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Marine Technology Society Journal110

TIntroduction he enhancement of coastal fishing grounds in South Korea has been in urgent demand, not only because of the shrinkage of fishing grounds along its coast as a result of international regulations and domestic modern industry development, but also be-cause of international competition resulting from the open economy under the World Trade Organization. In order to enhance the fishing grounds, the South Korean government has operated a marine biology habitat enhancement project since 1971. As a part of the project, artificial concrete reefs, selected because of their ease of construc-tion and stability, occupied a total of 1570 km2 of South Korean waters by 2001, at a cost of 515 million U.S. dollars. These artificial reefs provide marine creatures a place to live, breed, and hide from preda-tors as well as to create an environmentally friendly solution to coastal protection.

A U T H O R SH. S. Kim C. G. Kim National Fisheries Research and Development Institute, South Korea

W. B. NaJ. Woo Pukyong National University, South Korea

J. K. Kim1

Chonnam National University, South Korea

Physical and Chemical Deterioration of Reinforced Concrete Reefs in Ton�yeon�Concrete Reefs in Ton�yeon�in Ton�yeon� Coastal Waters, Korea

A B S T R A C TAs part of a marine habitat enhancement project, the physical and chemical deterioration

of reinforced concrete reefs that were fully immersed in Tongyeong waters of Korea was investigated. For the investigation, marine environmental factors such as seawater, salinity, pH, dissolved oxygen, sea-bottom materials, and water depth of the targeted sites were surveyed from 1997 to 2001. Then, four reinforced concrete reefs from four different sites were recovered and tested by using various destructive and nondestructive methods. Based on the observations and test results, it was seen that the reinforced concrete reefs have sound physical and chemical properties, except for chloride concentration and its associated factors. However, because of the lack of dissolved oxygen in the targeted seawaters and its continu-ous supply, it is concluded that the originally designed service life will be achieved, and in fact the concrete reefs will have an even longer service life than expected. By considering an extreme event such as impact loading under installation and construction, a new minimum concrete cover depth of 40 mm is introduced into practice.

Because the first generation of concrete reefs in South Korea was designed to have a service life of 30 years, most of the reefs were believed to have undergone severe deterioration by the present decade. Thus the Fisheries Resource Enhancement and Management Center, part of the National Fisheries Research and Development In-stitute (NFRDI) in South Korea, decided to inspect those aged reinforced concrete reefs, which had been immersed in seawater for 18–25 years. Four concrete reefs from four different sites in Tongyeong waters were identified by a side-scan sonar. The reefs were recovered by divers, transported to land, and tested by several scientific materials testing tools. This type of research had not been conducted in South Korea and other countries, although there was a concerted research effort on monitoring artificial reefs by SCUBA divers (Sherman et al., 2002; Baine, 2001). In investigating the integrity of the recovered reefs, the degree of their physical deterioration was determined by destructive and nondestruc-tive tests, such as FE-SEM (field emission scanning electron microscope) analysis,

the core compressive strength test, the Schmidt hammer test, and the ultrasonic velocity test. In addition, chemical dete-rioration was characterized by recording pH, chloride concentration and potential, and composition of the reinforced concrete reefs. From the experimental observations, a minimum cover depth of reinforced concrete reefs was proposed for securing a service life of 30 years or more. Those tests can be summarized in Table 1.

EnvironmentReinforced concrete reefs submerged

for 18 to 25 years were recovered from four different sites in Tongyeong coastal waters of South Korea, as shown in Figure 1. These sites were selected because the areas are known to have fewer marine environmen-tal variations. The marine environmental survey data from 1997 to 2002, provided by the NFRDI, were used to analyze the environmental characteristics of the inves-tigated sites. Marine environmental factors, such as seawater temperature, salinity, pH,

P A P E R

1Corresponding Author: Faculty of Marine Technology, Chonnam National University, Yeosu 550-749, South Korea, [email protected]

111Fall 2008 Volume 42, Number 3

and dissolved oxygen, were selected for the analysis. In addition, water depth and bot-tom materials were investigated. In Figure 1, site SK25 indicates the site name (San-yang Konli) and the age (25 years) of the concrete reef. Other sites have comparable names and ages, as denoted in Table 2. Thus the four different sites (SK, SY, YD, and HP) and ages (25, 23, 21, and 18) are considered in this study.

