Development of rational mathematical models for prediction of the likely present and future internal corrosion of reinforced concrete sewers requires understanding of the important physico-chemical processes, preferably based on field evidence. Samples of new and 70year old pre-corroded concretes were exposed for up to 31months in an aggressive sewer environment at 26°C, 98% relative humidity and 79ppm H2S concentration (averages). During the initial months of exposure the pH of the new concrete surfaces reduced rapidly however little corrosion loss was observed during this period. Subsequently pH reduced further, mass loss commenced and new concrete losses reached around 24mm after 2years. During the same period the old concrete corroded at an approximately constant rate. The presence of the corrosion product layer had negligible influence on corrosion losses. A bilinear corrosion loss model is proposed for practical applications.
No abstract is provided for this article.
The corrosion of steel in open seawater is influenced mainly by water temperature. If nutrients critical for bacterial activity are available, it also may be influenced by microbiological activity. The combined effect is investigated here for the case of large variations both in the seawater temperature and in concentration of dissolved nutrients. A theoretical analysis is given for these effects acting in conjunction, with or without time-dependent differences. Field observations of corrosion losses for steel sheet piling exposed at three different locations at sea sites near Vladivostok, Russia, are used in conjunction with local temperature and nutrient concentration data to obtain trends for longer term corrosion. The main objective of the paper is to build a new long-term predictive model of corrosion which may be effectively used in conditions with large variable seasonal in water temperature and in nutrient availability.
Of the some 1,000 almost identical, precast, reinforced concrete elements forming the 1.5-km-long, more than 60-year-old promenade railing immediately adjacent to the North Sea at Arbroath, Scotland, more than 90% still appear to be in remarkably good condition with little evidence of external rust staining or reinforcement corrosion. However, the elements that have been replaced, some only about 15 years ago, mostly show severe reinforcement corrosion. Concrete quality, density, and cover to the reinforcement are generally similar, but concrete permeability does not correlate well with reinforcement corrosion. Detailed investigations show some of the original, apparently sound concrete elements to have severe highly localized reinforcement corrosion, in some cases with a complete loss of steel. This was accompanied by limited amounts of mainly black corrosion product that x-ray diffraction (XRD) analysis showed to have a strong presence of magnetite. The ends of some reinforcement bars showed tunnelling corrosion, a phenomenon not previously reported for reinforcement. It is proposed that the observations show that the long-term corrosion of reinforcement in high-quality concretes involves processes more complex than simply oxidation of the steel under chloride conditions.
In this paper, firstly, existing methods for determining the behaviour factor of moment-resisting steel frames and their range of applicability for multi-degree-of-freedom frames are reviewed. Modifications of this factor for multi-degree-of-freedom moment-resisting steel frames are then indicated. Necessary modifications in determining the behaviour factor of frames involve the period and the base shear distribution factor for a given earthquake loading code. The effects of storeys, spans and connections of frames on the behaviour factor were considered over ranges of values for these factors for frames with semi-rigid connections up to five spans and ten storeys. On the basis of the results, new relationships for the period and the behaviour factor of moment-resisting steel frames with semi-rigid connections as a function of the main geometric parameters of the frame are presented.
No abstract is provided for this article.
The conditions for initiation and the subsequent development of the severity of corrosion of metal alloys in the short term continue to be of research interest. However, for most physical infrastructure the critical issue often is the development and progression of corrosion under some level of oxygenated conditions, over several decades. In many cases this has significant implications for safety and for economic loss. Increasingly, asset management decision-making requires robust tools or models to predict the effect of corrosion, including loss, pit depth and crevice severity. The present capability in this area is reviewed and available models generally compared, including their degree of empiricism and their relationship to corrosion science fundamentals. It is argued that in addition to the role of material imperfections and corrosion products, the immediate physical environment adjacent to the metal alloy also can play a major role. These aspects are explored and some speculation made about required future research directions.
