Density of Reinforced Concrete

Normal-weight plain concrete runs about 2300 kg/m³ (145 lb/ft³); ordinary reinforcement adds roughly 100 kg/m³, which is where the design figure of 2400 kg/m³ or 150 lb/ft³ comes from. Codes let you use 24 kN/m³ as the unit weight for self-weight, and a 150 mm slab therefore loads its supports at about 3.6 kPa (75 psf) before anything is put on it. Self-weight dominates concrete design in a way it never does in steel.

It is not one material. Structural lightweight concrete with expanded shale aggregate runs 1750–1900 kg/m³ and buys back a fifth of the dead load at the cost of stiffness and cost per cubic metre; heavyweight radiation-shielding mixes with magnetite or barite reach 3500 kg/m³ and above. Aggregate type, air entrainment and moisture state all move the figure by a few percent, so weigh a trial batch if you are checking a crane pick or a precast lift, and treat 2400 as a design convention rather than a measurement.