Mary Sansalone

Impact-Echo: The Complete Story

Impact-echos history is an interesting story of how a real need for nondestructive test methods for flaw detection in concrete structures led to a systematic and sustained basic and applied research effort to develop such a method, beginning in 1983, at the National Bureau of Standards, and continued since 1987 at Cornell University. This paper discusses the contributions of the people and the organizations who carried out the theoretical, numerical, laboratory, and field studies that established the method and who developed the software and instrumentation that gave rise to a patented impact-echo field system. It also documents how this effort was undertaken and sustained with government and industry funding. Subsequently, this paper draws on knowledge gained over 12 years of research to provide, for the first time, a unified explanation of impact-echo theory as it applies to the testing of structural elements, including plates (slabs, walls, bridge decks, and pavements), bars (beams and columns), and hollow cylinders (pipes and tunnel and mine shaft liners) and to the detection of flaws within these elements. The last key pieces fell into place in 1995, and it is now possible to explain in a concise and coherent way the principles upon which impact-echo testing is based.

Detecting Delaminations in Concrete Slabs With and Without Overlays Using the Impact-Echo Method

The paper demonstrates the feasibility of detecting delaminations in reinforced concrete slabs using the impact-echo method, a nondestructive testing technique based on transient stress wave propagation. The results of two laboratory studies are discussed. One study involved detecting artificial delaminations embedded at unknown locations in a reinforced concrete slab. All the artificial delaminations in the slab were located. The second study was aimed at showing the feasibility of detecting delaminations in reinforced concrete slabs with asphalt concrete overlays. Two reinforced concrete slab specimens with corrosion-induced delaminations were tested. Prior to overlaying the slabs with asphalt concrete, the depths of delaminations as determined by impact-echo testing were verified by drilling at selected points. After the asphalt concrete overlays were applied, the slabs were retested. It was found that the impact-echo method could successfully locate the delaminations in the slabs through the asphalt concrete overlays.

The impact-echo response of concrete plates containing delaminations: numerical, experimental and field studies

This paper describes the use of the impact-echo technique— a non-destructive testing technique based on the use of transient stress waves— for detecting delaminations in concrete plate-like structures such as bridge decks, slabs, and walls. Results obtained from numerical (finite element) analysis and controlled-flaw laboratory studies are presented and used to explain the elastic impact response of delaminated plates. Current impact-echo instrumentation is described, and results obtained from concrete bridge decks containing delaminations caused by corrosion of reinforcing steel are presented. These results provide a better understanding of the impact response of delaminated plates.ResumeOn décrit ici l’utilisation de la technique impact-écho-méthode d’essai non destructive basée sur l’utilisation d’ondes de pression acoustique transitoires-pour détecter les clivages dans des plaques de béton, telles que tabliers de pont, poutres et murs. On présente les résultats fournis par les méthodes numériques (élément fini) et l’étude en laboratoire sur éprouvette fissurée témoin, qui sont utilisées pour expliquer la réponse élastique à l’impact des plaques détériorées. On décrit l’appareillage courant et l’on expose les résultats obtenus sur des tabliers de ponts en béton qui présentent des clivages dus à la corrosion de l’armature. Ces résultats contribuent à mieux faire comprendre la réponse à l’impact de plaques détériorées.

Determining the Depth of Surface-Opening Cracks using Impact-Generated Stress Waves and Time-of-Flight Techniques

Recent research on the use of the impact-echo method for nondestructively determining the thickness of slabs and pavements has led to the development of a technique and instrumentation for determining the wave speed in concrete using impact-generated stress waves. This method is independent of the geometry of the structure and uses time-of-flight measurements for determining the depth of vertical, inclined, curved, and air- or water-filled surface-opening cracks in concrete. Knowledge of the dilatational (P-) wave speed is obtained independently from accurate travel time measurements of the P-wave along the surface of the concrete. This paper discusses the theory and equations that form the basis for the method. Numerical and laboratory results are presented to demonstrate the feasibility of the method. Experimental results are obtained using the same instrumentation (impactors, broadband transducer, and data-acquisition hardware and software) that is used in wave speed measurements and in impact-echo testing.

Transient response of thick circular and square bars subjected to transverse elastic impact

The objective of the studies presented in this paper was to understand the transient response of thick circular and square bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual cross‐sectional modes and their corresponding natural frequencies were determined using plane strain and three‐dimensional finite‐element models of circular and square cross sections. The first few modes for both circular and square cross sections are shown. Subsequently, three‐dimensional finite‐element models of circular and square bars were used to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical models, experimental studies were performed on a 0.4‐m‐diam circular bar and a 0.3‐m square bar. These specimens were representative of full‐size concrete columns—typical barlike structural elements. The results of these expe...

