International Concrete Abstracts Portal

A shear retrofitting method for reinforced concrete (RC) beams has beendeveloped that uses external CFRP straps to provide additional shear capacity.Research has been undertaken to develop an installation technique that allows theCFRP strap to encompass the full depth of the beam, without requiring access to the topsurface of the beam. The current testing scheme investigates the durability of the CFRPstrap system using the new installation technique. A long-term load test was conductedon a RC T-beam which indicated that the straps continued to provide shear capacityafter 7 months under a load equivalent to 80% of the ultimate capacity of the retrofittedbeam. A cyclic test conducted on another similar T-beam specimen demonstrated thatafter 1,000,000 cycles, under a load that varied between 0.5 and 0.8 times the ultimateretrofitted beam capacity, the straps continued to provide effective shearenhancement.

Fiber-reinforced polymers (FRPs) are effective in strengthening concretestructures. However, little work has examined the effects of cold regions on thebehavior of the strengthened members, particularly the combined effects of sustainedloading and freeze-thaw exposure. This paper presents the results of an experimentalstudy on the durability of 70 normal weight, low strength, and non-air entrainedconcrete cylinders (150 x 300mm). The cylinders were confined with glass-FRP (GFRP)sheets or carbon-FRP (CFRP) sheets and exposed to 300 freeze-thaw cycles while undersustained axial compression loads. FRP-wrapped cylinders showed exceptionaldurability performance after their extreme exposure to freeze-thaw and sustainedloading with a maximum of 12% reduction in strength. Some CFRP wrapped cylindersthat were exposed to freeze-thaw without longitudinal restraint, by means of sustainedloads, and all the plain concrete cylinders were completely disintegrated with virtuallyzero residual strength.

This paper presents an approach for considering a wide variety of compositematerials in reliability-based design of composite strengthening of concrete structures.The approach is based on the use of the mean value of composite properties as thedesign value and having a composite specific resistance factor that is a function of theCOV of the composite properties. Strengthening design of a simple beam with threemodel composites is used to demonstrate the feasibility of this approach. Forcomparison, designs are created using two existing strengthening guidelines, ACI 440and TR55. These designs are shown to have a high degree of variability in theirreliability and demonstrate the need for a reliability based design procedure. Some ofthe critical areas of further research required for further development of this designprocedure are briefly discussed.

Compared to ordinary steel reinforcement, Fiber-Reinforced Polymer (FRP)reinforcing bars have a lower stiffness, display a brittle-elastic response, and possessparticular bond characteristics. The dependence on these distinctive features makesdeflection control in FRP-reinforced concrete beams and one-way slabs a moreelaborate process compared to the traditional serviceability design of steel-reinforcedmembers. This paper reports the rationale and fundamental concepts backing theindirect deflection control procedure for concrete beams and one-way slabs reinforcedwith FRP bars adopted by ACI 440.1R-06. The fundamental procedure can be appliedregardless of the type of reinforcement; it is independent of the member’s stiffnessthrough the cracked stage; and it is expressed as a function of the deflection-spanratio, which allows designers to fully control deflections depending on applicableserviceability limits. The paper also explains the simplifications made to thefundamental procedure that led to the development of the indirect deflection controlprocedure in tabular form found in ACI 440.1R-06, including the method by whichtension stiffening effects are accounted for.

The development of different pre-stressed epoxy bonded CFRP-reinforcement systems for retrofitting RC-structures continues. In addition torequirements concerning the ultimate limit state (ULS) pre-stressed systems couldsatisfy demands in regard to the serviceability state (SLS) like crack limitation orfatigue. In comparison with the well known material behavior of concrete undersustained loading epoxy resins show material properties more sensitive to thermaleffects, for example loss of strength or creep deformations. In experimental tests theinfluence of permanent load and temperature is examined. The pure epoxy resinadhesive was evaluated in lab-shear tests and the structural behavior of the system inthe form of epoxy bonded pre-stressed near surface mounted CFRP-strips. Theexperiments reveal a significant influence of the epoxy resin adhesive behavior on thelong-term structural behavior in service. To evaluate the effects with regard to designsrelevant problems as long-term development of transfer lengths of pre-stressedreinforcement or of the slip between reinforcement and concrete under varying loadand thermal conditions results of a simulation model are presented.