Application of acoustic emission measurements in the evaluation of prestressed cast in-between decks

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APPLICATION OF ACOUSTIC EMISSION MEASUREMENTS IN THE

EVALUATION OF PRESTRESSED CAST IN-BETWEEN DECKS

Ir. P.H.A. van Hemert Delft University of Technology

Faculty of Civil Engineering & Geosciences

Stevinweg 1, 2628 CN Delft The Netherlands

Dr.ir. S.A.A.M. Fennis Delft University of Technology

Prof. Dr.ir. D.A. Hordijk Delft University of Technology

KEYWORDS: Acoustic emission, concrete, in-between decks, (punching) shear and cyclic loading.

ABSTRACT

A large number of concrete structures, that is built in the sixties and seventies of the twentieth century, need to be re-evaluated. It should be judged whether their capacity is still sufficient for the increased traffic loads. Acoustic emission (AE) is a non-destructive technique that can possibly be used to get a better insight in the structural state of these concrete structures. However, interpretation the AE measurements is challenging and is even more difficult when the concrete is cracked by for example alkali–silica reaction. Due to the existing cracks the wave attenuation affects the acoustic emission measurements. For an investigation into the capacity of pre-stressed cast in-between decks a 1:2 scale bridge was loaded in the Stevin laboratory of Delft University of Technology under two-way shear. In this preliminary research it was investigated whether AE-measurements can be used to get an idea about the structural condition of a structure, or more precise, to what extent the ultimate capacity is reached. For testing the concrete in-between decks a cyclic loading procedure is applied. It appeared that cyclic loading resulted in a lower capacity in comparison with the previously performed static loading experiments. There was no AE indication of early failure due to cyclic loading. Usually applied parameters, like ‘Kaiser Effect’ and ‘Calm Ratio’ are investigated. Furthermore, it was investigated whether the location of cracks could be determined by the emitted sound during the fracture process and applying a relatively large numbers of AE-sensors (so-called source location). In this article the performed AE measurements are reported and results discussed.

INTRODUCTION

In the Netherlands, a large number of the bridges originate from the sixties and seventies of the twentieth century. These bridges are now about 50 years old and need to be re-evaluated, not only because of their age, but also with respect to the increased traffic loads. Is their capacity still sufficient? About seventy of these bridges were built with precast-prestressed girders connected with slabs and cross-beams in between. The originally calculated failure capacity of this specific type of concrete structures, with cast-in-situ decks between the girders, is lower than the governing new design loads due to the traffic. For that reason it is important to determine as good as possible the real capacity of these concrete structures, by new calculations whether or not combined by testing.

One of the non-destructive testing techniques that can be used to get insight in the structural condition of concrete structures, like viaducts and bridges, is the application of acoustic emission measurements combined with proof loading (Gutermann, et al., 2003). However, interpretating the AE measurements is challenging, especially when the concrete is cracked by for example alkali–silica reaction. Due to existing cracks the wave attenuation affects the acoustic emission measurements. In a research project of Delft University of Technology into the application of AE-measurements for the assessment of existing structures, full scale tests on concrete structures are combined with laboratory testing. For the assessement of prestressed in-between decks laboratory tests on a 1:2 scale were performed. Acoustic emission measurements were performed in two experiments, which made it possible to test and evaluate a

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real structure, but in a controlled manner. In this article, the two laboratory tests are described. The main aim of the article is to show in what way the acoustic emission tests were performed and whether or not the results gave additional insight in the structural behaviour and/or whether there are parameters that might indicate upcoming failure of the structure. The final goal is to have, or develop, a technique based on AE-measurements that can be used to assess real structures during proof loading.

EXPERIMENTAL SETUP

Specimen

The specimen is a bridge deck, consisting of four precast-prestressed girders connected with three slabs and two cross-beams, see Fig. 1. The precast-prestressed girders are produced by Spanbeton, with dimensions of 12000 mm long, 1300 mm high and a width of 750 mm. It should be mentioned that these dimension of the specimen are on a 1:2 scale of the real bridge. After placing the precast-prestressed girders in position the cross beams are casted at the end of the girders. Then the deck slabs are casted and the specimen is pre-stressed in transverse direction. Two of three deck slabs have a skewed concrete-to-concrete interface on both sides, which is equal to the situation in the real bridge. The experiments described in this article were done on the in-between slab with skewed interfaces and a prestressing level of 2.5 N/mm2 in the decks. See Fig. 1 and Fig. 2 for location of loading points and applied prestressing.

