Ultrasonic testing of materials: Josef and Herbert Krautkramer Springer-Verlag Publishing Company, Berlin, Heidelberg, New York (1975) 669 pp, $63.00

....................................................................................................................................................... iii Preface .......................................................................................................................................................... v Table of

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Best Practices for Reducing the Potential for Progressive Collapse in Buildings | NIST

The impact-echo method has been developed over the past 20 years and is now widely used in the nondestructive evaluation of concrete. However, some practical issues remain unresolved, such as the physical basis for the empirical correction factor sbd used to obtain thickness mode frequency. A new approach based on guided wave theory is proposed in this paper: that the impact-echo resonance in plates corresponds to the zero-group-velocity frequency of the S1 Lamb mode. A numerical model is developed, verified by experiment, and then shown to adequately simulate the dynamic response of a concrete plate. Using this model the thickness resonance mode is identified and found to accurately match that particular Lamb mode in terms of shape and frequency. New values forb based on the Lamb mode model are computed and dependence on material Poisson's ratio is demonstrated.

Lamb wave basis for impact-echo method analysis

Chloride diffusivity of concrete cracked in flexure

The impact-echo method is a technique for flaw detection in concrete. It is based on monitoring the surface motion resulting from a short-duration mechanical impact. The method overcomes many of the barriers associated with flaw detection in concrete based on ultrasonic methods. The purpose of this paper is to provide an overview of the technique and to discuss the important parameters involved in this type of testing. One of the key features of the method is the transformation of the recorded time domain waveform of the surface motion into the frequency domain. The impact gives rise to modes of vibration and the frequency of these modes is related to the geometry of the test object and the presence of flaws. The principles involved in frequency analysis are discussed. The importance of the impact duration in relation to flaw detection and other factors affecting the smallest flaw that can be detected are also reviewed. The paper concludes with a summary of the ASTM standard governing the use of the impact-echo method for measuring the thickness of plate-like structures.

The Impact-Echo Method: An Overview

The structure of the current ACI Building Code has remained essentially unchanged since 1963 when ultimate strength was added. Knowledge has expanded with time through practice and research. In str...

The Framework of the 2014 American Concrete Institute (ACI) 318 Structural Concrete Building Code

Cracks in concrete have many causes. They may affect appearance only, or they may indicate significant structural distress or a lack of durability. Cracks may represent the total extent of the damage, or they may point to problems of greater magnitude. Their significance depends on the type of structure, as well as the nature of the cracking. For example, cracks that are acceptable for buildings may not be acceptable in waterretaining structures. The proper repair of cracks depends on knowing the causes and selecting the repair procedures that take these causes into account; otherwise, the repair may only be temporary. Successful long-term repair procedures must attack the causes of the cracks as well as the cracks themselves. Concrete inherits certain type of cracks in pre-hardening stage and also develops some other types of cracks in post hardening stage in due course of time due to various reasons, despite our utmost care in prevention of cracks. While concrete becomes older, these cracks become sources, of leakages and seepages and give easy access to the moisture, oxygen, chloride, carbon dioxide, and other aggressive chemicals and gases into the concrete leading to serious degradation of the structure and causing corrosion of steel and damage in the concrete in the form of spalling etc. and subsequently causing structural failure of the member. Cracking is the initial sign of distress of the structure baring other forms of distress and deterioration like deformation, surface deposits and construction defects etc. causing damage to structural strength, durability and serviceability.

Causes, Evaluation, and Repair of Cracks in Concrete Structures

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.

Detecting voids in grouted tendon ducts of post-tensioned concrete structures using the impact-echo method

Wind- and rain-induced vibrations of cables on cable-stayed bridges are a relatively new phenomenon. While some observers report hearing about such vibrations ten or 12 years ago, typically in Japan and Europe, it is only in the past few years that research has begun here in the United States. A study coordinated by the Texas Dept. of Transportation centers on vibrations at the Fred Hartman bridge, Baytown, Tex., which was opened in 1995, and the Veterans Memorial bridge in Port Arthur, Tex., which opened in 1991. The University of Texas, Austin, Texas Technological University, the University of Kentucky and Johns Hopkins University are contributing to the study and the Virginia firm of Whitlock, Dalrymple Poston & Assoc., working with these institutions. This article explores the possible reasons for the vibrations, as well as proposed remedial systems. Among them are restraining systems, dampers and strakers. The vibrations cause mororists concern, and can cause metal fatigue and other problems. Remediation can be costly. Design procedures are also suggested. A sidebar discusses the interim remediation work at the Fred Hartman bridge, and offers comments from engineers who have designed many cable-stayed bridges.

Cable-stay conundrum

This paper and a companion one summarize the findings of a feasibility study in which the impact-echo method was used to determine interfacial bond quality in layered concrete structures, such as bridge decks with overlays or at the interface between repair concrete and existing concrete. In this context, bond quality involves both the tensile strength and the amount of unbonded fraction of area at an interface. This paper focuses on how bond tensile strength impacts the results of impact-echo method. Results obtained from numerical, experimental, and field studies are presented. The numerical studies were used to quantify the stresses in the waves generated by elastic impact to determine whether these waves could be used as a measure of bond tensile strength. Experimental and field studies were conducted to examine impact-echo results obtained from a layered concrete plate with a variety of interface conditions varying from strongly-bonded to delaminated as well as conditions produced by removal of concrete by jackhammering and hydrodemolition. The pulloff test method was used to derive bond strengths in both the experimental and field studies. Findings revealed that the stresses generated by short-duration, elastic impacts are much too small to serve as a measure of bond tensile strength.

Randall W. Poston

Papers

The Framework of the 2014 American Concrete Institute (ACI) 318 Structural Concrete Building Code

Causes, Evaluation, and Repair of Cracks in Concrete Structures

Detecting voids in grouted tendon ducts of post-tensioned concrete structures using the impact-echo method

Cable-stay conundrum

Impact-echo studies of interfacial bond quality in concrete: part ii--effects of bond tensile strength