ASSESSMENT OF DAMAGE PROCESSES IN REINFORCED REACTIVE POWDER CONCRETE (RPC) BEAMS BY ACOUSTIC EMISSION PROCEDURE

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1 International Symposium on Structural Health Monitoring and Nondestructive Testing 4-5 October 2018, Saarbruecken, Germany More info about this article: ASSESSMENT OF DAMAGE PROCESSES IN REINFORCED REACTIVE POWDER CONCRETE (RPC) BEAMS BY ACOUSTIC EMISSION PROCEDURE Ninel Alver *, Sena Tayfur, Cihat Yüksel, Setenay Sürmelioğlu Ege University, Department of Civil Engineering, Izmir, Turkey * Corresponding Author: ninel.alver@ege.edu.tr ABSTRACT As structural usage of reactive powder concrete (RPC) increases, which is a high-performance concrete containing high amount of cement, its mechanical and fractural behaviors are needed to be understood. Although there are numerous material-based studies found in literature, limited number of studies has investigated fracture behavior of structural elements made of this superior concrete. This study was carried out with purpose of identification of damage processes in reinforced RPC beams by using mechanical and acoustical observations. In this context, two reinforced RPC beams were produced, tested under threepoint-bending and monitored with Acoustic Emission (AE) technique providing detection of even micro-scaled damages under the laboratory conditions. As a result of the mechanical and acoustical assessments, points where the AE parameters alter indicate the fracture mechanisms. In addition, more ductile RPC beams results in having much more micro cracks with less AE energy and more gradual AE energy behavior. Since the fibers retard the development of macro cracks, the beam behaves more ductile. KEYWORDS: Reactive Powder Concrete (RPC), Acoustic Emission (AE), Fracture Process. 1. INTRODUCTION Modern concrete is more than the mixture of cement, water and aggregate. It generally consists of also mineral components, chemical admixtures and fibers. Development of this concrete is a result of origination of admixture science and usage of these sciences to watch micro and nano structure of the concrete [1]. In order to provide low maintenance and, construction and management efficiencies, usage of reactive powder concrete (RPC) as a construction material came into prominence. Moreover, its usage became even compulsory for construction of long-span bridges and high-rise buildings due to safety, practical, durability and economical advantages [2]. As application of the RPC increased in structural purposes, to know and standardize its mechanical and fractural behavior is needed. However, though numerous material-based studies were conducted, studies investigating structural properties of RPC are limited. Identification of micro crack originations and propagations in RPC is possible by using Acoustic Emission (AE) technique. AE is propagation and detection of elastic waves released within the stressed medium due to fracture sources [3]. This phenomenon is also used in nondestructive testing industry for detection of even micro cracks and leakages and quality control of products. For these purposes, AE has been frequently applied on structures in the field and its development studies have been proceeding for concrete [4, 5]. However, number of the studies based on usage of AE for RPC is limited: [6] investigated self-healing of cracks originated in ultra-high performance concrete by AE. [7] researched on cracks in small members produced by ultra-high performance concrete statistically and by AE. [8] investigated explosion-caused void mechanisms by AE. As seen, even these studies are material-focused studies and mechanical behavior of RPC along with its fracture mechanisms should be clarified. This study was conducted in an attempt to assess damage processes in reinforced RPC beams. Accordingly, RPC mixture was prepared and two 125x250x1500 mm reinforced beams including different reinforcement details were manufactured from this concrete. Afterwards, they were subjected to three-point-bending and were simultaneously monitored with AE technique. Finally, their mechanical and acoustical behaviors were compared, different AE characteristics were stated and their damage processes were unveiled.

