Size Effect on the Mechanical Behavior and Failure of Plain Concrete Under Small-Eccentricity Compression

Concrete exhibits a pronounced size effect, but the influence of small eccentricity on the size dependence of compressive strength remains insufficiently understood. In particular, systematic experimental evidence for geometrically similar plain-concrete specimens under different small-eccentricity ratios is still limited. To address this gap, compression tests were conducted on C-shaped specimens under concentric loading (NA) and small-eccentricity compression with e0/h = 0.08 (NB) and 0.14 (NC). The effective cross-sectional dimensions were 50, 100, and 150 mm. A finite-element model based on the concrete damaged plasticity (CDP) constitutive law was established in Abaqus and calibrated against the test results, after which the numerical sizes were extended to 300–900 mm. The results show that nominal compressive strength decreases with increasing specimen size and that the size effect is more pronounced under eccentric compression than under concentric compression. As the effective cross-sectional dimension increased from 50 to 150 mm, the mean nominal compressive strengths of the NA, NB, and NC series decreased by 7.2%, 12.0%, and 11.7%, respectively. However, the similar normalized strength reductions in the NB and NC series indicate that increasing e0/h from 0.08 to 0.14 does not produce a clearly distinguishable further amplification of the size effect within the investigated range. The numerical results further indicate that nominal strength decreases rapidly at small sizes, whereas the rate of reduction gradually diminishes with increasing size. The three size-effect models capture the overall decreasing strength trend, but their fitting performance depends on the loading condition. The Carpinteri model provides the closest fit for concentric compression, whereas the Bažant compression relation based on the h−2/5 asymptotic scaling gives the best agreement for the two small-eccentricity series. These findings show that small-eccentricity compression produces a more pronounced size effect than concentric compression, while the influence of further increasing eccentricity on the normalized size-effect magnitude remains limited within the investigated range.