# Material Alternatives for Concrete Structures on Remote Islands: Based on Life-Cycle-Cost Analysis

Authors: Guiwen Liu; Jianmin Hua; Neng Wang; Wenjie Deng; Xuanyi Xue
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

Corrosion is an unavoidable problem for RC structures in harsh marine environment, which encourages the application of high-performance concrete (HPC) and corrosion-resistant reinforcements, such as stainless steel (SS), stainless-clad bimetallic steel (SB) and FRP. Especially, for the RC structures in remote islands or reefs, the use of local materials (coral, sea water, etc.) may bring cost benefits by avoiding the high expansive fee. In this paper, the cost effectiveness of five different material proposals is evaluated and compared based on the life-cycle-cost analysis (LCCA), incorporating not only initial cost but also long-term investment into account. The LCCA results of a 3-floor office building in the South China Sea shows that basically the SB + HPC proposal shows the highest cost benefits considering a 100-year period while the proposals using coral aggregate seawater concrete (CASC) are not that competitive. The sensitivity analysis is performed to address the influence of discount rate and construction-to-material cost ratio.


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## Body

## 1. Introduction

The marine environment is quite unfriendly for structures, leading to a shorter service life compared with the designed one [1]. The reinforced concrete (RC) structures have been widely used in marine structures, expected to perform better durability and ductility than steel structures. However, the corrosion problems due to the chloride penetration are still bothering the structural designers and users, as seen in Figure 1(a). Substituting the conventional black steel (BS) reinforcements by corrosion-resistant reinforcements (e.g., stainless steel (SS) reinforcements [3–5], epoxy-coated (EC) reinforcements [6, 7], fiber-reinforced polymer (FRP) reinforcements [8–10], etc.) is regarded as an effective approach to extend the service life of RC structure. Besides, alternatives for concrete also strongly influence the service life. For example, the high-performance-concrete (HPC), generally with larger cement-to-water ratio, better curing condition and the use of additions or fibers, has drawn attention extensively to improve the durability of the structure [11–14].Figure 1
The corrosion problem and location of the South China Sea. (a) The corrosion problem [2]. (b) The South China Sea.
(a)(b)On the one hand, the service life could be extended by using better materials, but on the other hand, the prices of the interested materials may not be acceptable. Although using corrosion-resistant reinforcements promotes the initial cost, the maintenance cost will be saved considering a long-term service. Hence, the cost effectiveness of each material proposal should be evaluated based on life-cycle-cost analysis (LCCA). LCCA considers not only the initial monetary investment but also the long-term expense such as the maintainence and reconstruction cost [15], providing the decision-makers with a competitive tool [16, 17]. Numbers of studies have been conducted focusing on the life-cycle-cost of RC structures [17–23]. Younis et al. [24] performed a LCCA for the structures using seawater, recycled concrete aggregate and GFRP reinforcements, whose 100-year cost was 50% lower than the conventional combination (black steel, fresh water, and natural aggregate). LCCA was carried out for a concrete water tank to determine the most cost effective reinforcement alternatives [25] and it was concluded that based on a 100-year period analysis, the use of GFRP, SS, and EC reinforcements could save an approximately 43%, 25% and 11% net present cost (NPC), respectively, compared with the black steel combination. Cadenazzi et al. [26] addressed a LCCA to verify the cost effectiveness of four reinforcement alternatives (BS, SS, FRP, EC) for the design of a demonstration FRP-RC bridge in Florida, namely, Halls River Bridge, where the FRP-RC and SS-RC combinations showed economic and environmental benefits.Although previous studied showed that the use of stainless steels or FRP reinforcements normally gain financial benefits in a long term, the cases for the RC structures in tropic islands or reefs have been not adequately discussed based on LCCA. The characters of such cases could be summarized as following: (1) high-temperature, high-humidity, high salinity and windy weather make up a particularly harsh environment for the serving RC structures, which accelerates the performance-degradation process compared to conventional ones; (2) the tropic islands or reefs could be far away from lands, resulting in a really high transportation fee, which encourages the use of local materials (e.g. coral aggregate, seawater). This paper focuses the South China Sea (Figure1(b)), a representative of harsh island or reef environment, where the reported corrosion degree of steel reinforcement could be up to 22.5% after 40 years [27] (Figure 1(b)), far greater than that of the marine structures in England [28], Singapore [29], Yellow sea [30], etc. Therefore, the necessity of conducting the LCCA to evaluate the cost effectiveness of different material alternatives has been upgraded. Though the stainless reinforcements have perfect structural and durable performance [31], the high price (5-6 times that of black steel) remains a huge obstacle for its wider application. Recently, the stainless-clad bimetallic steel reinforcement has drawn great attention (Figure 2(a)), where the stainless steel cladding layer, with thickness of 1.7–3.1 mm, is tightly integrated with the black steel substrate through the metallurgical bonding layer [32, 34, 35]. The stainless surface prevents the delicate substrate from premature corrosion, while reducing the material cost by 30%, showing great prospects of replacing stainless steel reinforcement in the marine structures [36, 37]. As mentioned earlier, utilizing the local materials could help cut down the material cost and transportation fee. On the beach of remote island or reef in the South China Sea, a mound of coral debris accumulates as time goes by, providing raw materials for manufacturing the coarse and fine coral aggregate after mechanical crushing, as shown in Figure 2(b). Moreover, by replacing fresh water by hands-down sea water, the high transportation fee could be saved. The mechanical properties of coral aggregate sea water concrete (CASC) have been investigated in recent years [6, 38–41], where it is concluded that the light-weight CASC is slightly more brittle than ordinary concrete and the strength is generally lower. As for the durability of the RC structures using CASC, since the coral aggregate and sea water are both rich of chloride ion, the metal reinforcements are easier to be corroded compared with ordinary concrete [42–46]. Hence, the service life of CASC structures is expected to be shorter than that of ordinary RC structures and the subsequent maintenance may outweigh the initial cost benefits. The LCCA carried out in this paper could bring valuable advices for the material proposals of RC structures in remote harsh marine environment, while providing recommendations for future research works.Figure 2
The material alternatives. (a) Stainless-clad bimetallic steel [32]. (b) Coral debris and crushed aggregate [8, 33].
(a)(b)


## 2. Overall Introduction on the Project

The life-cycle-cost analysis is based on a 3-floor office building in Subi Reef (also called Zhubi Dao). This island is at the middle of the South China sea (as shown in Figure1(b)), serving as an important military base and the administrative center of Nansha. The architectural design of the office building is referred to a real project in Sri Lanka [47]. The building height of the frame RC structure is 16 m, and the building area of the office is 897 m2 while the parking area is 430 m2. The material consuming of the office building is shown in Table 1.Table 1
Summary of the material consuming [47].
MaterialUnitQuantityMass (%)Concretem3576.5444.98Steel reinforcementkg114,760.003.73Clay brickskg855,439.0027.80Cement mortarkg492,376.5016.00Otherskg230443.007.49