2 2.1 METHOD 2. AE MONITORING OF REINFORCED RPC BEAMS UNDER FLEXURE Acoustic Emission (AE), which is defined as release of energy and propagation of it as elastic waves, is used in different field of applications. By this means, information about active damages such as their origination times, locations, types and magnitudes can be obtained. For these, elastic waves propagating within a stressed material are detected by AE sensors and are transformed into the digital signal forms. The features of these signals are AE parameters and they are used for AE analyses. Amplitude, energy, rise time, duration and average frequency are frequently used typical AE parameters. By processing these parameters with different techniques, significant knowledge concerning the fracture mechanism of the material can be obtained. 2.2 PRODUCTION OF THE TEST BEAMS & TESTS RPC mixture was produced by using CEM I 42.5R cement, silica fume, fly ash, plain and hooked steel fibers, 0-1 mm aggregate, hyper-plasticizer and water. From this mixture two test beams were manufactured with 125x250 mm in cross section and 1500 mm in length. They were designed considering different dominant failure mechanisms. Reinforcement details of the test beams are presented in Table 1. Table 1 Reinforcement details of the test beams. Test Beam Longitudinal Tension Rebar Longitudinal Compression Rebar Stirrup RPC/F 2Ø12 2Ø12 Ø8/10 RPC/FS 2Ø20 2Ø10 Ø8/10 The beams were tested under three-point-bending as their loading setup was given in Fig. 1. Besides, cracks were simultaneously monitored with Mistras DiSP AE system which consisted of 7 AE sensors, 7 preamplifiers, a recorder and a computer. The sensors were attached on different locations of the specimens with silicon grease and their coordinates were listed in Figure 1. Hydraulic jack Y Z X Load cell TEST SPECIMEN Potentiometer Sensor No X Y Z Fig. 1 Loading setup and coordinates of the AE sensors in mm. 2

3 3.1 MECHANICAL TEST RESULTS 3. RESULTS & DISCUSSION Longitudinal tension reinforcement of RPC/F yielded at 4 mm deflection level and the beam failed in flexure at 22.4 mm deflection level. In addition, its ultimate load capacity was kn. Afterwards, longitudinal reinforcement of RPC/FS yielded at 6 mm deflection level and it failed at 24 mm. Besides, its ultimate load capacity was kn. These results show that usage of RPC contributed to load and deflection capacities of the beams. Load vs deflection behaviors of both beams were presented in Fig. 2. Energy absorption capacities of the test beams were also calculated as J (RPC/F) and 5169 J (RPC/FS). As seen, the maximum energy was absorbed by RPC/FS and sufficient flexural and shear reinforcement increased energy absorption capacity of the beam. 3.2 AE TEST RESULTS Fig. 2 Load vs deflection curves of the test beams. Parameter analysis results show that AE parameters change at specific points during the tests when the critical cracks or failure mechanisms originated. However, their time-dependent distributions differed according to failure mechanisms of the beams. As seen from Fig. 3, RPC/F resisted up to 90 kn load (45 th sec) without any release of AE energy. Then, particularly after concentrations of micro cracks, it did not carry the load. Afterwards, the beam resisted up to 110 kn load level (75 th sec) due to strain hardening. This state can also be understood from continuous rising trend of AE energy. Gradual cumulative AE energy variations indicate that RPC/F generally presented a ductile behavior. On the other hand, AE activities having higher energy were observed in even low load levels for RPC/FS in contrast with RPC/F. However, its total AE energy was lower and cumulative AE energy curve was smoothened because of presence of both sufficient flexural and shear reinforcing rebars. Thus, more and frequent micro activities releasing lower energies were effective in RPC/FS. To comprehend failure magnitudes of the specimens, AE hit amounts and energies were compared by calculating their origination rates in a sec. As presented in Fig. 4, more micro activities having lower energy originated between and sec. However, macro activities having higher energy which were results of fewer hits were observed at between and sec. While lower energy releases were observed in RPC/FS, higher hit originated in this test specimen. Thus, it can be said that, activities formed in RPC/FS are more micro scale than those of RPC/F. 3

4 Fig. 3 AE energy distributions of the test beams. 4

5 Energy Release (1/sec) International Symposium on Structural Health Monitoring and Nondestructive Testing 2018, Saarbruecken, Germany Hit Origination (1/sec) Fig. 4 AE hit and energy originations in a sec. Fig. 5 presents Ib-value variations of the test beams with respect to amplitude distributions and their moving averages. This parameter is used to scale AE activities (Eq. 1) by calculating for certain groups of amplitude parameters. Ib=20(log10N 1-log10N 2)/(a 2-a 1) (Eq. 1) where N 1 indicates the number of maximum amplitude values (a 1) less than difference between mean and standard deviation of the amplitude distribution. N 2 also indicates the number of minimum amplitude values (a 2) higher than difference between mean and standard deviation of the amplitude distribution. As seen in Fig. 5, changes in moving averages of amplitude distributions cause fluctuations of Ib-values. Accordingly, although punctuations in Ib-value distributions were observed for both specimens; mean Ib-values of RPC/F changed from time to time, however that of RPC/FS remained approximately at 1.5 during the test. Thus, it can be said that same behavior as mechanical observations of RPC/FS was obtained from this feature which is a result of prevention of sudden failures. Moving average line of RPC/FS s amplitude distribution was also smooth. Nevertheless, it could not be possible to evaluate scales of cracking activities by using these Ib-value distributions because both specimens contained steel fibers which caused higher amplitude values. However, Ib-value variations of the beams are helpful for identifying specific points where critical mechanisms originate and to refer the points having higher AE energy releases. 5

