Hierarchical life-cycle design of reinforced concrete structures incorporating durability, economic efficiency and green objectives

Engineering Structures 157 (2018) 119–131

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Engineering Structures journal homepage: www.elsevier.com/locate/engstruct

Hierarchical life-cycle design of reinforced concrete structures incorporating durability, economic efficiency and green objectives

T

⁎

Zhujun Wanga, Weiliang Jina, , You Dongb, Dan M. Frangopolc a

Department of Civil Engineering, Zhejiang University, Hangzhou 310027, China Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong c Department of Civil and Environmental Engineering, ATLSS Engineering Research Center, Lehigh University, 117 ATLSS Dr., PA 18015-4729, USA b

A R T I C L E I N F O

A B S T R A C T

Keywords: Life-cycle design Durability Life-cycle cost Sustainability Green design CO2 emissions Reinforced concrete structures

Current structural design methods mostly emphasize the short-term structural behavior while neglect the longterm performance, social effects and environmental impacts. To address these problems, the Life-Cycle Design (LCD) method considering environmental impacts and structural deterioration could be adopted within the design process to ensure that the structural performance satisfies various objectives. Due to the complexity and the long lifespan of engineering structures, as well as the lack of standardized design approach, studies and application of LCD that cover all the design objectives are limited. This paper proposes a hierarchical LCD method for concrete structures by combining traditional design with green design and other engineering aspects. The design process is divided into six levels that cover the aspects of structural safety and reliability, durability, economic efficiency, local environment, social impacts, and global environment. The proposed design method is then applied to a reinforced concrete highway bridge in marine environment for the purpose of illustration, and a comprehensive comparison between traditional design and the hierarchical LCD approach is made within six design levels. A brief discussion on the hierarchical LCD framework and the future works is presented before conclusions are made.

1. Introduction From allowable stress design to limit state design, structural design concepts and methods have been developed and evolved for decades. However, current design methods still place major attention on the short-term structural performance, while neglect the long-term structural behavior and economic loss caused by structural deterioration, increasing live loads and environmental actions. The environmental and ecological impacts of structural activities have become an increasingly significant issue in modern design concept, and environmentally conscious design, assessment and management methodology aiming to ensure structural performance in a lifecycle context is needed. Structures’ green performance [1,2] is defined as the capability of efficient utilization of energy, water, and other resources; protecting occupants’ health and improving productivity; and reducing waste, pollution and environmental degradation. The 2005 World Summit on Social Development suggested that the structural sustainability is supported by three pillars, i.e. the economy, society and environment [3,4]. The underlying concept of structural sustainability is that our engineering activities should find ways to meet current needs without destroying the opportunity for the development of ⁎

future generations [18]. Significant achievements have been obtained in the establishment and application of structural green performance rating systems, such as LEED (Leadership in Energy and Environmental Design) [1] and BREEAM (Building Research Establishment Environmental Assessment Method) [5]. Sustainability and green performances of buildings and civil infrastructures have also been extensively reported in previous studies. Kibert [6] made a comprehensive discussion about green building design and sustainable construction from the backgrounds and foundations to green building assessment, design and implementation. Haapio and Viitaniemi [7] reviewed the environmental assessment tools of buildings considering the building types, users of tools, phases of lifecycle, databases and other aspects. The BEES (Building for Environmental and Economical Sustainability) software [8] is a freely available tool that assists the selection of building products with favorable performance in both environmental and economical aspects. Studies were also performed on the sustainable building materials, such as wood [9], new-type cement [10–12], unconventional insulation materials [13], and so on. For infrastructure, Mihyeon and Amekudz [14] reviewed sixteen sustainability initiatives for transportation systems and classified the indicators and metrics into five categories, namely economy, transportation, environment, safety and

Corresponding author. E-mail address: [email protected] (W. Jin).

https://doi.org/10.1016/j.engstruct.2017.11.022 Received 6 April 2017; Received in revised form 11 October 2017; Accepted 9 November 2017 0141-0296/ © 2017 Published by Elsevier Ltd.

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objectives and indicators. The proposed approach is then applied to the design of a coastal reinforced concrete bridge in Section 4. Based on results of the case study, a comparison between the proposed hierarchical method and traditional design method is made in Section 5, and both the advantages and disadvantages are identified. Before the conclusions are drawn, a brief discussion on the hierarchical LCD framework and future works is made in Section 6.

society. Sahely et al. [15] put forward a set of sustainability indicators for urban infrastructure systems by considering the interrelationships between infrastructure systems and surrounding environment, society and economy. Ugwu et al. [16] discussed the development of key performance indicators of infrastructure sustainability appraisal, and proposed an analytical decision model for sustainability evaluation. Shen et al. [17] developed the key assessment indicators for the sustainability evaluation of infrastructures by performing a questionnaire survey, and the sustainability indicators are produced by fuzzy set theory. Despite the abovementioned huge efforts and work, the quantification of sustainability characteristics and green performance is still needed [18]. More studies should be conducted to overcome the barriers [19,20] in the application of structural sustainability design. Efforts were also paid to set up the inventory [21] of the life-cycle environmental impacts of engineering structures. Life-cycle assessment (LCA) [22, 23] method is a tool to evaluate the environmental impacts of products over their entire lifespans, i.e., “from cradle to grave”, focusing on the materials and energy input, as well as the emissions to air, water and land. Application of LCA [24–26] in civil engineering area has been broadly witnessed on residential and commercial buildings, new and in-use bridges, as well as communities and infrastructures. Limitations of LCA and other environmental evaluation methods should also be highlighted. The environmental evaluation process does not consider the structural requirements, and none of these methods combines traditional structural design with green design, which marginalizes structural engineers [19]. In order to maintain the long-term performance of structures, while simultaneously minimizing the total cost and environmental impacts, a Life-cycle Design (LCD) method considering multi-objectives is needed. LCD has been widely used in the industrial product design for environmental performance improvement and risk reduction [27,28]. In the design of engineering structures, an integrated LCD methodology was proposed by Sarja [29] to optimize human conditions, and minimize financial costs and environmental impacts. Bergmeister [30] applied LCD to the Brenner Base Tunnel project with an emphasis on the service life of the structure, without considering the environmental performance and sustainability. Due to the complexity [32] and the long lifespan of engineering structures, as well as the lack of standardized design approach, studies and application of structural LCD that consider all the design objectives are limited. Compared with traditional structural design method, LCD covers not only the initial stage (e.g., design and construction), but the entire lifespan of a structure, which places more importance to structural durability and life-cycle cost (LCC). Being an interdisciplinary design approach [25], the objective system of LCD is greatly extended. It contains the knowledge from not only traditional civil engineering, but also the aspects that were overlooked in the past, such as project management, environmental evaluation, and economics [31]. Efforts were made to integrate different professionals and disciplines involved in the LCD of structures through concurrent design [32]. Life-cycle management [33] and maintenance [34,35] of infrastructures have been studied and performed, but based on limited objectives such as reliability, costs, benefits, etc. Multi-objective optimization approach [36,37], whose effectiveness has been verified by extensive application, can help with engineering decision-making involving multiple design objectives. However, this approach is currently not able to cover all the design objectives in the life cycle of a structure, since the computational efficiency and accuracy can seriously decrease with increasing numbers of design variables. Thus, an innovative and practical LCD approach that combines the traditional structural design with green design objectives and other engineering aspects is urgently needed. In light of the abovementioned research gaps, the authors proposed a hierarchical structural design methodology that systematically considers the multiple objectives associated with LCD. Section 2 gives a brief introduction of the LCD system. Section 3 presents the design process associated with six design levels considering different design

