International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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AE ENERGY ANALYSIS ON CONCRETE BRIDGE BEAMSIng. S. Colombo & Prof. M. C. Forde, School of Engineering and Electronics, The University of Edinburgh - UK
Prof. I.G. Main, School of GeoSciences, The University of Edinburgh - UK
J. Halliday, Highways Agency, London - UK
Assoc. Prof. M. Shigeishi, Department of Civil Engineering, Kumamoto University - Japan
Concrete bridges were built originally to have a cost-effective maintenance free life of 120 years, however both in Europe and the USA this has proved to be totally optimistic. Concrete bridges require major maintenance after twenty to thirty years in order to extend their life. The Acoustic Emission technique has been successfully applied to monitor and provide information on the safety of concrete bridges, but the processing of the AE data is often not trivial. This paper proposes a method of AE analysis to assess concrete bridges based on the fact that the AE energy is one of the effective parameters to evaluate the damage of a concrete structure. Results from experiments on concrete with different design and loading configurations are presented. The beams were loaded in cycles and the energy during the loading and unloading phase was analysed. It was observed during the tests that none or very low energy activity was recorded during the unloading cycles. On the other hand such activity was increasing as the failure was approaching and the damage on the beams increased. In the light of these observations the energy recording during each loading up and loading off cycle was computed and a new parameter, the "relaxation ratio" was introduced. The final results were compared and combined and a proposed assessment criterion for the damage of concrete structures was obtained. It is argued that in some cases the state of damage of bridge beams can be estimated from the AE energy analysis when approximately 45% of the ultimate load has been applied. This criterion was then evaluated in relation to the quantitative assessment criterion recommended by the Japanese Society for Non-Destructive Inspection. Full laboratory data and analyses are given in the paper.
Bridges represent a huge asset of a country with their importance lying in their functional and cultural significance. From the late 1960's to late 1970's the motorway construction boom in the UK gave rise to a large number of new concrete highway bridges, as concrete was perceived to be "maintenance free" and able to give bridges a 120 year life span. As a consequence, concrete bridges represent the majority of the UK motorway and trunk road bridge stock. Now that their average age is of the order of 25-35 years, they are starting to show signs of deterioration, demonstrating that the original construction expectation was too optimistic. The US bridge stock is older and faces a worse situation. Consequently, the necessity to monitor and verify the performance and safety for ongoing usage of concrete bridges is a matter of urgent concern. The Acoustic Emission (AE) technique has been successfully applied to monitor concrete bridges and to provide information about their structural condition, but the processing of the AE data is often not trivial. Moreover, during the evaluation of the safety of a bridge, it would be useful for the bridge engineer to be able to establish not only if the bridge is damaged, but also "how seriously" it is damaged. A quantitative way to assess the structural condition of a bridge is therefore highly desirable. The work presented herein approached this issue. A new type of parametric analysis of the AE data was developed on the base on a new defined parameter, the ``relaxation ratio'', that takes into account the AE energy recorded during the loading and unloading phase of an AE test. In the light of the results of this analysis, a criterion to assess the damage of concrete bridge beams was proposed. This criterion was then evaluated in comparison with the quantitative assessment procedure recommended by the Japanese Society for Non-Destructive Inspection (NDIS) (Ohtsu, 2002).
THE "RELAXATION RATIO" ANALYSIS
An AE test often consists of several load cycles, each of which generally includes two stages: a loading phase and an unloading phase. In the course of some experiments it was observed that during the early cycles of the tests, none or very low AE activity was recorded during the unloading of the specimens. The AE activity increased with the approach of failure and increasing damage of samples. In fact, AE activity observed during the unloading process is generally an indication of structural instability (Ohtsu, 2002).
A parallel can be drawn with earthquake sequences, recognised in seismology. Earthquakes generally are made up of foreshock and aftershock sequences which are closely associated with a larger event called the mainshock. A schematic illustration can be seen in Figure 1. Aftershocks follow the mainshock and are linearly proportional to the area of the mainshock rupture. Aftershocks typically begin immediately after the mainshock over the entire rupture area and its surroundings, or are generally concentrated around the rupture perimeter or in locations where the mainshock has newly produced high concentrations of stress. Therefore it can be said that aftershocks are a process of relaxing stress concentrations caused by the dynamic rupture of the mainshock. Foreshocks are smaller earthquakes that precede the mainshock. They generally occur in the vicinity of the mainshock hypocentre and are probably a part of the nucleation process (Scholz, 2002).