A wide range of literature data including in situ immersion, tidal, coastal atmospheric and industrial exposures is used to show that the trend for longer term corrosion of aluminium alloys is nearly always more consistent with a bi-modal model than with the classical power-law function. It is proposed the bi-modal characteristic results from the accumulation of corrosion products causing localised anoxic conditions. These permit a change from predominantly cathodic oxygen reduction to hydrogen ion reduction under anoxic autocatalytic conditions within pits. This mechanism is consistent with established theory for pitting corrosion in aluminium.
In the assessment of existing structures the likely future deterioration of strength and stiffness is of interest. For reinforced concrete beams one of the most important influencing factors is the loss of reinforcement cross-sectional area at critical locations. Because corrosion is a long-term process for well-designed structures, there is only limited field experience available on which to draw and with which to make empirical rules. The considerable research effort in corrosion of reinforcement in concrete structures has focused mainly on the rate of migration of chloride ions from the environment through the concrete to the corroding steel surface. This will estimate the time before reinforcement corrosion becomes significant but does not provide the rate of structural deterioration. In the present paper a method is proposed for estimating structural deterioration, based on evidence that (a) reinforcement corrosion causes internal damage to and loss of bond with the surrounding concrete and (b) chloride diffusion is influenced by stresses set up in the concrete by external loading. Both these factors affect the rate of chloride migration locally and hence the amount of corrosion and material strength loss at any point in time. The proposed method is outlined and an example application given. Reasonably good agreement between the results of the proposed procedure and the few experimental results was obtained.
The process of structural design has received very little attention from the research community despite its centralness in day to day structural engineering. Much of the research which has been done has been carried out in Australia over a number of years and with the cooperation of many structural engineers. The present paper reviews the reasons for being engaged in this relatively unusual research activity and provides an overview of the work to date. A review is given of the incidence and type of errors made in basic design tasks such as calculations, table look up and table interpolation, and how these errors relate to results obtained for more comprehensive tasks such as loading determination and member design. One of the outcomes of the work has been to suggest areas in certain current design codes which are prone to misinterpretation by practitioners. Preliminary results related to design computation checking and inspection are also reviewed. This work potentially has major implications for quality assurance programmes (A).
For redundant structural systems, the limit state equations for given failure modes may be obtained by the so-called incremental loading method (ILM), in which members fail progressively as the load increments. In this report it is shown that ILM is capable of dealing with structural systems having multiple random loadings. A wider range of member behaviour other than plastic and brittle types previously considered by others is presented. For the special case of member behaviour in which the post-yield stiffness is zero, the so-called artificial load concept for use in the truncated enumeration method (TEM) has been shown previously to be simpler in application than ILM. It is shown herein that the results obtained from this approach agree with those from ILM. For multiple load cases it is demonstrated (but not proved) that the choice of load for incrementation is not important; the limit states obtained are invariant, provided that the utilization matrix in ILM is not singular. It is proposed that an economical system reliability analysis would be obtained by combining the truncated enumeration method (TEM) and ILM. Then for a wide range of member behaviour with the possibility of unloading of member capacity following failure, either partial or total, stochastically dominant modes of structural failure, are automatically generated. Several examples are given to illustrate the validity of the approach. (Author/TRRL)
Parameters characterising the properties of soils used as backfill in immediate contact with cast iron pipes exhumed from a variety of soils after up to 129 years exposure are used together with average annual precipitation to estimate the average time of wetness. This is shown to correlate closely with pipe wall corrosion penetration. The possibility of microbiologically influenced corrosion is dismissed owing to lack of critical nutrients. The presence of chlorides in some soils most likely arises from pipe wall corrosion penetration and subsequent leakage of chlorinated drinking water. The predictions from the derived model are compared with an independent set of 16 pipe-wall corrosion penetration observations for pipes with up to 98 years exposures and found to agree well. The potential influences of soil temperature, physical damage and poor quality backfill are discussed.
Corrosion of external cast-iron pipe surfaces, a major contributor to pipe failure, has been attributed to the free water in the soil surrounding the pipe. Because observation at pipe depth is difficult, a potential proxy is the soil surface moisture. Herein highly accurate elevation data derived from airborne light detection and ranging is used to model the distribution of soil water in urban catchments containing pipe infrastructure. The results are compared with local soil moisture Theta Probe measurements along the pipe. The results show potential to identify wetter spots above underground infrastructure, which may inform its corrosion potential, without digging up the asset.
