Revisiting the effects of prefabrication on construction waste minimization: a quantitative study using bigger data
Weisheng Lu, Wendy M.W. Lee, Fan Xue, and Jinying Xu
This is the peer-reviewed post-print version of the paper:
Lu, W., Lee, W. M., Xue, F., & Xu, J. (2021). Revisiting the effects of prefabrication on
construction waste minimization: a quantitative study using bigger data. Resources,
Conservation & Recycling, 170, 105579. Doi: 10.1016/j.resconrec.2021.105579
The final version of this paper is available at https://doi.org/10.1016/j.resconrec.2021.105579.
The use of this file must follow the Creative Commons Attribution Non-Commercial No
Derivatives License, as required by Elsevier’s policy.
Highlights

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•
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•
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Exploit quantitative dataset of 114 high-rise building projects in Hong Kong
Re-evaluate the effects of prefabrication on construction waste minimization
Reconfirm the positive effects of prefabrication on construction waste minimization
Two types of precast components played bigger roles in waste minimization

Abstract

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Prefabrication has long been recognized as a green production technology to minimize
construction's adverse environmental impacts such as waste, noise, dust, and air pollution.
Previous studies reported the effects of prefabrication on construction waste minimization.
However, these studies relied primarily on small data obtained by ethnographic methods such
as interviews and questionnaire surveys. Research to evaluate the effects using bigger, more
objective quantitative data is highly desired. This research aims to re-evaluate the effects of
prefabrication on construction waste minimization by exploiting a quantitative dataset
stemmed from 114 sizable high-rise building projects in Hong Kong. It was discovered that
the average waste generation rates of conventional and prefabrication building projects were
0.91 and 0.77 ton/m² respectively. Compared with conventional construction, prefabrication
logged a 15.38% waste reduction. Further probing into specific prefabricated components
adopted in the samples, it is discovered that precast windows and walls are more conducive to
waste minimization. This is coincident with the fact that these components are also widely
adopted in the sample buildings. This study reconfirms the positive effects of prefabrication
on waste minimization and articulates that two types of prefabricated components play
relatively bigger role in minimizing construction waste. The strengths of this study lie in its
statistical analyses of a valuable and objective quantitative dataset measuring prefabrication
and waste generation rates. Future studies are recommended to prove the corollary - it is not
what category of prefabricated component, but the actual proportion of prefabrication in the
total construction volume that matters to waste minimization.
Keywords: Construction waste management; Waste minimization; Prefabrication; Big data
analytics; Hong Kong

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1. Introduction

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Construction waste, sometimes called construction and demolition (C&D) waste, means the
solid waste generated from construction activities such as site clearance, excavation,
construction, refurbishment, renovation, demolition, and road works (USEPA, 2020; Lu et
al., 2019). It mainly contains waste construction materials such as debris, rubble, earth, soil,
broken concrete, bamboo, timber, vegetation, packaging waste, and others (HKEPD, 2020).
Of the overall construction waste generated, the non-inert portion takes around 5%, but this
small portion often occupies around 25-30% of all the solid waste landfilled. Landfilling not
only takes up precious landfill space but also causes severe environmental problems due to
the production of greenhouse gases such as CO2 and methane (Wu et al., 2019a), as well as
leachate from anaerobic degradation of the waste. Hence, construction waste minimization
(CWM), e.g., via reuse, rethink, replace, reduce, refuse, and recycle (a.k.a., “6R”) (Boon and
Anuga, 2020), is always high on the governments' agenda. CWM is also one of the main
areas to rebuild a circular economy (Esa et al., 2016), which aims at extracting the maximum
value from resources and minimizing pollution and end-of-life waste.

Prefabrication has long been advocated as a green production strategy to alleviate the
negative environmental impacts caused by construction. "Prefabrication", as opposed to "cast
in-situ" construction, is the practice of assembling components of a structure in a factory or
other manufacturing site, and transporting complete assemblies or sub-assemblies to the
construction site where the structure is located (Beecher, 2014). Some prefer to use "offsite
construction", which refers to sub-structures or components of a structure built at an offsite
place other than the location of final erection (Gibb, 1999). For example, individual modules
of the building are constructed in an offsite place (e.g., a factory or a precast yard) then
transported to the site for assembling to a final building. Other terms such as "precast",
"industrialized construction", or "construction industrialization", are also seen. A detailed
account of the similarities and subtle differences among the concepts will be given later.
Prefabrication allows many traditional "cast in-situ" trades to be conducted in a factory
environment. Therefore, it is amenable to CWM, e.g., by reducing material waste (Lu and
Yuan, 2013), losses or misplacements of materials (Tam et al., 2005), or the impact from the
weather (Wuni and Shen, 2019).

