Publications

No abstract is provided for this article.

Many major city water supply distribution networks consist of buried cast iron pipes. In many cases the pipes are internally cement-lined and the predominant corrosion is by external pitting. This may cause leakage and eventual structural failure. It is conventional to use the Gumbel extreme value distribution to represent the statistics of maximum pits depth and to use it to estimate the probability of pipe wall perforation. Herein data obtained for maximum pit depths for large-sized (1–2m long) samples of 10 pipes exhumed from different, apparently randomly selected, locations after 34–129 years of service are examined for consistency with the Gumbel probability distribution. This was the case for the deepest pits, but the data for less deep pits show a consistent pattern of departure from the Gumbel distribution. Some extreme pit depth data, inconsistent with the rest are interpreted as possibly caused by material imperfections.

Empirical models are proposed for the progression of maximum depth of localised corrosion of cast iron pipes, based on data for pipes buried in 67 different backfill clay and sandy soils for up to 129 years. Early corrosion increases with increased inhomogeneity of the backfill soil and with greater availability of free water at the soil–pipe wall interface. Longer term corrosion is correlated with free water availability and occurs at a much slower rate. In most cases, the free water is fresh, oxygenated rainwater, known to be corrosive, but may also include some groundwater. Statistical uncertainty in pit depth is estimated and factors are proposed to allow for different annual rates of precipitation.

No abstract is provided for this article.

The expected maximum depth of corrosion pits in structural steel exposed to marine environments is important for estimating the reliability of engineered systems such as pipelines, tanks, ships and other containment structures. The conventional approach is to use observed data to determine the best fit extreme value (EV) distribution and then to estimate its parameters. In turn these may be used to estimate the maximum pit depth for other situations. However, in many cases there is insufficient data for this classical approach. Progress can be made by invoking a Bayesian statistics approach when a prior EV distribution can be invoked based on previous experience with similar data. The procedure is described herein for estimating the coefficients of variation for the maximum depth of pits on steel surfaces exposed to immersion, tidal, coastal atmospheric and marine inland atmospheric exposure conditions. For these situations previous experience for longer-term exposures has shown that Frechet is the most appropriate prior distribution.

The external corrosion of cast iron water supply pipes has been associated with the availability of free moisture at the pipe wall. This is difficult to observe or measure for extensive pipe networks such as in major cities. A proxy is soil surface moisture content. This may be estimated using terrain wetness indices with highly accurate elevation data derived from airborne Light Detection and Ranging (LiDAR). Such information was obtained for several pipelines and compared with data from 6 cm deep soil moisture Theta Probe measurements along 12 pipeline transects. Additionally, all soil moisture data (in volumetric water content, %) were combined and separated by thresholds of transect means into low (<20%), medium (20%–35%) and high (>35%). Good correlations were obtained (R values 0.49–0.93, all with p value < 0.05). Higher R values were found for wetter conditions (above ~20%). The wetness indices were also found to relate to pipe-wall maximum pitting corrosion rates (mm/yr) when soil classifications were taken into account, with separate trends for clay, silty clay, sandy clay and sand. Some outliers were detected. These were all associated with pipes for which there is evidence of poor workmanship or quality during installation. In total, 25 catchments from inner city to coastal and lakeside suburbs as well as semi-rural regions were assessed. Results show the potential for terrain indices to indicate relative soil wetness in urban areas and thus potential for pipe failure. This is the first study of its kind. This also may assist water authorities to improve life prediction, management and replacement schedules for water supply pipelines and other underground assets.

The methods of limit analysis have had little application to shell analysis. In attempting to develop Yieldline Theory for shells, the question of the importance of shell deformation must be considered since it complicates the analysis considerably. A sufficient condition for a stationary collapse load factor for shells with membrane forces is derived. A completely regular reinforced concrete shell composed of flat polyhedral faces is analyzed for collapse load. Under uniform external pressure, the simplification of ignoring shell deformation results in an error of up to 15% for a cubical shell, while under uniform internal pressure, the correct collapse load is obtained. Examination of parts of rectilinear shells suggests that for these, the neglect of shell deformation may be justified.

The corrosivity of an atmospheric environment is often determined by exposing metal coupons and measuring mass loss over a certain time, typically one year. It has been shown previously that orientation of the coupon can affect the derived corrosion loss, particularly at severe atmospheric locations. It also has been shown that the season of exposure will influence corrosion loss and that, depending on weather conditions, corrosivity may vary from one year to the next. Typically mild steel has been used as a determining metal but zinc, copper and alloyed steels also have been employed. To date there has been no investigation regarding the size of coupons used for corrosion rate determination. Reported herein are the results of a one-year trial in which four different sized coupons (50 mm x 100 mm to 200 mm x 400 mm) were exposed to the atmosphere at eight different locations, ranging from sub-alpine conditions to a severe marine environment. In almost all locations the smaller coupons showed higher corrosion loss for the same exposure period and conditions. At the more benign sites the difference in corrosion loss was marginal but noticeable. The change in corrosion loss became more obvious as the environment became more aggressive. After only 6 months exposure at the severe marine site the corrosion loss for the 50 mm x 100 mm coupons was more than three times that of the 200 mm x 400 mm coupons. Copper-bearing steel coupons of 50 mm x 100 mm size corroded at a similar rate to the same-sized mild steel coupons in milder environments but at a much higher rate near ocean locations.