2. Life-cycle design objective system The objectives of LCD are divided into the following two parts [38]: traditional objective that considers structure performance, service life, as well as economic efficiency, and green objective that considers local environmental impacts, social impacts, and global environmental impacts. The detailed content of the traditional and green objectives is explained in the following sections. 2.1. Traditional objective Traditional objective represents the most fundamental and common goals of structural design. It mainly consists of three correlated subobjectives, namely structural performance, service life and economic efficiency. The structural performance objective not only considers the structural behavior at project completion but also during the structural operation, maintenance and other future stages. Enhanced structural design usually can lead to a longer service life, but it also requires more monetary investment in the construction and maintenance activities. On the other hand, the life-cycle budget control should be carried out based on satisfying the precondition of the structural performance and service life requirements. 2.2. Green objective The green objective aims at improving structural green performance. As defined previously, a green structure is supposed to minimize local and global environmental impacts, as well as the social impacts. Thus, the green objective is related with the local environmental, social, and global environmental objectives. The local environmental objective focuses on the short-term, small-scale environmental quality around a structure, and the global environmental objective emphasizes the longterm effects of structural activities in a global range. The objective of social impacts aims to improve the quality of living and working environment related to the structure. The scope and detailed indicators of green objective are discussed in the following sections. 3. Hierarchical life-cycle design method The hierarchical relationships of LCD objectives are arranged by comprehensively considering the design concepts, the constraints in design codes and their relevancy degree to the structures. From the perspective of design concept, traditional design covers the fundamental purposes and primary drivers of a structural project, in which safety objective guarantees the functionality of structures, durability objective keeps the structural performance persistent enough to reach the designed service life, and the economic efficiency objective is in agreement with stakeholders’ primary demands of cutting down expenses. The green design is beyond the range of traditional structural design project [39,40] and aims to manage structures’ interrelationship with the environment and the human beings. The hierarchy of design objectives is also associated with the constraints corresponding to the design codes. The terms and regulations for structural safety and reliability are strictly mandatory to ensure adequate strength, stiffness and stability, whilst the durability codes are half-mandatory and half-optional, including both detailed structural design requirements for specified environmental conditions (e.g., the thickness of concrete cover) and recommended durability improvement 120

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and corrosion-prevention techniques (e.g., corrosion-prevention coatings). Although there are no specific economic constraints to a structure, project investors and stakeholders are always interested to minimize the cost and maximize the benefits. In the local environmental evaluation aspect, some compulsory indicators are available in manuals and guidelines, such as the minimum energy performance and air quality performance in LEED [1], but the threshold values of these objectives are subject to changes. The quantification of social and global environmental impacts is even more difficult due to huge uncertainties associated with many aspects including human behaviors and the global climate; hence these two indicators are basically used for comprehensive marking and solution selection in the green evaluation. The levels of green objectives can also be determined by their relevancy degree to the engineering structures. The local environmental objectives (e.g., indoor air quality, solid waste volume, water consumption) are strongly related to the structures, or directly derived from structural activities. The social impacts of a structure are not entirely or directly determined by the structure itself. They involve human factors and market factors. For example, the social impacts caused by bridge failure are associated with not only the probability of failure of the structure but also the traffic volume and the importance of the bridge to the regional economy, among others. Additionally, the structural activities are affecting the greenhouse effect, natural resources depletion or other global environmental problems. The identification and quantification of structures’ global environmental impacts can be difficult in a context as grand and as changeable as the entire planet, but it is still of vital importance to consider this aspect as an objective in the structural design process. Upon the above analysis, the fundamental objectives with stricter restrictions and higher relevancy with structures are placed on the basic levels (e.g., safety and reliability), and the design-assistant objectives with less restrictions and lower relevancy with structures are assigned to the upper level (e.g., global environment). The general flowchart of the hierarchical LCD is illustrated by the pyramid in Fig. 1, and the LCD can be carried out through six steps: Level 1-safety and reliability design, Level 2- durability design, Level 3- economic evaluation, Level 4local environmental evaluation, Level 5- social evaluation, and Level 6global environmental evaluation. Detailed information of these six levels is introduced in the following sections.

ϕRn ⩾

∑

γi Qi

(1)

i

where Rn is resistance; Qi is the ith load effect; γi is the load factor for the ith load effect; and ϕ is the resistance factor. Geometric dimensions and material properties are among the essential parameters in Level 1. Different combinations can lead to solutions with different durability, LCC and green performance. In the following design process, additional requirements will be raised to the initial design solutions considering more design objectives. 3.1.2. Durability design- level 2 Reinforced concrete structures in severe environment [43] are subjected to various degradation mechanisms including chloride attack, sulfate attack, frost attack, alkali-silica reaction, among others. The cracking and spalling of concrete cover, as well as the reinforcement corrosion are the two major durability problems for reinforced concrete structures in marine environment and deicing salt environment. To keep structures functional during their entire lifespan, initial durability design and future durability maintenance are both necessary. Usually, there are three steps in the durability design process. 3.1.2.1. Step one: structural requirements. For structures in aggressive environments, the Code for Durability Design of Concrete Structures [44] provides detailed structural requirements considering the type and intensity of environmental actions on structures. When the location of a structure is specified, the environmental actions can be determined, and the durability structural requirements mentioned above can be used to eliminate the solutions that do not satisfy the durability goal. 3.1.2.2. Step two: durable service life prediction. Another major issue associated with durability design is to predict the durable service life of a structure. For reinforced concrete structures located in chloride environment (marine or deicing salt), reinforcement corrosion induced by chloride penetration is one of the dominant durability problems. The chloride-induced deterioration of reinforced concrete structures can be divided into 4 stages, namely the initiation, propagation, cracking and degradation stage [45,46], as shown in Fig. 2. Cracks can happen during the whole lifespan of reinforced concrete structures. Early-age cracks are usually caused by the loss of moisture and concrete settlement. Other reasons of cracking include concrete shrinkage, thermal stresses, loads, and reinforcement corrosion. Reinforcement corrosion usually results in longitudinal cracks that are parallel to the bars. Once corrosion cracks reach the concrete surface, the mechanical performance and resistance to harmful ions of structure will begin to drop dramatically. The Durable Service Life TD of reinforced concrete structures is defined as the time period before corrosion cracks reach the concrete surface and can be computed as follows

3.1. Traditional design objectives 3.1.1. Safety and reliability design- level 1 The load and resistance factor design (LRFD) [41,42] method is widely adopted in current design codes, using partial coefficients on resistance and load effect to provide the structural components and/or system with adequate load-bearing capacity and reliability level. The safety and reliability requirement in LRFD is

(2)

TD = T0 + Tcr

where T0 is corrosion initiation time; and Tcr is the time interval between corrosion initiation and corrosion cracks reaching the concrete surface. When the chloride concentration around the reinforcements exceeds a critical level, the corrosion will start. Usually, Fick’s Second Law is used to predict the depassivation of reinforcements, which is computed as [47,48].

