Monitoring of cracking in reinforced concrete beam Under Shear Loadings by using Digital Image Correlation (DIC) and Acoustic Emission (AE)
BELBACHIR A./ PhD Student Civil engineering
RiSAM University of Tlemcen, Algeria GeM Ecole Centrale de Nantes, France Mail: ahmed.belbachir@ec-nantes.fr
MATALLAH M./Associate professor Civil engineering RiSAM University of Tlemcen, Algeria
SYED-YASIR A./ Assistant professor Civil engineering, GeM
Ecole Centrale de Nantes, Nantes, France
LOUKILI A./ Professor of universities Civil engineering, GeM
Ecole Centrale de Nantes, Nantes, France
Abstract— in this paper, an experimental study is performed on reinforced concrete beams without transverse reinforcement.
The kinematics of the diagonal crack was studied in beams, with a constant width of the cross section. The same longitudinal reinforcement ratio and the same shear-span ratio are retained.
Three point bending tests are performed in order to obtain the global mechanical behavior. During these tests, strains in longitudinal reinforcement bars were recorded using strain gauges embedded on the steel surface. The Digital Image Correlation (DIC) technique is used to monitor cracking during mechanical loading, in order to measure the intrinsic parameters of the cracking process (crack opening, crack length, slip between lips of cracks) at different stage. In the other face of the beams, sensors of the acoustic emission were placed to record the damage evolution and to locate the movement of the crack during test. Both AE and DIC are efficient techniques to study the failure process of reinforced concrete structures.
Keywords—reinforced concrete; Digital Images Correlation;
Acoustic Emission; shear cracks.
I. INTRODUCTION
Reinforced concrete structures are employed in bridges, buildings, dams and stadium…etc. These elements are subjected to different loading during services which conduct to the degradation and the failure of those structures [1-2]. The failure is due to the process of formation of different cracks (tensile, shear, or mixed cracks). During the process of failure, the cracks and damage of element can be identified and classified. In order to evaluate the behavior and testing the validity of theoretical model, several measurements techniques were developed to monitor and measure the cracking in reinforced concrete structures such as Acoustic Emission (AE) [3-5], microscopy, Dye Penetrate Inspection (DIP), and Digital Image Correlation (DIC) [5-7].
In this work, the Digital Image Correlation (DIC) and Acoustic Emission techniques were employed in order to study the fracture process in reinforced concrete elements without transversal reinforcement. The DIC technique was used to estimate the surface displacement field between the reference images and the deformed images taken during the test. Based on the measurements of displacement, the monitoring and openings of cracks can be determined [6]. The technique of Acoustic Emission (AE) is a non-destructive method [4]. This last is based on the recorder and acquisition of the phenomena inside the element which cannot be recorded by the DIC method. The AE is helpful for identifying the crack development in reinforced concrete elements.
The objectives of this paper are to understand and monitoring the cracking of reinforced concrete beams without stirrups. In order to get this objective, an experimental work was performed on three sizes of beam geometrically similar and with the same width. The same ratio of longitudinal reinforcement was installed in the beams. The DIC technique is employed to estimate the kinematics of cracks. In addition to that, the acoustic emission technique was also used to register the evolution of micro-cracking inside the specimens tested.
II. EXPERIMENTAL WORK A. Specimen geometry
To investigate and study the behavior of cracks in reinforced concrete beams without stirrups under shear loadings, an experimental work was carried out at Ecole Centrale de Nantes. The tested concrete specimens in this study are geometrically similar, where D1 present the smallest beam, D2 the medium beam and D3 the largest one. Geometry details are given in Table I. As shown in Table I, specimens
have the same ratio between the shear span a and the affective depth d (a/d=2.5).
Fig. 1. Tested specimen details (dimension in mm)
The longitudinal reinforcement ratio was kept constant (ρs=1.5%) in all the beams. At the ends of the reinforcing bars, right angled for hooks were performed. In addition, reinforcing bars were installed in the upper part of the beam with ratio of (ρc=0.5%) to minimize the compression and prevent the failure in crushing.
TABLE I. GEOMETRY DETAILS FOR THE THREE SPECIMENS.
Specim ens
b (mm)
h (mm)
d (mm)
L
(mm) ρs (%) As
(mm²)
D1 200 250 200 1000 1.50 3HA16
D2 200 450 400 2000 1.50 6H116
D3 200 650 600 3000 1.50 6HA20
B. Material properties
Specimens were cast by using a normal weight concrete made from the mix design illustrated in Table 2, where the maximum aggregate size is da=16 mm. In order to know the mechanical properties of the concrete used at 28 days, a test of simple compression on cylinders (diameter 110mm, length 220mm) was made to obtain the average of the compressive strength. Tensile strength was determined from the Brazilian test. The dynamic elastic modulus was obtained with the non- destructive method Grindosonic. The properties are summarized in table III.
TABLE II. CONCRETE MIX DESIGN DETAILS
Cement (Portland 52.5) 347 kg/m3
Sand 790 kg/m3
Coarse aggregate (6-10 mm) 245 kg/m3
Coarse aggregate (10-16 mm) 815 kg/m3
Water 185 kg/m3
The reinforcing bars used in this test are deformed steel bars where the properties of steel (yielding strength and elastic modulus) are listed in Table III.
TABLE III. REINFORCEMENT AND CONCRETE PROPERTIES. Concrete
Compressive strength 28.35 MPa
Tensile strength 3.31 MPa
Dynamic Modulus 30.00 GPa
Reinforcement
Yielding strength 500 MPa
Elastic Modulus 200 GPa
C. Test setup and instrumentations
Three points bending tests (Fig. 1) are carried out in order to get the global response of beams. The test setup is shown in Fig 2. The same vertical displacement rate of 0.02mm/min was kept constant during the test. A system has been adapted to measure the displacement of each beam in mid-span by using a laser sensor. At the tensile longitudinal reinforcement of each beam, strain gauges were placed in different places (see Fig. 1) for the acquisition of longitudinal strains during the test. Two gauges were installed near the supports where the loading shear is maximum and one gauge was installed in the middle of beams where the bending moment is maximum.
