Sustainable performance criteria for construction method selection in concrete buildings

Automation in Construction 19 (2010) 235–244

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Automation in Construction j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a u t c o n

Sustainable performance criteria for construction method selection in concrete buildings Ying Chen a,b, Gül E. Okudan c, David R. Riley b,⁎ a b c

Department of Construction Management, School of Civil Engineering, Tsinghua University, Beijing, P.R. China Department of Architectural Engineering, The Pennsylvania State University, University Park, 104 Engineering Unit A, University Park, PA, USA Department of Industrial and Manufacturing Engineering, The Pennsylvania State University, University Park, PA, USA

a r t i c l e

i n f o

Article history: Accepted 19 October 2009 Keywords: Construction methods Prefabrication Concrete construction Construction management Sustainable development

a b s t r a c t The use of prefabrication offers significant advantages, yet appropriate criteria for applicability assessments to a given building have been found to be deficient. Decisions to use prefabrication are still largely based on anecdotal evidence or simply cost-based evaluation when comparing various construction methods. Holistic criteria are needed to assist with the selection of an appropriate construction method in concrete buildings during early project stages. Following a thorough literature review and comprehensive comparisons between prefabrication and on-site construction method, a total of 33 sustainable performance criteria (SPC) based on the triple bottom line and the requirements of different project stakeholders were identified. A survey of U.S. experienced practitioners including clients/developers, engineers, contractors, and precast concrete manufacturers was conducted to capture their perceptions on the importance of the criteria. The ranking analysis of survey results shows that social awareness and environmental concerns were considered as increasingly important in construction method selections. Factor analysis reveals that these SPCs can be grouped into seven dimensions, namely, economic factors: “long-term cost,” “constructability,” “quality,” and “first cost”; social factors: “impact on health and community,” “architectural impact”; and environmental factor: “environmental impact.” The resultant list of SPCs provides team members a new way to select a construction method, thereby facilitating the sustainable development of built environment. © 2009 Elsevier B.V. All rights reserved.

1. Introduction With heightened awareness of environmental pollution, natural resource depletion and accompanying social problems, sustainable development and sustainable construction have become a growing concern throughout the world. Buildings are one of the heaviest consumers of natural resources and account for a significant portion of the greenhouse gas emissions. In the U.S., buildings account for 38.9% of primary energy use, 38% of all carbon dioxide emissions, and 30% of waste output [1]. Conventional on-site construction methods have long been criticized for low productivity, poor quality and safety records, long construction time, and large quantities of waste in the industry. Prefabrication is a manufacturing process, generally taking place at a specialized facility, with which various materials are joined to form a component part of the final installation [2]. Several benefits of applying prefabrication technology in construction were commonly discussed in previous literature [3–15], including: shortened construction time,

⁎ Corresponding author. Tel.: +1 814 863 2079; fax: +1 814 863 4789. E-mail addresses: [email protected] (Y. Chen), [email protected] (G.E. Okudan), [email protected] (D.R. Riley). 0926-5805/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.autcon.2009.10.004

lower overall construction cost, improved quality, enhanced durability, better architectural appearance, enhanced occupational health and safety, material conservation, less construction site waste, less environmental emissions, and reduction of energy and water consumption. These advantages provide opportunities for prefabrication to better serve sustainable building projects. Worldwide, the highest precast levels in 1996 were located in Denmark (43%), the Netherlands (40%), Sweden and Germany (31%) [16]. In the United States, the share of reinforced concrete construction supplied by precast producers is only 6% while the average across the European Union is 18% [8]. Although the U.S. precast concrete industry produces technologically and architecturally complex buildings and building elements, such as double tees, hollow-core slab elements, inverted tee and ledger beams, and facade panels, in building construction market, the percentage of precast concrete systems is pretty low (approximately 1.2%) [7,8]. It is more urgent to address prefabrication issues in concrete buildings while achieving sustainable construction in the United States. Pasquire and Connolly demonstrated that decisions to use prefabrication are still largely based on anecdotal evidence rather than rigorous data, as no formal measurement criteria or strategies are available [17]. Blismas et al. also indicated that holistic and methodical assessments of the prefabrication applicability to a particular project

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have been found to be deficient, and common methods of evaluation simply take material, labor and transportation costs into account when comparing various construction methods, without explicit regard for the long-term cost or soft issues, such as life cycle cost, health and safety, effects on energy consumption, and environmental impact of a project [10]. Additionally, for individual building projects, prefabrication technology is not always the only available option, nor is it always better than on-site construction method due to various project characteristics and available resources. If not employed appropriately, change orders, severe delays in production, erection schedules, substantial cost overruns, and constructability problems may be encountered in the use of precast concrete systems. All of these demonstrate that criteria for decisions regarding construction methods are unclear and unrecorded. There is a need to establish holistic criteria to select an appropriate construction method and stimulate the suitable use of prefabrication for a given building project. In this research, two prominent methods in building construction are reviewed and discussed: the conventional on-site reinforced concrete construction method, and the precast concrete building method. In the sections that follow, the former method is referred as the ‘on-site’ construction method, and the latter the ‘prefabrication’ method. The main objective of the research was to develop a holistic sustainable performance criteria (SPC) set to assist design team members in the selection of appropriate construction methods in concrete buildings during early project stages. These criteria enable applications of IT to support and automate the complex considerations of prefabrication on concrete building projects. As a result, the likelihood of sustainable construction is enhanced, both to meet society's environmental goals and account for the social and economic impacts of the project. 2. Research methodology Methodology selected for this research comprised of a questionnaire design, a questionnaire survey and interviews of the U.S. construction industry practitioners, and a statistical analysis of the survey data. Fig. 1 illustrates the methodology for the research.

that they consider to influence construction method selections but were not listed in the provided questionnaire (refer to Appendix A for questionnaire details).