Fig. 1: Lay-out Specimen

The four precast-prestressed girders are simply supported by eight supports. At each of these supports the reaction force is measured by a load cell. The directions of freedom are shown in the specimen lay-out, Fig. 1. Loading of the specimen was done by a hydraulic actuator, with a maximum capacity of 2000 kN. A loading plate is used to transfer the force from the hydraulic actuator to the slab. The loading plate is build-up from two steel plates 200x200x20 mm separated by a layer of teflon, on top of a rubber layer with a thickness of 8 mm. In Fig. 2 a 3D model of the experimental set-up is shown. The steel frame and hydraulic jack can be positioned freely for each individual loading test.

Ø 6 -2 5 0 Ø6-200 Ø 6 -2 5 0 Ø 6 -2 5 0 Ø6-200 Ø 6 -2 5 0 Ø 6 -2 5 0 1 3 0 0 6 4 0 0 Ø6-200 Ø 6 -2 5 0 201 A 101 B 301 C 401 8 7 5 1 0 5 0 7 5 0 1 0 5 0 7 5 0 1 0 5 0 8 7 5 12000 Freedom of movement Legend Skewed Joint 6800 3 2 0 0 10000 BB17 BB18

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The 28-days compressive concrete strength of the deck was 74.7 N/mm2. The decks were reinforced by bars φ6 every 250 mm in the longitudinal direction and every 200 mm in the transverse direction of the bridge. Reinforcing-steel B500B with an experimental yield value of 520 N/mm2 was used. The prestressing steel in the deck consisted of bars φ15 every 400 mm (van Hemert, 2014).

Fig. 2: 3D Experimental Setup

Loading Procedure

First, a static test was performed, which had a failure load of 321.4 kN (Amir & Van der Veen, 2013). The loading procedures of experiments described in this article, denoted as BB17 and BB18, are designed based on this failure load. A cyclic loading procedure is adopted according to (Liu & Ziehl, 2009) with which it might be possible to predict upcoming failure of the specimen. Following this approach and depending on the static failure load a cyclic loading procedure is designed which consists of 5 load levels with each three cycles, see Fig. 3. The level of each load cycle is determined based on the static failure load of 321.4 kN. In each cycle the load is increased with a constant loading rate of 1 kN/s. Every time the load level is reached the load is held constant for 10 minutes. After 10 minutes, the load is reduced with the same loading rate until 10 kN and again 10 minutes constant. During the third cycle of a specific load level the cracks are marked and measured at the bottom surface. Up until the start of the 5th load level, load control is used for the actuator. During the last part of the loading procedure the actuator is switched to displacement control.

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Fig. 3: Cyclic Loading Procedure

Measurements

During the experiments the following parameters are measured. 1. Applied load and displacement of the actuator;

2. Displacements of the deck slab close to the load;

3. AE signals during the loading procedure, registered by AE sensors.

The displacement of the actuator (item 1) is measured by a linear variable differential transformer (LVDT) connected to it. For a more accurate measurement of the vertical displacements, LVDTs’ are placed next to the loading plate, see Fig. 4. For both experiments the average of two LVDTs’ is used for the analyses of the results. These are LVDT 15 and 16, that are located the closest to the loading plate.

Fig. 4: Position lasers and LVDTs’ Fig. 5: Position of the AE sensors

During the experiments, the response of the deck in terms of signals due to energy release, is measured with AE sensors. The sensors are installed at the bottom of the slab, see Fig. 5. When the stress in the concrete due to loading exceeds the tensile stress, energy release results in a wave, that propagate in the concrete. Acoustic Emission is a method whereby these waves are measured at the concrete surface with piezoelectric sensors. The bandwidth of the frequency that is measured depends on the applied sensor. With the R6I-AST sensors the wave propagation between a frequency of 20-100 kHz is registered. These sensors also have a pre-amplifier inside of 40 dB to increase the signal strength.