T0 =

x2 4Df

−2

⎡erf −1 ⎛ CS−CCr ⎞ ⎤ ⎢ ⎝ CS ⎠ ⎥ ⎣ ⎦ ⎜

−1

⎟

(3)

where erf (x) is the inverse function of error function erf(x); Cs is the chloride concentration on concrete surface; CCr is the critical chloride concentration; x is the thickness of concrete cover; and Df is the chloride diffusion coefficient. Df depends on many factors, such as concrete

Fig. 1. Hierarchical relationships of LCD objectives.

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Fig. 2. Chloride induced corrosion process of concrete structures.

favorable effects in extending structures’ service life. Different combinations not only add up to different durable service lives, but also different LCCs, which is another major concern of the stakeholders. LCC evaluation will be performed in the next level to assess the economic efficiency.

quality (e.g., water cement ratio, porosity, etc.) [49,50], dosage of admixtures [51], temperature and moisture [52,53], among others. Different models can be used to compute the crack initiation time. Bazant [54] stated that the corrosion products can cause tensile stress to the concrete around the steel bar. Liu and Weyers [55] mentioned that part of the corrosion products can fill in the gaps and pores around the reinforcements before any pressure occurs. Generally, Tcr relies on multiple factors, such as the reinforcement corrosion rate, volume of corrosion products, quality of concrete, thickness of concrete cover, etc. [56–58]. Eq. (4) [59] is used herein to predict Tcr.

3.1.3. Economic evaluation- level 3 LCC is a comprehensive indicator for structural economic efficiency evaluation. Traditional engineering cost usually refers to the initial cost only, while LCC contains the costs associated with not only design and construction, but also the operation, maintenance, failure, as well as demolition. Several studies [64] showed that although design stage accounts for only 5–7% of the total LCC, the decisions made can determine the allocation of 70–80% of the future costs. In other words, initial design for structural reliability and durability can significantly affect the future costs. Based on the life-cycle stages of a structure, the LCC can be computed as [65,66]:

Tcr = 234762(d + kx )

(

⎧ 0.3 + ⎨ × ⎩

0.6x Ecef

)

ftk Ecef

(r + x )2 + r 2

⎡ 0 2 02 + μc ⎤ + 1 + ⎣ (r0 + x ) − r0 ⎦ (n−1) icorr

2 2δ0 ⎫ −1 d ⎬

⎭

(4)

where d is the diameter of reinforcements; x is the thickness of concrete cover; k is the correction factor considering that corrosion products can fill into the cracks; ftk is the standard tensile strength of concrete; Ecef is the effective elasticity modulus of concrete. Ecef = Ec /(1.0 + φ) , where Ec is the elasticity modulus of concrete, and φ is the creep coefficient of concrete; δ0 is the thickness of capillary cavity between reinforcement and concrete; r0 = d/2 + δ0; μc is the Poisson ratio of concrete; n is the volume expansion ratio of corrosion products; and icorr is the corrosion electric current density.

LCC = Cds + Ccon + Cop + Cmt + Cdm

(5)

where Cds is the design cost, including the cost of site investigation, research and design; Ccon is the construction cost; Cop is the operation cost; Cmt is the maintenance cost in terms of the direct and indirect cost of maintenance actions; and Cdm is the demolition cost, including the costs of demolition, landfill and recycling. Direct cost refers to the money spent directly on engineering activities, while the indirect cost is related with the deficiency or loss of structural functionality [67]. For instance, bridge failure can paralyze the traffic network, induce time loss to the commuters who travel through it, and economic loss to regional industry. These losses are considered as indirect costs.

3.1.2.3. Step three: durability improvement measures and maintenance plan. Coatings, mineral admixtures, as well as electrochemical techniques are among the most frequently used durability improvement methods. Studies [60] showed that adequate mineral admixtures can increase the concrete’s resistance to chloride penetration. The Code [44] and the Guide to Durability Design and Construction of Concrete Structures [61] also recommend the best dosage of admixtures for concrete structures in specific environment. For example, fly ash (FA) should be no less than 30% (wt. of cement, same below) if it is the only admixture, and blast furnace slag (SL) should be no less than 50%, while the most effective amount of silica fume (SF) is approximately 5%. Coatings are low-permeable organic paint, such as the epoxy coating, which can be used on either concrete or reinforcements to block the penetration of harmful ions. Corrosion resistant coatings can prolong TD of concrete structures effectively and conveniently. Electrochemical techniques can remove the chloride ions in concrete by connecting the reinforcements to an anode outside to form a current circuit [62]. It usually takes 8–12 weeks for the electrochemical chloride extraction (ECE) treatment to have a favorable effect. Based on ECE treatment, bidirectional electromigration (BE) can simultaneously move corrosion inhibitors into the concrete and provide longer protection [63]. Combined utilization of durability improvement methods also has

3.2. Green evaluation objectives 3.2.1. Local environmental evaluation- level 4 Indicators in this level are associated with controlling the harmful environmental impacts caused by structural activities in a life-cycle context. The evaluation indicators for local environmental impacts are developed based on the data categories of LCA [22], including the input of resources, energy and materials, as well as the emissions to air, water and land, as labelled in Table 1. For bridge structures, the scope of local environmental evaluation can include the energy, water, materials, solid wastes and toxic emissions. The sub-objectives and units are presented in Table 1. Energy section considers the life-cycle power consumption of bridge structures. To avoid double counting environmental impacts, the embodied energy of materials is put under the material section. Water section consists of life-cycle water consumption and water pollution. Material recycling rate and embodied energy are the evaluation 122

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Table 1 Green design indicators for bridge structures. Green objective

Sub-objective

Indicator

Unit

LCA categories

Local environmental evaluation

Energy Water

Power consumption Water consumption Water pollution Recycling rate Embodied energy Solid waste volume Particle matter (PM2.5) Ground-level ozone Sulfur dioxide Nitrogen dioxide Lead Carbon monoxide

MJ ton kg / kg water % MJ m3 μg/m3 ppm ppm, kg ppb, kg μg/m3 ppm

Input Input Output Input/Output

Travel time Extent of congestion Casualty Downtime Economic loss

min min persons days CNY (¥)

NA

Fossil fuel consumption Global warming potential Ozone depletion potential Deforestation Habitat destruction

MJ kg CO2eq kg CFC-11 m2 m2

Input Output

Materials Solid waste Toxic emission

Social evaluation

Public accessibility Health and safety Local development

Global environmental evaluation

Resources Atmosphere Ecology system

A = {a1,a2,…,an}T

v11 v21 ⎨… ⎩ vm1 ⎧

v12 v22 … vm2

… … ⋱ …

R = V ·A

(7)

where vij is the evaluation score of the jth (j = 1, 2, …, n) evaluation indicator of the ith (i = 1, 2, …, m) solution. In BEES software [8], a comparative scoring method is used by comparing the impact of a certain factor to the national annual per capita impact. A similar but simplified method is used here to determine the evaluation value of each indicator. The yardstick in the comparison is replaced by the biggest impact value among all alternative solutions, and the evaluation score is

vij =

Iij max(Ii1,Ii2,…,Iim)

