Blockchain for Smart Construction
23 July 2020
Wuhan, China
Frank Xue
Assistant Professor

iLab, REC, HKU, HK SAR

Outline
1

Construction: Distributed collaboration

2

Blockchain: Distributed trustworthy database

3

Two cases

Xue: BC for construction, HUST 2020 Summer Camp CM

2

0.1 HKU iLab: The urban big data hub
 iLab 实验室
 Director: Prof. Wilson Lu
 Urban big data hub at Faculty of Architecture, HKU

iLabHKU

 multi-dimensional and multi-disciplinary urban big data

collection, storage, analysis, and presentation to inform decisionmaking in urban development

fac.arch.hku.hk/iLab

 Focusing on information technology (IT)
o Building Information Modeling (BIM)

o Geographical Information System (GIS)

o Global Navigation Satellite System (GNSS)
o Urban Remote Sensing (URS)
o Internet of Things (IoT)
o Blockchain (BC/DLT)

Xue: BC for construction, HUST 2020 Summer Camp CM

2020 New Year dinner
3

0.2 About myself
 A mixed background 背景

 Engineering

 BEng in Automation, CAUC

2004

 MSc in Computer Science, CAUC

2007

o Advisor: Prof. W Fan

 Computer Science
 AI, OR, ML

 PhD in System Engineering, HKPU

2012

 PDF/RAP/AP in Construction IT

2016

 Research interests 方向

 ISE, CEM, EIE

 Economics
 SCM

 Urban sensing and computing
 Automation/IT in construction
 Applied operations research, ML, etc.

 Homepage: QR code for new updates
Xue: BC for construction, HUST 2020 Summer Camp CM

Homepage (free full-text)

4

0.2 My research projects
 On-going 在研

 Keywords

 PI: HK RGC GRF/ECS (17201717, 17200218, 27200520), HKU-Tsinghua

SPF (20300083), HKU (102009917, 201811159177, 201910159238)

 BIM/CIM
 3D point cloud

 Co-PI: Key R&D Guangdong (2019B010151001), HKU PTF (102009741)

 Derivative-free

 Co-I: NSFC (71671156), NSSFC (17ZDA062), HK SPPR (S2018.A8.010.18S),

optimization

HK ECF (111/2019)

 Completed 完成

 Urban semantics

 PI: HKU (201702159013, 201711159016)
 Co-I: NSFC (60472123), HK PPR (2018.A8.078.18D)

 Job vacancy (2 openings)
 PhD, HK$200,000~350,000/year
 RA, transferable to PhD (vision, rigor, & performance)
Xue: BC for construction, HUST 2020 Summer Camp CM

Sponsors of projects as PI/Co-PI

5

Section 1

CONSTRUCTION:

DISTRIBUTED COLLABORATION
Xue: BC for construction, HUST 2020 Summer Camp CM

6

1.1 Smart construction
 Construction is known as a “backward industry”
 Low productivity, labor-intensive (v.s. aging workers)
 Fatality, occupational hazards, management (e.g., cost overrun)

 A consensus of global research institutes (e.g., Harty et al., 2007)
 Effective (productive, automatic, age friendly) and efficient

(safer, profitable, on-time, sustainable) industry

 Construction smartization with new Information Tech.
 Computing power
 New devices
o RFID, LiDAR, GPS, UAV, smart phones…

USA’s gross value-added by sectors
source: economist.com

 New technologies

o BIM, GIS, CV, VR/AR, blockchain, …

Xue: BC for construction, HUST 2020 Summer Camp CM

Recent advances in IT

7

1.1 The distributed collaboration to smartize
 Multi-stage construction life cycle
 Architectural design
 Engineering design
 Construction
 Operation & maintenance
 Demolition

 Many stakeholders
 Even more decision makers, professionals
 Teaming  Coordination Collaboration

 Distributed collaboration
 Spatially and Temporally
 Semi-controlled working environment

Xue: BC for construction, HUST 2020 Summer Camp CM

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1.2 What if collaboration fails?
 Undermined project quality
 Overrun project period
 Harmed peers’ benefits
 Even scandals
 2018: Faked screwing of steel bars into couplers, by

cutting them shorter for an illusion
 2017: Faked concrete test results for Hong Kong-

