Concrete cracks when the tensile stress exceeds the tensile strength of the concrete. The crack is always at a right angle to the tensile stress. A split cylinder test is an indirect tension test that can help explain how vertical and horizontal cracks occur. A vertical compressive load is applied. The vertical diameter tends to get shorter and the horizontal diameter tends to get longer; the vertical compression induces horizontal tension. The cylinder splits along the vertical diameter, and compression induces cracks parallel to the compressive force.
Many structural cracks are diagonal or curved. Vertical shear forces, called here Forces 1 and 2, and horizontal shear forces, called here as forces 3 and 4, play a part in causing these cracks. Forces 1 and 3 combine into a single resultant force that acts diagonally upward to the left. The resultant of forces 2 and 4 acts diagonally downward to the right. These two resultants are aligned and they create tensile stresses across the diagonal that goes upward to the right. If the resultant forces are great enough, they create a diagonal crack.