Formula selection and scheduling for precast concrete production

Abstract

This paper considers the precast production problem with identical parallel molds. To produce precast elements, multiple processes including mold preparing, concrete mixing, curing, stripping and finishing must be completed in a mold. Some processes require an additional resource shared among molds. All the processing times are constant except the curing time, which increases when using a more expensive concrete mixing formula. Given a demand and a production time frame, this paper proposes a heuristics to select concrete formulas and schedule jobs to minimise the total product cost. A mixed integer programming (MIP) model is formulated to solve for the optimal solutions. The results demonstrate that the proposed heuristics finds the same optimal solutions as obtained from the MIP model but with many orders of magnitude less than computational times. In the problems where the MIP model cannot find the optimal solutions due to the problem complexity, the proposed heuristics can find feasible solutions that are within 3% differences for 4-mold problems and 9.5% differences for 8-mold problems from the approximate lower bound solutions and only uses less than a few seconds of computational time.