From the analysis of environmental factors, the seasonal seawater temperature ranges from 11.3 to 24.7°C at the surface, and 10.9 to 20.2°C at the bottom of Tongyeong coastal waters of South Korea. The temperature difference is not extreme except in August, when the difference is 4.2°C. The salinity ranges from 31.42 to 33.86%, and thus is not a wide variation. Dissolved oxygen (DO) ranges from 6.71

to 8.21 mg/L at the surface and 6.54 to 8.18 mg/L at the bottom. The pH ranges from 7.89 to 8.22 for the surface and the bottom, respectively, so this difference is negligible. Based on the investigation, the targeted areas are in 28 to 32 m water depth, and the bottom material is sandy mud, which is flat and ductile. This pattern is the same for all four sites. In summary, the four sites have similar marine environ-mental factors; hence it is concluded that these environmental factors are similar at the four sites.

Materials PreparationInitially, the concrete reefs were construct-ed with a 1:2:4 water/cement/aggregate ratio. The compressive strength was 210 kgf/cm2. In addition, the concrete reefs

were reinforced with steel bars. These reinforcing bars were D10 (nominal diameter, 0.953 cm) for the small-sized reefs and D13 (nominal diameter, 1.27 cm) for the medium-sized reefs. A total of 24 reinforcing bars were inserted in each reef. All of these concrete reefs are of small or medium size, as shown in Table 2, and all are cubes, as shown in Figure 2. The number of concrete reefs at each site varies from 900 to 1320. The locations of these targeted reefs were determined and confirmed by a side-scan sonar. At each confirmed site, divers inspected 10 reefs and selected one that was representative of each group, as shown in Figure 3. Then the reefs were recovered, so that a total of four concrete reefs were transported to Busan, the largest port city of South Korea, for scientific materials testing. During recovery and transportation, the reefs were covered by a mat to minimize water evaporation.

To investigate the contents of pH, CaO (calcium oxide), and potential, the concrete reefs were initially sampled from small-sized specimens, 150 × 150 × 500 mm, and medium-sized specimens, 250 × 250 × 700 mm. These samples were resized for different types of tests as fol-lows. To measure the pH of concrete reefs,

FigURE 1

Map showing sample sites HP18, YD21, SY23, and SK25. The two letters indicate the in-stallation areas of the concrete reefs, and the numbers are the ages after installation.

TABlE 1

Summary of physical and chemical tests for reinforced concrete reefs.

Test or calculation Target materials

Physical Visual inspection Concrete and steel barsmethod FE-SEM Concrete Tensile strength Reinforcing bars Compressive strength Concrete Water-absorption rate Concrete Apparent density Concrete

Pore volume Concrete

Chemcial pH measurement Concretemethod Chloride-concentration Concrete Diffusion-coefficient calculation Concrete Long-term prediction of chloride-ion penetration Concrete Chemical-composition measurement Concrete Corrosion test Reinforcing bars

TABlE 2

Information for the surveyed concrete reefs.

identity installation Specification

Area Year Shape Size (m)

SK25 Sanyang Konli 1978 Small cube 1×1×1SY23 Sanyang Yeondae 1980 Small cube 1×1×1YD21 Yokchi Donghang 1982 Medium cube 2×2×2HP18 Hansan Pijin 1985 Medium cube 2×2×2


beams, as shown in Figure 4. The exposed reinforcing bars were dark red, with some reduced cross sections. However, the rein-forcing bars that were still embedded in the concrete reefs did not show any evidence of corrosion.

For the composition test, the FE-SEM was used to view composition profiles at a depth of 5 mm or 73 mm from the surface

some of the samples were cut at depths of 5, 22, 39, 56, 73, 90, and 124 mm from the surface. Each of these seven cut specimens was powdered to less than 10 μm, using a jaw crusher, pulverizer, and vibrating miller. A 30 g aliquot of each powdered specimen was put into 100 mL of deionized water, and then the pH was measured, using an Autotitrimeter Metrohm (Model 216).

