## 2 State of the Art
### Smart Contracts and Automated Payment in Construction  
In the AEC industry, SCs can be used to automate different kinds of processes. For example, in an approach suggested by [PERSON] et al. [41], smart contracts were used to automatically verify the compliance of the digital deliverables provided by Exchange Information Requirements. The payment could then be authorized or denied based on the automatically checked results. However, they did not further discuss how to handle the payment after approval or rejection. Similarly, in the framework proposed by [PERSON] et al. [46], smart contracts (called chaincode by the Hyperledger Fabric project) were also used for automated compliance checking. These two papers introduced SCs only as pseudocode and need further validation. Another significant use case for the application of SCs is the improvement of supply chains. For example, the chaincode in a blockchain-based information management model built by [PERSON] et al. [40] was used for changing the status of the precast components for a precast supply chain. A further Hyperledger-Fabric-based blockchain framework developed by [PERSON] et al. [45] used chaincode to create or fill quality information forms, define quality acceptance results, and sign forms for trusted quality information management in construction. However, they only presented the initial smart contract function of \"create quality information form\". The smart contract in the framework from [PERSON] et al. [47] was proposed to include functions for automatically recording and calculating reimbursed costs, profits, and cost savings for each member of a construction project, especially in integrated project delivery projects. These papers provide ideas for using smart contracts in construction, but only a few of them explain their actual smart contract design. Smart contracts have proven to be particularly useful for payment automation in the construction industry. As [PERSON] et al. [9] pointed out, theoretically, the association between smart contracts and BIM can automate the entire delivery phase, and automatic payment can be issued via the connection between the information model (i.e., activity progress) and the computational contract. [PERSON] and [PERSON] [48] developed a data-flow-diagram framework for integrating BIM, a smart sensor, a blockchain, and a smart contract to address the payment security issues for subcontractors. In their framework, the smart sensor was used to capture the completed sub-tasks, and BIM data were used to validate the completed works. If the tasks and their corresponding transactions were validated, payment was automatically executed via the smart contract. The smart contract design was still in the form of a data-flow diagram, which should be further detailed. Similarly, [PERSON] and [PERSON] [10] introduced a smart-contract-enabled solution in order to autonomously translate on-site reality captures into direct payments between a general contractor and subcontractors. Their smart contract was used to handle payment settlements and transfer lien rights, an idea that should be further detailed and verified. [PERSON] et al. [38] proposed a blockchain-based framework to facilitate the execution of interim payments by deploying a logic to automatically initiate, validate, and disburse payments via smart contracts. The proposed framework had a high level of payment security, but it was unfortunately not contextualized in terms of the reality of interim payments in the construction industry. [PERSON] and [PERSON] [8] presented a smart contract payment security system for eliminating or reducing payment issues in construction contracts. An add-on module was developed in the system to transfer the required schedule and cost data to the smart contract through a project management software. Afterwards, the project progress payment amount would be blocked, and the actual payment amount would be paid monthly.