Most U.S. taxpayers do not give much thought to the details of proper roadway construction, but when concrete pavements fail prematurely, taxpayers who pay for these projects want to know why.
In one Texas municipality, taxpayers began to ask questions when multiple stretches of concrete roadway, each less than five years old, failed prematurely. 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 proper 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 extensive experience working with public works and engineering departments across the country on roadway construction projects. In this case, the municipality retained Terracon to research and determine what caused the failure and to recommend solutions that would reduce the chances of failure on future roadway projects.
Terracon performed a literature review that encompassed best practices for pavement subgrade, base preparation for rigid pavements, and design specifications across the Texas Gulf Coast region. Terracon also reviewed the Federal Highway Administration’s comprehensive recommendations for rigid pavement construction. The study revealed several possible upgrades that the client could make to their design manual to minimize future problems in roadway construction.
Project specifications have a major impact on the design and outcome of a project, but Terracon went beyond design and specifications to help the client. Terracon reviewed the municipality’s construction records and engaged in a series of field and laboratory tests to determine whether the project documents were followed correctly. Some of Terracon’s testing of the roadway 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 we were prepared to move to the sites quickly. Terracon mobilized a drilling rig to selected sites and obtained subsurface roadbed and subgrade 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 chemically treat the subgrade.
Proposed Changes to Pavement Design
Terracon used the obtained traffic data and geotechnical soil parameters to design new pavement sections. The objective was to compare the new design sections with their existing counterparts. Terracon provided the municipality recommended design sections to address observed pavement deficiencies and to provide longer lasting pavements.
Critiques of Design Manual
Terracon reviewed the municipality’s Design Manual, identified document deficiencies, and suggested recommendations to be added to the current manual and procedures. Terracon’s recommendations included specifying minimum design traffic and thickness of various pavement categories that included residential, collector, and thoroughfares.
Terracon recently completed this extensive study, which is being evaluated by the municipal client. If adopted, our proposed recommendations of changing the pavement design and modifying the design manual will enable the client to avoid similar premature pavement failures. This will ultimately reduce road maintenance costs and user delays. Upon implementation, we anticipate the municipality will achieve its target goal of providing longer lasting pavements to its residents with significant improvements to ride quality and drainage.