英文混凝土论文

Corrosion Process and Mechanisms of Corrosion- Induced Cracks in Reinforced Concrete identified by AE Analysis M. Ohtsu*, K. Mori† and Y. Kawasaki* *Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan †Research & Development Center, Taiheiyo Cement Corp. 2-4-2 Osaku, Sakura 285-8655, Japan ABSTRACT: Concrete structures could suffer from the corrosion of reinforcing steel bars (rebars) because of the penetration of chloride ions. For crack detection and damage evaluation in concrete, acoustic emission (AE) techniques have been extensively applied to concrete and concrete struc- tures. In the corrosion process of reinforced concrete, it is demonstrated that continuous AE monitoring is available to identify the onset of corrosion and the nucleation of concrete cracking because of the expansion of corrosion products. At the latter stage, the expansion of corrosion products generates corrosion-induced cracks in concrete. The generating mechanisms of these cracks are studied in accelerated corrosion tests of reinforced concrete beams. Kinematics of microcracks are identified by SiGMA (Simplified Green’s functions for Moment tensor Analysis) analysis of AE. It is demonstrated that AE activity at the onset of corrosion and at the nucleation of corrosion-induced cracks is in remarkable agreement with the phenomenological model of the corrosion process in steel. Then, mechanisms of corrosion-induced cracks are visually and quanti- tatively investigated by the SiGMA analysis. KEY WORDS: acoustic emission, corrosion of rebar, reinforced concrete, SiGMA analysis Introduction Corrosion of reinforcing steel bars (rebars) is known to be one of critical deteriorations in reinforced concrete structures. When chloride concentration at the level of rebar in concrete exceeds a range of values with the probability for onset of corrosion, a passive layer on the surface of rebar is destroyed and corrosion is initiated. Then, electrochemical reac- tion continues with available oxygen and water. According to a phenomenological model of rein- forcement corrosion in marine environments [1], a typical corrosion loss during the corrosion process is illustrated as shown in Figure 1 [2]. At phase 1, the onset of corrosion is initiated. As the rate of the corrosion process is controlled by the rate of trans- port of oxygen and water from the surface of rebar and the corrosion products build up on the cor- roding surface, the flow of oxygen is eventually inhibited, and thus the rate of the corrosion loss decreases at phase 2. The corrosion process proceeds further corrosion loss at phases 3 and 4 because of anaerobic corrosion. The corrosion penetrates inside the steel and the growth of corrosion products occurs. The phenomenological model of steel pre- sents a two-step process of the onset of corrosion and the growth of corrosion products. Acoustic emission (AE) techniques have been extensively studied in concrete engineering for approximately five decades [3]. They are applied to practical applications [4] and are going to be stan- dardised. This is because the increase in ageing structures and disastrous damages caused by the recent earthquakes urgently demand for mainte- nance and retrofit of reinforced concrete structures in service. It results in the need for the development of advanced and effective inspection techniques. Thus, AE techniques draw a great attention to diagnostic applications in concrete. Studies on fun- damentals of AE activity and the effects of mixture proportion were conducted [5–7]. A frequency analysis and a source location analysis were also reported [8–11]. In due course, applications to reinforced concrete structures were investigated [12]. These studies have resulted in practical appli- cations to monitor microcracks in concrete struc- tures and going to be made practical as diagnostic applications [13]. � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 179 doi: 10.1111/j.1475-1305.2010.00754.x An International Journal for Experimental Mechanics An application of the moment tensor analysis to AE waves was first reported on cracking mechanisms of glass because of the indentation [14], where only diagonal components of the tensor were assumed. This was because they dealt with only tensile cracks. From the definition, however, it is realised that the presence of all components is not associated with the type of a crack, but related with the directions of the coordinate system. Although the crack orientations are often assumed as parallel to the coordinate sys- tem, they are generally inclined to the coordinate system mostly because of the configuration of the specimen. As a result, the presence of all the com- ponents is consequent in AE waveform analysis. Both tensile motion of diagonal components and shear motion of off-diagonal are definitely present in crack motions as an