Most U.S. taxpayers do not give much thought to the details of proper roadway construction, but when concrete pavements fail prematurely, taxpayers want to know why.
In one Texas municipality, questions were asked when multiple stretches of concrete roadway, each less than five years old, failed. The distresses included ponding water (i.e. bird baths), cracking, and spalled concrete. At one site, an entire section of pavement was removed and replaced after five years in service. The municipality received several citizen complaints about construction repair delays. Complaints and concerns were well founded — properly designed and constructed reinforced concrete pavement should last between 20 and 30 years with limited maintenance.
When concrete fails before the end of its design life, it can be difficult to determine the exact cause. Possible root causes may include design flaws, shortcomings in project specifications, sub-quality material selection, poor workmanship, or inadequate quality control. In this case, the municipality needed to know why its roads were failing prematurely, what needed to be done to repair them, and how to prevent similar problems in the future.
Terracon has had extensive experience working with public works and engineering departments across the country on roadway construction projects. In this case, the municipality retained the company to research and determine what caused the failure and to recommend solutions that would reduce the chances of failure on future roadway projects.
A literature review was performed that encompassed best practices for pavement sub grade, base preparation for rigid pavements, and design specifications across the Texas Gulf Coast region. The Federal Highway Administration’s comprehensive recommendations for rigid pavement construction was also reviewed. This study revealed several possible upgrades that the client could make to their design manual to minimize future problems in roadway construction.
Project specifications have had a major impact on the design and outcome of a project, but Terracon went beyond design and specifications to help the client. The municipality’s construction records were reviewed and a series of field and laboratory tests were performed to determine whether the project documents were followed correctly. Some of the roadway testing included valuable non-destructive testing techniques. These tests are outlined in the sidebar to the right.
Traffic Data Collection
Traffic analysis is a key input parameter when designing pavements. Designers typically forecast traffic data based on expected and planned growth for the area. The designed roadways may accommodate anticipated traffic growth during the design period, but what if plans change over time and traffic patterns are heavier than expected? This may lead to traffic loading in excess of design traffic loading and possible premature roadway failure. Based on a comparison of current versus predicted traffic count, Terracon determined that underestimation of projected traffic was one of the root causes of reported premature pavement failure. The collected traffic data was also used to design new pavement sections for comparison with the existing pavement sections.
Roadway Sampling and Testing
Terracon maintains a fleet of more than 100 drill rigs, probe rigs, or CPT rigs, so it was prepared to move to the sites quickly. Drilling rigs were mobilized to selected sites to obtain subsurface roadbed and sub-grade soil samples for laboratory analysis. The objective was to characterize the existing pavement layer materials. The data was subsequently used as input parameters in designing and comparing the proposed new design sections with the existing pavement sections. Obtained data was also used to determine if the right type of chemical additive was used to treat the sub grade.
Proposed Changes to Pavement Design
The traffic data and geotechnical soil parameters were then used to design new pavement sections. The objective was to compare the new design sections with their existing counterparts. Terracon recommended design section changes to address observed pavement deficiencies and to provide longer lasting pavements.
Recommendations for Design Manual
Terracon reviewed the municipality’s design manual, identified deficiencies, and suggested recommendations including specifying minimum design traffic and thickness of various pavement categories such as residential, collector, and thoroughfares. If adopted, proposed recommendations will enable the client to avoid similar premature pavement failures in the future. This will ultimately reduce road maintenance costs and user delays. Upon implementation, it is anticipated that the municipality will achieve its target goal of providing longer lasting pavements to its residents with significant improvements to ride quality and drainage.