Ohtsu Acoustic Emission and Related Non-destructive Evaluation Techniques in the Fracture Mechanics of Concrete

Fundamentals and Applications
1. Auflage 2015
ISBN: 978-1-78242-345-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 318 Seiten

Reihe: Woodhead Publishing Series in Civil and Structural Engineering

The development of NDT (non-destructive testing) techniques used for the inspection of concrete structures is currently in high demand, because many existing structures have become aged and deteriorated in service. In order to formulate predictions on their stability and to estimate their safety, it is necessary to identify damage signals and to determine their causes. In this regard, the development and establishment of innovative and highly advanced non-destructive methods are required. Acoustic Emission (AE) and related NDE (non-destructive evaluation) techniques have been extensively used to determine crack detection and damage evaluation in concrete. With the move towards a more sustainable society, and the need to extend the long-term service life of infrastructure and aging and disastrous damage due to recent earthquakes, Acoustic Emission (AE) and Related Non-destructive Evaluation (NDE) Techniques in the Fracture Mechanics of Concrete: Fundamentals and Applications is a critical reference source for civil engineers, contractors working in construction and materials scientists working both in industry and academia. - Presents innovative Acoustic Emission (AE) and related non-destructive evaluation (NDE) techniques, used for damage detection and inspection of aged and deteriorated concrete structures - Contributions from recognized world-leaders in the application of acoustic emission (AE) and NDE techniques used for the damage assessment of concrete and concrete structures - With the move towards a more sustainable society, and the need to extend the long-term service life of infrastructure and damage due to recent earthquakes, this book is of critical importance - An essential knowledge resource for civil engineers, contractors working in construction and materials scientists working both in industry and academia

Autoren/Hrsg.

Ohtsu, Masayasu

Leseproben

1

Damage evaluation in concrete materials by acoustic emission

T. Suzuki     Niigata University, Niigata, Japan

Abstract

For detailed inspections of concrete structures, core samples are often drilled out and then mechanical properties of the concrete are measured. In this chapter, damage estimation for concrete materials by the compression test of core samples by applying acoustic emission (AE) measurement and the scalar damage parameter in damage mechanics is introduced. The procedure is named DeCAT (damage estimation of concrete by acoustic emission technique), which is based on estimating the intact modulus of elasticity in concrete. AE behavior of the concrete core under compression is analyzed by rate-process analysis, and mechanical behavior is approximated by the Loland model. Then, quantitative damage of the core concrete is evaluated as the relative damage by applying DeCAT.

Keywords

Acoustic emission; Concrete damage; Damage mechanics; Rate-process analysis

1.1. Introduction

The durability of concrete structures easily decreases as a result of environmental attacks. The degree of damage in concrete is, in most cases, evaluated by mechanical properties such as the strength and the modulus of elasticity. For effective maintenance and management of concrete structures, it is necessary to evaluate explicitly the degree of damage. To inspect concrete structures for maintenance, the acoustic emission (AE) technique is known to be useful (Grosse & Ohtsu, 2008). Crack nucleation and extension are readily detected by AE measurement. In this respect, monitoring of AE activity in the uniaxial compression test of core samples was proposed (Ohtsu, Kawai, & Yuji, 1988). AE behavior under compression is formulated by the rate-process theory.

Here, quantitative evaluation of damage has been performed by correlating AE rate-process analysis and damage mechanics. Concrete samples drilled out from structures are tested under compression, and AE measurement is carried out. By calculating Young's modulus of intact concrete, E*, from the AE database, the degree of damage was estimated as a relative damage in a road bridge (Suzuki, Shigeishi, et al., 2007). Thus, the procedure to estimate the relative moduli, E0/E*, of concrete is implemented as DeCAT (Damage Estimation of Concrete by Acoustic Emission Technique) (Suzuki, & Aoki, 2010; Suzuki, Ogata, Takada, Aoki, & Ohtsu, 2010).

1.2. Damage estimation of concrete by acoustic emission technique (DeCAT) procedure

1.2.1. Acoustic emission rate-process analysis

The concept that the AE activity of concrete under compression is associated with the rate-process theory was introduced (Ohtsu & Suzuki, 2004) in order to analyze AE-generating behavior associated with the generation of micro-cracks. It is well known that these cracks tend to gradually accumulate until final failure. Since this process could be referred to as stochastic, the following equation of the rate process is derived to formulate the number of AE events, dN, generated by the increment of stress from V to V+dV.

