Acoustic emission is an important nondestructive evaluation technique used in the field of structural engineering. In this study AE technique with a new approach was employed to investigate the process of fracture formation in reinforced concrete structure. A number of reinforced concrete (RC) frames were tested under cyclic load and were simultaneously monitored using AE. The AE test data using intensity analysis method were analyzed. This is based on calculating two values called the historic index (HI) and severity (Sr). The trend of HI and Sr showed that these parameters are able to indicate the levels of damage. Also, the results indicated that AE can be considered as a viable method to investigate the process of fracture formation in reinforced concrete structure.
Keywords: Reinforced concrete; Acoustic emission; nondestructive evaluation technique; AE source location; Intensity analysis
INTRODUCTION
AE method is a passive nondestructive technique that can be applicable in structural health monitoring (Proverbio, 2011). This technique has been used for source location and damage intensity predictions in field of civil engineering. The main goal of AE monitoring in structures is to detect, source, and assess the intensity of damage (Holford and Lark, 2005).
AE data can be evaluated by means of several methods. The Intensity analysis is a significant method for analysis of AE signals. This technique has already been success- fully applied to FRP and metal piping system evaluations (Nair and Cai, 2010). Also, a few works was found that IA method has been used for evaluation of the RC beam such as Golaski et al. (2002) and Proverbio(2011).
In main objective of this current study was evaluation of damage using Intensity analysis method. Commonly, previous works using has been focused on local evaluation of RC beams. However in this research, suitably of this method for global evaluation of RC frame was investigated.
2. METHODOLOGY
2.1. Intensity analysis
Intensity analysis (IA) evaluates the structural significance of an AE event and the level of deterioration of a structure by calculating two values called the historic index (HI) and severity (Sr) (Proverbio, 2011). The HI compares the signal strength of the most recent emissions to the signal strength of all emissions (Degala et al., 2009). Also, HI a measure of the changes in signal strength throughout the test which is an analytical method for estimating the changes of slop in cumulative signal strength against time (Proverbio, 2011) The Severity index, which is defined as the J largest signal strength emissions received at a sensor (Degala et al., 2009). HI is calculated using the following formulas (Blessing et al. 1992).
Where N is number of hits up to and including time, K is an empirical constant and is signal strength of ith hits. K is constant based on material. For concrete, N<50, K=0 ; 51<N<200, K=N-30; 201<N<500, K=0.85N; and N>501, K=N-75 as well as J valus for N<50, J=o and N>50 J=50 (Golaski et al., 2002).
.
Where, is the signal strength of the hit, J is an empirical constant based on material and based on magnitude of signal strength.
3. EXPERIMENTAL PROCEDURE
3.1. Material details
A series of experiments was conducted on reinforced concrete (RC) frame. A total of five RC frame specimens were built. The dimension of RC frames, were length of 2000mm, height of 1000mm and crocs section of 250x250 mm. The water to cement ratio was 0.5 and the material proportions were 1:3:4:0.6 by weight of cement, sand, aggregate and water respectively. The average compressive strength of concrete at 28 days was 240Mpa.
3.2. Test monitoring using AE technique
A total of five RC frame specimens described earlier were tested under loading cycle. In order to perform acoustic emission monitoring, an eight channel AE system (DISP-8PCI) manufactured by Physical Acoustics Corporation (PAC) was employed. Four R6I sensors with the resonance frequency of approximately 60 kHz were used. Figure1 shows sensor arrangements for the three point bending test. The AE systems hardware was set up was threshold level of 45dB for all channels in order to avoid the possibility of noise effect. The cyclic load pattern was determined. The load applied at one at mid span of the RC frame specimens. The load was applied in 10kN steps at mid span of RC frame. The load was applied from 0.5kN to maximum of each loading cycle (10kN increment) and held constant for one minute. Then, the load was unloaded from maximum of each loading cycle to 0.5kN and was held for 2 minutes. The test was monitored by AE throughout the test. The measurement include load, mid span deflection and AE data were recorded continuously during the three point bending test
Figure 1: Sensor arrangements for the three point bending test
4. RESULTS ANALYSIS AND DISCUSSION
4.1. Responses of test RC frame to cyclic loading
The RC frames described early were tested under loading cycle. Figure 2 shows a typical cracks development in the RC frames specimen. The behaviour of all RC frames under loading cycle can be divided into seven stages of failure namely:(I)Micro-cracking at the mid span of RC frame (II) First flexural cracks at mid span of RC frame (III) distributed flexural cracks at the mid span of RC frame (IV) first cracks at the beam-column connection zones (V)Distributed cracks at beam-column connection zones (VI) Damage localization at the beam-column connection zone (VII) Failure at beam-column connection zone .
4.2. Intensity Analysis
The AE data obtained in test was used in order to carry out Intensity Analysis (IA). The maximum of Severity (Sr) and Historic Index (HI) for all channels were calculated. These results are summarized in Table 1 and 2. Also, Figure 2 shows the maximum value of Sr and HI against loading cycle number for a sample of RC frame specimen. Data points shows that the maximum Sr and HI are increased with increasing of damage. Data points show that in stage micro-cracks, initiate cracks and distribution of cracks in mid-span of beam that HI is low level and without significant changes. Also, data points indicate that in stage initiate cracks and distribution of cracks in beam column connection, HI is high level with significant changes.
