Nondestructive Evaluation for Historic Preservation
Information gathered during the visual field investigation:
• helps to define the scope of repairs,
• determines whether components and systems are serviceable or must be replaced,
• uncovers opportunities to conceal new improvements.
An investigation usually begins with a walk-through survey of the entire site and structure. The survey, performed from grade and accessible building areas using binoculars, is used to ascertain the overall condition and configuration of the building, as well as to develop a plan for more detailed inspection of areas with visible signs of distress.
A close inspection can then take place, documenting all or selected areas of the building. This is the time to plan further inspection openings to expose concealed conditions.
An inspection of building systems may address all or only some elements and features of the building, including site and subsurface conditions; mechanical, electrical, plumbing, and structural systems; the superstructure; exterior wall enclosures, including roofs, chimneys, and drainage systems; walls, floors, and ceiling finishes; and interior and exterior architectural components.
The inspection can be organized with both elevation drawing documentation and in a schedule format for elements such as windows, doors, and hardware. The as-built drawings, photographic documentation, and schedules that are generated as a result of the inspection can subsequently serve as the basis for planning and design, as well as for the preparation of construction documents.
Information gathered in the field survey enables the design team to formulate a scope of work for further field testing. Field testing uses probes to examine concealed conditions, providing potentially valuable information on the existing conditions and the nature, causes, and extent of building deficiencies. Field-testing methods may be invasive or noninvasive.
Nondestructive evaluation techniques use methods that do not require openings or destruction of historic fabric. For example, metal detectors can be used to locate hidden or embedded structural members and anchors. Sounding with a rubber mallet is an effective way to identify areas of subsurface delamination of materials, such as terra cotta and stone.
Water and air infiltration studies can be used to locate leaks in roofing systems, mechanical ducts, and through doors and windows. Noninvasive testing is generally cost-effective, because it does not require disruption and repair of finish material.
The following nondestructive investigation techniques are state-of-the-art procedures developed to evaluate existing conditions in historic buildings, cladding, foundations, footings, and masonry chimneys. These methods provide engineering information about:
• the general arrangement of construction, material types, and layer thickness
• the extent of debonding and delamination
• variations in concrete compaction and voiding
• concrete slab support
• moisture content
• spalling or micro-cracking in masonry
• reinforcement details, mat size, lap detail, concrete cover, and bar sizes
• locations of cladding attachments, metal clamps and ties, wall ties, and chimney flues
Available nondestructive evaluation techniques include:
• Impulse Radar
• Impact Echo
• Pulse Induction Metal Detection
• Ultrasonic Pulse Velocity Measurements
• UV Inspection
The most useful techniques are the first three: thermography, impulse radar, and impact echo.
Thermography, or thermal imaging, is a rapid, remote, noninvasive technique used to obtain an image of heat distribution across the surface of an object. The outer skin of any object absorbs thermal energy from its surroundings. A building obtains thermal energy from the sun, atmosphere, and human activity.
This energy is emitted as thermal radiation which can be measured across a spectrum from "neat" infrared, or short wavelength, bordering on the red end of the visible light, to "far" infrared, or long wavelength.
The hotter an object or structure, the higher the thermal energy it emits, and the shorter the wavelength. The thermal output is measured by the camera and converted into a scanned video image.
Thermography is a versatile and useful tool. In many situations it is limited only by the resourcefulness of the project team. Thermography has been used to assess various structures from power station chimneys and concrete motorway bridges to listed historic buildings.
These surveys have been carried out to assess entirely different problems in structures that varied dramatically in age. The usefulness of thermography surveys must be assessed on a case-by-case basis.
Impulse radar, or ground-penetrating radar, was first developed for use in mapping near-surface geological formations. It is now recognized as a powerful and versatile technique that allows for the internal assessment of a wide variety of materials to depths of several feet. It provides for the identification of changes in materials and the condition of a structure and its elements.
Impulse radar provides a cost-effective solution to specific engineering problems, for example when the original design specification was not recorded or followed. It is often used to establish the location, arrangement, and condition of buried objects: flues and/or voids within walls, reinforcement within concrete, underground voiding, or services and service leaks.
Used properly, impulse radar causes no damage to a structure or delicate finishes and minimal disruption to the working environment. It can be used effectively on materials including wood, stone, brick, concrete, and ground strata.
Radar has been used as the principal investigative technique on bridges, ruined castles, roads, inhabited historic buildings, modern concrete structures, and ancient earthworks.
Impulse radar may uncover the following useful information:
• Size, nature, and disposition of structural components
• Location and condition of metallic inclusions (clamps, dowels, reinforcement)
• Location of flues and chases within walls
• Location of voiding within materials/structure
• Condition of mortar joints
• Microcracking of stone or concrete caused by chemical attack or expansive corrosion of contained metalwork
• Bulk moisture content of materials
Impulse radar works by inducing an electromagnetic pulse of energy into the structure under investigation and measuring the changes in wave velocity as the pulse passes from one type of material to another. This change causes energy to be reflected at the boundary between different materials so that it reaches the receiver, producing a record of the interference.
Both the transmitted and received signals are waves. The system applies the principle that radio waves travel at different velocities through different materials; the velocity is dependent on the electrical characteristics of that material. Impulse radar records the change in that electrical difference.
Two commonly found examples are:
1. Where a material condition changes (for example, where concrete becomes voided) there is a change in the electrical properties of that material.
2. Where the material itself changes (for example, a pipe within a uniform ground structure), there is a change in the electrical properties between the two materials.
Designing a Radar Investigation
When specifying any investigation, the focus should be on the answers sought by the survey, together with the uses to which those answers will be put. Structural, historical, or speculative information should be made available to the investigators as it will enhance the accuracy of their interpretations.
Radar does not directly measure the physical characteristics of a structure. It does, however, provide a means by which these characteristics can be inferred, cost effectively and with minimal disruption.
Impact echo, or dynamic impedance, testing was developed to calculate the depths and condition of large bodies of homogeneous solid material and is best suited to structures of uniform composition and thickness, such as slabs, blocks of material, or pipes. Impact echo records the holistic response of the object under investigation.
The name for this method stems from the test procedure used. "Impact" is the method used to excite the object (sometimes called the bump test). "Echo" refers to the characteristics of the transmitted and returned (echo) waves.
Designing an Impact Echo Investigation
When specifying any investigation, the focus should be the answers sought by the survey, together with the uses to which those answers will be put. Again, structural, historical, or speculative information gained from the investigation should be made available to the investigators, because this enhances the accuracy of their interpretations.
Impulse echo does not measure a point or material response of an
object to mechanical stimuli. It is therefore used qualitatively to identify anomalous responses from within the object as a whole prior to more detailed investigations to characterize the extent and nature of any defects.
Impact echo is a tool that can be of benefit when there is a need to determine the depths of large structures with approximately uniform composition (metallic or nonmetallic). It is less suited to fragile structures or detailed investigations of the nature and extent of detected defects.
Swanke Hayden Connell Architects are well known for their work on a wide range of residential, commercial, municipal, and institutional preservation projects.
GBG, Inc., of Cambridge, England, is a leader in state-of-the-art, nondestructive investigation and assessment techniques for existing structures.
This article was excerpted from Historic Preservation: Project Planning & Estimating, which is published by R.S. Means Company and available at Amazon.com.