640 publications from this institution
The quality of life-cycle assessments and civil and structural engineering reliability analyses depends much on the quality of the models on which they are based. Models for loads and material properties are well-established. For structural or material deterioration, including corrosion, good quality models are still being developed. Empirical models exist but often have large uncertainty and extrapolation is problematic. High quality models must be based closely on fundamental scientific principles and observations and be calibrated to real-world data. This approach has been exploited for some years to develop corrosion models for steels in marine environments, including the use of data from field experiments and selected laboratory observations. This work is reviewed and the effect of various influencing factors described, including that of microbiological corrosion. The approach has been extended to cast iron, aluminium and copper-nickels. Two practical application examples are described.
Laboratory studies have shown the influence of alloying on the pitting potential of modern mild- and low-alloy structural steels in a marine immersion environment. A similar effect would be expected infield studies, although these have not been able to isolate the effect of alloying on maximum pit depth as afunction of time, and correlation studies have been inconclusive. It is argued here that the reasons for this are twofold. One is that the nature of the pitting mechanics changes with time, from micropitting as typically observed in laboratory studies, to macropitting as seen in field studies. The second reason is that, so far, efforts at correlating field data have attempted to use a simplistic model for the growth of maximum pit depth as a function of time. A more realistic model that distinguishes between the corrosion mechanics in different phases has been introduced recently. This is applied to the literature data. It has been discovered that the effects of alloying depend on the corrosio...
The possibility of safely extending the life of existing infrastructure is a matter that has become of increasing importance for the work of structural engineers, particularly when seen in the widest context of structural engineering. This paper proposes the view that the profession must be able to provide defendable estimates of the safety of individuals and of the safety and performance of existing structural systems. It will be argued that to do so requires prediction of future structural behavior at a level of detail and accuracy not hitherto required from the profession. Meeting this challenge opens up very extensive basic and applied research needs in structural engineering. These include better understanding and modeling of load processes and their rational combination; better understanding of structural system behavior and the influences of largely unknowable boundary and construction conditions; and much better understanding and modeling of processes such as fatigue, corrosion, deterioration of paint, and other protection systems. Moreover, it is argued that assessment procedures still require fundamentally acceptable and rational decision frameworks able to justify structural adequacy in terms increasingly being demanded by safety and environmental regulators.
This paper deals with a technique to calculate the time-variant failure probability of reinforced concrete columns under the combined actions of bending moment and axial force when the applied loads are stationary or nonstationary stochastic processes. An outcrossing rate formulation is applied. Also the limit state surface for the structure is nonlinear. The technique presented is a simple approach suitable for checking the structural reliability of reinforced concrete columns. It avoids the use of multidimensional integration in the computation of the outcrossing rate; this is achieved by piecewise linearization of the limit state surface and the use of Gaussian processes. An example is given to illustrate the approach.
AbstractThe process of enclosed atmospheric corrosion within ship spaces is investigated. It is proposed that corrosion rates are influenced mainly by three parameters; time of wetness, salt deposition and temperature. Assuming atmospheric corrosion to be primarily cathodically controlled, it is shown that, as a first approximation, the rate of atmospheric corrosion is linearly proportional to both temperature and salt deposition. This is shown to be consistent with available experimental data. An equation is presented for the prediction of corrosion rates within ship spaces exposed to an enclosed atmosphere, such as cargo holds and ballast tanks, on a per voyage basis. The influencing parameters identified in this study, and the proposed model, provide a starting point for future probabilistic modelling of one component of ship corrosion, namely, enclosed atmospheric corrosion.
The time before visible evidence appears on the external part of a reinforced concrete structure is often used an indicator of the rate at which the reinforcement is corroding. The conventional wisdom is that this is a function of the rate at which chlorides from the marine environment (or from de-icing salts) reach the reinforcement. Current good practice requires high quality concretes with thick and low permeability cover to ensure long initiation times. This has been known for many years. Many modern structures show evidence of corrosion initiation already within 10-20 years. On the other hand, there is recorded evidence that many older reinforced concrete structures exposed to aggressive marine environments have survived for long periods of time with little or no evidence of reinforcement corrosion. In many cases this is despite thin covers and relatively poor quality concretes. This paper gives a summary of many structures recorded in the literature, both for cases with and without early corrosion evidence. It is proposed that understanding the differences in behaviour is the key to understanding longer-term durability of reinforced concrete structures in marine environments. The available evidence to date indicates that concretes with high pH buffering, such as imparted by calcareous aggregates, have considerably greater durability against reinforcement corrosion. Three Australian examples are given.
The theory for the estimation of the reliability of a structural system which is subject to one or more load processes requires knowledge of the surface defining the mechanical response of the system. For many systems this surface may be known only implicitly and defined through a method of structural analysis. In the formulation presented herein, the surface is obtained using an established probabilistic structural analysis technique. The reliability estimation is formulated in the load process space. In this space, directional simulation is employed to estimate the outcrossing rate and the initial (zero time) probability of failure and hence to estimate the probability of failure at any subsequent time. Two examples using discrete rectangular pulse (Poisson) processes are described.
No abstract is provided for this article.
