Q.: What can cause a wide variation in mixing water needed to get a uniform slump for ready mixed concrete? For a 4-inch slump, I normally need to batch about 150 gallons of water in a 5-yard load. Sometimes, though, it takes as much as 170 gallons to get the same slump. Why?

A.: Several factors can cause slump variations. If moisture content varies in the aggregate stockpile, switching from a wet to a dry portion of the pile can increase water demand. If admixtures are being used, a dispenser malfunction can result in not enough water-reducing or air-entraining admixture being added. If aggregate segregation has occurred in the stockpiles, a large amount of fines might be put in one batch.

Edward Wegner, formerly a technical services representative for a cement company, described his solution to a similar problem. He visited a plant plagued by erratic variations in water demand. Sand from a 40-foot-high stockpile was charged directly into a scale hopper with a front end loader. It was a dry, windy day and Wegner noted that the wind was depositing a drift of very fine sand near the toe of the pile. Instead of reblending this material, the operator periodically swung over, picked up the drift, and charged it directly into the hopper. The excess of fine sand dried up the mix and increased water demand. After the loader operator started blending drift sand back into the stockpile, there were only minor variations in water demand.