Keeping earthquake sequences in mind the failure of a specimen, or accumulated damage at the end of a load cycle, can be considered as the mainshock. The foreshocks and aftershocks can be seen as the acoustic emissions generated respectively during the loading and unloading phases. In the light of these preliminary observations, a "relaxation ratio" is proposed to quantify and compare the AE activity during the loading and unloading phases. As previous experiments had shown that the AE energy seems to be the most effective parameter to describe the damage of a beam (Colombo et al, 2002) the relaxation ratio is expressed in terms of energy and defined as:
RELAXATION RATIO = average energy during unloading phase/ average energy during loading phase
where the average energy is calculated as the cumulative energy recorded for each phase divided by the number of recorded hits. A relaxation ratio greater than one implies that the average energy recorded during the unloading cycle is higher than the average energy recorded during the corresponding loading cycle, therefore the relaxation (aftershock) is dominant. Vice-versa, the loading (foreshock) is dominant.
NDIS ASSESSMENT PROCEDURE
The Japanese Society for Non-Destructive Inspection (NDIS) has recently proposed a criterion based on the Kaiser Effect to quantitatively assess concrete structures (Ohtsu, 2002). The criterion is based on the definition of two parameters:
Load ratio = load at the onset of AE activity in the subsequent loading / the previous load
A The Load ratio is based on the concept of Kaiser Effect (Yuyama et al, 1998). A ratio greater than 1 is an indication of a structure in good condition, whilst a value less than 1 suggests the presence of damage. The Calm ratio refers to the generation of AE activity during unloading which is an indication of structural instability - as in a structure in good condition no acoustic emissions are generally recorded during this phase. The NDIS-2421 recommended an assessment chart based on a combination of these two parameters as shown in Figure 2.
The classification limits indicated by the dashed line in the figure need to be determined in advance, based on preliminary tests, in order to be used for practical applications (Ohtsu, 2002).
In order to have results representative of a significant variety of cases (in terms of type of failure, design, load configuration, concrete properties and type of sensor) the data related to several beams were analysed. Half of the beams were tested in Edinburgh (named beams BF2, BF3, BF2c, BF4, BF5 and BF6) whilst the remaining (named beams K1, K2, K3, K4, KL1 and KL2) were tested in Kumamoto, Japan. The beams were designed to be representative of the behaviour of concrete bridge beams. During the tests, they were simply supported and the load was applied at two points using hydraulic jacks that varied for the different specimens according to the type of failure that was to be achieved. The load was applied in cycles of varying steps, generally determined on the basis of the calculated designed failure load. A complete cycle consisted of two main sequential phases a loading up and an unloading phase. Beam BF2 was tested twice (renamed BF2c) - although it was seriously damaged it did not fail completely under the first load configuration. During the tests the beams were monitored using a PAC AE system and a varying number, location and type of sensors. A summarised description of all the tested beams is in Tables 1 and 2.
The results are shown in Figures 3, 4 and 5. The dots on each graph represent the values of the relaxation ratio for each set of data and for all the cycles that composed the whole experiment relative to the specific sample. The number of the cycle is indicated on the horizontal axis, whilst the value of the relative relaxation ratio is shown on the vertical axis. The red horizontal line corresponds to a relaxation ratio equal to one. It therefore divides the area above the line, in which the relaxation phase is dominant, from the zone below the line, where the loading phase is dominant. According to the preliminary observations, a dominance of the relaxation phase is then indicative of heavy damage on the beam.
Figure 3 shows the results related to the beams BF2, BF3, BF4 and BF2c. A common behaviour can be noted. Initially, the loading phase is dominant and the values of the relaxation ratio all lie below the red line. An inversion of trend occurs when the load reached a specific value of approximately 45% of the failure load of the sample and the relaxation phase then becomes dominant. The data related to the resonant sensors on beam BF2c indicate the change of trend at a lower percentage (36.9%). As the beam was pre-damaged, the resonant sensors might have been recorded more activity generated by the closing of the pre-existing crack during the relaxation phase. The general trend can be explained as a dominance of the primary AE activity (Iwanami et al, 1997) during the early stage of the fracture process when the cracks are forming and the damage is still restricted. Conversely, once the damage has seriously progressed, the secondary AE activity due to the friction of the existing cracks starts to prevail - manifesting itself during the relaxation phase of the tests.
