DETECTING VOIDS IN GROUTED TENDON DUCTS OF POST-TENSIONED CONCRETE STRUCTURES USING THREE DIFFERENT METHODS

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1 DEECING VOIDS IN GROUED ENDON DUCS O POS-ENSIONED CONCREE SRUCURES USING HREE DIEREN MEHODS Xianyan ZHOU Dr. Eng., Professor of Structural Engineering, President of Central South University of orestry and echnology. College of Civil Engineering and Mechanics, Central South University of orestry and echnology. 498 South Shaoshan Ave Changsha, Hunan P.R.China zxy560805@163.com * Zhifeng WANG Ph.D. Candidate, Lecturer of Bridge Engineering. College of Civil Engineering and Mechanics, Central South University of orestry and echnology. 498 South Shaoshan Ave Changsha, Hunan P.R.China wangzhifeng1984@gmail.com, @163.com Dahai ZHANG Master Candidates. College of Civil Engineering and Mechanics, Central South University of orestry and echnology. 498 South Shaoshan Ave Changsha, Hunan P.R.China @qq.com Abstract An increasing amount of post-tensioned concrete structures are used widely in China, and the grout condition inside tendon ducts attracts people s attentions. In order to assess internal grout quality of grouted tendon ducts in post-tensioned concrete structures accurately, and guarantee the lifetime of prestressed concrete structures, three different nondestructive testing (ND) methods, Impact-Echo Scanning (IES), Ultrasonic ransmission Method (UM), and Ground Penetrating Radar (GPR), have been employed to investigate the grouting defects qualitatively and quantitatively. A series of indoor model testing, in which different types of soft foams were placed in tendon ducts to simulate the flaws during in-situ construction, were carried out. Results show that the IES method can do a better job in situation where the walls of the ducts are metal rather than plastic. In contrast, and GPR technology may achieve a better performance in detecting voids within plastic ducts. Keywords: Detect, Impact-echo, Post-tensioned, Radar, endon Duct, Ultrasonic, Void 1. Introduction Statistical data provided by the Ministry of ransport of the People s Repubic of China on April 28, 2011 indicates that, the amount of highway bridges in mainland China has reached 658,100 with a total length of 30,483 km in later 2010, including 2,051 grand bridges, and 49,489 great bridges [1]. With the rapid development of highway constructions in China, the post-tensioned concrete structures are being widely used. However the grouting quality inside tendon ducts is a cause for concern. In order to guarantee the durability of prestressed concrete bridges, and prevent water and oil stain from entering the ducts, which may result in corrosion of tendons, the cable ducts should be fully grouted. or the actual injection, there exist voids in tendon ducts due to the block of ducts, unreasonable injection method, and human factors. hese voids will severely reduce the durability and load-carrying capacity of Page 1 of 10

2 concrete bridges. In 2001, poorly grouted tendon duct was classified as one of the top ten quality defects in construction of roads and bridges by the Ministry of ransport of the People s Repubic of China. here are no mature technologies for internal grout condition evaluation of tendon ducts in China. Some nondestructive tests and corresponding researches on injection quality inside post tensioned ducts have been carried out, identifying areas where there are full or partial voids in the tendon ducts to evaluate the internal grout condition accurately, and guarantee the quality of bridges [2,3]. he traditional, and still most widely used, test methods for concrete and masonry are destructive methods, such as coring, drilling or otherwise removing part of the structure to permit visual inspection of the interior. While these method are highly used reliable, they are also time consuming and expensive, and the defects they leave behind often become focal points for deterioration. In this paper, we restrict our interest to the nondestructive evaluation of internal injection quality in post-tensioned tendon ducts, and the conventional ND methods and new technology are also introduced. 2. Experimental Investigation As shown in igure 1, a concrete slab model was made to verify the effectiveness of these wave-based internal grouting quality evaluations of the grouted tendon ducts in posttensioned structures. he concrete mix ratio - that Gravel: Sand: Cement: Water is equal to 46: 29: 16: 9 - is set, and the dimension of the test model is 1220*850*200 mm. wo corrugated shells are embeded in the model with a concrete cover of 50 mm. he shells are located in halfway of the height-one is a metal duct, and the other a plastic one-in which a few soft foams of different sizes are placed to simulate the voids during the construction. he nominal wall thickness of the metal duct and plastic duct is 1.0 mm and 1.5 mm respectively, both of which have the same diameter of 100 mm, and the same void ratios (0%, 2.5%, 3.8%, 5.4%, 7.0%, 8.2%, 9.2%, 12.2%, 12.5%, 18.3% and 100%). More information about the arrangement of measuring points and simulation of flaws is presented as shown in able1 below Non-grouted ull grouted Plastic duct Metal duct igure 1. Simplified diagram of the specimen (Unit: cm). Page 2 of 10

