1 Introduction
Reinforced concrete structures play a vital role in the infrastructure systems around the world, but rebar corrosion is a major and increasing problem [
1–
5]. The rehabilitation of concrete bridges necessitated by chloride-induced rebar corrosion was estimated to cost US highway departments $5 billion per year [
6]. Cathodic prevention and cathodic protection (CP) are proven electrochemical techniques that can effectively prevent or mitigate rebar corrosion in concrete [
7–
11]. The Oregon Department of Transportation (ODOT) has employed impressed current cathodic protection (ICCP) to greatly reduce the corrosion of embedded steel rebar of existing major historic coastal bridges, with nine CP systems on decks, 11 on superstructures, nine on caps, and seven on columns [
12,
13]. The CP systems rely on passing an electric current into the concrete through zinc (Zn) anodes that have been arc sprayed onto the concrete surface.
Arc sprayed Zn anode on concrete surfaces has been an emerging technology for protecting reinforced concrete structures from rebar corrosion in coastal environments. In 1987, Apostolos et al. first used arc sprayed Zn as an impressed current anode for a structure maintained by the California Department of Transportation [
14]. After that, ODOT installed an ICCP system featuring arc sprayed Zn coating as the anode to protect the steel rebar in concrete on the 10,000-m
2 substructure of the Yaquina Bay Bridge, which is still one of the largest single substructure CP projects ever undertaken in the US According to McGill and Shike [
13], the “arc spray process was selected as it provided a coating that could be easily applied to the complex shapes found on substructure surfaces…. The gray color of Zn has the advantage of appearing very much like concrete—another important feature for historic bridges. Also, the low electrical resistivity of Zn allows uniform distribution of CP current, and the Zn system minimizes the dead load added to the structure, which is an important feature for older coastal bridges.”
To obtain improved understanding of the performance and service life of arc sprayed Zn anode, the National Energy Technology Laboratory in Albany, Oregon conducted accelerated electrochemical aging in the laboratory using a current density of 3 mA/ft
2 (0.032 A/m
2, a factor of 15 higher than the approximately 0.2 mA/ft
2 used by ODOT on coastal bridges). Electrochemical aging caused chemical and physical changes at the Zn–concrete interface, including acidification, oxidation, and secondary mineralization leading to its failure [
15]. The authors also reported that while preheating the concrete significantly improved the initial Zn bond strength to concrete, the beneficial effects disappeared after electrochemical aging of more than 200 KC/m
2 (5.2 A-h/ft
2, equivalent to three years of typical ODOT ICCP operations). The service life of arc-sprayed Zn was estimated to be approximately 27 years based on the bond strength measurements in accelerated ICCP tests. It was recommended to eliminate the supplemental heating of concrete surface and to reduce the Zn coating thickness from 20 to 10 mils (500 to 250
mm) since only 3.4 mils were expected to be consumed from electrochemical reactions in 27 years of ODOT ICCP operations [
15].
While initial bond strength between Zn and concrete does not always guarantee long-term performance of the CP system, it is an important initial condition to be achieved (as shown in Fig. 1). In practice, the ODOT-approved procedures use the initial Zn-concrete bond strength as a key parameter for quality assurance of arc spray Zn operations. The concrete surfaces are generally not wet or damp since they tend to be kept above 80°F due to the use of a heated main closure to contain the Zn-spray operations. A weed burner is typically used to achieve appropriately low moisture levels for isolated concrete areas. The target thickness of sprayed Zn falls in the range of 15 to 20 mils (375–500
mm) to ensure that the entire concrete surface (despite its roughness) is fully coated with Zn, which takes at least six passes of Zn spraying. Additional passes are needed for rough and irregular concrete surfaces. The SSPC-SP 13 details preparation procedures of concrete surfaces, inspection procedures and acceptance criteria prior to the application of protective coating or lining systems [
17]. For conductive coating such as Zn anode, a surface profile with appropriate anchor pattern, minimum moisture content, minimum bond-inhibiting substances (e.g., dust and oils), and adequate bond strength to the concrete substrate are the key to achieving desired performance of the CP system and long service life of the new anode. Profile is important because an irregular surface allows the coating to grip and affects the bond strength of the new anode to the concrete. The profile is also expected to affect the evolution of the anode–concrete interfacial chemistry over the duration of electrochemical aging. The adhesion of arc sprayed Zn to concrete was found to be mainly governed by the mechanical interaction of molten Zn droplets with the surface, and the root mean square (RMS) roughness obtained from a depth profile was the main parameter that can be related to the bond strength [
18].