Research to quantify the effects of prefabrication on CWM is hardly new. Tam et al. (2007),
for example, conducted one of the earliest studies in Hong Kong and found that "wastage
generation can reduce up to 100% after adopting prefabrication". Jaillon and Poon (2009)
conducted a comprehensive review of the evolution of prefabricated residential building
systems in Hong Kong. Using questionnaire surveys and case studies, Jaillon et al. (2009)
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discovered that CWM is one of the major benefits when using prefabrication construction;
"the average wastage reduction level was about 52%". Tam and Hao (2014) found that the
use of prefabricated components could reduce timber formwork waste and concrete waste by
up to 86.67% and 60%, respectively. Lu and Yuan (2013) discovered that "the waste
generation rate in the upstream processes of offshore prefabrication is around 2% or lower by
weight". Osman and Lee (2016) conducted a questionnaire survey and found that 74% of the
300 respondents agreed that prefabrication effectively reduced construction waste, especially
timber formwork waste. Eghbali et al. (2019) investigated waste generation levels of 17
projects and discovered that prefabrication could reduce 40% to 100% of waste.
Nevertheless, one common shortcoming of these studies is that they deployed small and
erratic data collected using ethnographic methods such as interviews, questionnaires, or
observations. Such small and erratic data may only reflect a snapshot of waste generation in a
project. Hence, using bigger, more thorough, and more objective data to examine the dyadics,
i.e., prefabrication and CWM, is highly desirable in literature, policy making, and
construction practices.

This study aims to re-evaluate the effects of prefabrication on CWM. It does so by making
good use of a big dataset collected from 114 large-scale high-rise building projects in Hong
Kong, including their categories of prefabricated components adopted and factual waste
generation. The remainder of this paper is structured as follows. Following this introductory
section is a literature review of prefabrication construction with particular focus on the
current status of prefabrication adoption in high-rise buildings, as well as an evaluation of its
potential in CWM. Section 3 is the methodology which describes the data used and analytical
methods employed, including cross-sectional comparison and correlation analyses. Section 4
reports the results and findings of cross-sectional analysis by way of independent samples ttest, Spearman correlation analyses, and comparison of the average waste generation rates
(WGRs) of projects with varying estimated levels of usage of different categories of
prefabricated components. Section 5 is an in-depth discussion, followed by the strengths and
limitations of this study highlighted in Section 6. Finally, conclusions are drawn in Section 7.

2. Literature review
2.1 Prefabrication for high-rise building construction
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Prefabrication represents a construction approach that is different from the traditional "cast
in-situ", where formwork is set up onsite for pre-mixed concrete to be cast in. The concrete
then needs to undergo a process of "hardening" in the formworks supported by falsework,
which normally takes a few weeks before it becomes a self-standing and permanent structure
(Kwan and Ling, 2015). Cast in-situ construction is predominantly of wet trades onsite (e.g.,
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tilelaying, bricklaying, concreting, and plastering), which cause much nuisance, including
construction waste. There are many other similar terms related to the concept of
"prefabrication". For example, "precast" is a construction product (mostly concrete) produced
by casting concrete in a reusable "mold" or "form", which is then cured in a controlled
environment and transported to the construction site for lifting to its final position (Chea et
al., 2020). "Industrialization" refers to the extensive use of large-sized factory-finished
elements and the conversion of production into a mechanized and continuously flowing
process of assembly and installation of buildings and structures made of prefabricated
assemblies and parts (Rostami et al., 2013). Nevertheless, it should be noticed that these
concepts are not mutually exclusive with one other. Instead, they emphasize different aspects
of prefabrication. For example, not all prefabrication must be offsite; there are also onsite
prefabrication works, particularly for road or bridge projects. With prefabrication conducted
in an offsite place, it is possible to realize construction industrialization. In this paper, these
terms are aligned into one, i.e., "prefabrication", but readers are reminded of their subtle
differences.

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Over the years, prefabrication has not only been used in low-rise, detached or semi-detached
houses, but also been adopted in high-rise building projects in metropolitan cities such as
New York, Tokyo, Singapore, Hong Kong, and many other densely populated places. Steel,
with its strong compress and tensile strengths and the ability for free extension (Chukin et al.,
2010; Zhang et al., 2019), is a perfect material for prefabrication in high-rise buildings. The
drawback is the high cost (Wu et al., 2010) and poor fire-resistance, as we learned from the
tragedy of "911" (Bement, 2002; Shuster, 2005). Therefore, on many occasions, full steel
prefabricated structures are replaced by composite structures using concrete and steel
materials. The composite structures have slightly inferior yet still acceptable structural
properties, but possess the strengths of much lower costs and superior fire resistance.

Being a densely populated compact city with limited land, Hong Kong has long been
employing offsite prefabrication, particularly in its massive public housing schemes. Around
half of the city's housing units are public housing, which has a long history of adopting
prefabrication, in one way or another (Ying, 2019). Figure 1 illustrates a typical floor of a
public rental housing (PRH) block in Hong Kong constructed with prefabrication technology.
The technology was historically considered as less stiff, less sound- and water-proof than cast
in-situ construction (Hao et al., 2020). Therefore, it is not uncommon that the structural parts
are still using the traditional construction while the precast components are manufactured
beforehand and installed into the positions on the main structure.

