EFFECT OF FREEZE-THAW DAMAGE ON CONCRETE MECHANICAL PROPERTIES

Transcription

1 EFFECT OF FREEZE-THAW DAMAGE ON CONCRETE MECHANICAL PROPERTIES Yanxia Liu (1), Gaixin Chen (1), Guojin Ji (1), Xiangzhi Kong (1), Lintao Ma (1) (1) State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing, 138 Abstract: Freeze-thaw damage was one of the main reasons for the deterioration and aging of concrete structures. In this paper, the deterioration rules of concrete mechanical properties in freeze-thaw process were studied, and the relationship between damage amount and concrete properties was established. In the tests, relative dynamic elastic modulus of 1%, 9%, 8%, 7%, 6% and 5% were taken as different damage points, at which concrete mechanical property tests were carried out. The test results showed that concrete mechanical properties decreased with the increase of freeze-thaw damage levels. Freeze-thaw damage had much higher adverse influence on splitting tensile strength, bending strength and axial tensile strength than that on compressive strength. Based on the theory of damage mechanics, damage models were established between concrete properties and damage amount that was characterized by dynamic modulus of elasticity. The damage models can not only be beneficial to understand the deterioration rules and aging mechanisms of concrete properties under freeze-thaw conditions, but also can provide a theoretical basis for the diagnoses and identification of aging state of structural concrete. 1. INTRODUCTION Concrete freeze-thaw damage is an important part of durability problems, and one of the main reasons for the deterioration and aging of concrete structures as well. Freeze-thaw cycles can give rise to the deterioration of material properties and the declining of mechanical performance of concrete structures, which consequently result in the decrease of structure life. Non-destructive detection methods, such as ultrasonic testing method and rebound method, are usually used in the detection and evaluation of deteriorated concrete structures [1], which, however, can only reveal the mechanical properties of concrete at the testing time, but not the

2 durability state at the testing time and the deterioration process and the residual life of concrete after the testing time. In this paper, methods and approaches that can solve the above problems were investigated. Concrete internal damage was introduced through rapid freeze-thaw tests to intentionally give rise to the declining of mechanical properties. The change of dynamic modulus of elasticity was used to characterize the freeze-thaw damage level of concrete, and mechanical properties under different damage level (damage point) were tested. According to damage mechanics, the relationship between mechanical properties and damage amount was established, and thus the damage model of concrete property under freeze-thaw cycles was built. 2 RAW MATERIALS, MIXING PROPORTIONS AND TESTING METHODS Portland moderate heat cement with a density of 3.23 g/cm 3 was used, and its chemical composition was shown in table 1. Fine aggregate was crushed sand with a density, fineness modulus and water absorption on saturated surface dry (S.S.D) basis of 2.62g/cm 3, 2.77 and 1.%, respectively. Crushed stone was used as coarse aggregate, with two size gradation ranges of 5-2mm and 2-4mm. The density of two kinds of aggregate was g/cm 3 and 2.62 g/cm 3, and water absorption ratio on S.S.D basis was.65% and.4, respectively. A kind of naphthalene-based water-reducing admixture was used. To avoid the self-healing of concrete resulting from the continuous hydration of fly ash at late age, no fly ash was used in concrete. Also, there was no air-entraining admixture used so as to shorten the duration of freeze-thaw test. The mixing proportion was shown in table 2. Table 1: Chemical composition of cement Chemical composition SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O SO 3 Loss Content (%) Table 2: Concrete mixing proportion Water to Sand Superplasticizer Quantity of raw materials for unit volume (kg/ m 3 ) cement ratio ratio (%) (%) Water Cement Fine aggregate Coarse aggregate Concrete specimens were prepared for freeze-thaw tests and mechanical tests. Specimens for dynamic modulus of elasticity and bending strength tests were prisms of 1mm by 1mm by 4mm, specimens for compressive strength and splitting strength tests were cube of 15mm by 15mm by 15mm, and specimens for axial tensile strength tests were in a shape of 8 with a size of 1mm by 1 13mm by 55mm. After 28 days cured in standard curing room, the specimens were divided into two groups. One group were for freeze-thaw tests (called FT Damaged Specimens), which were wrapped in silver paper bags