√ √ √

Input/output

(9)

(10)

Weight factors reflect the preferences of decision makers by identifying the importance of different impact categories. They bridge the gap between quantitative LCA results and the choices of decision makers by converting the evaluation results of various categories into a comprehensive score [69,70].The weight factors can vary significantly when determined by different groups of people, based on different cultural or political background, or in different places, and thus produce completely different evaluation results. For example, the BEES (Building for Environmental and Economical Sustainability) [8] software provides five choices of weight factors, including no weights, userdefined weights, equal weights, and two sets of predefined weights determined by BEES stakeholder panel and EPA Science Advisory Board. The former deems that global warming far outweighs other categories, while the latter places same importance on global warming and habitat alternation. Due to the inherent subjectivity and uncertainty, a global-scaled weighting set is not available [71], and LCIA (life cycle inventory assessment) also considers it an optional element in the assessment process [72]. In the cases where the decision makers’ preferences are affirmatively stated, weight factors are necessary for decision making. Due to the lack of reliable data, no weighting is applied to the illustrative example in this paper.

(6)

v1n ⎫ v2n … ⎬ vmn ⎭

√ √

where aj is the weight factor for uj, and ∑ aj = 1 (j = 1, 2, …, n). Consequently, the evaluation result vector is

where uj is the jth evaluation indicator, j = 1, 2, …, n. And the evaluation value set is

V=

√ √

score, and the weight vector is

indicators of material section. Solid waste volume is considered for disposal and landfilling. According to EPA’s air pollution challenges [68], particle matters, ground-level ozone, sulfur dioxide, nitrogen dioxide, lead and carbon monoxide are classified as six common toxic emissions to air. Apart from the abovementioned quantitative indicators, some qualitative indicators are also applicable in environmental evaluation, such as comprehensive scoring to encourage using native and natural materials, or pre-fabricated and industrial structural components. The qualitative indicators are not presented in Table 1. The evaluation factor set is

U = {u1,u2,…,un}

Output Output

Case study

(8)

3.2.2. Social evaluation- level 5 Civil infrastructure systems not only affect the environment, but also the society. Social impacts can include the changes to people’s way of life, culture, political systems, community, environment, health and wellbeing, personal and property rights and their fear and aspirations [73]. Social impact assessment [73–75] is defined as the analysis, estimation and management of future social consequences of a current change or development. For bridge structures, a simplified social evaluation indicator system is developed herein by considering the major characteristics of bridges, such as public accessibility, human health and safety, and local development [17,15]. Public accessibility of a bridge mainly refers to the traffic condition, such as travel time and

where Iij is the impact of the jth (j = 1, 2, …, n) evaluation factor of the ith (i = 1, 2, …, m) solution. The environmental evaluation is used for solution comparison and selection in the proposed hierarchical LCD approach. This simplified comparative scoring method can easily meet the request, even in circumstances where the annual per capita impact data is not available. It explicitly reveals the relative relationships between the impacts of different solutions. Given more information, other ranking methods could also be incorporated within the decision making process. To compare the overall environmental performance, weight factors can be allocated to each evaluation factor to reach a comprehensive 123

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4.1. Level 1: Safety and reliability design

extent of congestion. Human health and safety measures the injuries and casualties induced by structure failure or engineering activities during the life cycle of a bridge. The failure or engineering activities of a bridge can affect the local development by causing downtime and economic loss. Indicators and corresponding units for social evaluation of bridge structures are presented in Table 1. The comparative method [74] is one of the commonly-used approaches in the social impact assessment, which utilizes historical data of a similar project to predict the probable social impacts of the objective project. Due to the lack of historical data, the social evaluation process in this paper is also simplified. The comparative scoring method mentioned in local environmental evaluation section is applied here for the solution selection.

For internal stress analysis, special attention should be paid to the critical locations along the span of the beam, such as the bearing points, loading points and the mid-span, as shown in Fig. 3. Concrete and reinforcement strength level and the thickness of concrete cover are taken as design variables with notations shown in Table 2. The design process is based on the Code for Design of Concrete Structure [80]. Analysis results indicate that the biggest positive moment occurs at the mid-span of the beam, and the biggest shear happens near the left bearing. Considering solution iv-C as an example, its cross-sections near the bearing and loading points require the highest reinforcement ratio, with 6940 m2 for both top and bottom reinforcements. 4.2. Level 2: Durability design

3.2.3. Global environmental evaluation- level 6 The global environmental objectives, including resources, atmosphere and ecology system, are considered within the design process to minimize the long-term structural impacts on the global environment and ecosystem. Fossil fuels are the major unrenewable resources consumed by engineering structures, which can be measured by the embodied energy (MJ). The effect on atmosphere includes global warming potential and ozone depletion potential, and the impact on ecology system mainly considers deforestation and habitat destruction, as shown in Table 1. Carbon equivalent CO2eq is widely adopted as the quantitative indicator to measure the global warming potential [76,77], which includes the embodied carbon and other greenhouse gas emissions (CH4, N2O, etc.) over the life cycle of the bridge structure. The embodied carbon [78] of a material normally refers to the total CO2 emission during the process of extraction, manufacturing and transportation (i.e. cradle to gate). The life cycle CO2eq (LCCO2eq) of a bridge structure can be computed as

LCCO2 eq =

∑

Ai (αC ,i + αS,i γS + αN ,i γN ) +

i

+ βN ,j γN )

∑

4.2.1. Step one: structural requirements Based on the environmental classification in the Code [44] and the Guide [61], the following requirements are proposed toward the cap beam: Minimum reinforcement level should be HRB400, and minimum concrete level should be C50. Thickness of concrete cover should be no less than 60 mm. Maximum water-cement ratio (W/C) is 0.36. Mineral admixtures are recommended, and the integrated use of multiple anticorrosion strategies is recommended. Based on abovementioned requirements, the range of solution alternatives is narrowed down by excluding concrete level i and ii (i.e., C30 and C40), reinforcement level A (i.e., HRB335), concrete cover thickness a and b (i.e., 40 mm and 50 mm). 4.2.2. Step two: durable service life prediction Initial TD is investigated in this step. Relative parameters and TD of solutions are presented in Table 3 [81,82]. Results indicate that solutions with higher concrete level and thicker cover tend to have longer TD.

Mj (ECM ,j + βS,j γS

j

(11)

4.2.3. Step three: durability improvement measures and maintenance plan The effect of mineral admixtures, coatings and electrochemical techniques are investigated in this section, with corresponding notations and effects shown in Table 4. The mineral admixtures can change the chloride diffusion coefficient of concrete, as shown in the following equations [83]:

where Ai is the workload of ith engineering work; Mj is the consumption of jth material; αC,i, αS,i, αN,i is the emission factor of CO2, SO2, and N2O of the ith engineering work, respectively; ECM,j is the embodied carbon per unit of jth material; βS,j, βN,j is the emission factor of SO2 and N2O of jth material, respectively; and γS, γN is the global warming potential of SO2 and N2O, respectively.

