Non-destructive testing for masonry- part 1

While non-destructive evaluation (NDE) techniques have been applied to historic preservation projects in Europe as well as in other countries for many years, their use in the United States has been relatively limited. Most of the American literature about NDE is in the fields of manufacturing and civil engineering works. In manufacturing, for example, dye penetrant tests, x-ray analysis, and ultrasonic techniques aid the evaluation of welds in pipes and pressure vessels. Civil engineers use impact echo testing for the evaluation of concrete bridge decks; ultrasonic methods to determine steel thicknesses in other bridge elements; and electromagnetic equipment to verify the placement of reinforcing bars in concrete structures. NDE techniques now need to be recognized for their potential value to engineers and architects who work on historic structures. Historic construction hidden from view may be successfully understood and conditions assessed while minimizing destructive probe work. The data obtained from conventional probe techniques are generally more limited in accuracy because the data is collected at discrete locations and must be interpolated to estimate the conditions at points between the probes. While it may not be possible to eliminate completely the use of conventional methods to confirm data, the amount of invasive work can be minimized by the implementation of NDE within historic preservation projects. NDE techniques are particularly useful in historic preservation because original structural drawings are often unavailable or are deficient in detail and do not reflect "as built" conditions. The location and size of framing members and load-bearing elements can usually be determined through NDE. The condition and integrity of a building's structural members can also be determined with the aid of NDE techniques, thus avoiding destructive probe methods, such as test pits, material removal, and core drilling. This is important since conventional probing not only destroys historic fabric, it can be disruptive to the users of the building, and often requires some degree of repair work to reinstate the previous appearance and integrity of the affected building component. NDE available for use on historic masonry structures include radar (also referred to as impulse radar), impact echo, ultrasonic pulse velocity, spectral analysis of surface waves, electromagnetic detection, infrared thermography and fiber optics. Recent work on the New York State Capitol and on Whig Hall at Princeton University (and detailed in the two case studies that follow) demonstrates the practical application of NDE in historic preservation projects

Whig Hall, Princeton University
Whig Hall is located at Princeton University, in Princeton, New Jersey. An adaptation of a Greek Temple, the two story structure features an entrance portico with full entablature and a simple pediment supported on six fluted marble columns. Along with its adjacent twin, Clio Hall, Whig Hall was constructed of Vermont Montclair Danby Marble and completed in 1893. Designed by A. Page Brown, the buildings originally housed two rival, campus debating societies.

Problem
The portico of Whig Hall is primarily constructed of unreinforced, load-bearing masonry. Weathering over the past 100 years and a major fire resulted in extensive cracking of the column bases and plinths; sugaring, erosion and pitting of marble surfaces; and erosion of mortar joints. Although initial reports commissioned by the university concluded that the portico substructure and columns should be replaced, less intrusive methods of repair were sought.

Evaluation
Robert Silman Associates, P.C., was retained in 1994 to evaluate repair alternatives rather than undertaking a total reconstruction. In order to perform a load analysis of the portico structure, it was necessary to document the effective cross section of the columns. A program utilizing NDE techniques was established for both Whig and Clio Hall, although only the results of Whig Hall are presented here. The purpose of this evaluation was to determine the depth, extent, and nature of masonry cracking. Column A of Whig Hall, which measures 22 feet in height and 2 feet, six inches in diameter, appeared to have the most significant cracking. There was considerable concern for the integrity of this column and the unknown nature of the cracks. Initial condition investigations of the stone elements and the supporting structural members of the portico included a visual examination as part of the survey of the entire exterior of the building. All major elements were examined using "cherry-pickers," scaffolding, and ladders. The examination focused on the cracked pieces of stone at the entablature level; the cracked or spalled column capital volutes; the cracked stone columns; the cracked stone column bases and plinths; the deteriorated pieces of masonry that were immediately below or adjacent to the column plinths; and the deteriorated support structure below the portico floors. At Whig Hall, Column A exhibited severe surface cracking, and an extensive internal crack pattern within the plinths. Additional cracking was noted at the bases of a number of other column shafts. Some deterioration of the masonry walls beneath the column plinths was also observed, although without signs of differential movement between different portions of the portico foundations. Following this initial evaluation, the project team, including a stone conservator and stone fabrication specialist, recommended that the column bases and plinths be replaced; the upper portion of the column foundation system be reconstructed; and that localized stone repairs occur where cracks and/or spalls existed. This repair and restoration plan would require in situ support of the columns and pediment while their bases and plinths would be replaced and repairs conducted underneath. To better establish a probable scope of work, a more thorough study of the column interiors was undertaken using ultrasonic pulse velocity and impulse radar investigation. It was hoped that these techniques could more accurately determine areas of failure, voids, or other indications of stress.

