The chapter focuses on the development of mechano-responsive CNT-Epoxy concrete, a novel material designed to repair cracks in concrete structures while offering load sensing capabilities. The study begins with an introduction to the challenges posed by cracks in concrete structures and the need for advanced repair materials. It then delves into the fabrication process of CNT-Epoxy concrete, utilizing multi-walled carbon nanotubes (MWCNT) and epoxy resin. The electrical conductivity of the material is investigated, showing an increase with higher CNT content. The mechanical properties and electro-mechanical behavior under both static and cyclic loading conditions are extensively tested, demonstrating the material's sensitivity and stability. The results indicate that CNT-Epoxy concrete has the potential to revolutionize the repair and maintenance of concrete structures, offering both strength and the ability to monitor structural health.
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Abstract
Cracking in engineering concrete structures poses a significant problem as it not only accelerates the rate of deterioration but also diminishes the structural strength. The current repair materials being used are capable of short-term repairs only and may result in destroying the concrete structure due to additional damage. In this study, Carbon nanotube (CNT)-Epoxy concrete is proposed as a sensing repair material to address this problem. The authors assess the electrical properties of samples based on the CNT content and aggregates and report on the optimal mixing ratio of CNT-Epoxy concrete. The electro-mechanical characterization of CNT-Epoxy concrete was evaluated through a series of experiments such as static and cyclic loadings. CNT-Epoxy concrete exhibits accurate sensing response under compression and maintains a consistent cyclic response. The findings from the study demonstrate that CNT-Epoxy concrete is an effective material for repairing concrete cracks, offering favorable compressive strength, and showing reliable sensing ability.
1 Introduction
Engineering concrete structures such as bridges, tunnels, and dams are frequently exposed to service load, which leads to cracks initiated of the structure’s surface [1]. These cracks reduce local stiffness and create material discontinuities, which eventually destroy the concrete structure as damage accumulates [2, 3]. With an increasing focus on the maintenance of concrete structures, it has become crucial to address repair work promptly to prevent futher deterioration. Currently, repair work is being performed on cracks or damaged areas in various concrete structures [4‐6]. In recent years, polymer concrete has been increasingly recognized as a prominent material for repair work in various applications [7]. Polymer concrete has been widely used as a repair material because of its rapid curing property, excellent bond to cement concrete and reinforcing bars, and impressive strength and durability [8‐10]. In contrast, polymer concrete can only be repaired, and research on repair materials with sensing capabilities is limited.
CNT is one of the additions that can be used to create multifunctional materials [11]. The structural morphology of CNT can be used as a composite mixed with different materials to impart electric-mechanical properties. The self-sensing capability of CNT nanocomposites is regarded as a key requirement for future structural health monitoring technologies and applications [12, 13].
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In this study, we intend to fabricate CNT based epoxy concrete that can effectively repair cracks and enable load sensing in concrete structures. We calculated the electrical conductivity for CNT content to select the appropriate CNT content for CNT-Epoxy concrete with and without aggregate. We confirmed the mechano-responsive of CNT-Epoxy concrete when a static load was applied. We tested the cyclic load using this result and obtained a significant conclusion regarding the mechano-responsive performance.
2 Experimental
2.1 Materials
MWCNT was purchased from Nanolab, Inc. (MA, USA) and featured a diameter of 15nm, a length of 5-20μm, and a purity higher than 85wt%. Epoxy (DH-150) was provided by Daehwa Precision (Gyeonggi-do, South Korea) and was used as the base polymer matrix. Aggregate was obtained from Joomoonjin Silica sand Co., LTD (Gangwon-do, South Korea). Acetone was used as a dispersant and was provided by Samchun chemicals Co., LTD (Gyeonggi-do, South Korea) with 99.7% purity. All the materials were used as received.
2.2 Fabrication of CNT-Epoxy Concrete
Figure 1 showed the fabrication process of CNT-Epoxy concrete. A 50 ml of acetone was used as a dispersant to reduce the viscosity of the epoxy and to improve the dispersion performance of CNT [14]. The Q700CA ultrasonicator of Qsonica LLC. (CT, USA) and a TR50M three-roll mill of Trilos (CA, USA) were used to disperse the CNT in the epoxy concrete. A mixed sample of CNT, pure acetone, and epoxy resin was dispersed in an ultrasonicator using a 90% amplitude pulse model for 40 min. Then, the sample was placed on a hot plate to allow for the evaporation of acetone. Subsequently, epoxy hardener and aggregates were mixed with the sample using a three-roll mill. Note that 80wt% of aggregates were selected based on the optimal aggregate content reported by Jung et al. [8].
Fig. 1.
