What is a tension reinforced section and compression reinforced section?
Tension reinforced section is when you have steel reinforcement (rebar) on the tension controlled region where the rebar is used to counteract the lack of tensile strength in concrete. When reinforcement is only present in the tension zone, you have a singly reinforced beam.
Nevertheless, sometimes, rebar on the tension side only is not enough to resist the ultimate moment capacity of the beam, therefore, reinforcement in the compression region is needed. When reinforcement is present in both zones (tension and compression), you have a doubly reinforced beam. Refer to images below to have a better visual understanding since “pictures are worth a thousand words”.