By Matt Wilhelm
From the June 2023 Issue
Pavement for parking lots and structures must withstand the loads it will encounter to prevent premature failure. Too often, laying pavement follows a one-size-fits-all ideology.
Choosing the type of pavement and subgrade structure for parking structures must be done based on its specific needs.
Understanding a pavement’s intended use is essential to its design. Choosing the type of pavement and subgrade structure for parking structures must be done based on its specific needs. When there is a change of use for that structure, there also needs to be a meticulous evaluation conducted to determine if the current build will hold up.
How Change In Use Impacts Parking Structures
There may be an instance where a structure is perfectly suitable initially; if the structure use changes down the line, the initial serviceability time may reduce drastically, costing the owner money. This is common when a parking structure (typically tied to a facility) changes hands. The lot may be in pristine condition upon purchase, as it was being used for its intended purpose. However, if there is a drastic change in site use and load bearing on the lot, deterioration will likely begin. It is essential to look at the situation and discuss whether corrective actions or reconstruction is required once that previous design has started to fail.
For example, our pavement consultant company was contacted to address a “remedial” situation. A pavement project, completed without our involvement, was now failing long before its expected serviceable life had been reached. In this case, an 8-year-old facility was experiencing sudden premature pavement failure due to unintended loadbearing, where the damage was most prominent through the drive lanes. This wear on the pavement was first noticed roughly 18 months prior to their call. The owner was concerned that the existing pavement was already presenting structural failure after only 6½ years of service.
An investigation began to determine why the pavement in the drive lanes began failing. During the initial evaluation, the parking lot appeared to be well-designed and installed for a typical commuter parking lot. Some minor drainage issues were identified, which equated to localized subgrade erosion and related pavement failure, but this was not located near the areas of structural failure—the primary concern.
The investigation led us to a theory that the parking lot had a change in use. Following our site visit, we discussed our findings with the client and confirmed our theory was correct. The parking lot recently experienced a change in its use and traffic. The pavement structure in the driver lanes was designed for a different load than the pavement was now bearing. It transitioned into a bus pickup point for commuters, requiring busses to travel through the main drive lane many times a day. After this change in loadbearing, the original failure became apparent, approximately 18 months prior. We ultimately recommended reconstruction and essentially added 15 more years of serviceable life to the large pavement area.
Establishing An Action Plan
When a structure begins to fail, there are a few potential action plans for facility managers. This includes corrective actions, such as reconfiguring traffic patterns and loads to maximize the serviceable life of the pavement or total reconstruction.
In terms of pavement types, asphalt is the most common material used for parking lot pavement, driveways, and roadways.
The correct plan of action is determined through tests and assessments. One must understand both engineering principles and typical local designs, because the best way to assess the pavement’s suitability is through geotechnical testing and programs. This will determine if the pavement’s underlying structure can sustain its required use. Through geotechnical testing, facility managers are also able to determine the proper subgrade structure for the concrete or asphalt pavement.
In terms of pavement types, asphalt is the most common material used for parking lot pavement, driveways, and roadways. Installation can be relatively quick—it can be ready for use after a day or two. Asphalt is a flexible pavement that will conform to the subgrade over time. When the subgrade is yielding significantly, the asphalt moves in waves with the traffic and will cause it to break up. Understanding the native soils and incorporating stabilization and enough aggregate base will determine its stability.
Unlike asphalt, concrete is self-supporting. This means that it will not rely on the subgrade base. The serviceability is significantly longer than asphalt, lasting about 40 years as opposed to the expected 15-year life of asphalt, with proper maintenance. Maintenance and repairs can be rather complex and expensive for concrete. This means that concrete is not always the most cost-effective option for the job. Overall, aesthetic appeal, serviceability, use, surrounding local designs, and the installation timeline requirements will all play a part in determining what is best for the job.
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Change of use is an issue facility owners run into regularly. The parking lot failure mentioned previously is an excellent example of why having the proper initial and due diligence assessments conducted can save considerable time and effort. Situations like this are regularly overlooked, and premature failure is often inevitable.
Additionally, periodic site inspections by a pavement consultant can also help identify problem areas so they can be addressed before they compound and become more significant. Instead of throwing money at these issues to “patch” the problem, it is essential to fully understand the issue and make an educated decision about the best approach to repair. Consider this whenever designing a new lot or repurposing an existing one to help ensure that the pavement lasts longer, and long-term maintenance costs are reduced.
Wilhelm is the Lead Designer at Pavetech Consulting Inc. Based in St. Louis, MO, Pavetech Consulting Inc. is an independent “third-party” consulting group licensed and insured, with no ties, agreements, or other collusive interests. PaveTech specializes in the proper inspection, repair, maintenance, and design of new and existing roadways, parking lots, and parking structures. They also perform many Quality Assurance site visits to ensure that the design is carried out quickly and correctly in the field.
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