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Building Materials and Products / Строительные материалы и изделия
yflK 539.348
Makhmudov Kh., Benin A.
VIKTOR S. KUKSENKO, Doctor in Physics and Mathematics, Professor, Senior Researcher, Laboratory of Strength Physics, Ioffe Institute, RAS, St. Petersburg.
KHAIRULLO F. MAKHMUDOV, Candidate of Physical and Mathematical Sciences Laboratory of Strength Physics, Ioffe Institute, RAS, St. Petersburg. 26 Polytekhnicheskaya, St. Petersburg, Russia, 194021, e-mail: h.machmoudov@mail.ioffe.ru ANDREY V. BENIN, Ph.D. Head of the Mechanical Laboratory Petersburg State Transport University, St. Petersburg. 9 Moscow Av., St. Petersburg, Russia, 190031, e-mail: benin.andrey@mail.ru
Diagnostics of the stressed concrete beams instability
The article is concerned with the results of the comparative study aimed to detect the damage at the initial stage in reinforced concrete beams deformed with three-point bending. The beam deformation was measured with a set of strain indicators glued on the surface of the bean, which indicated the appearance of macroscopic cracking in. At the same time, the acoustic emission (AE) method was applied for monitoring the gradual accumulation of microscopic cracks, which also signalised the nucleation of a first visible crack. It has been found that the initial stage of damaging could be detected by those strain indicators that were situated in the close vicinity of newly-appeared cracks, while the AE transducers detected the loss of load carrying capability as being situated far from the initial small-scale failure.
Key words: concrete technology, beams & girders, failures.
Introduction
The diagnostics of constructions made of reinforced concrete is of particular importance for bulk components, sudden breakdown of which could result in great economic and human losses [3-5, 9]. There are a few control methods for assessing the current condition of stressed constructions [1, 2]. In this field, the acoustic emission (AE) technique, which allows one to control the appearance and extension of a locality of possible breakdown, gains momentum [6-8, 10]. In the present work, the AE method was applied for detecting the cracking in loaded concrete beams in a combination with the conventional strain gauging technique. Some advantages of the former technique for incipient failure detecting were demonstrated.
Samples and equipment
The tested beams were of rectangular cross-section made of reinforced concrete 0.3 m in height, 0.1 m in width, and 1.70 m in length. The beams were made of class V20 concrete (Russian nomenclature) reinforced with steel bars of class A400, 20 mm in diameter. The transverse reinforcement was performed with two-section binding ties made of ordinary reinforcing wire of 3 mm in diameter. These samples differed from a standard design in the presence of a particular zone. Under conditions of
© Kuksenko V.S., Makhmudov Kh.F., Benin A.V., 2015
Kuksenko V.
three-point bending tests, there were no ties in this zone along a significant length (300 mm) in the vicinity of cross-sections situated at a distance from supports equal to H of the bearing distance. The gaps between ties at other part of the bearing distance (in the vicinity of supports and in the neighborhood of the central cross-section) were equal to 50 mm. This pattern of reinforcing was used in order to initiate the nucleation of oblique cracks in the cross-sections, which could be expected in advance.
The experiments were carried out on a press PMM 250. Effective span between centers of supporting pads was equal to 1.50 m. The loading was produced with the concentrated force applied in the center of the bearing distance of the beam. The load was applied stepwise by equal increments of 9,81 kN (1 ton) with a time delay at every step.
A strain gauging system STKM-IS detected signals from 40 gauges glues onto the beams. The strain gauges were divided in four clusters, each of which contained ten differently oriented devices (Fig. 1). In addition, an indicating gage ICh 10 with measuring sensitivity of 0.01 mm was used for controlling the vertical deflection in the center of the beam. The crack appearance during the test was observed visually, and traced with a marker.
Four piesotransducers were used for detecting the AE pulses (locations of two of them are denoted in Fig. 1 by capital letters). The transducers were fixed on the beams with a rubber strap. In order to improve the sound pulse passage, special grease was applied on the beam surface under the transducers.
iLoad _
-A ^ L iUf _ — ^ Crack
s 0.375 m / 1.5 m \
1.7 m
Fig. 1. Layout of two sets of strain gauges (some of them denoted by lower-case letters) and two piezotransducers (capital letters) attached to loaded beam. A visible crack appeared in the vicinity of the right cluster of indicators
Results
A series of 5 beams was tested; all experiments brought similar results. Every time, the cracking occurred in the non-reinforced zone. An example of typical stress-strain dependences are depicted in Fig. 2. One can see that: i) only the strain gauges appeared to be in the close vicinity of the first visible crack, such as denoted by the lower-case letters a and b gave clear response to the local damage event; ii) the response of gauge a was stronger than that of the gauge b, which was inclined relatively the crack, and the angularly oriented gauge c did not show any response; iii) neither of differently oriented, just distant gauges d, e, and f demonstrated any response on the event of visible cracking.
