Masonry Magazine February 2006 Page. 36
Technology
Thermal sources can include the sun (solar energy) or an artificial heat source, such as a heater or heat lamp. Optimum conditions for IRT testing occur when there is a combination of both, such as testing an air-conditioned building in the summer on a hot sunny day.
Thermal sources can include the sun (solar energy) or an artificial heat source, such as a heater or heat lamp. Optimum conditions for IRT testing occur when there is a combination of both, such as testing an air-conditioned building in the summer on a hot sunny day. In this example, the exterior masonry surfaces are exposed to solar energy and develop high exterior temperatures, while the interior masonry surfaces are exposed to lower-conditioned interior air temperatures. The thermographic image would thus display hollow cells as hot (as they emit the thermal energy) and the solid grouted cells as cool (as the energy is transferred through the wall).
A sample photograph from IRT is illustrated in Figure 2 below. Note the hollow cells appear white (hot), while the solid cells appear black (cold). In this example, the thermographer can rapidly and easily assess the grout placement, particularly the grout voids and misalignment of grouted cells. With the use of a camcorder linked to an infrared camera, survey data can be recorded, taped and printed with ease for use in evaluating conditions.
Selecting the Best Test Method
NO SINGLE TEST method is applicable for all conditions and all construction types, and often several methods combined with selective destructive testing is required to evaluate a problem.
SPR testing is often limited by voids or air spaces within construction. The signal will not penetrate air spaces in typical masonry cavity wall construction; therefore, the method is often limited if access to the back-up wythe is not available.
Likewise, IRT is very dependent on the thermal conditions of the element being tested. If a sufficient thermal gradient cannot be developed across the component, IRT will be ineffective.
It is advisable to plan for some destructive testing to verify the results obtained from the nondestructive testing method selected. Probe openings will be required to calibrate equipment to accurately assess concrete cover depth. Probe openings are also required to verify reinforcement bar sizes, which cannot be determined nondestructively through the methods discussed.
Summary
NONDESTRUCTIVE TEST methods can be extremely valuable in the assessment of masonry structures, permitting the collection of more comprehensive data than could be collected through destructive testing alone. The test methods discussed can also be used for quality assurance testing and inspection of work in progress, as well as verifying as-built conditions of existing structures.
Gerald A. Dalrymple, P.E., is a principal of Whitlock Dalrymple Poston & Associates Inc. in Manassas, Va, a consulting engineering firm that specializes in evaluation, repair and restoration of existing structures. As a principal at WDP, Dalrymple is involved in the evaluation and repair of existing masonry and concrete structures suffering from water penetration, corrosion damage and material failures. His primary duties include repair design, bid solicitation and construction administration of full-scale repair programs. He can be reached via e-mail at adalrymple@wdpa.com, or by calling (703) 257-9280.