Experimental and Theoretical Serviceability of Strengthened and Nonstrengthened Unbonded Posttensioned Indeterminate I-Beams: Fid-Bau Portal

Experimental and Theoretical Serviceability of Strengthened and Nonstrengthened Unbonded Posttensioned Indeterminate I-Beams

Maghsoudi, A. A

Abstract

AbstractThe serviceability response of ultrahigh strength self-consolidating concrete (UHSSCC) in strengthened and nonstrengthened unbonded posttensioned indeterminate members is crucial, because little research has been conducted on such continuous members composed of this relatively new type of concrete. Moreover, most studies on continuous beams mainly focused on ultimate response prediction, because the results are not applicable to the serviceability response, and the design of these members is mostly governed by the serviceability limit state. Therefore, for evaluation of the serviceability response, flexural testing of unbonded posttensioned UHSSCC continuous nonstrengthened and strengthened I-beams, reinforced with one unbonded tendon of variable eccentricity and different area of tensile and compressive reinforcement bars, was conducted. One layer of unidirectional carbon-fiber-reinforced polymer sheet (CFRP) (ply) at sagging and hogging regions was externally applied to strengthened beams. To determine the service loads, materials stress and strain (tendon, concrete, steel rebar, and CFRP sheets), the flexural crack width and deflection of test beams were monitored at different stages of loading. Different standards were used to specify the permissible stress and strain of materials and flexural crack widths (0.1 and 0.2 mm) under the serviceability limit state. The end of the serviceability level was determined for beams with a new flexural crack width limitation of 0.3 mm. Considering this aspect, it was found that for the test beams, an average load capacity enhancement of 20% was pronounced at the serviceability state. An increase in propagation length and number of cracks with a decrease in crack spacing was monitored for strengthened beams compared with nonstrengthened ones. This was primarily caused by lower values of effective prestressing stress (fpe) and strengthening technique. Maximum flexural crack width equations were proposed and compared with experimental and theoretical values. The performances of the proposed equations were as good as that for the American Concrete Institute (ACI), but were superior to those recommended by the Eurocode 2 and Canadian Prestressed Concrete Institute standards. The ACI equation was more versatile because it was applicable to both nonstrengthened and strengthened beams. The predicted values for immediate deflection of the beams at the service limit state based on the simplified method were compared with the experimental deflection values, and the results showed good agreement up to the yielding of nonprestressed steel. The discrepancies between the predicted and experimental deflections were within ±20%.