3 filled with water to ensure the saturation state of specimens. The freeze-thaw test method was rapid freeze-thaw method, referring to <Test code for hydraulic concrete> (SL/T352-26). The wrapped specimens and freeze-thaw testing machine were shown in picture 1 and picture 2, respectively. The other group of specimens were still kept in standard curing room (called Standard-cured Specimens). Resonance method was used to measure the dynamic modulus of elasticity. In the process of relative dynamic modulus of elasticity decreasing from 1% to 5%, six damage points were set as evenly as possible. When the damage level reached the damage points, FT Damaged specimens were taken out from testing machine for mechanical tests. At the same time, corresponding mechanical tests of standard cured specimens were also carried out. Picture 1: Wrapping and sealing of specimens Picture2 : Freeze-thaw testing machine 3 TEST RESULTS AND DISCUSSION The curve of relative dynamic modulus of elasticity during the freeze-thaw process was shown in fig.1. Without air-entraining, the relative dynamic modulus of elasticity decreased quickly. Therefore, the test frequency was increased to once every 3-5 freeze-thaw cycles. 1 Relative dynamic modulus of elasticity Relative dynamic modulus of elasticity (%) Number of freeze-thaw cycles Fig.1 : Relative dynamic modulus of elasticity of concrete during freeze-thaw process

4 At the freeze-thaw cycles of, 33, 56, 8 and 98 times, the corresponding relative dynamic modulus of elasticity was 1%, 89.7%, 76.4%, 64.%, 56.1% and 44.4%, respectively. At these six damage points, mechanical property test results of FT Damaged and Standard-cured speciments were listed in table 3. The results indicated that the strength of FT Damaged speciments gradually decreased with the increase of the number of freeze-thaw cycles, while the strength of Standard-cured specimens increased with the growth of curing age. Table 3: Test results of concrete mechanical properties during freeze-thaw process No. of FTCs Compressive strength(mpa) Splitting tensile strength(mpa) Bending strength(mpa) Axial tensile strength(mpa) Standard-cured FT damaged Standard-cured FT damaged Standard-cured FT damaged Standard-cured FT damaged The comparison between mechanical properties of Freeze-thaw Ddamaged Concrete and Standard-cured Concrete were shown in fig.2 and fig Ratio of Properties of Freeze-thaw Damaged Concrete to that of Standardcured Concrete(%) compressive strength splitting tensile strength bending strength axial tensile strength modulus of elasticity relative dynamic modulus of elasticity Relative dynamic modulus of elasticity (%) Number of freeze-thaw cycles Fig.2: Comparison between properties of Freeze-thaw Damaged Concrete and that of Standard-cured Concrete

5 12 12 Ratio of Properties of Freeze-thaw Damaged Concrete to that of concrete with a FT cycle of time (%) compressive strength splitting tensile strength bending strength axial tensile strength modulus of elasticity Relative dynamic modulus of elasticity Number of freeze-thaw cycles Fig.3: Comparison between properties of Freeze-thaw Damaged Concrete and that of concrete with a FT cycle of times It can be concluded from fig.2 and fig.3 that all the mechanical properties of concrete subjected to freeze-thaw process declined with the increase of the number of freeze-thaw cycles, among which the deterioration level of splitting tensile strength, bending strength and axial tensile strength was relatively higher. It indicated that concrete splitting tensile strength, bending strength and axial tensile strength were more sensitive to freeze-thaw damage. This can be explained by Griffith s Theory. According to Griffith s theory, there are large numbers of micro-defects in concretes, which can give rise to stress concentration of loaded concrete and thus result in microscopic damages [2]. In tension state, concrete would be completely failed once cracking was induced, while in compression state, micro-cracks in concrete would be in a stable without any tendency of propagation under a load less than 3% of the ultimate stress [3]. Freeze-thaw cycles can bring about further development of micro-cracks in transition zones between cement and aggregates and new cracks in cement paste. With the inner damages accumulating continuously, freeze-thaw process consequently had much more adverse effect on splitting tensile strength, bending strength and axial tensile strength. 4 DETERIORATION RULES OF CONCRETE MECHANICAL PROPERTY BASED ON THEORY OF DAMAGE MECHANICS Dynamic modulus of elasticity, as an intrinsic property of concrete, was determined by its basic vibration frequency. Based on the theory of damage mechanics [4], damage amount of frost concrete was defined as follows: Relative dynamic modulus of elasticity (%) D = 1 - E E i (1) Where, D was damage amount, E was initial dynamic modulus of elasticity, and E i was

