International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE) September 15-17, 2015, Berlin, Germany

Transcription

1 September 5-7, 25, Berlin, Germany More Info at Open Access Database Novel Non-destructive Test method to Evaluate Air-Permeability Distribution in Depth Direction in Concrete -Development of Triple-Cell Air-Permeability Tester (TCAPT) Isao KURASHIGE Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry, Tokyo, Japan; Phone: ; Fax: ; kurasige@criepi.denken.or.jp Abstract The use of nondestructive testing techniques to evaluate the quality of concrete during the completion inspection of constructed structures has been studied extensively in recent years across the world. Air permeability tests have attracted the attention of many researchers as an easy-to-use technique for onsite tests. In this study, we have developed a novel nondestructive test method that can obtain the estimated air-permeability distribution in the depth direction in concrete from changes in the air pressure in a triple concentric cell. This paper presents the mechanism and estimation algorithm of the developed test and discusses the potential of the method as an onsite nondestructive test method based on the results obtained by using the new method. These show that the developed method is useful as a way to evaluate the air-permeability distribution in the depth direction in concrete which changes depending on water-cement ratio (W/C), curing condition, and drying degree of concrete. Keywords: Concrete, quality inspection, air permeability, triple-cell system, inverse analysis. Introduction In recent years, the social demands for ensuring the reliability and durability of infrastructure facilities have become stronger. The quality and performance of concrete structures are affected by factors such as environmental, operational, and human factors in the construction phase. Therefore, the careful control of material quality and construction processes such as casting and curing in concrete construction is essential for ensuring that the designed performance is realized. Moreover, some infrastructure facilities cannot be easily restored or repaired, and for some special facilities such as radioactive waste repositories, greater reliability and accountability can be achieved by checking the concrete quality through completion inspection as a post-test. In recent years, discussion and research activities on this issue have increased across the world, including in Japan []-[4]. In Japan, the Torrent air permeability test [5]-[7], which uses a double-cell system, has been increasingly used to evaluate the quality of concrete on experimental study and actual construction. The test provides rapid and nondestructive measurements of the quality of concrete in terms of its durability and is considered to be a relatively reliable inspection tool in the RILEM technical committee 89-NEC recommendation [2]. However, the problem with the test is that the kt values of air permeability coefficient measured by the test, which regards concrete to be homogeneous (i.e., air permeability is constant in the depth direction), can increase with age (Figure ); this increase is believed to be due to the drying from concrete surface. Thus, the inspections performed using this test method at an early age can incorrectly overestimate the quality of concrete, and it is difficult for the method to evaluate air-permeability distribution in the depth direction in concrete. The purpose of this study is to develop a non-destructive test method to evaluate air-permeability distribution in the depth direction depending on the drying progress in terms of the moisture dissipation from concrete surface.

2 September 5-7, 25, Berlin, Germany 2. Description of developed triple-cell test method 2. Testing device Air permeability kt ( -6 m 2 )... Ordinary Portland cement concrete exposed to condition at 2 o C % RH W/C(%) - demolding age (day) Age t (day) Figure. Increase in kt values of air permeability, measured by Torrent test method, during drying progress of concrete with age The Torrent test method measures a unidirectional flow of air to the inner cell through concrete, which is regarded to be homogeneous, as the pressure in the outer cell is balanced with that in the inner cell. The method calculates the value of the air permeability of concrete from the data of the increase in the pressure in the inner cell only. On the other hand, the testing device we developed is equipped with a triple concentric cell, as shown in Figures 2 and 3, and the test using this device measures the increases in pressure in every cell after the cells are vacuumed for a certain period of time ( s in this study) and each valve of the cells is closed. The behavior of the pressure increase in each cell can vary with the air-permeability levels and distributions in the depth direction, which is conceptually illustrated in Figure 4. Bottom view On-off valve Air gauge Diameter of outer rim: approx. 2 mm Test kit composed of a triple cell device, a data logger, and a PC (including a vacuum pump not shown in this picture). Side view Airflow Concrete Figure 2. Sketches of the developed triple cell testing device Top (left) and bottom (right) faces. Figure 3. Photographs of test kit

