Durability of steel/CFRP double strap joints exposed to sea water, cyclic temperature and humidity Tien-Cuong Nguyen a , Yu Bai a,⇑ , Xiao-Ling Zhao a , Riadh Al-Mahaidi b a Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia b Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia article info Article history: Available online 12 January 2012 Keywords: Durability CFRP Double strap joint Sea-water Cyclic temperature Humidity abstract This paper discusses the mechanical performance of steel/CFRP double strap joints subjected to harsh environments. Seventy-five joint specimens were exposed to various conditions, including (1) simulated sea-water at 20 °C and 50 °C up to 1 year in a temperature-controlled sea-water tank, (2) constant and (3) cyclic temperatures with a high level of relative humidity (RH) up to 1000 h in an environmental cham- ber. Tensile tests were undertaken to determine the mechanical properties of the unexposed and exposed specimens. Significant reductions in joint strength and stiffness were found for the specimens exposed to simulated sea-water after 1 year of exposure, while the specimens exposed to combinations of temper- ature and humidity in the environmental chamber showed little decrease in joint strength and stiffness after 1000 h of exposure. A model was developed to predict the strength and stiffness degradation of the joints in sea-water and the results compared well with the experimental results. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Many civil engineering structures are aging and deteriorating be- cause of the loss of material properties, exposure to severe environ- ments, or increases in service loads. Strengthening structurally damaged members has been traditionally accomplished by replacing members, adding supports, or bonding thin steel sheets to the tension face. However, since these solutions are usually costly, time-consum- ing and labour-intensive, there is a need to seek alternatives. The excellent properties of carbon fibre reinforced polymer (CFRP) composite such as its high strength-to-weight ratio, resis- tance to corrosion and environmental degradation, and flexibility in all kinds of shapes make CFRP an excellent candidate, particularly for rehabilitation. Because of its low density, CFRP composite not only adds less weight to existing structures, but it also offers great convenience during construction. In addition, as the modulus of CFRP is close to or higher than that of steel, the strengthening of steel structures with externally-bonded CFRP composite is increas- ingly favourable for the repair and rehabilitation of existing structures. A number of researchers have conducted experimental, analyt- ical and numerical investigations on the strength and stiffness of structural steel members that are bonded with CFRP composites. Most have shown that steel members, including beams, columns and joints, strengthened with CFRP exhibit much higher ultimate strength and stiffness than unstrengthened reference members. Two comprehensive reviews of applications of CFRP-strengthened steel members, conducted by Hollaway and Cadei [1] and Zhao and Zhang [2], have shown that this composite system has emerged as an attractive strengthening and retrofitting technique for civil engineering steel structures. However, one of the main restrictions to more widespread use of CFRP in strengthening and rehabilitating steel structures has been the uncertainty in the prediction of the durability of steel/CFRP sys- tems. This is as important as the initial strength and performance of steel/CFRP bonded systems since they may deteriorate upon expo- sure to service environments [3], because of change in the stiffness and strength of resin within the CFRP composite and the adhesive between CFRP and steel due to moisture and temperature. The principle of the strength degradation of CFRP/steel bonded joints by the presence of water is similar to that of general adhe- sively-bonded joints. By diffusion through the adhesive or trans- port along the oxide/adhesive interface or absorption through the porous adherend, water can penetrate to the interfaces between the adhesive and the adherend. Once moisture has penetrated the joint, degradation of the joint can occur due to two types of degradation; the degradation of the adhesive itself, and the degra- dation of the adhesive/adherend interface within an adhesively- bonded joint [4]. Within the bulk adhesive, moisture ingress leads to plasticiza- tion or chemical and physical break-down of the adhesive, causing a reduction in the adhesive’s mechanical properties. In considering the durability of adhesive joints, the water effect on the adhesive 0263-8223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compstruct.2012.01.004 ⇑ Corresponding author. Tel.: +61 3 9905 4987; fax: +61 3 9905 4944. E-mail address: yu.bai@monash.edu (Y. Bai). Composite Structures 94 (2012) 1834–1845 Contents lists available at SciVerse ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct