15.1 Reinforced, Precast, Pretensioned, and Post-Tensioned Concrete Materials

Concrete is strong in compression but cracks at relatively low tension. Reinforcement and prestressing control how that tension is resisted and how cracking, deflection, and strength develop. For July 2026, use the April 2024 PE Civil: Structural specification, current PE Civil Reference Handbook, ACI 318-14, and PCI Design Handbook 7th edition. Do not import the April 2027 reference set.

Reinforced Concrete Behavior

Conventional reinforcing bars act with concrete through bond and compatible strain. Before cracking, concrete can contribute tensile stiffness; after flexural cracking, tension is carried mainly by reinforcement crossing the cracks. Deformed bars, development length, hooks or mechanical anchorage, splice details, and confinement determine whether steel force reaches the surrounding member. Cover protects reinforcement and affects bond, durability, and fire response.

Steel yielding can provide ductile deformation and warning, but ductility is a system property as well as a material property. A bar with adequate elongation cannot compensate for a brittle shear failure, inadequate development, poor confinement, or a weak connection. Concrete compressive strength, reinforcement yield strength, modulus, shrinkage, creep, and temperature effects all enter at different stages. Keep specified properties separate from measured test values and design strengths.

Precast Is a Sequence, Not Just a Location

Precast concrete is cast away from its final position, often under controlled plant conditions. A member can have several governing configurations:

1. stripping from the form at an early concrete strength;
2. lifting from selected inserts, with bending that may reverse final-service signs;
3. transport with temporary supports and dynamic effects;
4. erection on temporary bearings or braces;
5. connection and diaphragm completion; and
6. final composite or noncomposite service.

Lifting inserts, bearing regions, temporary braces, welded or bolted embeds, and grouted joints need complete force paths. A topping slab does not automatically act compositely with a precast member. Interface roughness, reinforcement or connectors crossing the joint, cleanliness, curing, and compatibility must satisfy the ACI 318-14/PCI 7th-edition design model.

Pretensioning and Post-Tensioning

The two prestress methods differ primarily in when tendons are stressed and how force enters the concrete.

Pretensioning: High-strength strands are tensioned between fixed abutments before concrete is cast. Concrete is placed around the stressed strands. After the required concrete strength is reached, strands are released or cut from the bed. As each strand shortens, bond transfers compression into the concrete over a transfer region. Pretensioned members are commonly precast. Strand pattern, release sequence, transfer length, end-zone behavior, and temporary lifting condition matter.

Post-tensioning: Concrete is cast first with ducts, sleeves, or sheathed tendon paths. After the specified concrete strength is reached, tendons are stressed with jacks reacting against the hardened member and are locked at anchorages. A bonded system is typically grouted so the tendon bonds along its length after grouting; an unbonded tendon remains isolated in its sheath and transfers force mainly through anchorages. Anchorage-zone bursting, spalling, local bearing, tendon profile, friction, and grout or corrosion protection require attention.

Pretensioning is not “post-tensioning done in a factory,” and post-tensioning is not defined by whether the member is precast. The force-transfer sequence controls the name.

Prestress Losses and Stages

Jacking force is not the same as effective service prestress. Losses can include elastic shortening of concrete, concrete creep and shrinkage, and steel relaxation. Post-tensioned systems can also have anchorage seating and friction or wobble along curved ducts. The importance and timing of each loss differ between pretensioned and post-tensioned members. Use the method and coefficients in the applicable reference or problem rather than one blanket percentage.

Check at least transfer, handling or erection, and service. Concrete strength and allowable stresses change with age; self-weight and superimposed loads enter at different times; losses accumulate. A tendon eccentricity that reduces bottom tension at midspan can increase top tension near an end or during lifting.

Worked Eccentric-Prestress Calculation

A 12 in by 24 in rectangular member has effective prestress P_e = 300 kips acting 6 in below the centroid. Find elastic stresses from prestress alone at a section away from transfer-zone disturbances. Use compression as negative.

A = 12(24) = 288 in^2

I = bh^3/12 = 12(24^3)/12 = 13,824 in^4

S = I/c = 13,824/12 = 1,152 in^3

Uniform stress:

-P_e/A = -300/288 = -1.042 ksi

Eccentric moment:

M_p = P_e e = 300(6) = 1,800 kip-in

M_p/S = 1,800/1,152 = 1.563 ksi

Because the tendon is below the centroid, eccentric prestress increases bottom compression and offsets top compression:

f_bottom = -1.042 - 1.563 = -2.605 ksi

f_top = -1.042 + 1.563 = +0.521 ksi tension.

These are prestress-only elastic stresses, not a pass/fail result. Add self-weight and other loads at their actual stages, use the effective prestress appropriate to that stage, and compare with the ACI/PCI limits. Recheck signs with the expected curvature.

Exam Decision Check

Ask four questions: Was the tendon stressed before or after casting? Does force transfer by bond after bed release or through post-tensioning anchorages? What losses have occurred at the requested stage? Is the member precast, composite, temporarily supported, or fully connected? Those answers prevent methods and stages from being mixed.

Transfer zones require a final check: prestress is not uniform immediately at a pretensioned member end, while post-tensioning anchorages introduce concentrated local forces that require appropriate end-zone reinforcement.