Concrete is strong in compression but weak in tension. That single fact drives everything about reinforcement detailing. Steel reinforcing bars (rebar) handle the tension forces that concrete cannot, and placing them correctly determines whether a structural member performs or fails.

On the PDD exam, you need to know how to select the right rebar size for a given application, specify adequate concrete cover to protect against corrosion and fire, locate control joints to manage inevitable shrinkage cracking, and distinguish between slab types that serve different structural and programmatic purposes. These decisions show up in construction documents every day: the detail that calls out #4 bars at 12 inches on center with 1.5 inches of clear cover, the partition note specifying a control joint at 25-foot intervals, the structural plan showing a post-tensioned slab spanning 35 feet without intermediate columns.

Objective 1.3 asks you to determine the size of structural systems. For concrete, that means understanding rebar designations, applying cover requirements from ACI 318, placing joints where shrinkage cracking would otherwise occur, and selecting slab systems that match span, load, and depth constraints. You won't need to perform full structural engineering calculations, but you do need to apply standard sizing and detailing rules to project scenarios.