0 50 100 150 200 250 300 350 Displacement Controlled Load Controlled Time Load [k N ] LVDT16 LVDT04 LVDT12 LVDT03 LVDT10 250 280 280 1 0 1 0 0 1 0 1 0 0 4 7 5 1 0 0 250 200 200 250 LVDT 01 LVDT 15 LVDT11 LVDT02 LVDT09 Laser 13 Laser 14 AE1 AE2 AE3 AE4 AE5 AE6 AE7 AE8 AE9 AE10 AE11 AE12 AE13 AE15 AE14 R275 R550

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In Fig. 6 and Fig. 7 a typical waveform is presented. An AE hit is registered from the first threshold vT

crossing until the signal is damped below the threshold. Two other important parameters are the amplitude, the maximum of vmax and vmin, and the duration tD, which is defined as the absolute time

between start of the hit ts and the end of the hit te (NEN-EN, 2009), (Kocur, 2012). Both parameters are

illustrated in Fig. 7.

Fig. 6: AE Hit Fig. 7: Amplitude and Duration

RESULTS AND DISCUSSION

Experimental Results

The load-time curves of experiment BB17 and BB18 are presented in Fig. 8 and Fig. 9. After the load level of 240 kN the actuator was switched to displacement control. In experiment BB17 failure already occurred before the intended subsequent load level of 300 kN was reached, at a load of 275.1 kN. By using this cyclic loading procedure the failure load was 14.4% lower than the failure load of 321.4 kN in the static loading test (van Hemert, 2014). Therefore, the experiment BB18 was done with an additional load step of 270 kN. In this case the failure load was 290.7 kN. Compared to the static loading experiments, this was 9.6% lower.

Fig. 8: Load-Time Curve BB17 Fig. 9: Load-Time Curve BB18

In Fig. 10 the load-displacement relations of the LVDTs’ is plotted for experiment BB17. In Fig. 11 the load-displacement of experiment BB18 is presented. Comparison between the two experiments shows that the stiffness of the specimen at the measured locations is different. Experiment BB17 showed a lower stiffness than BB18. The difference between the stiffness might be caused by the cracked deck of

Hit 1 v_T v_max v_min _t D t_s t_e 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 0 50 100 150 200 250 300 Time [sec] Load [k N ] 0 0.5 1 1.5 2 2.5 0 50 100 150 200 250 300 Time [sec] Load [k N ] x 104

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experiment BB17. These cracks were the result of a previous experiment, see also Fig. 20. BB18 is done on a uncracked deck in-between two shrinkages cracks. In the load-displacement curve of the two experiments no visual signs were found which indicated upcoming failure.

Fig. 10: Load-Displacement Curve BB17 Fig. 11: Load-Displacement Curve BB18

AE-Measurements

After reaching each load level, the load is held constant for 10 minutes. During these 10 minutes the number of hits is registered. This information might possibly show whether the internal fracture processes at that specific load level continuous (Ohtsu, 1998). Therefore, in Fig. 12 and Fig. 13 the amount of AE hits are plotted against time. The figures show that for each load level the number of hits in the second and third cycle are lower than in the previous cycle. This holds for the hits during increasing of the load, but also when the number of hits during unloading are mutually compared. In general, the number of hits in the first loading peak of a load level are higher than in the first unloading peak, see Fig. 12 and Fig. 13. Especially in the case of BB17 this is visible in Fig. 12. However, during the experiment of BB18 this changes at the load levels of 240 and 270 kN. For these load levels, still the number of hits reduces in the second and third loading cycle, but for each cycle the number of hits during unloading is higher than during loading. To get more insight whether this might be related to the upcoming failure, the AE results are analysed based on energy and cumulative hits.