It is generally acknowledged that design checking reduces the incidence of structural failure. Because checking is often seen as nonproductive and costs money, the optimal degree of design checking is of interest. In the present paper, a decision model is developed to determine the optimal level of design checking. It is applied to the design of a steel rafter for a portal frame building. The model incorporates the effect of either an insured or an uninsured designer. Optimization was achieved by maximizing the expected utility. Using the model, it was found that the use of thorough self‐checking and overview checking only is the optimal strategy, unless the consequences of failure are expected to be catastrophic, in which case one independent “design check” is also necessary.
In structural system reliability analysis it is usually assumed that the elastic properties of the members are deterministic while the loading parameters and/or member resistances are assumed to be stochastic in nature. Theoretically, all potential modes of failure must be considered in an estimation of system failure probability. In reality, however, the probability of occurrence of many of these failure modes is so low, compared with the structural system failure probability, that they make only an insignificant contribution, and hence can be neglected. Thus it is usually sufficient to consider only the so-called dominant failure modes in a system reliability calculation. For rigid frames composed of ideal plastic or elastic-plastic material the dominant failure modes of interest in the reliability assessment are drawn from the set of all plastic collapse mechanisms. Several empirical procedures have been previously developed to enumerate these dominant mechanisms with varying degree of success. Although in principle it is possible to enumerate all dominant modes relative to some criteria, in practice no method has yet proved to be applicable to large frames, and the validity of the identified mechanisms is often uncertain. This paper proposes a modification of the existing truncated enumeration method (TEM) applied to plastic frames. An example is demonstrated, and the results compared to other methods recently developed. The results show that almost all the dominant mechanisms may be successfully found. The proposed procedure can be systematically programmed and may be applied to estimate reliability of large structural systems.
Recent progress has been made in developing a mathematical and probabilistic model for general corrosion of structural steels exposed to sea water immersion corrosion. It is based on corrosion science principles, with early corrosion controlled by oxygen concentration limitations that eventually evolve to anaerobic bacterial controlled corrosion behaviour. The model has been calibrated to extensive literature data and original field tests. Herein, three different sets of corrosion field data, for Halong Bay in Vietnam, for an offshore oil platform site and for three coastal sites along the China coast are examined for compatibility with the model. It is shown that all the data are consistent with the model provided allowance is made for the season of the first immersion, the effect of sea water velocity and the possibility of nutrient pollution. In its most general form, the model is probabilistic to allow for natural variability owing to unrecorded (and perhaps unrecordable) influences. The data considered herein are sufficient for comparing with the expected value of the model but not for considering statistical variability. Finally, the model provides plausible explanations for differences in observed corrosion at different sites, considered by others to be due to the action of hard marine fouling.Keywords: STRUCTURAL STEELCORROSIONSEA WATERTEMPERATUREOXYGENMODELSDATA
No abstract is provided for this article.
Conventional Monte-Carlo integration for high reliability systems is extremely inefficient. It is possible to use a rather simple technique, known in the literature as importance sampling, to dramatically improve sampling efficiency (or the in the integral for a given amount of sampling). This technique, unlike some other variance reduction techniques, can be applied in a systematic manner to problems with one or more limit state functions, including unions and intersections of limit states. These applications as well as the theory, are outlined in this report. (Author/TRRL)
The collapse of the Newcastle Workers Club during the Newcastle Earthquake in late 1989 was the subject of a Coronial Inquest, but no firm conclusions were reached. This paper describes an independent investigation of the reason(s) for the collapse. It outlines the investigation process, the data available for the investigation, eyewitness accounts not previously published, and gives a systematic overview of the progression of the investigation. It is concluded that while ground-shaking caused by the earthquake was the prime cause, failure would have been unlikely if there had not been significant deficiencies in the structure as built. These and the most likely sequence of failure events are described.