2.2. Questionnaire survey A pilot survey was conducted with experienced contractors and engineers to validate the final questionnaire. The questionnaire was then administered by email to 412 selected industry practitioners within the U.S. construction industry who are primary participants in the precast concrete supply chain, including construction clients/ developers, engineers, contractors, and precast concrete manufacturers. All of them have different opinions and focus on construction method selection. Obtaining views from the four categories ensures a holistic criteria set for construction method selection. Survey questionnaires were emailed to 84 construction clients/ developers, 71 engineers, 145 contractors, and 112 precast concrete manufactures in the United States. The email addresses of precast manufactures were obtained from the PCI's (Precast/Prestressed Concrete Institute) Membership Directory [18] and NPCA's (National Precast Concrete Association) Membership Directory [19] by selecting manufacturers who produce architectural precast units, such as architectural beams, facades, slabs, and stairs. Contact information for the construction clients/developers, engineers, and contractors were obtained from the Partnership for Achieving Construction Excellence (PACE) database. PACE is based in the Department of Architectural Engineering at The Pennsylvania State University and is a working partnership between Penn State students, faculty, and building industry practitioners. The developers, engineers, and contractors for the survey were selected from those who had worked on building projects and also had experience in prefabrication. To gain further understanding of the survey results, five selected respondents who had rich experience in concrete prefabrication and construction method selection process were interviewed after returning survey responses. Specifically, they were asked how they considered their selections, such as why some criteria are less or more important than others etc.

2.1. Questionnaire design A wide scope review of literature revealed that there was no comprehensive list of performance criteria developed specifically for construction method selection in concrete buildings. To compile a meaningful list of criteria, several researches in related areas were conducted. To ensure that prefabrication and on-site construction method are clearly distinguishable by the selected criteria, the comparison between the two methods was thoroughly explored. Combined with sustainable concerns and requirements of project stakeholders on construction method selection, a list of initial criteria was developed. Based on the derived criteria, an industry questionnaire survey was designed by Adobe Livecycle Designer, which enables user to create dynamic and interactive forms that are filled out on a computer. The survey, which consisted of two main parts, aims at investigating the perspective of the construction industry on the importance of the criteria. Part one sought background information about the respondents and their organizations, such as the experience of the respondent in the construction industry, and the number of projects using prefabrication the respondent has been involved in. In part two, respondents were asked to rate the level of importance of the derived criteria based on a scale of 1–5, where 1 is ‘least important’, 2 ‘fairly important’, 3 ‘important’, 4 ‘very important’, and 5 ‘extremely important’. To ensure a better understanding of the criteria, definition of each criterion was clarified and guidance on completion was given in the questionnaire. At the same time, respondents were encouraged to provide supplementary criteria

Fig. 1. Research framework and methodology.

Y. Chen et al. / Automation in Construction 19 (2010) 235–244

2.3. Data analysis methods To ensure that the rating scale (1–5) for measuring the criteria yields the same result over time, a reliability analysis using the internal consistency method was first examined. In order to identify the relative importance of SPCs based on the survey data, ranking analysis was performed. It must be noted that the ratings in the scale indicate only a rank order of importance of the criteria, rather than how much more important each rating is than the other. Using parametric statistics (means, standard deviations, etc.) to rank such data would not produce meaningful results, and therefore non-parametric procedures must be adopted [20]. Severity index analysis was selected in this study to rank the criteria according to their relative importance. The following formula is used to determine the severity index [21]: Severity Index ðSIÞ =

ð∑

fi 5 i = 1 ωi ·

n

Þ=ða·100Þ

·100

ð1Þ

where i is the point given to each criterion by the respondent, ranging from 1 to 5; ωi is the weight for each point (=rating in scale of points, which “1” is the least important and “5” is the extremely important); fi is the frequency of the point i by all respondents; n is the total number of responses; and a is the highest weight, in this study a = 5. Five important levels are transformed from SI values: High (H) (0.8 ≤ SI ≤ 1), High–Medium (H–M) (0.6 ≤ SI b 0.8), Medium (M) (0.4 ≤ SI b 0.6), Medium–Low (M–L) (0.2 ≤ SI b 0.4), and Low (L) (0 ≤ SI b 0.2). Recognizing that the derived SPCs are likely inter-related through an underlying structure of primary factors, and to obtain a concise list of SPCs under these circumstances, a factor analysis was also utilized. Factor analysis is an effective statistical method used to describe variability among observed variables in terms of fewer unobserved variables (latent variables) called factors. In other words, it reduces variables with similar characteristics together into a smaller set of uncorrelated dimensions or factors, which are capable of explaining the observed variance in the larger number of variables [20]. This analysis was performed with the assistance of Predictive Analytics Software (PASW) Statistics 17.0 (formerly SPSS Statistics). Kaiser–Meyer–Olkin (KMO) measure and Bartlett's Test of Sphericity were conducted to examine the sampling adequacy, ensuring that factor analysis was going to be appropriate for the research. The principal component analysis was chosen to extract the latent factors based on the criterion that the associated eigenvalue should be greater than 1. To interpret the relationship between the observed variables and the latent factors more easily, the most commonly used rotation method, varimax rotation, was selected. 3. Development of sustainable performance criteria 3.1. Previous studies on related criteria An extensive review of literature in related areas including PPMOF (Prefabrication, Preassembly, Modularization and Offsite Fabrication), modular construction, HCC (Hybrid Concrete Construction), precast/ prestressed concrete construction, prefabricated building components, etc. has been conducted. In PPMOF areas, general processes for evaluating the use of PPMOF, which were highly project specific, ranging from very systematic studies of feasibility, cost, and schedule for several alternatives, to quick decisions based on intuition and judgment were described [22]. PPMOF feasibility using ten decision factor categories, varying from four to ten detailed questions for each category, was also evaluated [23]. Regarding modular construction, the feasibility of using modular construction technology for a particular project or a petrochemical power plant based on various factors classified into five influencing categories were determined [24,25]. In terms of hybrid concrete