## 3. The Lifespan Prediction

### 3.1. The Performance Degradation Process of RC Structures

Basically, the lifespan of an RC structure depends on the service environment and the durability of materials. For the service environment, the extremely high or low temperature, the rapid change of humidity and the high concentration of corrosive substances (such as chloride and sulfate) could all accelerate the performance degradation and the resultant lifespan-shortening. As for the materials, as introduced earlier, the high-performance concrete and corrosion-resistant reinforcements are effective to delay the performance degradation process and extend the service life of RC structures.A typical structural performance degradation process of RC structures is shown in Figure3. Generally, during the early stage of service life (namely the initial period: t = 0 ∼ ti), the performance remains at a high level. The threshold (t = ti) represents the corrosion of steel reinforcement, induced by carbonation of the concrete cover or chloride ingress. For marine structures, the chloride ingress is the main reason [5]. The passivity on embedded steel protects the reinforcement from corrosion until the moment that the chloride concentration on the surface of rebar (Cb) reaching the critical value (Ccr). The corrosion products accumulate around the reinforcements, resulting in the cracking along the reinforcements (t = tc) since the volume of corrosion products is larger than that of the reactants. The longitudinal crack propagation later induces the perpendicular crack, which further accelerates the degradation of structural performance (t = tc ∼ tr). When the performance decreases to an unacceptable level, the structure needs to be repaired (t = tr) or rebuilt.Figure 3
The typical performance degradation process of RC structures.


### 3.2. Service Life Prediction Model

The chloride diffusion process in RC structures is usually described by Fick’s second law [48]:(1)∂C∂t=Da∂2C∂x2,where C is the chloride content, t is time, Da is the apparent diffusion coefficient, and x is the depth from the exposed concrete surface.A large number of studies were conducted on the service life prediction of RC structures, and several kinds of software such as Life-365 [49], DuraCrete [50], etc., are available for practical use. However, the feasibility of these models for predicting the service life of CASC structures has not been validated by field data. Yu et al. [2, 51] proposed a new chloride diffusion equation by modifying Fick’s second law, as shown in equation (2). Compared with previous models [52–54], this model could consider: (1) the decreasing of free diffusion coefficient (Df) as exposure time (t) going on; (2) the chloride binding capacity of concrete; (3) the microdefects and scraping in concrete structures, both accelerate the chloride diffusion.(2)∂Cf∂t=KD0t0,Tt0m1+Rt−m∂2Cf∂x2,where Cf is surface-free chloride concentration, K is deterioration coefficient of chloride diffusion behavior, D0 (t0, T) is free chloride diffusion coefficient when exposure time is t0 (usually t0 = 28 day) and temperature is T, D0(t0, T) is expressed in equation (3), m is the time-dependent index of chloride diffusion coefficient, R is the chloride binding capacity and R = ∂Ct/∂Cf  − 1 where Ct is the total chloride concentration.(3)D0t0,T=D0t0,T0TT0eq1/T0−1/T,where T0 = 293.15 K, q is the activation constant, when W/B = 0.4, q = 6000 K, when W/B = 0.5, q = 5450 K, and when W/B = 0.6, q = 3850 K. The time-varying boundary condition is used to solve the new equation, where the power function relationships between surface free chloride concentration (Cs) and exposure time (t) are expressed as follows:(4)Cs=kt0.51−m+C00<t<tc,ktc0.51−m+C0t≥tc,where k is fitting parameter and tc is time-varying termination point, usually tc = 7, 15 or 25 a.More details about the model were introduced in references [55] and [56]. Based on this modified chloride diffusion theory and the reliability theory, a software named ChaDuraLife V1.0 [57] was developed for predicting the service life of concrete structures in chloride environment. The reliability of the model has been proven by numerous field statistic validation and engineering calculation on ordinary concrete structures [58, 59] and coral aggregate concrete structures [60]. Therefore, the service life of the concrete structures in this study is predicted by ChaDuraLife V1.0 [57].


### 3.3. The Durability Design Requirements

For reinforced concrete (RC) structures, the durability requirements in GB/T 50476–2019 [61] should be met. According to Section 3.2.1, the exposure class of the office building is III, since the chloride-induced corrosion is the main concern. The more accurate class based on the specific service condition of structural component should be determined by Section 6.2.1, where class III-E is defined for this building (at the air zone within 15 m from sea level). For structures at class III-E, when the service life is 100 years, the concrete grade should be higher than C50 (Section 3.4) while c ≥ 65 mm and W/B ≤ 0.36 should be satisfied (Section 6.3.2), where c is the concrete cover thickness and W/B is water-to-binder ratio.


### 3.4. The Service Life Prediction

Although ChaDuraLife V1.0 [57] is quite suitable for predicting the service life of RC structures in this study, this software has not been open access. In so doing, we are not able to analyze any case we want for this study. Fortunately, the developers have used this software to predict the service life of innumerable RC structures, and the results have been published in detail [55, 60, 62–65]. Among all, we choose some of the cases by following principles: (1) the durability requirements in Section 3.3 should be met; (2) the compressive strength of all the concrete should be close to ensure the basic mechanical performances of the structures using different material proposals are approximately the same; (3) the concrete cover thickness for all the cases is the same; (4) the service life prediction results should be based on the same failure probability (pf = 5%).Totally, five material combinations extracted from references [60] and [65] are included in this study, as shown in Table 2. Three kinds of reinforcements are considered, namely, black steel (BS), stainless-clad bimetallic steel (SB), and stainless steel (SS). The FRP is also a durable material notwithstanding, the FRP reinforcement is not involved in this study. The reason is, FRP is quite different from that of metals in terms of the mechanical properties such as elastic modulus, strength, ductility and bond behavior with surrounding concrete. In this paper, however, to make the material proposals comparable, the LCCA among the material proposals is based on the similar mechanical behaviors of the RC structures. As for the concrete, high-performance concrete (HPC) and concrete aggregate sea water concrete (CASC) are adopted, and the material properties of them are presented in Table 3. It should be noticed that the combination of BS + CASC is not listed, since the service life of this combination is too short (1∼2 years [60]) to be compared in this study.Table 2
The service-life prediction for the RC structures using different material proposals.
Material proposalti (year)Referencetr (year)BS + HPC33[65]39SB + HPC100[65]—SS + HPC100[65]—SB + CASC17[60]23SS + CASC17[60]23BS: black steel; SB: stainless-clad bimetallic steel; SS: stainless steel; HPC: high-performance concrete; CASC: coral aggregate seawater concrete.Table 3
Concrete mixture and properties.
ConcreteGradefcc (MPa)Density (kg/m3)W/BComponents (kg/m3)ReferenceCementFly ashSlagStoneRiver sandFresh waterWRAEGHPCC5055.20.3220003001168657160.54.50.04[65]CementFly ashSlagCoral aggregateCoral sandSea waterWRCASCC5053.822000.286206012036986022416[60]HPC: high-performance concrete; CASC: concrete aggregate sea water concrete;fcc: 28-day cube compressive strength; W/B: water-to-binder ratio; WR: water-reducer; AEG: air-entraining agent.Given a 100-year period, the service-life prediction results for the concrete structures using different material proposals are shown in Table2 and Figure 4. As seen in Figure 4, the failure probability-service time (pf -t) relationships vary among material proposals, and the t value corresponding to the point of the curve where pf = 5% is recorded as ti in Table 2. Identical influence of using stainless-clad bimetallic steel (SB) or stainless steel (SS) on the service life of RC structures is noticed, since the service life prediction is based on the beginning of rebar corrosion (Cr ≥ Ccr) and Ccr value is the same for SB and SS (Ccr-SS ≈ 0.7% [55]). It should be noticed that the stainless steel discussed in this paper is 304 stainless steel, which is much more commonly used in structures than the other types. As it should be, the application of SB/SS is effective for the corrosion delay, since the Ccr value for black steel (BS) is much smaller (Ccr-BS ≈ 0.05% [55]) than that of SB/SS. Besides, for the same reinforcement, using HPC could significantly extend the service life of RC structures, compared to those using CASC. It is due to the fact that the initial chloride concentration (Cb0) of CASC is much higher than that of HPC, which makes the rebar corrosion easier to happen in CASC. Thanks to the excellent corrosion resistance of reinforcement (high Ccr-SS value) and initial chloride concentration in concrete (low Cb0 value), the proposals SB/SS + HPC have the longest predicted service life (100 years).Figure 4
The failure probability-service time (pf  − t) plot of the RC structures.


