научная статья по теме APPLICATION ACOUSTIC EMISSION METHOD DURING CONCRETE FROST RESISTANCE Общие и комплексные проблемы технических и прикладных наук и отраслей народного хозяйства

Текст научной статьи на тему «APPLICATION ACOUSTIC EMISSION METHOD DURING CONCRETE FROST RESISTANCE»

УДК 620.179.16

APPLICATION ACOUSTIC EMISSION METHOD DURING CONCRETE FROST RESISTANCE

Lubos Pazdera, Libor Topolar Brno University of Technology, Faculty of Civil Engineering, Department of Physics, Veveri 95, 602 00, Brno, Czech Republic Е-mail: pazdera.l@fce.vutbr.cz, topolar.l@fce.vutbr.cz

Abstract. Frost resistance is one of the most frequent characteristics of concrete. It is a very complex subject and the test methods themselves are still in development and the international consensus on methodology is still sought, too. The determination concrete frost resistance takes several weeks, months or even more than a year. However micro-structural changes as micro-cracks have not been described sufficiently. Acoustic Emission Method as unusual Non-Destructive Methods can help to monitor structural changes during common frost resistance measuring. Note the Acoustic Emission Method detects only active "defects" into monitored structure. Thus when e. g. crack grows some acoustic waves spread from crack place, i. e. from acoustic emission source. The method does not detect geometric discontinuities and "passive" defects. Selected Non-Destructive Methods as Ultrasound, Non-Linear Ultrasonic Spectroscopy, Impact Echo etc. are used to confirm micro-structural changes. The article describes the first experiment with its imperfections, difficulties and possibilities.

Key words: acoustic emission, concrete, frost resistance, impact-echo method.

ПРИМЕНЕНИЕ МЕТОДА АКУСТИЧЕСКОЙ ЭМИССИИ ДЛЯ ОЦЕНКИ МОРОЗОСТОЙКОСТИ БЕТОНА

Любош Паздера, Либор Тополар Технологический университет Брно, строительный факультет, кафедра "Физика ", Вевери 95, 602 00, Брно, Чешская Республика Е-mail: pazdera.l@fce.vutbr.cz, topolar.l@fce.vutbr.cz

Морозостойкость является одной из наиболее часто встречающихся характеристик бетона. Это очень сложный вопрос, и методы оценки морозостойкости до сих пор находятся в развитии, а международный консенсус по вопросам методологии до сих пор не достигнут. Определение морозостойкости бетона занимает несколько недель, месяцев или даже более года. Однако микроструктурные изменения, такие как микротрещины, не были описаны в полной мере. Метод акустической эмиссии, как нетрадиционный метод неразрушающего контроля, может помочь контролировать структурные изменения во время проведения обычного измерения морозостойкости бетона. Обращаем внимание, что метод акустической эмиссии обнаруживает только активные дефекты в исследуемой структуре. Таким образом, когда, например, трещина растет, некоторые акустические волны распространяются от месторасположения трещины, то есть от источника акустической эмиссии. Метод не обнаруживает дефекты геометрии — пассивные дефекты. Избранные неразрушающие методы, такие как у.з., нелинейная у.з. спектроскопия, отклик на удар и т.д., используют для подтверждения микроструктурных изменений. В статье описан первый эксперимент с его несовершенствами, трудностями и возможностями.

Ключевые слова: акустическая эмиссия, бетон, морозостойкость, метод отклика на удар.

INTRODUCTION

Concrete is one of the most popular building materials [1]. Frost resistance is one of the most important characteristics of concrete [3]. Frost resistance of concrete made with durable aggregate is determined by the air-void system ability to prevent development of destructive pressures due to freezing and associated movement of moisture in the concrete pores. The specific requirements of the air-void system depend on the amount and mobility of the water in the pores. An investigation of the frost resistance of concrete made with durable aggregates should identify the air-void system necessary to protect a variety of concrete water-pore systems [4]. The mechanisms of concrete damage from repeated cycles of freezing and thawing are not well understood and continue to be intensively

studied [5]. The frost resistance of concrete is a very complex subject and the test methods themselves are still the object development work, at the same time as international consensus on methodology is sought. From the available data and from examining existing constructions, three important factors in the production of frost-resistant concrete are [2]:

concrete exposed to frost attack should be air-entrained;

the proper use of silica fume will increase the frost resistance of the concrete;

it is important that the structural detailing prevents pending on horizontal parts of the structure.

Acoustic emission is the term for the noise emitted by material and structures when they are subjected to stress. Types of stresses can be mechanical, thermal or chemical [5]. This emission is caused by the rapid release of energy within a material due to events such as crack formation, and the subsequent extension occurring under an applied stress, generating transient elastic waves which can be detected by piezoelectric sensors [6, 7].

