1 INTRODUCTION
The benefits of ready mixed concrete (RMC) in light of attaining consistent quality standards, being environmentally-friendly, and demanding less site space have accounted for its ubiquitous application in infrastructure and residential building projects. The scheduling of RMC production and delivery is essentially a problem of materials logistics planning, which is “a decision process for strategically managing the procurement, movement and storage of raw materials, finished product inventory and the related information flows throughout the organization and its marketing channels in such a way that the current and future profitability is maximized by cost-effective fulfillment of orders” (Christopher 1992) [6]. In Hong Kong, a RMC plant generally requires a three-day advance notice for ordering concrete delivery service, with the order confirmed one day before the actual pour date. Raw material stocks at a plant are replenished on a daily basis in preparation for the next-day's production which is pulled by site orders placed by multiple clients. A RMC plant transforms raw materials into concrete in its production facility and is committed to delivering concrete to different construction sites by truckmixers so as to match the onsite concreting progresses. The elapsed duration from the introduction of water to the final placement of the concrete is of particular importance to the quality control of ready mixed concrete. For example, ASTM C94 (ASTM 2000) [5] allows a maximum of 1.5 hours (90 minutes), or before the drum has made 300 revolutions, whichever comes first. The Hong Kong common practice is to unload concrete into the forms on site within 1 hr 45 min of first mixing as a maximum time limit. Due to the perishable nature of concrete, the batching and delivery operation of the RMC industry is a classic example of Just-In-Time (JIT) construction system (Tommelein and Li 1999) [21].