ICC Prestressed Concrete SI cheat sheet

Prestressing steel strength grades in SI practice: wire and strand commonly 1570, 1670, 1770, 1860 MPa; high-strength alloy bars typically 1030 MPa (plain) or 1080 MPa (threaded).

Seven-wire strand nominal diameters (mm): 9.3, 11.1, 12.7, 13.0, 15.2, 15.7, 18.0; area and mass are tabulated per EN 10138 or ASTM A416 dual-unit tables.

Strand construction: six outer wires helically wrapped around one straight king wire; lay length and direction affect elongation at fracture and anchorage seating behavior.

Low-relaxation strand (class 2 per EN 10138) loses about 2.5 percent or less of initial load after 1000 h at 0.7 fpk at 20 degrees C; stress-relieved (class 1) loses about 8 percent.

Duct systems: corrugated galvanized or plastic sheathing; plastic ducts reduce friction and give better corrosion protection; duct internal diameter must exceed strand bundle diameter by a code-specified margin.

Anchorages: wedge-type (chuck) for strand, button-head or swaged for wire, nut-and-threaded for bar; each anchor transfers load by wedge action, bearing, or thread engagement, not by bond.

Couplers, grout caps, and bearing plates are part of the anchorage assembly; mismatch of wedge and anchor head from different suppliers is not permitted unless tested as a system.

Corrosion protection options: cement grout, grease or wax, and pre-grouted or plastic-coated strand; galvanized strand is not standard for bonded post-tensioning.

Nominal strand area formula for seven-wire strand: Ap = 6 times area of one outer wire plus area of king wire, computed from nominal wire diameter; use the tabulated nominal area, not measured area, for design.

Ultimate tensile load of a tendon: Fpk = Ap times fpk; characteristic yield is taken at 0.1 percent proof stress f0.1k, typically about 0.85 to 0.9 fpk for strand.

Minimum elongation at maximum load for strand is commonly 3.5 percent; lower values indicate a non-conforming heat and require retest.

Relaxation test basis: initial load 0.6, 0.7, or 0.8 fpk held at constant strain at 20 degrees C for 1000 h; extrapolation to 100,000 h allowed by standards with stated factors.

Anchorage efficiency: the assembly must develop at least 95 percent of the strand's actual breaking load in a static tensile test; failure inside the anchor head or wire breaks near the wedges are non-conforming.

Fatigue performance of anchorages is verified by cyclic load tests (commonly 2 million cycles between 0.45 and 0.65 fpk depending on the standard) after which the assembly must still reach the required static load.

Bars: high-strength alloy bars use threaded ends or upset ends with nuts; bar tendons are coupled with threaded couplers and can be restressed, unlike bonded strand.

Wire systems use button heads or wedge grips; button-head anchorages rely on the enlarged head bearing on a plate and require controlled heading temperature to avoid embrittlement.

Transfer strength at prestress transfer is typically at least 0.6 fci or 0.7 fci per code; the actual cylinder strength at transfer must be verified, not assumed from mix design.

Release strength governs mix design: high early strength is achieved with low water-cement ratio (0.35 to 0.40), Type III or rapid-hardening cement, and sometimes steam curing.

Permitted concrete strength range for prestressed members is commonly 30 MPa minimum at transfer and up to 60 MPa or higher for pretensioned elements; higher strength reduces elastic shortening losses.

Elastic shortening loss: delta = (Ep/Eci) times average concrete stress at tendon level; Eci is the modulus at transfer, not the 28-day modulus.

Creep and shrinkage losses depend on volume-to-surface ratio, ambient relative humidity, and age at loading; higher humidity reduces shrinkage and creep.

Concrete cover and chloride limits are stricter for prestressed members than for reinforced concrete because strand is highly sensitive to pitting corrosion and stress corrosion.

Aggregate: use non-reactive aggregates and limit alkali content where reactive silica is present; prestressed members are thin and more vulnerable to alkali-silica expansion.

Temperature limits during curing: steam curing typically caps concrete at about 60 to 65 degrees C; higher temperatures increase later-age strength loss and can cause thermal cracking.

Pretensioning sequence: place strand, tension to the calculated jacking force (including overpull for losses), lock off, place reinforcement and mild steel, cast concrete, cure, then release.