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construction, salient criteria for contractors in their choice and use of HCC were identified [26], and HCC performance indicators, which would help to inform the process of deciding whether or not to adopt HCC technology rather than more traditional alternatives, were selected [27,28]. For prefabrication, Pasquire et al. [9] recommended six factors of measurement when comparing prefabrication and traditional construction: cost, time, quality, health and safety, sustainability and site issues. A total of 97 detailed items and considerations for the six factors were included in the research. Idrus and Newman [21] conducted a survey within the UK construction industry to investigate the construction related factors influencing the choice of concrete floor systems: in situ, precast and hybrid construction. Ultimately, 12 factors were identified as being directly related to the construction process. Findings from the above studies suggest that factors or criteria of decision-making for PPMOF, modular construction, and HCC have been well documented. For prefabrication and on-site construction method, although Pasquire et al. [9] recommended 97 detailed items categorized in six factors for comparing prefabrication and traditional construction, the most significant challenge in using the criteria is the very limited information available at an early stage of a project. The project team also found that in a follow-up survey, “many of the items listed were not currently recorded in any meaningful way”. Idrus and Newman [21] tried to investigate factors influencing the construction method selection of concrete floor systems, but the study was limited to investigating construction related factors only. The performance criteria developed specifically for construction method selection in concrete buildings has yet to be examined. This research attempted to establish a list of holistic criteria based on the sustainable triple bottom line and requirements of different project stakeholders, which may better capture the potential performance of construction methods and facilitate the sustainable development of built environment. 3.2. Comparison between prefabrication and on-site construction method The on-site construction method consists of extensive cast-in-place activities being widely used. It is characterized by labor-intensive, wettrade activities, resulting in poor safety, lengthy construction time and a large quantity of waste. The prefabrication method is featured by cleaner and tidier site environment, and the reduction of construction waste and time. In the research, assessment criteria for construction method selection should have the capability that prefabrication and onsite construction method can be clearly distinguishable by the selected criteria. Thus, there is a need to comprehensively compare the two construction methods. The comparisons were divided into three categories based on the sustainable triple bottom line. It is not practical to compare all differences between prefabrication and on-site construction method in this paper due to space constraints. Instead we take “construction time” under economic criteria as an example, and provide an in-depth comparison as presented in Table 1. In the table, several reasons for explaining the difference were identified in matched pairs. For example, in prefabrication, factory fabrication and site preparation can occur at the same time, while on-site construction work procedures cannot start until the previous procedure is completed. On the other hand, the quantification of the difference between prefabrication and on-site method on construction time is also listed in Table 1. Similar formats and procedures were implemented for all other differences between prefabrication and on-site construction method. 3.3. Sustainable performance criteria The comparisons categorized by economic aspects, social aspects, and environmental aspects between prefabrication and on-site

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Table 1 Examples of comparisons between precast/prefabrication and on-site/cast-in-place construction method. Factors

Precast/Prefabrication

Economic criteria Reduction of construction time 1. Prefabrication and site preparation can occur simultaneously, and the erection process is fast;

Positive/ On-Site/Cast-in-place Negative

√

2. Independent of adverse weather which has √ almost no impact on the schedule of the prefabrication manufacturing; 3. Prefabrication usually has a greater potential for √ Construction automation (e.g., digital fabrication) and intelligent time management systems; 4. Workers in a prefabrication plant are able to be √ more proficiently experienced at specific tasks; 5. Precast beams and slabs eliminate field forming and shoring requirements; 6. Less finish works (e.g., external wall finishes, laying tiles and plastering) is required to be completed on-site.

√ √

Positive/ Quantification of differences Negative

Time consuming × 1. All construction occurs on site, and one work cannot start until the previous work is completed; 2. The construction process can be easily × delayed by adverse weather or scheduling conflicts; 3. Most on-site construction procedures × heavily depend on manual methods;

1. Construction time for prefabrication is less than half of on-site construction [29]. 2. Up to 70% time saving can be achieved when compared in-situ construction [30]. 3. Average reduction in construction time can achieve 20% when compared with onsite construction [16].

4. Most of workers on site are × temporary, and their craft and technical skills are more variable; 5. Formwork and shoring installation × are necessary for on-site construction; 6. All finish works need to be finished on × site.

Note: positive for project goals: √; negative for project goals: ×.

construction method have been identified. This creates a valuable base for the development of sustainable performance criteria. Further, project team members have different perspectives and needs during the construction method selection process. To establish a list of holistic criteria for the appropriate construction method decision, various requirements from project stakeholders should also be captured and considered appropriately. Combined with a range of criteria reviewed previously, a matrix was formed, where X featured sustainable aspects: economic, social, and environmental aspects; Y featured different perspectives and requirements of key stakeholders: clients, engineers, contractors, and precasters. Overall, as shown in Table 2, a total of 33 SPCs were selected for construction method assessment, with 16 SPCs in economic criteria, 8 SPCs in social criteria, and 9 SPCs in environmental criteria, respectively. To better understand the criteria, definition of each criterion was provided in the questionnaire (see Appendix A). 4. Analysis and discussion 4.1. Sample characteristics After the questionnaire was delivered and a follow-up reminder letter was sent to the respondents who had not returned the survey, a

total of 97 responses were received. Two of the questionnaires were not properly completed, thus only 95 questionnaires were valid for the analysis. Among those 95 responses, 15 completed responses were from clients/developers, 19 from engineers, 39 from contractors, and 22 from precast concrete manufacturers, with an overall response rate of 23.1%, and sub-groups response rates ranged between 18% and 27%, as shown in Table 3. The respondents were from different organizations/ institutions in a number of states (United States), including architectural firms, engineering firms, consulting firms, general contractors, construction managers, AECs (Architecture, Engineering, and Construction), precast concrete manufacturers, and government agencies. The diversity of the samples is a guarantee of obtaining holistic criteria. In the present study, architectural designers and structural engineers are referred to as “engineer”; general manager, construction manager, project manager, and superintendent were categorized as “contractor” due to their similar job characteristics. All of the survey participants were experienced construction experts. About 16% of them had more than 35 years of experience in the construction industry, 24% had experience between 26 and 35 years, and 45% between 11 and 25 years. The respondents' experience in the construction industry is shown in Fig. 2. The participant history of projects using prefabrication that the respondents have been involved in was also impressive. As shown in Fig. 3,

Table 2 Sustainable performance criteria for construction method selection in concrete buildings. Economic criteria

Social criteria

Environmental criteria

Focus of clients/engineers

E1: E2: E3: E4: E5: E6: E7: E8: E9:

S1: health of occupants (indoor air quality) S2: influence on job market S3: physical space S4: aesthetic options

P1: P2: P3: P4: P5: P6:

Focus of contractors/ precasters

E10: loading capacity E11: integration of building services E12: lead-times E13: material costs E14: labor costs E15: constructability (buildability) E16: integration of supply chains (logistics)

S5: workers' health and safety S6: labor availability S7: community disturbance S8: traffic congestion

