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External prestressing is widely employed in structural strengthening engineering due to its numerous advantages. However, external prestressed steel bars are prone to corrosion when exposed to the service environment. This paper is dedicated to examining the use of fiber-reinforced polymer (FRP) bars as external prestressing materials to strengthen one-way concrete slabs. Five one-way concrete slabs were strengthened with externally prestressed FRP bars with different prestress levels and different amounts of FRP bars, while one non-strengthened slab was used for comparison. The effects of strengthening on the flexural behavior, specifically the cracking load, ultimate load, stiffness and failure mode, were analyzed systematically. Moreover, the ductility and cost–benefit optimizing properties of the reinforcing design were discussed. The results show that external prestressed FRP bars significantly improve the cracking load, ultimate load and stiffness of one-way concrete slabs. The absence of a bond between the concrete and FRP bars overcomes the brittleness of the FRP bars, while the strengthened slabs exhibit satisfactory ductility and a higher post-yield stiffness and bearing capacity. Additionally, the cost/benefit ratio is optimized by increasing the prestress level, while a higher number of prestressed FRP bars is beneficial to ductility. Finally, a method for calculating the stress in prestressed FRP bars at ultimate loads was proposed. Irrespective of the prestressing material, this method is applicable to both strengthened beams and one-way slabs. [ABSTRACT FROM AUTHOR]

• Plastic Concrete under true tri-axial compression. • Failure mode, constitutive curve and loading path were investigated. • The failure criterion of plastic concrete under true tri-axial compression was put forward. Based on the tests among 45 group specimens of five mixes under true tri-axial compressive stress, the characteristics of strain–stress curve, the impact of the stress path and the failure criterion of the plastic concrete were illustrated. The results indicated that the stress - strain curves can be generally divided into four segments, the initial segment was equipped with significantly elastic characteristics, while the last segment indicated severely plastic deformation had occurred. Specimen failure pattern was similar to that of axial compression specimen when the levels of the second and third principal stress were smaller. Otherwise, specimen failure pattern looked like eight diagonal crack. Principal stresses, cohesive forces and angles were influenced dramatically by different stress paths. Based on 93 group test data, three formulas with four parameters, three parameters and two parameters are put forward to express the failure criterion of plastic concrete under true tri-axial compression. [ABSTRACT FROM AUTHOR]

• An anchor system is provided for strengthening RC one-way slabs by FRP tendons. • The flexural behavior of the strengthened slab is improved efficiently. • The improvements are different at different loading stage. • The failure form of strengthened slabs is distinguish from that of beams. • A calculation method for the stress of external FRP tendon is proposed. External prestressing is an efficient technology to strengthen existing structures, and using fiber-reinforced polymers instead of steel in external tendons is advantageous to avoid corrosion-related problems, though anchoring external fiber-reinforced polymer tendons is still a major issue. An innovative anchoring technology is proposed in this paper for the external fiber-reinforced polymer tendons aimed to strengthen one-way reinforced concrete slabs. Six slab specimens were designed and manufactured to check the feasibility and reliability of the proposed technology. In one slab no external prestressing was applied (reference slab), while the remaining five slabs were strengthened by means of external post-tensioned glass fiber-reinforced polymer tendons, using the proposed anchoring system. The strengthened slabs differ for the prestressing level and external-tendon ratio. All slabs were tested up to failure and their flexural behaviors and failure modes were thoroughly investigated. The tests show that the proposed anchoring technology is reliable and efficient. A method for the calculation of the stress in the tendons at ultimate is also proposed. Finally, the numerical results are shown to agree fairly well with the test results. [ABSTRACT FROM AUTHOR]