A Lasting Bond: Ultra-High Performance Concrete and Steel Reinforcement

A Lasting Bond: Ultra-High Performance Concrete and Steel Reinforcement

A Lasting Bond: Ultra-High Performance Concrete and Steel Reinforcement

By Doug Darling

The global Ultra-High Performance Concrete (UHPC) market is expected to reach $1.9 billion by 2025 as it grows in popularity. One of the many ways UHPC is transforming the concrete and construction industries is its exceptional bonding quality. This is being put to good use in steel-reinforced concrete, with UHPC drastically increasing the life of the product.

The superior bonding quality of UHPC is already being used to repair steel-reinforced concrete in existing roads and bridges. It is also being used in new construction as well, with many state and federal governments favoring this stronger and more durable concrete.

A match made in heaven: Ultra-High Performance Concrete and steel reinforcements

Traditional concrete is known for its compressive strength, but it lacks tensile strength. When a structure requires both compressive and tensile strength, reinforced concrete is used instead. Steel reinforcement bars or concrete-reinforcing steel mesh are embedded in the concrete to add the required tensile strength.

Steel and concrete are an excellent pair because they have a similar coefficient of expansion, a substance’s change in shape or size in response to fluctuations in temperature. Concrete and steel expand at about the same rate under heat, so when they are used together in reinforced concrete, the composite material is stronger and less likely to break.

However, there is a fundamental weakness in traditional concrete that decreases the longevity of steel-reinforced concrete. Regular concrete is very porous. Oxygen, moisture, and chemicals penetrate the concrete and eventually reach the steel reinforcement bars.

The steel rusts, which causes it to expand and break the bond between the steel and the concrete. The two materials no longer function as a single unit that provides both compressive and tensile strength.

Ultra-High Performance Concrete solves this problem. UHPC is extremely non-porous; it is essentially waterproof. Moisture from the environment, chemicals used to treat concrete surfaces during the winter months, and other environmental gases do not easily penetrate UHPC.

Tests performed by the US Army Corps of Engineers monitored several UHPC mixtures at a marine exposure site. Over a period of five to 15 years of exposure to more than 1,500 freeze/thaw cycles, the UHPC showed no signs of deterioration. The steel reinforcements are protected, and the complementary strengths of the two materials stay intact. Regular concrete begins to deteriorate in as few as 28 freeze/thaw cycles.  

Additional testing has been done to examine the bond strength between UHPC and steel reinforcement bars. The tests have shown that UHPC with greater compressive strength results in an even stronger bond between the concrete and the steel. UHPC with a greater percent of steel fiber (2 percent versus 1 percent) also further improves the bond strength. This makes it important to use high-quality UHPC for reinforced concrete.

Real world applications of UHPC in steel-reinforced concrete

Bridge repairs

Reinforced concrete is often used in bridge construction. Over time, the traditional concrete breaks down due to moisture, deicing chemicals in snowy regions, and environmental gases penetrating the concrete. The chemicals and moisture cause the steel reinforcements to rust, and the bond between the concrete and steel is broken.

UHPC is being used to repair the reinforced concrete already in place in these bridges. The portion of the traditional concrete that is deteriorated is removed, and the steel reinforcement bars are cleaned with a rust-removing solution. UHPC is then used to replace the area of traditional concrete that was removed around the steel reinforcement.

This repair job ends up being stronger than the original construction, thanks to the enhanced bond strength of UHPC. Examples of bridges that have used UHPC for steel reinforcement repair include the CN Rail Bridge in Montreal, Quebec, and the Francis Lewis Blvd. Bridge in Queens, New York.

Corroded steel girder repair

Steel girders and beams used in bridge construction become corroded over time from environmental conditions such as moisture and chloride. UHPC can be used to repair the corrosion without having to replace the girder.

The corrosion is cleaned and removed from the steel, and it is encased in UHPC. The lasting bond between UHPC and steel protects the girder or beam from future deterioration. This is a newer application of UHPC that is being spearheaded by the Connecticut Department of Transportation.

Ultra-High Performance Concrete has many benefits over traditional concrete, but the superior bonding quality is one that makes it ideal for use in steel-reinforced concrete. A stronger, more durable end product increases the lifespan of bridges and other structures. Repair jobs extend the life of existing structures, actually making them stronger than they were before. The unique bonding capabilities of UHPC and steel make this a winning combination. It is no wonder UHPC is becoming a favored material in the construction industry.

Photo Source: Peter Buitelaar Consultancy

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