Solution (Test Results)
NDE was undertaken by a firm specializing in the evaluation of older buildings, G.B. Geotechnics of Cambridge, England. They performed a survey of both porticos over a three-day period using a combination of ultrasonic pulse velocity and impulse radar. The Whig Hall project benefited from the previous NDE work at the New York State Capitol; however, here, only two tests were determined to be appropriate. Additionally, a single firm that offered a range of NDE services was used.

Ultrasonic Pulse Velocity
The ultrasonic pulse velocities recorded for the marble were in the range of 0.157 to 0.256 inches per microsecond. In some areas of the column shafts, high amplitude internal reflections were noted, indicative of discontinuities or cracks within the column. The orientation of the various cracks, e.g., horizontal, was identified. The variable depths of the cracks were calculated, with the average maximum depth being about 2 inches.

Impulse Radar
The radar readings indicated that the column shafts were constructed of monolithic marble, with the same type of marble used for the column bases and capitals. The radar detected anomalies near the junctions of the column shaft with the base and the capital. The readings also indicated that there may be cramps or ties between the columns and the bases/capitals. It could not be determined whether the objects detected were metallic, or if they were possibly "Lewis holes" that had been used to maneuver the stone blocks during construction. Magnetic detection equipment with the ability to penetrate large diameter columns could have been used in conjunction with the radar to verify the nature of the internal anomalies.

Project Summary
The consulting NDE firm provided a written report describing its methodology and findings; it also provided graphics to assist the engineer, architect and conservator in the interpretation of the results. To help maintain the integrity of the masonry during the repairs, it was recommended that Column A, as well as several areas of other stone elements be pinned. It was necessary to support the column shafts while the bases and plinths were removed. This could be accomplished by the use of high strength steel pins through the column shaft, or the placement of steel clamps around the column shaft to establish a friction hold around the column.



The non-destructive evaluation of Whig Hall allowed the diagnosis of the condition of the masonry and the specification of a relatively low impact repair.

Conclusion
The case studies provided different perspectives on the use of NDE. At the New York State Capitol, the primary purpose was to research nondestructive methods of locating and imaging hidden structure in a massive masonry building. The research project supplemented information acquired during a visual survey with destructive probe work. At Princeton's Whig Hall, the specification of NDE was part of a contract to assess structural capacities and design repairs, and to prepare construction documents. The point of the comparison is that there are techniques currently available and experienced firms to perform field investigations, but continued research is needed. Ways to improve the portability and simplify the use of equipment should continue in the manufacturing community. Additional applications and improvement of accuracy for the techniques should be researched through a combination of academic, non-profit research, and commercial organizations. Standards and guidelines need to be developed to aid the architect, engineer, or conservator in the specification of NDE in historic preservation projects. It is hoped that through the combined efforts of preservation professionals, researchers, and equipment manufacturers and operators, that NDE will become generally available and increasingly economical for use on landmark structures. For projects where these techniques should be implemented, the preservation professional must impress upon the client the need for NDE in conjunction with limited conventional probes before beginning project work.