By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Dec 9 2024A recent article published in Case Studies in Construction Materials reviewed the current research on nanocarbon material-engineered electrically conductive cement composites (ECCCs), with a focus on their self-sensing applications from 2014 to 2024.
Study: Nanocarbon-Enhanced Cement Composites for Self-Sensing and Monitoring in Transport Infrastructure. Image Credit: 88studio/Shutterstock.com
Nanocarbon Materials-Engineered ECCCs
ECCCs are multi-phase, multi-component materials composed of a cementitious matrix integrated with conductive fillers, forming a network that facilitates electron transfer. These conductive fillers are typically derived from graphene, carbon nanotubes (CNTs), and carbon nanofibers (CNFs). Meanwhile, the cementitious matrix, made up of cement and aggregates, provides the necessary structural support.
To prepare ECCCs, nanocarbon materials are either pre-dispersed in a solution before being combined with cement and aggregates or directly blended with dry cement and subsequently mixed with water and aggregates. The performance of ECCCs heavily depends on achieving a uniform and stable dispersion of nanocarbon materials within the matrix. However, their high surface energy often results in challenges with dispersion.
Various methods have been developed to enhance the dispersion quality of nanocarbon materials in aqueous solutions. These include ultrasonic dispersion, surfactant treatments, and combined techniques. Despite these advancements, agglomeration of nanocarbon materials remains a persistent issue.
To address this, researchers have proposed nanocarbon-coated aggregates as an alternative solution. For example, a conductive aggregate can be synthesized by spraying carbon nanotube-latex (CNT-latex) ink onto the surface of standard aggregates. Another approach involves impregnating modified gelatin and carbon black into porous ceramic materials to create conductive aggregates. These innovations aim to improve the uniformity and conductivity of ECCCs while overcoming dispersion challenges.
Working Mechanism of Self-Sensing Cement Composites (SSCCCs)
The self-sensing properties of ECCCs, also known as self-sensing cement composites (SSCCCs), stem from changes in their internal conductive network when subjected to external forces. Nanocarbon materials embedded in ECCCs facilitate electrical conduction by forming efficient pathways through direct contact.
Even in the absence of direct contact, conduction pathways can emerge due to quantum effects such as tunneling conduction. This phenomenon occurs when charged particles exhibit wave-like behavior, allowing them to "tunnel" through the gap between nanocarbon materials separated by distances smaller than 10 nm. Additionally, field emission, a specific manifestation of tunneling conduction, arises from localized strong electric fields in certain nanocarbon materials, such as CNTs.
The self-sensing ability of ECCCs is characterized by measurable electrical properties, including volumetric resistivity, resistivity index, reactance, impedance, and capacitance. These properties are influenced by the types and distribution of conductive additives, as well as external factors such as electrode selection and placement, signal acquisition methods, curing age, and environmental conditions (temperature and humidity).
Under external loads, ECCCs experience a redistribution of conductive fillers, leading to changes in resistivity. This resistivity shift is further influenced by tunneling effects caused by variations in the spacing between conductive fillers. Additionally, local deformations of the cement matrix under stress alter the intrinsic resistivity of the nanocarbon materials. Together, these factors dynamically impact the sensing performance of SSCCCs, enabling accurate detection and monitoring of structural changes.
Performance and Applications
The performance of ECCCs is evaluated using parameters such as repeatability, hysteresis, sensitivity, and signal-to-noise ratio. Among these, sensitivity and repeatability are particularly critical, as different loading conditions can significantly influence their evaluation.
The electrical and self-sensing behavior of ECCCs is primarily governed by the development and distribution of their conductive network. Enhancing the performance of ECCCs involves optimizing various factors, including the type, geometric shape, concentration, and surface treatment of conductive fillers. Additionally, improving the uniform distribution of these fillers and their interface bonding with the cementitious matrix can further enhance their self-sensing capabilities.
SSCCs can detect and monitor structural parameters such as strain, stress, cracks, and damage based on the recorded electrical signals. This makes them ideal for transportation information detection. For example, integrating SSCCs into bridge pavement layers or road surfaces allows for real-time monitoring of traffic information, such as vehicle speed, traffic volume, and dynamic weight. These insights contribute to improved road-use efficiency, enhanced safety, and the intelligent operation of transportation networks.
Currently, CNTs are the most commonly used conductive filler in engineered SSCCs due to their superior conductivity and ease of incorporation. They can be directly mixed into cement or applied as coatings on aggregate surfaces. However, SSCCs prepared specifically from CNT-coated aggregates remain relatively rare in practical applications, highlighting an area for further development and innovation.
Conclusion and Future Prospects
The study provides a comprehensive overview of the current research on nanocarbon material-engineered ECCCs, with an emphasis on their SSCC applications. These materials show great promise, particularly in traffic monitoring applications, but their fabrication continues to face significant challenges.
While methods such as ultrasonic dispersion, mechanical stirring, and surfactants have shown potential in achieving high-quality nanocarbon material dispersions, maintaining the same level of dispersion in cementitious matrices remains a challenge. The dispersion issue, coupled with the tendency of nanocarbon materials to agglomerate, often compromises both the mechanical and electrical properties of ECCCs.
Moreover, the inclusion of nanocarbon materials can negatively affect the workability of cement mixtures, complicating transportation and casting processes. These factors limit the widespread adoption of ECCCs in practical applications.
To overcome these challenges, future research should focus on developing more convenient and cost-effective processes to ensure uniform and stable dispersion of nanocarbon materials within cementitious composites. Such advancements will enhance the mechanical, electrical, and sensing capabilities of ECCCs, paving the way for broader applications in smart infrastructure and beyond.
Journal Reference
Yuan, J. et al. (2024). Nanocarbon-Enhanced Cement Composites for Self-Sensing and Monitoring in Transport Infrastructure. Case Studies in Construction Materials, e04082. DOI: 10.1016/j.cscm.2024.e04082, https://www.sciencedirect.com/science/article/pii/S2214509524012348
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