## 3.1. The Performance Degradation Process of RC Structures

Basically, the lifespan of an RC structure depends on the service environment and the durability of materials. For the service environment, the extremely high or low temperature, the rapid change of humidity and the high concentration of corrosive substances (such as chloride and sulfate) could all accelerate the performance degradation and the resultant lifespan-shortening. As for the materials, as introduced earlier, the high-performance concrete and corrosion-resistant reinforcements are effective to delay the performance degradation process and extend the service life of RC structures.A typical structural performance degradation process of RC structures is shown in Figure3. Generally, during the early stage of service life (namely the initial period: t = 0 ∼ ti), the performance remains at a high level. The threshold (t = ti) represents the corrosion of steel reinforcement, induced by carbonation of the concrete cover or chloride ingress. For marine structures, the chloride ingress is the main reason [5]. The passivity on embedded steel protects the reinforcement from corrosion until the moment that the chloride concentration on the surface of rebar (Cb) reaching the critical value (Ccr). The corrosion products accumulate around the reinforcements, resulting in the cracking along the reinforcements (t = tc) since the volume of corrosion products is larger than that of the reactants. The longitudinal crack propagation later induces the perpendicular crack, which further accelerates the degradation of structural performance (t = tc ∼ tr). When the performance decreases to an unacceptable level, the structure needs to be repaired (t = tr) or rebuilt.Figure 3
The typical performance degradation process of RC structures.


## 3.2. Service Life Prediction Model

The chloride diffusion process in RC structures is usually described by Fick’s second law [48]:(1)∂C∂t=Da∂2C∂x2,where C is the chloride content, t is time, Da is the apparent diffusion coefficient, and x is the depth from the exposed concrete surface.A large number of studies were conducted on the service life prediction of RC structures, and several kinds of software such as Life-365 [49], DuraCrete [50], etc., are available for practical use. However, the feasibility of these models for predicting the service life of CASC structures has not been validated by field data. Yu et al. [2, 51] proposed a new chloride diffusion equation by modifying Fick’s second law, as shown in equation (2). Compared with previous models [52–54], this model could consider: (1) the decreasing of free diffusion coefficient (Df) as exposure time (t) going on; (2) the chloride binding capacity of concrete; (3) the microdefects and scraping in concrete structures, both accelerate the chloride diffusion.(2)∂Cf∂t=KD0t0,Tt0m1+Rt−m∂2Cf∂x2,where Cf is surface-free chloride concentration, K is deterioration coefficient of chloride diffusion behavior, D0 (t0, T) is free chloride diffusion coefficient when exposure time is t0 (usually t0 = 28 day) and temperature is T, D0(t0, T) is expressed in equation (3), m is the time-dependent index of chloride diffusion coefficient, R is the chloride binding capacity and R = ∂Ct/∂Cf  − 1 where Ct is the total chloride concentration.(3)D0t0,T=D0t0,T0TT0eq1/T0−1/T,where T0 = 293.15 K, q is the activation constant, when W/B = 0.4, q = 6000 K, when W/B = 0.5, q = 5450 K, and when W/B = 0.6, q = 3850 K. The time-varying boundary condition is used to solve the new equation, where the power function relationships between surface free chloride concentration (Cs) and exposure time (t) are expressed as follows:(4)Cs=kt0.51−m+C00<t<tc,ktc0.51−m+C0t≥tc,where k is fitting parameter and tc is time-varying termination point, usually tc = 7, 15 or 25 a.More details about the model were introduced in references [55] and [56]. Based on this modified chloride diffusion theory and the reliability theory, a software named ChaDuraLife V1.0 [57] was developed for predicting the service life of concrete structures in chloride environment. The reliability of the model has been proven by numerous field statistic validation and engineering calculation on ordinary concrete structures [58, 59] and coral aggregate concrete structures [60]. Therefore, the service life of the concrete structures in this study is predicted by ChaDuraLife V1.0 [57].


## 3.3. The Durability Design Requirements

For reinforced concrete (RC) structures, the durability requirements in GB/T 50476–2019 [61] should be met. According to Section 3.2.1, the exposure class of the office building is III, since the chloride-induced corrosion is the main concern. The more accurate class based on the specific service condition of structural component should be determined by Section 6.2.1, where class III-E is defined for this building (at the air zone within 15 m from sea level). For structures at class III-E, when the service life is 100 years, the concrete grade should be higher than C50 (Section 3.4) while c ≥ 65 mm and W/B ≤ 0.36 should be satisfied (Section 6.3.2), where c is the concrete cover thickness and W/B is water-to-binder ratio.


