Validating the Durability of Corrosion Resistant Mineral Admixture Concrete (04-GS108)
Started: June, 2006 Ended: June, 2010 Project ID #4W1495 Status: Completed
Results & Findings
Corrosion of reinforced and prestressed concrete structures is a major and increasing problem worldwide. Possibly half of the US’s 500,000 bridges require immediate attention and the total repair bill is estimated at $90 billion (Dunker and Rabbat, 1993). The remediation of concrete bridges in the US, undertaken as a direct result of chloride-induced corrosion of the reinforcing steel, would cost the US highway departments $5 billion per year (FHWA, 1999). Caltrans owns and maintains approximately 15,000 bridges with spans over 20 feet, and there are an equal number in the city and county systems. Caltrans construction (primarily bridge construction) averaged almost $1.3 billion per year over the 1988-1992 period, and the majority of California highway bridges are prestressed or reinforced structures (Hampson and Fischer, 1997). In both types of structures, the corrosion of steel reinforcement in concrete is a significant problem. The cost of maintenance and rehabilitation required to reserve the structural integrity and overall safety of Caltrans highway structures is phenomenal. Repeated rehabilitation and repair also incur a significant environmental toll, as well as the delays caused by closing roads or bridges. On the other hand, appropriate design for corrosion protection would generate substantial cost savings for the Department by minimizing the premature rehabilitation or failure of highway bridges and reducing the construction costs. The research will produce valuable information to be used by the Caltrans Design Engineer and may lead to improvements to the current Caltrans BDS in mitigating chloride-induced corrosion and deterioration. In addition to the validation of corrosion mitigation design assumptions, the research results will also assist the Department to rapidly evaluate concrete mixes designed with new mineral admixtures, allowing implementation of cost-effective corrosion mitigation strategies while providing safe and reliable structures for the traveling public. This research could lead to additional research phases as necessary, such as the development and field evaluation of various types of high performance corrosion resistant concretes. With extended service life and reduced need for costly and difficult repair and rehabilitation of bridge structures, the implementation of better design practices will have immediate positive impact on the California highway system, including cost savings, enhanced traveler safety, reduced traveler delays, and minimized environmental impacts.
The purpose of this project was to assist Caltrans by validating chloride diffusion coefficients of mineral admixture concrete mix designs currently specified for corrosion mitigation. Furthermore, WTI will establish an accelerated test method to evaluate the chloride permeability of concrete mix design containing mineral admixtures.
As our country’s growth and development spurs the expansion of the nation’s highway system, each year hundreds of miles of public roads and highways are built, widened, realigned, patched, modified or reconstructed. As concrete is the most widely used manmade material on earth, the concrete industry has great potential for recycled materials. Recent years have seen increasing interest in Environmentally-Friendly Concretes (EFCs), which utilize alternative or recycled materials and thus benefit the environment. Among them, mineral admixtures such as fly ash, silica fume, and slag – have been used to replace cement in concrete while shown to enhance concrete durability and improve resistance to chloride diffusion. Caltrans has developed concrete mixes for corrosion mitigation of structures to meet AASHTO Load and Resistance Factor Design (LRFD) Bridge Specification requirement of a 75-year design life. However, the work to date has been based on diffusion coefficient data for low permeability, mineral admixture concretes selected from available literature, which may not represent the materials and exposure conditions seen in California. Additional research is thus needed to validate the corrosion mitigation design assumptions by Caltrans. There is also a significant amount of variability in determining chloride diffusion coefficients as an indicator of concrete durability, and existing chloride permeability tests are either very time-consuming for high quality concrete mixes or too biased to provide reliable chloride diffusion coefficients. WTI will establish a modified Rapid Chloride Permeability Test (RCPT) to simultaneously determine the chloride diffusion coefficient and chloride depassivation thresholds in concrete mixes. This electrically accelerated method will force the penetration of chlorides into concrete and thus significantly reduces the time to initiate corrosion of rebar in steel. As such, this accelerated test will provide valuable information about the durability of the reinforced concrete, especially those with mineral admixtures, in a quick and reliable fashion. Appropriate design for corrosion protection may generate substantial cost savings. The accelerated test method could be used for developing performance-based specifications for these types of concrete mix designs and for evaluating environmentally friendly concrete mix designs that could help reduce greenhouse emissions from the production of cement.
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