The sampling process and experi-mental setup for measuring the chloride concentration were the same as for measuring the pH. The 10 μm powders were dissolved by HNO

3 (nitric acid) and

diluted with 100 mL of deionized water. Then the voltage was measured with an ion selectivity electrode, and the chloride content was measured by 0.05 N AgNO

3

(silver nitrate) solution.

Physical Tests and ResultsVisual inspection showed that the external views of the recovered concrete reefs were globally normal. Minor deterioration, such as three microcracks, four fractures, and three places where the steel bars were exposed, were observed; however, they were not severe enough to cause struc-tural malfunctions, as shown in Table 3. It is noteworthy that the mortar of one surface of the medium-sized reefs was eroded about 10 to 35 mm, exposing the aggregates. This erosion probably occurred because of faulty fabrication; hence the bottom was weakened, and finally eroded. Visual inspection revealed that some reinforcing bars were corroded. A total of three reinforcing bars were exposed in the medium-sized cubes: one was on a column, and the other two were on bottom

FigURE 2

Diagram of a concrete reef.

FigURE 3

Field operations: (a) search, (b) confirmation, (c) observation, and (d) recovery of concrete reef.

TABlE 3

Results of concrete surface observation.

(a)

(b)

(c)

(d)


of each specimen. During the microscopic testing, magnifications of 1000, 5000, and 10,000 were used. As a result, CSH (calcium silicate hydrate), monosulfate, and pores were observed in all the specimens. Although it was not easy to quantify the difference in composition profiles with respect to the age of the concrete reefs (18, 21, 23, and 25 years), the oldest showed the weakest composition. In addition, the composition at a depth of 5 mm had less monosulfate and fewer pores than the composition at a depth of 73 mm, because hydration-induced new materials occupied the pores. This was also the case in a core volume test, which is introduced later.

The tensile strength tests of the reinforc-ing bars, which were mechanically extracted from the reefs, were carried out. From the tests the tensile strengths vary from 43 to 55 kgf/mm2, the yield strengths vary from 31 to 39 kgf/mm2, and the elongation ranges vary from 26 to 31%. These ranges are within the standards denoted by KS D 3504 (2001), although the tensile strengths of the 18-year and 23-year concrete reefs are slightly below the minimum standard value of 45 kgf/mm2. The range of tensile strengths does not present a significant

structural problem, as the reefs are usually not under severe external loading condi-tions. Thus it is seen that the reinforcing bars deteriorated only slightly.

To determine the compressive strength of the concrete reefs, measurements were made by the core compressive strength test, the Schmidt hammer test, and the ultrasonic velocity test. The core test was conducted by KS F 2405 (2001), 2412 (1990), and 2422 (2002). For the hammer test the NR-type Schmidt hammer was used, and the standards of DIN 1048-2 (1991) and ASTM C805-02 (2002) were adopted. The ultrasonic velocity test was carried out according to BS 1881 Part 203 (1986).

For the core compressive strength test, cores were sampled from each member (upper beam, column, and bottom beam) of the reinforced concrete reefs. Test results showed that the averaged compressive strength is 157 kgf/cm2 for the bottom beams, 148 kgf/cm2 for the upper beams, and 149 kgf/cm2 for the columns. These values are 19 to 34% lower than the standard 28 days’ compressive strength of 210 kgf/cm2.

The compressive strength values were also obtained by the Schmidt hammer test. Test results showed that these values were higher than those from the core compres-sion test. Finally, the ultrasonic velocity test was performed to measure the compressive

strength. Depending on ultrasonic veloc-ity, we can classify the strength into five different levels (Whitehurst, 1951). The test results showed that the compressive strengths range from 3 to 122 kgf/cm2, which were very low. One interesting factor here is that the youngest concrete reef had the lowest ultrasonic velocity; hence the levels are very poor for all the members. In addition, depending on the velocity, some of the compressive strengths were negative, which means that the condition of the initial concrete was not fully known; thus the formula for estimating the compressive strength was not fully applicable. So in this case the ultrasonic velocity method was not suitable for measuring the compressive strength. The test results obtained from the core compressive strength and Schmidt hammer tests are shown in Figure 5. The figure shows that the mean compressive strength decreases with time, although the trend is flat for sites SY23 and SK25.