## 3.4. The Service Life Prediction

Although ChaDuraLife V1.0 [57] is quite suitable for predicting the service life of RC structures in this study, this software has not been open access. In so doing, we are not able to analyze any case we want for this study. Fortunately, the developers have used this software to predict the service life of innumerable RC structures, and the results have been published in detail [55, 60, 62–65]. Among all, we choose some of the cases by following principles: (1) the durability requirements in Section 3.3 should be met; (2) the compressive strength of all the concrete should be close to ensure the basic mechanical performances of the structures using different material proposals are approximately the same; (3) the concrete cover thickness for all the cases is the same; (4) the service life prediction results should be based on the same failure probability (pf = 5%).Totally, five material combinations extracted from references [60] and [65] are included in this study, as shown in Table 2. Three kinds of reinforcements are considered, namely, black steel (BS), stainless-clad bimetallic steel (SB), and stainless steel (SS). The FRP is also a durable material notwithstanding, the FRP reinforcement is not involved in this study. The reason is, FRP is quite different from that of metals in terms of the mechanical properties such as elastic modulus, strength, ductility and bond behavior with surrounding concrete. In this paper, however, to make the material proposals comparable, the LCCA among the material proposals is based on the similar mechanical behaviors of the RC structures. As for the concrete, high-performance concrete (HPC) and concrete aggregate sea water concrete (CASC) are adopted, and the material properties of them are presented in Table 3. It should be noticed that the combination of BS + CASC is not listed, since the service life of this combination is too short (1∼2 years [60]) to be compared in this study.Table 2
The service-life prediction for the RC structures using different material proposals.
Material proposalti (year)Referencetr (year)BS + HPC33[65]39SB + HPC100[65]—SS + HPC100[65]—SB + CASC17[60]23SS + CASC17[60]23BS: black steel; SB: stainless-clad bimetallic steel; SS: stainless steel; HPC: high-performance concrete; CASC: coral aggregate seawater concrete.Table 3
Concrete mixture and properties.
ConcreteGradefcc (MPa)Density (kg/m3)W/BComponents (kg/m3)ReferenceCementFly ashSlagStoneRiver sandFresh waterWRAEGHPCC5055.20.3220003001168657160.54.50.04[65]CementFly ashSlagCoral aggregateCoral sandSea waterWRCASCC5053.822000.286206012036986022416[60]HPC: high-performance concrete; CASC: concrete aggregate sea water concrete;fcc: 28-day cube compressive strength; W/B: water-to-binder ratio; WR: water-reducer; AEG: air-entraining agent.Given a 100-year period, the service-life prediction results for the concrete structures using different material proposals are shown in Table2 and Figure 4. As seen in Figure 4, the failure probability-service time (pf -t) relationships vary among material proposals, and the t value corresponding to the point of the curve where pf = 5% is recorded as ti in Table 2. Identical influence of using stainless-clad bimetallic steel (SB) or stainless steel (SS) on the service life of RC structures is noticed, since the service life prediction is based on the beginning of rebar corrosion (Cr ≥ Ccr) and Ccr value is the same for SB and SS (Ccr-SS ≈ 0.7% [55]). It should be noticed that the stainless steel discussed in this paper is 304 stainless steel, which is much more commonly used in structures than the other types. As it should be, the application of SB/SS is effective for the corrosion delay, since the Ccr value for black steel (BS) is much smaller (Ccr-BS ≈ 0.05% [55]) than that of SB/SS. Besides, for the same reinforcement, using HPC could significantly extend the service life of RC structures, compared to those using CASC. It is due to the fact that the initial chloride concentration (Cb0) of CASC is much higher than that of HPC, which makes the rebar corrosion easier to happen in CASC. Thanks to the excellent corrosion resistance of reinforcement (high Ccr-SS value) and initial chloride concentration in concrete (low Cb0 value), the proposals SB/SS + HPC have the longest predicted service life (100 years).Figure 4
The failure probability-service time (pf  − t) plot of the RC structures.


## 4. Life-Cycle-Cost (LCC) Modeling

### 4.1. Model Structures

Life-cycle-cost analysis undertakes the integrated cost proposal for the construction. Life cycle cost (LCC) principally consists of construction cost, maintenance cost, and end-of-life cost [10, 18, 24, 25, 66]. The detailed components can be added or refined accounting for a specific case such as material, inspection as so on [25, 66]. In this paper, the life cycle cost comprises material cost (Cm), construction cost (Cc), repair cost (Cr), end-of-life cost (Ce), which is shown in equation (5). The material cost is extracted from construction cost because the choice of reinforcement and concrete explicitly affects the initial cost and long-term cost in LCCA of marine constructions. Also, detailing the type and amount of materials are useful to evaluate the construction cost and maintenance cost which could be expensive in marine infrastructures. Indeed, the marine environment affects each cost at a different level. The content of each cost component is discussed in this section corresponding to the construction in the marine environment.(5)LCC=Cm+Cc+Cr+Ce,where Cm is material cost, Cc is construction cost, Cr is repair cost, and Ce is end-of-life cost.


### 4.2. The Material Cost (Cm)

This section evaluates the material cost of a 3-floor office building at the selected island. The material consumption for cost analysis is shown in Table1. Specifically, concrete and steel reinforcements are the basic unit of building, the cost of which explicitly decides the total material cost. The unit cost of concrete is calculated by the mix proportion of CASC and HPC that is shown in Table 3. Moreover, the clay bricks and cement mortar are counted due to the large proportion of mass. The rest of materials insignificantly contribute to material cost, but the mass of them is later counted for transportation cost estimation. The material unit costs are summarized in Table 4 in CNY (¥), which includes reinforcements, each component of concrete, and other basic building materials. The prices are obtained from paper, website, government, and manufacturers. These prices try to be close to market prices as much as possible in Hainan, the province nearest to the construction site.Table 4
Unit cost.
MaterialUnit cost (¥, CNY)SourceBlack steel bar HRB4004210 ¥/t[67]Stainless-clad bimetallic steel bar21000 ¥/t[68]Stainless steel bar30000 ¥/t[67]HPC861 ¥/m3CalculatedCement for HPC3200 ¥/t[69]Fresh water3.2 ¥/t[70]Fly ash90 ¥/t[71]Slag150 ¥/t[72]River sand132 ¥/t[71]Water reducer12000 ¥/t[73]Air-entraining agent8000 ¥/t[74]CASC591 ¥/m3CalculatedCoral aggregate90 ¥/t[75]Stone67 ¥/t[71]Cement C50640 ¥/t[67]Coral sand115 ¥/t[75]Sea water10 ¥/t[75]Cement mortar503 ¥/t[67]Clay brick246 ¥/t[67]Scrap steel1800 ¥/t[72]Scrap stainless steel6000 ¥/t[72]Scrap stainless-clad bimetallic steel2640 ¥/t[32]


### 4.3. The Construction Cost (Cc)

Construction cost generally incorporates labor, machine, formwork, and transportation. In LCCA, the construction cost can be assumed as the proportion to material cost. Younis et al. [24] conservatively estimates that construction cost is 150% of material cost, in which labor, machine, formwork, and transportation are all included. Val and Stewart [1] specify the labor cost as 60% of material cost for the marine RC structure, but the machine and formwork are not counted. In this paper, the construction cost equation is calculated by(6)Cc=Cbase+CL,EX+CT,where Cbase is basic cost of labor, machine, formwork, CL,EX is the extra labor cost caused by hard marine environment, and CT is the transportation cost.The basic cost of labor, machine, and formworks are estimated as 125% of the BS + HPC proposal material cost. It is less than Younis’ assumption about 25%, because the difficulty of building a 3-floor office is less than that of a 20 to 70-floor building. According to the unit costs introduced in [66], the proportions of labor, machine, and formworks cost are assumed as 0.46, 0.20, 0.34, respectively. Furthermore, the extra 10% material cost is added to labor cost due to latitude marine environment such as high temperature, strong ultraviolet ray, server convection weather, as so on. The final labor cost is 67.5% of material cost which is slightly expensive than Dimitri’s assumption [1]. In marine construction, not only hard environment can cause the extra cost of employing labors, the inadequate transportation is also a challenge for transferring materials. Lin [69] investigates an island building in the similar region with this paper, and the transportation cost is specified as 0.38 ¥/km for cement, sand, and aggregate; 0.54 ¥/km for fresh water. In this paper, the distance from site to Sanya is about 1010 km, thus, the transportation rate for solid and fresh water are 383.8 ¥/t, 545.4 ¥/t, respectively. It should be noted that the coral sand and coral aggregate are collected in site, the considerable transportation cost is expected to be saved.


