Datum Beta
Start free

← Construction & Evaluation

Sheet S-105
PPDPDD CE

Concreteconcept

Concrete reinforcing: rebar, WWF, pre- vs post-tensioning

One-line orientation

Because concrete is weak in tension, steel does the pulling — and the exam tests rebar sizing (eighths of an inch) and the pre- vs post-tensioning sequence above all else.

Key points

  • Rebar (hot-rolled steel):
    • Size = eighths of an inch of diameter: bar # ÷ 8 = inches. #8 = 1”, #4 = 1/2”, #3 = 3/8”.
    • Grades name the yield strength in ksi — 40 / 60 / 75; Grade 60 is most common, Grade 75 typical in columns.
    • Stock length ~60 ft.
    • Cover (concrete over the bar) protects against fire and corrosion; development length bonds the bar enough to transfer its force; a lap splice overlaps two bars ~30× the bar diameter to be continuous.
  • Welded-wire reinforcing (WWR / WWF): a prefabricated steel mesh for distributed reinforcement, common in slab bottoms; grid spacing roughly 2”–12”.
  • Corrosion protection in chlorides (de-icing salt, marine): epoxy-coated, galvanized, stainless, or polymer-coated bar.
  • Prestressing — putting the steel in tension to pre-compress the concrete:
    • Pre-tensioning: tendons are stressed before the pour; after curing the jacks release and the steel’s pull transfers to the concrete by bond. Done in a casting bed → typical of precast (cold-rolled, high-strength cables).
    • Post-tensioning: tendons run in ducts/sleeves through the cured concrete and are stressed after curing, then anchored. Typical of cast-in-place systems.

Pre- vs post-tensioning: stress the steel before the pour (bond) or after the cure (anchorage)

The blue stressing jack is the same tool in both storyboards — read where it sits in the sequence.

View diagram Hide diagram 2 panels

2 panels side by side — swipe to compare

Pre-tensioning and post-tensioning compared as paired vertical construction storyboards Two side-by-side vertical construction storyboards, three scenes each, read top to bottom. Left column, pre-tensioning on a casting bed (precast): scene 01, the blue stressing jack pulls a straight steel strand tight between two buttressed ground-anchored casting-bed abutments, bearing against the right abutment, with no concrete present yet; scene 02, concrete is cast around the still-taut strand while the abutments hold it, so the strand stays blue; scene 03, the abutments are removed, the released member arches into a small schematic camber, the strand turns to ink, and a dense train of small arrowheads shows the prestress transferring into the concrete by bond, distributed along the whole length. Right column, post-tensioning in a cured member (cast-in-place): scene 01, concrete is cast first around a draped hollow duct whose channel is empty, with no tendon stressed; scene 02, the same blue stressing jack pulls the draped tendon inside the hollow duct after the concrete has cured, with no anchor plates present; scene 03, the jack is gone, real end anchorages appear for the first time as a bearing plate with a wedge cone gripping the tendon and a protruding button-head end, the tendon turns to ink, and two large block arrows bear on the two anchorages showing the prestress held concentrated. Blue marks jack-held tension in both columns and stops the moment transfer happens; the jack sits at the top of the left column but the middle of the right, showing the steel is tensioned before the pour in pre-tensioning and after the cure in post-tensioning. Pre-tensioningon the casting bed · precastPost-tensioningin the cured member · cast-in-place01Tension steelcasting-bed abutmentsteel stressed before the pour02Cast + cureconcrete poured around the taut steel03Release → bondcamber — schematicForce transfers by bondspread in along the whole length01Cast + cure around ductconcrete cast first, around a draped hollow duct02Stress after curethe jack stresses the tendon after cure03Anchor endsForce held at end platesconcentrated at the two anchorages
  • blue = jack-held tension — ends where transfer happens

The names say WHEN the steel is tensioned relative to the pour — not where the work happens: bond = precast, anchorage = cast-in-place.

Confusions / comparison

Pre-tensioningPost-tensioning
When steel is stressedBefore concrete is pouredAfter concrete has cured
Force transferBond (release tendons)Anchorage at duct ends
Tendons in ducts?No — cast directly into concreteYes — in ducts/sleeves
Typical constructionPrecast (plant casting bed)Cast-in-place
Rebar factValue
Size unitEighths of an inch (#8 = 1”)
Common grade60 (Gr. 75 in columns)
Lap splice~30× bar diameter

→ Properties & curing (this topic): why steel is needed (tension) and steel-concrete synergy · Precast concrete (this topic): why precast is pre-tensioned · Slab systems (this topic): where WWR and rebar sit in each floor type.

Spotted an issue with this card? Tell us →

Related units

Cement typesFormwork & placementPrecast concreteProperties & curing