P7: waste P8: pollution generation P9: water consumption

construction time initial construction costs maintenance costs disposal costs life cycle costs defects and damages durability the speed of return on investment flexibility (adaptability)

site disruption recyclable/renewable contents energy efficiency in building use (thermal mass) reusable/recyclable elements material consumption energy consumption in design and construction

Y. Chen et al. / Automation in Construction 19 (2010) 235–244

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Table 3 Questionnaire distribution and response. Respondents

Clients/Developers Engineers Contractors Precasters Total

Number of questionnaires Final sent-out

Valid responses

84 71 145 112 412

15 19 39 22 95

Percentage (%)

Response rate (%)

15.8% 20.0% 41.0% 23.2% 100%

17.9% 26.8% 26.9% 19.6% 23.1%

Fig. 3. Participant history of projects using prefabrication.

about 48% respondents were involved in more than 55 concrete prefabrication projects, and the average number of prefabrication buildings was 33. As the experience of the respondents in the concrete prefabrication buildings and in the construction industry is quite respectable, opinions and views on relevance of SPC obtained through the survey can be regarded as important and reliable. 4.2. Reliability of the questionnaire With the help of PASW 17.0, Cronbach's alpha was calculated to test the internal consistency reliability of the generated scale. The alpha reliability coefficient normally ranges between 0 and 1. The closer alpha is to 1 the greater the internal consistency reliability of the criteria in the scale. Cronbach's alpha values for economic criteria, social criteria, environmental criteria, and all criteria are 0.834, 0.836, 0.941, and 0.939, respectively. All alpha values are greater than 0.7, indicating that all reliability coefficients are acceptable and the internal consistency of the criteria included in the scale is excellent. 4.3. Ranking analysis By feeding the survey results into PASW 17.0, severity index values were calculated using the formula in Eq. (1). Based on the magnitude of the severity indices, the ranking results for each criteria category (e.g., economic), and for all criteria are presented in descending order as shown in Table 4. Based on these ranking results, five criteria were highlighted to have “High” importance levels in evaluating construction methods with an SI value between 0.810 and 0.852. These five criteria are “construction time (E1)”, “initial construction costs (E2)”, “constructability (E15)”, “material costs (E13)”, and “lead-times (E12)”. “Construction time” was ranked as the first priority in economic category with an SI value of 0.852, and it was also the highest among all criteria and was highlighted at “High” importance level; “lead-time”, the coordination time during preconstruction, has been neglected in traditional projects, and now has

Fig. 2. Experiences of survey participants in the construction industry.

become a major concern when selecting a construction method; “initial construction costs” and “material costs” have been, and will continue to be, major concerns for a project team, as well as important traditional performance measures; “constructability”, the extent of the facility of construction, basically, has close relationships with time, cost, and quality performance. Apart from “lead-time”, prefabrication positively impacts the other four criteria when compared to the on-site construction method. Among the top five criteria, it is observed that four criteria are concerning time and cost, indicating time and cost remained as the most important factors for choosing a construction method. According to Table 4, a total of 24 criteria, consisting of 10 economic criteria, 7 social criteria, and 7 environmental criteria, were recorded to have “High–Medium” importance levels. Although these 24 criteria were in the same importance level category, the environmental criteria (average SI = 0.640) were considered to be less important compared to the economic criteria (average SI = 0.712) and social criteria (average SI = 0.697). However, it should be noted that both the social criteria and environmental criteria account for 29.2% in this importance level. The result is an example of evidence pointing to the trend that social and environmental aspects are no longer the least important factors for construction method selection in concrete buildings. Some criteria in the two categories were ranked relatively higher in the “High–Medium” level. For example, “workers' health and safety (S5)” was rated as first in the social subcategory, and ranked as second in the 24 criteria with an SI value of 0.787. Workers' health and safety issues are of paramount importance to all project participants. Improved site safety is found to be a major benefit of prefabrication due to cleaner and safer working environments when compared to on-site construction method. Supporting this, Jaillon and Poon showed a significant reduction in accident rates (63% lower) when compared to the industry figures with an average of 22 accidents per 1000 workers [15]. Among the criteria with “High–Medium” importance levels, another good example is “energy efficiency in building use (P3)”, which was ranked as the most important criterion in environmental subcategory with an SI value of 0.724. One of the reasons for the high level of importance of this criterion may be that the respondents realized that the energy consumption in building use has become dominant in total energy use. Indeed, studies [31,32] revealed that the amount of energy consumption during the building use phase, such as natural gas or electricity for heating and cooling, accounts for 84%– 94% of life cycle energy use. Energy savings can be easily achieved in prefabricated buildings by combining the thermal mass of concrete with the optimal amount of insulation in precast concrete walls. Doyoon [33] showed that prefabricated buildings had 7% less energy consumption over their service life in comparison to conventional buildings. Four criteria had “Medium” importance levels with SI values between 0.474 and 0.598: “influence on job market (S2)”, “pollution generation (P8)”, “water consumption (P9)”, and “disposal costs (E4)”. One characteristic of these criteria is that the items are longer-

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Table 4 Rank of sustainable performance criteria for construction method selection. Sustainable performance criteria

Valid percentage for score of (%)

Severity Index

Ranking by category

Overall ranking

Importance level

41.0 41.1 33.7 43.1 33.7 36.8 33.7 18.9 26.3 15.8 24.2 15.8 16.8 6.3 11.6 3.2

0.852 0.835 0.834 0.833 0.810 0.793 0.773 0.732 0.726 0.722 0.705 0.699 0.688 0.655 0.625 0.474

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 2 3 4 5 6 8 9 11 13 16 17 18 21 24 33

H H H H H M–H M–H M–H M–H M–H M–H M–H M–H M–H M–H M

23.2 36.8 30.5 33.8 27.4 25.3 25.3 23.1

41.0 21.1 27.4 18.9 14.7 14.7 9.5 5.3

0.787 0.726 0.722 0.710 0.672 0.646 0.613 0.598

1 2 3 4 5 6 7 8

7 10 14 15 19 23 26 30

M–H M–H M–H M–H M–H M–H M–H M

28.4 32.6 37.9 32.6 29.5 25.3 26.3 24.2 15.8

29.5 14.8 6.3 8.4 9.5 10.5 8.5 10.5 7.4

0.724 0.663 0.646 0.617 0.611 0.611 0.609 0.596 0.537

1 2 3 4 5 6 7 8 9

12 20 22 25 27 28 29 31 32

M–H M–H M–H M–H M–H M–H M–H M M

1

2

3

4

5

Economic criteria E1: construction time E2: initial construction costs E15: constructability (buildability) E13: material costs E12: lead-times E10: loading capacity E7: durability E14: labor costs E8: the speed of return on investment E11: integration of building services E5: life cycle costs E6: defects and damages E3: maintenance costs E16: integration of supply chains (logistics) E9: flexibility (adaptability) E4: disposal costs