Figure 3 shows a time sweep of the AE intensity recorded by the transducer A, which was remote significantly from the future visible crack. The pronounced regular oscillations reflect the step-wise mode of loading. At the initial stage of the test (prior to the appearance of macroscopic damage), the AE intensity increased in periods of the load increase, and dropped to the level of acoustic noise when the load was kept constant. About 2 minutes prior to the observation of the first visible crack (2-3 loading cycles before), the AE signal ceased reaching the noise level even in quiescent periods. The non-zero sound emission evidenced the latent accumulation of microscopic cracks at this stage. As beginning from
Fig. 2. Stress-strain plots as detected by strain gauges indicated in Fig. 1. Dashed horizontal line indicates the load matched to the visual observation of the first crack
the moment of the appearance of the visible crack, the pattern of oscillations changed qualitatively. The lower level of the AE intensity during periods of constant load increased significantly, and revealed a trend to further gradual increase. This effect can be explained by a progressive fracturing in macroscopically damaged beam even under the constant load.
T
_I_I_I_I_I_I_
300 400 500 600 700
Time, s
Fig. 3. Time series of the AE intensity detected by transducer A in stepwise loading beam
Further loading resulted in pulling of steel out of concrete. Therefore, a transition from the stage characterized by zero AE activity in periods between further loading cycles to the stage of permanently growing AE intensity under the constant load can be regarded as the loss of load carrying capability of the beam [8]. The AE technique allows one to determine reliably this transition with a minimal number of transducers established on the beam to be monitored.
Conclusion
The forthcoming breakdown of loaded carried structures manifests itself in persistent changes of strain in the vicinity of primary small-size damages and in emission of elastic waves that propagate from nucleated small cracks over distances comparable in length with dimensions of bulk components. The strain gauging system and the acoustic emission technique are sensitive, respectively, to these manifestations. A direct comparison of results of destructive testing of reinforced concrete beams, which were obtained with the noted techniques, demonstrated the applicability of both methods for detection the pre-failure conditions in bulk structures. However, the strain gauging system should be composed of many sensors to provide the reliable detection short-range structural distortions preceding the catastrophic loss of load carrying capability. Application of the acoustic emission technique allows one to resolve the problem of monitoring with a minimal number (1-3) of piezotransducers established on a structural element.
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Строительные материалы и изделия
Х.Ф. Махмудов, A.В. Бенин
КУКСЕНКО ВИКТОР СТЕПАНОВИЧ - доктор физико-математических наук, профессор, главный научный сотрудник лаборатории физики прочности (Физико-технический институт им. А.Ф. Иоффе РАН, Санкт-Петербург).
МАХМУДОВ ХАЙРУЛЛО ФАЙЗУЛЛАЕВИЧ - кандидат физико-математических наук, научный сотрудник лаборатории физики прочности (Физико-технический институт им. А.Ф. Иоффе РАН, Санкт-Петербург). Политехническая ул., д. 26, Санкт-Петербург, 194021. E-mail: h.machmoudov@mail.ioffe.ru
БЕНИН АНДРЕЙ ВЛАДИМИРОВИЧ - кандидат технических наук, заведующий механической лабораторией (Петербургский государственный университет путей сообщения, Санкт-Петербург). Московский пр., 9, Санкт-Петербург, 190031. Е-mail: benin.andrey@mail.ru
Диагностика потери устойчивости нагруженных железобетонных балок
Железобетонные балки длиной 1,7 м, сечением 0,3х0,1м нагружались изгибом по трехточечной схеме. Тензодатчиками регистрировалась деформация, в том числе локальная. Методом акустической эмиссии регистрировались микротрещины. Интенсивность акустической эмиссии возрастает при нарастании нагрузки и спадает практически до нуля при поддержании нагрузки постоянной. Потеря несущей способности балки надежно выявлена по качественному и количественному изменению акустической эмиссии.
Ключевые слова: балка, деформация, изгиб, железобетон, тензодатчик.
В.С. Куксенко