Many CP systems with arc sprayed Zn anodes will reach or exceed their design life in the near future and thus may function improperly or insufficiently, making it necessary to replace the aged anodes. When the rate of active corrosion resumes, the unprotected sections are on the path to concrete spalling and steel section loss- conditions that required agencies to undertake expensive repairs and protection schemes. Currently, there is no procedure established by ODOT or other agencies to guide the proper removal of old Zn anodes or the sandblasting (profiling) of old or new concrete surfaces. A preliminary study by the authors [
19] suggested achieving an optimum surface with a micro-roughness of 3.5–5 microns (0.014–0.02 centi-inches) and a moderate level of exposed rock in order to ensure high initial bond strength of new Zn to concrete. For concrete protected by a Zn layer with more than 8 years of electrochemical aging, the bond strength would be better if the old reaction layer were completely removed by 4 mm grinding before profiling and arc spraying.
In this context, this work aimed to address questions underlying the preparation of new concrete surfaces and replacement of arc sprayed Zn anodes on cathodically protected steel reinforced concrete bridges and to develop criteria to prepare old or new concrete surfaces for the new anode. The Yaquina Bay Bridge, an arch bridge spanning Yaquina Bay south of Newport, Oregon, had a CP system installed in 1994, and several sections had prematurely failed. One of these sections was the entire surface of Pier 9 on the south end of the bridge, which was used for the field evaluation in this work. In addition, a number of experiments were conducted in the laboratory, followed by modeling analyses of laboratory and field data using artificial neural networks.
2 Methodology
2.1 Artificial neural network modeling
To study the complex cause-and-effect relationships between the potential influential factors and the initial Zn–concrete bond strength, artificial neural networks (ANNs) were elected as a modeling alternative to establish predictive models. ANNs are powerful tools to model the nonlinear cause-and-effect relationships inherent in complex processes where conventional modeling techniques (e.g., multiple regression) fail, as demonstrated in our previous works [
20–
23]. This is generally accomplished without knowing the form of the predictive relationship
a priori [
24]. A three-layered feed-forward neural network with an input layer, a hidden layer, and an output layer, as depicted in Fig. 2, was adopted in this study. The layers consist of several neurons and interconnected by sets of correlation weights. The neurons produce an output based on some function of a linear combination of outputs from neurons in a previous layer. A common transfer function is the sigmoid function expressed by
f (
x) = 1/(1+
e−x). In this study, a modified back propagation (BP) algorithm was employed for the ANN training, during which the interconnection weights are adjusted with an error convergence technique to obtain a desired output for a given input. The error at the output layer propagates backward to the input layer through the hidden layer to obtain the final desired outputs; a gradient descent method is used to adjust the weight of interconnections to minimize the output error. A thorough treatment of the BP ANNs is beyond the scope of this paper, and the detailed description of data normalization and error propagation algorithm is provided elsewhere [
25]. Note that the ANN models are typically considered as “grey box” which cannot be readily described in the form of equations [
26].
2.2 Mix design and fabrication of laboratory samples
Portland cement concrete (PCC) and mortar samples were produced in the laboratory to represent the actual concrete used to construct the Yaquina Bay Bridge decades ago. These materials, concrete or mortar, were cast into rectangular blocks that measured 12″ × 18″ × 1.5″ (305 mm × 457 mm × 38 mm), before being placed into the forms and vibrated with an external vibration stinger. The maximum size aggregate for these new concrete mixes was 3/4″ (19 mm), whereas the maximum aggregate size allowed during the construction of the Yaquina Bay Bridge was as big as 1.5″ (38 mm). Both the coarse and fine aggregates met the requirements of ASTM C33 for concrete aggregate. The minimum coarse aggregate size was a No.4 (0.19 inches or 4.8 mm). Mix proportions for 1 cubic yard (0.764 m3) of concrete included Bozeman City water (310 lbs or 140.6 kg), type I/II Portland cement (563 lbs or 255.4 kg), coarse aggregate (2,108 lbs or 956.2 kg) and fine aggregate (1,218 lbs or 552.5 kg). The quality of each batch of concrete was examined in the plastic state by measuring the slump, air content and mix temperature. The average slump for this mix design was 5.0″ (127 mm) and the total air content ranged between 1.3 and 2.2 percent for the 17 batches fabricated. The final surface tooling did not occur until the bleed water dissipated from the surface. Once the surface was troweled smooth the samples were covered with plastic for 24 h. The samples were then uncovered, de-molded and subsequently placed into a fog-curing room with 100% humidity for six days. Thereafter, the samples were placed on pallets and stored outside until testing began.