1.1 1.1 0.0 1.1 2.1 0.0 4.2 2.1 3.2 2.1 5.3 7.4 2.1 5.3 9.5 25.3

1.1 1.1 2.1 0.0 3.2 9.5 5.3 9.5 11.6 8.4 13.7 7.4 19.0 12.6 17.9 28.4

9.5 17.9 12.6 23.2 15.8 21.1 24.2 27.4 30.5 31.6 28.4 29.4 28.4 37.9 34.7 33.6

47.3 38.8 51.6 32.6 45.2 32.6 32.6 42.1 28.4 42.1 28.4 40.0 33.7 37.9 26.3 9.5

Social criteria S5: workers' health and safety S4: aesthetic options S1: health of occupants S3: physical space S6: labor availability S7: community disturbance S8: traffic congestion S2: influence on job market

3.1 2.1 7.4 2.1 4.2 7.4 10.5 5.3

5.3 11.6 9.5 12.6 12.6 16.8 16.8 24.2

27.4 28.4 25.2 32.6 41.1 35.8 37.9 42.1

Environmental criteria P3: energy efficiency in building use P2: recyclable/renewable contents P1: site disruption P7: waste P6: energy consumption P4: reusable/recyclable elements P5: material consumption P8: pollution generation P9: water consumption

7.4 4.2 5.3 10.5 13.6 10.5 10.5 13.7 22.1

10.5 22.1 16.8 20.1 15.8 20.0 17.9 20.0 17.9

24.2 26.3 33.7 28.4 31.6 33.7 36.8 31.6 36.8

term (e.g., E4) and ‘softer’ (e.g., S2 and P8). An interesting observation is that “disposal costs (E4)” is the last one in all criteria with the lowest SI value of 0.474, although it is linked to cost. From the results in Table 4, it is clearly seen that all SPCs are important. All criteria are rated with “High”, “High–Medium”, or “Medium” importance levels for use when selecting a construction method. During the interview discussion, the respondents also asserted that the criteria they rated lower did not mean they are not important for selecting construction methods, but rather they wanted to highlight the relative importance of criteria from their vantage point. 4.4. Factor analysis Although the most significant criteria were identified using ranking analysis, some of them are likely to be inter-related with each other through an underlying structure of primary factors. In order to obtain a concise list of SPCs, a factor analysis was performed. For the economic criteria, 95 valid survey opinions pertaining to 16 SPCs were entered into the PASW 17.0 to conduct the factor analysis. The analysis results showed that the Kaiser–Meyer–Olkin (KMO) measure of sampling adequacy was 0.757, larger than 0.5, suggesting that the sample was acceptable for factor analysis. The Bartlett Test of Sphericity was 680.290 and the associated significance level was 0.000, indicating that the population correlation matrix was not an identity matrix. Both of the tests showed that the obtained data in economic criteria supported the use of factor analysis and these could be grouped into a smaller set of underlying factors.

Using principal component analysis, the factor analysis extracted four latent factors with eigenvalues greater than 1.0 for the 16 economic criteria, explaining 65.8% of the variance. The rotated factorloading matrix based on the varimax rotation for the four latent factors is shown in Table 5. The component matrix identifies the relationship between the observed variables and the latent factors. The relationships are referred to as factor loadings. The higher the absolute value of the loading, the more the latent factor contributes to the observed variable. Small factor loadings with absolute values less than 0.5 were suppressed to help simplify Table 5. For further interpretation, the four latent factors under the economic criteria (shown in Table 5) are given names as: Factor 1: long-term cost; Factor 2: constructability; Factor 3: quality; and Factor 4: first cost. Similar factor analyses were performed to identify the underlying structures for social criteria and environmental criteria. For social criteria, both the KMO measure of sampling adequacy test (0.768) and Bartlett's sphericity (p = 0.000) were significant, which indicated that factor analysis was also appropriate for the social criteria. Two factors under social criteria were extracted from the factor analysis, namely, Factor 5: impact on health and community, and Factor 6: architectural impact. Along with rotated factor-loading matrix, the percentage of variance attributable to each factor and the cumulative variance values are shown in Table 6. From the table, it can be seen that the two factors accounted for 61.1% of the total variance of the eight social criteria. In the environmental category, the results for the factor analysis showed that the KMO measure was 0.916 and the Bartlett's test (p = 0.000) was also significant, which indicated that the factor analysis was also appropriate in identifying the underlying structure

Y. Chen et al. / Automation in Construction 19 (2010) 235–244 Table 5 Factor loadings for the economic criteria after varimax rotation. Observed economic variables

Table 7 Factor loadings for the environmental criteria after varimax rotation.

Latent economic factors Longterm cost

Observed environmental variables

Constructability Quality First cost

E8: the speed of return 0.798 on investment E10: loading capacity 0.755 E9: flexibility (adaptability) 0.744 E7: durability 0.677 E5: life cycle costs 0.603 E3: maintenance costs 0.550 E12: lead-times 0.770 E1: construction time 0.757 E15: constructability (buildability) 0.691 E16: integration of supply chains 0.558 (logistics) E11: integration of building 0.521 services E4: disposal costs E6: defects and damages E13: material costs E2: initial construction costs E14: labor costs Eigenvalues 3.223 2.756 Percentage of variance (%) 20.145 17.226 Cumulative of variance (%) 20.145 37.371

0.794 0.681

2.320 14.498 51.869

0.887 0.869 0.572 2.228 13.922 65.791

4.4.1. Factor 1: long-term cost The first dimension focuses on the “long-term cost”. Long-term costs include criteria such as the speed of return on investment, loading capacity, durability, life cycle costs, and maintenance costs. This factor suggests that in addition to focus on the cost of materials and labor, design team members are taking long-term expenses into account when selecting a construction method. Long-term costs can be reduced when prefabrication is used. Combined with good compaction and curing in a controlled factory environment, these factors ensure a dense, highly durable concrete that can increase resistance to weathering and corrosion. Additionally,