Zhuhai-Macau bridge project

 Because, in the project organization
 Conflicts of interest exist as always
 Physically distributed, hard to manage
 The culture encourages covering small problems up
Xue: BC for construction, HUST 2020 Summer Camp CM

Two recent scandals in Hong Kong (Source: SCMP) 9

Section 2

BLOCKCHAIN:

DISTRIBUTED TRUSTWORTHY
DATABASE
Xue: BC for construction, HUST 2020 Summer Camp CM

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2.1 What is a blockchain?
 Blockchain 区块链
 Linked-list-like incremental Block

data storage systems

o Saved distributed, identically on each “node”
o Verified by “miners” for rejecting Attack

o Each “solved” Block becomes immutable

 Less related to

Bitcoin 比特币,

Ethereum 以太坊, ….

 By generation
o Blockchain 1.0; 2.0; 3.0; 4.0 (?) …

time

 Three old components “wine” in any blockchain “bottle”
新瓶旧酒？鸡尾酒?

Sect. 2.2

 Consensus mechanisms (1990s)

Sect. 3.1

 Distributed storage (1970s)

Sect. 3.2

 Cryptographic tools (1990s) / smart contract (1990s)

Xue: BC for construction, HUST 2020 Summer Camp CM

Root
#n
#2
Node #1
Diagram of a blockchain
(miners hidden)

Block
Attack
11

2.2 PoW consensus: Invented against Email spam
 Email spam, junk email
 Appeared in early 1990s
 90+% world emails were spam by 2014
 Reason 1: Spamming cost ~ 0;
 Reason 2: Assuming-people-are-good Email protocols

 Dwork & Naor (1992): ‘Proof of computational efforts’
计算量证明

 “If I don’t know you and you want to send me a message, then

you must prove that you spent, say, ten seconds of CPU time,
just for me and just for this message.” (Dwork et al. 2003)

 Jakobsson & Juels (1999): ‘Proof of work’
工作量证明

 Where a prover demonstrates to a verifier that he has expended a

certain level of computational effort in a specific time interval
Xue: BC for construction, HUST 2020 Summer Camp CM

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2.2 PoW consensus: On hard-to-solve, easy-tocheck math problems
 Sudoku puzzle
 Each column, each row, and each of the nine 3×3 grids
o All nine digits (1-9)

 Hard to solve, easy to check
 Nondeterministic Polynomial time-Complete (NPC) when n > 3

 And Max clique, Boolean satisfiability, Subset sum, …
 NPC

 And, e.g., hashcash PoW (Back 1997; 2002)
 PUBLIC: H(), k
o k: difficulty

 MINT: solving = O(2k) complexity
 VALUE: checking = O(k) complexity

Xue: BC for construction, HUST 2020 Summer Camp CM

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2.2 PoW consensus: How it works
 Proof of work (PoW) 工作量证明
 A class of consensus
 Sender / prover / miner side
o Hard to solve (e.g., Soduko, hashing, …)

 Server / verifier / node side
o Easy to check

 Examples
1.

Hashcash PoW (Back 1997; 2002)
o X-Hashcash: 1:52:380119:calvin@comics.net:::9B760005E92F0DAE

o $ echo -n 1:52:380119:calvin@comics.net:::9B760005E92F0DAE | openssl sha1

$ 0000000000000756af69e2ffbdb930261873cd71 ( correct; 13 hex (52 binary) 0s in <1us)

2.

Email attaches a key to the Sudoku’s initialized by sender + content + Email time

Xue: BC for construction, HUST 2020 Summer Camp CM

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2.3 Nakamoto (2008)’s Bitcoin (Blockchain 1.0)
 Bitcoin is a typical application of BC
 Immutable
 1 block = many transactions
 1 trans = 1 sender + 1 receiver + amount

 Anonymous
 Hash “wallets”

 Secure (and expensive)
 ~ 125EH/s (1.2 ×1020 H/s) computational power
 ~100TWh/year
o 2× Google, 4× Ireland, or US$10B bill

 Decentralized (pseudo?)