### 4.4. The Repair Cost (Cr)

Repair is necessary to keep structures in good working condition [76] during the long service period. Basically, the repair cost (Cr) consists of the cost for inspection, repair and rebuild [24].(7)Cr=CIN+CRP+CRB,where CIN is the inspection cost, CRP is the repair (including replacement) cost, and CRB is the rebuild cost, all in present values.The inspection helps monitoring the operating functions and safety conditions of the structures [77]. The inspection cost depends on the inspection strategies planned by the stakeholders [23]. It is advised that general inspection could be conducted at 5-year intervals for detecting obvious defects which may lead to safety problems while the detailed inspection is carried out just before the repair to provide fundamental information [25]. Given this inspection strategy, the inspection cost could be estimated as 3% (Cm + Cc) [25].The repair strategy depends on the damage of the structure, with no standard schedule to be referred. Although a tailor-made schedule could be made by using Bayesian techniques based on the monitoring data from field inspection for obtaining the real-time performance of the concerned structure [77–80], it is not feasible to be used in this study for the lack of field data. In Life-365 software [49], a simplified repair schedule is provided for common application. The default repair schedule considers a fixed area to repair (10%) and a fix repair interval (10 years), starting from tr. A more accurate schedule is also available if the material properties and environmental parameters are input by users. Nevertheless, the service-life prediction model adopted in this paper is different from that in Life-365, the repair schedule calculated by the software may have potential conflict with the predicted results previously.Since the case of the actual marine structure is believed to be more valuable for the present study, the repair/rebuild schedule is based on the LCCA of the Progreso Pier [81], as presented in Figure 5. The first repair is planned at tr, where tr = ti + tp and propagation period (tp) is fixed at six years [49], as calculated in Table 2. The second repair is arranged 15 years after the first repair, which means tr2 = tr1 + 15. Similarly, the third repair time tr3 = tr2 + 15, and the complete rebuild will be 10 years after that (tr3 = tr2 + 10). Therefore, the RC structure using BS + HPC proposal will be repaired at Year 39, 54, 69, respectively, before the complete rebuild at Year 79. It should be noticed that the repair costs for the SB/SS + HPC proposals are not considered, because: (1) the structural components are of much higher quality than those of other proposals (see Figure 4); (2) based on reason 1), the performance degradation of nonstructural components (such as partition/infill walls) is resultantly slowed down; (3) the actual inspection costs for SB/SS + HPC proposals may have been overestimated. As for the SB/SS + CASC proposal, Year 23, 38 and 53 are the repair years, after which the structure will be rebuilt at Year 63. It should be noticed that the newly built structure needs to be repaired at Year 86 (tr−1 = trb + tr = 2tr + 40 < 100). Given the fact that the damaged area of the RC structure will expand as time goes by, the area to be repaired is assumed to be 10%, 15% and 20% of total construction area for each repair, respectively. Besides, not all the materials in the damaged area need to be replaced, it is assumed that 50% [25] of the damaged materials will be replaced and equivalent new materials will be installed.Figure 5
The repair/rebuilt schedule.According to the data reported in reference [82], the repair/maintenance cost (Cr) is two times the sum of material cost (Cm) and construction cost (Cc) plus the quality assurance cost (CQA), namely Cr = 2 (Cm + Cc) + CQA. Considering the case in this study (fc = 50 MPa, c = 70 mm), CQA is 0 [25], which means Cr = 2 (Cm + Cc). As a result, the repair cost for the first, second and third repair is 10% (Cm + Cc), 15% (Cm + Cc) and 20% (Cm + Cc), respectively.As for the rebuild cost, it is assumed to be equal to the present value of the sum of initial material and construction cost.


### 4.5. The End-of-Life Cost (Ce)

End-of-life cost includes the cost of demolition, disposal, and landfill. The common equation for calculating end of life cost is:(8)Ce=CD+CLF−CRCC,where CD is demolition cost, CLF is landfill cost, CRCC is recycle value.The demolition cost is usually site-specific, which depends on location, type of construction, scale of construction. Guo [83] points out that the prefabricated building has less demolition cost than cast-in building, which are 20 and 37 ¥/m2, respectively. Younis et al. [24] specifies demolition rate for RC building as 122 $/m3 (789 ¥/m3) that including 20-mile haul to dump, excluding dumping fees. According to the expert engineer in design institution, in China, the demolition unit cost can vary from 100 to 400 ¥/m3 depending on dismantling by machine or labor. In this case, the building is assumed to be fully dismantled when reaches the end of lifetime. Although the machine could be the main force of demolition, there still remains additional obstacles for transferring machine and labors to the remote islands and reefs. Thus, the demolition unit cost is assumed as 350 ¥/m3, including potential transportation cost and labor cost. As for the disposal and landfill, the concrete is abandoned in the end of lifetime and the landfill rate is 35 ¥/t [84]; steel is however the perfectly recyclable material and it is assumed that 90% of reinforcements can be recycled [24, 26]. Hence, the recycle rate of black steel, stainless steel, and stainless-clad bimetallic steel are 1800, 6000, 2640 ¥/t, respectively, as shown in Table 4, where the scrap stainless-clad bimetallic steel rate is calculated based on the steel-stainless proportion introduced in Ref. [32]. Additionally, the rate of transferring scrap steel is 383.8 ¥/t, the recycle value of scrap steel is the repurchase rate minus transportation cost.


### 4.6. The Net Present Cost (NPC)

NPC illustrates the current value of the future investment, which is usually calculated by equation (9) [85]. Typically, the discount rate (r) is the crucial parameter of NPC, which correlates with social discount rate and inflation rate. They all express the value of currency varying. The discount rate varies among different countries or regions. In the early studies, the discount rate is used to take as 3∼6% [85, 86]. In recent studies, the discount rate is set as a lower number that is 0.6∼1% [24–26]. In this study, the baseline discount rate is assumed to be 0.8%.(9)NPC=∑n=0NCn1+rn,where Cn is all cost incurred at the year n, r is the discount rate, and N is the studied year.


## 4.1. Model Structures

Life-cycle-cost analysis undertakes the integrated cost proposal for the construction. Life cycle cost (LCC) principally consists of construction cost, maintenance cost, and end-of-life cost [10, 18, 24, 25, 66]. The detailed components can be added or refined accounting for a specific case such as material, inspection as so on [25, 66]. In this paper, the life cycle cost comprises material cost (Cm), construction cost (Cc), repair cost (Cr), end-of-life cost (Ce), which is shown in equation (5). The material cost is extracted from construction cost because the choice of reinforcement and concrete explicitly affects the initial cost and long-term cost in LCCA of marine constructions. Also, detailing the type and amount of materials are useful to evaluate the construction cost and maintenance cost which could be expensive in marine infrastructures. Indeed, the marine environment affects each cost at a different level. The content of each cost component is discussed in this section corresponding to the construction in the marine environment.(5)LCC=Cm+Cc+Cr+Ce,where Cm is material cost, Cc is construction cost, Cr is repair cost, and Ce is end-of-life cost.