New mortar samples were also prepared using the same basic mix design with some modifications. The primary change to the mix design was in the ratio of cement to aggregates. For the concrete mixes the ratio was 0.46, and for the mortar mixes this ratio was 0.26. By removing the aggregates with diameter 0.25″ (6.35 mm) or bigger, the mortar samples had one of the influential variables removed, prior to surface profiling and Zn spraying. The mortar was tested for compressive strength at 7 days to ensure that it exceeded the minimum 28-day strength of 2200 psi (15.2 MPa). Much like the concrete mixes, the mortar exceeded 4000 psi (27.6 MPa) at 7 days.
2.3 Surface sandblasting, arc spraying, and bond testing of laboratory samples
To determine what surface features have significant influence on the initial bond strength between arc sprayed Zn and concrete, 24 concrete samples and 24 mortar samples were sandblasted (as shown in Fig. 3(a)), arc sprayed with Zn and bond tested. This work was completed at the McCullough Bay Bridge in North Bend, Oregon, according to a statistical design of experiments shown in Table 1 (http://sites.stat.psu.edu/~rli/DMCE/UniformDesign/). Note that these PCC and mortar samples were 1 month old at the time of sandblasting; as such, they may not be fully cured and are thus defined as “New PCC” and “New Mortar” in the discussions later.
The PCC and mortar samples were loaded into a containment enclosure on the bridge and allowed to acclimate for 24 h. The containment room temperature ranged between 80°F (26.7°C) and 90°F (32.2°C) throughout the work, and the average sample surface temperature was in the low 80s in °F. The test matrix of sandblasting was separated into groups based on nozzle size, number of passes and sand volume. For this study, three nozzles sizes were used (#4, #6, #8). The sand volume (adjusted by pressure) had three levels: low (L), medium (M), high (H). The number of passes across the surface also had three levels: 1, 2, and 3. All of the samples (concrete and mortar) with the same treatment settings were sandblasted at the same time to minimize equipment changes.
After sandblasting, the samples were arc sprayed with Zn (Fig. 3(b)), alternating the direction of travel from top to bottom and side to side for each pass. The goal was to achieve a layer of Zn 17 mils (0.017 inches or 0.43 mm) thick. Additional passes were made if the layer was too thin. To achieve the 17-mil thickness, a minimum of eight passes, four in each direction, were required, and in some instances as many as 12 passes were made. Prior to applying the zinc, the sample surface was blown down with compressed air to remove any loose debris. The standoff distance for Zn application averaged at 10 inches (254 mm).
After the samples were arc sprayed with Zn, they were allowed to acclimate for 4 h. The dollies, 50 mm in diameter, were then glued to the Zn coating using a high strength fast setting epoxy (DevconTM by ITW Performance Polymers, with a published tensile strength of 1250 psi or 8.6 MPa at 1 h). The epoxy was allowed to cure for 14 h prior to the bond test using a DeFlesko Posi-Test adhesion tester.
For practical reasons, the surface profile was quantified in root mean square (RMS) macro-roughness (Fig. 3(c)), in place of 2-D macro-roughness or micro-roughness [
27]. Once the sample surface was fully profiled, three bond test sites were identified on each surface. Each bond test site was outlined with a permanent marker on the surface so that macro-roughness measurements could be made both before the application of Zn and after the bond test in the exact same location (defined as “pre-roughness” and “post-roughness”, respectively). Each RMS macro-roughness value was averaged from three measurements taken from the same bond test site. For this study, a destructive chemical method [
28] was used to quantify the percent exposed aggregate area. As such, only the percent exposed aggregate area after the bond testing was measured. As illustrated in Fig. 4, the percent of exposed aggregates at each bond test site was evaluated, by first treating the site with a phenolphthalein solution to provide contrast between the cement paste and the coarse aggregate phase for photographic analysis.