Additional values for the physical condi-tion of the concrete reefs were obtained by measuring water absorption rate, apparent density, and pore volume. Because the role of water is decisive in the deterioration of concrete structures, characterizing different types of moisture transport is relevant for studies of the alteration of a porous mate-rial, i.e., concrete. To quantify these absorp-tion processes, various working procedures have been proposed. However, all the

FigURE 4

Corrosion and cracks on the surface of reinforced concrete reefs.

FigURE 5

Comparison of averaged compressive strength of concrete reefs between core compression test and Schmidt hammer test.


procedures are based on the same principle: measuring the evolution of a sample mass at preset time intervals. Apparent density is the weight per unit volume of concrete, which includes the volume of voids inher-ent in concrete. Pore volume is the volume of water required to replace (flush out) water in a certain volume of a saturated porous medium, i.e., concrete. Those three measurements are closely related.

In this study we used the Korean Stand-ard KS F 2518 (1980) for the measurement of the water absorption rate. From the test the absorption rate was 7.19%, 6.12%, 5.74%, and 5.69% for sites HP18, YD21, SY23, and SK25, respectively. The young-est concrete reef (HP18) shows the biggest absorption rate (7.19%), although HP18 has the highest concrete strength, as shown in Figure 5. This indicates that HP18 has a greater chance for rapid deterioration than the other concrete reefs. The appar-ent densities of the concrete reefs were also measured, based on the Korean Standard KS F 2459 (2002). The test result shows that the range of the apparent densities of all the concrete reefs is between 2.27 and 2.30 g/cm3. In addition, pore volumes of the concrete reefs were measured by the mercury porosimeter method, as seen in Figure 6, which shows that the pore volumes at a depth of 22 mm have higher values among the concrete depths (5 mm, 22 mm, 39 mm, and 73 mm). Among the concrete reefs, SY23 has the highest values except for a depth of 5 mm. In global, at a depth of 5 mm, near the top surface, the pore volume is low. The reason for this is that after Ca(OH)

2 was removed as a result

of chemical action on sapropel, seawater, and concrete hydrate, the salinity was re-produced so that the pores were relatively contracted. A similar observation was made by Mohammed et al. (2003). According to Figures 5 and 6, the relation between pore volume and concrete strength is not clear. In general, concrete strength is closely related to concrete porosity; hence, many equations have been suggested for describ-ing this relationship (Khatib and Mangat, 2003). However, the relationship was not observed in this study.

Chemical Tests and ResultspH values are shown in Figure 7. For all the specimens (HP18, YD21, SY23, and SK25), the value increases as the depth of the specimen increases. Based on the pH values, the concrete reefs are still alkaline, although they were immersed in seawater for 18, 21, 23, and 25 years.

In the measurement of chloride con-tent, water-soluble and acid-soluble chlo-ride ion penetration decreases as the depth increases for all the specimens. In addition, the water-soluble concentration is lower than the acid-soluble concentration, at about 460 to 1302 ppm. Here, the term “acid-soluble” refers to the chloride ion concentration measured when the sample

is prepared by acid dissolution. There is controversy over whether the threshold value should be on “acid-soluble” or “wa-ter-soluble” chlorides (Carino, 2004). By converting the chloride ion concentration into the concrete content, the chloride content ranges from 11 to 20 kg/m3 at a depth of 22 mm, 9 to 16 kg/m3 at a depth of 56 mm, and 8 to 15 kg/m3 at a depth of 73 mm, respectively.

These results are quite similar to the measures conducted by Okada (1986), who reported that the chloride ion con-tent of 10- to 20-year-old concrete reefs, collected from 30 to 40 m water depth, ranged from 12 to 28 kg/m3. Stratfull et al. (1975) reported that the limiting value of

FigURE 7

pH values at different depths from the surface of concrete reefs.

FigURE 6

Pore volumes of concrete reefs.


chloride ion content capable of corroding reinforcing bars of a bridge deck located on a pier was 0.89 kg/m3. Thus, our chloride ion content, ranging from 8 to 20 kg/m3, is believed to be sufficient to cause corro-sion. In addition, the ranges might cause leaching of Ca(OH)

2 (portlandite).

