What material is added to cast concrete today to give it the tensile strength that modern concrete otherwise lacks?

Steel

Explanation:

    In a contemporary reinforced concrete structure, steel's basic function is to resist tensile stresses. This is because the concrete does not have sufficient tensile strength to absorb the stresses of a building. In this logic, during the performance of the structural calculation, the regions subject to the tensile forces must be armed. For example, in the case of simply supported slabs, under the action of vertical loads directed from top to bottom, the reinforcement must be placed on the bottom face of the slab.