Understanding Prestressing Steel Tendons

Prestressing steel tendons are the backbone of modern prestressed concrete construction. Composed of high-strength steel wires, strands, or bars, they are tensioned before or after concrete placement to induce compressive stresses that counteract tensile forces from service loads. Typical grades include 1860 MPa (270 ksi) low-relaxation strands, seven-wire strands (0.5” or 0.6” diameter), and threaded bars. Their exceptional tensile strength — often five to seven times that of ordinary reinforcing steel — demands rigorous handling protocols. Any defect introduced during site handling can propagate under tension, leading to sudden failures. Understanding the metallurgical sensitivity of these tendons is the first step toward safe site practice. The steel is susceptible to stress corrosion cracking, hydrogen embrittlement, and pitting if exposed to chlorides, moisture, or mechanical abuse. Therefore, every site activity — from unloading to tensioning — must prioritize protection of the tendon surface, geometry, and material properties.

Best Practices for Receiving and Unloading Tendons

Upon delivery, verify that the tendons match the project specifications — size, grade, and certification. Inspect packaging for signs of damage during transit. Coils or reels should be lifted using spreader bars and slings rated for the weight; never use chains or wire ropes that could gouge the steel. Unload directly onto a clean, dry surface or onto prepared storage cribbing. Avoid dropping coils, as even a short free fall can cause permanent bends or surface indentations. Immediately after unloading, perform a visual check for bent wires, broken strands, or rust staining. Document the condition with photos and notes. If damage is found, quarantine the material and notify the supplier before proceeding.

Handling Tendons on Site: Core Principles

Handling includes moving tendons from storage to the work area, cutting, threading, and stressing. Follow these fundamental rules to preserve tendon integrity:

Use Correct Lifting and Rigging Gear

All lifting equipment must have a safe working load (SWL) exceeding the tendon coil or reel weight. Use nylon or polyester slings with protective sleeves to avoid abrasion. For long bundles, use a spreader beam to distribute load evenly. Never hoist a coil by passing a sling through the center eye alone — use a lifting beam or a purpose‑built coil lifter. Tag lines help control swing and prevent impact with adjacent structures.

Avoid Surface Damage

Dragging tendons across rough ground, over edges of formwork, or through sharp openings produces nicks and scratches that become stress raisers. Always transport tendons on dollies, pallets, or with slings under full support. When threading tendons through ducts or anchorages, use pulling grips designed for prestressing steel, never choker hitches or wire rope clips. Lubricate ducts with water‑based lubricants per manufacturer recommendations. Protect tendon ends with plastic caps or tape to avoid splaying wires.

Prevent Kinking and Bending

Minimum bend radii must be observed — typically 10 to 15 times the tendon diameter for strands, and larger for bars. Coils should never be straightened by bending over a sharp edge. Instead, unroll tendons from a rotating payout reel or use a coil straightener. For bars, use bending machines with proper radius dies. Hand‑bending over a knee or pipe is prohibited. Any kink or permanent set creates a weak point that can fail under tension.

Wear Appropriate Personal Protective Equipment (PPE)

Prestressing steel has sharp ends and can store elastic energy. Personnel must wear heavy‑duty leather gloves, safety glasses with side shields, hard hats, and steel‑toed boots. When cutting tendons — using abrasive saws or hydraulic shears — wear hearing protection and a face shield. Never allow bare‑hand contact with oiled or greased surfaces to avoid skin irritation, and always wash after handling.

Follow Manufacturer Instructions Exactly

Each tendon product may have specific handling limits regarding temperature, humidity, and chemical exposure. Some epoxy‑coated or galvanized tendons require extra care. Always keep the manufacturer’s data sheet accessible at the worksite. For imported tendons, check for compatibility with local environmental conditions.

Storage Best Practices for Prestressing Tendons

Even short‑term storage on a construction site can degrade tendons if not managed properly. A dedicated, well‑organized storage area is essential.

Choose a Dry, Sheltered Location

Store tendons under a roof or under heavy‑duty tarpaulins that allow air circulation. Avoid low‑lying areas where water pools. The ideal environment has relative humidity below 60% and no standing water. In coastal or industrial zones, extra protection against salt spray or airborne chemicals is required. Use dehumidifiers or desiccant packs inside enclosed storage sheds if necessary.

Elevate Tendons Off the Ground

Place tendons on wooden skids, concrete blocks, or plastic pallets at least 100 mm (4 inches) above grade. This prevents wicking of ground moisture, contact with spilled chemicals, and damage from construction traffic. For long bars, use multiple supports at uniform spacing to prevent sagging. Wrapping the supports with cushioning material (old conveyor belt, rubber matting) avoids point loading.

Organize by Type and Size

Group tendons by strand diameter, grade, and intended use. Label each stack with the material identification number, receipt date, and batch number. This streamlines inventory control and ensures the correct tendon is used for each stressing operation. Separate clean, unused tendons from cut‑offs or damaged stock. A simple log sheet hung in the storage area helps track usage.

Protect from Physical Abuse

Erect barriers such as bollards, fences, or heavy‑duty netting around the storage area to prevent forklifts, dump trucks, or excavators from hitting the piles. For vulnerable tendons (epoxy‑coated or low‑relaxation strands), wrap them in breathable fabric to shield from UV radiation and dust. Do not store other construction materials — especially chemicals, fuels, or corrosives — in the same area.

Implement Regular Inspections

Inspect stored tendons weekly for rust, pitting, discoloration, or mechanical damage. Pay special attention to contact points with supports. If surface rust appears, clean it with a brass wire brush or approved rust remover. For deeper pitting, consult the engineer; such tendons may be rejected. Maintain a written inspection log with dates, findings, and corrective actions. Any tendon with visible corrosion beyond light surface rust should not be used.