## 4.2. The Material Cost (Cm)

This section evaluates the material cost of a 3-floor office building at the selected island. The material consumption for cost analysis is shown in Table1. Specifically, concrete and steel reinforcements are the basic unit of building, the cost of which explicitly decides the total material cost. The unit cost of concrete is calculated by the mix proportion of CASC and HPC that is shown in Table 3. Moreover, the clay bricks and cement mortar are counted due to the large proportion of mass. The rest of materials insignificantly contribute to material cost, but the mass of them is later counted for transportation cost estimation. The material unit costs are summarized in Table 4 in CNY (¥), which includes reinforcements, each component of concrete, and other basic building materials. The prices are obtained from paper, website, government, and manufacturers. These prices try to be close to market prices as much as possible in Hainan, the province nearest to the construction site.Table 4
Unit cost.
MaterialUnit cost (¥, CNY)SourceBlack steel bar HRB4004210 ¥/t[67]Stainless-clad bimetallic steel bar21000 ¥/t[68]Stainless steel bar30000 ¥/t[67]HPC861 ¥/m3CalculatedCement for HPC3200 ¥/t[69]Fresh water3.2 ¥/t[70]Fly ash90 ¥/t[71]Slag150 ¥/t[72]River sand132 ¥/t[71]Water reducer12000 ¥/t[73]Air-entraining agent8000 ¥/t[74]CASC591 ¥/m3CalculatedCoral aggregate90 ¥/t[75]Stone67 ¥/t[71]Cement C50640 ¥/t[67]Coral sand115 ¥/t[75]Sea water10 ¥/t[75]Cement mortar503 ¥/t[67]Clay brick246 ¥/t[67]Scrap steel1800 ¥/t[72]Scrap stainless steel6000 ¥/t[72]Scrap stainless-clad bimetallic steel2640 ¥/t[32]


## 4.3. The Construction Cost (Cc)

Construction cost generally incorporates labor, machine, formwork, and transportation. In LCCA, the construction cost can be assumed as the proportion to material cost. Younis et al. [24] conservatively estimates that construction cost is 150% of material cost, in which labor, machine, formwork, and transportation are all included. Val and Stewart [1] specify the labor cost as 60% of material cost for the marine RC structure, but the machine and formwork are not counted. In this paper, the construction cost equation is calculated by(6)Cc=Cbase+CL,EX+CT,where Cbase is basic cost of labor, machine, formwork, CL,EX is the extra labor cost caused by hard marine environment, and CT is the transportation cost.The basic cost of labor, machine, and formworks are estimated as 125% of the BS + HPC proposal material cost. It is less than Younis’ assumption about 25%, because the difficulty of building a 3-floor office is less than that of a 20 to 70-floor building. According to the unit costs introduced in [66], the proportions of labor, machine, and formworks cost are assumed as 0.46, 0.20, 0.34, respectively. Furthermore, the extra 10% material cost is added to labor cost due to latitude marine environment such as high temperature, strong ultraviolet ray, server convection weather, as so on. The final labor cost is 67.5% of material cost which is slightly expensive than Dimitri’s assumption [1]. In marine construction, not only hard environment can cause the extra cost of employing labors, the inadequate transportation is also a challenge for transferring materials. Lin [69] investigates an island building in the similar region with this paper, and the transportation cost is specified as 0.38 ¥/km for cement, sand, and aggregate; 0.54 ¥/km for fresh water. In this paper, the distance from site to Sanya is about 1010 km, thus, the transportation rate for solid and fresh water are 383.8 ¥/t, 545.4 ¥/t, respectively. It should be noted that the coral sand and coral aggregate are collected in site, the considerable transportation cost is expected to be saved.


## 4.4. The Repair Cost (Cr)

Repair is necessary to keep structures in good working condition [76] during the long service period. Basically, the repair cost (Cr) consists of the cost for inspection, repair and rebuild [24].(7)Cr=CIN+CRP+CRB,where CIN is the inspection cost, CRP is the repair (including replacement) cost, and CRB is the rebuild cost, all in present values.The inspection helps monitoring the operating functions and safety conditions of the structures [77]. The inspection cost depends on the inspection strategies planned by the stakeholders [23]. It is advised that general inspection could be conducted at 5-year intervals for detecting obvious defects which may lead to safety problems while the detailed inspection is carried out just before the repair to provide fundamental information [25]. Given this inspection strategy, the inspection cost could be estimated as 3% (Cm + Cc) [25].The repair strategy depends on the damage of the structure, with no standard schedule to be referred. Although a tailor-made schedule could be made by using Bayesian techniques based on the monitoring data from field inspection for obtaining the real-time performance of the concerned structure [77–80], it is not feasible to be used in this study for the lack of field data. In Life-365 software [49], a simplified repair schedule is provided for common application. The default repair schedule considers a fixed area to repair (10%) and a fix repair interval (10 years), starting from tr. A more accurate schedule is also available if the material properties and environmental parameters are input by users. Nevertheless, the service-life prediction model adopted in this paper is different from that in Life-365, the repair schedule calculated by the software may have potential conflict with the predicted results previously.Since the case of the actual marine structure is believed to be more valuable for the present study, the repair/rebuild schedule is based on the LCCA of the Progreso Pier [81], as presented in Figure 5. The first repair is planned at tr, where tr = ti + tp and propagation period (tp) is fixed at six years [49], as calculated in Table 2. The second repair is arranged 15 years after the first repair, which means tr2 = tr1 + 15. Similarly, the third repair time tr3 = tr2 + 15, and the complete rebuild will be 10 years after that (tr3 = tr2 + 10). Therefore, the RC structure using BS + HPC proposal will be repaired at Year 39, 54, 69, respectively, before the complete rebuild at Year 79. It should be noticed that the repair costs for the SB/SS + HPC proposals are not considered, because: (1) the structural components are of much higher quality than those of other proposals (see Figure 4); (2) based on reason 1), the performance degradation of nonstructural components (such as partition/infill walls) is resultantly slowed down; (3) the actual inspection costs for SB/SS + HPC proposals may have been overestimated. As for the SB/SS + CASC proposal, Year 23, 38 and 53 are the repair years, after which the structure will be rebuilt at Year 63. It should be noticed that the newly built structure needs to be repaired at Year 86 (tr−1 = trb + tr = 2tr + 40 < 100). Given the fact that the damaged area of the RC structure will expand as time goes by, the area to be repaired is assumed to be 10%, 15% and 20% of total construction area for each repair, respectively. Besides, not all the materials in the damaged area need to be replaced, it is assumed that 50% [25] of the damaged materials will be replaced and equivalent new materials will be installed.Figure 5
The repair/rebuilt schedule.According to the data reported in reference [82], the repair/maintenance cost (Cr) is two times the sum of material cost (Cm) and construction cost (Cc) plus the quality assurance cost (CQA), namely Cr = 2 (Cm + Cc) + CQA. Considering the case in this study (fc = 50 MPa, c = 70 mm), CQA is 0 [25], which means Cr = 2 (Cm + Cc). As a result, the repair cost for the first, second and third repair is 10% (Cm + Cc), 15% (Cm + Cc) and 20% (Cm + Cc), respectively.As for the rebuild cost, it is assumed to be equal to the present value of the sum of initial material and construction cost.