But, > 65% in China, > 35% in Xinjiang (Gogo 2020)


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HUST 2020 Summer Camp CM

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2.3 Bitcoin’s consensus: Hashcoin PoW
 A “miner” is a prover
 Solves the hashcash PoW
o Data content = trans + hash pointer
o Return ‘nonce’ to server

 Receives reward as BTC

 Server / validator

 Collects and packs transactions
 Opens a puzzle for millions of machines
o Flexible difficulty: every 10 mins per block

 Awards the winner with 6.25 BTC (now)

 The ledger (> 200 GB now)

 Live on millions of devices (Space redundant)
Xue: BC for construction, HUST 2020 Summer Camp CM

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2.3 PoW’s cons: 51% attack and more
 A malicious miner
 Tries to modify transactions
o E.g., change his/her -100 BTC to 0
o (by US$1M goods for free)

 Can succeed if > 50% computing power

 Other cons

 Competitiveness between miners
o Root cause of 51% attack
o Too much energy cost

 21 million hard cap BTC
 Easy coins before 2010

o 97% bitcoins were held by 4% of addresses

o Regardless
of those
unregistered
Xue: BC for construction, HUST
2020 Summer Camp
CM

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2.4 Blockchain as a distributed trustworthy
technology
 Some characteristics meet smart construction requirements
 Immutability
 Distributedness
 Transparency
 Security

 Blockchain is not equal to “crypto-currency” (not currency)
 Good medium of exchange 
 Poor store of value 
o See the right picture

 Inappropriate unit of account 
o Countless new ‘coins’ (> 5,000 now)

Xue: BC for construction, HUST 2020 Summer Camp CM

Bitcoin price
18

Section 3

TWO CASES

Xue: BC for construction, HUST 2020 Summer Camp CM

19

3.1 Case 1: Blockchaining supply chain (Yang et al. 2020)
 Construction supply chain
 Multi-stakeholder, distributed
 Having possible trust/compliance problems
 Involving payment, quality assurance

 Yang et al.’s (2020) example
 Purchasing a distillation tower
 In five steps

 Objective
 Blockchaining the procurement
o E.g., “pay AU$ 30,100”

 On Ethereum (Blockchain 2.0)
Xue: BC for construction, HUST 2020 Summer Camp CM

Processing of purchasing construction equipment
(Reprinted with permission)

20

3.1 Data storage: From two parties to >1 million
 Smart contact
 Modeled in “App server”
 A “World state” computer in a Ethereum “virtual machine”

 Data in the application layer (top left)
 Two parties
 6 world states, 5 steps

 In Ethereum layer

Two parties

Data saved by >1M users,
each user has one copy;

 > 1M user (data copies)
 ~ 30,000 similar smart

contracts per day

Buried in ~30,000 smart
contracts/day
Ethereum-based architecture
(Reprinted with permission)

Xue: BC for construction, HUST 2020 Summer Camp CM

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3.1 ETH transactions under the hood
 Each step is transformed
 To a ETH transactions

 ETH’s Transaction fee
 ~HK$0.1 / step
 ~HK$0.5 for each procurement

 Note:
 Payment was offline

Xue: BC for construction, HUST 2020 Summer Camp CM

Procurement (Steps 4, 5) was transformed into Ethereum transactions
(Reprinted with permission)

22

3.2 Case 2: Blockchaining BIM changes (Xue & Lu, 2020)
 Rome wasn’t built in a day; so wasn’t BIM.

(a) Incremental in geometry, (b) in geometric + non-geometric semantics (Ellis 2019)

Xue: BC for construction, HUST 2020 Summer Camp CM

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3.2 Semantic differential transaction of local BIM
 IFC (Industry Foundation Classes)
 The best open BIM standard
 STEP (Standard for the Exchange of Product Data) format
 Clear, [hardly] readable
 But massive, involving many random global IDs