4. Illustrative example The proposed hierarchical life-cycle design approach is applied to a reinforced concrete highway bridge in marine-atmosphere environment. The designed service life is 100 years. The beam is supported by two columns with center distance of 7.5 m, and holds a 15 m deck. Forces are transferred to the beam through two bearings with the central distance of 6.9 m. The main cross-section of the beam is 1.5 m × 1.5 m, and the total length is 15 m. The structure layout and load condition are shown in Fig. 3. Average daily traffic (ADT) volume of the bridge is assumed to be 18,000 vehicles, and the average daily truck traffic rate (ADTT/ADT) is 0.12[79]. Average occupancies of cars and trucks is 1.5 persons and 1.05 persons, respectively [79]. Based on the monthly salary of various industries in China, the average wage of car drivers and truck drivers is supposed to be ¥49.23/h and ¥64.10/h, respectively. Average running cost for cars and trucks is ¥0.42/km and ¥0.896/km, respectively. In cases when the bridge is closed, the detour length is 2.9 km, and the average detour speed is 50 m/h [79].

t m D (t ) = D28 ⎛ 28 ⎞ ⎝ t ⎠

(12a)

m = 0.2 + 0.4(%FA/50 + %SL/70)

(12b)

DSF = DPC ·e−0.165·% SF

(12c)

where D(t) is the chloride diffusion coefficient at time t ; D28 is the chloride diffusion coefficient at 28d (t28); m is the diffusion decay index; DSF is the chloride diffusion coefficient of concrete with SF; DPC is the chloride diffusion coefficient of Portland cement; and %FA, %SL and %SF is the mass percentage of fly ash, slag and silica fume to all cementitious materials, respectively. Reports [44,61] stated that the effect of epoxy coating for reinforcements (ER) could last at least 20 years, depending on the thickness and bonding strength of the coating. Epoxy coating [84,85] for concrete (EC) is also an effective way to insulate chloride intrusion, which can provide protection for at least 10 years. Water repellent surface impregnation with silane [86,87] (SC) can be effective for at least 20 years. After 8–12 weeks of electrochemical chloride extraction (ECE) treatment, chloride ions around the reinforcement could reduce by nearly 80% [88,89]. With the help of corrosion inhibitor in bidirectional electro-migration (BE) treatment, the TD could be prolonged for 124

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Fig. 3. Cap beam of a coastal highway bridge (the dashed box) and load condition.

Table 2 Classifications and notations of initial design parameters. Concrete levela

Notations a b

Reinforcement levelb

Concrete cover

C30

C40

C50

C60

HRB335

HRB400

HRB500

40 mm

50 mm

60 mm

70 mm

i

ii

iii

iv

A

B

C

a

b

c

d

C30 represents that the standard compressive strength of a cubic sample (150 mm × 150 mm × 150 mm) of the concrete is 30 MPa. HRB335 represents the hot-rolled ribbed steel bar with standard yield strength of 335 MPa.

durability improvement methods accordingly. For example, given that decision makers require an initial TD of at least 50 years, then the combined utilization of different durability improvement measures is inevitable, and Table 6 shows several eligible solutions. In order to reach the designed service life of 100 years, it is assumed that the component should avoid corrosion cracks for at least 75 years. Thus, durability maintenances should be performed regularly based on the conditions presented in Table 4. Corresponding maintenance costs are analyzed in Level 3.

at least 5 more years [63,90]. TD of solutions using single durability improvement measure are listed in Table 5. The TD under the combined utilization of multiple techniques are also calculated, which lead to the following conclusions: ● Increasing the thickness of concrete cover is an effective way to improve durability; ● When applied alone, coatings show the best durability improvement effect, followed by mineral admixtures and electrochemical techniques; ● The combination of ER and EC/SC have the strongest resistance to chloride ions; and ● The combination of mineral admixtures and other durability improvement strategies makes highly effective corrosion-prevention measures.

4.3. Level 3: Economic evaluation In order to meet the requirements of designed service life and to predict the future costs, a maintenance plan should be determined based on the initial design solution. Unit cost of construction materials, durability improvement techniques and construction works are presented in Table 7 in CNY (¥), collected from various dealers and

Based on the TD prediction, decision makers can choose the

Table 3 Durability related parameters and TD (yrs) of solutions. Solutions

W/C

d (mm)

fc (N/mm2)a

ft (N/mm2)b

x (mm)

Df (m2/s)

Cs (%)

CCr (%)

T0 (yrs)

Tcr (yrs)

TD (yrs)

iii -c iii -d iv -c iv -d

0.34

25

23.1

1.89

2.74 × 10−12

2.14

0.20

27.5

2.04

60 70 60 70

7.4 10.1 9.2 12.5

4.4 4.8 4.5 5.0

11.8 14.9 13.7 17.5

a b

0.30

2.20 × 10−12

fc is the compressive strength of concrete; ft is the tensile strength of concrete.

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Table 4 Notations and effects of durability improvement techniques. Admixture/technique

Notation

Effect

Repeat condition

Silica fume (6%) Fly ash (30%) Blast furnace slag (50%) Mixture of 20% fly ash & 40% blast furnace slag Epoxy coated reinforcement (full coverage) Epoxy coating for concrete (full coverage) Water repellent surface impregnation with silane for concrete (full coverage) Electrochemical chloride extraction (8–12 weeks)

SF FA SL FA&SL ER EC SC

Eq. (12a, b and c) [83]

NA

Prolong TD for approximately 20 yrs [44,61] Prolong TD for approximately 10 yrs [84,85] Prolong TD for approximately 20 yrs [86,87]

Old coating fails Old coating fails

ECE

Remove 80% of Cl−. [88,89]

Bidirectional electro-migration (18–22 weeks)

BE

Remove 80% of Cl−; and prolong TD for at least 5 yrs [63,90]

different initial durability designs and future maintenance plans can contribute to different service lives and LCCs, for the sake of fair comparison, the total cost of each solution are divided by its TD to reach an average annual cost, as listed in Table 8. Solution iv-d-SL-ER is a special case among others. It fulfills the requirement of minimum TD, but the initial design does not provide reference for future maintenance plan, so this solution will keep deteriorating, and be restored directly at the 85.9th year. The following conclusions are obtained from the results in Table 8:

Table 5 TD (yrs) of solutions using single durability improvement technique. Solution

Plain concrete

SF

FA

SL

FA&SL

ER

EC

SC

ECE

BE

iii -c iii -d iv -c iv -d

11.8 14.9 13.7 17.5

24.3 31.9 29.3 38.8

22.3 29.1 26.8 35.0

25.5 33.6 30.9 40.9

35.2 46.8 43.0 57.4

31.8 34.9 33.7 27.5

21.8 24.9 23.7 27.5

31.8 34.9 33.7 37.5

18.4 23.9 22.0 28.7

23.4 28.9 27.0 33.7

● Concrete coating is the most economical method, and also performs well when combined with ER; ● 50% SL shows better durability effect and higher economic efficiency than 30% FA; and ● Although electrochemical measures can greatly prolong structures’ service life, their unit costs are too high to be economical.