## 4.5. The End-of-Life Cost (Ce)

End-of-life cost includes the cost of demolition, disposal, and landfill. The common equation for calculating end of life cost is:(8)Ce=CD+CLF−CRCC,where CD is demolition cost, CLF is landfill cost, CRCC is recycle value.The demolition cost is usually site-specific, which depends on location, type of construction, scale of construction. Guo [83] points out that the prefabricated building has less demolition cost than cast-in building, which are 20 and 37 ¥/m2, respectively. Younis et al. [24] specifies demolition rate for RC building as 122 $/m3 (789 ¥/m3) that including 20-mile haul to dump, excluding dumping fees. According to the expert engineer in design institution, in China, the demolition unit cost can vary from 100 to 400 ¥/m3 depending on dismantling by machine or labor. In this case, the building is assumed to be fully dismantled when reaches the end of lifetime. Although the machine could be the main force of demolition, there still remains additional obstacles for transferring machine and labors to the remote islands and reefs. Thus, the demolition unit cost is assumed as 350 ¥/m3, including potential transportation cost and labor cost. As for the disposal and landfill, the concrete is abandoned in the end of lifetime and the landfill rate is 35 ¥/t [84]; steel is however the perfectly recyclable material and it is assumed that 90% of reinforcements can be recycled [24, 26]. Hence, the recycle rate of black steel, stainless steel, and stainless-clad bimetallic steel are 1800, 6000, 2640 ¥/t, respectively, as shown in Table 4, where the scrap stainless-clad bimetallic steel rate is calculated based on the steel-stainless proportion introduced in Ref. [32]. Additionally, the rate of transferring scrap steel is 383.8 ¥/t, the recycle value of scrap steel is the repurchase rate minus transportation cost.


## 4.6. The Net Present Cost (NPC)

NPC illustrates the current value of the future investment, which is usually calculated by equation (9) [85]. Typically, the discount rate (r) is the crucial parameter of NPC, which correlates with social discount rate and inflation rate. They all express the value of currency varying. The discount rate varies among different countries or regions. In the early studies, the discount rate is used to take as 3∼6% [85, 86]. In recent studies, the discount rate is set as a lower number that is 0.6∼1% [24–26]. In this study, the baseline discount rate is assumed to be 0.8%.(9)NPC=∑n=0NCn1+rn,where Cn is all cost incurred at the year n, r is the discount rate, and N is the studied year.


## 5. Results and Discussion

Table5 and Figure 6 summarize the LCC results and details about each cost component. The total material cost varies from ¥1.4 million to 4.4 million. This variation is principally due to the steel reinforcement alternatives. The reinforcement cost of black steel is ¥0.48 million, which is the least. The reinforcement cost of stainless steel is approximately 7 times that of black steel, which is ¥3.4 million. Considering such an obvious difference in the price of black and stainless steel, it is necessary to identify a reinforcement alternative which combines corrosion-resistance with cost-effectiveness. The results show that the use of stainless-clad bimetallic steel saves considerable material cost (about ¥1 million) compared with the stainless steel proposal. Moreover, replacing HPC by CASC could save about ¥0.16 million, account for only 3.6% of the total material cost, since the cost of corrosion-resistant reinforcement takes the lead.Table 5
Life-cycle-cost components for different material proposals (¥, CNY).
Material proposalCm (¥)Cc (¥)Cr (¥)Ce (¥)NPC (¥)BS + HPC1437739281535335714972127068037297SB + HPC336456028153531274581735996480971SS + HPC43974002815353148760171697378683SB + CASC32090002429818564736617359911459783SS + CASC4241840242981866817711716913370599Cm: material cost; Cc: construction cost; Cr: maintenance/repair cost; Ce: end-of-life cost; BS: black steel; SB: stainless-clad bimetallic steel; SS: stainless steel; HPC: high-performance concrete; CASC: coral aggregate seawater concrete; NPC: net present cost, here, assuming r = 0.8%.Figure 6
Construction cost for each proposal.Furthermore, the construction costs of the CASC and HPC proposals are ¥2.4 and 2.8 million, respectively. Different proposals show the insignificant difference in construction cost due to similar preparation techniques. It should be noticed that the transportation cost accounts for a considerable proportion (20.1%–31.1%) of total construction cost, as shown in Figure6. The CASC proposals are assumed to use seawater and sea sand locally, which saves the transportation cost by approximately ¥0.48 million.Maintenance is necessary for a building to keep acceptable serviceability during its lifetime [76]. In general, the cost of maintenance strongly correlates with the choice of material proposal. As shown in Table 5, the maintenance cost varies from ¥1.3–6.7 million. SS + HPC and SB + HPC have satisfying performance in the service stage and the costs are only caused by inspection, which are ¥1.3 million and ¥1.5 million, respectively. In contrast, due to the inherent drawback of durability, CASC proposals are uneconomic in serviceability. The maintenance cost of SB + CASC and SS + CASC are ¥5.6 million and ¥6.7 million, respectively, about 4 times those of corresponding HPC proposals. In conclusion, the high-performance materials improve the durability of RC structures and as such, help save future cost. The variation of the end-of-life cost is principally caused by the recycled value of scrap steel. As shown in Table 5, the SS proposals have the least end-of-life costs, which is about ¥17 thousand. A large proportion of the end-of-life cost is canceled by recycling stainless steel reinforcements. The SB proposals do not save the cost as much as SS proposals because only 20% mass of stainless-clad bimetallic steel rebar is stainless steel. Doubtlessly, the BS proposal causes the highest end-of-life cost, namely ¥0.21 million, which is 92% and 18% higher than that of the SS and SB proposals, respectively.Figure7 presents the LCC results of each material proposal for the baseline scenario (r = 0.8%, C = 1.25 M). Basically, the SB + HPC proposal is the most competitive solution. The NPC of SB + HPC proposal is 18.8% lower than that of BS + HPC proposal in the integrated lifetime. The SB + HPC proposal not only has an acceptable initial cost but also shows a stable performance in its lifetime, which saves the maintenance cost. The cumulative NPC of SS + HPC proposal is close to the BS + HPC proposal, where the difference due to the expensive unit cost of stainless is 7.5%. It seems that rather than stainless steel, stainless-clad bimetallic steel is the effective option to save the cost without the reduction on building durability.Figure 7
Life-cycle-cost results (assumingr = 0.8%, C = 1.25 M).In terms of the CASC proposals, they are not as economic as the HPC proposals. The LCC of the SB + CASC proposal is ¥11.5 million, which is 43% higher than the SB + HPC proposal; similarly, the LCC of SS + CASC proposal is ¥13.4 million, about 67.5% higher than the SS + HPC proposal. However, in the case of using the same reinforcement, the initial costs of CASC proposals are less than HPC proposals about 20%. Indeed, although the CASC proposals save considerable transportation costs, the maintenance cost is so expensive in terms of a long period that the final cost jumps to a high level. In other words, the durability of CASC needs to be improved.Figure8 shows the cumulative NPC of each material proposal corresponding to time. The ranking of the initial cost from low to high is BS + HPC, SB + CASC, SB + HPC, SS + CASC, SS + HPC. Before Year 39, the curves of the CASC proposals stay behind the corresponding HPC proposals while the reverse situation is seen after Year 39. The next intersection appears in Year 53, where the NPC of SB + CASC proposal exceeds the SS + HPC proposal after three repairs. An important breakeven point of BS + HPC is observed in Year 80 when this proposal experiences rebuilding, which makes it rank below the other proposals using HPC. Younis et al. [25] also specifies the same cost relationship between BS and SS proposals. In so doing, the SB + HPC proposal remains to be the most cost-effective after Year 80.Figure 8
The cumulative NPC for different material proposals (assumingr = 0.8%, C/M = 1.25).In conclusion, rebuilding is the crucial activity to affect the cost ranking of different material proposals. Under using the same steel reinforcement, it should be noted that the CASC proposals show the cost-effective in the first 39 years compared to HPC proposals, and the BS + HPC proposal shows the largest cost-effectiveness considering an 80-year or shorter study period. Nonetheless, given the studied time period of 100 years, the SB + HPC proposal should be recommended based on the LCCA.