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Figure 1. Typical floor of a PRH block in Hong Kong constructed with prefabrication
technology (Source: HKHA, 2018)
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An important footnote to understand prefabrication’s effects on CWM is the level of
prefabrication. It would form a misconception if considering that construction will adopt
either cast in-situ or prefabrication. Actually, there are different levels of prefabrication
adopted, ranging from full cast in-situ to complete prefabrication. An index-style
measurement could offer a clear understanding of the proportion of prefabrication in the
overall construction volume. Alinaitwe et al. (2006), for example, suggested that the level of
prefabrication could be measured by the ratio of the value of work done onsite to offsite.
Hong et al. (2016) proposed a "prefabrication rate" that is derived from the adopted
prefabrication volume over the total construction volume. Gibb (1999) provides one of the
most widely used approaches to measuring prefabrication levels (see Table 1). Using this
taxonomy, the PRH in Hong Kong adopts a Level 2 prefabrication, and some of the projects
adopted a Level 3 prefabrication. The city is enthusiastically advocating Modular Integrated
Construction (MiC), which can be considered as Level 4 in Gibb's (1999) taxonomy.

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Table 1. Taxonomy of different levels of prefabrication
Level
0
1
2

Description
Cast in-situ
Component and subassembly
Non-volumetric
assembly

3

Volumetric
assembly

4

Modular building

Source: Gibb (1999) and others.

Examples
Conventional construction technologies, using formwork and
falsework, rebar and cast in premixed concrete
Components like doorsteps, lintels, and so on, to be formed as
functional components such as doors and windows
2-dimensional precast concrete wall panels, precast components
with no usage space enclosed, e.g., precast concrete partition wall,
precast lost form, semi-precast slab, non-structural prefabricated
external wall
Encompassing all 3D preassembled units enclosing usable space
but do not constitute part of the building structure, e.g., volumetric
precast bathroom, precast kitchen, precast water tank, precast
refuse chute
3D preassembled volumetric units forming part of or the complete
building structure, like motel rooms, MiC

2.2 Prefabrication as a key player in CWM

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Adoption of prefabrication is one of the key contributors to CWM (Lu and Yuan, 2013;
Wang et al., 2015; Ajayi et al., 2017). There are several explanations. Firstly, prefabrication
construction minimizes the reliance on traditional wet trades (Poon et al., 2004). Secondly,
the use of prefabricated components reduces the amount of waste resulting from negligent
handling. Since conventional building materials are often unpacked during transportation,
incidents of fragile materials being damaged, which render them unfit for purpose, are
rampant (Tam et al., 2005). Thirdly, prefabrication avoids possible wastage arising from
over-ordering of materials. Over-ordering of readily-mixed concrete due to improper
planning or ensuring sufficient buffers was found to be one of the major causes of
construction waste (Wang et al., 2008; Tam and Hao, 2014). Fourthly, the use of precast
components eliminates construction waste arising from poor workmanship and cutting of
materials. In conventional construction, off-cuts from the cutting of materials such as steel
bars, bricks, and blocks can easily end up as waste rather than being reused (Osmani et al.,
2006; Nadoushani et al., 2016). There are also instances where the steel bars and bricks being
cut are unfit for their purposes (Tam et al., 2005). Lastly, factory conditions for prefabrication
avoid the impact of weather and other onsite exposure, thereby reducing the generation of
construction waste (Tam et al., 2005).

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Despite the benefits being discussed above, the decision as to which level of prefabrication
should be adopted is also contingent on political, economic, social, and environmental factors
(Lu et al., 2018). Governments around the globe have been promoting prefabrication akin to a
political agenda. They offer economic incentives to developers in recognition of their

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projects' fulfillment of certain requirements of using precast components (Kamar et al., 2009;
Li et al., 2017; Lu et al., 2018). Prefabrication often incurs higher setting-up and higher
transportation costs, which must be offset by the possibility of mass production, i.e.,
"economy of scale" (Chen et al., 2010; Bildsten, 2011). Prefabrication requires skilled labor
with expertise in the lifting and onsite assembly of precast components, such as connecting
them with the in-situ ones (Chiang et al., 2006). Thus, prefabrication adoption can be
hindered by the shortage of relevant skilled labor in the market. The implementation of
precast construction can also be impeded by the lack of site space for the temporary storage
of precast units (Azhar et al., 2013).

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Most previous studies have reached the conclusion that prefabrication construction is
effective for CWM. They adopted such methods as interviews, questionnaires, or
observations to collect the first-hand data. Undoubtedly, interviews can elicit in-depth
information from respondents (Milligan et al., 2005; Meyer et al., 2017), and questionnaire
survey enables the gathering of data from a large sample within a short period at a relatively
low cost (Reynolds and Sponaugle, 1982). Nonetheless, the accuracy and quality of data
collected by such ethnographic methods might be hampered by respondents' possible memory
lapses (Snijkers, 2002). The interview data may just reflect a generation impression or a
snapshot of the waste saving by adopting prefabrication. In fact, both prefabrication
construction and its waste generation are tangible and amenable to objective measurement. It
will be more desirable to measure the effects of prefabrication on CWM using more
comprehensive and more objective secondary data that naturally happens on site.