Table 6 Factor loadings for the social criteria after varimax rotation. Latent social factors Impact on health and community S5: workers' health and safety S8: traffic congestion S6: labor availability S7: community disturbance S1: health of occupants S4: aesthetic options S3: physical space S2: influence on job market Eigenvalues Percentage of variance (%) Cumulative of variance (%)

Architectural impact

0.808 0.749 0.714 0.707 0.577

2.851 35.632 35.632

Latent environmental factor Environmental impact

P5: material consumption 0.905 P4: reusable/recyclable elements 0.888 P6: energy consumption in design and construction 0.863 P8: pollution generation 0.856 P2: recyclable/renewable contents 0.848 P9: water consumption 0.846 P7: waste 0.820 P3: energy efficiency in building use 0.737 P1: site disruption 0.642 Eigenvalues 6.148 Percentage of variance (%) 68.316

of the environmental criteria. The results of the analysis are presented in Table 7. Just one factor named Factor 7: environmental impact was extracted, explaining 68.3% of the total variance of the nine environmental criteria. Overall, a total of seven latent factors were extracted to present the underlying structure of the criteria used for selecting a construction method in concrete buildings. Four factors were under economic category, two factors belong to social category, and one factor for the environmental dimension. Description and findings of the seven latent factors are presented in the following sections.

Observed social variables

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0.909 0.669 0.604 2.041 25.513 61.145

incorporating the architecture into the structure enlarges panel sizes, and thus enables significantly the reduction of the chance for water penetration that can weaken a structure and cause unsightly staining and mold problems. Such long-term durability advantages require little or no maintenance over the building's life to preserve the original look and limit rust, corrosion, stain or fading. This is a great benefit to owners who understand the need to consider long-term costs during the design phase. Many professionals believe that by adopting a life cycle costing approach, the first cost of prefabrication can be largely offset by other factors such as potential reductions in construction time, on-site activities and labor requirements, waste and resources [15]. 4.4.2. Factor 2: constructability The second dimension concerns integration of supply chains, integration of building services, lead-time, construction time, and buildability, which collectively could be interpreted as “constructability”. Constructability can be referred to as the extent to which a design facilitates efficient use of construction resources and enhances ease and safety of construction on site while the client's requirements are met [34]. Generally, an efficient construction method with good constructability means there is an improved management flow of building materials and other sources from suppliers, an excellent integration of mechanical/plumping/electrical services, a smooth construction process, and a reduced construction time. Prefabrication contributes positively to these aspects. Building materials in conventional buildings are usually acquired through project-based purchases from wholesalers or retailers, resulting in a lack of long-term cooperation between contractors and suppliers. On the other hand, prefabrication procures building materials mostly from stable manufacturers, and mass production enables purchases in bulk quantities. This makes the relationship between prefabrication plants and suppliers more reliable and longterm, facilitating an improvement in constructability. Most importantly, the continuous, uninterrupted manufacturing and erection of precast structural components lend prefabrication perfectly to track construction schedules. Attesting to this, previous studies [16,29,30] showed that average reduction in construction time ranges between 20% and 70%. Developers and general contractors appreciate the efficiencies of buildings with precast concrete. However, interviews also showed that tighter and longer periods of advanced coordination between architects, general contractors, and prefabrication subcontractors are required to allow for a cohesive structural design, construction planning, and procurement when adopting prefabrication. This could delay the beginning of the project on site. Goodier and Gibb recommended that prefabrication needs to be integrated from the start to the whole design and construction process, and offsite suppliers and constructors need to be more aligned in order to minimize lead-time [14].

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4.4.3. Factor 3: quality Variables loading on latent Factor 3 focus on disposal cost, defects and damages. Therefore, latent Factor 3 is named “quality”. Obviously, defects and damages and disposal cost are strongly connected with each other (i.e., more defects and damages will contribute to a higher disposal cost). “Quality” has been considered an important difference between prefabrication and on-site construction in previous studies [15,16,35,36]. For prefabrication components, several measures ensure higher quality. First of all, in a climate-controlled environment using advanced facilities operated by well-trained people, quality is easier to control. Second, prefabrication plants have more rigorous quality supervision systems and better structured environments than on-site construction. In the United States, PCI-certified precast concrete fabricators must undergo two annual, unannounced and stringent inspections. This certified quality process combined with the factorycontrolled conditions assure owners and designers that their project will receive high-quality products with uniform color and texture along with tight tolerances. Third, in prefabrication, the key stakeholders of the project, such as owner representatives, engineers, contractors, and prefabrication subcontractors, usually provide valuable input on the project design. The high level of integrated design environment contributes to fewer defects. However, in the interviews, some respondents argued that the on-site output quality may not be lower than prefabrication as long as a rigorous quality supervision system is implemented. In spite of this, we cannot deny the fact that a better structured environment as in a manufacturing plant is a guarantee for higher quality of concrete components. 4.4.4. Factor 4: first cost Factor 4 is labeled “first cost”, which is associated with material costs, transportation costs, and labor costs. First cost has been a traditional project driver when selecting a construction method, whether intentionally or unintentionally. For prefabrication, economic benefits may not be easy to evaluate [10,14], despite the assertions in previous research [37] showing that the cost of prefabrication is higher than on-site construction by 10%– 20%. According to Hsieh's research [38], the cost per unit area in prefabrication is 5–20% more than that of the on-site construction method. The higher first cost is mainly due to the transportation cost of prefabricated elements and extra lifting equipment requirements for the installation of precast elements [39]. The transportation cost is directly proportional to the number of truckloads used in the delivery process and the unit cost of delivery. Other constraints related to transportation cost are the limited production scale of precast components and the allowable weight and size of loads that are specified by highway agencies. However, if maximum repetition of standardized building elements are designed at early stages, mass production of precast components can be implemented, and a good communication between engineers, contractors, and precast concrete manufacturers is achieved, the first cost of prefabrication can be reduced. Indeed, several investigations [15,16] indicated that the average construction cost for prefabrication is only slightly higher (0.25%–3%) than on-site construction method. In a recent survey, Gul revealed that today's contractors are quite aware that they can achieve cost savings when they use precast concrete systems [39]. 4.4.5. Factor 5: impact on health and community Performance criteria in this cluster concern health and community such as workers' health and safety, health of occupants, labor availability, traffic congestion, and community disturbance. Therefore, this factor is named “impact on health and community”. It is essential that a selected construction method has minimal negative impact on workers, potential occupants, and surroundings. As stated earlier, prefabrication can improve workers' health and safety due to cleaner and safer working environments. It also