Cutting, Threading, and Tensioning: Site Practices That Impact Long‑Term Integrity

Once tendons leave storage, the handling process continues. Cutting and threading introduce new risks.

Cutting Tendons

Cut strands and wires with an abrasive cut‑off wheel or a mechanical shear designed for high‑strength steel. Never use an oxy‑fuel torch — heat destroys the temper. After cutting, deburr the ends with a file to remove sharp edges. Mark each cut length immediately to avoid confusion. For bars, use a cold saw with coolant to prevent overheating.

Threading Through Ducts

Before threading, blow out ducts with compressed air to remove debris. Apply a lubricant recommended by the duct manufacturer to reduce friction and prevent wear on the tendon. Push rather than pull whenever possible; if pulling is necessary, attach a pulling head that distributes load evenly and does not crush the strand. Check for obstructions; if a tendon becomes stuck, do not force it — remove and inspect the duct.

Pre‑Stressing Inspection

Just before tensioning, examine the full length of the tendon for any new damage. Ensure that the tendon does not cross sharp edges of anchorage hardware. Straighten any minor bends by hand (using approved methods). Verify that the wedges are clean and properly seated. Any doubt about tendon condition should prompt rejection — the cost of a failed tendon during stressing far outweighs the cost of replacement.

Environmental Controls and Corrosion Prevention

Corrosion is the primary enemy of prestressing steel. Even small pits can nucleate stress corrosion cracks under tension. Control the environment at every stage.

Moisture Management

Keep tendons dry. If stored outdoors temporarily, cover with waterproof tarps that are vented to prevent condensation. Do not allow water to pool on the cover. In high‑humidity climates, consider using a portable dehumidifier inside a storage tent. During periods of rain or snow, halt cutting and threading operations unless work can be sheltered.

Chemical Protection

Keep tendons away from chlorides (deicing salts, seawater spray), nitrates, and sulfates. Do not store near or under areas where concrete admixtures, curing compounds, or form‑release agents are handled. If tendons become contaminated, wash immediately with clean water and dry. For bonded tendons, ensure that grout is injected promptly after tensioning to seal the steel permanently. Never delay grouting beyond 24 hours (or as specified).

Temperature Considerations

Direct sunlight can heat tendons to temperatures that accelerate oxidation and affect relaxation properties. In hot climates, store under reflective covers or in shaded structures. For post‑tensioning work, avoid stressing tendons when the steel temperature exceeds 50°C (122°F). Cold weather does not damage the steel itself, but ice on the surface can trap moisture — remove ice before handling.

Training and Documentation

No amount of best practice guidelines will protect tendons if the workforce is not trained. Develop a site‑specific handling and storage procedure and conduct toolbox talks with all personnel involved. Include:

  • Recognition of tendon damage (bends, nicks, rust, broken wires).
  • Proper use of lifting equipment and PPE.
  • Emergency response for tendon failure or snapping.
  • Reporting and quarantine procedures for damaged material.

Maintain a detailed log of tendon receipt, storage location, inspection results, and usage. This documentation serves as quality assurance evidence for the client and engineer. It also helps identify recurring issues — for example, if multiple tendons show rust from a particular storage area, corrective action can be taken.

Common Mistakes and How to Avoid Them

Even experienced crews sometimes fall into bad habits. Here are frequent errors:

Storing Tendons Directly on the Ground

This causes moisture wicking, mud splatter, and damage from vehicles. Always elevate. If skids are not available, use plastic pallets or even a layer of clean gravel covered with plywood.

Using Wire Rope Clips to Handle Strands

Wire rope clips crush strands and cause permanent deformation. Use only grips approved for prestressing strand, such as Rockwell or Varco grips with smooth jaws.

Allowing Tendons to Stay Unprotected Overnight

Left on the ground or partially threaded, tendons can be damaged by weather or construction traffic. Always cover or store properly at the end of the shift.

Ignoring Small Surface Rust

Light surface rust that wipes off may be acceptable, but pitting or flaking rust indicates active corrosion. Have a qualified inspector decide. The cost of replacing a strand is minor compared to a post‑tensioning failure.

Failing to Document Handling History

When a problem arises, the root cause is often untraceable without records. A simple daily checklist can save weeks of investigation later.

Case Studies and Lessons Learned

Industry experience underscores the importance of these practices. In one documented project, a batch of strands stored for three weeks on a gravel bed without covers developed deep pitting from chlorides in the groundwater. On site, the damage was not noticed until after stressing; one strand snapped during tensioning, requiring a complete replacement of the tendon group and the associated concrete. The delay cost over $50,000.

In another case, a contractor used chains to lift a coil, creating deep gouges. During stressing, the strand failed at a gouge, causing a whiplash that injured a worker. Investigation showed that the chains had been used because “it was quicker.” These incidents are avoidable with proper equipment and training.

Conclusion

Handling and storage of prestressing steel tendons are not mere logistics — they are critical to structural safety and project success. By following best practices: using rated lifting gear, preventing surface damage, storing in dry elevated areas, conducting regular inspections, and training all personnel, you can ensure that tendons arrive at the stressing jack in the same condition they left the factory. The investment in protective measures, documentation, and quality control pays dividends in reliability, longevity, and peace of mind. For further reading, consult the PTI (Post‑Tensioning Institute) guides on handling and storage, or the Post‑Tensioning Institute website. Additionally, the American Concrete Institute (ACI) provides detailed specifications. Always refer to your project’s quality assurance plan and the manufacturer’s technical data sheet for material‑specific instructions.