Concrete is strong in compression, however is generally weak in tension. Therefore when a load is placed upon it it may bend. To make sure this doesn’t happen, steel reinforcement is put in on the tensile face, normally the bottom edge if a floor or beam.
Tension can also occur outwards, so for example a basement wall needs reinforcement on the tension face to hold back the ground. A column loaded at the top is likely to flex into an S-shape, so reinforcement is put in on both sides and tied together using horizontal steel bars called stirrups. These, however, are replaced with helical (spiral) reinforcement in earthquake zones.
One consideration with reinforcement is that coarse aggregate in cement must be able to fit through the steel gaps. A different aggregate may have to be used and this may have other implications. Also, if a great deal of reinforcement is needed it may be likely that a steel building alone would be easier, cheaper and stronger.
Reinforcement in stone is possible, however it requires drilling and is liable to cracking or fracturing.
