Admixtures help pave the way to longer-lasting bridge decks

Thousands of bridges across the country need repair and modernization, with the impact on the economy going way beyond the cost of reconstruction, particularly for the supply chain. These blockages have cost hundreds of millions of hours of productivity from trucks idling in traffic when bridges fail or while road crews are working on them. Aging bridges are part of the overall national decaying infrastructure that has added $75 billion annually to the country’s cost of transporting goods. 

Part of this problem is the cracks that form on the surface of bridge decks, which will eventually need repair and replacement. Thanks to developments in admixtures, concrete practitioners are now able to significantly reduce cracking and keep traffic moving. 

Interstate 85 bridge just ahead of reopening to Atlanta commuter and freight traffic.

Engineers have recognized bridges as a significant part of the overall infrastructure problem. According to the American Society of Civil Engineers’ Report Card for America’s Infrastructure, there are more than 617,000 state and federal bridges across the U.S., and 42 percent are at least 50 years old. As bridges age, they become substandard; roughly 15,000 bridges have become a growing problem nationwide. Likely every county has at least one substandard crossing.

To help unclog the supply chain, along with facing the ongoing problem of aging infrastructure throughout the country, the U.S. Department of Transportation enacted the Bridge Replacement, Rehabilitation, Preservation, Protection, and Construction Program (Bridge Formula Program). It provides $26.5 billion to states, the District of Columbia, and Puerto Rico over five years and $825 million for Tribal transportation facilities. 

Transportation Secretary Pete Buttigieg sees this expenditure as the single most significant dedicated bridge investment since the construction of the Interstate Highway System. The goal is for the country’s bridges to provide a 75-year service life vs. the previous 50-year standard to minimize costly traffic disruptions. Having a bridge in service for this long is an important goal, but frequent deck maintenance requirements minimize the benefits.

A massive issue around the repair and modernization of bridges is the effect of concrete cracking on decks. Cracking creates stress on a bridge deck and allows water and salt ingress, leading to rust, crumbling, compromised structural integrity, and ongoing costly maintenance, which impedes bridge traffic. 

Most deck or slab cracking is from shrinkage that occurs as the freshly poured concrete cures. Recent developments in admixture compounds enable projects to minimize cracking from the start, significantly reducing costs while extending a bridge deck’s service life.

The recently completed Marquette Interchange runs past the Marquette University Department of Civil & Environmental Engineering just west of downtown Milwaukee. The school used the building’s proximity to analyze the 15 bridge structures in this very complex project. University observers sought to investigate the issue of premature bridge deck cracking, using 21 variables throughout the interchange that cause the condition. 

Finite element simulations ruled out average traffic load as the sole cause of concrete deck cracking. However, investigators found that tensile stress introduced by concrete shrinkage may cause transverse cracks as early as four to eight days after pouring concrete. A report they issued to reduce shrinkage cracking includes these observations or recommendations:

  • Designing supported bridge superstructures to reduce the tendency for early-age deck cracking; 
  • The use of mix designs that are known to have a lower tendency for shrinkage, such as those with a lower amount of cement and relatively low cement/water ratio;
  • Controlling the strength gain of the bridge deck concrete; and,
  • Providing a more extended curing period before opening a bridge superstructure to traffic.

The Marquette University recommendations and others from industry experts, associations, and reports show the need for innovation around concrete mixtures. Moreover, these findings align with the aim of governments for bridges to carry traffic longer while striving to minimize closures by accelerating the construction timeline. While this goal is admirable, if engineers cannot address the cracking problem, recurring bridge deck maintenance will diminish the impact of this target. 

One solution is a magnesia-based admixture that mitigates shrinkage cracks and curling by leveraging two widely used technologies—expansive components and pore water surface tension reduction. This proven magnesium oxide admixture technology reduces shrinkage cracks by more than 90 percent, depending on the mix design and admixture dosage.

A recent American Concrete Institute report, which includes reviews of several categories and types of chemical admixtures, cites a new category for magnesium oxide as the compound gains acceptance in North America. This recognition is due to the admixture’s ability to expand at a rate closely matching the drying shrinkage of cement. As an example of their durability, dam construction in China has used magnesium oxide admixtures for several decades.

