Structural Integrity of Steel Bridges: Environment-Assisted Cracking

Abstract

Steel bridges are generally subjected to degradation due to corrosive environments. Uniform corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, microbiologically influenced corrosion and environment-assisted cracking (EAC) are commonly applicable to steel bridges. EAC is classified into corrosion fatigue (CF), stress corrosion cracking (SCC) and hydrogen embrittlement (HE). The recent failures and collapses of bridges show that EAC significantly affects the structural integrity of steel bridges. The found knowledge gaps are the lack of detailed guidelines for assessing structural integrity due to EAC, the unavailability of generalized S-N curves for structural details xposed to corrosive environments, and the lack of investigations into the EAC susceptibility of steel types used in bridges. This study aims to contribute these gaps. The objectives of the thesis are to (i) propose a conceptual framework to assess the structural integrity due to EAC of ageing steel bridges; (ii) derive a generalized formula of S-N curve for structural details which are corroded and/or exposed to corrosive environment; (iii) investigate the EAC susceptibility of ST52 steel in 3.5wt% NaCl media with different hydrogen-charging environments; (iv) investigate the HE susceptibility of a tempered martensitic steel by slow strain rate tensile testing of hydrogen pre-charged specimens under hydrogen charging while straining. In the first part of the thesis, a framework is presented for assessing the structural integrity of steel bridges for EAC damage. The framework consists of four major steps, i.e. identification of factors affecting the corrosion and/or EAC; identification of forms of corrosion, their causes and effects, checking for structural integrity/estimation of remaining life, and proposing remedial measures to control EAC and/or strengthening techniques. The conceptual framework consists of proven bridge inspection and investigation methodologies, except for EAC. As a major contribution, a formula for an S-N curve is proposed, to predict the fatigue life of members and joints of steel bridges exposed to corrosive environments. The concept is identified by the corrosion fatigue results of different types of steel specimens tested in air, fresh water and seawater. The corrosive parameters of the S-N curve are determined for marine and urban environments and tabulated for the detailed categories given in the Eurocode and the DNV GL code. The proposed S-N curve formula is compared with full-scale fatigue test results of several structural details, and the validity of the formula is confirmed. The formula does not require any material parameter other than the code-given fatigue curves. The fatigue life of a case-study bridge is estimated by using the new formula, and the results are compared with conventional approaches. The applicability and significance of the proposed curve are confirmed. SCC and HE susceptibility of ST52 steel is experimentally investigated by slow strain rate tests (SSRT). Stress versus strain curves are plotted, and characteristic parameters are determined. Hence, EAC susceptibility indexes are calculated. Fracture surface analyses are conducted. The study reveals that the cathodic protection (CP) process accelerates the EAC susceptibility of ST52 steel in a 3.5 wt% NaCl water solution. The change in the mechanical properties of tempered high-strength carbon steel AISI 4130 due to HE is investigated by SSRT. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are conducted for fracture surfaces, and such samples show a transition of failure mechanism from brittle intergranular cracking at the lowest tempering temperature to ductile microvoid coalescence at the highest temperature. The material was subjected to HE when the tempering temperatures were 400 oC and lower, corresponding to a hardness value of 450 HV and greater. Hence, the limiting tensile strength and fracture strain are determined.