*Article* **E** ff**ect of PBO–FRCM Reinforcement on Sti** ff**ness of Eccentrically Compressed Reinforced Concrete Columns**

### **Tomasz Trapko \* and Michał Musiał**

Faculty of Civil Engineering, Wroclaw University of Science and Technology, Pl. Grunwaldzki 11, 50-377 Wroclaw, Poland; michal.musial@pwr.edu.pl **\***Correspondence:tomasz.trapko@pwr.edu.pl; Tel.:+48-71-320-35-48

Received: 20 January 2020; Accepted: 5 March 2020; Published: 9 March 2020

**Abstract:** This paper examines the e ffect of PBO (P-phenylene benzobisoxazole)–FRCM (Fabric Reinforced Cementitious Matrix) reinforcement on the sti ffness of eccentrically compressed reinforced concrete columns. Reinforcement with FRCM consists of bonding composite meshes to the concrete substrate by means of mineral mortar. Longitudinal and/or transverse reinforcements made of PBO (P-phenylene benzobisoxazole) mesh were applied to the analyzed column specimens. When assessing the sti ffness of the columns, the focus was on the e ffect of the composite reinforcement itself, the value and eccentricity of the longitudinal force and the decrease in the modulus of elasticity of the concrete with increasing stress intensity in the latter. Dependences between the change in the elasticity modulus of the concrete and the change in the sti ffness of the tested specimens were examined. The relevant standards, providing methods of calculating the sti ffness of composite columns, were used in the analysis. For columns, which were strengthened only transversely with PBO mesh, reinforcement increases their load capacity, and at the same time, the sti ffness of the columns increases due to the confinement of the cross-section. The sti ffness depends on the destruction of the concrete core inside its composite jacket. In the case of columns with transverse and longitudinal reinforcement, the presence of longitudinal reinforcement reduces longitudinal deformations. The columns failed at higher sti ffness values in the whole range of the eccentricities.

**Keywords:** column; sti ffness; FRCM; PBO mesh; PBO–FRCM
