Impact-echo

Impact-echo is a method for non-destructive evaluation of concrete and masonry structures, based on the use of impact-generated stress (sound) waves that propagate throngh a structure and are reflected by internal flaws and external surfaces. 

It can provide the thickness of concrete slabs up to an accuracy of 3% and can determine the location and extent of flaws such as cracks, delaminations, voids, honeycombing and debonding in plain, reinforced and post-tensioned concrete structures It can also locate voids in the subgrade beneath slabs and pavements. For masonry it can determine thickness and locate cracks, voids and other defects where the brick or block units are bonded together with mortar. Impact-echo is not adversely affected by the presence of steel reinforcing bars. 

The typical equipment is portable and battery powered enabling it to be used in most locations. The eqnipment comprises a control unit and transdncer with the system being run from a laptop computer, the energy sonrce used consists of spherical impactors (ball bearings) which are used to strike the concrete surface to set up standing waves within the structnre. The smaller the impactor the higher the frequency and therefore the higher resolution. 

If the speed of sound throngh the concrete is known or can be determined on site then it is possible to determine the depth of features immediately as 

As with several other techniques discussed in the last few sections, this technique requires specialized expertise and is therefore usually conducted by test houses who use it on a regular basis. Intimate contact is required between the impactor and the surface. 

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Ultrasonic wave speed, Impact Echo and Spectral Analysis of Surface Waves... 

Ultrasonic wave speed, SASW and Impact Echo... 

Ultrasonic wave speed and Impact Echo High-Energy Radiography... 

The common civil engineering seismic testing techniques work on the principles of ultrasonic through transmission (UPV), transient stress wave propagation and reflection (Impact Echo), Ultrasonic Pulse Echo (UPE) and Spectral Analysis of Surface Waves (SASW). 

A problem obviously exists in trying to characterise anomalies in concrete due to the limitations of the individual techniques. Even a simple problem such as measurement of concrete thickness can result in misleading data if complementary measurements are not made In Fig. 7 and 8 the results of Impact Echo and SASW on concrete slabs are shown. The lE-result indicates a reflecting boundary at a depth corresponding to a frequency of transient stress wave reflection of 5.2 KHz. This is equivalent to a depth of 530 mm for a compression wave speed (Cp) of 3000 m/s, or 706 mm if Cp = 4000 m/s. Does the reflection come from a crack, void or back-side of a wall, and what is the true Cp ... 

Impact Echo -result showing waveform (top), single wave spectrum with peak echoes (middle) and average frequency spectrum for a series of tests at the same point. 

Several methods have been proposed to nonintrusively measure the thicknesses of walls, corrosion profiles, and macrodefects [i.e., 49], Two methods at room temperature that require point contact with the cold face of the furnace are known. The first is impact-echo method, used in construction concretes and pavements (Sect. 1.4). The second method is the frequency-modulated continuous-wave (FM-CW) radar technique [50], which can produce wall thickness data in real time. 

M.-T. Liang, P.-J. Su, Detection of the corrosion damage of rebar in concrete using impact-echo method, Cem. Concr. Res., 31, 1427-1436 (2001). 

M.J. Sansalone, and W.B. Street, Impact-Echo Nondestructive Evaluation of Concrete and Masonry, Bullbrier Press, Ithaca, NY, 1997. 

A. Sadri, Application of impact-echo technique in diagnoses and repair of stone masonry structures, NDT E Int. (in press). 

ASTM C 1383-98 Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates using the Impact-Echo Method, The American Society for Testing and Materials, West Conshohocken, PA. 

M. Ohtsu, and T. Watanabe, Stack imaging of spectral amplitudes based on impact-echo for flaw detectionNDT E Int., 35, 189-196 (2002). 

With the recent collapse of two posttensioned bridges in the United Kingdom and one in Belgium the impact-echo nondestructive evaluation (NDE) technique was developed to detect voids in posttensioned ducts. In addition, a complementary magnetic-based nondestmctive technique (NDT) for assessing section loss in... 

The pulse velocity approach is best done in a transmission mode with the pulse created on one side of a concrete member and detected on the other side. It can be used at corners or in a reflective mode if necessary, but it loses effectiveness and interpretation gets harder. The impact-echo technique can be used with pulse generator and detector side by side as the echo is reflected back from defects. 

Figure 4.27(a) shows typical equipment nsed for impact-echo measurement and interpretation... 

Figure 4.27 (a) Impactor/transducer, electronic interface and laptop computer for generating the impact and recording the echo spectrum. Courtesy Physical Acoustics Ltd. (b) Impact echo response showing a delamination at 94 mm depth (lower frequency domain plot). Courtesy Aperio Ltd. 

ASTM C597-02 standard test method for pulse velocity through concrete and ASTM Cl383-04 test method for measuring the P-wave speed and the thickness of concrete plates using the impact-echo method cover the application of impact echo to concrete structures. 

In the 12 years since the first edition I have been fortunate to continue to work with many knowledgeable and experienced engineers and specialists on the committees of NACE, the Corrosion Prevention Association, Concrete Society, Institute of Corrosion, the Transportation Research Board and CEN. I must specifically acknowledge Andrew Trafford of Aperio who kindly supplied a lot of useful information as well as photographs about radar, radiography, pulse velocity, ultrasonic and impact-echo techniques. My thanks also go to my publisher, and to Nick Clarke whose helpful suggestions strengthened the first draft of the book considerably. 

Sansalone MJ, Streett WB (1997) Impact-Echo. NY Burier Press, Ithaca... 

Various electromagnetic and acoustic techniques are used for nondestructive examination of concrete. These include radar for locating delaminations, radiography for mapping the rebar network, ultrasonic techniques like pulse velocity and PUNDIT, which can estimate concrete strength and quality and find defects like voids, honeycombing, crack.s, etc. All of these techniques require. specialized equipment and most require specialists to operate and interpret them. Figure.s 4.18(a) and (b) illustrate the equipment and use of pulse velocity and impact echo. 

Impact echo/ spectral analysis of surface waves Examines condition of concrete Objective measurement system Does not give direct information on wire breaks Slower inspeotions than remote field effect Requires an empty pipeline... 

In addition to the major techniques used for evaluation of corrosion of the reinforcing steel described above, non-destructive techniques such as computed tomography, impact echo and magnetic field disturbance, electrochemical noise and other techniques have also been used for inspection and detection of corrosion of reinforcing steels. Reference on these techniques along with the references on major techniques are cited at the end of this chapter. 

Cheng, C. and Sansalone, M. (1993). Effect of impact-echo signals caused by steel reinforcing bars and voids around bars, ACT Materials Journal, 421-434, Sep-Oct. 

Delamination, voids, and other hidden defects Impact echo, infrared thermography, pulse echo, and radar... 

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