Jacking force is verified from pressure gauge calibration plus measured elongation; the elongation tolerance is typically plus or minus 5 to 7 percent of calculated value, and discrepancies require investigation.

Release of pretensioning must be gradual and symmetrical to avoid shock loading; abrupt release can cause strand slip and end-zone cracking.

Post-tensioning sequence: install ducts and anchorage, cast and cure concrete, verify concrete strength, tension in the specified order, record elongation and pressure, then grout.

Grouting: use approved grout, vent high points, grout within the time limit after stressing (often 7 to 14 days), and verify complete filling by vent flow or inspection.

Tensioning records must include tendon identification, jacking force, gauge pressure, measured elongation, slip at anchorage, and ambient temperature at the time of stressing.

Duct and anchorage placement tolerances affect tendon profile and therefore losses; deviations beyond the specified tolerance require engineering review before stressing.

Cold weather and hot weather limits: stressing below about 5 degrees C or above about 35 degrees C may be restricted; grout temperature limits are tighter than concrete limits.

Strand tests: tensile strength, yield at 0.1 percent proof, elongation at maximum load, and relaxation; sampling frequency is per lot mass, commonly one sample per 20 to 50 tonnes.

Anchorage assembly tests include static tensile, fatigue, and sometimes dynamic or corrosion tests; test assemblies must match the production system, including wedge and head supplier.

Concrete tests at transfer: cylinders or cubes cured alongside the member, tested to confirm the release strength before detensioning.

End-zone inspection: check for horizontal cracks along the tendon path, spalling at anchorages, and splitting cracks; these indicate inadequate confinement reinforcement.

Post-tensioning records are inspected against the calculated elongation; a measured elongation outside the tolerance band triggers a friction or wedge-slip investigation.

Grout inspection: verify vent flow, check for voids by drilling or nondestructive methods where specified, and confirm grout cube strength.

Dimensional checks: camber, sweep, member length, and tendon eccentricity; camber is measured after detensioning and compared with predicted values to detect strand slip.

Documentation review: mill certificates, heat numbers, calibration records for jacks and gauges, and test reports must be traceable to the specific member or lot.

Lifting points are specified by the designer and are usually at or near the ends of pretensioned members; lifting elsewhere can cause cracking or overstress.

Lifting inserts and lifting loops must be rated for the member weight including dynamic allowance, commonly 1.25 to 1.5 times static weight.

Storage: support members at the designated bearing points, do not stack directly on strand or on thin edges, and keep members off the ground on dunnage.

Long-term storage can cause camber growth and creep deflection; check bearing elevations and adjust shims before erection.

Transport: secure members against longitudinal and lateral movement, protect strand ends and anchorage from damage, and check route clearances for length and weight.

Erection: verify bearing pad placement, seat elevation, and member orientation; check that sweep and camber match the erection plan before final seating.

Post-tensioned members: do not lift by tendons or anchorage; grout only after the specified stressing sequence and strength verification.

Site handling of strand: protect from weld spatter, ground contact, and contamination; damaged or kinked strand must be rejected, not straightened and reused.

Core referenced standards: EN 1992-1-1 for design, EN 13391 for prestressing systems, EN 10138 for steel, ASTM A416 for strand, ASTM A722 for high-strength bar, and ACI 318 for US practice.

Inspector duties: verify materials against certificates, witness tensioning and record elongation, check concrete strength at transfer, and document nonconformances.

Nonconformance handling: reject non-conforming material or work, issue a report to the engineer of record, and do not proceed until disposition is approved.

Calibration: jacks and pressure gauges must be calibrated at defined intervals, commonly every 6 months or after repair; calibration records must be available at the plant.

Traceability: each tendon and member must be traceable to heat numbers, test reports, and tensioning records; missing records are a compliance failure.

Hold points: concrete strength at transfer, tensioning, grouting, and final dimensional inspection are typical hold points requiring inspector sign-off before the next step.

The inspector does not redesign; deviations from drawings or specifications are referred to the engineer, and verbal instructions are confirmed in writing.

Reporting: daily reports should record weather, operations witnessed, tests performed, and any deviations, signed and dated for the project record.