Compressive behavior of UHPC confined by both spiral stirrups and carbon fiber-reinforced polymer (CFRP).

• Failure modes of UHPC and CUHPC were compared. • Stress-strain behavior of UHPC and CUHPC was analyzed. • Effects of concrete strength, volumetric ratio of stirrups, and CFRP on compressive strength and ductility of UHPC and CUHPC were analyzed. • Existing models for peak stress and peak strain of confined UHPC should be modified. To understand the compressive behavior of UHPC confined by both lateral steel and CFRP, an experiment was presented with the compressive strength of UHPC up to 120.92 MPa in this paper. The specimens were confined by Grade 970 with volumetric ratio in a range of 0.0–2.0% and two layers CFRP laminates. The experimental results showed that the failure modes of confined UHPC were failure in brittle, shear failure, and expansion failure corresponding to specimens with no, low, and high volumetric ratios, respectively, while the failure modes of CFRP confined UHPC were expansion failure. The stress-strain behavior of confined UHPC were divided into three stages included linear stage, nonlinear stage, and descending stage. The stress-strain behavior of CFRP confined UHPC had five stages: linear stage, near linear stage, the stress declined gradually until stress decline sharply, the stress dropped abruptly to a stress platform, and a gradual stress decrease until specimens failed. Concrete Strength, transverse reinforcements volumetric ratio, and CFRP had influence on the load capacity and the ductility of specimens. Furthermore, by comparing experimental results with existing models, none of existing models was suitable for predicting the peak stress and peak strain of confined UHPC without fibers, and the existing model should be modified. [ABSTRACT FROM AUTHOR]