Nondestructive Construction Inspection

Defects such as cracks, honey combing and voiding occasional occur in the construction of new concrete structures. Many of these defects can be repaired with little or no adverse affects on the performance of the structure. However, before repairs are initiated, it is important to know the extent and severity of the defects. It is also important to have a noninvasive or nondestructive means to quantify and document the effectiveness of repairs.

The quality/strength (chart above) of concrete is determined from the velocities at which the compressional and shear waves propagate through the concrete. Concrete with an average strength of 4000 to 5000 psi has average compressional wave velocity values in the range of 13,000 ft/sec and shear wave velocity values in the range of 7,000 ft/sec. Concrete with open cracks, honey combing, and voiding has lower compressional and shear wave velocity values because the waves travel through or around flaws (usually cracks). Depending on the size and severity of the flaw, the compressional velocity can be as low as 1,000 ft/sec and there may be no shear wave. Moderately low compressional and shear wave velocity values are indicative of micro or macro cracking or areas of honey combing and voiding. Dramatically lower compressional and shear wave velocity values or the loss of shear wave signal is indicative of open horizontal delamination or vertical cracks.

NDT Corporation uses sonic/ultrasonic measurements to document concrete strength and detect, map and characterize areas of cracking, honey combing, and voiding. After repairs such as epoxy injection are completed, the sonic/ultrasonic measurements are repeated to determine and document the affectedness of the repair.

Sonic/ultrasonic testing includes the following measurements simultaneously from a single energy input; velocity of the compressional and shear waves in the horizontal direction; the frequency of reflected waves which represents the velocity in the thickness direction from the back of the concrete. The combination of these 3D measurements can be used to assess the concrete strength and integrity, locate and map weak areas; identify delaminations and determine the dimensions of structural elements.

On structures where there is access to multiple sides, sonic/ultrasonic data is acquired at multiple grid nodes to develop a tomographic image. This image is used to analyze the data and produce a 3D velocity image of the interior of the structure that can be presented as 2D velocity profiles in the transverse, longitudinal or horizontal direction. On structures where there is access to only one face velocity and resonate frequency, data are combined to identify defect areas in the concrete.

NDT Corporation has successfully used the sonic/ultrasonic measurements to:

• identify areas of delamination and/voiding where closely spaced reinforcing did not allow concrete to be properly compacted by vibration;
• determine the extent of cracking and document repairs to bridge beam seat poured in cold weather;
• document repairs to a building wall poured with aggregate too large to fit between the forms and reinforcing;
• identify weak thin areas in a leaking tunnel slurry wall;
• document the strength of concrete in parking garages and bridge piers, walls and barriers to collaborate test results or when no test data were available;
• map the extent of cracked concrete due to frozen water filled tendon ducts;
• evaluate the strength and map voids in a dry pack grout coffer dam footing support.

For additional information call NDT Corporation at 508-754-0417 or check our web site www.ndtcorporation.com.