Determination of the diffusion coef-ficient of chloride penetration from the top surface of concrete to a certain depth of concrete is important for predicting the amount of corrosion of reinforcing bars. Es-timation of the diffusion coefficient can be divided into two categories, stationary state and nonstationary state. In the stationary state, chloride is believed to diffuse in the same direction in all of the cross sections, and the diffusion coefficients do not change in each cross section. In such a case the diffusion equation from Frick’s first law is used. In the case of the nonstationary state, the nonstationary diffusion equation and the initial and boundary conditions from

Frick’s second law are used. In this study, three methods for estimating the diffusion coefficients of chloride in the nonstationary state were used: (1) the SLEM (simplified linear error function-based method), as used by Suryavanshi et al. (2002); (2) the graphical method; and (3) numerical methods, such as the Newton-Raphson and the least square. Table 4 summarizes the diffusion coefficients of the chloride ion solved by the above methods. It is seen that the overall values are small, although the coefficients are slightly decreased in the old concrete reefs in comparison with those of the 18-year-old reef.

Using the diffusion coefficients, the chloride ion concentration can be predicted with respect to time. The coefficient is converted to a constant, and then the chloride ion concentration is obtained us-ing the Newton-Raphson method. Figure 8 shows the chloride ion concentration with respect to time for three different concrete

FigURE 8

The long-term (50-year) prediction of the chloride ion concentration at different depths in concrete reefs with different ages: (a) HP18, (b) YD21, (c) SY23, and (d) SK25.

cover depths, which are the locations of reinforcing bars from the top concrete surface. The three cover depths are 40 mm (the proposed design cover depth in the study), 50 mm (the current standard cover depth of reinforced concrete reefs in South Korea), and 80 mm (the current standard cover depth of reinforced concrete structures in splash zones of South Korea), respectively. In other words the figure shows the chloride ion content with respect to time. Because the timescale ranges up to 50 years, this work is a kind of long-term prediction of chloride ion concentration (or penetration). At 30 years, and for 80 mm cover depth, the chloride ion concentration ranges from 8.5 to 17.0 kg/m3. This range is believed to cause corrosion of the reinforc-ing bars. Thus, with other cover depths of 40 and 50 mm, reinforced concrete reefs are at risk of corrosion.

The chemical composition of concrete varies with respect to time. This variation


results in the changes of concrete strength and other deterioration. Because the original detailed mix design of the concrete reefs was not recorded, it is not possible to pinpoint the exact variation in chemical composition with respect to time. Thus the approximate trend was analyzed by the energy dispersive microprobe analyzer (EDMA), as shown in Table 5. This table shows the chemical composition near the surface (5 mm) and inside (73 mm) of each concrete reef. From the results, SK25 has the highest values and SY23 has the lowest of CaO, and SK25 and HP18 have lower values of SiO

2 (silicon dioxide). From a

loss on ignition, the contents of concrete hydrates, such as CaCO

3 (calcite), CSH

(calcium silicate hydrate), and Ca(OH)2

(portlandite), at the concrete surface were calculated. In addition, using XRF (X-ray fluorescence) spectroscopy, the amount of CaO is 6.24%, 5.36%, 3.23%, and 4.60% for SK25, SY23, YD21, and HP18, respectively. Except for HP18, the amount of CaO increases as time passes. This shows that the concrete reefs have undergone a reduction in concrete strength and dete-rioration with time.

XRD (X-ray diffraction) measurements were taken to determine the presence of Friedel’s salt, portlandite, and quartz. From the results, Friedel’s salt was not observed, which means that the materials preparation for the test was faulty. However, portlandite was observed from HP18 and YD21. In addition, concrete strength tests show that HP18 has the highest strength. These facts indicate that the relatively young concrete reefs show less deterioration.

Electrochemical methods for measuring the corrosion of reinforcing bars embedded

in concrete structures typically can be clas-sified into the half-cell potential, concrete resistivity, and polarization resistance methods (Carino, 2004). Among these methods the half-cell potential method has been widely used in the practice of science. For this study the potentials proposed by DIN 50900-2 (2002) were used as the reference values. The average potential of the reinforcing steel bars in concrete reefs is shown in Table 6. YD21 and SK25 have

higher average potential and standard deviation; hence the bars at these two sites are more likely to be corroded. However, by examining the reinforcing bars extracted from the concrete reefs, severe corrosion was not observed.