The measurement of the depth of carbonation in reinforced and prestressed concrete structures is typically determined via freshly cut cores or broken concrete surfaces (often adjacent to superficial evidence of corrosion) and sprayed with a pH indicator phenolphthalein solution. These measurements are representative of only a small percentage of the entire concrete surface and rarely give insight into the overall carbonation profile. This may therefore lead to an over- or underestimation of the depth of carbonation, and presents difficulties in assessing the likely corrosion risk due to carbonation of a reinforced or prestressed concrete structure. The current paper presents unique information on the progression of the overall carbonation front in cross-sections taken from three 45-year-old precast, prestressed concrete T-beams located in an aggressive marine environment. Deeper carbonation was found with increasing height of the beam cross-section, particularly at junctions, and towards the more sheltered areas of the diaphragms. Maximum carbonation depths of 50mm were recorded in some locations, exceeding reinforcement depths and approaching the depth of the prestressing. Carbonation fronts were irregular, but were not always influenced by aggregate. Zones of carbonation were observed around cracks but this was not always the case. The present study highlights that carbonation can be influenced locally by geometry, environment, orientation, and properties of the concrete. Further research is required to assist in the development of improved carbonation field investigations.
According to conventional wisdom reinforcement corrosion commences when the inward diffusion of chlorides causes a critical concentration at the steel or when the pH of the concrete falls below about 11. These criteria are inconsistent with basic corrosion science and with field observations. A 10 year study showed only negligible corrosion for concretes made with seawater. Compaction was high. It influences contact between reinforcement and concrete. Loss of alkalis is the main cause for corrosion initiation. This is consistent with classical observations that in stagnant conditions, as inside well-made concretes, oxygen rather than chloride is the governing electron acceptor. Mechanisms for alkali loss are described. Although these mechanisms depart from the conventional wisdom, the requirements for durable reinforced concrete structures do, however, agree.
No abstract is provided for this article.
Steel pipes used in the oil and gas industry when subject to corrosion pitting may fail as a result of internal pressure and metal cracking influenced by hydrogen embrittlement. The progressive shape of the crack, including at the instant of failure, is predicted under increasing internal pressure. It is proposed that when hydrogen-assisted cracking (HAC) is involved the shape of the crack front may be estimated from a parallel numerical analyses in the absence of HAC. The theory for this is outlined and an example given.
Calcareous deposits on and within corrosion products tend to inhibit the (abiotic) corrosion of steels in seawater. Herein, it was considered whether this inhibition effect extends to microbiologically influenced corrosion (MIC) for extended (long-term) exposure periods. Quantitative estimates of corrosion rates were made from reported observations for 46 iron and steel shipwrecks, and other iron and steel objects immersed in seawater at various depths and for extended periods (many around 60 years and some up to 160 years). The observations are correlated with observations of the occurrence of calcareous deposits and information about dissolved inorganic nitrogen (DIN), a critical micronutrient for MIC. The results show that calcareous deposits can inhibit both long-term abiotic corrosion and long-term corrosion influenced by conditions suitable for MIC. The practical implications are briefly reviewed.
Numerical modelling is used to trace the development of cracks starting from pits that from on the surface of corroded, brittle metal pipe. Under internal pressure applied to the pipe, two general cracking patterns are found to occur and these are related to pit shape. The numerical analysis used allows estimation of the area of the cracked region and of the pressure under which failure occurs. These results also facilitate Stress Corrosion Cracking (SCC) studies by estimating the critical area of the crack stemming from a pit. This may result in a more realistic approach to estimating the likelihood of occurrence of critical cracks in brittle pitting corroded pipes.
A model is proposed for general corrosion of mild and low-alloy steels under fully aerated at-sea immersion conditions as measured by weight loss. Because all factors influencing corrosion have inherent uncertainties, the proposed model is probabilistic, composed of a mean-value function and a zero-mean uncertainty function. The mean-value function is nonlinear in time, controlled by distinctly different phases—kinetic, diffusion, and anaerobic. Literature field data as well as new field observations indicate that seawater temperature has a strong influence on each phase. The model is used to interpret literature data together with additional inferred or collected environmental data. These are then used to calibrate the parameters for the mean-value function of the model. The calibrated parameters are used to predict a consolidated plot of expected corrosion loss—time-temperature. An example application is given.
For existing reinforced concrete structures exposed to saline or marine conditions, there is an increasing engineering interest in their remaining safety and serviceability. A significant factor is the corrosion of steel reinforcement. At present there is little field experience and other data available. This limits the possibility for developing purely empirical models for strength and performance deterioration for use in structural safety and serviceability assessment. An alternative approach using theoretical concepts and probabilistic modeling is proposed herein. It is based on the evidence that the rate of diffusion of chlorides is influenced by internal damage to the concrete surrounding the reinforcement. This may be due to localized stresses resulting from external loading or through concrete shrinkage. Usually, the net effect is that the time to initiation of active corrosion is shortened, leading to greater localized corrosion and earlier reduction of ultimate capacity and structural stiffness. The proposed procedure is applied to an example beam and compared to experimental observations, including estimates of uncertainty in the remaining ultimate moment capacity and beam stiffness. Reasonably good agreement between the results of the proposed procedure and the experiment was found.
For reinforced concrete structures the conventional wisdom is that after some years of exposure to marine conditions reinforcement corrosion is inevitable. Much attention is paid in the literature to the rate of ingress of chlorides through the concrete cover to the reinforcing bars and to ensuring highly impermeable cover and/or deeper cover, to try top prevent chloride-induced or carbonation-induced corrosion initiation. Actual field experience shows that there are many reinforced concrete structures that have survived remarkably well for many decades despite having very high chloride concentrations next to the reinforcing bars. Even with very modest concrete cover by modern standards, exhumation often finds bars free from corrosion. Detailed investigations of a number of such cases showed no corrosion if the concrete pH levels are above about 9. On the other hand, very severe reinforcement corrosion was observed in the few cases where the concrete had cracked right through the cover to the bars. Often there was no external evidence or signs of interior corrosion, including longitudinal cracking. The implications of these findings for practice are discussed.
