Students rebuilt a failing permeable concrete path at a Eureka school, and it held up

A permeable concrete path in the Peace Garden at Catherine Zane Middle School in Eureka, California, was failing: uneven, sunken, impermeable and shedding loose gravel. A student team replaced it, and follow-up visits in September 2018 and September 2019 found the new path working.

Photo: Jps19 on Appropedia, CC BY-SA 3.0.

Why the old path failed

Team^2 designed and installed the new path. Its members were Ailynn Andersen, Jesse Bobrow, Catherine Carbajal and Julian Sicaud, in Humboldt State University's Engineering 215: Intro to Design course in Spring 2018, taught by Lonny Grafman. The client was the school, a STEAM school with an ongoing relationship with ENGR 215 projects, represented by administrator Trevor Hammons.

An earlier engineering team had installed the path incorrectly. The new team's hypothesis was that their mix had too much water, so the cement sank to the bottom and hardened into an impermeable layer, leaving too little at the top to hold the aggregate. The aggregate eroded into loose gravel that pedestrians tracked across campus.

Criteria

The client's goal was to correct the failed path. The team weighted the criteria from 1 to 10:

Testing the mix

Permeable concrete is a mix of round aggregate of equal size and Portland cement. A typical mix is three parts 3/8" pea gravel to one part cement, or 3:1. The team also tested 4:1 and 2:1, pouring one-square-foot samples three inches deep.

Photo: Jps19 on Appropedia, CC BY-SA 3.0.

To test permeability they built a sealed wooden box around each sample, levelled it, put in two water balloons holding about 2 gallons, popped both at once and timed how long the water took to drain through. They ran three tests per ratio. All three ratios gave a strong, highly permeable sample, so the exact ratio mattered less than getting the moisture right and curing the mix properly after installation.

The path is approximately 62 feet long, in ten sections each about six feet long, two and a half feet wide and three inches tall. The team worked out a little under forty cubic feet, just over a cubic yard, of concrete.

Cost

The budget was $400. Donations and community help held actual expenses to $197.25; most gravel, cement, weed cloth, wheelbarrows, stakes, tarps, shovels and bender board were donated, and the rest came from local hardware stores.

The result

Photo: Aily on Appropedia, CC BY-SA 3.0.

Photo: Kathy Nativi on Appropedia, CC BY-SA 3.0.

How to build one

1. Remove the unwanted path, or dig to the depth of the concrete layer.
2. Level and compact the subgrade to the depth of the porous layer, set the blocks off with bender board and 1x4 wood dividers, and cover the dirt with weed cloth.
3. Mix the porous concrete in a wheelbarrow or mixer, pour it into the sections, level it and cover it immediately with plastic to cure for a week.

Photo: Jps19 on Appropedia, CC BY-SA 3.0.

Photo: Jps19 on Appropedia, CC BY-SA 3.0.

Photo: Jps19 on Appropedia, CC BY-SA 3.0.

Recommended maintenance is power-washing twice a year so small objects do not clog the holes.

Follow-up

September 2018. The path looked good, gravel did not come loose when walked on, and it felt softer than the parking lot concrete, though a hard kick loosened gravel. A water test of 2/3 cup on the 2015 walkway and on the 2018 walkway showed the older one draining almost at once, while the new one took a couple of minutes to drain completely.

September 2019. The path seemed fully functional and very lightly worn. Ten to fifteen minutes after rain it was already dry, while the asphalt road still had puddles. The school's counseling services director said the paths were not causing any problems, and maintenance appeared low to none.

Sources

• Appropedia, "Zane Middle School/Permeable concrete 3" (https://www.appropedia.org/Zane_Middle_School/Permeable_concrete_3), by its contributors (https://www.appropedia.org/w/index.php?title=Zane_Middle_School%2FPermeable_concrete_3&action=history), under CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/). Changed here: condensed and reworded, tables and photos reduced, photos resized; each photo is under the licence and credit shown in its caption; rewritten in hubnx's own words.