Fabrication process of CNT-Epoxy concrete
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2.3 Characterization
The electrical resistance of CNT-Epoxy concrete was measured using a 2450 Sourcemeter of Kethley (OH, USA) with the two-probe measurement method. Electrical conductivity was calculated to select the most suitable amount of CNT to impart the desired function to CNT-Epoxy concrete. The electrical conductivity, σ, was calculated using the Eq. (1):
$$ \sigma = L/AR $$
(1)
where σ (S/m) was the electrical conductivity of the sample, A(m2) was the cross-sectional area of the specimen, L(m) was the sample thickness, and R(Ω) was the evaluated electrical resistance.
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The 8861 universal testing machine of Instron (MA, USA) was used for the static and cyclic load tests to evaluate the electro-mechanical properties of CNT-Epoxy concrete. Static loading was performed under displacement control of 1.3 mm/min. Cyclic loading was performed under lading frequency 0.5Hz. During the static and cyclic loading, the change in resistance of the specimens was recorded using a 2700 multimeter of Kethley (OH, USA) with a data acquisition (DAQ) system (Fig. 2).
Fig. 2.
Electro-mechanical test set up
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3 Result and Discussion
Figure 3 showed the electrical conductivity of CNT-Epoxy concrete. The electrical conductivity of CNT-Epoxy concrete increases as increasing CNT content. This is because the electrical conductivity of CNT-Epoxy concrete was determined by the conductive path of dispersed CNTs [15]. Both CNT-Epoxy concrete with and without aggregates exhibited variations in electrical conductivity but demonstrated a similar increasing trend. CNT-Epoxy concrete with aggregate was also selected for its conductive and economic feasibility. At 0.5 wt% of CNT, the electrical conductivity started to converge, and the electrical conductivity was stable. Therefore, CNT content 0.5 wt% was selected for reasons of economic efficiency and stability.
Fig. 3.
Electrical conductivity of CNT-Epoxy concrete as a function of CNT content
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The mechanical properties and electrical changes of CNT-Epoxy concrete under the compression are shown in Fig. 4. The compressive strength-strain behaviors of CNT-Epoxy concrete are plotted in Fig. 4(a). Compared with CNT-Epoxy concrete and South Korea repair material compressive strength standards, CNT-Epoxy concrete has a much higher compressive strength. Figure 4(b) showed the relative resistance of CNT-Epoxy concrete under the compressive loading. The sensitivity of the S was determined by
where ΔR was the change in the resistance, R0 was the initial resistance when no compressive strength is applied, and ΔP was the applied compressive strength. The S was defined as the slope of the curve and is divided into two ranges based on compressive strength. A sharp decrease in resistance was observed in the range of 0–5 MPa, and then a stable decrease in resistance was observed in the range of 5–10 MPa. The compressive sensitivities of samples were calculated as 11.83 and 0.73, respectively. This is attributed to the structural variation of CNT-Epoxy concrete during the compression process, leading to different rates of change in the contact area between CNT in CNT-Epoxy concrete.
Fig. 4.
CNT-Epoxy concrete in compression test (a) Mechanical properties (b) Electro-mechanical properties
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Figure 5 showed the electro-mechanical properties of CNT-Epoxy concrete under the cyclic loading conditions, and CNT-Epoxy concrete under stepwise cyclic loading levels was explored as well. Figure 5(a) showed the stepwise resistance response of CNT-Epoxy concrete at intervals of 5MPa. The maximum relative resistance of samples decreased with each compressive strength level and the resistance recovered to its initial value after external compressive strength release. Similar to the resistance response shown in Fig. 4(b), the minimum resistance response for each strength exhibited decreasing trend with stepwise cyclic loading. Figure 5(b) showed the cyclic loading resistance response of CNT-Epoxy concrete. The results indicated that CNT-Epoxy concrete achieved good resilience and reproducibility for every cyclic loading level.
Fig. 5.
Electro-mechanical properties of CNT-Epoxy concrete under cyclic loading (a) Stepwise cyclic loading response of CNT-Epoxy concrete up to 25MPa (b) Cyclic response of CNT-Epoxy concrete under 15MPa
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4 Conclusions
In summary, we developed mechano-responsive CNT-Epoxy concrete by mixing CNT into epoxy concrete. CNT-Epoxy concrete showed a clear decrease in electrical resistance as increasing compressive strength and accurate and stable compressive sensing performance. Under cyclic loading conditions, the minimum resistance response of CNT-Epoxy concrete varies with the change in stepwise cyclic levels. CNT-Epoxy concrete shows a constant resistance response repeatability under repeated load and secures stability and responsivity. Therefore, CNT-Epoxy concrete exhibits the potential to be used as a multifunctional construction material for repair.
Acknowledgements
This work was supported by the Technology Innovation Program (20014127, Development of a smart monitoring system integrating 3D printed battery-free antenna sensor technology with AI optimization) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1C1C1005273).
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