Modelling the Evolution of Reinforcing Steel Corrosion and Concrete Structure Deterioration as Affected by Chloride Ingress
Statement: The life cycle costing of highway structures is increasingly required by state and federal agencies to ensure that limited capital and maintenance funds are allocated and spent effectively. This approach relies on accurate models of the relevant structure deterioration processes. For reinforced concrete the single most significant deterioration mechanism is corrosion of the reinforcement due to the ingress of chlorides from deicing salt or from marine exposure (or both) and resultant metal loss, build-up of corrosion by-product and concrete cracking and spalling. At the same time, interventions required for maintenance of structures are generally expensive, disruptive, and involve health and safety risks for highway agency personnel, contractors, and the traveling public. It is therefore important that state and federal agencies have the best information on the optimum time and type of intervention. Planned maintenance is the safest and most cost effective approach. It is important that personnel from state and federal highway agencies have the most up to date information on when their structures will need maintenance and repair to allow them to optimize maintenance planning and budgeting.
Reinforcement corrosion starts once the concentration of chloride ions at the reinforcing steel surface, termed the critical chloride concentration, exceeds a threshold value. However, research and field experience in North America and elsewhere has shown that this value can vary widely and is a probabilistic function rather than being fixed or constant. From a deterministic perspective, the critical chloride concentration is a largely unknown function of concrete component materials, mix design parameters, and exposure variables. Consequently, corrosion has been observed to initiate at chloride concentrations from 0.1 to over 1 weight percent by mass of cement (about 0.5 to 5 pounds of chloride per cubic yard of concrete). Such a wide variation causes service life estimates to differ by as much as this same amount (an order of magnitude) depending upon model input parameters, meaning that life cycle costing calculations can exhibit such large uncertainties that they are effectively meaningless. This uncertainty also affects the validity of the choice and projected scheduling of maintenance, repair and rehabilitation alternatives. As an example, consider that barriers are applied or repaired to minimize further chloride ingress into the concrete. However, if the chloride concentration at the steel depth already approaches or exceeds the critical value, then such effort is of little or no benefit and the associated resources are, in effect, wasted. Once started, corrosion progresses by, first, progressive spread of attack outward from the initiation site and, second, initiation at other sites followed by spread from these. The proposed research effort will involve the collection and analysis of existing data from reinforced concrete bridges in the U.S. and other countries.
Objective: The objective of the proposed study is to develop a quantitative understanding as to how reinforcing steel corrosion and resultant concrete damage depends upon progressive chloride intrusion, as affected by 1) concrete material properties, 2) concrete mix design, 3) type of reinforcing steel, 4) component and structure type, 5) exposure conditions (e.g. temperature, oxygen availability/moisture content), and 6) other possible factors. To accomplish this, a critical review of existing critical chloride threshold data, particularly those obtained from actual highway structures but also from other reinforced concrete structures and laboratory tests and existing reviews, will be collected and evaluated. Specific focus would be placed upon the following:
* Aggregate properties and cement type and composition (for example, cement equivalent alkalinity and C3A content (although some reports show this to be less important for chloride threshold levels than previously thought).
* Concrete mix design variables and in-place properties (water-cement ratio, presence of pozzolans and corrosion inhibiting admixtures, resistivity, permeability, entrapped air voids at the steel surface , self consolidating and high performance versus conventional concrete).
* Reinforcement type (black steel, epoxy coated (ECR) and galvanized bar, and corrosion resistant steels (stainless steels).
* Definition of the distribution function for the corrosion threshold and for corrosion induced concrete damage progression; that is, determination of the probability of corrosion (steel section loss) and concrete cracking and spalling at different chloride concentrations.
* Effect of environment (temperature, relative humidity, time of wetness, deicing salt versus marine substructure exposure).
* Macrocell effects.
