Mark G. Stewart

Structural reliability of concrete bridges including improved chloride-induced corrosion models

Abstract A structural deterioration reliability (probabilistic) model has been used herein to calculate probabilities of structural failure. New reinforced concrete corrosion initiation, corrosion rate and time-variant load models are proposed. Three durability design specifications are considered in a lifetime reliability analysis of a RC slab bridge. Time-variant increases in loads are considered also. It was found that the application of de-icing salts causes significant long-term deterioration and reduction in structural safety for poor durability design specifications. A reduced cover or increased water-cement ratio increases failure probabilities. When compared to the case of “no deterioration”, it was observed also that the probability of failure only marginally increased for good durability design specifications. The approaches described herein are relevant to other physical infrastructure also.

Time-dependent reliability of deteriorating reinforced concrete bridge decks

A structural deterioration reliability model is developed to calculate probabilities of structural failure (flexure) for a typical reinforced concrete continuous slab bridge. Corrosion may be initiated from the application of de-icing salts or atmospheric exposure in a marine environment. It is assumed that corrosion will lead to a reduction in the cross-sectional area of the reinforcing steel. Monte Carlo simulation is used to calculate probabilities of failure for annual increments over the lifetime of the structure (75 years). The application of de-icing salts or atmospheric marine exposure is shown to cause significant long-term deterioration and reduction in structural safety. Reducing the cover from that currently specified for design results in a significant increase in failure probabilities.

Risk assessment for civil engineering facilities: critical overview and discussion

Abstract The present paper should be seen as a basis for discussion of important aspects of risk analysis and assessment, as well as attempting to describe risk assessment in accordance with the present state of the art. Risk assessment is thus presented in an overview form from the viewpoint of being a means for decision-making and thus within the formal framework of decision theory. First the motivation for risk analysis is given and the theoretical basis together with the practical aspects, methodologies and techniques for the implementation of risk assessment in civil engineering applications are explained and discussed. The paper furthermore addresses the problems associated with risk acceptance criteria, risk aversion and value of human life and attempts to provide suggestions for the rational treatment of these aspects. Finally a number of problem areas are highlighted and the needs for further education, research and dissemination are stressed.

Corrosion-Induced Cracking: Experimental Data and Predictive Models

The corrosion of reinforced concrete (RC) is usually obvious, with cracking, rust staining, and spalling appearing on the concrete cover. However, even by the time of excessive cracking, the structural capacity is only diminished by 10 to 20%. The structural collapse of RC is rare. This article reports on an accelerated corrosion testing program undertaken on eight reinforced concrete (RC) specimens in order to simulate reinforcement corrosion of a section of a typical RC bridge deck. A constant corrosion rate was applied to accelerate the corrosion process in chloride contaminated concrete. The authors compared their experimental results to existing crack initiation and propagation models. They propose a new empirical model to predict the time to excessive cracking for RC structures subjected to corrosion for cracks up to 1 mm in width by considering concrete quality and cover as influencing variables such as water/cement ratio (w/c). The model is also able to account for time-variant corrosion rates and the effect of a high rate of loading often associated with extrapolating accelerated corrosion test results to real RC structures. The authors observed that the accuracy of the predictive model for crack initiation will not significantly influence the time to excessive cracking. The time to excessive cracking is most influenced by the correction factor for rate of loading. However, the authors caution against using direct extrapolation of accelerated test results to realistic (field) conditions.

Life-cycle cost analysis of reinforced concrete structures in marine environments

Chloride-induced corrosion of carbon steel reinforcement is the main cause of deterioration of reinforced concrete (RC) structures in marine environments. One of the ways to protect RC structures from corrosion is to use corrosion-resistant stainless steel reinforcing bars. However, stainless steel is between six and nine times more expensive than carbon steel. Thus, its use can only be justified on a life-cycle cost basis. In the paper a time-variant probabilistic model was presented to predict expected costs of repair and replacement which was then used to calculate life-cycle costs for RC structures in marine environments under different exposure conditions. Results of the life-cycle cost analysis can be applied to select optimal strategies improving durability of RC structures in marine environments, including the use of stainless steel reinforcement.

