As an intrinsically self-sensing material, carbon fibre concrete has promising applications in structural health monitoring. Investigating the long-term stability of carbon fibre concrete in terms of resistivity changes under the influence of factors such as ageing effects and environmental conditions can help assess its feasibility for long-term monitoring applications. In this paper, the specimens were designed based on the variables of carbon fibre admixture (0.05 vol%, 0.08 vol%, 0.1 vol%) and carbon fibre length (3 mm, 6 mm, 9 mm, 12 mm), and the DC four-electrode method was used to study the self-sensing of low admixture carbon fibre reinforced hydraulic concrete containing coarse aggregate (5–20 mm) under long age (180d) in order to analyze the resistivity change law when it was loaded under long age condition. The results show that the fraction of resistivity change during loading (in compression and flexure) is low for all specimens in the long-age condition, but the resistivity change is consistent with its internal change. Uniaxial compressive cyclic loading in the elastic phase, each specimen exhibits a piezoresistivity under each loading cycle, while the material's own resistance under bending loading increases irreversibly as the loading progresses. In addition, for the flexural toughness evaluation test of fibre concrete, an adjustable deflection test device is designed based on existing codes, which can effectively solve the drawback of the existing bracket for fixing the deflection test instrument in the span, which needs to know the type of instrument in advance and then reserve the through-holes that can be clamped to the instrument.