Size effect tests of normal-strength and high-strength RC columns subjected to axial compressive loading

• Global mechanical properties of 43 RC columns under axial compression are reported.
• The cross-sectional size of the RC columns tested ranges from 200 mm to 800 mm.
• Test results confirm the existence of size effect in larger-sized RC columns under axial compression.
• The nominal strength of RC columns follows closely the size effect law proposed by Bažant.

Geometrically similar reinforced concrete (RC) columns subjected to axial compressive loading were tested for the study of size effect. Herein the present work, a series of axial compressive tests of 43 RC columns with different sizes (in the ratio 1:2:3:4), different concrete strength grades (i.e. the average compressive strengths were 33.2 MPa for NSC and 65.8 MPa for HSC, respectively), different longitudinal (i.e. 1.5% and 2.5%) and transversal (in the volume-stirrup ratios of 0%, 0.65%, 0.94% and 1.2–1.31%) reinforcement ratios and different slenderness ratios (i.e. 3 and 4.5) were conducted. In the tests, the width of the square cross-section of the RC columns was between 200 mm and 800 mm, and the length varied from 600 mm to 2700 mm. The overall mechanical performances of the RC columns, involving the failure patterns, the nominal compressive stress–strain relationships, the peak load-carrying capacity, the nominal compressive strength, the post-peak softening behavior and the buckling and necking of steel rebar were observed and explored. From the test observations one can find that, for both the normal-strength and the high-strength RC columns, the nominal compressive strength decreased with the increase of structure size. The test observations indicated the existence of size effect. Specifically, RC columns with a higher slenderness ratio, a lower reinforcement ratio and a higher concrete strength grade presented a stronger size effect. It was concluded that the bi-logarithmic plots of nominal compressive strengths for different RC columns followed closely the “size effect law (SEL)” proposed by Bažant.