NDT in Construction and Infrastructure

What can go wrong, and why it matters

Structures fail rarely, but when they do the consequences are public, costly and sometimes deadly. Much of what determines whether a steel frame or concrete deck performs is invisible once construction moves on. A weld can look acceptable on the surface while hiding lack of fusion or cracking inside. Reinforcing steel can end up with too little concrete cover. A void can form inside a grouted duct. Once the next floor goes up, or the deck is paved, these conditions are very hard to reach.

Finding problems during construction is almost always cheaper than finding them later. A rejected weld repaired in the fabrication shop costs hours. The same defect discovered after erection can mean shoring, access equipment, schedule delay and disputes over who pays. Discovered decades later in service, it can mean load restrictions or closure.

Where NDT is used

Structural steel. Welded connections are the heart of steel construction, and they receive the most inspection.

• Visual testing (VT) by a qualified welding inspector covers fit-up, preheat, welding procedure compliance and the finished weld profile. On most projects it is the inspection applied to every weld.
• Ultrasonic testing (UT) is the usual volumetric method for complete joint penetration groove welds in thicker members, such as moment connections and column splices. Phased array UT is increasingly used for its imaging and recorded data.
• Magnetic particle testing (MT) finds surface and near-surface cracks in fillet and groove welds and is often specified at critical connections or where cracking is a known risk.
• Liquid penetrant testing (PT) serves the same purpose on non-magnetic materials such as stainless steel and aluminum.
• Radiographic testing (RT) is used on some shop welds, but radiation safety and access make it less common on an active job site than UT.

Concrete and masonry. Concrete cannot be inspected like steel, but a family of methods helps engineers see inside it:

• Ground-penetrating radar (GPR) and electromagnetic cover meters locate reinforcing steel, post-tensioning tendons and conduits, and estimate cover depth. They are also used before coring or cutting to avoid hitting embedded steel.
• Ultrasonic pulse velocity and impact-echo help assess concrete uniformity and locate voids, honeycombing and delaminations.
• Rebound hammer readings give a quick comparative indication of surface hardness, useful for mapping areas that deserve closer investigation.

Bolts, anchors and other components. Ultrasonic methods can check bolt length and anchor rod integrity, and visual inspection verifies bolting procedures. For bridges and older structures, thermography and acoustic sounding help find delaminated concrete, and ultrasonic thickness measurement tracks section loss from corrosion.

How inspection fits into a project

On a typical building or bridge project, inspection happens in layers.

Fabrication. Steel is usually welded in a fabrication shop under the fabricator's own quality control, with NDT at the frequency the contract documents call for.

Erection. Field welds and bolted connections are inspected as the frame goes up. Building codes commonly require special inspection of certain structural work, performed by an inspection agency working for the owner rather than the contractor, so the person who checks the work is independent of the person who built it.

Acceptance and records. Rejected welds are repaired and re-inspected, and the reports become part of the project record. Good records matter long after handover: they are what an engineer reaches for when the building is altered, damaged or questioned.

In service. Bridges in particular are inspected on a recurring cycle, with more detailed hands-on inspection of members whose failure could affect the whole structure. NDT supports those inspections where visual checks raise questions.

Codes and standards

• AWS D1.1, the Structural Welding Code for steel, is the reference for welding and weld inspection on most steel buildings in the United States. Related codes cover bridges (AWS D1.5), aluminum (D1.2) and seismic supplements (D1.8).
• AISC specifications for structural steel buildings, including provisions on quality control and quality assurance.
• The International Building Code, adopted in many jurisdictions, which sets out special inspection requirements that local authorities enforce.
• ASTM test methods for concrete and steel, and the ACI guidance documents on nondestructive evaluation of concrete.
• The National Bridge Inspection Standards, administered by the Federal Highway Administration, which govern bridge inspection programs for public highway bridges.

Personnel who perform UT, MT and PT on these projects are commonly qualified under a written practice following ASNT SNT-TC-1A (Level I, II and III), and visual weld inspection is often performed by an AWS Certified Welding Inspector. Always check which edition of a code your contract actually invokes; they differ.

What this means for owners and developers

Owners often assume inspection "just happens." It does, but its quality varies. Before construction, make sure the contract documents say which welds and elements get which inspection, at what frequency and to what acceptance criteria. During construction, make sure the inspection agency is independent, its personnel are qualified for the methods they perform, and reports reach the engineer of record promptly rather than at the end of the job. At handover, keep the records.

For existing structures, NDT is a way to answer specific questions — how much section has corrosion taken, where is the reinforcing steel, is that crack growing — before committing to repairs or demolition.

How NDT LLC can help

NDT LLC provides Certified Welding Inspection, NDT program and procedure development, and independent quality assurance of inspection work. If you are planning a project, reviewing an inspection agency's reports, or investigating an existing structure, contact us to talk it through.