Figure 4 illustrates the results of the data related to the beams K1, K2, K3, K4, KL1 and KL2. The data are more scattered and this is due to the fact that a higher threshold value was used than in the Edinburgh experiments (see Tables 1 and 2). The higher threshold generates a higher ``noise to signal '' ratio, resulting in the dispersion. However, no clear pattern can be identified in any of these six cases. In some graphs (beams K1, K4 and KL2), the relaxation ratio never exceeds the threshold value of one represented by the red line. In the remaining cases, some values go beyond the horizontal red line, but without a constant trend.
Two possible reasons can explain this different behaviour. The first possibility is related to the characteristics of the concrete. The concrete used in Japan had a much higher strength and thus it cracked at a higher level of stress. As AE derives from the energy released from the material, it seems reasonable to expect that higher stress should release a higher amount of energy. This could therefore be the reason for the dominance of activity recorded during crack nucleation and forming, i.e. during the loading phase. Secondly, the load rate used in the test was different. An average value of loading rate was calculated by dividing the sum of the maximum load applied in each cycle for the total duration time of each cycle. An average value of approximately 0.06-0.07 kN/sec was found for the experiments carried out in Edinburgh, whilst a value of approximately 0.04-0.05 kN/sec was found for the experiments in Kumamoto. This could in fact have affected the generation of AE activity and therefore the results.
To have a further confirmation, the data related to the beams BF5 and BF6 were analysed and the results are shown in Figure 5. To verify the influence of the load rate, the load was applied and removed as slowly as possible. The average value of load rate was in fact calculated as approximately 0.04 kN/sec. By looking at the graphs referring to the resonant sensors, it is possible to note that a similar trend to the one observed in the previous Edinburgh experiments is present. The percentage of ultimate load to which the change of dominant phases occurs is however different, being 31.8% for beam BF6 and 83.6% for beam BF5. Looking at Table 1, the lower wave velocity measured on these beams suggests a lower quality of the concrete compared to all previous cases. The characteristics of the concrete and the lower load rate might have once again influenced the results.
To evaluate the feasibility of the described analysis in the above section, a comparison with the quantitative assessment criterion proposed by the NDIS was carried out. Two cases were considered, beam BF4 for which the proposed method appeared to work and beam K2 for which it was not successful. The NDIS assessment procedure was then applied, using the data of the most active channel (number 5). The limits were determined graphically. The results are shown in Figure 6 with the load ratio and the calm ratio indicated on the horizontal and vertical axes respectively. The number inside the circle represents the corresponding loading cycle number. For beam BF4, the change of trend during the relaxation analysis occurred during cycle number 5. It can be noted on the NDSI assessment table that that cycle corresponds to the last one falling into the intermediate damage area - after which serious damage takes place. The two results appear then to support each other.
At this stage, only partial conclusions can be drawn. The method of analysis seems to be very promising and able to define the state of damage of concrete beams. A structurally serious damage condition should in fact result in the relaxation phase being dominant and thus a relaxation ratio greater than one. However, some of the results showed however a disagreement which is thought to be due to a combination of different properties of the concrete and load rate adopted during the tests. Further experiments are needed to evaluate in which specific conditions the analysis is successful and to establish the limits of its application. Once the limits are defined, a practical application to a full scale assessment load test on a real bridge is suggested. A new procedure is suggested using the AET and the relaxation ratio analysis. The assessment test would consist of cycles of loading and unloading, whilst monitoring it with an AE system. The load values should be based on the 45% of the expected ultimate load of the bridge where the significant change of trend would be expected. The relaxation ratio of each cycle should be calculated. A value greater than one would indicate serious damage, whilst a value minor than one would reassure about the condition of the bridge and its capacity to hold the expected load. Further work is needed to obtain final conclusions and thus confirm the feasibility of this type of analysis and eventual test procedure. On site trials would also be indispensable.
The authors acknowledge the facilities and the technical and support staff of the University of Edinburgh (UK) and of the University of Kumamoto (Japan). The financial support of Highways Agency (London) Contract No.3/320, the Small Project Grants of the University of Edinburgh Development Trust and the Academic Frontiers Student Exchange Promotion Program Scholarship of the Japanese Ministry of Education and Culture is greatly acknowledged.