A number of the basic principles of good concrete practice are too little understood and appreciated. A better understanding of these principles will make clear the wisdom of following established practices. Although there are many other factors of concrete proportioning that affect the strength- such as kind of aggregate, aggregate grading, type of cement and whether an admixture is used- the effect of the water-cement ratio is by far the most important. If a strong and well graded aggregate had been mixed into each of two cement pastes, using as much aggregate as each mix would accommodate, the strengths of the concrete cylinders would have been essentially the same as those of the corresponding pastes. The obvious conclusion is that the water-cement ratio of concrete should be kept low in order to obtain high strength. The use of low water-cement ratio has other benefits also. One is that it concentrates more matter into the same amount of space, making the paste more dense. This means that the cement paste will be less permeable and the concrete more watertight. A strong, dense paste has less drying shrinkage and makes the concrete less susceptible to cracking. When concrete is mixed it contains a larger amount of water than is actually needed for hydration of the cement content. It is nevertheless important to keep most of this water within the concrete during the curing period in order for there to be continuous contact between water and the unhydrated surfaces of cement particles. As hydration proceeds, the new products of hydration which make up the cement paste, or binder, migrate throughout the water-filled space to fill it with the gelatinous filaments which hold the aggregate together. Thus one of the reasons for keeping water in the concrete during the curing period is to provide a medium in which these reaction products can be uniformly distributed.