Let's Talk About Slump
from Mike Stanzel at Lehigh Hanson covers everything about slump that you could be wondering.
One portion of the article that I found most helpful covers the factors that affect the workability of concrete.
There are several factors that influence the workability of concrete including: the water-to-cementitious materials ratio, entrained air content, the paste and mortar contents, the aggregate size, gradation, shape, and texture, cement and supplementary cementing materials types and proportions, mixing time and intensity, and the use of chemical admixtures, particularly water reducing and super-plasticizing admixtures.
Generally, the higher the slump requirements, the higher the fines portion in the concrete (powder and sand) and the higher the water-reducing admixture dose to balance slump and cohesion while maintaining strength and durability. You might be spending a little more upfront, but you can save bottom line dollars over the course of a placement under the right application.
Temperature, and other environmental conditions such as moisture are the biggest external factors which can significantly impact on the workability and plastic lifespan of the concrete. During the hydration process, atmospheric circumstances will accelerate or slow down those reactions over time.
Stanzel is a plethora of knowledge when it comes to concrete consistency testing. Read the article for more insight on slump testing and the concreting system.