Unfortunately not all materials are sourced dry so their water content can vary and be considerable, particularly sand and aggregate which are usually in the weather and frequently sprayed in dryer conditions to reduce dust. If materials are bagged and stored for later use and also include cementious material it's imperative that all the materials are completely dry. I have two large bins for sand and aggregate. Unfortunately although covered to prevent rain, some moisture can get it by wicking up from the base. I have just modified them to include drainage to prevent this. The pics should tell the story.