Example IFC

ISO-10303-21;
HEADER;
FILE_DESCRIPTION(('ViewDefinition [CoordinationView, …);
FILE_NAME('example.ifc','2008-08-01T21:53:56',('Architect…);
FILE_SCHEMA(('IFC2X3'));
ENDSEC;
DATA;
#1=IFCOWNERHISTORY(#84,#71,$,.ADDED.,$,$,$,1217620436);
#2=IFCAXIS2PLACEMENT3D(#11,#4,#8);
#3=IFCCARTESIANPOINT((0.0,0.0));
#4=IFCDIRECTION((0.0,0.0,1.0));
#5=IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.0E-5,#75,$);
#6=IFCWALLSTANDARDCASE('3vB2YO$MX4xv5uCqZZG05x',#1,'Wall …);
#7=IFCWINDOW('0LV8Pid0X3IA3jJLVDPidY',#1,'Window xyz’,’…);
#8=IFCDIRECTION((1.0,0.0,0.0));
#9=IFCOPENINGELEMENT('2LcE70iQb51PEZynawyvuT',#1,'Opening …);
#10=IFCCARTESIANPOINT((0.75,0.0));
# 11 =IFCCARTESIANPOINT((0.0,0.0,0.0));
#12=IFCCARTESIANPOINT((0.0,0.3));
#13=IFCORGANIZATION($,'TNO','TNO Building Innovation',$,$);
#14=IFCPROPERTYSINGLEVALUE('AcousticRating','AcousticRating’,…);
#15=IFCPROPERTYSINGLEVALUE('Reference','Reference',IFCTEXT(''),$);
#16=IFCPROPERTYSINGLEVALUE('FireRating','FireRating',IFCTEXT(''),$);
#17=IFCPROPERTYSINGLEVALUE('IsExternal','IsExternal',IFCBOOLEAN(.T.),$);
#18=IFCPROPERTYSINGLEVALUE('ThermalTransmittance’,…);
#19=IFCQUANTITYLENGTH('Height','Height',$,1.4);
#20=IFCQUANTITYLENGTH('Width','Width',$,0.75);
#21=IFCLOCALPLACEMENT($,#2);
#22=IFCBUILDING('0yf_M5JZv9QQXly4dq_zvI',#1,'Sample Building’,…);
#23=IFCBUILDINGSTOREY('0C87kaqBXF$xpGmTZ7zxN$',#1,…);
#24=IFCLOCALPLACEMENT(#21,#2);
…
END-ISO-10303-21;

 Our in-house program for the semantic difference trans.

Xue: BC for construction, HUST 2020 Summer Camp CM

(Xue & Lu 2020)

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3.2 SDT tests
 Changing a window’s size

Xue: BC for construction, HUST 2020 Summer Camp CM

Input
IFC
(7.4KB
each)

Item
Size (KB)
Time (s)
SH?*
Output

Line-by-line file comparison
1.00
0.041

6 changed lines:

The proposed SDT
0.36
0.003

4 changed properties:

IFCXML
(32.9KB
each)

Size (KB)
Time (s)
SH?*
Output

0.56
0.042

6 changed lines:

0.89
0.012

6 changed properties:

*: With semantic hierarchies?

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3.2 BIM change contract as a smart contract
 BIM change contract (BCC)
 BCCt: All BIM changes at time t
o BCCi = ⊕n σi

o Note: ⊕ is the simplest operation for proof-of-

concept

 A BIM can be created from the model at t - 1 and

changes at t

o ifct = ifct-1 + BCCt.

 BIM at any time can be recovered from base BIM

and the chained BCCs
o ifct = ifc0 + Σt BCCi.

