Technical paper DOWEL EFFECTS ON INTERFACE SHEAR BOND FORCE TRANSFER IN CONCRETE BEAMS STRENTHENED WITH FRP SHEETS Jianguo DAI * 1 , Tamon UEDA* 1 and Yasuhiko SATO* 1 ABSTRACT: This paper discusses the effects of dowel force acting on FRP sheets on the shear bond force transfer capacity of FRP sheet-concrete interface. It is found that the dowel force affects the initiation of interface peeling significantly. Moreover, dowel deformation ability of FRP sheets in concrete beams strengthened with FRP sheets decreases greatly with increasing the pullout force in FRP sheets. However, ultimate interface shear bond force capacity is negligibly affected by the dowel deformation if a sufficient anchorage length is provided and if the dowel deformation is limited under a suggested value. KEYWORDS: FRP sheet, flexural strengthening, dowel effects, interface shear bond 1. INTRODUCTION RC beams flexurally strengthened with FRP sheets have high possibilities to fail due to the peeling initiating from the tips of cracks in the mid-span of beams rather than the debonding at the ends of FRP sheets near two supports, because FRP sheets have rather small thickness and they are easy to be extended toward supports. To prevent the mid-span debonding, stress in FRP sheets is given limitations in various design specifications. Those limitations are generally determined based on shear bond stress-slip law of FRP sheet-concrete interface, flexural crack spacing, anchorage length and so on [1,2,3]. Through pullout test, the interface bond performances can be evaluated under the failure caused by interface slip. However, the real cracks in RC beams strengthened with FRP sheets are flexural-shear ones, which are inclined and include both crack opening and crack sliding displacements. Thus the latter exerts dowel force on FRP sheets and may cause vertical interface peeling accompanied with slip failure. It has been mentioned by many researchers that mix-mode interface failure has complex mechanisms and design details should be given to avoid it in the FRP flexural strengthening cases. Up to now, however, no appropriate experimental or analytical study has been performed to observe directly the interface peeling-off that is triggered by combined actions of dowel force and pullout force in FRP sheets. It is not clear as well how the dowel force on the FRP sheet-concrete interface affects the interface shear force transfer (flexural strengthening efficiency of FRP sheets) in RC beams strengthened with FRP sheets. The objective of this paper is to study quantitatively the bond performances of FRP sheet-concrete interfaces under combined loading conditions based on a new test method. 2. EXPERIMENTS 2.1 TEST MATERIALS Properties of CFRP (Carbon FRP) sheets used in the study are shown in Table 1. The applied resin (FR-E3P) and primer (FP-NS) have the elastic modulus of 2.41GPa and the Poisson ratio of 0.38. The mixing ratio of resin/hardner by weight is 2:1. Concrete with mixing ratio of W:C:S:A=160:301:742:1160 was used. The concrete has the test compressive strength of 45MPa. *1 Division of Structural and Geotechnical Engineering, Hokkaido University, Dr. Eng., Member of JCI コンクリート工学年次論文集，Vol.26，No.2，2004 -883-