Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsGenerally, higher pulse velocities indicate good quality material, while decreasedvelocities or poor return signals (decreased high frequencies) indicate poor quality,such as voids, cracks, or deterioration. A large number of transducers used aroundthe circumference of the conduit will improve the accuracy and the resulting imageresolution.The pulse-velocity method is widely used and has provided reliable in situdelineations of the extent and severity of cracks, areas of deterioration, and generalassessments of the condition of concrete structures for many years. The equipmentcan penetrate thick concrete sections with the aid of amplifiers, is easily portable, andhas a high data acquisition-to-cost ratio. Although most applications of thepulse-velocity method have been under dry conditions, the transducers can bewaterproofed for underwater surveys. Tests have shown that the pulse-echo systemis capable of delineating sound concrete, concrete of questionable quality, anddeteriorated concrete, as well as delaminations, voids, reinforcing steel, and otherobjects within concrete. Also, the system can be used to determine the thickness ofa concrete section in which only one surface is accessible. The system will work onvertical or horizontal surfaces. However, the present system is limited to a thicknessof about 1.5 feet with only one side accessible. For maximum use of this system, theoperator should have had considerable experience using the system and interpretingthe results (USACE, 1995b, p. 2-17).The pulse echo is limited to about a 1 foot thickness. The pulse-echo method is avariation of the pulse-velocity method and is best suited for characterizing voids andcracks parallel to the conduit surface. The pulse-velocity method is suitable fordetecting cracks and voids in other directions. The pulse-echo method can be usedfrom a single face of the conduit, whereas the pulse-velocity method requires accessto both faces of the conduit. Both methods are commonly used concurrently to geta complete evaluation of the conduit (Promboon, Olson, and Lund, 2002).Note that recent MASW (section 10.1.1) and GPR (section 10.4) advances haveallowed multiple different imaging methods to be used on a given site, allowinggreater interpretation confidence in difficult problem areas.10.6.1 Ultrasonic thickness surveyThe thickness of an existing metal or steel pipe can be measured using an ultrasonicthickness survey. The survey is conducted by using a pulse-echo ultrasonic thicknessgauge (figure 146). An ultrasonic gauge determines the thickness of metal or steelpipe by accurately measuring the time required for a short ultrasonic pulse generatedby a transducer to travel through the thickness of the pipe, reflect from the back orinside surface, and be returned to the transducer. Different types of materials havedifferent acoustic velocities. The advantages of performing an ultrasonic survey are:256