Table 6 Selected solutions using combined durability improvement measures with TD longer than 50 yrs. Solutions

TD (yrs)

iii-c-ER&SC iv-d-SF-BE iv-d-FA-SC iii-d-FA&SL-EC iv-d-SL-ER iv-d-FA&SL-ECE iii-c-FA&SL-ER&EC iii-d-SF-ER&SC

51.8 52.7 55 56.8 60.9 64.4 65.2 71.9

The indirect costs of the maintenances include the user cost and the socioeconomic cost [91]. The vehicle operation cost and monetary value for time loss due to loss of traffic capacity can be expressed as [79]:

Table 7 Unit costs (¥, CNY) of materials, durability improvement techniques and construction works.

Concrete (¥/m )

C50 C60

332.7 342.4

Reinforcement (¥/t)

HRB400 HRB500 ER

1850 2150 4930

Admixture (¥/t)

SF FA SL

2950 300 293

Coatings (¥/m2)

EC SC

90 71.5

Electrochemical technique (¥/m2)

ECE BE

600 650

Construction work (¥/m2)

Reinforcement work Concrete work Formwork Labor Crane work Transportation

300 165 150 200 90 30

T T ⎤ ⎞ + cRun,truck CV = ⎡cRun,car ⎛1− Dl Adt (1 + γ )t 100 ⎦ ⎝ 100 ⎠ ⎣

(13)

T T ⎤ Dl Adt ⎞ + CW ,truck Otruck CT = ⎡cW ,car Ocar ⎛1− (1 + γ )t 100 100 ⎝ ⎠ ⎣ ⎦ S

(14)

where CV and CT is the vehicle operation cost and cost of time loss, respectively; cRun,car and cRun,truck is the operation cost of cars and trucks, respectively (¥/km); cW,car and cW,truck is the wage of car drivers and truck drivers, respectively (¥/h); Ocar and Otruck is the average occupancies of cars and trucks, respectively; A is the average daily traffic (ADT); T is the average daily truck traffic rate (ADTT/ADT); Dl is the detour length (km); dt is the downtime (days); S is the average detour speed (km/h); γ is the monetary discount rate; and t is the time. According to Table 4, the downtime of electrochemical maintenance measures can be several months. During electrochemical treatment, the component should be steadily soaked in electrolyte [89]. The surface strength of treated concrete drops temporarily during the operation, making the lane closure or speed limit unavoidable during the work period. The restoration of failed girder using pre-fabricated component requires shutting down the bridge traffic, and the downtime is supposed to be 4 days. Painting coatings, on the other hand, have less strict requirements. The application of new coating can take place even when the structure is in service. The indirect cost induced by maintenance and restoration is presented in Table 9. Those schemes in bold have relatively lower annual cost, and are selected for green evaluation. The following conclusions are reached based on results in Table 9:

Unit cost 3

Cl− concentration around rebar reaches critical value Cl− concentration around rebar reaches critical value

contractors. It should be noted that the unit costs can vary with time, places and markets. The future costs are transferred to present value by a monetary discount rate of 2% and an inflation rate of 1.2%. Since

● Due to long operation time of electrochemical approach, the indirect 126

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Table 8 Total direct cost (¥) and annual direct cost (¥/yr) for solutions with maintenance activities. Solutions

Mt.a

TD (yrs)

Initial cost

1st Mt. Time

1st Mt. cost

2nd Mt. time

2nd Mt. cost

Total Dir. costc

Annual Dir. cost

iii-c-ER&SC iv-d-SF-BE iv-d-FA-SC iii-d-FA&SL-EC iv-d-SL-ER iv-d-FA&SL-ECE iii-c-FA&SL-ER&EC iii-d-SF-ER&SC

SC BE SC EC – ECE EC SC

91.8 80.5 75 76.8 60.9 78.4 75.2 91.9

45917.7 92489.0 41231.3 41953.0 40989.3 84949.5 47396.2 48230.5

51.8 52.7 55 56.8 85.9b 64.4 65.2 71.9

3990.6 36021.6 3891.3 4829.2 102473.3 30324.3 4520.1 3406.4

71.8 66.6 NA 66.8 NA 71.4 NA

3409.1 32287.1

53317.4 160797.7 45122.6 51245.6 93092.2 143971.9 51916.2 51636.9

456.5 1524.1 451.2 503.4 541.8 1392.4 518.1 441.7

a b c

4463.5 28698.1

“Mt.” is short for “Maintenance activities”; Scheme iv-d-SL-ER has no maintenance plan and hence faces structure failure at the 85.9th year, with restoration cost (discounted) ¥102473.3; “Dir. cost” is short for “direct cost”.

comparative scoring method, and the points of all factors add up to the total local environmental scores. Lower score means better environmental performance. The total score in environmental evaluation is denoted as RLE in Table 10. The two schemes who score 2.03 points both apply multiple admixtures and have the lowest cement consumption. The embodied energy and NO2 emission are dominated by cement, and contribute to big part of the environmental scores. The SO2 emission factor of FA is 0.72 kgSO2/t FA [93], which is much higher than that of cement (0.53 kgSO2/t cement) [93], so the scheme with higher FA dosage will emit more SO2. The three schemes with the lowest RLE are selected for higher-level evaluation, as marked in bold in Table 10.

cost can be extremely large. Based on the comparison of both direct and indirect cost of this example, the electrochemical measure is the least economic maintenance approach for chloride-induced durability problems. ● The indirect cost of restoration mainly depends on the damage state of the bridge and the restoration method. For instance, using prefabricated girder in restoration can largely shorten the downtime compared to cast-in-site approach. ● Concrete surface coatings have obvious economic advantages in both direct and indirect concept. 4.4. Level 4: Local environmental evaluation For the sake of solution selection, a simplified environmental evaluation is performed herein to identify the distinction of the environmental performances of available schemes, while the similarities are omitted. The same applies to the social evaluation and global environmental evaluation. No preferences are given to specific evaluation indexes, and no weighting is applied. The indicators used in this environmental evaluation include: embodied energy (EE), solid waste (SW), sulfur dioxide (SO2) emission and nitrogen dioxide (NO2) emission, which are checked in Table 1. Embodied energy of material is the primary energy consumed during the life cycle, including the energy for extraction, manufacturing and transportation [78,92]. Embodied energy of basic construction materials is collected from the data inventory of Athena software [92] and Bath University [78]. Solid waste is mainly consist of the discarded construction materials at the end of structural service life. The recycling rate of steel reinforcement is assumed to be 42.7% [78], and no concrete fragment will be reused. SO2 and NO2 emissions [68] can induce acidification and respiratory problems, the latter also contributes the eutrophication of coastal waters. The emissions of these air pollutants are common in the manufacturing process of construction materials [93] and during the combustion of fossil fuels [94]. Results of the environmental evaluation are presented in Table 10. Numbers in the brackets are evaluation values, which are determined by the