Departments of Transportation use different criteria and thresholds for deciding on intervention strategies. Particularly important is that the effectiveness of a specific strategy may vary with the progression of chloride uptake. For example, as noted above, coatings and membranes can be effective in prolonging corrosion initiation, provided they are applied before the threshold is exceeded. Accurate definition as to when such an approach is no longer effective and other alternatives should be considered is both technically and economically critical if resources are to be conserved and disruption minimized. Information relating to this issue is available from highway agencies and researchers in the USA and worldwide; however, there is a need for this information to be integrated, consolidated, and critical decision paths identified. Only in this manner can highway agencies ensure that their specifications correctly assess the consequences of different levels of chloride ingress on effectiveness of repair and maintenance strategy alternatives. Additionally, in the U.S., state DOTâs conduct bridge deck condition surveys as a part of their required bi-annual National Bridge Inspection Survey (NBIS); possible correlation to the NBIS data may provide bridge engineers with a valuable, readily available and recognizable tool for determining cost-effective bridge deck maintenance and repair. A statistically-sufficient number of field investigations will be undertaken in order that predicted corrosion thresholds and levels of concrete distress are correlated to what transpires on actual structures. The final report from the project will provide suggested amendments to Specifications for implementation by AASHTO committees and state DOTs.
Key Words: Reinforcement corrosion, bridge decks, bridge substructures, self consolidating concrete, high performance concrete, chloride threshold, black steel, epoxy coated reinforcement, galvanized steel, stainless steel reinforcement, corrosion initiation, life cycle modeling.
Related Work: Some work on corrosion thresholds has been conducted in Europe (Glass firstname.lastname@example.org) and a summary of current standards is reported in ACI Report 222-1. There are related projects being conducted in for the Danish Roads Administration (Arne Henriksen (email@example.com)) and a project at Virginia Transportation Research Council (Michael Brown (Michael.Brown@VDOT.Virginia.gov)). Also relevant is the work of Vassie (firstname.lastname@example.org) (Vassie , P.R., The chloride concentration and resistivity of eight reinforced concrete bridge decks after 50 years service,â Transport and Road Research Laboratory (UK) Research Report 93, 1987; Vassie, P.R.. Corrosion of reinforcement: an assessment of twelve concrete bridges after 50 years service. Transport and Road Research Laboratory (UK) Research Report 78, 1986.). Pennsylvania DOT have offered the resources of their own bridges and bridge survey data for the purposes of this research
Urgency/Priority: Despite recent advances in understanding corrosion performance of reinforcement in bridge deck and substructure applications, uncertainties remain such that an inclusive predictive model in which confidence can be placed is lacking. Any information that allows highway structures to be cost-effectively maintained rather than replaced will have major resource savings for DOTs and reduce disruption on the transportation network. Information on how to elevate the chloride threshold for corrosion could lead to extending both the life of structures and the time to repair which will save lives, money, and inconvenience to the traveling public.
Cost: It is expected that this will be primarily a desk study along with a few site investigations of corroding structures and will require funding of the order of $350,000 over two years.
User Community: The information and recommendations from this work will inform State DOT and AASHTO specifications on the effect of chloride content of the concrete at rebar depth, on the suitability of overlays, coatings and sealers and the accuracy of life cycle modelling and life cycle costing.
Implementation: The report will provide suggested amendments to Specifications, Standards and Policy that can be implemented by AASHTO committees and state DOTs. The report will correlate levels of chloride in concrete at reinforcement depth with different repair options. These chloride levels can be adopted into AASHTO and State DOT Specifications and Standards. The results of the investigation will be presented at suitable venues, e.g. TRB annual meeting.
Effectiveness: For a modest cost this research can provide evidence based guidance to maximize the cost effectiveness of maintenance, repair, and rehabilitation techniques by ensuring that the correct treatment is selected for a given chloride concentration â now and for the residual service life. It will also improve the accuracy (and our understanding of the inaccuracy) of life cycle costing for reinforced concrete highway structures subject to chloride induced reinforcement corrosion.
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