Posted on May 31, 2018
Many of Pennsylvania's bridges were constructed decades ago, a time when usage was vastly different than it is today. As a result, many bridges carry much more traffic than their designers ever intended. Since the days of the Eisenhower administration, America's vehicles have tripled in number.
The most recent National Bridge Inventory (NBI) from the Federal Highway Administration identifies 22,783 bridges in Pennsylvania. Forty percent of them are classified as either "structurally deficient" or "functionally obsolete." To rectify the situation, billions of dollars in repairs, restorations and replacements are needed.
For instance, it's common for water to migrate into the structural components of bridges, potentially weakening them in a variety of ways. Freeze-thaw cycles that are so common in this part of the country further increase the width and depth of many of these cracks, accelerating deterioration of the concrete. Chlorides can also work their way deep into the bridge deck, where they corrode reinforcing steel.
Cracks are often perpendicular to the girders, although diagonal cracks are also sometimes present but tend to be restricted to the bridge ends. Here are some other key characteristics of bridge deck cracks:
- Cracks exist in both steel girder bridges and prestressed concrete girder bridges
- Cracks are most commonly found near the pier location
- Cracks are commonly spaced at three to eight feet
- Crack leaching indicates the cracks are full depth
Increasing the longevity of updated bridge decks is key to collectively improving Pennsylvania's bridges for the long-term. To achieve this goal, reductions in early-age cracking are imperative. Early-age cracking occurs after construction but before the new bridge deck is put into service. Research conducted at Penn State University (PSU) identifies ways to improve the longevity of concrete bridge decks.
What causes early-age cracking in concrete bridge decks? These are some of the conditions that contribute to the problem:
- Excessive temperature changes
- Plastic shrinkage due to excessive evaporation
- Autogenous shrinkage
- Drying shrinkage
Researchers Tackle Bridge Deck Cracking
Certainly, bridge deck cracking is a complex issue with multiple causes, including adverse changes in concrete volume. Traditionally, undesirable changes in concrete volume were often attributed to:
- Problems with the proportions and properties of the concrete mix
- The unwanted structural interaction between the deck and other bridge components
- Adverse environmental influences during construction
- Problematic construction techniques
Beginning in 2008, the PA concrete industry and PennDOT collaborated to better understand and mitigate cracking, including the development of AAAP mixes that limited changes in concrete volume. In 2009-2010, 10 field tests compared AAAP and AAA. Field testing of AAA and AAAP concrete mixes led to "The AAAP Bridge Deck Concrete Initiative." The study was sponsored by three major agencies, the Pennsylvania Department of Transportation (PennDOT), U.S. Department of Transportation (DOT) and Federal Highway Administration (FHA).
AAAP mixes address adverse changes that increase cracking. Field testing revealed its superiority over AAA in a number of areas:
- Shrinkage cut by 15 percent
- Higher 28-day strength
- Better resistance to penetration by chloride ions
- Slower more desirable rates of strength gain
The results were conclusive. Today, the American Council of Engineering Companies of Pennsylvania (ACEC/PA) specifically states there should be no AAA concrete on new bridge decks.
Use of Pozzolans
A key to the success of AAAP in mitigating bridge deck cracking is the use of a pozzolan to reduce permeability. Three pozzolans have been used:
- Fly ash
- Ground granulated blast furnace slag (GGBFS).
- Silica fume (less common due to cost)
Pozzolans limit the potential for thermal cracking by reducing the rate of hydration. Lower hydration rates reduce set times, temperature increases and premature strength development.
A 2015 study yielded a report entitled "Bridge Deck Cracking: Effects on In-service Performance, Prevention and Remediation," which further identified ways to mitigate cracking on bridge decks.
Mitigating Early-Age Cracking
At the conclusion of their work, PSU researchers recommended various methods for optimizing AAAP concrete performance on bridge decks. Crack mitigation is related to mix, structural and construction factors:
More effective AAAP mixes are possible. Recommended AAAP concrete mixes feature a reduced cement paste content and the inclusion of supplementary cementitious materials (SCM) readily available in the Commonwealth. Researchers suggest the following:
- Use aggregate optimization to improve aggregate packing
- Increase air content to greater than 6 percent
- Select lower water-to-cement ratios
- Use water reducers
- Reduce slump
Specifically, the maximum cementitious content can be reduced from 690 pcy to 620 pcy. Also, it is ideal to limit evaporation rates to 0.10 lbs/SF/hr. The use of fine lightweight aggregates (FLWA) promotes internal curing.
Designs can also be improved/updated:
- Avoid excessively stiff concrete
- Use maximum #5 rebar sizes
- Increase restraint with interior spans
- Better assess girder depth and spacing
- Carefully consider cross-frame positions
- Avoid excess concrete cover
- Girders and end restraints with simpler support systems
- Use of expansion joints, especially on longer continuous spans
Since both shrinkage and thermal effects cause bridge deck cracking, updated construction techniques are valuable as well. Contractors can reduce bridge deck cracking through the use of the following:
- Earlier concrete finishing
- Wet curing for 14 days, not seven
- Mechanical vibration
- Decreased placement time (single pour whenever possible)
- Awareness of wind speed impact on evaporation and thermal stress
The CP Road Map identifies additional sources of bridge deck cracking. Ultimately, it is incumbent upon all stakeholders to better deter moisture and chloride ingress.
The Pennsylvania Aggregates and Concrete Association (PACA) is dedicated to the dissemination of emerging news about the effective use of concrete. For further information, please contact us.