Addressing Stubborn Longitudinal Cracking
Longitudinal cracking is ubiquitous on Texas highways, and any heavily trafficked road in the state, for that matter. These expansive cracks emerge parallel to the road’s edge and can extend for many feet before terminating. They tend to widen with time, so longitudinal cracks can be a pest for motorists and Houston pavement teams alike.
Recently, the Texas A&M Transportation Institute studied longitudinal cracking on FM 443, near Shiner, Texas. The goal was to diagnose the cause of extensive longitudinal cracking in the area and determine mitigation measures to prevent future cracking. The hope is that the survey team’s work could help other Houston Texas pavement teams avoid similar issues.
What the Texas A&M Survey Team Found
The survey team used a variety of analysis tools to determine the root cause behind the FM 443 cracking. They included ground-penetrating radar (GPR), falling weight deflectometer (FWD) and coring analysis.
Both GPR and FWD data agreed on one critical detail – that longitudinal cracking wasn’t caused by poor subgrade compaction. Instead, both GPR and FWD data showed a locally weakened zone deep in the subgrade – weakening that extended beyond 48 inches in some spots. These weak areas were spotted outside the pavement team’s work area, also suggesting that cracking was due to environmental factors and not the pavement crew’s installation methods.
Coring samples taken from areas around longitudinal cracking determined that cracks extended down to the weakened zones identified by GPR and FWD analysis.
What Is Causing the Cracking on FM 443?
With installation and compaction methods ruled out, the Texas A&M survey team concluded that FM 443’s longitudinal cracking was likely the result of environmental factors around the installation site. Specifically, the presence of high plasticity index soils in the subgrade was identified as the primary culprit.
In Texas, large swaths of land contain high amounts of clay soil. Clay is a highly plastic material that tends to swell and shrink with precipitation cycles. When conditions are dry, clay shrinks. When it rains, clay takes on water and swells. This constant shrink/swell cycle wreaks havoc on pavement, causing severe tensile stresses that can lead to longitudinal cracking.
For pavement teams, identifying high-clay soils and implementing mitigation measures is recommended.
What To Do About Clay Soils
Clay soils pose a significant challenge to Texas pavement teams, so experience in dealing with these soils is a must when building on top of them. For property owners, this means working with a pavement contractor that knows what to do when soil conditions are unfavorable.
The Texas A&M transportation team also made several specific recommendations for pavement crews dealing with clay soils. They include:
- Extensive soil analysis prior to beginning work. The first step in mitigating the effects of clay soils is determining where they are present. As the Texas A&M team’s analysis showed, clay soils can lead to localized weak spots that can accelerate crack formation. Finding these areas prior to starting work is critical for implementing mitigation methods.
High plasticity soils are one risk factor pointing to potential longitudinal cracking. Other risk factors include steep front slopes, narrow pavement, and high water-uptake vegetation close to the pavement’s edge.
- Build out a wider shoulder or a wider stabilized layer with a membrane. This provides additional support and moves the cracking away from the pavement area.
- Install stress-relief measures between the subgrade and surface. One way to do this is to add a geogrid to the top of the subgrade, then install a flexible base and final surfacing on top of that. This approach doesn’t completely eliminate longitudinal cracking, but it does have a strong performance history of slowing it down.
- Consider using a crack-resistant thin overlay mix along with the above measures (use of a geogrid and flexible base materials). Advancements in asphalt mixing have provided pavement teams with better performance in this area.
Longitudinal Cracks Can Be Avoided with The Right Approach
The Texas A&M team’s work showed that longitudinal cracks are difficult to avoid, especially in Texas, but they don’t have to be inevitable. With comprehensive pre-project analysis and various mitigation techniques, an expert Houston pavement team can greatly reduce the prevalence and severity of longitudinal cracks.
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