In addition, there is a universal lack of unified test methods to assess the effectiveness of self-healing technologies. The purpose of this study is to develop non-destructive testing methods and procedures for evaluating the performance of self-healing concrete. A supplementary experimental study is then carried out to monitor the changes in crack size and parameters of surface wave transmission in the process of self-healing.
INTRODUCTION
Research Background
In addition, uniform testing methods have not been established worldwide to assess the effectiveness of self-healing technologies. In particular, with regard to the use of self-healing concrete in actual construction, non-destructive testing methods are required so that evaluation tasks do not require any interruption in the use of structures [46]. Clear classifications of non-destructive tests that take into account self-healing purposes can support the development of correlation models.
Objectives and Scope
During the last decade, various ultrasonic non-destructive testing methods have been attempted to evaluate self-healing performance and include measurement of ultrasonic pulse velocity (UPV), surface wave transmission, acoustic emission (AE), ultrasound diffusion, interferometry of code waves and nonlinear ultrasonic techniques. However, the applicability and limitation of different test methods for specific cases (eg, self-healing targets and types of damage and cracks) have rarely been investigated to date. Thus, the first step in developing an evaluation model related to specific self-healing targets (eg, crack depth and permeability) using non-destructive parameters involves defining and classifying the technical characteristics and differences in different methods. testing.
Description of Thesis Chapters
PREVIOUS STUDIES FOR THE EVALUATION OF SELF-HEALING EFFICIENCY
Previous Studies for the Self-Healing Technologies in Cementitious Materials
Initially, expansive agents of the CSA type are used as self-healing agents in concrete with a low water-to-cement ratio. Next, CSA-type expansive agents with inorganic carbonate (eg, NaHCO3 and LiCO3) are studied to improve self-healing efficiency. Nowadays, geomaterials that promote swelling mechanisms with waterproofing concepts, chemical agents that precipitate recrystallizations, and CSA-type expansive agents are used to improve self-healing performance in cementitious materials with normal water-to-cement ratio as 0.5 [10].
Previous Studies for the Performance Evaluation of Self-Healing in Cementitious Materials
Also, SEM is used to confirm the shape of crack-filling materials because different self-healing agents result in different shapes (eg, needle shape) of crack-filling materials. In terms of stiffness recovery evaluations, measurement of resonant frequency is the most suitable assessment technique among non-destructive techniques to date. Also, electrical impedance measurements were attempted to evaluate self-healing efficiency of engineered cementitious composites (ECC).
THEORY AND APPLICATION OF ULTRASONIC-BASED NON-DESTRUCTIVE
Theories and Case Studies of Ultrasonic Non-Destructive Tests
- Theories of ultrasonic non-destructive tests
- Case studies of ultrasonic non-destructive tests
The measured coda wave signal scattered in concrete internal structures will change due to the self-healing process [26,27]. The main objective of this study is to monitor the changes in concrete in the process of self-healing using surface wave transmission. Therefore, the author would like to use the self-healing materials for crack repair.
Applicability and Limitations of Different Ultrasonic Non-Destructive Tests for Self-Healing
- Evaluation of change in crack size
- Evaluation of regained durability properties
- Evaluation of regained mechanical properties
- Self-healing assessment for in situ structures
- Applicability for different self-healing agents
Chapter Summary
Pulse velocity measurements are most commonly used as non-destructive testing methods to evaluate crack characteristics, durability properties, mechanical properties and self-healing performance. First, monitoring partially closed cracks due to self-healing with UPV does not give good grades. The target structure of self-healing concrete is similar to the structure of a tunnel into which sufficient water is supplied due to water leakage.
The water-infused conditions to describe the self-healing process are not adequate to predict the recovered mechanical properties. The AE analysis is suitable in laboratory-scale performance evaluation of self-healing concrete using capsules to locate capsule fractures. That is, AE can only be applied to selected self-healing element technology using capsules.
It is impossible to assess the self-healing ability of bacteria or other chemical additives on the basis of rehabilitated concrete. Moreover, the variability of measured diffusivity is high, although diffusivity shows the most ideal behavior for monitoring internal changes and self-healing results. Finally, different types of targets (e.g., crack depth, durability properties, mechanical properties) are proposed and analyzed to evaluate the performance of self-healing.
Along with the in-depth review of non-destructive evaluation technologies attempted for self-healing concrete, the author conducted an experimental study using surface waves.
Theoretical Background
The main aim of this study was to evaluate the feasibility of accurate crack depth estimation by characterizing the transmission of surface wave across cracks in concrete. Here, the signal consistency index to determine useful frequency ranges for multiple surface wave measurements can be defined as follows. The lower and upper integral ranges in these equations can be determined based on the signal consistency index.
