Performance of damaged concrete stub columns strengthened with post-tensioned CFRP under freezing-thawing environment

Abstract

Reinforced concrete structure with directly bonded fiber reinforced polymer (FRP) on the surface showed obvious stress hysteresis under load, which caused that the strengthening materials cannot be fully utilized. The strengthening system formed by carbon fiber reinforced polymer (CFRP) and shrinkage compensating self-consolidating concrete (ESCC) applied posttensioning stress to the external CFRP shell as a stay-in-place strengthening method, which eliminated stress hysteresis and exerted the restraint effect immediately after being loaded. This paper explored whether this method can be applied to concrete compression members damaged by freeze–thaw environment and its effect. Experimental variables include the number of freezing-thawing cycles, FRP shell plies and types of strengthening concrete. The experimental results showed that the damage of concrete under freezing-thawing environment was an incremental process: the dynamic elastic modulus loss rate and the mass loss rate of specimens damaged by 50 freeze–thaw cycles were 5.9% and 0.4%, respectively. When the freezing-thawing cycles reached 125, the dynamic elastic modulus loss of concrete compression members reached 24.8% and the mass loss rate was 5.3%. Compared with concrete compression specimens after 125 freeze–thaw cycles, the load capacity of specimens strengthened with single-ply and double-ply CFRP were increased by 63.1% and 114.1% respectively, which fully compensated the damage caused by freeze–thaw cycles. It was indicated that when single-ply CFRP was used for strengthening, ordinary self-consolidating concrete specimens exhibited higher hoop strain and less loss of ultimate load capacity than ESCC. When two-ply CFRP was used for strengthening, concrete compression members strengthened with ESCC showed higher strength and less loss of ultimate load-bearing capacity than specimens strengthened with ordinary self-consolidating concrete (SCC). With the increase of freezing-thawing cycles, the normalized inflection stress, intercept stress as well as ductility coefficient of strengthened specimens showed decreasing trend. The theoretical value of normalized peak stress and corrected strain were in good agreement with experimental value and it shows that the maximum error were 10.74% and 24.88%, respectively