3 September 5-7, 25, Berlin, Germany Airflow * Airflow Concrete *2 Homogeneous concrete of lower W/C Concrete Airflow Concrete Airflow Concrete Inhomogeneous concrete of lower W/C, more permeable nearer the surface. Homogeneous concrete of higher W/C Inhomogeneous concrete of higher W/C, more permeable nearer the surface. * A longer arrow represents a larger volume of airflow. *2 A lighter color shows higher permeability. Figure 4. Measurement concept of air-permeability distribution in depth direction for triple cell test method 2.2 Algorithm for estimation of air-permeability distribution in depth direction The estimation algorithm consists of two steps: the simulation of air inflow through concrete to each cell, and the optimization calculation of coefficients of a power function expressing the air-permeability distribution in the depth direction in concrete. For the simulation of air inflow, in this study, Equations () and (2) were used as governing equations for the law of conservation of mass and the generalized Darcy s law that describes the airflow through concrete, respectively. Then, Equation (3) [8] was adopted as a relational expression between permeability and porosity considering the degree of water saturation of concrete. Furthermore, the air-permeability distribution in the depth direction was expressed by a power function (Equation (4)) under the assumption that moisture dissipation from within concrete follows Fick s law of diffusion. + = () = (2) = (3) = (4) where φ: the porosity of concrete; ρ: the density of air; : the volume velocity of air; J: the flux of air; K: the permeability of concrete; : the viscosity of air; P: the pressure of air; a and b: the coefficients to be optimized; and d: the distance from the surface of concrete. In the optimization step, the coefficients were optimized by iterative calculation (Figure 5). For this nonlinear optimization problem, in this study, the steepest descent method was adopted. Airflow simulation based on the assumed values of coefficients a and b in Equation (4) Comparison between simulation result and measured data Iterative calculation Optimization of the values of coefficients a and b in Equation (4) Figure 5. Optimization algorithm for coefficients a and b, which represent air-permeability distribution in depth direction in Equation (4)

4 September 5-7, 25, Berlin, Germany 3. Measurement and estimation results 3. Comparison between measured and calculated air pressure in each cell Figures 6-8 show the results of the measured and calculated air pressure in each cell of the testing device developed in this study. It is observed that the air pressure in the outer cell increases much earlier than in the other cells; the air pressure in the inner cell increases most slowly. The measurements were performed until the air pressure in the outer cell increased to around - kpa. The calculation estimating the values of coefficients a and b was performed for the period same as the measurement duration; the air-permeability distributions based on the estimated values for mortar under various conditions are described in subsection mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age 9 8 mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age (a) mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at (b) mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at (c) (d) Figure 6. Measurement and calculation results for air pressure in each cell in test for mortar with W/C of % for various curing conditions and measurement age. The W/C value, demolding age, and measurement age for different panels are as follows: (a) %; day; 3 months, (b) %; day; year, (c) %; 5 days; 3 months, (d) %; 5 days; year.

5 International Symposium mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age (a) mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at (c) mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at 28-day age September 5-7, 25, Berlin, Germany (e) (f) Figure 7. Measurement and calculation results for air pressure in each cell in test for mortar with W/C of % for various curing conditions and measurement age. The W/C value, demolding age, and measurement age for different panels are as follows: (a) %; day; 3 months, (b) %; day; year, (c) %; 5 days; 3 months, (d) %; 5 days; year, (e) %; 28 days; 3 months, (f) %; 28 days; year, (g) %; 9 days (in water); year (inserted in the next page) (d) 9 mortar with W/C of %exposed to conditions 8 of 2 o C and % RH after being demolded at 28-day age 2 mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age (b) mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at