6 dynamic modulus of elasticity after freeze-thaw cycles of i times. Here damage amount D was characterized by the relative dynamic modulus of elasticity of concrete. In the freeze-thaw process, the decrease of relative dynamic modulus of elasticity meant the increase of damage level and damage amount. The relative value of concrete mechanical properties was defined as f i R = (2) f Where, R was the relative value of concrete mechanical properties, f i was concrete mechanical properties after i freeze-thaw cycles, and f was concrete mechanical properties before the start of freeze-thaw test. According to above definitions, damage amount and relative values of concrete mechanical properties after certain number of freeze-thaw cycles were calculated, and shown in table 4. Table 4: damage amount and relative values of concrete mechanical properties Number of FTCs Damage amount D Compressive strength R Splitting tensile strength R Binding strength R Axial tensile strength R Compressive modulus of elasticity R The relationship between relative values of mechanical property of FT Damaged Concrete and damage amount can be analyzed through fitting. When the relative dynamic modulus of elasticity was less than 6%, concrete performance were considered to be failed. Therefore, the data at the damage point of.556 were not adopted in fitting analysis. The relationship between relative mechanical properties and damage amount can be favorably built up with modified power function (Equation 3). In the fitting process, damage amount was plotted as abscissa x, and relative mechanical properties of concrete as ordinate y. The fitting results were listed in table 5, and fitting curve in fig.4-fig.7. The correlation coefficients of fitting equations were higher than.96 and the standard errors less than.82, showing favorable correlation. 1 y c (3) a bx Where, a, b and c were parameters of the fitting equation.

7 Table 5 Fitting results of relationship between R and D Parameters of fitting equation Fitting correlation Items Standard error Correlation a b c S coefficient r Compressive strength Splitting tensile strength Binding strength Axial tensile strength relative compressive strength Fig. 5: Relationship between relative splitting tensile.8 strength and damage amount relative splitting tensile strength damage amount Fig. 4: Relationship between relative compressive strength and damage amount damage amount Fig. 5 Relationship between relative splitting tensile strength and damage amount relative bending strength relative axial tensile strength damage amount damage amount Fig. 6 Relationship between relative bending strength and damage amount Fig. 7 Relationship between relative axial tensile strength and damage amount Through fitting analysis, it can be seen that parameter a of the fitting equations was 1.. Therefore, the deterioration model of mechanical properties in freeze-thaw process can be written as :

8 1 R 1 bd c (4) For the concrete without air-entraining in the study, its deterioration level of mechanical properties can be predicted according to equation 4, when the damage amount was in a range of On the other hand, based on the test results of its mechanical properties in service and the results of its initial mechanical properties before the start of service, deterioration level of actual structure with this kind of concrete can be concluded through equation 4, providing theoretical basis for the diagnosis and evaluation of durability state and aging level of concrete structures. 5 CONCLUSION For concrete without air-entraining admixture, its mechanical properties gradually decreased with the increase of the number of freeze-thaw cycles. The deterioration level of splitting tensile strength, bending strength and axial tensile strength was relatively higher than that of compressive strength. For concrete without air-entraining admixture, deterioration model of mechanical properties under freeze-thaw conditions was put forward. The model can be used to predict its deterioration process, or to conclude its aging level. The deterioration model of other kinds of concretes and real structures should be calibrated or further studied. ACKNOWLEDGEMENTS The authors would like to express our gratitude to the staff and other colleagues of IWHR. In particular, we would like to acknowledge the financial support from Ministry of Water Resources of China (Project Number: 27141). REFERENCE [1] Compiling Group of New Concrete Non-destructive Testing Techniques. New Concrete Non-destructive Testing Techniques [M]. Beijing: China Environmental Science Press, (in Chinese). [2] Yu Xiaozhong. Rock and concrete fracture mechanics, Central South University Press, Dec. 1991, Hunan Province : pp (In Chinese) [3] A.M.Neville. Concrete property, China Building Industry Press, Oct. 1983, Beijing : pp (Chinese Version) [4] Li Zhaoxia. Damage mechanics and its application, Science Press, Jul. 22, Beijing. (In Chinese)