gif video from: Marta Ambrosetti, DensityDesign Research Lab
Concrete is a man-made material
Concrete is made from natural materials but concrete itself is a man-made material. It consists of cement, steel reinforcing, fine aggregate (sand) and gravel or rock (large aggregate).
Concrete is neither homogeneous nor isotropic
A homogeneous material has the same physical and mechanical properties at any location in a specimen. Isotropic material has the same properties in any direction.
For design purposes, concrete is considered a homogeneous and isotropic material. It is important to understand that this is a simplification. In materials science, a homogeneous material is a This is necessary to get a practical design, but it is unrealistic. What do structural engineers mean by a homogeneous material? A homogeneous material is a material that has the same characteristics throughout the material. An example of a true homogeneous material is steel.
Concrete is made of cement, water, and aggregate. Cement is like a glue that holds everything together. Water is necessary to initiate a chemical reaction in the cement. Aggregate includes sand (fine aggregate) and a coarse aggregate of stone.
How engineers mitigate the lack of homogeneity
Concrete is initially mixed at a concrete plant where all the components are placed into the barrel of a mixer truck. The mix consists of a precise amount of rock, sand, water, and cement. Inside the barrel, the concrete is continuously mixed.
Next, the concrete is placed into a Ready-Mix truck barrel or rotating drum. The rotating drum continuously keeps the concrete mixed. The more the concrete is mixed, the more it will resemble a homogeneous mass.
Inside the rotating drum, a version of Archimedes screw shown below) is used to pump the mixed concrete to a chute where it directed to where the concrete is needed.
Marta Ambrosetti, DensityDesign Research Lab