### 5.1. Sensitivity Analysis

The sensitivity analysis in this study incorporates the discount rate (r) and construction-to-material cost ratio (C/M), which have the most influence on NPC [25]. Figure 9 presents the sensitivity analysis of the discount rate. As expected, the SB + HPC and SS + HPC proposals are insensitive to the variation of the discount rate since the future investment is rather low. In contrast, the NPCs of BS + HPC, SB + CASC, and SS + CASC proposals decrease with the discount rate. Among all, the BS + HPC proposal has the highest cost benefit when the discount rate (r) exceeds 1.5%, while the SB + HPC proposal ranks the first when r < 1.5%. As for the CASC proposals, the cost effectiveness compared with the corresponding HPC proposals is expected to emerge when r > 3.5% or 6.1%, but still lower than the BS + HPC proposal.Figure 9
Sensitivity of LCC results to the discount rate (C/M = 1.25).Figure10 presents the LCC corresponding to the C/M ratio for the discount rate equals to 0.8%, where C is the basic construction cost, and M is the material cost. The SB + HPC proposal however remains to be the most cost-effective regardless of the variation of the C/M ratio. Also, the SS + HPC proposal is more economical than the BS + HPC proposal when C/M is larger than 0.7. The C/M ratio explicitly affects the initial cost, meanwhile the maintenance cost shows a positive correlation to initial cost. Thus, with the increase of the C/M ratio, the advantage of the corrosion-resistant proposals is more outstanding. Particularly on the remote islands and reefs, the construction cost increases due to hard environment and inconvenient transportation.Figure 10
Sensitivity of LCC results to theC/M ratio (r = 0.8%).


## 5.1. Sensitivity Analysis

The sensitivity analysis in this study incorporates the discount rate (r) and construction-to-material cost ratio (C/M), which have the most influence on NPC [25]. Figure 9 presents the sensitivity analysis of the discount rate. As expected, the SB + HPC and SS + HPC proposals are insensitive to the variation of the discount rate since the future investment is rather low. In contrast, the NPCs of BS + HPC, SB + CASC, and SS + CASC proposals decrease with the discount rate. Among all, the BS + HPC proposal has the highest cost benefit when the discount rate (r) exceeds 1.5%, while the SB + HPC proposal ranks the first when r < 1.5%. As for the CASC proposals, the cost effectiveness compared with the corresponding HPC proposals is expected to emerge when r > 3.5% or 6.1%, but still lower than the BS + HPC proposal.Figure 9
Sensitivity of LCC results to the discount rate (C/M = 1.25).Figure10 presents the LCC corresponding to the C/M ratio for the discount rate equals to 0.8%, where C is the basic construction cost, and M is the material cost. The SB + HPC proposal however remains to be the most cost-effective regardless of the variation of the C/M ratio. Also, the SS + HPC proposal is more economical than the BS + HPC proposal when C/M is larger than 0.7. The C/M ratio explicitly affects the initial cost, meanwhile the maintenance cost shows a positive correlation to initial cost. Thus, with the increase of the C/M ratio, the advantage of the corrosion-resistant proposals is more outstanding. Particularly on the remote islands and reefs, the construction cost increases due to hard environment and inconvenient transportation.Figure 10
Sensitivity of LCC results to theC/M ratio (r = 0.8%).


## 6. Recommendations on Future Works

The LCCA carried out in this study is a valuable supplement for decision-makers to determine the most cost-effective material proposals for the RC structures in remote islands or reefs under harsh environment. Nevertheless, there still remains lots of future works:(1)
The results of the LCCA in this study have shown indistinct cost benefits for using coral aggregate sea water concrete (CASC), but the cases should be further discussed when the durability of CASC is technically improved, where the performance-improvement technology will be hotspots in the near future.(2)
This study incorporates the newly-introduced stainless-clad bimetallic steel reinforcement, which shows outstanding cost-effectiveness in a long term, other corrosion-resistant materials should be tracked and involved in the future LCCA.(3)
The construction cost highly depends on the structural type and construction method. This study focuses on the cast-in-situ concrete structure, which is a common type. Future investigations are expected to conducted on steel structures, composite structures, etc. where the influence of prefabrication should be discussed.(4)
This paper takes an island in the South China Sea as an example, where the harsh marine environment and middle-distance from land were considered. However, other cases based on remote islands with different features should be investigated in the future.(5)
The seawater considered in this study is used without desalination. Although the desalination of seawater will bring additional cost, it could extend the service life of structures using CASC, which may cause lower LCC. This point should be researched using more reliable data in the future.


## 7. Conclusions

This paper focuses on the material alternatives for RC structures on a remote island in the South China Sea based on the life-cycle-cost analysis (LCCA). Totally, five material proposals are considered, namely BS + HPC, SB + HPC, SS + HPC, SB + CASC and SS + CASC, where BS, SB and SS donate the black steel, stainless-clad bimetallic steel and stainless steel, respectively while HPC and CASC denote the high-performance concrete and coral aggregate seawater concrete, respectively. The conclusions towards the cost-effectiveness of the material proposals are drawn as following.(1)
Given a discount rate (r) of 0.8% and construction-to-material cost ratio (C/M) of 1.25, the initial cost ranking from low to high is BS + HPC, SB + CASC, SB + HPC, SS + CASC, SS + HPC, while considering a time period of 100 years, the final ranking changed to: SB + HPC, SS + HPC, BS + HPC, SB + CASC, SS + CASC.(2)
Although the high prices of corrosion-resistant reinforcements (stainless-clad bimetallic steel and stainless steel) push up the initial cost, the cost benefits emerge after Year 80, since a large amount of maintenance costs could be saved compared with the black steel proposal. Specifically, the SB + HPC proposal is the most cost-effective, saving the net present cost (NPC) by 7.5% and 18.8% compared with SS + HPC and BS + HPC, respectively.(3)
The CASC proposals are assumed to use seawater and sea sand locally, which saves the material cost and transportation fee by approximately ¥0.16 million and ¥0.48 million, respectively, compared with the proposals using HPC. However, the saving could not outweigh the maintenance cost due to the reduced structural performance.(4)
The LCC results of proposals BS + HPC, SS + CASC and SB + CASC are sensitive to the discount rate (r) while the other two are not significantly influenced, the BS + HPC proposal will be the most cost-effective if r > 1.5%.(5)
The SB + HPC proposal is recommended regardless of theC/M ratio, but the cost-effectiveness of BS + HPC will exceed that of SS + HPC when C/M < 0.7.



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*Source: 7329408-2022-05-12.xml*