(V)?V=?NN

(1.1)

where N is the total number of AE events and f(V) is the probability function of AE generation at stress level V%. For f(V) in Eqn (1.1), the following hyperbolic function is assumed:

(V)=aV+b

(1.2)

where a and b are empirical constants. Here, the value a is named the rate. As shown in Figure 1.1, the probability varies, in particular at low stress level, depending on whether rate a is positive or negative. In the case that the rate a is positive, the probability of AE generation is high at low stress level. This indicates that a tested concrete core could be damaged. In contrast, in the case that the rate a is negative, the probability is low at low stress level, implying that the concrete is in sound condition. Therefore, it is possible to quantitatively evaluate the damage of concrete from AE activity under uniaxial compression by AE rate-process analysis.

Substituting Eqn (1.2) into Eqn (1.1), a relationship between the total number of AE events, N, and stress level, V, is obtained as follows:

=CVaexp(bV)

(1.3)

where C is the integration constant.

Figure 1.1 Two possible relations of probability function f(V).

1.2.2. Damage mechanics

Damage parameter O in continuum damage mechanics is defined as a relative change in the modulus of elasticity, as follows:

=1-EE*

(1.4)

where E is the modulus of elasticity and E* is the modulus of concrete that is assumed to be intact and undamaged. Loland (1989) assumed that the relationship between damage parameter O and strain e under uniaxial compression is as follows:

=O0+A0e?

(1.5)

where O0 is the initial damage at the onset of the uniaxial compression test, and A0 and ? are empirical constants of concrete. The following equation is derived from Eqns (1.4) and (1.5):

=(E0-E*A0e?)e

(1.6)

where

0=E*(1-O0)

(1.7)

c=E0-E*A0ec?

(1.8)

From Eqn (1.7), to estimate the initial damage, O0, it is essential to obtain the modulus of intact concrete E*. However, it is not feasible to determine E* of concrete in an existing structure. To estimate E* from AE measurement, the relation between the total number of AE events and the stress level in Eqn (1.3) is correlated with the Loland model. In the compression test, a relation between stress and strain is shown in Figure 1.2. The modulus of elasticity varies from E0 to final Ec. Note that the former is defined as a tangential modulus, whereas the latter is a secant modulus. Following Eqn (1.5), damage O increases from O0 to Oc, as shown in Figure 1.3.

1.2.3. Database development

The static initial modulus of elasticity E0 is to be quantitatively determined as a tangential gradient of the stress–strain curve, from Eqn (1.6):

=E0e-E*A0e?+1

(1.9)

Thus, the static modulus, E0, is uniquely determined as a tangential modulus: ds/de at e=0.

As shown in Figure 1.2, two moduli of elasticity, E0 and Ec, are determined in the core test. Then, the rate-process analysis is conducted in the stress level range from 30% to 80%. This is because AE events at initial loading below 30% stress are normally associated with contact with the loading plate, and those at an accelerated stage above 80% have little to do with the damage.

Figure 1.2 Stress–strain relation and two Young's moduli.

Figure 1.3 Evolution of damage in concrete.

Figure 1.4 Acoustic emission database.

We have found the highest correlation between the damage parameter ? and the rate a (Suzuki & Ohtsu, 2011). Results of all samples damaged due to the freezing–thawing process in previous experiments are plotted with open circles in Figure 1.4. The AE database consists of 200 samples tested at Kumamoto University from 1988 to 2013. A linear correlation between ?' and the rate a' value is reasonably assumed. Thus, the equation of ?' is expressed as follows:

'=a'X+Y

+(a×100)=(a×100)X+Y

(1.10)

where

=EcE0-Ec

(1.11)

Here, it is assumed that E0=E* when a=0.0. This allows us to estimate Young's modulus of intact concrete E* from the AE database as follows:

*=Ec+EcY

(1.12)

In this study, the damage of concrete is evaluated by relative moduli E' as presented.

'=E*E0×100

(1.13)

Here E0 is the tangent modulus of elasticity in the compression test. The procedure to estimate the relative moduli E' is named DeCAT (damage estimation of concrete by acoustic emission technique).

1.3. Comparison...