6 Figure 7. (g) (right) (Caption of this graph is shown on the previous page) International Symposium mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age (a) 9 mortar with W/C of %exposed to conditions 8 of 2 o C and % RH after being demolded at September 5-7, 25, Berlin, Germany mortar with W/C of %exposed to conditions of 2 o C and % RH after being cured in water until 9-dayage mortar with W/C of %exposed to conditions of 2 o C and % RH after being demolded at -day age (b) 9 mortar with W/C of %exposed to conditions 8 of 2 o C and % RH after being demolded at (c) (d) Figure 8. Measurement and calculation results for air pressure in each cell in test for mortar with W/C of % for various curing conditions and measurement age. The W/C value, demolding age, and measurement age for different panels are as follows: (a) %; day; 3 months, (b) %; day; year, (c) %; 5 days; 3 months, (d) %; 5 days; year. 2

7 September 5-7, 25, Berlin, Germany 3.2 Effects of W/C and curing conditions on air-permeability distribution in depth direction The estimation results of the air-permeability distributions in the depth direction for mortar under various conditions are shown in Figure 9. It was experimentally verified that the testing method we developed was valid for estimating the distributions; the differences in the estimated air-permeability distributions for mortar for different W/C values, curing conditions, and measurement ages were clear and the air permeability of higher-w/c mortar changed more substantially at greater depths with the age of mortar than that of lower-w/c mortar, as shown in Figure 9(a). Figure 9(b) shows the estimation results for the mortar with W/C of % demolded at ages of and 5 days. It is observed that the air-permeability level of the mortar demolded early at an age of day is higher at greater depths than the other mortar, especially at the older measurement age of year. For the mortars with W/C of % (Figure 9(c)) and % (Figure 9(d)), similar negative impacts of early demolding on the air permeability are observed. On the other hand, Figure 9(c) indicates that additional curing, such as the 28-day age demolding and 9-day age water curing, over the standard demolding can decrease the permeability; especially, the effect of water curing on the permeability is strong. As described above, the test method we developed can nondestructively estimate the air-permeability distribution in the depth direction for mortar under various conditions; the method is expected to have wide applications in the onsite inspection of constructed concrete structures. Concrete hardens and becomes denser because of the reaction of cement with water, which means that cement requires sufficient water to react and perform at its full potential. Therefore, it is believed that the estimation method for the air-permeability distribution in the depth direction in concrete, which is dependent on the moisture content of concrete, is a beneficial way to inspect the quality of concrete. However, some topics such as appropriate measurement time duration, types of model function for the air-permeability distribution and mesh model on simulation should be discussed continuously as future issues.

8 September 5-7, 25, Berlin, Germany Permeability coefficient ( -6 m 2 ) W/C (%) - demolding age - measurement age -5d-3m -5d-y -5d-3m -5d-y -5d-3m -5d-y Permeability coefficient ( -6 m 2 ) W/C (%) - demolding age - measurement age -d-3m -d-y -5d-3m -5d-y Permeability coefficient ( -6 m 2 ) (a) W/C (%) - demolding age - measurement age -d-3m -d-y -5d-3m -5d-y -28d-3m -28d-y 系列 8 -W9d-y Permeability coefficient ( -6 m 2 ) (b) W/C (%) - demolding age - measurement age -d-3m -d-y -5d-3m -5d-y (c) (d) Figure 9. Estimation results of air-permeability distribution in depth direction for mortar for various W/C values and curing conditions. (a) Comparison among different W/C values in the case of 5-day demolding, (b) Comparison of effect of curing conditions for mortar with W/C of %, (c) Comparison of effect of curing conditions for mortar with W/C of %, (d) Comparison of effect of curing conditions for mortar with W/C of %.

