Damage Evaluation of Core Concrete by AE

Transcription

1 Vol. 2 (3) Sep Damage valuation of Core Conrete by A Tetsuya Suzuki 1 and Masayasu Ohtsu 2 1 Faulty of Agriulture Niigata University, JAPAN 2 Graduate Shool of Siene and Tehnology Kumamoto University, JAPAN Abstrat For a detailed inspetion of a onrete struture, ore samples are usually drilled out and then their physial properties are measured. In this study, damage evaluation method for onrete materials is studied, applying aousti emission (A) method and omputerized tomography (CT) sanning proedure. We have proposed a quantitative damage evaluation of onrete, based on A measurement and damage mehanis in the ompression test. The proedure is named DeCAT (Damage stimation of Conrete by Aousti mission Tehnique), whih is based on estimating an intat modulus of elastiity in onrete. Conrete-ore samples were taken from reinfored onrete walls in a anal. These samples are strongly damaged by a freezing and thawing proess. Crak distributions in the samples were inspeted with helial CT sans, whih were undertaken at one-millimetre interval. After the helial CT san, damage of freeze-thawed samples was evaluated by applying the DeCAT analysis. These results demonstrate that the derease in physial properties ould be evaluated by omparing the averaged CT number with the durability index by A. The both values learly reflet amount of internal raks in ore samples. Keywords: Aousti emission; Rate-proess analysis; Damage mehanis; DeCAT; CT number. 1. Introdution The durability of onrete strutures dereases easily due to suh environmental effets, as arbonation and freeze-thawed proess [1]. The degree of damage in onrete is, in most ases, evaluated by an unonfined ompression test or a ultrasoni test. For effetive maintenane and management of onrete strutures, it is neessary to evaluate not only the strength of physial properties but also the degree of damage. Quantitative damage evaluation method for onrete is proposed by applying aousti emission (A) method and damage mehanis [2]. The proedure is named DeCAT (Damage stimation of Conrete by Aousti mission Tehnique) [3]. The damage is evaluated as a durability index. In this study, damage estimation method for onrete is studied, applying DeCAT analysis and CT sans to ore-onrete samples. These samples were taken from reinfored onrete walls Corresponding Author: Tetsuya Suzuki mail: suzuki@agr.niigata-u.a.jp Telephone: Fax: All rights reserved. ISSR Journals 275

2 Damage valuation of Core Conrete by A in a anal, whih were strongly damaged by a freeze-thawed proess. Crak distribution is inspeted with helial CT sans, whih were undertaken at one-millimetre intervals. After helial CT san, damages of freeze-thawed samples are evaluated, based on the and the durability index by A. 2. Analytial Proedure 2.1. A rate-proess analysis A behaviour of a onrete sample under unonfined ompression is assoiated with the generation of miro-raks. This proess is dependent on the number of raks at a ertain stress level and the progress rate of the frature stage, and ould be referred to as a stohasti proess. Thus, the rate proess theory is introdued to quantify A behaviour under unonfined ompression [3]. The following equation of the rate proess is formulated to represent A ourrene dn due to the inrement of stress from V to V+dV, dn f ( V) dv =, (1) N where N is the total number of A events and ƒ(v) is the probability funtion of A at stress level V(%). For ƒ (V) in q.1, the following hyperboli funtion is assumed, a f ( V) = + b, (2) V where a and b are empirial onstants. Here, the value a is named the rate Damage stimation by sholar damage parameter A damage parameter Ω in damage mehanis an be defined as a relative hange in modulus of elastiity, as follows, Ω = 1, (3) * where is the modulus of elastiity of onrete and * is the modulus of elastiity of onrete whih is assumed to be intat and undamaged. Loland assumed that the relationship between damage parameter Ω and strain ε under unonfined ompression is expressed as [4], λ Ω = Ω +, (4) 0 A 0 ε where Ω 0 is the initial damage at the onset of the unonfined ompression test, and A 0 and λ are empirial onstants of the onrete. The following equation is derived from qs. 3 and 4, σ = ( * ε λ 0 A0 ) ε, (5) As given in q. 4, the initial damage Ω 0 in damage mehanis represents an index of damage, and in Loland s model (q. 5) it is fundamental to know Young's modulus of the intat onrete (*). However, it is not easy to obtain * from an existing struture. Therefore, it is attempted to estimate * from A monitoring in the ompression test. Two relations between total number of A events and stress level and between stress and strain are taken into aount. Based on a orrelation between these two relationships, a proedure is developed to evaluate the intat modulus from A analysis. A orrelation between the damage parameter λ and the rate a derived from A rate proess analysis is shown in Fig. 1. Good orrelation between the oeffiient λ and the rate a is shown, as results of samples damaged due to the freeze-thaw proess in experiments are plotted by gray irles. A linear orrelation between λ and the rate a is reasonably assumed, as follows: λ = a X + Y 276

