International Concrete Abstracts Portal

Upgrading often becomes a necessity due to changes in usage of buildings due to factors such as deterioration and aging, change in occupancy, or the need for installation of facilities such as air-conditioning, heating, escalators, elevators, additional skylights, or new façade structures. In a number of cases upgrading is related to changes which affect the load bearing components of the structure. Fiber reinforced polymer matrix composites provide an efficient means of both strengthening slabs for enhanced load carrying capacity and for strengthening slabs after installation of cut-outs. This paper reports on a series of tests conducted to assess the comparative efficiencies of a commercially available strip form and a fabric form of material vis-à-vis strengthening ability and ductility. It is shown that material tailoring can result in significant changes in efficiencies. The extension of this to the rehabilitation of cut-outs is also detailed and aspects of an on-going full-scale test program in that area are elucidated.

When the serviceability or ultimate strength of a reinforced or prestressed concrete beam is assessed to be inadequate, fibre reinforced polymer (FRP) sheets may be suitable for strengthening these beams. FRP sheets exhibit high strength-to-weight ratios and are non-corrosive. When bonded to the tensile face of a concrete beam, FRP sheets supplement the flexural reinforcement of the beam, increasing the beam’s strength. To improve the efficiency of this strengthening technique, FRP sheets may be applied with an induced prestress. This paper presents results from an on-going experimental investigation that examines the effectiveness and feasibility of using prestressed carbon fiber reinforced polymer (CFRP) sheets to increase the capacity and improve the serviceability of damaged concrete members. A practical mechanical anchorage system for prestressing the CFRP sheets against the concrete beam is presented and the results of the prestressing process are discussed. The flexural behaviour of one 4.5 metre T-section prestressed concrete beam strengthened with prestressed sheets and loaded to failure at room temperature (22×C) is presented and compared to that of an unstrengthened control beam. The paper describes the on-going investigation into the behaviour of beams strengthened with prestressed CFRP sheets and tested at low temperature (-27×C). Aspects of the research program related to the long-term behaviour of beams strengthened with prestressed FRP are also discussed.

In this paper, a new method to use Carbon Fiber Sheets(CFS) which is prestressed before they are bonded to the concrete surfaces based on the concept of prestressing technique is developed. A prestressing system is first designed to be suitable for strengthening existing concrete structures. An experimental program is carried out to verify the reinforcement effects of beams such as on the improvements of flexural strength, ductility, stiffness and crack resistance. To avoid the debonding failure near the ends upon releasing the pre-tensioned force, the anchorage zone are dealt with by several proposed reinforcing methods and the effects of anchorage treatments are also discussed. An effort is also made to investigate the determination of appropriate presressing stress level of CFS and structural optimization of reinforcement.

In this paper, uniaxial tension tests on CFS(carbon fiber sheet)-strengthened concrete specimens with and without steel bar reinforcement and bending tests on CFS strengthened RC beams were conducted to examine the crack behavior in the concret. The testing showed that the crack spacing of the uniaxial tension members strengthened with CFS was only slightly affected by the diameter of the reinforcing steel bars, the thickness of the concrete covering and the stiffness of the CFS. The crack spacing and the crack width both for tensile and flexural members was significantly smaller when the CFS was used. And also the cracks were distributed in the plain concrete tension member without steel reinforcement strengthened with CFS. Finally, it was recognized that the tension stiffening effect is realized to be improved by the non-liner behavior of the CFS-concrete interface.

An experimental study was conducted to determine the transfer length, development length and flexural behavior of fiber-reinforced polymer (FRP) tendons in prestressed concrete beams. Three kinds of nominally 5/16 in (8 mm) diameter FRP tendons were included in the study: Carbon Leadline, Aramid Technora and Carbon Strawman. Thirty beams were pretensioned using a single FRP tendon. In addition, twelve control beams were pretensioned with a seven-wire steel strand (ST). Transfer length observations from this study were based on concrete strain measurements with a DEMEC gage system. Development length observations were based on three-point flexural tests. Four-point flexure tests were also performed on each material to gain additional understanding of the bond behavior between concrete and the PC reinforcing materials. The "95% average plateau strain" method of using concrete strain results was shown to be an effective way to determine transfer length. By using an appropriate flexural model and extrapolating results from over-reinforced tests to situations where the tendon would actually fail, it was possible to determine development length in this investigation. Despite differences in tendon material properties and prestressing forces, both the measured transfer lengths and the development lengths were almost identical for all tendon materials tested. The development length for FRP tendons was reasonably predicted by the ACI design equation, although transfer length appears to be underestimated.