Reliability-based assessment of ageing bridges using risk ranking and life cycle cost decision analyses

Abstract Information about present and anticipated bridge reliabilities, in conjunction with decision models, provides a rational and powerful decision-making tool for the structural assessment of bridges. For assessment purposes, an updated reliability (after an inspection) may be used for comparative or relative risk purposes. This may include the prioritisation of risk management measures (risk ranking) for inspection, maintenance, repair or replacement. A life-cycle cost analysis may also be used to quantify the expected cost of a decision. The present paper will present a broad overview of the concepts, methodology and immediate applications of risk-based assessments of bridges. In particular, two practical applications of reliability-based bridge assessment are considered — risk ranking and life-cycle cost analysis.

Effect of reinforcement corrosion on reliability of highway bridges

This paper presents several aspects of a method for reliability assessment of reinforced concrete (RC) slab bridges with corroded reinforcement. It is assumed that corrosion will lead to a reduction in cross-sectional area of the reinforcing steel and/or a reduction in bond strength. Two types of corrosion are considered: general and localized corrosion. The method includes a non-linear finite element structural model and probabilistic models for traffic loads, corrosion propagation, bond characteristics, material properties, element dimensions and reinforcement placement. Reliability is estimated in terms of the reliability index using the first-order reliability method (FORM). For illustrative purposes, bridge reliabilities are calculated for a deteriorating, simple-span RC slab bridge, for ultimate strength and serviceability limit states.

Pitting corrosion and structural reliability of corroding RC structures: Experimental data and probabilistic analysis

A stochastic analysis is developed to assess the temporal and spatial variability of pitting corrosion on the reliability of corroding reinforced concrete (RC) structures. The structure considered herein is a singly reinforced RC beam with Y16 or Y27 reinforcing bars. Experimental data obtained from corrosion tests are used to characterise the probability distribution of pit depth. The RC beam is discretised into a series of small elements and maximum pit depths are generated for each reinforcing steel bar in each element. The loss of cross-sectional area, reduction in yield strength and reduction in flexural resistance are then inferred. The analysis considers various member spans, loading ratios, bar diameters and numbers of bars in a given cross-section, and moment diagrams. It was found that the maximum corrosion loss in a reinforcing bar conditional on beam collapse was no more than 16%. The probabilities of failure considering spatial variability of pitting corrosion were up to 200% higher than probabilities of failure obtained from a non-spatial analysis after 50 years of corrosion. This shows the importance of considering spatial variability in a structural reliability analysis for deteriorating structures, particularly for corroding RC beams in flexure.

The Terrorism Delusion: America's Overwrought Response to September 11

The reaction of Americans to the terrorist attacks of September 11, 2001, has been massively disproportionate to the actual threat posed by al-Qaida either as an international menace or as an inspiration or model for homegrown amateurs. An examination of the activities of international and domestic terrorist “adversaries” reveals that exaggerations and distortions of the threat have inspired a determined and expensive quest to ferret out, and even to create, the nearly nonexistent. The result has been an ill-conceived and remarkably unreflective effort to react to an event that, however tragic and dramatic in the first instance, should have been seen to be of only limited significance at least after a few years. Not only has the terrorism delusion had significant costs, but the initial alarmed perspective has been so internalized that anxieties about terrorism have persisted for more than a decade despite exceedingly limited evidence that much fear is justified.

A TWO PARAMETER STRESS BLOCK FOR HIGH-STRENGTH CONCRETE

The rectangular stress block parameters in the current ACI Code are limited to concrete strengths in the range 20-50 MPa (2,900-7,250 psi). This paper looks at the applicability of the ACI rectangular stress block parameters to high-strength concretes. New rectangular stress block parameters are proposed that are based on a probabilistic analysis using a stress-strain relationship for high-strength concrete and that include estimates of variability and distribution of the input properties. A sensitivity analysis is also carried out to ascertain the effect of parameter uncertainty. The probabilistic models proposed can be used in a code calibration of design formula for high-strength concrete. It is shown that for a ductile singly reinforced rectangular section, the ultimate moment capacity is relatively insensitive to the stress block model. Estimates of the ductility level at both ultimate and column capacity in primary compression failure, however, are significantly affected by the choice of the stress block model.