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[Display omitted] • In situ observation of the cement-based materials under hydraulic pressure with image analyses revealed the volumetric strain development due to hydraulic pressure varied greatly with location, owing to heterogeneous water infiltration in the mortar specimens. • The behavior of strains and the resultant internal damage to a mortar specimen under hydraulic pressure are strongly influenced by hydraulic pressure as a confinement mechanism and water infiltration as a stress-relaxation mechanism. • In situ observation and fluorescent imaging of the small cylindrical mortar specimens showed no visible damage within the spatial resolution (50–172 μm) up to 20 MPa whilst the reduction in Young's modulus suggested the occurrence of microcracks. The influence of hydraulic pressure (max. 20 MPa) on the internal damage of cement-based materials is experimentally studied. In order to observe in situ the internal damage of cement-based materials under hydraulic pressure, a new testing apparatus equipped with an X-ray micro-computed tomography (X-μCT) scanner is developed. The behaviors of water infiltration and strain development are evaluated for mortar specimens with different water-to-cement ratios. Although the internal damage of the mortar specimens under hydraulic pressure was not detected by the X-μCT scanner and fluorescent imaging, the mechanical properties tested after hydraulic pressure loadings were altered, indicative of the occurrence of microcracks (<50 μm) below the resolution of these observation techniques. It is concluded that the internal damage of mortar specimens under hydraulic pressure is strongly influenced by two mechanisms: hydraulic pressure as a mechanism of external confinement and water infiltration as a mechanism of stress relaxation. The volumetric strain development due to hydraulic pressure varies greatly with location, owing to heterogeneous water infiltration in mortar. Therefore, the balance of these two mechanisms must be taken into account to assess the risk of internal damage of cement-based materials under hydraulic pressure. [ABSTRACT FROM AUTHOR]