3. Methodology
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Basically, the methodology adopted in this study is a typical comparative study. There are
five steps included in the comparative study. The first three steps are about data: (1) sampling
the building projects; (2) collecting the data related to construction waste generation; and (3)
collecting the data related to prefabrication. The rest two steps analyze the data to find out (4)
whether prefabrication makes any difference in terms of CWM performance amongst
different clients and building types; and (5) which type(s) of prefabricated components
contribute most to CWM, if there is any. They are elaborated in greater details below.
3.1 Project sampling

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Firstly, sample projects were identified with the aid of a project database obtained from the
Hong Kong Environmental Protection Department (HKEPD) comprising basic information
(e.g., contract number, contract name, contract sum, site address) of all six prevailing types of
construction projects (i.e., demolition, building, renovation, site formation, the foundation
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works, others). We selected 157 projects ("initial sample projects") from the database based
on four criteria:

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1. The project type was restricted to "building", as prefabrication is mainly adopted in
building projects;
2. The projects must be commenced and completed in the period from 2006 to 2019, as
that is the period our database straddles;
3. The projects must be relatively sizeable, e.g., contract sum > HK$ 100 million and
GFA > 3,000 m², to allow steady CWM patterns to surface; and
4. The projects had credible sources of data.
Data cleansing was conducted to remove projects with incomplete data from the sample. As a
result, 114 projects, including 85 prefabricated and 29 conventional ones, were ultimately
chosen for further analysis. Table 2 lists the profiles of the building projects. There were:
(1) 85 prefabrication and 29 conventional projects;
(2) 30 public projects (all being prefabrication projects) and 84 private projects (including 55
prefabrication and 29 conventional projects); and
(3) 91 residential projects (including 81 prefabrication and 10 conventional projects) and 23
commercial projects (including 4 prefabrication and 19 conventional projects).
Table 2. Basic profiles of the 114 sample projects
Prefab.?
Yes
No
Subtotal

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By client
Public
30
0
30

Private
55
29
84

By nature of building
Residential
Commercial
81
4
10
19
91
23

Subtotal
85
29
114

Then, basic information of the initial sample projects was compiled in the form of an excel
spreadsheet. The contract number, contract name, site address, clients (e.g., public or private),
contract sum, Construction/Gross Floor Area (CFA/GFA), and waste disposal account
number were distilled from the project database and supplemented from various sources, e.g.,
sales brochures of private developments, official websites of contractors, developers and
government departments, as well as other public documents. Notably, this research managed
to secure the CFA/GFA information. According to Tam and Lu (2016), the WGR per GFA
(unit = ton/m2) is a more objective indicator than tons/$ of the contract sum, as floor area is
an objective indicator while contract sum as the denominator can change in line with different
material prices in different regions.
3.2 Waste generation data collection
The total amount of waste generated by each project was collected with the aid of the
HKEPD's Waste Disposal database, which records every truckload of construction waste
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received by government waste disposal facilities. We obtained all the truckload records of
individual projects to calculate the total construction waste generation from specific projects.
Afterward, we also computed the WGR of each project, which is a widely recognized
indicator of construction waste management performance of building projects. In this study,
the WGR of each project was calculated by dividing the total amount of waste generated (in
tons) by the GFA (in m2) of the project (Formoso et al., 2002; Lu et al., 2016; Zheng et al.,
2017). The lower the WGR, the higher the CWM performance.
3.3 Prefabrication data collection

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The types of prefabricated components adopted in these projects were identified using
various public sources. The prefabrication information collection and cleansing processes
took the research team around eight months with assistance from several summer interns,
who browsed the initial sample projects' structural and building plans via HeBROS and
BRAVO. These are two online systems operated by the Hong Kong Housing Authority
(HKHA) and the Buildings Department of the Hong Kong Government (HKBD),
respectively to store drawings and other information of all the buildings in Hong Kong.
Figure 2 is a screenshot of the data collected for waste generation and prefabrication
adoption. It is an expansive spreadsheet containing detailed information about the projects
and prefabricated components.

Proje Id

Contract
No.

Account
No.

Contract
name*

P1

20120141

7016997

P2

20110039

7015648

P3

20120263

P4

Precast
Total
Gross
Prefabricat façades
amount of
(part of
Floor Area
ed
waste
(GFA)
component window
generated
(m2)^
used? (Y/N) system)
(tons)
(Y/N)

Precast
staircases
(Y/N)

Semiprecast
slabs
(Y/N)

Y

Y

Y

N

Y

N

N

Y

Y

Y

Y

N

Y

N

N

Y

Y

Y

Y

Y

N

Y

N

Y

Y

347503.98

Y

Y

Y

Y

N

N

N

Y

Y

85772.974

33566.87

Y

Y

Y

Y

N

Y

N

Y

Y

120454.63

41119.79

Y

Y

Y

Y

N

Y

Y

Y

Y

Contract
sum (HK$)