2.4 Field investigation at Yaquina Bay Bridge
The intent of field trials was to investigate the bond strength of arc sprayed Zn on old concrete after various levels of abrasive blasting which led to various levels of roughness and aggregate exposure. The test sections on the Yaquina Bay Bridge were successfully sandblasted (profiled), sprayed and bond tested. These were conducted on the Pier 9 (zone 16, with a surface area of 5,419 ft2), which had been protected under ICCP with a nominal current density of 0.2 mA/ft2 (2.2 mA/m2) for approximately 14 years (1996–2010). The test sections included 12 ft by 12 ft (3.7 m by 3.7 m) areas that were profiled to different roughness and then arc sprayed with Zn. These bridge sections were selected mainly based on the quality of existing concrete (e.g., level of cracking and air void characteristics), condition of the old Zn anode, and accessibility of the sections. Multiple bond tests were conducted on each section to further the effort to correlate surface condition to initial Zn–concrete bond strength. The Great Western Corporation (GWC) constructed the containment enclosure (Fig. 5(a)), removed the failed anode by sandblasting, and re-applied the 17-mil Zn anode (Fig. 5(b)) to the south side of the pier structure from the ground level up to about 12 feet (3.7 m).
2.5 Surface sandblasting, arc spraying, and bond testing of field sections
The pier structure was divided into nine sections and each section was used to try different blasting equipment configurations with respect to Zn removal and surface profiling. The entire south face on the west pier was used as the control. GWC was instructed to remove the failed Zn anode on this section using methods and techniques currently being employed at the McCullough Bay Bridge in North Bend, Oregon (#8 nozzle and high pressure). The remaining sections, the pile cap between the piers and the east face of the east pier were divided into eight experimental sections to accommodate the different sandblasting equipment configurations. When the pressure was set at high, the #8, #6, and #4 nozzles featured an average sand volume of 11.8, 10.4, and 2.9 lbs/min (i.e., 5.4, 4.7, and 1.3 kg/min) for sandblasting operation, respectively. When the pressure was set at medium, the #8, #6, and #4 nozzles featured an average sand volume of 10.5, 7.3, and 2.8 lbs/min (i.e., 4.8, 3.3, and 1.3 kg/min) for sandblasting operation, respectively. When the pressure was set at low, the #8, #6, and #4 nozzles featured an average sand volume of 9.8, 7.3, and 2.4 lbs/min (i.e., 4.4, 3.3, and 1.1 kg/min) for sandblasting operation, respectively.
Re-application of the Zn anode to the bridge structure was completed using methods currently in practice at the McCullough Bay Bridge (Fig. 5(b)). The entire concrete surface was cleaned with compressed air to remove any dirt or loose debris. This step was repeated for every spray set-up, which was about every 9 square feet. The process of arc spraying of new Zn onto the west pier/pile cap section of pier structure has been given elsewhere [
28], featuring the standoff distance of about 12 inches (305 mm) and actual thickness of new Zn between 16 and 18 mils.
After all of the sections were profiled, bond test sites were located and outlined on the surface with a permanent marker. The west pier was used as the control and had 12 bond test sites; the east pier was divided into two sections each with six bond test sites. Both of the pier sections were profiled with a #8 nozzle with the three different sand contents. The west pier (control) was profiled with a high volume of sand and the east pier had two levels of sand content, medium and low. The pile cap was split into six sections, three for the #6 nozzle at the three sand volumes and three sections for the #4 nozzles at the three sand volumes.
3 Results and discussion
3.1 Bond strength of new mortar samples
For ANN modeling of new mortar samples (1-month old, with equivalent electrochemical age of 0 year), three influential factors were chosen as input variables, i.e., % exposed aggregate area, RMS pre-roughness, and thickness of new Zn. The Zn-mortar bond strength was chosen as the output variable. The three-layered ANN model was trained and tested using the 24 data points obtained from measurements of the mortar samples. Each data point was averaged from measurements from three bond test sites that were treated under a certain set of operating parameters, following the design shown in Table 1. The topological structure of the ANN model was determined to be 3-6-1 on the basis of trial-and-error. The sum of the mean squared error (SMSE) from the training data set (23 samples) and testing data set (1 sample) was 0.105 and 0.081 respectively. The model performance of bond strength of new mortar (Fig. 6) shows that the established model has good “memory” and the trained matrices of interconnected weights and bias reflect the hidden functional relationship well. As such, the model was then employed to predict the bond strength as a function of influential factors. In the following sections, the model predictions are presented in the form of 3-D response surfaces to illustrate the complex interactions between influential factors.
