Prestressed CFRP Sheets for Strengthening Concrete Slabs in Fatigue

Abstract

Six one-way reinforced concrete slabs were constructed and tested to investigate the effectiveness of using prestressed carbon fibre reinforced polymer (CFRP) sheets to strengthen concrete slabs subjected to fatigue loading. The slabs were 90mm × 1000mm × 3000mm and reinforced with 11.3mm diameter steel reinforcing bars. The slabs were tested in two groups of three slabs. In each group, one slab was unstrengthened and used as a control, the second slab was strengthened with non-prestressed FRP sheets and the third slab was strengthened with prestressed sheets. For the first group, the loading range for the three slabs was based on the loads that induced in the reinforcing steel of the control slab stresses that ranged between 10 and 95 percent of the steel yield strength. For the second group, the loading range chosen for the three slabs induced steel stresses in its control slab that ranged between 30 and 95 percent of the steel yield strength. Therefore, the first group of slabs was subjected to a higher load amplitude but lower mean stress than the second group. The CFRP strengthened slabs of the second group were externally loaded while applying the CFRP sheets, to represent the sustained load that may be associated with field strengthening conditions. The test results confirmed that strengthening reinforced concrete slabs with bonded CFRP sheets, especially prestressed CFRP sheets, increases fatigue life. Common to strengthening with non-prestressed and prestressed CFRP sheets are that compared to the control slabs, strains in the steel reinforcement, plastic creep strains in the concrete, and deflections are all decreased, with greater decreases in all these observed with prestressed CFRP sheets. Crack widths are greatly reduced when the CFRP sheets are prestressed. Since fatigue failure of the internal steel occurred at the location of large cracks that were repetitively opened and closed, reducing the crack widths contributes to achieving longer fatigue life. Finally, the presence of anchors at the ends of the CFRP sheets can delay the ultimate fatigue failure of CFRP strengthened slabs by delaying sheet debonding failures.