3 here Tetsuya Suzuki & Masayasu Ohtsu Conrete Researh Letters Vol. 2 (3) Sep ( a 100) = ( a ) X + Y λ + 100, (6) λ =. (7) 0 Then, it is assumed that 0 = * when a = 0.0. This allows us to estimate Young's modulus of intat onrete * from A rate proess analysis as, * = + Y. (8) This damage evaluation proess is named DeCAT. The DeCAT is appliable to evaluate onrete damage based on estimation of an intat modulus of elastiity from A database shown in Fig. 1. A database onsists of 200 samples tested in the Kumamoto University and Niigata University from 1988 to Figure 1. A database of DeCAT system. 3. xperimental Proedure Cylindrial onrete samples of 5m in diameter and about 10m in height were taken from onrete walls in an open anal wall in Hokkaido prefeture, Japan. The walls were onstruted about 40 years ago, and heavily damaged by freezing and thawing yles. Here-in-after, ore samples severely raked are named Type A. The samples with a few raks are named Type B, and sound samples are named Type C. These ore samples were inspeted with helial CT sans at the Animal Medial Center, Nihon University. The helial CT san was undertaken with one-millimeter intervals before the ompression test. After CT san, a uniaxial ompression test of the sample was onduted. SAMOS-A system (manufatured by PAC) was employed. A hits were deteted by using an A sensor R15 (resonane frequeny: approx. 150 khz). 4. Results and disussion 4.1. Physial properties of onrete samples Physial properties are evaluated by mehanial parameters, X-ray and the rate a in A analysis. Mehanial parameters obtained are summarized in Table 1, with the maximum, and the minimum values of all speimens. 277

4 Damage valuation of Core Conrete by A Sine these mehanial properties are affeted by rak distributions, s were obtained by the helial CT sanner. Figure 2 shows harts of s in onrete ores. In Type A sample, the s learly derease at raked portions. The averaged s inrease from Type A to Type C. The of a non-raked sample (Type C) is as the average, while the averaged s are in Type A and in Type B. TABL1. MCHANICAL PROPRTY OF COR SAMPLS. Sample Sample Compressive Vp* Name Number Strength* (N/mm 2 ) (m/se) Type A ,645-1,654 Type B ,873-2,089 Type C ,397-1,568 *Minimum-Maximum TYP A TYP A TYP B TYP B Crak AV 1,742.5 RANG 2,836 Std TYP C TYP C AV 1,859.4 RANG 1,702 Std AV 1,895.1 RANG 1,160 Std Figure 2. Results of X-ray density analysis from top to bottom in ore-sample A rates in ompression tests The rate proess analysis of A generating behavior shows the positive a values in all samples, whereas the rate a denoted (a = ) for normal onrete with 28-day moisture uring [2]. The rate obtained were for the speimen with the maximum strength (Type C), and for the speimen with the minimum strength (Type A). The rate a is so positive that the probability of A ativity is high at a low stress level in the ompression test. This implies that the onrete walls are damaged, and that an inreasing trend of the rate a is demonstrated with the inrease in damage Quantitative damage evaluation based on estimation of intat modulus * A durability index is defined as the ratio ( 0 / * ) of the initial Young s moduli 0 to the intat * estimated by the DeCAT analysis. Results are ompared with the ompressive strengths in Fig. 3. Here, all previous results are plotted along with those of the present study. As seen, the durability index less than 100% means the state of heavily damage. The baseline of the strength is 278

5 Tetsuya Suzuki & Masayasu Ohtsu Conrete Researh Letters Vol. 2 (3) Sep Type C Non-Damage Type A Damage Type B Figure 3. Comparison of durability index and ompressive strength. set to 21 N/mm 2, whih is defined as the standard design-strength for onrete in water anals in Japan [5]. It is learly observed that durability indies estimated show a reasonable relationship with the ompressive strengths, as the relative moduli ( 0 / * ) are positively orrelated with the ompressive strengths [3]. Results of Type A and Type B are plotted in the damaged zone (Durability index < 100%, Compressive strength < 21N/mm 2 ), and that of Type C is denoted in the non-damage zone. 5. Conlusion For quantitative evaluation of onrete ores in a water-anal struture damaged by freezing and thawing ations, rak distributions in the ore samples were inspeted with X-ray CT method. The damage of onrete was evaluated by the DeCAT analysis in the ompression tests. It is demonstrated that the damage an be quantitatively evaluated by the DeCAT analysis and X-ray CT values. The durability index ( 0 /*) is evaluated in reasonable agreement with the strengths of damaged onrete. Referenes [1] JCI C65, valuation of Conrete Performane in Natural nvironmental Conditions, Japan Conrete institute, 2005: pp [2] Ohtsu, M. and Suzuki, T. Quantitative Damage valuation of Conrete Core based on A Rate-Proess Analysis, Journal of Aousti mission, : pp [3] Suzuki, T., Ohtsu, M. and Shigeishi, M., Relative Damage valuation of Conrete in a Road Bridge by A Rate-Proess Analysis, Materials and Strutures, : pp [4] Loland, K.., Continuous Damage Model for Load Response stimation of Conrete, Cement and Conrete Researh, 1989, 10: pp [5] Ministry of Agriulture, Forestry and Fisheries of Japan (MAFF), Japanese standard for design and onstrution of agriultural water anal struture (JSIDR), 2001: pp