As shown in Fig. 7(a), for new mortars with 28% exposed aggregates the bond strength exhibited two main trends with 12–20 mils of new Zn. For rougher surface, the thicker the Zn layer, the higher the bond strength; for the smoother surface, the thinner the Zn layer, the higher the bond strength. As shown in Fig. 7(b), for new mortars with 17.5 mils of new Zn, higher bond strength values generally coincided with mortar surfaces with a moderate level of surface macro-roughness, but the dependency of bond strength on exposed fine aggregates was less significant. Both figures suggest that too rough or too smooth a mortar surface would not lead to high bond strength of the new Zn.
3.2 Bond strength of new PCC samples
For ANN modeling of new PCC samples (1 months old, with equivalent electrochemical age of 0 years), three influential factors were chosen as input variables, i.e., % exposed aggregate area, RMS pre-roughness, and thickness of new Zn. The Zn–concrete bond strength was chosen as the output variable. The model was trained and tested using the 24 data points obtained from measurements of new PCC samples. The topological structure of the ANN model was determined to be 3-5-1 on the basis of trial-and-error. The SMSE from the training data set (23 samples) and testing data set (1 sample) was 0.093 and 0.076 respectively. The model performance of bond strength of new PCC (Fig. 8) was satisfactory and as such, the model was then employed to predict the bond strength as a function of influential factors, as detailed later.
As shown in Fig. 9(a), for new concretes with 13.4% exposed aggregates the highest bond strength values were found with the rough surfaces (0.9–1.1 centi-inches or 0.23–0.28 mm), in which case the effect of Zn thickness (12–20 mils) was negligible. For the smooth surfaces (0.2–0.3 centi-inches or 0.05–0.08 mm), the relative high bond strength values corresponded with intermediate Zn thickness (15–17 mils), which coincided relatively well with the target thickness of current ODOT specifications. Figure 9(b) reveals that for new concretes with 16.8 mils of new Zn the highest bond strength values were found for surfaces with less exposed aggregates (<13%), in which case the rougher surfaces were generally beneficial. The rougher surface likely provided more and larger “anchor points” for the molten Zn to adhere to. Another preferred combination featured a very smooth surface (0.2–0.3 centi-inches or 0.05–0.08 mm) and a moderate level of exposed aggregates (18%–34%). The exposed aggregates on the very smooth surface likely provided more “anchor points” for the molten Zn to adhere to.
3.3 Bond strength of fully cured concrete
For ANN modeling of fully cured concrete samples, three influential factors were chosen as input variables: % exposed aggregate area, RMS pre-roughness, and equivalent electrochemical age. The Zn–concrete bond strength was chosen as the output variable. The three-layered ANN model was trained and tested using the data points from the preliminary investigation (31 ten-year old NETL samples, 18 nine-month old PCC samples, and three rock samples) [
19] as well as 42 data points from the 80-year old Yaquina Bay Bridge sections. Each data point was averaged from measurements from three bond test sites that were treated under a certain set of operating parameters. Note that the missing pre-roughness data of some samples were estimated from post-roughness and % exposed aggregates using the predictive model discussed elsewhere [
28].
The topological structure of the ANN model was determined to be 3-11-1 on the basis of trial-and-error. The SMSE from the training data set (91 samples) and testing data set (three samples) was 0.079 and 0.095, respectively. The model performance of bond strength of fully cured concrete (Fig. 10) was satisfactory and as such, the model was then employed to predict the bond strength as a function of influential factors, as detailed later.
As shown in Fig. 11, for fully cured concrete with a moderate level of exposed aggregates (35%) the ideal surface macro-roughness varied significantly with the electrochemical age of the concrete. For concrete with an electrochemical age of 0–5 years, the surface macro-roughness after sandblasting should be maintained at higher than 1.1 centi-inches (0.28 mm). The ideal surface macro-roughness changed to 0.8–1.5, 1.3–1.5, 0.4–0.8, and 0.4–1.3 centi-inches (i.e., 0.2–0.39, 0.33–0.39, 0.1–0.2, and 0.1–0.33 mm), for concrete with an electrochemical age of 10, 15, 20, and 25 years respectively. As the electrochemical age of concrete further increased, the highest bond strengths tended to gradually shift to smoother surfaces.
The change of ideal surface roughness with electrochemical age was likely linked to how the chemistry and microstructure of the concrete surface layer beneath the arc sprayed Zn evolved over the duration of electrochemical aging [
29,
30]. Figure 11 also shows that the lowest bond strength values are expected near the electrochemical age of 14, which coincides with that of the Yaquina Bay Bridge Pier 9. It is cautioned that the bridge concrete that provided the data points for electrochemical age of 14 years had been in service for 80 years, whereas the laboratory concrete samples that provided the data points of other electrochemical ages (0, 5, 9, 22, 38, 45) was never in service. As such, the model might have been skewed by the data points from the bridge sections.