Site
address#

Client+

A

542500380

A1

H

25318.42

58209.3

Y

B

2233000000

B1

H

171223.42 161018.77

Y

7017381

C

2580000000

C1

H

183003.09 118018.97

20110333

7016204

D

2105000000

D1

H

117992.4

P5

20120095

7016593

E

1142800000

E1

H

P6

20120096

7016758

F

1232468000

F1

H

Precast
Precast
Precast
Precast
balconies partition lost form bathrooms
(Y/N) wall (Y/N)
(Y/N)
(Y/N)

Precast
beams
(Y/N)

Note: *, #, and + There are specific project names, site addresses, and client names, which are pseudonymized for
protecting privacy
^ Including both domestic and non-domestic GFA

Figure 2. An excerpt of the dataset collected in this study

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Then, the prevailing prefabricated components were grouped into eight categories, as listed in
Table 3. It can be seen that buildings in Hong Kong have adopted a wide range of precast
components, ranging from big, sophisticated precast facades, balcony, kitchen, and bay
windows, to small, simple ones such as slabs and partitioning walls. Precast façades are the
most popular components. Hong Kong has developed a good knowledge of precast façades
by integrating windows, sinks, and washing benches, and left pipes and openings for utilities
to fit in. They represent a high level of non-volumetric assembly (Level 2) of prefabrication.
Non-structural prefabricated external walls are also widely adopted. 51 out of the 85
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prefabrication projects have adopted non-structural prefabricated external walls. Precast
partition walls, as non-structural members, are also widely adopted. Simple precast slabs are
popular components in a building. Owning to the conservative attitudes towards
prefabrication's strengths, sound- and water-proof, semi-precast slabs are more popular than
full precast slabs. On the top of the semi-precast slab, they often add an extra layer of rebar
and concrete using cast in-situ technology. It can also be seen from Table 3 that Level 3
prefabrication components such as volumetric bathroom and kitchen are also adopted,
although they are not prevailing. The widespread use of water tanks, lost forms, window
frames, and refuse chutes demonstrate that Hong Kong's building clients are endeavored to
embrace prefabrication as much as possible.

Table 3. Eight categories of prevailing prefabricated components
Category of components
Id Name
Abbr. Range
{0, 1}
1
Façade
Fac.
{0, 1}
2
Staircase
sta.
3
Beam
Beam {0, 1}
{0, 1, 2}
4
Window
Win.
5

Slab

Slab

{0, 1, ..., 4}

6

Wall

Wall

{0, 1, …, 7}

7

Volumetric

Vol.

{0, 1, …, 5}

8

Others

Oth.

{0, 1, …, 4}

Type of prefab. component

Num. of projects
Public Private Total

Precast façade
Precast staircase
Precast beam
Precast bay window
Precast window frame
Semi-precast slab
Full precast slab
Precast plank
Semi-precast plank
Precast partition wall
Precast structural wall
Semi-precast structural wall
Non-structural prefabricated
external wall
Precast hanger wall
Precast parapet wall
Precast lost form
Precast bathroom
Precast kitchen
Precast water tank
Semi-precast water tank
Precast refuse chute
Precast balcony
Precast half-landing
Semi-precast utility platform
Miscellaneous types

30
29
29
4
0
21
14
17
1
23
0
2

49
2
0
23
10
3
0
1
0
13
1
0

79
31
29
27
10
24
14
18
1
36
1
2

19

32

51

3
6
1
8
2
19
3
15
4
7
0
–

0
1
14
0
0
0
0
0
1
0
1
–

3
7
15
8
2
19
3
15
5
7
0
–

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315

One may notice that prefabrication has an unbalanced adoption in public and private projects.
All public projects in the sample had adopted prefabrication, whereas only less than twothirds of private projects had incorporated precast components. There are some drawbacks
associated with prefabrication construction, e.g., higher cost, heavy transportation and
hoisting, lack of flexibility, inadequate skillsets, and poor sound-/water-proof, some of which
are true while others are stereotypical ideas. The Government, however, asserts that
prefabrication is the direction to achieve a greener, safer, and more productive construction
industry in Hong Kong, so it enacted various measures to boost prefabrication adoption.
Particularly, prefabrication is recognized as one of the green features that can enjoy the
exemption from GFA calculation pursuant to Joint Practice Notes No. 1 & 2 promulgated in
2001 and updated in 2019 (HKBD, 2019a and 2019b). This can increase the actual
permissible GFA to be constructed and sold, thereby maximizing developers' monetary
return. In contrast to public projects, which had incorporated a wide range of precast
components covering both Levels 2 and 3 prefabrication, the private projects had employed a
much narrower range of precast components, excluding any volumetric assemblies.
Influential private sector clients, e.g., real estate developers in town, have found a delicate
balance between enjoying the GFA exemption and maintaining their flexibility.

3.3 Analyzing the effects of prefabrication on CWM in different building types and clients

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Cross-sectional comparison studies are performed to investigate the effect of prefabrication
on CWM among different types of projects and clients. Basically, they encompass
independent samples t-tests using the IBM SPSS (version 27). For comparing the effects
measured by average WGRs, the 114 projects were grouped in the following ways in the ttests:
(i) Prefabricated versus conventional projects
(ii) Prefabricated private versus conventional private projects
(iii) Prefabricated residential versus conventional residential projects
(iv) Prefabricated office versus conventional office projects
3.4 Analyzing the effects of different types of prefabrication on CWM

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Firstly, Spearman correlation analyses are conducted to unravel the contributions of adopting
different types of prefabricated components to CWM. A higher correlation coefficient means
the use of such prefabricated components has a lower level of contribution to CWM (Sachs,
2012). In addition, independent samples t-tests are performed to explore whether there is any
significant difference in the effect on CWM by adopting varying levels of precast window,
slab, wall, and volumetric components.