AE source. Consequently, general treatment on the moment tensor components of diagonal and off-diagonal components has been developed as SiGMA (Simplified Green’s functions for Moment tensor Analysis) software [15, 16]. The pro- cedure is already applied to clarify fracture mecha- nisms in concrete members [17, 18]. By applying AE techniques, recently it has been reported that concrete cracking arising out of rebar corrosion is effectively detected [19], [20]. AE detec- tion because of the corrosion of rebar is illustrated in Figure 2 [21]. Recently, it is demonstrated that high AE activities are observed twice during the corrosion process [2]. As seen in Figure 3, a curve of total AE hits (counts) is in remarkable agreement with the curve shown in Figure 1. In the case of reinforced concrete, AE activity at phase 1 reasonably corre- sponds to the onset of corrosion in reinforcement. During phases 3 and 4, not only the growth of cor- rosion products, but also corrosion-induced cracks in concrete could be generated because of the expan- sion of corrosion products in reinforced concrete. In the figure, these periods are compared with the chloride concentration at rebar, where two threshold values of 1.2 and 0.3 kg per 1 m3 concrete are de- noted. The latter is equivalent to the lower-bound value for nucleation of corrosion in the Japanese standard [22] and is very low compared with the threshold values assigned for corrosion initiation in many reports. But, right after the chloride concen- tration becomes higher than the lower-bound, 1st high AE activity is observed which corresponds to phase 1 and the onset of corrosion. At the stage over the upper-bound value of chloride concentration, 2nd high AE activity is observed as phases 3 and 4. It is easily realised that AE hits arising out of concrete cracking is detected at the stage, because the expan- sion of corrosion products could occur. By applying the two-domain boundary-element method (BEM), extension of the corrosion-induced crack in an arbi- trary direction was analysed [23]. Here, experiments were conducted by simulating corrosion-induced cracks in expansion tests. With respect to the orien- tations of crack extension, results of the BEM analysis were compared with those of SiGMA, introducing the normalised stress intensity factors. It is demonstrated that extension of the corrosion-induced crack is governed by the mode-I failure in the meso- and macroscale. Based on these findings, here the SiGMA analysis is applied to an accelerated corrosion test of a rein- forced concrete beam, and thus kinematics of AE sources in the corrosion process are clarified and discussed. Figure 2: AE generation because of the corrosion of rebar Figure 3: Total number of AE hits and chloride concentration per 1 m)3 concrete [4] Figure 1: Typical corrosion loss for steel in sea water immer- sion [1] 180 � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 doi: 10.1111/j.1475-1305.2010.00754.x Corrosion Process and Mechanisms of Corrosion-Induced Cracks : M. Ohtsu et al. Experiments A reinforced concrete specimen tested was of dimensions 100 mm · 100 mm · 400 mm. One de- formed steel-bar (rebar) of 13-mm nominal diameter was embedded with 45-mm cover-thickness. Config- uration of the specimen is illustrated in Figure 4. In total, six specimens were prepared and tested. These were applied to estimate chloride concentration in concrete and measure half-cell potentials during accelerated tests. For chloride concentration, core samples of 30-mm diameter were taken, and sliced and crashed. Mixture proportion of concrete was that water: cement: sand: gravel = 0.55, 1.0, 2.15: 3.58 by weight. The maximum size of gravel was 20 mm, and the slump value (6 cm) and air content (5%) were controlled by using air-entrained admixture. The compressive strength at 28-day standard curing was 35.4 MPa. Right after the standard curing for 28 days, all surfaces of the specimen were coated by epoxy, except the bottom surface. An accelerated corrosion test conducted is shown in Figure 5. The specimen was placed on a copper plate at the bottom of a container filled with 3% NaCl solution. Between the copper plate and the rebar, 40 mA electric current was constantly charged. The current density chosen corresponds to a current density of 245 lA cm)2 charged into rebar, which is very common for accelerated tests. Previously, the accelerated corrosion tests of reinforced concrete were conducted by applying wet–dry cycles and by applying the electric charge [21]. It is realised that corrosion on rebar because of the electric charge is different from that of the wet–dry test. The former generated corrosion cracking inside rebar, because of the electric current and heat inside. In contrast, only the surface of rebar was corroded because of the wet and dry cycles. Consequently, a fairly