Acoustic emission method can monitor changes in materials behavior over a long time and without moving one of its components i. e. sensors. This makes the technique quite unique along with the ability to detect crack propagations occurring not only on the surface but also deep inside the material. The acoustic emission method is considered to be a "passive" nondestructive technique, because usually identifies defects while they develop during the test. The acoustic emission method is often used to detect a failure at a very early stage of damage long before a structure completely fails [7, 8].

Fracture in a material takes place with the release of stored strain energy, which is consumed by nucleating new external surfaces (cracks) and emitting elastic waves, which are defined as acoustic emission waves. The elastic waves propagate inside a material and are detected by an acoustic emission sensor. Except for contact less sensors, acoustic emission sensors are directly attached on the surface [8, 9].

Application Acoustic Emission Method during Frost Resistance Testing can help to describe structural micro-changes of this phenomenon. It is supposed that structural changes will follow temperature changes.

The impact-echo method is a technique for flaw detection in concrete. It is based on monitoring the surface motion resulting from a short-duration mechanical impact. The method overcomes many of the barriers associated with flaw detection in concrete based on ultrasonic methods. One of the key features of the method is the transformation of the recorded time domain waveform of the surface motion into the frequency domain. The impact gives rise to modes of vibration and the frequency of these modes is related to the geometry of the test object and the presence of flaws [10, 11].

EXPERIMENTAL SETUP

Two samples of length, 400 mm, high, 100 mm, and width, 100 mm, were measured simultaneously. Four acoustic emission sensors were placed on the surface of both samples (Fig. 1). Two sensors were placed on the first sample and the other two ones on the second sample [12]. Each sensor was kept by specially made holder, so that the contact between sensor and sample surface was as good as possible [13].

The four channels acoustic emission device named XEDO was used for detection of acoustic emission activity. Frequency range of two channels was set from 40 kHz to 2 MHz and another two from 100 kHz to 2 MHz. All four sensors with integrated preamplifier had frequency range up to 500 kHz (see Fig. 1 left). Their construction was resistant to aggressive salt solution and was used in temperature range from -30 °C to 30 °C without any troubles [3]. Both samples were put into freezer as long as temperature fell down to -25 °C and

twelve hours were kept there at this temperature. Then the samples were put into water 10 °C ~20 °C for two or three days to soak up water.

Fig. 1. Acoustic emission sensor (on the left) and location of acoustic emission and temperature sensors and electrodes for monitoring electrical properties (on the right).

Temperatures S [°C] inside the samples were measured by negative thermal resistors during whole experiment and computed by equation

d = 0.0342)

( R ^ 4

V R25 У

-0.827:

( R V

V R25 У

+ 7.4917:

( R V

V R25 У

-33.918

( R Л

V R25 У

+ 51.689, (1)

where Rx is measured resistance [Q], R25 is reference resistance at 25 °C [Q]. These sensors were embedded in concrete samples.

RESULTS

In the diagram (Fig. 2) is shown the dependence cumulative counts (Nc) on time (t). The specimen with worse frost resistance has got significantly higher the acoustic emission count event than specimen with better frost resistance during

N

С

40 000 35 000 30 000 25 000 20 000 15 000 10 000 5000 0

_ Specimen with better frost resistance . - Specimen with worse frost resistance

0 48 96 Fig. 2. Dependences of acoustic emission activity (N) on time (t)

144 192 240 288 336 384 t, h

at the measurement period (6 freezing cycles). It can be assumed that the greater numbers of acoustic emission events are caused by higher numbers of micro cracks in the concrete specimen.

Time histories of acoustic emission activity and temperature of two different concrete mixtures are shown in Figs. 3 and 4 (left). The specimen made from mixture, at which is expected worst frost resistance (Fig. 4), is indicated by bigger number of acoustic emission activity. Higher acoustic emission activity is noticed at increasing temperature. When temperature decreases the acoustic emission activity is usually not changed very much.

N

t, h

Fig. 3. Dependences of acoustic emission activity (N — left axis) and temperature (9 — right axis) on time (t) — (specimen with better frost resistance).

N 1014

10

10

AE event — Temperature

.......-4

o bo

20 10

0 O

o -10

-20 -30

0 48 96 144 192 240 288 336 384 t, h

Fig. 4. Dependences of acoustic emission activity (N — left axis) and temperature (9 — right axis) on time (t) — (specimen with worse frost resistance).

Acoustic emission activities, i. e. micro-changes in structure,

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