The Yaquina Bay Bridge concrete sections featured much lower bond strengths (43–248 psi, averaged at 151 psi or 1.0 MPa) than the other PCC samples (104–309 psi, averaged at 196 psi or 1.4 MPa). One likely reason is the difference in the maximum aggregate size between the two (38 mm
vs. 19 mm). These results imply that once exposed by surface sandblasting, the aggregates larger than 19 mm degraded the bond strength of new Zn. Another possible reason was that the current sandblasting protocol was too aggressive and might have “bruised” the bridge concrete surfaces and generated defects that led to poor bond of Zn to them. This was suggested by the fact that the bridge concrete samples had much rougher surfaces after sandblasting and validated by later findings related to the modeling of operating parameters for PCC and bridge concrete [
28]. It is hypothesized that a less aggressive sandblasting protocol and/or a more abrasion-resistant mortar treatment to the concrete surface after the old Zn was removed would lead to higher Zn–concrete or Zn–mortar bond strength.
The predicted bond strength of fully cured concrete as a function of pre-roughness and surface composition with the changes of electrochemical age (0, 8, 14, 20, 27 years) are listed in Table 2. Based on the model predictions, for existing concrete with relatively high electrochemical age (14 years under CP current density of 2.2 mA/m2) and 17 mils of new Zn, the highest bond strength values generally coincided with surfaces with a moderate level of macro-roughness (0.28–0.38 mm) and a moderate level of exposed aggregates (44%–67%). If too much aggregate phase were exposed (e.g., 67%–75%), it would expose a proportionally high concentration of large aggregates and thus negatively affect the bond strength of new Zn. This unravels the complex role of exposed aggregates in affecting the initial bond strength of arc sprayed Zn to the profiled concrete surface, with the surface of large aggregates providing poor bonding to the new Zn coating but small aggregates providing beneficial “anchor” spots around them. In light of field observations and the caveats of the modeling discussed earlier, the ideal level of exposed aggregates should avoid the higher end of the model predictions (i.e., maintained at 44%–55%). Wherever possible, large aggregates (e.g., diameters 19 mm and bigger) should be avoided for exposure by surface sandblasting.
4 Concluding remarks
CP of reinforced concrete can be used to effectively mitigate chloride-induced corrosion of rebar, and in this study, efforts were directed toward developing and testing a method for determining the “suitability” of a concrete surface for applying arc sprayed Zn. The key finding are provided as follows:
1) ANN was used to achieve better understanding of the complex cause-and-effect relationships inherent in the Zn-mortar or Zn–concrete systems and was successful in finding meaningful, logical results from the bond strength data.
2) For one month old (“new”) mortars, a moderate level of macro-roughness is key to achieving a high initial bond strength of newly applied Zn layer which is approximately 17.5 mils (444 mm) thick. For one month old (“new”) concretes with 16.8 mils (427 mm) of new Zn, the highest bond strength values were found for rough surfaces with less exposed aggregates (<13%) or a very smooth surface (0.05–0.08 mm) with a moderate level of exposed aggregates (18%–34%).
3) For fully cured concrete with a moderate level of exposed aggregates (35%) the ideal surface macro-roughness varied significantly with the electrochemical age of the concrete. For concrete with an electrochemical age of 0-5 years, the surface macro-roughness after sandblasting should be maintained at higher than 0.28 mm. The ideal surface macro-roughness changed to 0.1–0.2 mm for concrete with an electrochemical age of 20 years.
4) To achieve high initial bond of arc sprayed Zn, agencies should adjust the anode removal and surface profiling based on the electrochemical age of the existing concrete. In all cases, minimize the exposure of large aggregates (e.g., those bigger than 19 mm in diameter). These targets can be achieved in the field operations by adjusting the nozzle size, air pressure, number of passes and possibly other operating parameters.
5) Continued research is needed to investigate how the surface preparation affects the evolution of Zn–concrete interface and its bond strength (as well as circuit resistance) over the process of electrochemical aging by the operations of CP system. There is also the need to search for innovative materials and cost-effective methods for anode removal and surface profiling, in order to better protect atmospherically exposed bridge substructures in coastal and deicer environments.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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