Prestressed and Post-Tension Inspection
Non-destructive testing (NDT) provides critical field data to help identify visible and hidden conditions within reinforced concrete structures, supporting condition assessments without invasive investigation.
NDT Corporation provides non-destructive testing services to evaluate concrete durability, locate internal defects, and detect corrosion-related deterioration. Our skilled technicians work with engineers and asset owners to identify the appropriate testing technologies for each project and deliver accurate, quantitative datasets that support structural assessments, repair design, rehabilitation planning, and long-term asset management.
METHODS
Grout Void and Defect Identification
Inspecting Internal Post-Tensioning
Impact Echo (IE) testing is used to examine internal post-tensioning tendons and identify grout defections. Conditions such as voids or soft grout within the tendon duct can reduce corrosion protection and affect the long-term durability of the prestressing system.
During testing, stress waves are placed on the concrete near the post-tensioning duct. The resulting wave response is recorded and analyzed to determine the resonant frequency of the system. When voids or soft grout are present within the duct, the wave travel path and material response are altered, resulting in a measurable reduction in resonant frequency.
By collecting IE measurements along the length of a tendon, variations in resonant frequency can be used to identify locations where grout defects may exist.
Inspecting External Post-Tensioning
Pulse Velocity (PV) testing is used to evaluate grout conditions in external post-tensioning tendons. The method involves introducing a stress wave into the tendon duct and measuring the time required for the wave to travel between sensors.
During testing, a wave source and receiver are positioned on opposite sides of the tendon duct to measure wave transmission through the system. Changes in the recorded pulse velocity signal may indicate grout conditions such as voids, water infiltration, or soft grout within the duct.
Measurements collected along the tendon length can be used to identify locations where grout characteristics differ and where further investigation may be required.
IE/PV testing of internal PT tendons on a bridge.
Borescope Inspection
Once potential post-tensioning (PT) grout anomalies such as voids or soft grout have been identified through prior testing, access points can be created in the tendon duct to allow for borescope inspection. This typically involves drilling a small opening to insert the borescope and visually observe internal conditions.
Borescope inspection provides direct visual documentation of grout conditions within the duct while minimizing physical disturbance to the PT system.
In cases where grout conditions warrant further examination, larger access openings may be created to obtain grout samples for laboratory analysis, based on project requirements.
Acoustic Inspection Methods
Acoustic inspection of concrete pipe is performed using Impact Echo and Pulse Velocity (IE/PV) testing. This approach measures compressional and shear wave transmission velocities to document mechanical characteristics of the concrete.
When applied along a a concrete pipe, IE/PV testing can be used to identify sections exhibiting changes in wave response that may be associated with reduced wall thickness or a loss of pre-stressing force. The collected data provides objective information that owners and engineering teams can use to support asset management activities and reduce the risk of unexpected pipeline failures.
Experience You Can Rely On
Founded in 1994, NDT has built a reputation for delivering reliable data, practical insight, and responsive service on technically complex projects. Our team combines decades of field experience with advanced testing technologies to produce results that are clear, defensible, and actionable for engineers and owners.