low current density was selected, although the accelerated test was adopted to encounter two high AE activities earlier. Acoustic emission measurement was continuously conducted, by using AE analyser (DiSP, PAC). Six AE sensors (R15, PAC) of 150 kHz resonance were at- tached to the surface of one specimen as shown in Figure 5. The choice of six sensors resulted from the fact that the region for the analysis is as small as targeted area was of dimensions 100 mm · 100 mm · 100 mm. Frequency range of the measurement was 10 kHz–2 MHz, and total amplification was 60 dB gain. For AE counting, the dead-time was set to 2 ms. and the threshold level was 40 dB gain. The SiGMA analysis was applied to the one specimen, because AE activity during the corrosion process was already known. To monitor just the activity, other specimens were monitored by employing one-channel AE sys- tem. AE Analysis Parameter analysis Acoustic emission activity was analysed by AE hits and AE event. Here, AE hit is the term to indicate that a given AE channel has detected and processed one AE transient signal. Counting methods of AE signals are ringdown-counting by setting the threshold. By employing a multichannel system, AE wave can be detected in the form of hits on one or more channels. One event is a group of AE hits received from a single source by two or more channels, of which spatial coordinates could be located. Characteristics of AE signals were estimated by using two indices of RA value and average frequency [24]. These are defined from such waveform param- eters as rise time, maximum amplitude, counts and duration shown in Figure 6. AE sources of active cracks are classified, based on the relationship be- tween these indices. Two indices are defined as follows: RA ¼ Rise time=Maximum amplitude; (1) Average frequency ¼ Counts=Duration: (2) It is reported that the relationship between two indices is effective to classify cracks into tensile cracks and shear cracks [24]. Here, when the RA value is small and the average frequency is high, AE source is Figure 4: Sketch of reinforced concrete specimen tested Figure 5: Experimental set-up of accelerated corrosion test � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 181 doi: 10.1111/j.1475-1305.2010.00754.x M. Ohtsu et al. : Corrosion Process and Mechanisms of Corrosion-Induced Cracks classified as a tensile crack. In the other case, AE source is referred to as other cracks than a tensile crack. This criterion is applied to classify AE events detected in the corrosion process. To evaluate the size distribution of AE sources, the amplitude distribution of AE events is applicable. A relationship between the number of AE events, N, and the maximum amplitudes, A, is statistically rep- resented as Log10N ¼ a� bLog10A; (3) where a and b are empirical constants. The latter is called the improved b value (Ib value), which is pro- posed on the basis of cumulative distribution [25]. For 100 hits, the Ib value is defined, assuming aver- aged amplitude l and standard deviation r, Ib ¼ ½log10Nðl� rÞ � log10Nðlþ r�=2r; (4) where N(l ) r) and N(l + r) represent the number of hits with the amplitudes higher than l ) r and l + r, respectively. In the case that the Ib values are large, small AE events are mostly generated. In contrast, the case where the Ib values become small implies nucleation of large AE events. SiGMA analysis with AIC picker The SiGMA analysis consists of 3-D (three-dimen- sional) AE source location procedure and moment tensor analysis for AE source. Two parameters of the arrival time (P1) and the amplitude of the first mo- tion (P2) are read and applied to the analysis. In AE source location procedure, AE source is located from the arrival time differences ti between the observation point xi and xi+1, by solving equations, Ri � R iþ1 ¼ jxi � x0j � jxiþ1 � x0j ¼ mpti: (5) Here, vp is the velocity of P wave. After determining the AE source location, the amplitudes of the first motion (P2) are substituted into the following equation. AðxÞ ¼ CS � Refðt; cÞ R � cpcqMpq �DA (6) Here, A(x) is the amplitude of the first motion and CS is the calibration coefficient of the sensor sensitivity and material constants. The reflection coefficient Ref(t,c) is obtained as t is the direction of sensor sensitivity. DA is area of crack surface, Mpq is the moment tensor and c is the direction vector of dis- tance R from the source to the observation point x. As the moment tensor Mpq is symmetric and of the sec- ond rank, the number of independent unknownsMpq is six. To determine the moment tensor components, waveforms are to be detected at more than six sen- sors. The classification of a crack is performed by the eigen-value analysis of the moment tensor [15]. Eventually, microcracks are visualised by employing the Light Wave 3D software (New Tek) as shown in Figure 7. Here, an