No abstract is provided for this article.

The corrosion loss of mild steel in the marine tidal zone varies considerably with elevation relative to mean tide. Classical studies on steel piles and electrically isolated coupons show quite different vertical corrosion profiles. This usually is attributed to the effect of differential aeration. In many practical applications, steel structures may be confined to just part of the tidal zone or may extend only partly into either the atmospheric zone or the immersion zone. This paper presents mass-loss profiles for steel strips of different lengths (0.20, 0.50, 1.0, and 3 m) exposed at various elevations in the atmospheric, tidal, and immersion zones at Jervis Bay, Australia, where the tidal range is around 1.2 m. Relatively little difference in the corrosion profile was observed in the corrosion profiles for 100-mm-long coupons and for the shorter strips, mainly because each strip or coupon was confined to exposure only in a single zone. However, very high, localized corrosion was observed for a 1-m strip with the lower end extending just into the immersion zone. The 3-m-long strip and the individual coupons, all extending from the immersion zone to the atmospheric zone, displayed corrosion loss profiles generally consistent with those reported in classical studies.

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. In general, models for loading systems and for properties of materials are well-established. Models for structural or material deterioration are not in the same class. Ideally they must be based on good understanding of the underlying principles, have a degree of scientific validity and be calibrated to real-world data. Experimental laboratory data must be used with caution as it seldom represents long-term behaviour. Data from field experiments must be relevant and be statistically homogeneous. The influence of microbiological corrosion may be important, particularly in seawater environments. Examples of developed models for corrosion of steel, aluminium and copper-nickels are presented. Examples of practical applications discussed include steel sheet piling and cast iron piers and pipes in soils. The models also can sometimes give rise to new interpretations for observed phenomena. The latter is illustrated for the corrosion of stainless steels.

Plastic design for steel structures is a design approach accepted by most structural design codes as leading to economical and safe structures capable of supporting at least the ultimate loads for which they have been designed.

No abstract is provided for this article.

The contribution of bacteria to the corrosion mass loss and to pitting of mild steel was observed over 2.5 years using parallel streams of unpolluted natural (biotic) and nominally sterilized (abiotic) Pacific Ocean coastal seawater. As also observed by others, in artificial laboratory exposures, corrosion mass loss within the first few days of exposure was much greater in the biotic stream. However, after only about 10 days the difference in mass losses were gradually reduced and were very similar up to about one year of exposure. Thereafter, the corrosion loss in the biotic stream again became more severe. Pitting corrosion in the biotic stream was more severe from the very first exposure throughout the 2.5 years. Corrosion in both seawater streams exhibited three distinct but transient time-dependent phases. Of these only the first and third obviously involve bacteria. Similar longer-term observations in real seawaters have not been described previously but are generally consistent with some long-term field data. The results show that longer-term corrosion behavior and possible microbial influences cannot be predicted from short-term laboratory observations, even if natural seawater is used.

The reliability of structures subjected to multiple time-varying random loads is considered herein. It is well-known that the reliability of such systems may be evaluated by considering outcrossings of the load process vector out of a safe domain, and the contribution of individual loads to structural failure may be evaluated by considering outcrossings caused by combinations of one or more loads. In this paper the ‘Directional Simulation in the Load Space’ approach to reliability analysis is developed to consider explicitly outcrossings caused by all possible combinations of loads, during analysis of systems comprising stationary continuous Gaussian loads. To do this, the direction of the load process vector is ‘fixed’ at each point of outcrossing (to physically represent the particular combination of loads causing the outcrossing), and, by considering each possible load combination, all loads not causing an outcrossing are then held constant during radial integration (to model correctly that they do not contribute to each outcrossing). A numerical example demonstrating the validity of the proposed formulation is presented.

No abstract is provided for this article.

An overview of current practices and recent advances in corrosion assessment and management for ships and offshore structures is provided in the present paper. Relevant corrosion mechanisms, corrosion wastage models, design considerations (design corrosion margin values) and preventive measures with the emphasis on marine structures are presented.

A Monte Carlo procedure for reliability estimation of structural systems, subject to multiparameter time‐varying loading and formulated in the space of the load processes, is described. For most realistic reliability problems, this space is of low order, significantly simplifying the required multidimensional integration. The structural resistance is considered to be time‐ and structural‐behavior‐invariant, and is described only in terms of time‐independent, random‐variable strength parameters. The integration to obtain both the outcrossing rate and the zero‐time failure probability is performed using directional simulation of a general type. Example applications are given for loadings modeled as continuous Gaussian processes and for a two‐bay, rigid‐plastic, portal frame structure. Both completely dependent member resistances and member resistances having a given correlation structure are considered. The effect of changes in member strength variance on zero‐time failure probability and outcrossing rate is described. The rate of convergence of the simulation results is noted.