Fig. 12: Hits vs. Time Graph BB17 Fig. 13: Hits vs. Time Graph BB18

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 50 100 150 200 250 300 Displacement [mm] Load [k N ] LVDT 15&16 0 0.5 1 1.5 2 2.5 3 3.5 0 50 100 150 200 250 300 Displacement [mm] Load [k N ] LVDT 15&16 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 500 1000 1500 2000 2500 3000 3500 Time [sec] H its [-] x 104 60 120 180 240 First loading 120 kN First unloading 120 kN 0 0.5 1 1.5 2 2.5 0 500 1000 1500 2000 2500 3000 3500 Time [sec] H its [-] x 104 60 120 180 240 F irs t l oa di ng 120 kN F irs t unl oa di ng 120 kN 270

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In Fig. 14 and Fig. 15 the energy against the time is presented as also was done by (Kapphahn & Slowik, 2008). The energy of an AE hit is represented by the area under the curve that connects the peaks of the waves (see Fig. 7) and is related to the amplitude vmax and duration tD of a signal. In general, waves with a

long duration time have a high amount of energy. For plotting the energy against the time an energy threshold of 100 [10 µvolt-sec/count] is used. Because of the AE program settings, the maximum energy release per wave is about 6.5x104 [10 µvolt-sec/count]. As can be seen, high energy waves occur mainly during increasing the load to a new load level. In this respect it can be mentioned that in concrete two possible causes for energy release can be distinguished. First of all the energy releases when new cracks are created. Secondly, during loading and unloading friction in existing cracks will also be accompanied with energy release. The results in Figs. 14 and 15 may indicate that hits during creation of new cracks are accompanied with higher energy content then those due to friction in cracks. Especially during failure of the specimens there is a high release of high energy waves. However, there is no clear trend in the increasing of the number of these high energy wave hits during the previous load levels.

Fig. 14: Energy-Time Graph BB17 Fig. 15: Energy-Time Graph BB18

The ‘Kaiser effect’ is defined as “the phenomenon where a material under load emits acoustic waves only after a primary load level is exceeded. During reloading, these materials behave elastically before the previous maximum load level is reached. If the Kaiser effect is permanent for these materials, little or no AE will be recorded before the previous maximum stress level is achieved” according to (Grosse & Ohtsu, 2008). With the behaviour of cracks in concrete during loading and unloading, as discussed before, it can be understood that for concrete the Kaiser effect will not exist, at least not in a pure way. Nevertheless, in Fig. 16 and Fig. 17, the cumulative number of hits is plotted versus the applied load. In case of a Kaiser effect the amount of AE hits during loading and unloading of second and third cycles should have been constant. This is not the case for both experiments. In case of these experiments there are a certain amount of AE hits during unloading and reloading to the previous load level, as expected and was already clear from previous presented results. Nevertheless, it can be seen that at each higher load level the number of cumulative hits increases, indicating an increasingly higher level of internal damage. To get a better understanding whether these hits occur during loading and unloading or during the constant load phase, the ‘calm ratio’ is investigated.

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Fig. 16: Cumulative AE Hits vs. Load BB17 Fig. 17: Cumulative AE Hits vs. Load BB18

The calm ratio is defined as “the ratio of the number of cumulative AE activity during unloading to that of the complete loading cycle” (Grosse & Ohtsu, 2008), whereby in this article the AE activity is defined as the number of AE hits. In Fig. 18 and Fig. 19 the calm ratio per cycle is presented. It is interesting to see that the difference in calm ratio between the first and the third cycle of each load level decreases with increasing load level. For instance, load level one of BB17 the ratio between calm ratio of the first and third cycle is 0.55/ 0.18 = 3.08. For load level two this was 0.59/ 0.35 = 1.69, load level three 0.65/ 0.46 = 1.40, and load level four 0.61/ 0.53 = 1.14. Both parameters might in the future be used as indicators for upcoming failure, however many research in this area is required before it is that far.

