Texas Christian University Evaluates 3 Pavement Systems for Long Term Performance & Maintenance
Concrete pipe and catch basins has been a common tool for engineers to convey runoff from impervious areas, but traditional grey infrastructure isn’t always the solution. This is a scenario that plays out regularly during downpours in older regions of Fort Worth, especially on Texas Christian University’s (TCU) campus. Flooding in North Texas during 2015 and 2016 made this problem even more apparent, with some areas receiving more than double the average annual rainfall. TCU took steps to solve this problem by taking advantage of next generation Low Impact Development (LID) practices.
Permeable pavements are one of several Green Infrastructure systems that are being used to manage runoff and help mitigate flooding. TCU decided to experiment with this tool by kicking off a pilot program in a parking lot on campus. Since there are several permeable pavement options on the market, TCU decided to test 3 different systems. Several factors were taken into account such as costs, performance, maintenance, and constructability. In the summer of 2016, TCU demolished a small portion of a parking lot and retrofitted it with pervious concrete, permeable interlocking concrete pavers (Pavestone), and permeable articulating concrete blocks (PaveDrain). All three systems were installed with an underdrain that discharged into the street.
After construction was complete, TCU tested the infiltration rate of all three systems to obtain a baseline rate that can be evaluated for performance over time. TCU conducted 3 infiltration tests for each pavement section, the results are listed in the table below.
The best performing systems were porous concrete and PaveDrain, with Pavestone having the lowest infiltration rate. A high infiltration rate is necessary if the stone subgrade is being used for detention, but it also helps reduce long-term maintenance costs. Typically, a high infiltration rate system will take longer to clog and require less frequent cleaning.
TCU employed the same contractor for all three systems, but each one came with a learning curve. Various blends of pervious concrete were tested in other parking lots to determine the correct blend. Some blends had too many fines, others dried out quickly and a few contained too much binder. The paver systems didn’t require testing various blends. PaveDrain required the contractor to compact 6” of stone to a smooth, flat surface before placing a geogrid and the blocks. The Pavestone pavers were placed on top 4” of stone and a 2” bedding layer, but did require small chip rock to be swept into the joints of the pavers.
TCU returned in January of 2017 to see how the pavements held up 6 months of heavy traffic during the Fall semester.
As expected, the infiltration rate decreased for each system. The Pavestone pavers failed at the first location, but the second location tested at 309 in/hr. Pervious concrete decreased the least amount out of all three systems, but suffered from severe spalling and crumbling. Large areas of the porous concrete turned into loose gravel, which is a sign of not using the right amount of binder during installation.
PaveDrain had about a 43% decrease in performance, but still maintained an infiltration rate above 800 in/hr. During construction the contractor used a small bedding layer on top of the #57 stone, which is not needed. The decrease in performance can be attributed to the bedding layer, since it is prone to clogging from fine sediments that are generated on parking lots.
Lessons Learned:
Porous concrete requires many variables to go right to be installed correctly. The correct amount of binder, moisture content, and aggregate quality are a few variables that make installation challenging.
Aggregate interlock used for permeable pavers can clog quickly and are not a good choice if being used for subsurface detention.
Bedding layers are not necessary for articulating concrete blocks and will negatively affect performance.
Higher performing pavements require less maintenance and reduce life cycle costs.
Many popular porous pavement systems don’t have an effective maintenance protocol that will effectively keep/return the product to its original specified flow rate. The PaveDrain product far exceeds the others and is why we are now proud to represent them.
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