Soil difficulties are mainly related to soil moisture and voids in soil. Tests on soil in laboratory are dealt with in different ways to develop analyses. Some of them deal with engineering properties, like shear strength, soil composition and structure. Soil composition is different from other materials. Soil consists of three phases: solid, liquid, and gaseous. There are special diagrams for expressing particle size distribution (Cu), uniformity coefficient (Cc) and coefficient of gradation (Gc). A real soil rarely contains similar-size particles only, but different sizes are mixed together. For any soil, there is a rather flat zone in the size distribution curve. For example, by the particle size distribution curve (D₁₀). Such soils are described as gap-graded because they are missing particles in a certain size range. Gap-graded soils are sometimes considered a type of poorly graded soil. Aggregates used to make concrete are typically gap-graded soils.