# Proactive Production Scheduling Approach for Off-Site Construction with Due Date Uncertainty
## 1 Introduction  
Off-site construction has increasingly gained attention as one of the solutions to address low productivity, high rates of accidents, and low quality in traditional on-site construction [1]. In precast concrete (PC) construction, which is a representative of the off-site construction methods, the PC components produced via an off-site manufacturer are delivered to an on-site one and then assembled. It can replace the traditional structural works consisting of formwork, rebar, and concrete pouring on-site by just the assembly of the PC components produced under the controlled off-site environment so that it can improve productivity and quality, with the possibility of accidents decreased [2]. PC production scheduling is vital for the successful completion of a project. It enables PC components produced by an off-site manufacturer to be delivered to each contractor's site within their due date. Since PC component assembly is on the project's critical path, any delay in delivering the PC components to the site will delay the subsequent work and extend the overall construction duration [3]. However, contractors' schedule variability negatively influences the PC production schedule. A construction task can be delayed--or, in rare cases, completed ahead of schedule--due to various factors, such as unreliable planning, weather, equipment failures, labor shortages, or missing and incorrect data. These schedule changes related to on-site construction affect the due dates in off-site production and change the performance of the initial production schedule. The challenge of handling these inevitable disruptions is frequently viewed as a significant reason for the discrepancy between scheduling theory and its practical implementation in the industry [4]. The research in scheduling under uncertainty can be classified into the reactive scheduling approach and the proactive scheduling approach [5]. The reactive scheduling approachadjusts the schedule based on the actual conditions of the plant once uncertainties arise or unexpected events occur. However, implementing rescheduling strategies to address disruptions can be challenging and may result in ineffective or costly reconfigurations, as well as causing anxiety among the manufacturing staff [6]. In particular, PC suppliers might have to cancel a pre-scheduled job order or change the sequence of job orders due to a customer's altered delivery schedule. Proactive scheduling generates schedules that are, in some sense, robust or insensitive to anticipated uncertainties, and it does not change schedules in the face of uncertainty. In many precast concrete (PC) suppliers, the production schedule is typically rigid, with monthly schedules fixed in advance, dictating resource planning. Despite occasional changes in delivery schedules requested by customers (construction contractors), most PC suppliers lack the flexibility to adjust their production schedules mid-cycle, highlighting the importance of developing proactive scheduling approaches. However, proactive scheduling approaches have not yet been sufficiently explored in PC production scheduling problems. Therefore, this study aims to develop a robust PC production scheduling model for PC construction projects with schedule uncertainty by adopting a proactive scheduling approach. The proposed model consists of two modules: (1) a Monte Carlo (MC)-method-based PC production simulation module, which simulates and evaluates the total tardiness of the schedule with contractor schedule uncertainty; and (2) a genetic algorithm (GA)-based production schedule optimization module that finds a near-optimal schedule through iterative generation and evaluation using a PC production simulation module. It would help PC production schedulers to achieve robust schedule performance with contractors' schedules changed, thereby leading to the PC components' reliable and economical supply chain. The rest of this study is organized as follows.