DETECTING VOIDS IN GROUTED TENDON DUCTS OF POST-TENSIONED CONCRETE STRUCTURES USING THE IMPACT-ECHO METHOD

A nondestructive testing technique that uses transient stress waves is part of an ongoing research program at Cornell University, which is aimed at developing the theoretical basis and practical applications for the impact-echo method. This paper describes a phase of the program involving detection of voids in grouted tendon ducts in post-tensioned concrete structures. Findings from the numerical finite element studies, controlled-flaw laboratory studies, and a field study are presented. Three-dimensional dynamic finite element analyses were performed to examine the response of fully grouted, partially grouted, and ungrouted tendon ducts to transient stress waves. Laboratory specimens were constructed and tested to confirm the numerical results. Numerical and laboratory studies showed that the impact-echo method could be used successfully to detect both complete and partial voids in grouted tendon ducts. A field study was then conducted on an existing post-tensioned bridge. Fully grouted, partially grouted, and ungrouted tendon ducts located in actual bridge girders were examined. The results of the impact-echo tests were verified by invasive testing; the ducts were opened up and visually inspected. The impact-echo results correctly located fully grouted, partially grouted, and ungrouted tendon ducts.

IMPACT-ECHO RESPONSE OF CONCRETE SHAFTS

Numerical and experimental studies of the transient response of concrete shafts subjected to elastic impact were carried out using the finite element method and the impact-echo testing technique. Two- and three-dimensional finite element studies of concrete shafts were performed for: (a) solid shafts; (b) shafts containing cracks, voids, layers of low-quality concrete, and changes in cross section, such as bulges and necks; and (c) shafts in soil. These studies were carried out to gain an improved understanding of the impact response of concrete shafts containing flaws, problems for which there are currently no analytical solutions. It was also the intent of the numerical studies to determine whether information in addition to that obtained with existing nondestructive testing techniques could be obtained during impact testing of concrete shafts. Laboratory studies of solid concrete shafts were carried out to verify the finite element models. Subsequently, experimental studies were carried out on concrete shafts embedded in soil. These shafts contained flaws at known locations. The results of these studies show that additional information about the integrity of drilled shafts and piles can be obtained using impact techniques. The numerical studies also show that the finite element method is a powerful tool for studying the impact response of concrete foundation elements.

A PROCEDURE FOR DETERMINING P-WAVE SPEED IN CONCRETE FOR USE IN IMPACT-ECHO TESTING USING A P-WAVE SPEED MEASUREMENT TECHNIQUE

The dilatational or P-wave speed in concrete is needed in impact-echo testing if the dimensions of the structural elements or the location of flaws are to be determined. Previously, the P-wave speed was determined from cores or from performing a test on a portion of the structure having known dimensions and no flaws. In cases where neither approach was possible, an estimate had to be made of the wave speed. This paper presents the details of a method for independently determining P-wave speeds in concretes by determining the time it takes the P-wave to travel between two points along the surface of a structure. This procedure is more accurate than procedures that involve the use of Rayleigh wave speed measurements, and thus it is a better technique for use in quality control situations and other testing situations where more accurate results are needed.

Transient response of thick rectangular bars subjected to transverse elastic impact

The objective of the research presented in this paper was to study the transient response of thick rectangular bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual modes and their corresponding natural frequencies were determined using plane‐strain, finite‐element models of rectangular cross sections. The effect of the cross‐section aspect (depth/width) ratio on the natural frequencies is shown. Three‐dimensional, finite‐element models of rectangular bars were analyzed to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical studies, experimental studies were performed on rectangular bars with aspect ratios of 0.60, 0.75, 1.33, and 1.67. The bars used in the experimental study were representative of full‐scale concrete beams and columns—typical barlike structural elements. Excellent agreement was o...

Finite element studies of the impact-echo response of plates containing thin layers and voids

Quantitative nondestructive evaluation based on the use of transient stress waves generated by point impact is hindered by the fact that the governing partial differential equations admit closed-form solutions for only the most trivial cases, an infinite half-space and an infinite plate. In previous studies carried out by the authors, the finite element method has been shown to provide useful numerical solutions for a variety of cases involving bounded solids containing flaws. Numerical results have been verified with carefully controlled experiments. Currently, the method is being used to establish the basis for a new nondestructive evaluation technique for civil engineering structures. This technique is called impact-echo, and it is based on the use of low frequency, transient stress waves generated by elastic point impact. In this paper, the impact-echo response of plates containing thin layers is studied using finite element models. The purpose of these studies was to determine the applicability of using the method for detecting voids in layered civil engineering structures. Results of the numerical studies show that it is feasible to use the impact-echo method for this application.