3 able 1. Arrangement of measuring points & simulation of flaws. MEASURING POINS NUMBER OAM HICKNESS (mm) OAM WIDH (mm) OAM LENGH (mm) SIMULAED VOID RAIO % % % % % % % % % 2.1 Impact-Echo Scanning (IES) Impact-echo is a method for nondestructive evaluation of concrete and masonry structures, invited at the U.S. National Bureau of Standards (NBS) in the mid-1980's, and developed at Cornell University, in Ithaca, New York, from [4]. It s based on the use of transient stress waves generated by elastic impact. A short-duration mechanical impact, produced by tapping a small steel sphere against a concrete surface, is used to generate low-frequency stress waves that propagate into the structure and are reflected by flaws and/or external surfaces. Surface displacements caused by reflections of these waves are recorded by a transducer, located adjacent to the impact. he resulting displacement versus time signals are transformed into the frequency domain, and plots of amplitude versus frequency (spectra) are obtained. Multiple reflections of stress waves between the impact surface, flaws, and/or other external surfaces give rise to transient resonances, which can be identified in the spectrum, and used to evaluate the integrity of the structure or to determine the location of flaws. Impact-echo signals recorded from the normal concrete, fully grouted ducts and partially grouted ducts will present different patterns [5], as shown in igure 2. V P Amplitude s V P / 2 (a) requency (Hz) d ully grouted Amplitude steel requency (Hz) s V P / 2 V / 4 d steel s P (b) d Not fully filled Amplitude - move steel move 2 move V / 2 steel - move s P move (c) requency (Hz) V / 2 d steel move steel 2 s steel P igure 2. he resonant echoes of displacement responses for different ducts If flaws in grouted tendon ducts are present, these patterns present in the waveforms and Page 3 of 10

spectra (especially the latter) are disrupted and changed, in ways that provide qualitative and quantitative information

Once the wave speed is known, tests could be performed systematically along the measuring lines.

In the 3D views, as shown in igure 3, the run of the ducts is mainly imaged and the detailed information about the

$At poorly-grouted areas, the lower dominant frequency indicates that impact-generated stress waves diffract around the$ As the interval between the soft foams is about 4 cm while that of the two adjacent measuring points is approximate 5

As the interval between the soft foams is about 4 cm while that of the two adjacent measuring points is approximate 5

Compared igure 4 with igure 5, the three-dimension views of the plastic ducts with the abilities of absorption of the

4 spectra (especially the latter) are disrupted and changed, in ways that provide qualitative and quantitative information about the existence and location of the flaws. Once the wave speed is known, tests could be performed systematically along the measuring lines. he three-dimension views are obtained using the IE scanner field data analyses system. In the 3D views, as shown in igure 3, the run of the ducts is mainly imaged and the detailed information about the artificial grouting faults can be obtained. At poorly-grouted areas, the lower dominant frequency indicates that impact-generated stress waves diffract around the defects. As the interval between the soft foams is about 4 cm while that of the two adjacent measuring points is approximate 5 cm, the intended defects presented in the igure 3 seem to be in a tight pack. Compared igure 4 with igure 5, the three-dimension views of the plastic ducts with the abilities of absorption of the signal are less clear than that of metal ducts, which corresponds to the research outcomes achieved by the lorida Department of ransportation in When the testing is carried out perpendicular to the duct instead of parallel to it, the location and the size of the flaws can be obtained more accurately and the 3D views can be depicted more clearly, as well. igure 3. 3D map and top view perpendicular to the ducts igure 4. 3D map and top view parallel to the metal ducts Page 4 of 10

