Cracking Up

Feb. 1, 2023
How prevention can protect investment

By Jim Preskenis, Contributing Author

Concrete cracking on bridge decks impacts the repair and modernization of bridges. Cracking creates stress on a bridge deck and allows water and salt ingress, leading to rust, crumbling, compromised structural integrity, and ongoing costly maintenance.

This impact can be seen across the country. More than 15,000 federal highway bridges in the United States need repair, according to the U.S. Department of Transportation (USDOT).

Bridges are expected to last 50 years. As bridges age, they become substandard, and 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.; 42% are at least half a century old.

The USDOT launched the Bridge Replacement, Rehabilitation, Preservation, Protection, and Construction Program (Bridge Formula Program) because of the critical need to repair and modernize highway.

This initiative provides $26.5 billion to states, the District of Columbia, and Puerto Rico over five years and $825 million for Tribal transportation facilities. U.S. Transportation Secretary Pete Buttigieg said that this expenditure is the single most significant dedicated bridge investment since constructing the interstate highway system.

Prevention can protect that investment. Most concrete cracking is from shrinkage that occurs as the freshly poured concrete cures. Therefore, minimizing cracking from the start can reduce costs and extend a bridge deck's service life.

Cracking Occurs Within the First Week

The Wisconsin Department of Transportation sponsored a Highway Research Program Report titled, "Concrete Cracking in New Bridge Decks and Overlays." The report found that new bridge decks in Wisconsin were developing early-age transverse cracks and map cracks in concrete overlays.

In Milwaukee, the Marquette University Department of Civil & Environmental Engineering looked at the recently completed Marquette Interchange, which passes right by their building. They analyzed 15 bridge structures using 21 variables thought to cause early-age bridge deck cracking.

Finite element simulations indicated that the average traffic load alone would not cause concrete deck cracking. However, tensile stress introduced by concrete shrinkage may cause transverse cracks as early as four to eight days after pouring concrete.

The report recommended designing supported bridge superstructures to reduce the tendency for early-age deck cracking. It also suggested using 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 ration.

The report stated that controlling the strength gain of the bridge deck concrete and providing a more extended curing period before opening a bridge superstructure to traffic were crucial to success.

Concrete Innovation

The Marquette University recommendations and others from industry experts, associations, and reports show the need for innovation in concrete mixture. Governments are building bridges to carry traffic longer while striving to minimize closures by accelerating the construction timeline.

According to the industry manual produced by the American Association of State Highway Transportation Officials, the guidelines specify that new bridge designs are required to offer a 75-year service life as opposed to the previous 50-year service life.

However, it is essential to note that current life cycle models used in the industry do not consider concrete cracking. Cracking leads to chemical, drying, or autogenous shrinkage, significantly shortening the service life of the bridge. Instead, engineers calculate deterioration over time based on concrete as a continuous material. So, their service life estimates are higher than most bridge realities.

A magnesia-based admixture 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%, depending on the mix design and the amount of admixture utilized. 

By successfully decreasing or eliminating cracks, bridge decks require less repair work, improved durability, and a better defense against water and chemical access. State and federal governments experience fewer schedule headaches and an improved construction sequence.

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.

Here are two examples of how working with admixtures can increase bridge lifespan and decrease construction time: 

Accelerated Construction Timelines

In 2017, a fire led to an Interstate-85 bridge collapse in Atlanta. The disaster became a transportation crisis for commuters, trucking, and residents. 

To minimize the effects on local commerce scheduling and driver frustration, the Georgia Department of Transportation (GADOT) decided on accelerated construction techniques to reduce the economic impact of the detour. The contractor added the magnesium oxide admixture to the concrete mix for the I-85 project to speed up the process. The compound gave the project an efficient construction timeline and a strong, effective bridge repair that would not require extensive and costly maintenance soon.

GADOT selected an accelerated mix and a three-day curing period versus the standard seven-day curing period. The chosen mix design allowed for a shorter construction schedule leading to a much earlier than anticipated interstate opening. Typically, when a project uses accelerated strength and minimal curing, cracking would be visible immediately following the end of curing. 

Not in this case. When the crew removed the curing from the I-85 bridge, inspectors found no evidence of cracking.

Thin Bridge Deck Sections and Overlays

The Rogers Road bridge over I-95 in Wilmington, Del., was around 40 years old when the bridge deck was scheduled for maintenance to preserve structural integrity. After assessing the different bridge deck rehabilitation options, the project chose a mill-and-overlay repair.

Overlays are an economical option compared to a full-depth replacement. However, overlays typically have a high occurrence of shrinkage cracks due to the highly restrained condition of the underlying roughened surface combined with the thin 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 the first couple of years after rehabilitation. Therefore, the contractors chose a magnesium oxide-based shrinkage-reducing/compensating admixture to provide a dual-mechanism mitigation technology. 

Meanwhile, contractors rehabbed a sister bridge deck using the same method but without blending in the admixture. Inspectors performed a side-by-side comparison of cracking to evaluate admixture effectiveness. The admixture strategy reduced the 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%.

Bridge contractors know that concrete is prone to cracking, especially 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. R&B

Jim Preskenis is the Dover, Delaware-based vice president of sales of Premier Construction Products Group.

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