Axial behavior of reinforced concrete columns encased with UHPC stay-in-place cylindrical forms

This study investigated the axial loading performance of composite columns consisting of normal strength concrete (NSC) cores encased with ultra-high-performance concrete (UHPC) jackets as stay-in-place forms. Ten columns were concentrically loaded to failure and jacket strains in the two orthogonal directions were measured. The columns exhibited a predominantly diagonal shear failure near mid-height, through both the NSC and UHPC, accompanied by buckling of the longitudinal steel rebars in the core. A nonlinear finite element model was developed, verified, and was used in a comprehensive parametric study. Results showed that UHPC jackets increased axial load capacity significantly. However, this increase is attributed to the axial load sharing of the jacket, rather than its confining effect of the NSC core. In this study the maximum confinement effectiveness was 1.12 for UHPC jackets with hoop steel reinforcement ratio of up to 2.2%, at a diameter-to-thickness ( D o / t ) ratio of 15. The jacket experiences a bi-axial state of stress which weakens it in the hoop direction. When loading was applied to the NSC core only, the UHPC hoop stress at failure increased from 24 to 70% of its ultimate value, increasing confinement. However, in this case the gain in axial load from core confinement is far smaller than the lost axial load sharing of the UHPC jacket. As such, loading the inner core, only, is not justified. A bi-axial stress interaction failure envelope was established for the UHPC jacket and used to explain the reduced confinement when the jacket was axially loaded. The UHPC axial load sharing of the total load reduced from 68 to 23% as its diameter-to-thickness ratio increased from 6 to 25 and reduced from 40 to 31% as NSC compressive strength increased from 30 to 60 MPa.