HI is a measure of the changes in signal strength throughout the test (Proverbio, 2011). Also, a significant increase in HI can indicate the onset of more serious structural damage as the loading progresses (Lovejoy, 2008). Furthermore, The AE knees may be used to identify possible damage mechanisms and to locate the onset of failure (Gostautas et al., 2005).
The results of this study show that early stage of failure that load cycles is less than 50% ultimate load, HI haven’t significant change. Also, the results indicated that is in stage in stage initiate in beam â€" column until specimen failure that that load cycles is more than 50% ultimate load, HI have significant changes. Thus, using the interpretation described above it is clear to see that HI can indicate serious structural damage in RC frame.
With respect to Sr that is average signal strength, the value of Sr can be used to show the level of damage. The primary advantage of using both HI and Sr in this application is the high sensitivity to stage of failure. Figure 3 shows the intensity chart for a sample. This chart can be divided three zones.
Zone 1 that consist of results HI and Sr during loading cycle 1 to 6 and initial of stages of failure. Also, Zone 2 that consist of results HI and Sr during loading cycle 7 to 9 and initial and distribution of cracks at the beam-column connection zones. Furthermore, Zone 3 that consist of results HI and Sr during loading cycle 10 to 12 and stage of damage localization and failure at beam-column connection zone. Data points show that stages of failure is recognizable using IA chart.
Table 1 a summary of maximum Historic index during cycle loading
Cycle no.
Stage of failure.
Maximum Severity Index
SPRCF1
SPRCF2
SPRCF3
SPRCF4
SPRCF5
C1
I
1.47E+06
1.24E+06
3.25E+06
1.35E+06
1.01E+06
C2
I
2.95E+06
2.10E+06
6.05E+06
1.68E+06
1.25E+06
C3
I
6.58E+06
4.84E+06
1.37E+07
4.15E+06
3.10E+06
C4
II
1.04E+06
1.04E+06
2.48E+06
1.39E+06
1.04E+06
C5
III
5.40E+06
4.96E+06
1.24E+07
6.07E+06
4.53E+06
C6
III
1.55E+07
1.29E+07
3.40E+07
1.37E+07
1.02E+07
C7
IV
1.94E+07
1.58E+07
4.22E+07
1.65E+07
1.23E+07
C8
V
4.02E+07
2.54E+07
7.88E+07
1.43E+07
1.07E+07
C9
V
4.46E+07
7.07E+07
1.38E+08
1.30E+08
9.68E+07
C10
V
6.87E+07
5.39E+07
1.47E+08
5.23E+07
3.90E+07
C11
VI
7.08E+07
6.01E+07
1.57E+08
6.62E+07
4.94E+07
C12
VII
2.32E+08
1.40E+08
4.46E+08
6.41E+07
4.79E+07
Table 2 a summary of maximum historic index during cycle loading
Cycle no.
Stage of failure.
Maximum Historic Index
SPRCF1
SPRCF2
SPRCF3
SPRCF4
SPRCF5
C1
I
2.33E+00
4.09E+02
4.94E+02
1.09E+03
8.16E+02
C2
I
1.10E+02
6.11E+02
8.66E+02
1.49E+03
1.11E+03
C3
I
3.48E+02
1.16E+03
1.81E+03
2.64E+03
1.97E+03
C4
II
5.90E+02
1.02E+03
1.94E+03
1.96E+03
1.46E+03
C5
III
3.40E+02
7.88E+02
1.35E+03
1.66E+03
1.24E+03
C6
III
1.13E+03
1.29E+03
2.90E+03
1.94E+03
1.45E+03
C7
IV
6.73E+03
3.97E+03
1.28E+04
1.62E+03
1.21E+03
C8
V
4.41E+03
5.23E+03
1.16E+04
8.11E+03
6.05E+03
C9
V
8.45E+03
8.38E+03
2.02E+04
1.11E+04
8.30E+03
C10
V
5.47E+03
8.88E+03
1.72E+04
1.65E+04
1.23E+04
C11
VI
1.41E+04
9.42E+03
2.82E+04
6.39E+03
4.77E+03
C12
VII
3.40E+04
2.18E+04
6.70E+04
1.28E+04
9.56E+03
Figure 2: The Intensity chart â€"SPRCF1
Figure 3: The Intensity chart â€"SPRCF1
5. CONCLUSIONS
This paper provides the results from tests on RC frame under loading cycle and was monitored by AE throughout the test. On the basis of AE activities, the analysis of signal characteristics using intensity analysis and with regard to damage levels, the conclusions are presented below:
Three levels of damage in concrete structure can be identified using intensity analysis
The trend of historic and severity index during loading cycle showed that these parameters are strongly sensitive with cracks growth in RC frame specimens and were able to indicate the levels of damage.
Results showed that AE can be considered as a viable method to predict the remaining service life of reinforced concrete.
ACKNOWLEDGMENT
The authors would like to thank Universiti Sains Malaysia (USM) for providing support through the short term Grant [304/PAWAM/6039047]