Cover depth should be controlled to prevent reinforcing bars from corrosion. For this purpose the quality of the concrete should be maintained by controlling the ratio of water and cement (W/C) within 0.4. For concrete substructures installed underwater, whether they are in coastal waters or offshore, the range of 50 to 80 mm minimum cover depth is used in South Korea (Ministry of Construction and Transportation, 2005), Japan (Tanaka et al., 2001), Britain (HSE, 2001), and Nor-way (Helland et al., 2006). These criteria are aimed at marine concrete structures ex-posed to splash zones with a specific service life. However, concrete reefs are totally im-

TABlE 4

Comparison of diffusion coefficient (D) of chloride ion solved by different methods in concrete reefs.

Year 18-year-old 21-year-old 23-year-old 25-year-oldMethod concrete reef concrete reef concrete reef concrete reef

SLEM 1.9853E-7 1.0850E-7 3.0335E-8 5.7866E-8

Graphical method 6.3231E-7 1.7572E-7 1.6761E-7 2.6587E-7

Newton-Raphson 2.9290E-7 1.5074E-7 3.8954E-8 8.4591E-8

Least square fit 2.9345E-7 2.5153E-7 1.3314E-7 3.0638E-7

TABlE 5

Chemical composition at the surface (5 mm) and inside (73 mm) the concrete reef by energy dispersive microprobe analyzer.

TABlE 6

Average potential of steel bars in concrete reefs.

Chemical HP18 (%) YD21 (%) SY23 (%) SK25 (%)

composition Surface inner Surface inner Surface inner Surface inner

Na2O 0.85 1.31 0.79 1.65 1.31 1.02 0.66 0.49

MgO 3.15 2.18 3.93 2.02 3.82 2.65 1.96 2.13

Al2O3 5.69 6.05 6.94 7.38 6.41 8.39 3.59 3.78

SiO2 23.11 27.00 26.48 29.71 26.83 31.92 21.73 21.41

Cl 0.45 0.92 1.30 2.96 1.97 5.92 3.26 2.60

K2O 0.68 0.84 0.57 0.84 0.56 0.63 0.51 0.66

CaO 61.39 57.53 55.38 52.02 55.57 49.82 64.66 65.32

Fe2O3 4.68 4.17 4.62 3.42 3.53 4.00 3.63 3.61

Artificial reef Potential (mV)

Average Standard deviation Maximum Minimum

HP18 –565 11 –540 –585

YD21 –633 51 –560 –765

SY23 –645 34 –595 –715

SK25 –621 54 –585 –765


mersed in seawater, where variation in the marine environment, such as temperature and pH, is much less than in a splash zone. In addition, there is less oxygen than in a splash zone. The oxygen dissolved in the targeted seawaters ranges from 6.54 to 8.18 mg/L at the bottom, which is much smaller than the 300 mg/L in air. Moreover, in air the consumption and supply of oxygen are continuous, but in seawater the oxygen in the concrete reefs is separated by porous structures so that diffusion does not occur easily. In summary, as shown in Figure 9, the concrete reefs (HP18, YD21, SY23, and SK25) do not show any evidence of severe corrosion because of the lack of oxygen and the porous structure of the concrete.

ConclusionsThe physical and chemical characteris-

tics of reinforced concrete reefs, which were fully immersed in seawater for 19 to 25 years, were investigated. For the investiga-tion, marine environmental factors, such as seawater temperature, salinity, pH, dis-solved oxygen, sea-bottom materials, and water depth of these sites, were surveyed from 1997 to 2001. Then four reinforced concrete reefs recovered from four differ-ent sites in Tongyeong waters were tested with various destructive and nondestructive methods. From the test results, the follow-ing observations are made.■ The four selected sites in Tongyeong waters have similar marine environmental factors acting on the concrete reefs.■ Visual inspection showed three microcracks, four fractures (possibly because of mechanical impacts), and three areas of exposed reinforcements; however, the latter were not severe enough to cause structural malfunction of the concrete reefs.■ As a result of FE-SEM (field emission scanning electron microscope) exam- ination, CSH (calcium silicate hydrate), monosulfate, and pores in all the specimens were observed; however, it was not easy to quantify the difference in composition profiles with respect to the ages of the concrete reefs.