Different material compositions of concrete can cause differences in the propagation of surface waves in concrete. 55] found that the transfer function across cracks is more sensitive to the proportions of the concrete mix than to the spectral ratio of energy transfer. The spectral power transfer ratios measured for the various materials are provided as line fits to the previous regression curve.
Test Descriptions
The dimensions of the specimen are determined from the results of numerical simulations using ABAQUS. The same process was applied to determine the width of sample using a three-dimensional finite element model, and the width of sample was determined to be 250 mm. A single artificial crack was generated in the center of the specimen using a 1 mm wide stainless steel panel.
DAQ devices for digitizing and conditioning signals were used to implement surface wave transmission across a crack. All signals from the receivers were measured using LabVIEW software and analyzed using Matlab software. Two miniature accelerometers (PCB 353 B15 with flat frequency responses greater than 30 kHz) used as receivers were attached at a location 15 mm from the crack surface on each side, and impacts were located at a location 30 mm from the nearest sensor excited
In order to recognize the sample as a semi-infinite solid medium, the wavelength of the generated excitations used was shorter than the thickness of the sample. For the waves to be transmitted across the cracks, wavelengths longer than the maximum crack depth (e.g. 45 mm) were used. To generate the surface wave, a steel ball with a diameter of 8 mm was dropped at a height of 200 mm from the surface.
Most of the dominant frequencies of shock sources fall in the frequency range between the 20 to 27 kHz components.
Data Processing
Experimental Results
- Effects of crack depth
- Effects of mix proportioning
- Normalized transmission coefficient as a function of crack depth
- Crack depth estimation using spectral energy transmission ratio
- Correlation between surface-wave parameters and crack depth
An analytical model between normalized transmission coefficients and normalized crack depth was proposed by Angel and Achenbach in 1984. Normalized transmission coefficients are useful to estimate the crack depth, when the crack depth over lambda has a value less than 0.3 [57]. Normalized transmission coefficients as a function of normalized crack depth in mortar, normal strength concrete and high strength concrete are plotted as shown in Figure 2.
The results of normalized transmission coefficients with respect to normalized crack depth showed a relatively fine compliance with the theoretical model. Furthermore, the regression curves between crack depth and spectral energy transfer ratio are derived as follows:. In this study, spectral energy transfer ratios as a function of crack depth in mortar, normal strength concrete and high strength concrete are plotted with previous empirical solutions as in Fig.
The results of spectral energy transmission ratios with respect to crack depth showed a relatively high agreement with previous empirical solutions. The correlation model between spectral energy transmission ratio and crack depth was derived using regression analysis. A minimum of twenty cases of the spectral energy transmission ratio at each crack depth and specimen are used in order to improve the reliability of the regression model.
RMSE (Root Mean Square Error) values are analyzed to confirm which crack depth has the smallest error.
EVALUATION OF SELF-HEALING PERFORMANCE BY SURFACE-WAVE
Test Descriptions
To minimize the effects of the arrival of L-wave signals from the bottom and side faces of the specimens, the Hanning window is applied. To improve the resolution of the Fourier-transformed signal in the frequency domain, 800 zeros are added after the Hanning window. Finally, the time domain signals are transformed into frequency domain data using Fast Fourier transform functions and then the transmission coefficients are calculated.
Experimental Results
CONCLUSIONS AND FUTURE STUDY
Conclusions
Capsules are broken in the initial break test and leakage of healing agents is assumed at the exposed sites. In all specimens, for a given wave frequency, the surface-to-wave transmission ratio decreased significantly with increasing crack depth. The presence (or absence) of coarse aggregate as well as the strength of the concrete seemed to have no effect on the propagation of surface waves over cracks of different depths in the concrete.
However, the useful frequency range of surface wave transmission data was observed to be limited for the purpose of crack depth estimation. In addition, the effects of the presence of coarse aggregate as well as concrete strength were found to be less sensitive than the effects of crack depth in estimating crack depth based on spectral energy.
Research in Progress and Future Study
- Needs of research in surface-wave experimental program
- Application on the assessment of crack healing repaired by self-healing materials
As the first step to monitor crack healing repaired by self-healing materials, plain concrete samples are prepared and cracks are generated in the center of sample by means of three-point bending tests. A review of intrinsic self-healing ability of engineered cementitious composites: Recovery of transport and mechanical properties. Effect of crystalline admixtures on the self-healing ability of early-age concrete studied by means of permeability and crack closure tests.
The effect of varying volume fraction of microcapsules on fresh, mechanical and self-healing properties of mortars. Self-healing ability of fiber-reinforced cementitious composites for restoration of waterproofing and mechanical properties. Experimental characterization of the self-healing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis.
Detection of the activation of a self-healing mechanism in concrete by acoustic emission and digital image correlation. The effectiveness of self-healing concrete using alternative manufacturing procedures and more realistic crack patterns.