5 igure 5. 3D map and top view parallel to the plastic ducts 2.2 Ultrasonic ransmission Method (UM) Ultrasonic testing uses high frequency sound energy to conduct examinations and make measurements. It s based on the propagation, and in some cases reflection, of waves in solid. A typical Ultrasonic testing system consists of several functional units, such as the pulser /receiver, transducer, and display devices. A pulser/receiver is an electronic device that can excite high voltage electronic pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. he wave propagates through the medium and is reflected by material defects or by interfaces between regions of different densities and/or elastic moduli. he reflected waves are monitored by a transducer, and the reflected wave signal is transformed into an electronic signal by the transducer and is displayed on a screen. Using the time base of the display, the travel time of the wave is determined. Signal travel time can be directly related to the distance that the signal travelled. By analysing the signal, information about the location of the defect or interface where the reflection occurs can be obtained. igure 6 outlines the details of the points for detecting flaws and gives a general view of the layout lines for testing. o simplify the data collection, a number of nominally measuring lines are established on the slab model, and a naming convention is used to designate each line. A total of 11 measuring lines, 1 through 11, are placed with 5 points (labeled as Ai, Bi, Ci, Di, and Ei) for each one. A1 B1 C1 D1 E1 A2 B2 C2 D2 E2 A3 B3 C3 D3 E3 A4 B4 C4 D4 E4 A5 B5 C5 D5 E5 A6 B6 C6 D6 E6 A7 B7 C7 D7 E7 A8 B8 C8 D8 E8 A9 B9 C9 D9 E9 A10 B10 C10D10 E10 A11 B11 C11D11 E11 Plastic duct Metal duct igure 6. Distribution of test points for detecting voids here are no ducts inside the slab in zone A, C and E, as shown in igure 7. At the locations of B and D, the plastic duct and metal duct exist respectively. By comparing the tests of B and D with that of A, C and E, the grouting quality of the post-tensioned ducts can be evaluated. he ultrasonic waves propagate through the medium and are reflected by material defects or by interfaces between regions of different densities and/or elastic moduli. hus, if voids are present, these acoustic parameters are changed, in ways that provides qualitative information about the existence and location of flaws. According to the principle of ultrasonic testing, the voids in grouted tendon ducts are detected by analysing the relative variation of the acoustic parameters when the waves propagate through the structures. Page 5 of 10

6 A B C D E Plastic Duct Metal Duct A B C D E igure 7. wo opposite sides of the concrete member for UM testing. able 2 shows the results of the ultrasonic testing. or each measuring line, these points, in zones A, C and E where there are no ducts in the model, have nearly equal travel time and amplitudes, which can verifies the fact that the slab is homogeneous. able 2. Unprocessed testing results of UM for the slab model. A 1 A 2 A 3 A 4 A 5 A 6 A 7 A 8 A 9 A 10 A 11 IME VELOCIY AMPLIUDE C 1 C 2 C 3 C 4 C 5 C 6 C 7 C 8 C 9 C 10 C 11 IME VELOCIY AMPLIUDE E 1 E 2 E 3 E 4 E 5 E 6 E 7 E 8 E 9 E 10 E 11 IME VELOCIY AMPLIUDE AVG AVG 1 AVG 2 AVG 3 AVG 4 AVG 5 AVG 6 AVG 7 AVG 8 AVG 9 AVG 10 AVG 11 IME VELOCIY AMPLIUDE B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B 11 IME VELOCIY AMPLIUDE Page 6 of 10