Fig. 18: Calm Ratio BB17 Fig. 19: Calm Ratio BB18

Source Location

Source location is a technique whereby four or more sensors are used to calculate the position of an event. This is a well-known technique for steel structures. It is investigated whether in the current experiments this technique can be used to localise the crack development. The success of source localization depends on the applying the correct velocity and the effect of attenuation of the wave. With respect to the latter, cracks between the sensors influence the wave paths, which can result in incorrect location designation. In Fig. 20 until Fig. 29 the crack development and the number of AE hits during the loading part of the first cycle of a new load level is shown. The outer circle sensors, see Fig. 5, are used as guard sensors, which means the hits that arrive first at the outer circle sensors are not registered. Therefore, the amount of hits

1 2 3 4 5 6 7 8 9 10 11 12 13 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Load Cycle [-] C al m R ati o [-] 0 2 4 6 8 10 12 14 16 18 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Load Cycle [-] C al m R ati o [-]

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within the outer circle is larger in comparison to the hits outside this circle. Cracks plotted with the colour green are existing cracks from a previous experiment. In the beginning of the experiment the hits are around the purple cracks, which were the shrinkages cracks that were already present before the start of the experiment. Between the load step of 60 and 120 kN the concrete below the loading plate starts to crack. In this stadium of the experiment sound sources were close to the existing cracks and the new cracks under the loading plate. After a load of 120 kN, no correlation could be found anymore between new cracks, old cracks and the indicated sound source locations.

Fig. 20: Crack Pattern 60 kN and AE Hits BB17 Fig. 21: Crack Pattern 60 kN and AE Hits BB18

60 kN 60 kN

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180 kN 180 kN

240 kN 240 kN

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CONCLUSION

Acoustic emission measurements are often seen as a possible valuable additional tool in the evaluation of existing concrete structures. However, analysing the AE results is a challenge. In general cyclic loading is applied for the AE analyses, while changes in AE parameters, like hits, energy and amplitude are used to investigate the condition of the structure. More specifically, it is intended to find a parameter that can be used to define to what extend failure of the structure is reached. Although, a long way has to be gone, before we are that far, in literature phenomena like Kaiser effect and calm ratio are proposed. In the Stevin laboratory of the Delft University of Technology a research project on this is being executed. Since a 1 to 2 scale bridge experiment with respect to the capacity of in-between decks was performed, the opportunity was taken to use this experiment to do some preliminary tests with AE. The AE findings for the cyclic tests, that failed at a ca. 10% lower capacity than the previous applied quasi static tests, were:

• There were no indications found in the hits vs time or energy vs time graph that the experiments is close to the failure load;

• Because of AE hits during (un)loading, the Kaiser effect does not exist in a pure way. Probably that has something to do with friction in the existing cracks during (un)loading.

• It appeared that for higher load levels the increase of calm ratio with number of load cycles decreases. Although in these preliminary experiments a real trend could not yet be found, the calm ratio seems to be an interesting parameter to further investigate.

• As far as source location is concerned in the beginning of the experiment to some extent the location of already existing cracks could be found. However, for higher load levels no relation was found.

FUTURE WORK

In the AE research project it is intended to further investigate the possibilities of AE measurements during proof loading for the assessment of existing concrete structures. In that respect, recently a full scale proof loading was performed on a ASR-affected viaduct. The AE signals measured during this experiment still need to be evaluated. However, based on the experiments described in this article, it is shown that it is already a challenge to evaluate AE measurements in controlled laboratory experiments. By investigating graphs like: energy-time, hits vs time, etc. there were no indications found that the experiment is close to failure. The other investigated technique, source location, results in questions about how velocity, attenuation, etc. influence the results. To thoroughly understand the results of the case study on the ASR-affected viaduct, it is important to solve these problems first. For that reason future work will also contain investigations on small concrete beams.

ACKNOWLEDGEMENTS

This research is supported by the Dutch Technology Foundation STW, applied science division of NWO and the Technology Program of the Ministry of Economic Affairs. The Dutch Ministry of Infrastructure and the Environment (Rijkswaterstaat) is greatly acknowledged for giving the opportunity to use the experiments on in-between decks for the preliminary AE-tests. The work of the laboratory personal of the Stevin Laboratory of Delft University of Technology is very much appreciated. Finally, Prof.dr.ir. J.C. Walraven is greatly acknowledged for initiating the project and by that giving me the opportunity to do this research.

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