. The electric field at one side of the steel network has a positive effect on the process of the corrosion inhibitor on the other side migrating to internal concrete. It produces an addition-    al electric field to promote its migration to the internal concrete. This phenomenon is more pronounced when conducting double-sided electroosmosis.

Conclusions
In this paper, the influence of steel mesh configuration on the durability of concrete is tested with the BIEM. The migration rule of the chloride ion and the migrating corrosion inhibitor in both single-layer steel mesh and doublelayer steel mesh are investigated. The following conclusions can be drawn.
(1) BIEM treatment on the specimen is more conducive to remove chlorine on both sides of steel. However, in terms of the efficiency of removing chlorine from one side of the steel, the single-sided electroosmosis is better than the double-sided.
(2) The effect of chloride ion migration in the steel bar of the monolayer steel mesh is better than that of doublelayer. The chloride within the protective layer of doublelayer steel mesh specimen is excluded more than that of monolayer. However, the double-layer steel mesh enables the anti-inhibitor agent more facile movement. The chlorine removal efficiency of the BIEM achieved more than 50%. It can be enhanced by an appropriate increase in power to im-prove the sample chloride ion exclusion rate during the experimental process.
(3) After the BIEM treatment, the effect of the migrating corrosion inhibitor in the specimen with monolayer steel mesh or double steel mesh indicates is obtained. It reveals that the concentration of migrating corrosion inhibitor decreases as the depth increases with the peak in the back of the steel mesh. Meanwhile, it can be used to simulate ion transport by increasing the reinforcement ratio, so as to study the effect of multilayer electroosmosis.   PAN Chong-gen et al. China Ocean Eng., 2017, Vol. 31, No. 5, P. 631-638