4.5. Level 5: Social evaluation Social evaluation of a whole bridge structure is based on many indicators. For instance, casualties can be used to estimate the life loss during the life cycle of bridge structures, especially in hazards. The traffic capacity of a bridge can be weakened when part of the lanes are closed due to pavement work or bridge deck repair, and the congestion extent and travel time are helpful in assessing the residual traffic capacity and corresponding social effect. However, the social impact of a bridge girder is relatively small, only its failure can affect the public. For this reason, the social evaluation factor is limited to the downtime (DT) in this illustrative example. As can be seen from Eqs. (13) and (14), the indirect costs are closely related to the downtime. Downtime also has great influence on the local economic development, since the supply chain of local industries can be interrupted by the bridge closure. In this illustrative example, the downtime for scheme iv-d-SL-ER is 4 days owing to girder restoration, but the traffic will not be interrupted by the concrete surface treatment of other solutions. The social evaluation score is denoted as RSC, as presented in Table 10. The social impact of all the solutions is evaluated, but only those considered qualified in environmental evaluation are ranked in this level. No obvious distinction is witnessed among the

Table 9 Total indirect cost (¥), LCC (¥) and annual cost (¥/yr) for solutions with maintenance activities. Solutions

1st Mt. Ind. costa

2nd Mt. Ind. cost

Total Ind. cost

LCC

T (yrs)

Annual costb

iii-c-ER&SC iv-d-SF-BE iv-d-FA-SC iii-d-FA&SL-EC iv-d-SL-ER iv-d-FA&SL-ECE iii-c-FA&SL-ER&EC iii-d-SF-ER&SC

0 9354247.8 0 0 205782.4 4265489.3 0 0

0 8384469.8 NA 0 NA 4036744.3 NA

0 17738717.6 0 0 205782.4 8302233.7 0 0

53317.4 17899515.3 45122.6 51245.6 298874.6 84462.05 51916.2 51636.9

116.8 105.5 100 101.8 171.8 103.4 100.2 116.9

456.5 169663.7 451.2 503.4 1739.7 81684.8 518.1 441.7

a b

“Ind. cost” is short for “indirect cost”; Values in bold represent solutions with economical advantages, which are selected for next level of evaluation.

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Table 10 Local environmental, social and global environmental evaluation results.a Local environmental evaluation

iii-c-ER&SC iv-d-FA-SC iii-d-FA&SL-EC iv-d-SL-ER iii-c-FA&SL-ER&EC iii-d-SF-ER&SC

a

Social evaluation

Global environmental evaluation

EE /GJ

SW /m3

SO2/kg

NO2/kg

RLE

DT/days

RSC

FF /GJ

LCCO2eq/t

RGE

84.3 (0.59) 72.6 (0.50) 50.6 (0.35) 144.0 (1) 50.6 (0.35) 78.6 (0.55)

29.0 (0.50) 29.1 (0.50) 29.0 (0.50) 58.2 (1) 29.0 (0.50) 29.0 (0.50)

7.6 (0.19) 39.1 (1) 30.4 (0.78) 26.7 (0.68) 30.4 (0.78) 7.1 (0.18)

52.1 (0.81) 42.2 (0.66) 25.4 (0.40) 64.0 (1) 25.4 (0.40) 48.9 (0.76)

2.09

0

–

3.0

12.4

–

2.66

0

–

2.0

12.7

–

2.03

0 (1) 4

1

3.0 (1) 15.0

6.9 (0.59) 18.4

1.59

0 (1) 0 (1)

1

2.0 (0.67) 2.0 (0.67)

6.8 (0.59) 11.6 (1)

1.25

3.68 2.03 1.99

–

1

–

1.67

Values in bold represent solutions with better green performance, which are selected for next level of evaluation.

the local environmental, social, and global environmental objectives, respectively. The best solution that passes 6 levels of design and evaluation turns out to be iii-c-FA&SL-ER&EC. The best solution and other preferable solutions have some obvious characteristics:

solutions, hence all will advance in the next level. 4.6. Level 6: Global environmental evaluation Global environmental evaluation can be performed on a whole bridge, but a small-scaled component, such as the girder in this example, can have relatively limited global impacts. For this reason, two representative indicators are selected for the global environmental evaluation of the girder, which are fossil fuel consumption (FF) and Greenhouse Gas (GHG). The cradle-to-gate embodied energy of materials already includes the fuel consumed by extraction, manufacturing and transportation of materials. The embodied energy of fuels used for worker transportation and in-site construction work can be considered in the global environmental evaluation. Greenhouse gas emissions (CH4, N2O, etc.) are usually transformed into carbon equivalent (CO2eq) to measure the global warming potential (GWP), which can be computed by Eq. (11). In order to compute the fossil fuel consumption, several assumptions are made concerning transportation and construction works. The transport of workers and materials is completed by passenger cars (24.1 km or 15 miles) and light-duty trucks (12.9 km or 8 miles), respectively. With respect to in-site work, concrete surface treatment (surface cleaning, painting, etc.) and component restoration requires light-duty and medium-duty construction equipment, respectively. Based on data of embodied carbon [78,93,95], GHG emission factors [92,93], GWP [94] and heat value of fossil fuels [94], the global environmental evaluation is performed, as shown in Table 10. The global environmental evaluation score is denoted as RGE. Results indicate that schemes with highest cement replacement rate (i.e., admixture dosage) and least maintenance activities have the best global environmental performance. Replacing cement with industrial byproduct such as fly ash and blast furnace slag is an effective way to reduce GHG emissions, since cement is the most primary material of reinforced concrete structures, and its dosage usually dominates the CO2eq. The embodied carbon of construction materials contains large variations, which means there are also chances to reduce GHG emission by improving the manufacturing techniques. Fossil fuel consumption in this case depends on the transportation of materials from factories to construction site, transportation of workers and in-site construction works. Thus, the scheme with minimum transportation distance and frequencies, as well as the lightest construction duties will have the best performance. The scheme with the lowest RGE has the best global environmental performance, as marked in bold in Table 10.

● ● ● ●

Strong initial durability design; Reasonable future maintenance plan; Low future maintenance frequency; and High cement replacement rate.

From a life-cycle perspective, strong initial durability design can largely prolong the TD, which means the service life requirement can be fulfilled more easily. With a reasonable maintenance plan, structures can be maintained with higher efficiency and lower environmental burden. Reducing maintenance frequency not only has better economic benefit, but also decreases the energy and resources input, as well as harmful impacts. With respect to materials, the combination of multiple admixtures can improve structural durability and replace a large proportion of cement, which shows significant advantage on durable, environmental and economic aspects. The combination of these characteristics enhances structural durability on the one hand, and minimizes material and resource consumption, toxic emissions, social impacts and global warming potential on the other hand, which contributes to the best solution in hierarchical LCD.

