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MATERIAL MATTERS Continued from page 17
and 50% were constructed using pre- stressed concrete, where the concrete's load-bearing capacity is increased with embedded reinforcing steel tendons un- der tension. Prestressed concrete struc- tures can be built with pre-tensioned concrete beams, where the concrete is cast in a mold around lengths of steel cables or rods that are stretched, and then released once the concrete sets; or con- structed of post-tensioned concrete beams, where the steel tendons are pulled through ducts in the hardened concrete and then stretched, and the ducts are ÅTTML _Q\P OZW]\ IN\MZ_IZL \W XZW\MK\ \PM steel tendons. Corrosion of the steel reinforcement,
typically caused by water and chlorides from road deicing salts and/or seawater environments that enter the concrete through surface cracks, can affect the service life as well as the safety of the entire structure. For post-tensioned concrete struc-
tures, corrosion of the steel tendons can occur if voids are present in the protective grout and the tendons are exposed to _I\MZ IVL KPTWZQLM[ \PI\ PI^M QVÅT\ZI\ML these voids. A number of corrosion- related tendon failures, often associated with grouting problems, have occurred in post-tensioned concrete bridge structures over the past two decades, which have either led to the structure's collapse or significant reduction in strength that could have resulted in a collapse. Assessing the condition of the steel in
these structures is particularly challenging because few NDT inspection tools lend themselves to inspecting tendons in ducts buried deep in concrete. Often, early signs of deterioration are not detected. The CROSS-IT project, which aims
to reduce the incidence of steel compo- nent failures in embedded concrete and increase the safety of bridges, buildings, and other structures designed for the public use, will utilize both ground pen- etrating radar (GPR) to locate the rein- forcing steel embedded in the concrete, and ultrasonic guided waves (UGW) to inspect the steel for signs of corrosion or degradation.
18 MATERIALS PERFORMANCE November 2012 According to Kamer Tuncbilek, se-
nior project leader with TWI, the unique aspect of the project is utilizing radar technology to locate steel tendons in the concrete so they can be assessed with the guided wave technology. Positioning of the tendons in concrete beams can be complex, with prestressing steel cables frequently situated on top of each other. Tuncbilek notes that the precise loca-
tion of the steel cables may differ from the bridge's original design due to the nature of prestressed concrete beam construc- tion, and differentiating one tendon from IVW\PMZ KIV JM LQNÅK]T\ ;PM KWUUMV\[ that using UGW to evaluate steel tendons in concrete has required drilling through the concrete to access the tendons and conduct the guided wave test along the length of the tendon. The researchers' goal is to conduct the test and assess the health of the tendons without damaging the concrete. During the two-year project, NTUA
will develop the GPR technology, which uses electromagnetic waves to create a real-time image or map of objects that are visually hidden beneath a surface. GPR typically works by emitting radio- frequency electromagnetic pulses from an antenna into a subsurface. As the pulses or waves move through the subsurface UI\MZQIT \PMa IZM ZMÆMK\ML JIKS \W \PM antenna from the location where there is a contrast in the subsurface's conductivity caused by a change in material. The penetration depth and resolution quality of the waves are determined by the an- tenna size and the subsurface material's electrical properties. The technology being developed for this project is ex- pected to penetrate concrete depths of up to 500 mm (~20 in). The UGW method, an accepted NDT
technique for inspecting metal pipelines to determine local pipe wall damage, commonly utilizes mechanical stress waves with very low ultrasonic frequen- cies (usually between 10 to100 kHz) transmitted by ultrasonic transducers in contact with a metal structure. The gen- erated waves propagate along the length of a structure while being guided by the
boundaries of the structure's shape.