Post Tension Slab Failure Modes and Effects

Post-tensioned concrete slabs have become increasingly popular in commercial and residential construction over the past few decades. By prestressing the concrete with high-strength steel tendons or cables, post-tensioned slabs can span greater distances without intermediate supports and minimize cracking. However, these systems are vulnerable to failure if not designed, installed, and maintained properly. Slab failure can result in serious structural damage and safety hazards.

By understanding the mechanisms and modes of failure, building professionals can take steps to avoid potential problems.

Causes of Post-Tension Slab Failure

There are several factors that can lead to failure in post-tensioned concrete slabs:

Corrosion

Corrosion of the prestressing steel cables and anchorage components is a major cause of failure. Moisture infiltration, carbonation, chlorides, or stray electrical currents can corrode the steel over time and cause it to lose strength and eventually fracture. Lack of adequate concrete cover over the cables also increases corrosion risk.

Concrete Deterioration

Cracking, spalling, efflorescence, and other deterioration of the concrete itself can progressively weaken the slab. Contributing factors include freeze-thaw cycles, reactive aggregates, construction errors, and inadequate curing of the concrete.

Construction Defects

Mistakes during design or construction such as under-designed tendons, excessive prestress forces, improper tendon profile, inadequate anchorage, and poor workmanship can lead to failure. Using incorrect materials or deviating from specifications also compromises performance.

Overloading

Exceeding the design loads, improper load distribution, dynamic loads, and addition of unanticipated weights can overstress the tendons and anchorage, resulting in failure.

Ground Movement

Settlement, heave, lateral movement, slope instability, and seismic activity can impose excessive stresses on post-tensioned slabs not designed to accommodate ground displacement.

Fire Damage

Fire exposure can severely degrade the strength of both the concrete and the prestressing steel. Even if the structure remains standing, fire damage requires assessment to determine residual load capacity.

Signs and Symptoms of Post-Tension Slab Failure

Recognizing the warning signs of an impending failure can help avoid catastrophic collapse. Warning signs include:

Cracking

Cracking in a post-tensioned slab, especially over or near tendon positions, can indicate overstressing, seismic damage, foundation movement, or material deterioration. Cracks require further evaluation by an engineer.

Leaking

Water infiltration through cracks may signal deterioration of the concrete or compromised waterproof membranes. Leaking can lead to corrosion of internal steel components.

Spalling

Spalling and delamination of the concrete surface may be caused by excessive prestress, reinforcement corrosion, aggregate reactivity, or freeze-thaw damage. It decreases concrete cover over internal tendons.

Deflection

Excessive sagging or deformation under live loads can mean the slab is overstressed and at risk of collapse. Deflection should not exceed code limits.

Floor Vibration

Perceptible floor vibration could result from loss of stiffness due to hidden tendon damage or anchorage slippage. This warrants further evaluation of the slab's structural integrity.

Consequences of Post-Tension Slab Failure

Failure of a post-tensioned slab can have serious consequences including:

Safety Hazards

Sudden collapse poses an immediate danger to the safety of occupants and can result in injuries or fatalities.

Structural Collapse

Local failure of a slab can progress to collapse of floors above or destabilize adjacent structural elements. This may lead to partial or total building collapse.

Water Damage

Broken tendons or severed pipes may lead to large volumes of water leakage into the building interior, resulting in extensive damage to contents.

Expensive Repairs

Remediation of a failed post-tensioned slab is complex and costly, often requiring complete slab replacement. Associated damage can add substantially to repair costs.

Preventing Post-Tension Slab Failure

While post-tensioning defects cannot always be prevented, the following practices help minimize failure risk:

Proper Design

Work closely with qualified structural engineers experienced in post-tensioned slab design to comply with applicable codes and standards.

Quality Materials

Use high-strength, non-reactive concrete and corrosion-resistant prestressing steel along with effective waterproofing measures.

Skilled Installation

Hire experienced contractors thoroughly trained in post-tensioning techniques to properly place and tension the tendons.

Regular Inspections

Inspect slabs regularly for signs of deterioration, corrosion, cracking, leakage, or deflection issues.

Load Testing

Conduct load tests at project completion and periodic intervals to verify slab performance under live loads.

Maintenance

Promptly repair any water intrusion, concrete damage, or corrosion to prevent further deterioration.

Forensic Investigation of Failures

When a post-tensioned slab does fail, forensic engineers employ various methods to determine the cause:

Visual Inspection

Inspection of damage patterns, concrete deterioration, and cable corrosion provides clues to the sequence and mechanism of failure.

Corrosion Testing

Electrical resistance and carbonation testing check for section loss in prestressing steel due to corrosion.

Core Samples

Extracting concrete core samples tests compressive strength and identifies internal defects like honeycombing or inadequate consolidation.

Load Testing

Applying test loads to slabs can diagnose issues with excessive deflection, tendon slippage, or loss of prestress.

Computer Modeling

Advanced 3D finite element analysis models help determine critical failure loads and simulate collapse mechanisms.

Repairing Failed Post-Tensioned Slabs

Repair options for failed post-tensioned slabs include:

Partial Slab Replacement

Removing and replacing damaged portions while keeping surrounding slabs and tendons intact.

Additional Post-Tensioning

Installing supplemental tendons to reinforce slabs near collapse.

External Post-Tensioning

Applying external post-tensioning on the slab surface to improve capacity.

Bonding carbon fiber sheets or plates to the slab soffit can enhance stiffness and strength.

While prevention is ideal, early detection along with proper investigation and repair of post-tension slab failures can mitigate damage and avoid catastrophic collapse. Careful design, construction, inspection, and maintenance is critical for the safe, long-term performance of these structural systems.