Development Length and Bond Strength of FRP Bars Embedded in Normal, High Strength, and Ultra-High-Performance Concrete

Abstract

This thesis examines experimentally the development lengths (Ld) of small diameter (db) (4 mm, 6 mm, and 8 mm) basalt fiber reinforced polymer (BFRP) deformed bars embedded in concrete with compressive strength ranging from 41 to 70 MPa. It also examines the development length of 17 mm diameter sand coated glass fiber reinforced polymer (GFRP) bars embedded in ultra-high-performance concrete (UHPC) with 2% and 4% steel fiber and 2% and 3% polyvinyl alcohol (PVA) fiber content. The compressive strength of the UHPC ranged from 87 MPa to 132 MPa. Both studies used the notched beam testing method.

For the first study investigating small diameter BFRP bars, embedment lengths varied from 5 to 109 db and the clear concrete cover was 30 mm. The 44 notched beam specimens measured 101 x 203 mm in cross-section and varied in length from 610 to 1980 mm to accommodate various embedment lengths. The bar axial stress at failure was plotted against embedment length for each set and linear regression was used to establish Ld based on the guaranteed tensile strength. It was shown that as concrete strength increased from 41 to 70 MPa, Ld reduced from 32 to 20 db for the 4 mm bar, from 30 to 25 db for the 6 mm bar and from 52 to 23 db for the 8 mm bar. ACI440.1R-15 and CAN/CSA S806-12 design guides overestimated Ld of these small diameter bars. Alternative equations for splitting and pullout are proposed and explained herein.

For the second study investigating sand coated GFRP bars, the embedment length was varied from 4 db to 16 db and the clear concrete cover was kept constant at 1.0 db. The specimens experienced splitting bond failure. The average bond strength (τu) was higher in UHPC with steel fibers than PVA fibers. It also increased with fiber content but reduced with embedment length. Consequently, Ld was smaller in UHPC with steel fibers than PVA fibers and was also reduced as fiber content increased. The Ld values for UHPC with 4% steel, 2% steel, 3% PVA and 2% PVA were 20 db, 29 db, 35 db and 55 db, respectively. ACI440.1R-15 (2015) and CAN/CSA S806-12 (2012) design equations provided very conservative estimations for Ld by factors of 1.5-5.7. An alternative simple equation has been suggested for Ld.