6 Fig. 5 Ib-value and amplitude variations of the test beams. By localizing AE activities, cracking patterns of the specimens were scattered and Fig. 6 was composed. As seen, AE crack localization results are in accord with mechanical observations; in addition, invisible activities could be clarified by AE which were estimated as to be hairline cracks or steel fiber ruptures. 6

7 Fig. 6 Crack patterns of the test beams: mechanical observations and AE crack localization results. 3. CONCLUSION This paper is focused on identification of damage processes of reinforced RPC beams exposed to three-point-bending by AE technique. While one of the beams was flexure-deficient, the other was properly designed having right amount of reinforcement. By evaluating both mechanical and AE behaviors of the test beams, following conclusions were obtained from the study: Enhancing flexural reinforcement design of the RPC beam increased its ultimate load and energy absorption capacities with ratios of 134% and 147%, respectively. Hence, in order to fully use bearing capacity of a reinforced RPC beam, reinforcement amount should also be optimally increased. AE is an effective tool to clarify damage processes of RPC beams by distinguishing different failure mechanisms. Points, when the AE parameters alter, highlight the critical mechanisms. With increasing of ductility of RPC beam, more micro cracks releasing less AE energy originate. Even though the beam is ductile, it cannot absorb sufficient energy if it has lower flexural reinforcement. Besides, good reinforcement designation also smoothed time-dependent behaviors of the AE parameters. Thus, as the good reinforcement design prevents the sudden damages, total AE energy released decreases. In addition, activities are more micro scale than those of the flexural deficient beam. Ib-value variations are helpful for identifying specific points where critical mechanisms originate and to refer the points having higher AE energy releases. AE crack localization is very effective for identifying crack patterns of RPC beams, particularly detection of invisible fractures or steel fiber rupture events. REFERENCES [1] Pierre-Claude Aїtcin. Cements of Yesterday and Today Concrete of Tomorrow. Cement and Concrete Research. 2000;30: [2] Na-Hyun Yi, Jang-Ho Jay Kim, Tong-Seok Han et al. Blast-Resistant Characteristics of Ultra-High Strength Concrete and Reactive Powder Concrete. Construction and Building Materials. 2012;28: [3] American Society for Testing and Materials. Standard Terminology for NDT [4] Kentaro Ohno, Masayasu Ohtsu. Crack Classification in Concrete based on AE. Construction and Building Materials. 2010;24: [5] Masayasu Ohtsu, Yuma Kawasaki. AE-SiGMA Analysis in Brazilian Test and Accelerated Corrosion Test of Concrete. Journal of Acoustic Emission. 2010;28: [6] Granger, Sebastien, Louikili, Ahmed, Pijaudier-Cabot, G., Chanvillard, Gilles. Experimental Characterization of the Self-Healing of Cracks in an Ultra-High Performance Cementitious Material: Mechanical Tests and Acoustic Emission Analysis. Cement and Concrete Research. 2012; 37: [7] Dai, Qingli, Ng, Kenny, Zhou, Jun, Kreiger, Eric L., Ahlborn, Theresa M. Damage Investigation of Single-Edge Notched Beam Tests with Normal Strength Concrete and Ultra High Performance Concrete Specimens using Acoustic Emission Technique. Construction and Building Materials. 2012; 45: [8] Ozawa, Mitsuo, Uchida, Shinya, Kamada, Toshiro, Morimoto, Hiroaki. Study of Mechanisms of Explosive Spalling in High-Strength Concrete at High Temperatures using Acoustic Emission. Construction and Building Materials. 2012; 37: