# Applications of Cement-Based Smart Composites to Civil Structural Health Monitoring: A Review
## 1 Introduction  
Most infrastructures around the world are made of concrete, which represents one of the most used materials for human purposes, second only to water [1]. Despite continuous and significant advances both in technology and design of reinforced concrete (RC) structures, there is a large stock of existing structures and infrastructures that are experiencing deterioration phenomena or functional obsolescence [2, 3]. As a matter of fact, in the last decades the effective management and maintenance of existing structures has become increasingly crucial in Europe and worldwide, and Structural Health Monitoring (SHM) has been recognized as a paramount tool in this concern. Indeed, SHM systems are aimed at automatically identifying damage occurrence on the monitored structure, evaluating also location, severity and potential consequences, by means of continuous in-situ non-destructive sensing and analysis of damage features characterizing the health state of a structure [4]. The timely detection of structural distress at very early stages has an impact on both safety and economic management [5]. Nevertheless, the desirable large-scale application of SHM technologies has been hindered by the combination of several limits, such as the large dimensions of civil structures and practical limits of traditional monitoring technologies. Accordingly, successful SHM systems applied to large infrastructures often need several conventional off-the-shelf sensors to ensure adequate spatial resolution of measurements, and they often cannot be easily integrated in the host structure [6, 7]. Within this framework, the development of innovative integrated biomimetic SHM systems for civil structures based on the use of cement-based smart composites (CSC) has gained more and more attention in the last decades, also following the relevant progressin the field of nanomaterial synthesis [8]. Basically, CSCs exploit the piezoresistive effect induced by internally dispersed conductive fillers, namely the capacity of the electrical resistivity to change due to mechanical actions. Thus, by measuring the variation of electrical resistivity, strain can be monitored. Self-sensing materials for SHM allow capitalizing a series of potential benefits if compared to off-the-shelf sensors: high gauge factor, material compatibility, lower disturbance of the mechanical behavior of the hosting member, reduction of costs and complexity of installation because sensors can be embedded without varying the original design of the hosting member, limited probability of sensor failure [9; 10]. From an economic point of view, in recent years there has been a significant reduction of filler fabrication costs whereby carbon-based nanomaterials are currently characterized by affordable unit costs (ranging from 10 $/g to 150 $/g depending on type, purity and dimensions); taking into account the very low filler concentration (typically 1% by weight of cement) and the little volume usually characterizing CSC sensors, they can be fabricated in large amount at affordable cost nowadays. As a result of the previously mentioned advantages, CSCs are ideal candidates for the development of biomimetic SHM systems able to mimic structure and functions of the human nervous system by means of densely distributed CSC sensors which capture local strain signals continuously processed by a central unit (Figure 1). CSCs also find other relevant applications related to civil SHM, such as traffic detection [11] or weigh-in-motion [12]. However, these applications are out of the scope of the present study. Since the first studies on the piezoresistive behavior of CSCs, which date back to the early Nineties [13], significant research efforts by many research groups from all over the world have focused on numerous different aspects in the development of self-sensing CSCs, mostly related to material optimization.