Concrete is strong in compression (around 4,000 psi typical) but pathetic in tension (only ~10% of compressive strength). Plain rebar lets concrete crack first, then the steel takes over once the crack opens. Prestressing skips the cracking step entirely by squeezing the concrete before any service load arrives. The concrete now starts life in compression, so applied tension just reduces that compression instead of creating cracks.

Two methods dominate:

• Pre-tensioned: High-strength steel strands (typically 270 ksi) are stretched between abutments in a casting bed, concrete is poured around them, and once it cures, the strands are cut. The strands try to shrink back to original length but bond friction transfers that force into the concrete as compression. Used for precast double-tees, hollow-core planks, bridge girders.
• Post-tensioned: Strands run through plastic ducts cast into the concrete. After curing, hydraulic jacks pull the strands and anchor them at the ends. Used for cast-in-place slabs, parking garages, and long-span buildings where you can't precast.

Rule of thumb: A prestressed beam can span roughly 1.5× to 2× what a conventionally reinforced beam of the same depth can span. A 24-inch-deep PT slab can routinely cover 30+ foot spans with no beams — try that with regular rebar and you'll get a sagging, cracked mess or need 36+ inches of depth.

Quick calculation: A 0.5" diameter 7-wire strand has a cross-section of ~0.153 in² and is typically stressed to 0.74 × fpu = 0.74 × 270 ksi ≈ 200 ksi. That's 30,600 lbs per strand — a single strand the diameter of a pencil holds up 15 tons.

Real-world example: The post-tensioned parking garage at almost any modern shopping mall. Those 60-foot column-free spans with only an 8-inch slab are only possible because PT cables, tensioned a few days after the pour, compress the slab against itself. Watch for the small grout-patched anchor pockets along the slab edges — that's where the jacks pulled the strands.

Failure modes to respect: Anchorage zone bursting (huge concentrated forces at the ends), strand corrosion in unbonded systems (a hidden break can release lethal energy — "zipper failure"), and long-term losses from concrete creep and steel relaxation that can shed 15–20% of the initial force over decades.