PSI - Issue 83

Henry Ssenyonjo et al. / Procedia Structural Integrity 83 (2026) 47–56

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2. UHPC as a host matrix for self-sensing UHPC represents an extreme end of high-performance concrete, distinguished by its refined granular mix and low porosity. A typical UHPC mix contains a high cement content combined with fine supplementary cementitious material (SCM) like silica fume to fill small scale voids and enhance strength. Only fine sand is used as aggregate as no coarse aggregate is required to improve homogeneity (B. Han et al., 2015). A very low water-to-binder ratio is employed, and the matrix is made workable by high-range water-reducing admixtures (superplasticizers). The result is a compact, virtually pore-free microstructure giving compressive strengths multiple times greater than conventional concrete. In fact, commercial UHPC mixes have reached 150–200 MPa compressive strength with approximately 15 MPa flexural strength (Lian et al., 2021). Steel fibres at approximately 2% by volume are commonly used in UHPC to overcome brittleness (Zhao et al., 2022), bridging cracks and providing pseudo-ductile post-cracking behaviour. Fibre reinforcement transforms UHPC from an extremely strong but brittle material into a strong yet ductile composite capable of sustaining and bridging cracks without immediate failure. The resulting microstructure offers low permeability and high compressive strength. These attributes also affect electrical movement through the composite by controlling the morphology of cracks, probability of fibre contact, and the stability of the conductive network. Whereas steel fibres are the default choice, non-metallic fibres are also emerging as promising alternatives for UHPC reinforcement. Research on replacing steel fibres in UHPC with polymer fibres has been limited until recently, but interest is growing due to the potential for cost reduction and improved durability (Nana et al., 2021). Fig. 1 illustrates the various components of a UHPC mix.

Fig. 1. Materials used in UHPC

SS-UHPC is typically achieved by introducing one or more conductive phases that may include metallic fibers, conductive particles (fine steel slag aggregates, nickel or copper particles), and nano/micro carbon-based fillers such as carbon nanotubes or carbon black (Lee et al., 2024). Hybrid designs aim to reduce percolation threshold and improve both sensitivity and signal stability (Chun & Yoo, 2019). Some of the typical conductive phases used for self-sensing purposes in UHPC have been discussed in Table 1 below.

Table 1.Conductive phases used to enable self-sensing in UHPC

Conductive phase Metallic fibers (steel, stainless, coated) Carbon nanomaterials (CNT, carbon black, graphene oxide) Conductive particles (slag, Ni/Cu aggregates) Piezoelectric inclusions (PZT cement sensors)

Dominant roles

Typical benefits

Key constraints

Contact conduction; crack bridging Tunneling + contact networks; multiscale pathways. Bulk conduction pathways; stress sensing Electromechanical impedance response

Durability + mechanical synergy Corrosion; orientation variability; contact resistance effects

Lower percolation threshold; high strain sensitivity

Dispersion difficulty; cost; workability impact

Lower cost options; improved conductivity Strength/condition monitoring via impedance shifts

Density / segregation; influence on rheology Embedding/compatibility; calibration complexity