■ From the tensile strength test of the reinforcing bars, the tensile strength varies from 43 to 55 kgf/mm2, so that some of the reinforcing bars have less strength than the minimum standard value of 45 kgf/mm2. However, it is believed that the tensile strengths do not constitute a significant structural problem because the reefs are usually not under severe external loading conditions. In addition, the yield strength and elongation ranges were within the standard values. Except for three exposed reinforcing bars, the reinforcing bars embedded in the reefs did not show any indication of corrosion; hence the condition of the reinforcing bars was sound enough to maintain their role in the reinforced-concrete reefs. ■ Measurements of the compressive strength of the concrete through the core compressive-strength test, the Schmidt hammer test, and the ultrasonic velocity test showed different results. The compressive strength measured by the core compressive strength test showed slightly lower values than the standard 28-day compressive strength of 210 kgf/cm2, whereas the Schmidt hammer test showed higher values than the standard. Based on the two measurements, it is

concluded that the concrete reefs have undergone some deterioration but not enough to cause failure.■ The other physical tests for acquiring water absorption rate, apparent density, and pore volume did not give any negative results for the physical condition of the concrete reefs. ■ From pH measurements the concrete reefs are still alkaline, although they have been immersed in seawater for 18 to 25 years. ■ From chloride concentration measurements, these concentrations, ranging from 8 to 20 kg/m3, are believed to be sufficiently high to cause corrosion. In addition, this range might cause Ca(OH)

2 leaching.

■ From diffusion coefficient calculations, the overall values are small, and the coefficients are slightly lower in the older concrete reefs in comparison with the 18-year-old reef. ■ From long-term prediction of chlori- deion penetrations, it is observed that the chloride-ion concentration ranges from 8.5 to 17.0 kg/m3 at 30 years, with an 80 mm cover depth. This range is believed to cause corrosion of the reinforcing bars. Thus, with other cover depths of 40 and 50 mm, reinforced concrete reefs are at risk of corrosion.

FigURE 9

Reinforced steel bar used in 25-year-old concrete reef.


■ From chemical composition measurements by XRF spectroscopy, the CaO contents increase with time, except for those at site HP18. This shows that the concrete reefs have been undergoing concrete strength reduction and deterioration with time. Portlandite was observed at sites HP18 and YD21; hence the relatively young concrete reefs show less deterioration.■ From corrosion tests, it is shown that the reinforced concrete reefs have a greater chance of having corroded reinforcing bars, but only when the concrete cover has broken and cracking has occurred. Otherwise, corrosion was not observed in most places, even where the concrete cover was 23 mm or 25 mm.

On the basis of all the observations and test results, it is shown that globally the reinforced concrete reefs have sound physical and chemical properties except for chloride concentration and its associated factors. However, as mentioned earlier, chloride alone cannot cause corrosion of the reinforcing steel bars; it needs other necessary factors such as oxygen and its continuous supply to invoke corrosion. The oxygen dissolved in the targeted seawaters ranges from 6.54 to 8.18 mg/L, which is much lower than 300 mg/L in air. Thus, the originally designed service life will be achieved, and the concrete reefs will have a longer service life than expected.

Based on a service life of 30 years, the safety factor of 1.5, and from the test results given above, the proper concrete cover over bars is estimated at about 34.5 mm. This number is lower than the current Korean practice of 50 mm for the reinforced con-crete reefs and the other standards ranging from 50 to 80 mm under splash zones. It should be noted here that the first generation of the reinforced concrete reefs in Korea had variable minimum concrete covers ranging from 23 to 50 mm, which was dependent on the size of the concrete reefs and the location of the members. However, by considering an extreme event, such as impact loading conditions under installation and construc-tion, a range of 40 to 50 mm cover depth is quite safe for field work. Thus, the new

proposed minimum concrete cover depth of 40 mm is introduced into practice.

Acknowled�mentsAcademic or technical support from

the following institutes, universities, and companies are greatly acknowledged: National Fisheries Research and Develop-ment Institute, Pukyong National Uni-versity, Chonnam National University, Taesung ENG, Misung CNS, and Ssangyong Technology Research Center. The financial support from Gyeongsang-nam-do is also greatly acknowledged.

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