9 September 5-7, 25, Berlin, Germany 3.3 Comparison of estimated air-permeability distributions in depth direction with measured carbonation depths Figures and show the measured carbonation depths (represented as circles) on distribution curve lines of air permeability for concrete columns with ages of.5 and 2-3 years, respectively. These figures indicate that the permeability coefficients at different carbonation depths were almost in the same range of 5 to -6 m 2 in both the cases of considerably different concrete column ages and environmental conditions. This indicates that the carbonation progress depends largely on the air-permeability distribution in the depth direction and is suppressed when the permeability is lower than around 5-6 m 2. In other words, the developed method may have the potential for the nondestructive estimation of the carbonation depth of concrete structures on site. Permeability coefficient ( -6 m 2 ) Demolded at -day age Demolded at, Part Demolded at, Part 2 Seal-cured until 28-day age Measurement results at.5-year age for ordinary Portland cement concrete mockup columns with W/C of 58%constructed in winter and exposed to outdoor environment; but not to rain Figure. Measured carbonation depths (represented by circles) on estimated distribution curve lines of air permeability for concrete mockup columns with age of.5 years Permeability coefficient ( -6 m 2 ) Site A Column Site A Column 2 Site B Column Site B Column 2 Site C Column Site C Column 2 Measurement results at 2~3-year age for in service cocrete columns with W/C of approximately 39% Figure. Measured carbonation depths (represented by circles) on estimated distribution curve lines of air permeability for in-service concrete columns with age of 2 ~ 3 years 4. Conclusion In this study, we developed a novel nondestructive test method to evaluate the air-permeability distribution in the depth direction in concrete and verified the practical effectiveness of the method. The main results of this study can be summarized as follows: () A testing device equipped with a triple concentric cell was fabricated, and using this device, the nondestructive testing method for concrete was developed. (2) An algorithm was built for the estimation of the air-permeability distribution in the depth direction in concrete based on the measurement data of air pressure in all the cells. (3) From experimental verifications, it was confirmed that the developed test method can nondestructively estimate the air-permeability distribution in the depth direction for mortar under various conditions. The method is expected to have wide applications in the onsite inspection of constructed concrete structures. (4) From the comparison of the measured carbonation depths with the air-permeability distribution, it was found that the developed method may have the potential for the nondestructive estimation of the carbonation depth of concrete structures on site.

10 September 5-7, 25, Berlin, Germany Acknowledgements This research was partially supported by the Yoshida Award for Research Encouragement from the Japan Society of Civil Engineers (JSCE Award 2) through the study titled Development of nondestructive air-permeability testing method for in-depth quality profiling of concrete structures. The authors gratefully acknowledge the opportunities for onsite research provided by Professor T. Kishi and the East Japan Railway Company. References. IAEA. Long term behaviour of low and intermediate level waste packages under repository conditions: results of a co-ordinated research project IAEA-TECDOC-397, RILEM. RILEM report : non-destructive evaluation of the penetrability and thickness of the concrete cover. State of the Art Report of RILEM Technical Committee TC 89-NEC: Non-destructive Evaluation of the Concrete Cover, JSCE. Working report on sub-committee on the quality evaluation of successive concrete structures and the quality inspection system for newly-constructed RC structures. Concrete Engineering Series 87, 29 (in Japanese). 4. JSCE. Working report on sub-committee on the verification system for surface quality and durability performance of RC structures. Concrete Engineering Series 97, 22 (in Japanese). 5. Torrent, R.J. A two-chamber vacuum cell for measuring the coefficient of permeability to air of the concrete cover on site. Materials and Structures. Vol 25, No 6, pp , Torrent, R.J. and Jacobs, F. Swiss standard SIA 262:23, a step towards performance-based specifications for durability. Proceedings of the International RILEM TC-2-PAE Final Conference on Concrete in Aggressive Aqueous Environments Performance, Testing and Modeling, pp , Torrent, R.J. Non-destructive air permeability measurement: from gas-flow modeling to improved testing. Proceedings of the Second International Conference on Microstructural-related Durability of Cementitious Composites, Kohno, S. and Ujike, I. Study on change of air permeability coefficient due to drying. Proceedings of the Japan Concrete Institute, Vol 2, No 2, pp , 999.