 Data storage

 Permissioned nodes, not public
Xue: BC for construction, HUST 2020 Summer Camp CM

Permissioned blockchain architecture
(Reprinted with permission)

26

3.2 Another test case
 Autodesk Revit 2018’s sample BIM (a modern villa, 27.4 MB in IFC)
 Sequential / simultaneous roof window changes
 By two BIM users, from t = 0 to 4
 t2 → t3: Simultaneous changes by two users

Xue: BC for construction, HUST 2020 Summer Camp CM

A test case
(Reprinted with permission)

27

3.2 SDT/BCC tests
 t2 → t3
 User A: Added a roof window
o σA = top left block

 User B: Added text comments to

another window

o σB = bottom left block

 BCC as the conflict-free merge
 BCC = right block

Combining SDTs to BCC at t2 → t3
(Reprinted with permission)

 BCC is efficient (<0.02%)
 3.37KB out of 27.4MB IFC

Input

Change

 Good for blockchaining

IFC
(27.4MB
each)

t2→ t3
(Arch.)
t2→ t3
(Client)

Xue: BC for construction, HUST 2020 Summer Camp CM

Line-by-line file comparison
Size (KB) Time
SH?#
(lines)
(s)*

54,700
0.789
(533,923)

53,900
0.756
(514,192)

The proposed BCC/SDT
Size
Interop.
SDT
(KB)
time (s)*
time (s)*

3.45

6.681

0.463

SH?#



28

3.2 Blockchained BIM changes
 On a simplest blockchain
 Web-based
 Easy nonce
 Visualized blocks
o Green = verified; red = wrong / hacked

 BIM was immutable from

 claiming false authorships,
 destroying evidence, or
 being hacked, etc.

Xue: BC for construction, HUST 2020 Summer Camp CM

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3.3 Discussion
 Existing blockchain applications for smart construction
 Works, e.g., blockchaining SCM and BIM changes
 but preliminary and infantile

 The characteristics of blockchain are appropriate for
construction
 Immutability, transparency, security (e.g., data loss)

 Challenges ahead
 Culture, regulation, governance
 Cost and efficiency (e.g., not widely used to fight spams)
 Security (e.g., business secrets, privacy)
 Understanding and acceptance
Xue: BC for construction, HUST 2020 Summer Camp CM

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References
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Back A. (1997). Hashcash. http://www.cypherspace.org/hashcash/
Back A. (2002). Hashcash—A Denial of Service Counter-Measure. http://www.hashcash.org/hashcash.pdf
Brambilla, G., Amoretti, M., & Zanichelli, F. (2016). Using blockchain for peer-to-peer proof-of-location. arXiv preprint arXiv:1607.00174.
Dwork, C., & Naor, M. (1992). Pricing via processing or combatting junk mail. In Annual International Cryptology Conference (pp. 139-147).
Springer, Berlin, Heidelberg.
Dwork, C., Goldberg, A., & Naor, M. (2003). On memory-bound functions for fighting spam. In Annual International Cryptology Conference (pp.
426-444). Springer, Berlin, Heidelberg.
Ellis, M. (2019, July 12). Level of Detail or Development: LOD in BIM. Retrieved November 6, 2019, from REBIM: https://rebim.io/level-of-detail-ordevelopment-lod-in-bim/
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future studies. Construction Management and Economics, 25(5), 477-493
Gogo, J. (2020). 65% of Global Bitcoin Hashrate Concentrated in China, Blockchain News, https://news.bitcoin.com/65-of-global-bitcoinhashrate-concentrated-in-china/
Jakobsson, M., & Juels, A. (1999). Proofs of work and bread pudding protocols. In Secure information networks (pp. 258-272). Springer, Boston,
MA.
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Facilities.
Xue, F., & Lu, W. (2020). A semantic differential transaction approach to minimizing information redundancy for BIM and blockchain
integration. Automation in Construction, 118, 103270.
Xue, F., Guo, H., & Lu, W. (2020). Digital twinning construction objects: Lessons learned from pose estimation methods. In The Joint Conference
ICCCBE and CIB W78 2020.
Xue, J., Shen, G.Q., Yang, R.J., Wu, H., Li, X., Lin, X., & Xue, F. (2020). Mapping the knowledge domain of stakeholder perspective studies in
construction projects: A bibliometric approach. International Journal of Project Management 38 (6), 313-326.
Yang, R., Wakefield, R., Lyu, S., Jayasuriya, S., Han, F., Yi, X., ... & Chen, S. (2020). Public and private blockchain in construction business process
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If you want to go fast, go alone.
If you want to go far, go together.
— African proverb

Q&A
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