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4. Data analyses, results, and findings
4.1 Prefabrication’s effect on CWM in different building types and clients
4.1.1 Cross-sectional comparisons between prefabrication and conventional buildings
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345

The average WGRs and other statistics of such comparisons can be found in Table 4. The
average WGRs of the prefabrication and conventional construction were 0.77 ton/m2 and 0.91
ton/m2, respectively. On an average scale, prefabrication construction reduced 15.38% waste
generation. The results echo the findings of Jaillon et al. (2009) as well as Tam and Hao
(2014), which reported that prefabrication could reduce the amount of waste generated.
However, the 15.38% reduction was not statistically significant (p = 0.492 > 0.05), implying
that other confounders such as project management, time, site, and technologies may also
determine the CWM performance.

Table 4. Comparisons of CWM performance
Group by

Project type

Overall
Client

Public
Private

Building
nature

Residential
Commercial

Prefab.?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No

No. of
projects

Average WGR (tons/m²)
Mean
Stdev
Δ (%)

85
29
30
0
55
29
81
10
4
19

0.77
0.91
0.81
–
0.75
0.91
0.79
0.93
0.43
0.90

0.86
1.10
0.63
–
0.97
1.10
0.87
0.84
0.64
1.24

Significance#

-15.38

0.492

–

–

-17.58

0.497

-15.05

0.633

-52.15

0.476

# The p values of independent sample t-tests (Two-tailed)
350

4.1.2 Cross-sectional comparisons between client types

355

As listed in Table 4, there is no public project without adopting prefabrication. Therefore, to
compare the effects of prefabrication on CWM against conventional construction is null.
There are 55 and 29 private projects that adopted prefabrication and conventional
construction respectively, and their average WGRs are 0.75 ton/m2 and 0.91 ton/m2
respectively, showing that prefabrication can reduce 17.58% waste generation. The difference
was not statistically significant either (p = 0.497 > 0.05).

12

360

365

370

375

The results indicate that the private prefabrication projects had outperformed their
counterparts in the public sector in CWM. The reason might be the private sector's
predominant reliance on precast window and wall components, which were found to be the
most effective in minimizing waste if they are widely adopted. Private clients also value the
construction cost seriously. They will do enormous diligence to reduce material waste.

4.1.3 Cross-sectional comparisons among the nature of buildings
As listed in Table 4, both residential and commercial developments adopting prefabrication
had considerably lower average WGRs than their conventional counterparts, with the
percentage differences being 15.05% and 52.15%, respectively. Furthermore, the average
WGR of residential prefabrication projects is over 80% greater than that of commercial
developments with precast components. Amidst the low proportion of commercial projects
incorporating precast components, commercial projects have exhibited a much greater
reduction in average WGR compared with residential developments by only using precast
wall and window components. On the contrary, despite using a much wider variety of precast
components, the residential prefabrication projects' average WGR was nearly two times that
of commercial projects. It might be the high proportion of precast wall and window
components in commercial projects’ total construction volume that makes a difference in
CWM.
4.2 The effects of different types of prefabrication on CWM

380

385

4.2.1 Spearman correlation analysis
Figure 3 visualizes the data, and the results of the tests conducted to compare WGRs as
caused by different types of prefabricated components adopted. The dataset was the eight
categories of prefabricated components defined in Table 3. The sample size was 114 projects.
From the distributions of the components in the diagonal cells in Figure 3, it can be seen that
seven categories had declining trends in the distribution, while the only exception was the
façade category.

13

390

Figure 3. Results of Spearman correlation analyses, where each diagonal subfigure shows the
distribution of a prefabrication category, the abbreviations are described in Table 3, a upper
tringle subfigure at (i, j) presents the bivariate box-whisker plot between the i-th and j-th
categories, and a lower tringle subfigure at (j, i) shows Spearman coefficient ρ and
significance (*** p ≤ 0.001; ** p ≤ 0.01; * p ≤ 0.1, two-tailed, N = 114) between the i-th and
j-th categories.

395

400

The first column of the cells in Figure 3 illustrates the major results of Spearman correlation
analyses. Two significant positive correlations were found: (1) between the “volumetric”
category and WGR, where the coefficient was ρ = 0.19 at a significance level p ≤ 0.1 (N =
114, two-tailed); and (2) between “beam” category and WGR, with coefficient being ρ = 0.18
at a significance level p ≤ 0.1 (N = 114, two-tailed). In other words, rather than contributing

14

to waste reduction, the use of “precast beam” and “precast volumetric components” had the
reverse effect by yielding a slightly higher WGR.

