Definitions Radar/Impulse Radar: Electromagnetic waves in the band from 50 megahertz to 1.5 megahertz are pulsed into a material by means of a transducer and read by an antenna receiver. In this technique, the receiver reads signals reflected off changes in materials, voids, or buried objects. The raw output display shows the velocity at which the transducer is moved across the surface as the horizontal axis. The vertical axis is the time, in nanoseconds, required for the waves to travel through the material, reflect off an anomaly, and be picked up by the receiver. Access to both sides of the test material is not required. Impact Echo: A sophisticated version of "sounding" a material enables the user to make judgments about internal conditions. This technique involves a hammer striking a masonry surface, with a receiving transducer located near the striking point. The hammer and receiver are wired to a computer that records the input energy from the hammer and the reflected compression wave energy from the receiver. The output is processed by computer using Fast Fourier Transform operations to produce a frequency domain display where reflections (or echoes) are seen as peaks in the wave signals. This technique has been used extensively to locate voids and defects in concrete slabs and bridge decks. With regard to overall soundness of the masonry, generally speaking, the denser the material, the higher the wave velocity response. It should be noted that this is a qualitative measurement that is useful in surveys for comparative purposes. The thickness of a material can be readily determined if the characteristic wave velocity is known (see Ultrasonic Pulse Velocity, below). Ultrasonic Pulse Velocity: Also referred to as pulse velocity, this is a standard ASTM test for assessing concrete (ASTM C597-83). This technique uses a transmitter and receiver to pass ultrasonic energy (frequency greater than 20 megahertz) directly through test material. In general, the faster the velocity, the denser the material. If certain engineering properties of the material are known, such as the modulus of elasticity, other mechanical properties can be empirically computed. This technique is also useful for obtaining characteristic wave velocities for various sound masonry materials that can be used in conjunction with other tests, such as impact echo and spectral analysis of surface waves. Accessibility to both sides of the material is required. Spectral Analysis of Surface Waves: This seismic NDE method was developed to read shear wave velocities and modulus profiles in layered systems, such as pavements and earth where access to the material is only available on one side. It is useful for tasks such as locating delaminations parallel to the surface, and measuring the thickness of a slab on grade or the thickness of multiple layers. Since identical equipment is used for the spectral analysis of surface waves and impact echo techniques, the two tests are often undertaken in concert. The surface of the test material is struck with a hammer at a point adjacent to two receivers (versus one point for impact echo). As with ultra pulse velocity, the higher the measured velocity, the higher the material modulus and, thus, the better the quality of the masonry. The information provided is qualitative; transformation of the wave data into the frequency domain user Fast Fourier Transform methods provides information more closely related to other mechanical properties of the test material. Electromagnetic Detection: Commonly used for locating rebar, this technique involves passing an alternating current through a coil to generate a magnetic field. When a magnetic material is encountered, the field is disturbed. The magnitude is related to the size of the object and proximity to the probe. This NDE technique is recommended for use in conjunction with other techniques to determine the presence of hidden metallic objects. Infrared Thermography: Also known as heat imagery, this method has often been used to perform energy efficiency studies of buildings, and to evaluate roofing membranes. The operating principle is that an object having a temperature above absolute zero will radiate electromagnetic waves. Wavelengths fall within certain bands, depending on temperature. Wavelengths at room temperature are outside the visible spectrum, while those at very high temperatures are shorter and fall within the visible spectrum. Cameras or video equipment are used to photograph the surface temperature of the subject. The resulting photograph or video images indicate surface temperature variations, which are also affected by ambient humidity, time of day, and other micro-environmental factors. In masonry construction, the different wavelengths often indicate the presence of moisture. The technique can distinguish between dense and porous or deteriorated masonry, as well as identifying voids in walls, such as flues. Fiber Optics: This technique allows the viewing of the interior of inaccessible areas by inserting a fiberscope (a bundle of flexible optical fibers) or a borescope (a bundle of rigid optical fibers) into the void. Both carry the high intensity light along their length. Some manufacturers have coordinating lines between their "structure scopes" and optional camera equipment. While the technique is marginally useful for solid masonry structures with limited voids, it has been used successfully on masonry cavity wall construction.
Site Map Home | Search | E-mail | Technical Preservation Services | NPS History & Culture
| |