7 D 1 D 2 D 3 D 4 D 5 D 6 D 7 D 8 D 9 D 10 D 11 IME VELOCIY AMPLIUDE he numerical results show that, compared with the acoustic parameters gained from zones A, C and E, the travel time, velocity and amplitude of ultrasonic waves in zones B and D become longer, smaller and lower respectively. While this is feasible, it is found to be timeconsuming. However, if the relative change value ( Ratio) is calculated according to the following Eq. (1), it can be employed as a measure of change of tested acoustic parameters. hese relative values listed in able 3 can be used to evaluate the internal grouting condition of tendon ducts at each test point. Value Ratio 100% (1) AVG Where Value is the results of tests carried in zones B and D, AVG is the average value of measuring zones A, C and E. able 3. Processed testing results of UM for the slab model. Ratio B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B 11 IME 116% 138% 115% 115% 117% 122% 112% 119% 121% 113% 108% VELOCIY 86.3% 72.3% 87.2% 87.2% 85.3% 81.9% 89.4% 84.0% 82.9% 88.3% 92.8% AMPLIUDE 87.5% 87.3% 88.1% 89.5% 88.4% 88.3% 89.2% 89.5% 90.7% 92.8% 92.8% D 1 D 2 D 3 D 4 D 5 D 6 D 7 D 8 D 9 D 10 D 11 IME 110% 101% 112% 111% 114% 110% 153% 110% 108% 110% 111% VELOCIY 91.1% 99.1% 89.2% 89.9% 87.9% 91.0% 86.7% 91.3% 92.2% 91.1% 89.9% AMPLIUDE 89.6% 92.7% 95.3% 86.1% 85.7% 86.6% 94.7% 89.5% 93.9% 89.5% 88.3% Seeing from the above table, we can find that UM may achieve better performance in detecting voids within plastic ducts because ultrasonic waves are reflected by the steel corrugated shell, which prevent waves from propagating into the ducts. he parameter amplitude is more sensitive to the change of propagation path of ultrasonic waves, which can be used to evaluate the general internal grouting condition of steel tendon ducts in posttensioned concrete structures. 2.3 Ground Penetrating Radar (GPR) GPR systems use separate, man-portable, transmitter and receiver antennas, which are placed and moved in a known pattern on the surface of the ground or material under investigation, and an image can be generated, in real time, on a display either in grey scale or in color. By systematically surveying the area in a regular grid pattern, a radar image of the ground can be built up. GPR images are displayed either as two-dimensional representations, using horizontal (x or y) and depth (z) axes or a horizontal plane representation (x, y) at a given Page 7 of 10

depth (z) or as a three-dimensional reconstruction. GPR provides the ability to see through solid material and detect internal voids without affecting the material in an adverse manner.

he measuring lines in both lateral and longitudinal directions are arranged as shown in igure 8. o simplify the GPR tests, a naming convention is used to designate each measuring line.

8 depth (z) or as a three-dimensional reconstruction. GPR provides the ability to see through solid material and detect internal voids without affecting the material in an adverse manner. In this study, not only the location and dimension but the grouting quality of the tendon ducts need to be determined. he measuring lines in both lateral and longitudinal directions are arranged as shown in igure 8. o simplify the GPR tests, a naming convention is used to designate each measuring line. he top measuring line in non-grouted zone is labeled as A, and bottom one in full grouted zone labeled as D. In addition, measuring lines B and C can be found in the middle of the partially grouted zone. A Non-grouted Non-grouted B Partially grouted C D ull grouted ull grouted Plastic duct Lateral testing trail Metal duct Plastic duct Longitudinal testing trail Metal duct igure 8. Schematic representation of GPR testing. Unprocessed GPR images often show high spots caused by multiple internal reflections as well as a distortion of the aspect ratio of the image of the target caused by variations in the velocity of propagation. Symmetrical targets, such as spheres or pipes, cause migration of the reflected energy to a hyperbolic pattern, as shown in igure 9. GPR images can be processed to compensate for these effects, and this is usually carried out offline. (A) (B) Page 8 of 10

or normal incidence the reflection coefficient, R, is given by the following: ( Z 2 Z1) R (2) ( Z Z ) 2 Where Z 2 =specific acoustic impedance of material 2 and Z 1 =specific acoustic impedance of