4.2.2 Independent samples t-tests
405

410

The lack of significant correlation between CWM and different types of prefabricated
components may be attributed to their different levels of usage in different projects. For
example, both Projects A and B have used two types of volumetric components. While the
two volumetric components used by Project A comprise precast bathrooms and kitchens, the
two volumetric components used by Project B were precast water tanks and refuse chutes.
Given that each flat unit requires at least one bathroom and kitchen, an entire housing
development can have up to hundreds or even thousands of precast bathrooms and kitchens.
On the contrary, each building block only requires a few water tanks and refuse chutes.
Therefore, it follows that the actual number of volumetric components used by Project A
must be greater than that of Project B, so their effects on CWM should be different.

415

420

425

The ideal situation would be calculating the actual volumes of different types of prefabricated
components vs. the volumes of cast in-situ construction to examine their different
contributions to CWM. However, this is extremely onerous. One must read the drawings and
calculate the proportions of prefabricated vs. cast in-situ volumes. Having considered the
existence of such circumstances, in this study, we estimate the level of usage of “precast
windows”, “slabs”, “walls”, and “volumetric components” based on their nature and intensity
in a typical building. We then employ the independent samples t-test to compare the effects
on CWM amongst projects with high, low, and no use of each of the four categories of
prefabricated components. Table 5 lists out the results. Readers are reminded that the second
column is estimated values while other columns contain factual data.

15

Table 5. Average WGRs among projects with varying levels of usage of precast window,
slab, wall and volumetric components
Category of
prefabricated
component
Window

430

435

440

445

450

Estimated level
of usage

Average WGR
(tons/m2)

High
0.59
Low
0.43
No
0.91
Slab
High
0.90
Low
0.49
No
0.81
Wall
High
0.58
Low
0.92
No
0.86
Volumetric
High
0.96
Low
0.78
No
0.79
# The p values of independent samples t-tests (Two-tailed)

Stdev

Significance# of comparing the
average WGRs of three levels

0.71
0.19
1.00
0.74
0.44
0.98
0.39
1.06
1.05
0.79
0.61
0.98

High vs Low usage:
Low vs No usage:
High vs No usage:
High vs Low usage:
Low vs No usage:
High vs No usage:
High vs Low usage:
Low vs No usage:
High vs No usage:
High vs Low usage:
Low vs No usage:
High vs No usage:

0.70
0.41
0.06
0.19
0.40
0.72
0.07
0.79
0.10
0.55
0.96
0.63

Regarding precast window components, with a difference in average WGR of 0.32 ton/m2,
projects with greater estimated usage yield significantly lower average WGR than projects
with no usage at all (p ≤ 0.1). As for precast wall components, with differences in average
WGR being 0.28 ton/m2 and 0.35 ton/m2 respectively, projects with greater estimated usage
had significantly lower average WGR than projects with lower estimated usage and no usage
at all (p ≤ 0.1). It follows that incorporation of greater amounts of precast window and wall
components is indeed conducive to waste reduction. Furthermore, with difference in average
WGR being 0.01 ton/m2 only, the average WGRs of projects with no and lower estimated
usage of precast volumetric components were significantly equal (p > 0.9). This indicates that
the deployment of only small amounts of precast volumetric components yields nearly no
effect on waste minimization.

Precast window and wall components. There are no significant correlations between variety
of precast window and wall components used and WGR. Nevertheless, the results of
independent samples t-test illustrates that usage of greater estimated levels of precast window
and wall components is conducive to construction waste minimization. This also provides a
probable explanation for the higher popularity of precast bay windows, non-structural
prefabricated external walls, and precast partition walls among the 114 sample projects.
However, on closer inspection, all prefabrication projects with higher estimated levels of
usage of precast window and wall components are residential developments. Therefore, it can
only be inferred that usage of greater amounts of precast window and wall components can
facilitate minimization of waste in residential building projects.

16

455

460

465

470

Volumetric precast components. Volumetric precast components, which have been widely
promoted by the Government in recent years (HKEPD, 2009; HKSARG, 2018), were only
used by public residential development projects. A probable explanation for the unpopularity
of volumetric fabrication units among private developers is that most private development
sites are situated in dense urban areas of Hong Kong, which lack temporary storage areas for
such precast volumetric units. Public housing projects are often sizable to achieve the scale of
the economy with land pre-assembled and provided freely by the Government. In this study,
the results of Spearman correlation analysis suggest that projects incorporating wider variety
of precast volumetric components tend to have slightly higher WGR. However, the
independent samples t-test results suggest that the average WGR of projects using lower
estimated levels of precast volumetric components were significantly equal to that of projects
not using any precast volumetric components at all. This provides an additional explanation
for the lack of popularity of precast volumetric components in private projects.

Precast beam component. Although precast beam is prevalent in public projects, the results
of Spearman correlation analysis indicate that there is slight positive correlation between
precast beam usage and WGR. This provides a probable explanation for the lack of
popularity of precast beams in private projects.