The most effective admixture in mitigating shrinkage cracks and curling is a magnesia-based product that works in multiple ways to reduce and prevent cracks altogether. These compounds blend the expansive components of magnesia with a glycol ether agent that reduces surface water tension to defend against cracks. 

According to the Marquette report, “Magnesium oxide behavior will vary depending on the quality and the heat of calcination. However, in general, high-quality light-burnt magnesium oxide is most successful in compensating for the drying and autogenous shrinkage without harmful effects on the concrete. In addition, projects have succeeded using a hybrid material that combines shrinkage-reducing admixtures to reduce the pore water tension, specifically lightly burnt magnesium oxide, to provide a controlled expansion.”

Typical addition rates for these admixtures range from 2 percent to 7 percent, based on cement content, or 2 lbs. to 7 lbs. for each 100 lbs. of cement in concrete, mortar, or grout mix designs. When used at a 5 percent addition rate, the compounds have reduced shrinkage cracking from 90 percent to 100 percent. These crack-free results also improve the construction sequence and project durability while significantly reducing repair and maintenance costs over the structure’s life. By successfully decreasing or eliminating cracks, bridge decks require less repair work while exhibiting improved durability and a better defense against water and chemical access. State and federal governments experience fewer schedule headaches.

These admixtures prevent shrinkage cracking on any bridge deck application but are especially effective in two areas where governments are looking to drastically reduce timelines and costs: accelerated construction and thin bridge deck sections or overlays. Following are two examples of how working with admixtures can increase bridge lifespan and decrease construction time.

In 2017, a large fire led to a partial Interstate 85 bridge collapse in Atlanta. The disaster became a transportation crisis for commuters, trucking, and local motorists. To minimize the effects of the incapacitated crossing on local commerce scheduling and frustration to drivers, the Georgia Department of Transportation decided on accelerated construction techniques to reduce the economic impact of the detour. 

To speed up the process, the contractor added the magnesium oxide admixture to the concrete mix for the I-85 project. The compound contributed to an efficient construction timeline and a strong, effective bridge repair that would not require extensive, frequent, and costly maintenance.

The Georgia DOT selected an accelerated mix and a three-day versus the standard seven-day curing period. The chosen mix design allowed for a shorter construction schedule leading to a much earlier than anticipated roadway opening. Typically, when a project uses concrete designed for accelerated strength development and minimal curing, cracking would be visible immediately following the curing period. Not in the case of the I-85 crossing: When crews removed curing blankets from the deck, inspectors found no evidence of cracking.

The Rogers Road Bridge over Interstate 95 in Wilmington, Del., was around 40 years old when its deck was scheduled for maintenance to preserve structural integrity. After assessing rehabilitation options, project officials chose a mill-and-overlay method.

Overlays are an economical option compared to a full-depth replacement. However, they typically have a very high occurrence of shrinkage cracks. This problem is due to the highly restrained condition of the underlying roughened surface combined with the thin 2-in. top section.

The shrinkage cracks can quickly lead to freeze-thaw deterioration and eventually delamination of the new surface, requiring additional maintenance as soon as a few years after rehabilitation. Therefore, the contractors chose a magnesium oxide-based shrinkage-reducing/compensating admixture to provide a dual-mechanism mitigation technology. 

Contractors rehabbed a sister bridge deck down I-95 from Rogers Road using the same method but without blending in the admixture. The admixture strategy reduced necessary future maintenance and preserved the original structure as much as possible with the overlay. In addition, inspectors determined that using the magnesium oxide reduced shrinkage cracking by 85-90 percent.

Bridge contractors know that concrete is prone to cracking, primarily due to drying and autogenous shrinkage, leading to costly mitigation efforts and repairs of slabs and structures. Therefore, preventing cracks from day one when cars roll over the roadway is the long-term solution to bridge maintenance, integrity, and extended service life.

With this country’s significant investment in bridge repairs and renovation, magnesium oxide admixtures will enable governments to reach their goals for extended bridge life and minimal project time. The results are a strengthened national infrastructure and a more efficient supply chain. 

Jim Preskenis is Vice President of Sales for Premier Construction Products Group, which provides an easily adopted solution to reduce the occurrence of shrinkage cracks in concrete. He is based in Dover, Del. and can be reached at 302/218-4987; [email protected]