contributes to the health of future occupants during the building use phase. Prefabricated elements are completed in a factorycontrolled setting using dry materials, and the low levels of moisture in new buildings correlate to lower risks of chronic health issues of occupants. But for the new on-site buildings, the potential of high levels of moisture trapped in the site-built elements leads to many indoor air quality issues. A building using substantial prefabrication contributes to reductions of on-site construction activities and construction duration, thus definitely reducing the nuisance factor such as construction noise, dust, light pollution and other pollutants faced by the nearby community. The construction method is particularly beneficial in urban areas where minimal traffic disruption is critical. Precast concrete units are normally large components, so it takes only a limited number of trips to the construction site through the congested city traffic, creating less disruption overall. However, for an on-site construction, intense cast-in-place activities result in untidiness, dust, noise, and air pollution. Furthermore, frequent transportation trips of materials and equipments to construction sites are required that add vehicular traffic to an already congested roadway situation.

4.4.6. Factor 6: architectural impact Factor 6 is described as “architectural impact”, which refers to the influence of a construction method on the physical space, decorative finishes and architectural look. This suggests that with a higher standard of living, the growing demand of comfort of potential occupiers is becoming an important criterion for construction method selection. When compared to on-site construction method, prefabrication can provide open space for both engineering systems and potential occupants. The use of prestressing provides much longer spans and wider openings with no obstructions than those that can be achieved using an equivalent on-site construction method. For the floor loads and spans required, prestressed concrete beams and floor slabs provide reductions in thickness, resulting in savings in height and overcoming the restricted floor-to-ceiling heights of nine-story buildings in some areas [4]. Additionally, with the help of a greater potential for automation (e.g., digital fabrication) and more intelligent management systems under controlled factory conditions, prefabrication offers aesthetic solutions with various shapes and curves, precise dimensional accuracy, and consistency in finishes and textures that are usually unattainable in cast-in-place reinforced concrete. 4.4.7. Factor 7: environmental impact The final factor is related to environmental effects, from site disruption, material and energy consumption to waste, pollution, and recyclability. With the increased awareness of greenhouse gases, global warming and scarcity of natural resources, environmental impact has become an important performance improvement agenda in construction. Prefabrication has many environmental benefits during construction as well as the life cycle phase of buildings. Waste reduction was thought to be one of the most significant environmental benefits when adopting prefabrication in many previous studies [15,35,40,41]. Most of the work is conducted at the manufacturing plant, where tight control of quantities of constituent materials is achieved and waste materials are more readily reused/recycled, resulting in effective waste reduction. Tam et al. revealed that the use of prefabrication reduces waste arising from plastering, timber formwork and concrete works by about 100%, 74%–87% and 51%–60%, respectively [42]. Although the magnitude of waste reduction depends on the level of prefabrication [41], waste levels have an average reduction of 65% and up to 70% when compared with on-site construction method [15].

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Material conservation is another important environmental advantage for prefabrication. According to Yee's research [4], the substitution of alternative prefabrication designs for conventional onsite construction can result in savings as much as 55% of the concrete quantities, 40% of the reinforcing steel, and 70% timber formwork. Yee also indicated that the material savings vary according to span lengths and loading requirements. In general, longer spans and higher live loads result in larger material savings when prefabrication is employed. In addition to the savings in material quantities, savings in formwork are also realized because the precast slab serves as formwork and simultaneously becomes a large portion of the composite slab structure. Furthermore, recycled materials such as fly ash, slag cement, silica fume, and recycled aggregates tend to be more easily incorporated into precast concrete, thereby diverting materials from the landfill and reducing use of virgin materials. Effective waste reduction, less material consumption, and recyclability definitely contributes to a reduction in energy consumption and environmental emissions. In short, prefabrication results in less environmental impact than in the case of a conventional on-site construction method. 4.5. Further discussion The ranking analysis presented reflects current industry emphasis on construction method selection in concrete buildings. Although the average rankings of social criteria and environmental criteria are not as high as economic criteria, the results showed that social awareness and environmental concerns were considered to be increasingly important when selecting construction methods. For example, workers' health and safety, health of occupants, energy efficiency in building use, and reusability/recyclability issues were rated with higher importance. A project seeking a higher LEED (Leadership in Energy and Environmental Design) certification would put more concentration on environmental factors. During the interviews, respondents claimed that the reason for highlighting some environmental criteria was that they were closely related to LEED points. This also can be reflected in the ranking of environmental criteria, for example, “energy efficiency in building use” and “site disruption” received more attention than other items. As the LEED green building rating system becomes more popular, increased environmental consideration in construction method selection is an inevitable trend in the future. Four economic factors, two social factors, and one environmental factor were extracted after factor analysis was performed on economic criteria, social criteria, and environmental criteria, respectively. To find out a better underlying structure of the criteria, a similar factor analysis was also conducted using all the 33 sustainable performance criteria. As a result, a total of eight latent factors were extracted, accounting for 73.7% of the total variance, in which six factors have striking resemblance to the factors obtained from the different categories analyzed; however, two additional factors were found to be difficult to interpret even when initial factor matrix was rotated. Consequently, the seven latent factors extracted for three sustainable categories using factor analysis were adopted. Although some additional criteria provided by survey respondents, could not be included in the analysis, most of the suggested criteria were already captured in the original survey. For example, the criterion “longevity” was recommended, which was described in the original criterion of “durability”. “Ease of installation” and “panel size” can be seen as related to the “constructability” criterion. Compared to other published prefabrication research studies [15,28,39], the response rate (23.1%) obtained in this study is considered to be good. Considering that the questionnaire was completed by nearly 100 industry practitioners, the level of distribution of the varying participant groups is acceptable, and the experience of the respondents in the concrete prefabrication buildings