5. Comparison between traditional and hierarchical LCD approach Current structural design process mainly focuses on the safety and reliability design. Only critical projects facing severe environmental actions will move one step further to take the durability requirements into account. The attention of decision makers is still focused on the initial cost of a project, neglecting the possible future cost caused by durability deficiency and structural failure. The hierarchical LCD approach is proposed to fill the abovementioned research gap. A comparison between the hierarchical LCD and traditional structural design is conducted in Table 11. Solution iv-d-SL-ER is a typical solution under traditional structural design, which satisfies the safety and reliability design, and meets the requirement of minimum durable service life TD. The solution iii-c-FA&SL-ER&EC is a result based on hierarchical LCD method. The checked item (√) means better or equal performance, and the crossed item (×) refers to the opposite. As indicated in Table 11, solution iv-d-SL-ER does not have a future maintenance plan. As a consequence, it will gradually deteriorate and fail 25 years after corrosion cracks appear. Restoration is conducted to replace the degraded component. Although it has relatively lower initial cost, the LCC and average annual cost are much higher due to

4.7. Green evaluation discussion Up to this point, all the solutions have been evaluated considering 128

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Although it is built with slightly higher initial cost, the LCC and average annual cost are much less. What’s more, green evaluation process shows that it has better performance in local environmental, social and global environmental aspects. In general, hierarchical LCD is a more comprehensive design method considering life-cycle requirements.

Table 11 Comparison between hierarchical LCD and traditional structural design. Design process

Traditional structural design iv-d-SL-ER

Hierarchical life-cycle design iii-c-FA&SL-ER&EC

6. Discussion

Safety and reliability

Safety

√

Reliability

√

Structural requirements TD

√

Maintenance plan

×

Initial cost LCC Average annual cost

Meet safety requirements Meet reliability requirements

√ √

Meet safety requirements Meet reliability requirements

Meet structural requirements Meet predetermined TD requirement TD = 60.9 yrs No maintenance plan

√

× × ×

¥ 40989.3 ¥ 298874.6 ¥ 2035.9

√ √ √

Local environmental evaluation RLE

×

RLE = 3.68

√

RLE = 2.03

Social evaluation (Downtime)

×

4 days

√

0 day

LCCO2eq FF

× ×

18.4 t 15.0 GJ

√ √

6.8 t 2.0 GJ

Durability

Economic efficiency

Global environmental evaluation

√

√

√

The proposed hierarchical LCD method provides an organized framework for the design of engineering structures. Detailed design and evaluation methods within this framework can be flexible depending on the project requirements. For instance, multiple models can be applied for service life prediction catering to diverse assumptions, environmental conditions or project requirements. The environmental evaluation process can vary in different countries and organizations, with different understandings and backgrounds, different types of structures and projects, different needs of designers and stakeholders, and different emphasis of studies. Hence the evaluation methods, indicators and weight factors used in this study are not exclusive. Environmental evaluation tools such as ATHENA [96], BEES [8], BEAT [97] (Building Environmental Assessment Tool) and others, can be easily used to support the decision-making process. The comparative scoring method used in this paper can also be replaced by other environmental evaluation methods as needed, such as the LEED [1], BREEAM [5], the SETAC (the Society of Environmental Toxicology and Chemistry) model of life-cycle impact assessment [98], GBTool [99] of the international Green Building Challenge process. The same applies to the social evaluation and the global environmental evaluation section. In addition, there are several limitations in this paper that need to be addressed in future work. The limit states of durable service life can be adjusted according to different project requirements and other environmental conditions. Stochasticity in structural models and parameters should be considered to perform probabilistic design and assessment. A more complete assessment inventory that includes both qualitative and quantitative indicators for local environmental, social and global environmental evaluation of engineering structures should be set up. Given additional information, decision makers’ preferences should be considered by applying weight factors in the green evaluation process. Considering the complexity in solution selection, user-friendly software should be developed to facilitate the hierarchical LCD approach. In addition, lifetime strategies based on system reliability have to be developed, sensitivity studies have to be performed, and reliability-based inspection /maintenance optimization studies have to be carried out [100–103] in the context of life-cycle design of reinforced concrete structures incorporating durability, economic efficiency and green objectives.

Meet structural requirements Meet predetermined TD requirement TD = 65.2 yrs Repaint EC at 65.2th year TD′ = 75.2 yrs ¥ 47989.2 ¥ 51916.2 ¥ 518.1

7. Conclusions Traditional structural design method fails to cover all the design objectives. This paper proposes the hierarchical LCD approach based on the theory of LCD and LCA, which comprehensively considers the aspects of safety and reliability, durability, economic efficiency, local environmental impacts, social impacts and global environmental impacts. The design objectives are arranged hierarchically into six levels according to design concept, design constraints, as well as their relevancy degree to the structure. The fundamental objectives with stricter restrictions and higher relevancy are placed on the basic levels, and the decision-support objectives with less restrictions and lower relevancy are placed on the upper level. Level 1 performs structural safety and reliability design, which is the most fundamental requirement in structural design process. Initial durability design and durability improvement measures are proposed in Level 2 to ensure the durable service life of structures. In Level 3, LCC and average annual cost are used to identify the economic efficiency of various solutions. The green evaluation indicators are set up in Level

Fig. 4. Structural performance and LCC evolution of cases designed by traditional and hierarchical approaches.

direct and indirect economic impacts of restoration, as shown in Fig. 4. Restorations can cause other negative effects. As restoration requires a large amount of materials and construction work, the structure’s normal function can be interrupted, and can induce heavy environmental burden and extensive social impacts. The solution designed based on hierarchical LCD method shows advantages in wide range of aspects. It can maintain the demanded structural performance with less intervention for a longer time. 129

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4–6 based on the concept of LCA, which mainly focus on the effects on the local environment, the society and the global environment. Detailed methods and indicators for design and evaluation are addressed in the paper. The hierarchical LCD method provides an organized framework for the design of engineering structures. Design methods, indicators and weight factors within this framework can be flexible and subject to change depending on the project requirements. A girder of a reinforced concrete bridge in marine-atmosphere environment is investigated to illustrate the practical application of the proposed design method. Safety and reliability-based design considers the strength level of concrete and reinforcements, as well as the thickness of concrete cover. All adequate solutions are considered through durability-based design by measuring the effect of mineral admixtures, corrosion-prevention coatings and electrochemical techniques on structural durable service life. Solutions that fulfill the durability requirements are selected for economic evaluation, where both the direct and indirect costs are calculated. Local environmental evaluation is performed on the qualified solutions by considering the embodied energy of materials, solid waste volume, as well as the emission of sulfur dioxide and nitrogen dioxide. No weighting is applied in the evaluation, and the local environmental score is determined by comparative scoring method. For social evaluation, the indicator is limited to downtime, and the global environmental evaluation takes greenhouse gas emission and fossil fuel consumption into account. At the end of the hierarchical LCD process, only the best solution remains. Hierarchical LCD results indicate that solutions with strong initial durability design, reasonable future maintenance plan, minimum future maintenance frequency, and high cement replacement rate are more likely to have better comprehensive performance. A comparison is made between the results of two specific solutions designed by traditional method (iv-d-SL-ER, or ‘Solution T’ for short) and hierarchical LCD method (iii-c-FA&SL-ER&EC, or ‘Solution H’ for short), respectively. The average annual cost of Solution H is around 25% of Solution T. The local environmental evaluation score of Solution H is 2.03 points, shows better performance than the 3.68 points of Solution T. Solution H causes no downtime in social evaluation, while Solution T has a 4-day downtime. Solution H also emits less greenhouse gases and consumes less fossil fuels compared with Solution T. According to the comparison, the hierarchical LCD method is able to obtain solutions that meet the requirements in major design aspects.

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