Rarely will a laboratory mix be entirely suitable under the variable field conditions facing a concrete contractor. Adjustments to the amount of water and admixtures are commonly required. These adjustments aren't necessarily due to poor quality control or material variations. Variations in slump between trucks can be the result of other factors such as batching tolerances, aggregate moisture content, concrete temperature, mixing, and delivery, waiting, and unloading times. Water addition must not increase the water-cement ratio above the maximum permitted by specifications. Adding just 1 gallon of water per cubic yard can: Increase slump 1 inch, decrease compressive strength 150 to 200 psi, waste about 1/4 bag of cement, and increase shrinkage by 10%. But water isn't the only way to adjust slump. Consider adding a superplasticizer at the site to increase slump. Using admixtures maintains the water-cement ratio, but provides a workable slump. Sometimes, slump needs to be decreased. Ready mix drivers typically use a couple of extra sacks of cement to try to dry up the mix. But the process is time-consuming and usually not very effective. An effective method for reducing slump is to add powdered silica fume.