## 4 BIMcontracts Approach
### Smart Contract in the Execution Phase  
After the successful signing of the contract, the system is ready for use, and the execution phase of the construction project begins. The process of the delivery and acceptance of this phase consists of the following main steps (see Figures 1 and 2): (i) reporting the completion of a construction task by the contractor; (ii) checking the results and a notification of the decision (confirmation or rejection) by the client; (iii) automatic triggering of the payment in the case of a (partial) confirmation. Each activity is handled as a transaction and is stored in the blockchain to ensure the validity and traceability of the processes. Thus, the progress of contract fulfillment is continuously and transparently visible to both contracting parties.  
During the execution phase, the executed work and the problems, defects, remedying, and repair procedures that occur are documented. Many digital tools already exist to support construction documentation, final acceptance, and issue management. Meanwhile, model-based reporting is already provided by several tools, which are often referred to as field BIM tools [57]. Thus, the concept of BIMcontracts takes into account such model-based reporting and the use of current tools based on open standards. The contractor has the ability to report completion by selecting billing units, items from the BoQ, or even model elements, as the link between a billing schedule, the BoQ, and a BIM model is always provided.  
In addition, the contractor should be able to include detailed documentation as evidence, such as measurements in the case of a unit price contract. The documentation can be done by means of text messages, voice messages, or photos and videos. After receiving a completion notification, the client checks within the agreed period whether the work has been carried out without any significant defects. At this point, three scenarios are possible:  
1. The construction work is complete and free of defects, so the client confirms and rewards it.
2. Refusal of the confirmation of completion due to existing defects.
3. Partial confirmation of completion with retention.  
Figure 5: Sequence chart of the generation and deployment of the smart contract.  
At the lowest level of complexity is the correctly executed construction process: The client accepts the completion and confirms that the work has been performed without any significant defects. Thus, a payment that is related to the respective billing unit or billing unit element (in the case of an agreed partial payment) is automatically triggered. The security deposit will be subtracted accordingly.  
Nevertheless, during the construction, defects may occur. In general, retentions in the case of defects and open work are to be treated in the same way. The billing procedure follows the same logic: The client can either refuse the payment of the reported work or define retention of the amount to be paid. Both refusal and retention due to defects or outstanding remaining work must be justified and documented as precisely as possible by the client. In the event of a refusal, according to the client's opinion, the work has not yet been performed as per the agreement. The contractor can now repeat the work and submit a new notification of completion. A partial confirmation is also possible, which legally corresponds to justified retention of payment due to defects. Through retention, the contractor remains more liquid against a denial of the payment.