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A number of recent studies have shown that reinforcing steel corrosion due to salt ingress is a primary cause of premature degradation in concrete bridges and overpasses. In reinforced concrete structures, corrosion gradually reduces the cross-sectional area of the reinforcing steel and thus reduces its load-carrying capacity. In addition, the corrosion products cause expansion, which in return causes additional stress that often result the cracking of the concrete surface. Once begun, this process is difficult to stop, and further maintenance and repair costs, not to mention safety hazards, are the inevitable consequences.
Figure 1 – Corroded structure covered with a net
For owners and managers, corroded reinforced and prestressed concrete structures impose a major financial burden at a time of unprecedented budget restrictions. It is therefore imperative to find cost-effective ways to improve new structure designs and optimize the maintenance of existing structures. Hence, the Civil Infrastructure Systems Technology Roadmap 2003-2013, backed by a public consultation process and a national consensus to preserve Canada’s public infrastructure, has recommended several actions for the next decade. The overall goal is accelerated development and introduction of innovative new technologies and processes to ensure the quality, durability, and efficiency of our civil infrastructures. In particular, a need was identified for technical tools to predict the remaining service life of concrete structures in support of asset management and decision-making.
STADIUM® is a one-of-a-kind leading-edge technology and currently the most efficient and accurate world-wide tool for predicting the short and long-term behaviour and service life of concrete civil infrastructures. STADIUM® was developed by SIMCO Technologies Inc., a subsidiary of Service d'Expertise en Matériaux (S.E.M.) inc. A sophisticated, finite element analysis software, which reliably predicts time to initiate corrosion, identifies causes and progress of degradation (e.g., due to deicing salts), and develops optimal rehabilitation strategies to extend service life.
STADIUM® is a reliable, accurate and important strategic tool designed to help owners and managers of civil infrastructures identify and assess concrete degradation over time and within stringent maintenance, repair, and rehabilitation budgets.
The STADIUM Approach to the Design of Concrete Durability
A durability analysis involving concrete exposed to ionic contaminants like chloride requires information on two sets of parameters: material properties and environmental conditions. The material properties are directly related to the concrete mixture characteristics and can be assessed by a series of laboratory tests. The environmental conditions include temperature, humidity, and ionic exposure (chloride, sulfate, magnesium, etc.) in the vicinity of the structure.
As illustrated, the two sets of data are then used as input parameters for the service life prediction software called STADIUM®. The software outputs allow estimating the service life of the structure based on parameters like the concrete cover and the type of rebar. One of the main advantages of the software is that it can account for fluctuations of the exposure condition over a given period of time (e.g. seasonal changes in temperature).
Illustration of the various steps of the durability analysis
The following paragraphs outlines the steps needed to perform a durable analysis.
Material properties
The material properties depend on the characteristics of the concrete mixture: water to cement ratio, type of binder, use of supplementary cementitious materials, etc. To perform a durable analysis with STADIUM® the following material parameters need to be evaluated for the different concretes:
• Cement and additional cementitious materials chemical composition
• Porosity
• Ionic diffusion coefficients
• Water diffusivity coefficient
Each of these parameters is obtained through well-defined laboratory procedures.
One of the key elements in a durability analysis involving high performance concrete is the effect of hydration, which decreases the transport properties through time. This phenomenon thus contributes significantly to extend the service life of a structure. This effect can be evaluated by performing the material properties testing after different curing period, e.g. 28, 91, and 180 days.
Statistical variations are also part of any concrete due to its heterogeneous nature. To include these variations in the analysis, the testing program is made over several samples, which leads to the estimation of average parameters along with their distribution.
Standard test procedures such as:
• Compressive strength
• Air void content
• Shrinkage
• Rapid Chloride Permeability Test (RCPT)
• Freeze/thaw durability
are also performed to have a complete characterization of the material.
Environmental conditions
The environmental conditions consist in the temperature, relative humidity, and ionic contaminant (chloride, sulfate, magnesium, etc.) levels to which a structure is exposed. Temperature and relative humidity can easily be obtained from local weather services. More specific data can be obtained by installing data loggers in the vicinity of the structure.
The contaminants concentration level is usually unknown around a structure. To gather this information, the proposed approach consists in coring concrete in structures exposed to the same (or similar) type of environment. Analyzing the contaminant profile in the core allow evaluating the exposure conditions, which are then used as input parameters to determine the durability of the new structure. Similar to the material properties, having multiple cores allow determining the mean exposure level and its distribution around this value.
Durability simulations
Once material properties and environmental conditions are known, STADIUM® is used to evaluate the service life of the structure. The software predicts the ingress of chloride and/or other contaminants depending on the values of the input parameters over a period of time determined by the user.
By taking into account the chemical and physical characteristics of the material as well as the local exposure and environmental conditions, STADIUM® provides a reliable analysis of the structure durability. With the software, it is possible to determine the time to initiate corrosion based on different concrete cover values. Coupled with the values or the material properties and exposure conditions, the analysis provides a range of service life expectations.
Web site: http://www.simcotechnologies.com/
