## 4 Self-Sensing Geosynthetics
### Carbon-Black-Based Self-Sensing Geosynthetics  
Carbon black (CB, CAS No. 1333-86-4) is prepared by rigorously controlling the thermal decomposition of carbon-rich feeds in an oxygen-depleted (partial combustion) or inert (pyrolysis) atmosphere [108; 109; 110]. CB is the most cost-effective and industrially manufactured nano-carbon, with an annual output expected to reach 15 Mt by 2025. CB dwarfs the carbon nano-tube (CNT) market (15 kt by 2022), and at 1 USD/kg, it is considerably less expensive than the cheapest grade of CNTs currently available on the market (600 USD/kg) [108]. [PERSON] et al. [111; 5, 112] investigated the piezoresistivity of conductive-polymer-doped geobelts, which serve not only as reinforcements but also as monitoring devices. The self-sensing geobelt was composed of CB-filled high-density polyethylene (HDPE) and was fabricated for both industrial and laboratory applications. For the laboratory-developed self-sensing geobelt, the optimal CB filler content was 47.5%. For the industrial specimens, the optimal CB concentration was slightly less. The authors also performed tensile and pull-out tests to investigate the mechanical properties and piezoresistivity of the self-sensing geobelt and found that quadratic polynomial and linear functions defined the relationship between the self-sensing geobelt normalised resistance and strain in the ranges 0-10% and 0-7%, respectively. Furthermore, the authors showed that both the tensile strength and elongation at the break of the cyclically loaded self-sensing geobelt both decreased with the increasing number of loading cycles and amplitude compared with the non-loaded self-sensing geobelt. However, the pre-strain only slightly affected the mechanical properties of the self-sensing geobelts. The authors also reported that by increasing the pre-strain and the number and amplitude of the loading cycles, the electrical conductivity of the self-sensing geobelts became more strain sensitive after cyclic loading. [PERSON] et al. [113] evaluated the effects of the CB type on the mechanical performance, percolation threshold, and piezoresistivity of low-density polyethylene (LDPE) and polypropylene (PP) composites for application to self-sensing geosynthetics. The CB structure is influenced by the primary CB particle shape, size, and aggregate size. Owing to the higher attractive interactions of the inter-aggregates in smaller primary particles,CB exhibits higher agglomeration and a larger structure. In contrast, when aggregates are composed of only a few primary particles, a smaller CB structure is generated [114]. Owing to the low cost of CB, slightly changing the CB concentration in existing geosynthetic formulations (i.e., 1-4%) is unlikely to markedly impact the geosynthetic manufacturing cost, nor does it necessitate major changes in the manufacturing processes of existing geosynthetic products. The observations of [PERSON] et al. [113] supported the theory that the CB percolation threshold concentration was inversely related to the CB structure. Furthermore, their studies showed that composites containing larger CB particles exhibited sharper piezoresistivity. In addition, the piezoresistivity response of PP composites was higher than that of LDPE composites, as listed in Table 3. Regardless of the host polymer utilised, the unitised self-sensing geosynthetic specimens indicated greater gauge factors (GFs) relative to normal commercial strain gauges (i.e., GF = 2). In the mechanical property characterisation, a CB percentage of up to 20 wt.% reduced the PP composite tensile strength by approximately 50% while leaving the LDPE composite tensile strength virtually intact. The increased PP crystallinity (45-55% for LDPE vs. 70-80% for PP [115]) could explain the more drastic drop in tensile strength. Furthermore, incorporating 30% CB into PP reduces the PP crystallinity by approximately 33% [116].