5. Discussions
475

480

485

490

A cross-sectional comparison using a two-tailed t-test discovered that an overall 15.38%
waste reduction was logged in prefabrication vs. conventional building projects. Further
analyses also discovered that prefabrication led to a 17.58% waste reduction in private, a
15.05% waste reduction in residential, and a 52.15% waste reduction in commercial building
projects. Although they are not statistically significant (i.e., p < 0.05), the analyses echoed
with previous studies that prefabrication does have a positive effect on CWM. However, in
any case, it seems that the reduction is not as "exciting" as reported before. A possible
explanation is that in high-rise buildings the main structural parts are still using cast in-situ
technology. The effect is not to be understood as the comparison between a purely cast in-situ
building and a fully prefabricated building project. Nevertheless, the findings well echo Poon
et al. (2004), which reported that prefabrication can replace around 20% wet trades in
reducing the total amount of construction waste generation.

The research further investigated which category of precast components is more conducive to
CWM. The Spearman correlation analyses discovered that projects using precast beams and
volumetric components tended to generate slightly more waste. Additionally, we also
17

495

500

505

510

deployed independent samples t-test to investigate the difference in average WGR between
projects with varying estimated levels of usage of precast window, slab, wall and volumetric
components. Contrary to the orthodox knowledge that modular precast components should be
highly conducive to CWM, the average WGR of projects with greater usage of precast
volumetric components were not significantly different from that of projects without such
components. Furthermore, projects with greater usage of precast window and wall
components have significantly lower average WGRs. It is coincident with the fact that they
are also the most widely used ones in an overall building. The temptation is to say that it is
not what type of prefabricated components but the proportion of prefabrication in the total
construction volumes that matters to CWM.

Overall, the analytic results show that prefabrication has a positive effect on CWM. This
effect is consistently observed in different groups of building projects (e.g., public vs. private;
residential vs. commercial). Nevertheless, they are unbalanced samples with different
numbers of building projects in each group. This research tells in greater detail and
confidence which categories of prefabricated components have better effects on CWM if they
are widely employed. This can provide insightful guidelines for informing policies to boost
the adoption of this low waste construction technology. However, as an unfortunate fact,
CWM is often of low prioritization in real-life construction (Wu et al., 2019b). Costs,
buildability, and logistics and supply chain tend to be more important considerations than
CWM in real-life practices.

6. Strengths and limitations of this study

515

520

525

The research has its strengths and weaknesses. Firstly, it is a strength that we are able to
derive the objective measurement of waste generation in each project. Nevertheless, extrinsic
factors affecting the total amount of waste generation, e.g., waste being recycled onsite,
illegally disposed, and temporarily stored either onsite or offsite for future reuse, have not
been considered. Secondly, this study is superior to previous studies in that it builds upon a
sample of 114 real-life projects with an objective measurement of prefabrication adopted.
However, we are only able to count the types of precast components and subcategories of
different types of precast components being adopted. As the actual frequencies of usage and
proportions in the total construction volumes are not obtainable for a more meticulous
assessment, our categorization of projects with greater and lower usages of particular
categories of prefabricated components was based on estimated rather than actual levels of
usage. Thirdly, the mechanism through which prefabrication affects CWM is yet to be
explored. It is desired to derive an index-style measurement of prefabrication to test the
corollary mentioned above.
18

7. Conclusion
530

535

540

545

550

555

Prefabrication has long been acclaimed as a clean production technology to minimize
construction waste generation. Previous studies have proved the positive effects of
prefabrication on CWM. However, their data tends to be qualitatively collected using
interviews, questionnaire surveys, or observations, which may not be ideal, given the fact that
both prefabrication and construction waste generation can be objectively quantified. This
paper aimed to revisit the effects of prefabrication on CWM by harnessing a set of valuable
quantitative, secondary data collected from 114 building projects in Hong Kong.

Overall, it is discovered that prefabrication contributes to 15.38% reduction in construction
waste. Further analyses found that prefabrication led to 17.58% waste reduction in private
building projects, 15.05% in residential projects, and 52.15% in commercial building
projects. Thus, this study reconfirms the positive effects of prefabrication on CWM as
reported by previous studies. Unlike previous studies reporting an overall CWM rate though,
this research was able to probe into the specific categories of precast components adopted as
well as the contribution of varying levels of usage of four categories of prefabricated
components to CWM. It is discovered that greater deployment of precast window (e.g., bay
windows, window frames) and wall (e.g., non-structural prefabricated external walls,
partition walls) components can significantly reduce the WGR. A possible explanation is
their high level of adoption in the sample building projects. We thus draw a corollary the
level of prefabrication usage is more important than what prefabricated components used in
minimizing C&D waste generation. In comparison to existing studies, this study tells in
greater confidence and clarity how prefabrication contributes to CWM. The research findings
can be used as evidence to inform the policies (e.g., subsidiaries, saleable floor area
exemption) to incentivize prefabrication adoption.

Acknowledgments
This research is jointly supported by the Strategic Public Policy Research (SPPR) Funding
Scheme (Project No.: S2018.A8.010.18S) and the Environment and Conservation Fund
(ECF) (Project No.: 111/2019) of the Government of the Hong Kong Special Administrative
Region.

560

19

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