9 (C) (D) igure 9. he radar image within plastical ducts. When an electromagnetic wave traveling through material 1 is incident on the interface between a dissimilar material 2, a portion of the incident wave is reflected. or normal incidence the reflection coefficient, R, is given by the following: ( Z 2 Z1) R (2) ( Z Z ) 2 Where Z 2 =specific acoustic impedance of material 2 and Z 1 =specific acoustic impedance of material 1. he reflection coefficient given by Eq. (2) can be negative or positive depending on the relative values of the acoustic impedances of the two materials. If the waves traveling through concrete encounter an interface with air, the reflection coefficient is negative. his means that the phase of the reflected wave is opposite to the phase of the incident wave. Voids can be detected on the lateral testing trail A" according to color sequence(red-blue) of the radar image in igure 9(A), and full grout determined on the trail D" according to color sequence(red-blue-red) of the radar image in igure 9(D). he representation in igure 9(B) and 9(C) indicate the tendon ducts are not grouted fully on the trail B and C. hese differences of color sequence are important in distinguishing between reflection from a concrete-air interface and from a concrete-steel interface. his is why ND methods based on wave propagation have proven to be successful for checking a post-tensioned structure for voids in the grouted tendon ducts. 2.4 Conclusions Results show that the IES method can do a better job in detecting voids in steel tendon ducts of post-tensioned concrete structures. A scientific internal injection quality evaluation can be made after analysing the IE signals accurately. In contrast, the GPR technology may achieve a better performance in detecting voids within plastic ducts because the steel ducts have great electromagnetic shielding functions, which prevent electromagnetic waves from propagating into the ducts. he accuracy of impact-echo in testing post tensioned ducts is related to P-wave speed, diameter of steel spheres and sampling frequency. Accurate testing parameters and a reasonable mechanical impact used to generate low-frequency stress waves can lead to good results. Evaluating injection quality of grouted tendon ducts with nondestructive testing methods is a new project. here are no perfect technologies and corresponding specifications in China. A large number of experimental researches have been carried out to apply impact-echo testing to internal grout condition evaluation of post-tensioned ducts. 1 Page 9 of 10

10 2.5 Acknowledgements he writers gratefully acknowledge the financial support from National Natural Science und of China under the Project Also, this project was supported by Youth Scientific Research undation of Central South University of orestry & echnology award QJ B. 2.6 References [1] he statistical communique (Years: 2000 to 20 10) of Chinese Road Administration of Ministry of ransport. he web site of Ministry of ransport of the People s Republic of China: [2] ZHOU Xian-yan, WANG Zhi-feng, YAN Ban-fu, Nondestructive esting Method of Grouting Quality for Prestressed Pile, China Journal of Highway and ransport, Vol. 24, 6, November 2011, pp [3] YANG ian-chun, YI Wei-jian, LU Guang-yin, HUANG Hua-lin, Experimental Study of Nondestructive esting for Grouting Quality in Prestressed Concrete -beam, Journal of Vibration Engineering, Vol. 19, 3, September 2006, pp [4] Mary J. Sansalone, William B. Streett, Impact-Echo: Nondestructive Evaluation of Concrete and Masonry, Bullbrier Press, Jersey Shore, PA, [5] WANG Zhi-eng, YAN Ban-u, ZHOU Xian-Yan, XIAO Yun-eng, esting Grouting Quality in Prestressed Ducts with Impact-Echo Method, Journal of Vibration and Shock, Vol. 28, 1, January 2009, pp Submission of papers he paper will be submitted through the Author s personal area in the website. Page 10 of 10

IMPACT ECHO TESTING OF IN-SITU PRECAST CONCRETE CYLINDER PIPE

IMPACT ECHO TESTING OF IN-SITU PRECAST CONCRETE CYLINDER PIPE Dennis A. Sack; Larry D. Olson, P.E. 1 Abstract This paper presents recent research and field work performed in the area of Nondestructive