International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Damage Estimation of Concrete by AE Rate Process Analysis in Core TestTetsuya Suzuki, Nippon Suiko Consultants CO., LTD., 3-15-48 Goryo, Kumamoto, Japan
Masayasu Ohtsu, Graduate School of Science & Technology, Kumamoto Univ., 2-39-1 Kurokami, Kumamoto, Japan
The durability of concrete structures decreases easily due to such external effects, as neutralization and freezing-thawing process. The degree of damage in concrete is, in most cases, evaluated by an unconfined compression test and a rebound hammer test.For effective maintenance and management of concrete structures, it is necessary to evaluate not only the strength of concrete but also the degree of damage. Quantitative damage evaluation of concrete is proposed by applyingacoustic emission (AE) method and damage mechanics.
In the present study, damaged concrete samples are examined, based on fracturing behavior under unconfined compression. AE behavior of the concrete under compression is dependent on the degree of damage, and could be approximated,applying the rate process analysis. Using Loland's model, a relationship between stress and strain is modeled, and a relation between AE rate and the damage parameter is correlated. By calculating the relative damage from the database based on this relation,the initial damage of the concrete in an actual structure is successfully estimated.
With increasing necessity of maintenance and management techniques for concrete structures, it has become important to evaluate not only the strength but also other physical properties of concrete. For detailed inspection of concrete structures, unconfined compression tests have been frequently conducted, taking out core samples.Through a comparison between test results and the reference strength specified in the design standards, the degree of damage is conventionally estimated. However, the strength is not good enough for practical evaluation of damage degree in concrete structures. In this respect, Acoustic Emission (AE) method is known to be promising for determining the degree of damage .
In the present study, AE measurements are conducted during uniaxial compression tests. Concrete-core samples were taken from a thrust block of an agricultural pipeline constructed in 1967 and repaired in 1979. AE activity under unconfined compression is approximated by the rate process analysis, and the damage parameter derived from the stress-strain behavior is evaluated by using Loland's model. Then, a database is applied to make the proposed method applicable to a limited number of samples taken from an existing structure.
2. Analytical procedure
where N is the total number of AE events and f(V) is the probability function of AE at stress level V(%). For f(V) in Eq.1, the following hyperbolic function is assumed,
where a and b are empirical constants. Here, The value 'a' is named the rate.
In Eq.1, the value of 'a' reflects AE activity at a designated stress level, such that at low stress level the probability varies, depending on whether the rate 'a' is positive or negative. Two possible relations of probability function f(V) is shown in Fig.1. In the case that the rate 'a' is positive, the probability of AE activity is high at a low stress level, indicating that the structure is damaged. In the case of the negative rate, the probability is low at a low stress level, revealing that the structure is in stable condition. Therefore, it is possible to quantitatively evaluate the damage in a concrete structure using AE measurement under unconfined compression by the rate process analysis.
Based on Eqs.1 and 2, the relationship between total number of AE events N and stress level V is represented as the following equation,
Where C is the integration constant.
where E is the modulus of elasticity of concrete and E* is the modulus of elasticity of concrete which is assumed to be intact and undamaged.
Loland assumed that the relationship between damage parameter W and strain e under unconfined compression is expressed 
where W0 is the initial damage at the onset of the unconfined compression test, and A0 and l are empirical constants of the concrete.
The following equation is derived from Eqs. 4 and 5,
Here Wc is the damage at the final stage. Based on a linear correlation equation in Fig.2,
Here, it is assumed that E0=E* when a=0. This allows us to estimate Young's modulus of intact concrete E* from AE rate process analysis as,
4. Results and discussion
In the present study, the initial tangential Young's modulus E0, was quantitatively determined as a tangential gradient of the stress-strain curve by approximating as,
Here, a1 and a2 are empirical constants. By the approximation of the stress-strain relation in Eq.11, the initial modulus E0 is derived as tangential modulus : ds / de at e = 0,
Thus, the moduli of elasticity, E0 and Ec were determined. Here, Ec is the secant Young's modulus at final fracture. Table 2 shows mechanical properties of all the samples. Initial Young's modulus E0 varies from 8.7 to 34.0 GPa, while the unconfined compression strength varies from 8.0 to 20.4 MPa.
The rate process analysis was conducted at stress level in the range from 30% to 80%. This is because AE events occurring at initial loading below 30% strength due to contact with the loading plate and at an accelerated stage above 80% has little to do with the damage.
It is demonstrated that Young's modulus of intact concrete E* can be estimated by Eq.10 [4, 5]. Thus, to estimate relative damage of the concrete, ratios E0/E* are obtained, and are summarized in Fig.4. To determine E* from the relation in Fig.2, it is necessary to analyze a large number of specimens. However, the number of concrete cores available is limited in existing structures. Therefore a database, which could allow even a single concrete core to be evaluated, is constructed as shown in Fig.2. By using this database, Young's modulus of the normal concrete E* and the relative damage of the samples are calculated . The samples enrolled in a database in Fig.2 are all tested in the previous research. Fig.4 shows results, by employing the database. E0/E* of the sound specimens is to be obtained as equal to 1.0 or over. In Fig.4, relative moduli E0/E*, vary from 0.48 to 1.25. Relative damages in 9 samples are estimated as below 1.0. Though the degree of damage in the concrete samples is not clearly identified by the unconfined compression test in the figure, it is quantitatively estimated, using the relative damage evaluated by the AE measurement.
Thus, the effectiveness of the relative damage based on Young's modulus of intact concrete E* using the database is demonstrated. The database is applicable to evaluate the relative damage even in the case that there are not enough the number of specimens available from existing structures.
Unconfined compression tests are conducted using the AE measurement in the core samples taken from a pipeline structure, in order to evaluate the degree of damage quantitatively. The close relation is confirmed between the AE generating behavior and the internal damage of concrete, which is analyzed based on the rate process theory and damage mechanics. Thus, the degrees of damage in concrete samples are quantitatively evaluated, even when the initial physical properties of concrete structure at the time of construction are unknown. Conclusions are summarized below.