Fig. 2. Test configuration.
D. Digital Images Correlation (DIC)
Digital Images Correlation technique was applied in the experimental work. It is an optical technique used to measure the full field surface displacement between the reference image, which was taken without applying the load, and image taken after loading (deformed image) [5-6]. In addition, DIC technique can perform the crack opening of existing cracks during the test. In one side of the beam, this technique was
applied during this experimental work by employing two cameras with a resolution of 1040 x 1392 pixels. Each camera captured one side of the beam (interested area) as shown in Fig. 3. Both of the cameras were placed away from the specimen surface and perpendicular to the centre of the interested area, in order to record digital images every second.
Fig. 3. Zone captured by cameras.
E. Acoustic Emission (AE)
The Acoustic Emission technique is also used to detect the initiation and propagation of crack during loading test[3-4]. This method is based on installation of sensors on the surface of material to detect elastic waves produced by the internal displacement during fracture. In order to assess this method, 8 piezoelectric sensors of type R15a, having the same frequency ranged from 50 to 200 kHz and the same resonance of 150 kHz are used. These sensors were placed on the surface of the beam as show in Fig. 4. The distance between the sensors was changed in each beam.
Fig. 4. Location of acoustic emission sensors in beam D1.
III. RESULTS AND DISCUSSIONS A. Results of DIC
For each beam, the recorded images at different level of loading were analyzed. Based in this analysis, it was observed
a formation of flexional cracks in the mid-span du to presence of maximum value of bending moment. Later, some vertical cracks appeared in a shear span zone and continue to incline towards the point of loading. Close to the supports, a diagonal- crack that causes the failure was the continuity of the propagation of the bending crack. In the post-peak portion, it was noticed a third crack that propagates along the reinforcement caused by the delimitation between the steel and concrete (decrease in the bond strength).
Fig. 5. Cracking in reinforced concrete beam.
Fig. 5 shows the evolution and monitoring of the cracks during the test. The same mode of failure was observed in all the beams tested in this work. The location of cracks can determinate by the study of the major principal strain (as shown in Fig. 5). The opening of the existing cracks can be estimated by measuring the horizontal or the vertical displacement, and in the case of diagonal crack the crack width should be calculated using both of them. Those displacements could be extracted by tracing a section line in the interested location. Fig. 6 shows the results of distribution of horizontal displacement at different stage of loading. The sudden jumps are caused by crack openings, and the value of this jump can be used to estimate and calculate the crack width. In our experimental work, the technique was found able to estimate the position and the opening of cracks during the test.
Fig. 6. Horizontal displacement measured by DIC vs horizontal position of the created line.
B. Results of AE
In this section, the recorded results of Acoustic Emission are presented and discussed for tested beam. We have presented in this paper the totally results obtained from the software AEwin and treated by Matlab to plot the different parameters of acoustic emission. For each beam size the evolution of absolute energy versus the time was presented in Fig. 7. The absolute energy is used to indicate of the formation of large fracture surfaces.
In the elastic behavior of the beams, it is noticed that by increasing the load only few, AE hits with lower absolute energy arise, after that, they increase slower than before and it was also observed a large number of AE hits with higher absolute energy. In all the curves the presence of peaks was observed which can be explained by the formation of micro- cracking in the case of lower value of absolute energy.
However, the highest value of absolute energy corresponds to a large number of AE hits which should be attributed to the growth of macro-cracking and propagation of cracks. The larger beam was presented a less number of peaks and a higher value of the absolute energy released during the test comparing the smallest beam which have a lower value of the absolute energy with more peaks. It appears that as the size of specimen increase, the absolute energy released is increasing also.
(a)
(b)
(c)
Fig. 7. Applied Load and Absolute Energy of all the beams vs Time of testing: (a) beam D1, (b) beam D2 and (c) beam D3.
AE parameter analysis was made to classify the mode and nature of existence cracks. This analysis is based on two parameters: the RA value which is the ratio between the rise time and the maximum amplitude, and the second parameter is the average frequency which can be estimated from the AE counts and the duration [8-9].
(a)
(b)
(c)
Fig. 8.Relationship between Average Frequency and RA value for all the beams: (a) beam D1, (b) beam D2 and (c) beam D3.
Fig. 8(a-c) shows the Average frequency in function of the RA value for all the beam size and during all the test, and the diagonal line in the figures presents the limits between traction and shear cracks. Based on these analyses, the values at the right of the diagonal line are the events where the shear is dominant; and the values at the left side of the diagonal line represent the events for the tensile process. In the adjacent left and right of the diagonal line, the values are dues to the mixed
mode. For all the specimens tested, the events for the shear are higher than the events where the tensile mode is dominant. By comparing the number of events (shear or tensile or missed mode) in the largest beam, it is observed more events compared to the medium and smallest beams. It appears that as the size of reinforced concrete beams increase, the events also increase.
IV. CONCLUSIONS
In this paper an experimental work is presented in order to monitor and identify the crack growth in reinforced concrete beams without stirrups. Two techniques: Acoustic Emission and Digital Images Correlation were used. It is observed that:
-
Digital Images Correlation technique is able to describe the propagation, profiles, strains, and slipping of various cracks (diagonal and flexional cracks) at different states of load.-
The technique of Acoustic Emission was able to classify the different mode of failure (tensile or shear or mixed modes) for events detected.References
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