The experimental results of twenty-one plain and reinforced concrete columns with 200mm square section are presented. The columns were tested under two different strain profiles: A uniform strain representing a concentric load situation and a triangular strain profile with the neutral axis at one edge of the cross-section. The main variables are concrete strength, ranging from 45 to 100 MPa, and confinement reinforcement parameters with volumetric ratio ranged from 0.7% to 2.4%. It was found that the confinement effect only became significant when the columns underwent large deformation far from the commencement of cover spalling. Stress block parameters of the concrete in compression are provided. The coefficient of the average stress α_1 was found to decrease with the increase of concrete strength while the coefficient of the effective depth β_1 was not sensitive to the change of concrete strength.
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