Insights

Post-tensioned (PT) System concrete bridges play a vital role in today’s transportation networks. Many serve dense urban cores, airports and high-volume corridors where reliability is critical. While PT systems offer significant structural advantages, they also pose inspection challenges that differ substantially from those of conventional reinforced concrete and steel bridges. Some of the most serious risks develop inside the structure, hidden from view during routine visual inspections. 

B&N helps owners uncover potential hidden risks so they can avoid surprise shutdowns, protect public safety, and make informed, economical decisions about when to repair, strengthen or even replace their post-tensioned (PT) bridges. This work requires specialized experience, targeted testing and a deep understanding of how PT system bridges behave over time, particularly in older structures. 

What is a Post-Tensioned System Bridge? 

Post Tensioned (PT) System concrete bridges are structures that the industry has been building for more than 50 years. Unlike prestressed concrete, where high-strength steel is tensioned before concrete placement, PT steel is stressed after the concrete has been placed and cured. 

By tensioning high-strength steel strands and strategically positioning them within the concrete, PT increases the strength and load-carrying capacity of concrete. This allows bridges to span longer distances between piers and abutments, often making PT concrete a cost-effective solution for medium- to long-span bridges. 

An internal PT tendon typically consists of: 

• An anchorage at each end of the tendon to maintain tension. 
• Multiple seven-wire high-strength steel strands. 
• In older structures, a steel pervious duct encasing the tendon. 
• Cementitious grout that fills the duct and protects the steel from contaminants, especially water, bonding the steel throughout the entirety of the tendon duct. 

More recently designed PT System bridges may include both internal and external tendons. External tendons function similarly but are encased in impervious high-density polyethylene (HDPE) ducts and are located outside the structural concrete, making them significantly more accessible. 

Regardless of tendon type, the greatest concern remains the same: deficiencies such as water intrusion, voided grout and corrosion that are often not visually obvious but can significantly reduce load-carrying capacity if left undetected and not addressed in a timely manner. 

Why Post-Tensioned Bridge Inspection is Increasingly Important 

PT concrete bridges have grown steadily in prevalence across the United States over the last two decades. Concrete is often the least cost alternative for medium- to long-span bridges and, when properly constructed, provides a durable solution.

Many PT bridges in service today were built before the early 2000s, when construction standards and detailing differed from current practices. These older structures often lack the flexibility of modern designs, such as provisions for adding tendons if capacity is reduced. As a result, once deterioration begins, repair options are typically limited and costly.

This has become a reality for many public agencies. For example, when B&N assessed PT bridges for the Ohio Department of Transportation, several structures required further evaluation after initial inspection. Identifying deficiencies early allowed owners to pursue targeted, cost-effective rehabilitation rather than reacting to advanced deterioration. 

Common Deficiencies in Post-Tensioned Structures 

Modern PT System bridges may include additional details that allow owners to add ducts or tendons if load-carrying capacity is reduced. Structures built prior to the year 2000 typically do not include these features. Once capacity begins to decline, deterioration and, therefore, repair costs can escalate quickly. Common deficiencies include: 

Vented High Points in Tendon Profiles 

PT tendons often contain high points in regions of negative bending moment. At these locations, air voids can form as water separates from grout during placement. Over time, these voids compromise the ability of the grout to protect the steel. 

Concrete decks will typically crack with age. Many tendon high points are located near the roadway deck surface, where water and contaminants can more easily migrate through cracks and enter the duct system. Where grout is missing or compromised, steel strands may corrode, reducing capacity.  

Anchorage Deficiencies 

Anchorages are usually located at high points and may have similar void-related issues. Anchorage heads grip the strands and maintain tendon force. If an anchorage deteriorates or fails, the entire tendon may lose significant capacity.

These conditions were a focus of post-tensioned bridge inspections completed by B&N for the City of Cincinnati. On two City bridges, invasive testing and non-destructive testing were used to evaluate internal tendon conditions and anchorages. Identifying issues early allowed the City to plan rehabilitation and maintenance strategies before capacity loss became critical, avoiding potentially costly future outlays. 

Limited Time for Remediation 

Highly tensioned steel deteriorates more rapidly than conventional mild reinforcement once corrosion initiates. If multiple tendons are affected, bridges can quickly approach critical condition, leaving little time for remediation and increasing the likelihood of major rehabilitation or replacement. 

A Proactive, Data-Driven Approach 

To avoid these outcomes, B&N takes a proactive approach to PT bridge inspection. Before any fieldwork begins, B&N conducts a detailed literature review, including: 

• Original drawings 
• Previous inspection and testing reports 
• Construction records and repair documentation 
• Available photographs and historical information 

This review provides insight into construction practices and standards at the time the bridge was built. Because different eras followed different designs, grouting materials and grouting practices, historical data can be very helpful in predicting locations where deficiencies are more likely to occur. 

This approach was particularly important during the inspection of the Boise Airport Terminal viaduct, a PT box girder bridge carrying vehicular departure and drop-off lanes. Understanding the original tendon profiles and construction details allowed targeted invasive testing at tendon high points in the PT crossbeams and pier caps, where grout samples were collected and evaluated. 

Field Inspection and Testing 

Once in the field, inspection efforts focus on identifying distress indicators and verifying internal conditions. 

Tendon Profiles and Deck Cracking 

Using information from the literature review, inspectors locate tendon high points and examine the concrete deck above them. Cracking or distress along these profiles can indicate internal tendon issues and the need for further investigation. 

Invasive Testing at High-Risk Locations 

Ground Penetrating Radar (GPR) and pachometers are used to accurately locate reinforcing steel, strands and ducts. Small drill holes allow access to the duct system. If the grout does not fully fill the duct, a borescope is inserted to visually assess the internal conditions. 

This testing provides direct evidence of exposed strands, corrosion, section loss, grout quality and the presence of water. These methods were used extensively on the City of Cincinnati bridges, where detailed narrative reports and photographic documentation supported rehabilitation planning and cost estimation. 

Anchorage Investigations

Anchorage heads are evaluated by removing concrete blockouts surrounding the anchorage heads as needed and using borescopes to inspect grout ports and anchorage hardware. Because anchorages are often more exposed to environmental conditions, they require additional scrutiny.

Where appropriate, additional protective measures can be applied to seal anchorage areas and prevent future deterioration.

Why This Work Matters

With the growing and aging inventory of PT concrete bridges in the U.S., this specialized inspection and testing work has become increasingly important. Identifying deficiencies early allows owners to plan cost-effective repairs, avoid emergency closures and extend the service life of their structures.

Ultimately, uncovering hidden risks inside PT bridges is more than protecting infrastructure investment; it is about protecting the public and ensuring these critical structures continue to serve their communities safely and reliably.