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and in the construction industry is quite respectable, the data analysis results are considered reliable and satisfactory. It should be noted that traditional participation framework requires attention when prefabrication is adopted. Several interviewees argued that to achieve the potential advantages of prefabrication and to successfully integrate prefabrication techniques, precast concrete suppliers and contractors should work with the developers and architects at the earliest stage of the building design, and the participants should possess cooperative and effective communication throughout the building project life. 5. Conclusions One challenge facing construction practitioners during early project stages is that of selecting an appropriate construction method. The significant advantages of prefabrication construction method are commonly cited when justifying the use of prefabrication, yet appropriate criteria for assessments of the applicability to a given building have been found to be deficient. Identifying a list of holistic criteria is valuable for assisting construction practitioners in the selection of appropriate construction methods for a given concrete building. This research identified 33 performance criteria based on the sustainable triple bottom line and requirements of different project stakeholders, consisting of 16 economic criteria, 8 social criteria, and 9 environmental criteria. All of the criteria were derived from a thorough related literature review and comprehensive comparisons between prefabrication and the on-site construction method. To obtain the perceived importance of the criteria, a questionnaire was distributed to a large sample of U.S. experienced practitioners including clients/developers, engineers, contractors, and precast concrete manufacturers, and selected sub-set of respondents were interviewed. Ranking analysis revealed that all criteria were highlighted at “High”, “High–Medium”, or “Medium” importance levels in selecting a construction method. A total of five criteria were highlighted at the “High” importance level: “construction time (E1)”, “initial construction costs (E2)”, “constructability (E15)”, “material costs (E13)”, and “lead-times (E12)”, and four criteria were categorized under the “Medium” importance level. Among the “High–Medium” level criteria, social criteria and environmental criteria made up a large percentage. Although time and cost remained as the most important criteria for choosing a construction method, social awareness and environmental concerns were considered to be increasingly important. Factor analysis of the data generated a total of seven latent factors from the criteria. Four of these factors are under the economic category: “long-term cost”, “constructability”, “quality”, and “first cost”; two factors belong to the social category: “impact on health and community”, and “architectural impact”; and one factor for environmental dimension, “environmental impact”. Among the seven factors, “first cost” and “quality” are the traditional project criteria for clients. Trends showed that clients were changing the way of their thinking and were taking “long-term cost” into account when selecting a construction method. “Constructability” is the criterion that contractors may care most about, and architectural engineers may value “architectural impact”. “Impact on health and community” and “environmental impact” are two of the most salient characteristics in sustainability, and are now increasing in importance for all project participants. This research demonstrates the current U.S. industry emphasis on construction method selection and identifies seven dimensions of sustainable performance criteria assisting construction practitioners in selecting an appropriate construction method. The proposed criteria based on the sustainable triple bottom line include both “hard” and “soft” factors, which may better capture the potential

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performance of construction methods, as opposed to the traditional measures of cost, time and quality. This will allow project teams to have an appropriate balance between economic, social, and environmental issues, changing the way construction practitioners think about the information they use when selecting the construction method for future concrete buildings. Additionally, the list of criteria also appropriately captured the concerns of different project stakeholders. The optimum construction method should benefit all project stakeholders. More importantly, the proposed criteria require only a minimum of information, usually available in the early stages of conceptualization, and thus enable quick and easy data collection. This paper lays the groundwork for automated tools to help make project level decisions regarding prefabrication strategies and facilitates the achievement of a healthy built environment, and thus the likelihood of sustainable construction. A tool is currently being developed based on the derived criteria to help improve the decision-making process for an appropriate construction method selection in concrete buildings. This tool will focus on the feasibility analysis of prefabrication and finding an optimal prefabrication strategy. Acknowledgements Thanks are due to building industry practitioners for their generous contributions and participation in the questionnaire survey and interviews. Without their input, this research would not have been possible. The authors wish to thank the anonymous reviewers for their valuable comments. The financial support of the China Scholarship Council is also gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.autcon.2009.10.004. References [1] USGBC (U.S. Green Building Council), Green Building Research, 2009 http://www. usgbc.org/DisplayPage.aspx?CMSPageID=1718. [2] CIRIA (Construction Industry Research and Information Association), Standardization and Pre-Assembly Adding Value to Construction Projects, Report 176, London, 1999. [3] A.A. Yee, Structural and economic benefits of precast/prestressed concrete construction, PCI Journal 7–8 (2001) 34–42. [4] A.A. Yee, Social and environmental benefits of precast concrete technology, PCI Journal 5–6 (2001) 14–20. [5] L.S. Pheng, C.J. Chuan, Just-in-time management of precast concrete components, Journal of Construction Engineering and Management 127 (6) (2001) 494–501. [6] W.T. Chan, H. Hu, Constraint programming approach to precast production scheduling, Journal of Construction Engineering and Management 128 (6) (2002) 513–521. [7] C. Eastman, R. Sacks, G. Lee, Development and implementation of advanced IT in the North American precast concrete industry, Journal of Information Technology in Construction 8 (2003) 247–262. [8] R. Sacks, C.M. Eastman, G. Lee, Process model perspectives on management and engineering procedures in the precast/prestressed concrete industry, Journal of Construction Engineering and Management 130 (2) (2004) 206–215. [9] C. Pasquire, A. Gibb, N. Blismas, What should you really measure if you want to compare prefabrication with traditional construction? Proc. IGLC-13, IGCL, the International Group for Lean Construction, Sydney, Australia, 2005, pp. 481–491. [10] N. Blismas, C. Pasquire, A. Gibb, Benefit evaluation for off-site production in construction, Construction Management and Economics 24 (2) (2006) 121–130. [11] M. VanGeem, Achieving sustainability with precast concrete, PCI Journal 51 (1) (2006) 42–61. [12] S. Kale, D. Arditi, Diffusion of ISO 9000 certification in the precast concrete industry, Construction Management and Economics 24 (5) (2006) 485–495. [13] J.D. Manrique, M. Al-Hussein, A. Telyas, G. Funston, Case study-based challenges of quality concrete finishing for architecturally complex structures, Journal of Construction Engineering and Management 133 (3) (2007) 208–216. [14] C. Goodier, A. Gibb, Future opportunities for offsite in the UK Construction, Construction Management and Economics 25 (6) (2007) 585–595.

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