arrow vector indicates a crack motion vector, and a circular plate corresponds to a crack surface, which is perpendicular to a crack nor- mal vector. In the conventional SiGMA, determination of the two parameters of P1 and P2 for the SiGMA analysis has been carried out one-by-one via a software package named ‘‘wave-monitor’’. To process many AE waveforms, easy and quick determination of the first motion is in great demand. An auto-picker is developed by combining the auto-regressive model with AIC (Akaike Information Criterion) method [26]. Here, a direct AIC method is applied to deter- mination of the arrival time for the SiGMA analysis [27]. As the number of amplitudes of a digitised AE wave is N and values of amplitudes are Xi (i = 1, 2, ..N), AICk at point i = k is represented as AICk ¼ k � logfvarðX½1;k�Þg þ ðN � kÞ � logfvarðX½k;N�Þg; (7) where var(X[1,k]) indicates the variance between X1 and Xk, and var(X[k,N]) is also the variance between Figure 6: AE waveform parameters Figure 7: Crack models in SiGMA analysis 182 � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 doi: 10.1111/j.1475-1305.2010.00754.x Corrosion Process and Mechanisms of Corrosion-Induced Cracks : M. Ohtsu et al. Xk and XN. The AIC method defines the onset point as the global minimum. In the automated detection of the first motion (auto-picker), this value was adopted. Then, the parameter P1 of arrival time is determined by applying an equation, P1 ¼ Tk MinðAICkÞgf � DT ; (8) where Tk[Min(AICk)] represents the period when AICk becomes the minimum value at i = k and DT is sampling time which is set to 1 ls in our experi- ments. As for the determination of the parameter P2, amplitude Xi which satisfies the following equation is adopted. P2 ¼ Xi ; when ðXi � Xi�1ÞðXiþ1 � XiÞh0 ; (9) where small index i represents between k + 1 and N. Results and Discussion AE activity Total number of AE hits and AE events, which were located reasonably inside the specimen, are shown in Figure 8. Generating process of AE hits observed is classified into four stages, referring to the curve of corrosion loss in Figure 1. After 48 h elapsed, the increase in AE hits is observed as Stage 1. Following the decrease in AE hits at Stage 2, high AE activity is again observed at Stage 3. AE events, which were simultaneously detected at six channels and suc- cessfully located, are mostly observed at Stage 4. The number of these events observed is denoted in the figure. It is confirmed that the curve of AE activity (total AE hits) is in remarkable agreement with the curve in Figure 1. This implies that the stages in Figure 8 reasonably correspond to the phases in Fig- ure 1. Accordingly, it leads to the fact that the onset of corrosion started at Stage 1 after 48 h elapsed, and the expansion of corrosion products occurred at Stage 3, continuing at Stage 4. It clearly implies that AE events observed at Stage 4 result from corrosion- induced cracking in concrete. It suggests that the occurrence of corrosion-induced cracking in concrete is readily detected and located by AE measurement. Parameter analysis Variations of the RA values and the averaged fre- quency are given in Figure 9. Although trends of the variations are not clear at Stages 1, 2 and 3, the RA values start to decrease and the averaged frequencies are increasing from Stage 3. At Stage 4, the RA values are constantly low and the average frequencies are high, suggesting the nucleation of tensile cracks. From 72 to 96 h elapsed at Stage 1, an abrupt de- crease in the average frequency is observed, corre- sponding to the period when the 1st increase in AE hits is observed in Figure 8. This suggests that cracks other than tensile cracks are nucleated because of the onset of corrosion in rebar. At Stage 4, as tensile cracks are actively nucleated, the generation of cor- rosion-induced cracks is evident. According to the Ib values in Figure 10, the values suddenly decrease at around 288 h elapsed, when the most activity of AE events is observed in Figure 8. This implies that large-scale tensile cracks are gener- ated as corrosion-induced cracks. The Ib values be- come large after the sudden decrease in Figure 10, while the RA values are consistently low in Figure 9. It suggests that large-scale tensile cracks are domi- nantly generated around 288 h elapsed. It has been reported [2] that at the 1st high AE activity of Stage 1, AE sources were of small amplitudes and classified as other-type cracks. At this stage, chloride concentra- tion at the level of rebar was just higher than the lower-bound level for the initiation of corrosion. At Figure 8: AE activity during the test � 2010 Blackwell Publishing Ltd j Strain (2011) 47 (Suppl. 2), 179–186 183 doi: 10.1111/j.1475-1305.2010.00754.x M. Ohtsu et al. : Corrosion Process and Mechanisms of Corrosion-Induced Cracks the 2nd high AE activity