Mechanical and Corrosion Properties of a High-Strength, High Chromium Reinforcing Steel for Concrete

Abstract

The corrosion of reinforcing steel in highway structures results in maintenance and replacement costs in the United States that are measured in billions of dollars. The use of deicing salts has resulted in the steady deterioration of roadway bridge decks due to corrosion. One method to reduce corrosion is the use of corrosion-resistant alloys. A high-strength, high chromium reinforcing steel, MMFX Microcomposite steel, is evaluated for corrosion resistance, mechanical properties, applicability for structural applications, life expectancy and cost effectiveness. The steel is compared with conventional mild steel reinforcement, and epoxy-coated reinforcement. Principal emphasis is placed on corrosion performance of the steel, which is evaluated using rapid macrocell tests of bare and mortar-wrapped reinforcement and initial test results for Southern Exposure and cracked beam tests. Life expectancy and cost effectiveness are evaluated based on experience and costs in South Dakota, combined with laboratory results for the chloride content required for corrosion initiation and the rate of corrosion in cracked concrete. MMFX microcomposite steel has average yield strengths between 120 and 140 ksi. Most of the specimens tested satisfy the requirements for high-strength steel bars for prestressing concrete, as specified under ASTM A 722. In most cases, the steel also satisfies the requirements for conventional reinforcement under ASTM A 615, although some samples do not meet the requirements for elongation. MMFX steel bars satisfy the ASTM requirements for bar geometry and will provide satisfactory bond strength with concrete. Depending on bridge deck design, the use of MMFX Microcomposite steel provides few or no alternatives that satisfy all AASHTO bridge design criteria. The corrosion threshold of MMFX Microcomposite steel is approximately four times higher than that of conventional reinforcement. It has a corrosion rate between one-third and two-thirds that of uncoated conventional reinforcing steel. Epoxy-coated steel provides superior corrosion performance to MMFX Microcomposite steel. Bridge decks containing MMFX Microcomposite reinforcing steel will require repair approximately 30 years after construction, compared to 10 to 25 years for conventional steel and 40 years for epoxy-coated reinforcement under typical conditions in South Dakota. Bridge decks containing MMFX Microcomposite steel do not appear to be cost effective when compared to bridge decks containing epoxy-coated reinforcement. MMFX Microcomposite steel is not recommended for use in bridge decks without the use of a supplementary corrosion protection system.