Commercial Building Products - May 2015 - 23

PROJECT

rban streetscapes are a challenging en-

rous Pave Inc., Grant, MI. Porous Pave XL is a

vironment for trees, and no trees are

hard, durable material made from 50% recycled

as challenged as street trees planted in

rubber chips and 50% stone aggregate, with a

small openings in hardscapes such as sidewalks.

moisture-cured urethane binding agent. Porous

These openings can be as small as 15 to 25 sq.

Pave is engineered with 29% void space.

ft. yet they are the only way water and oxygen

Independent testing indicates that the mate-

can reach the tree roots. Metal tree grates are

rial allows as many as 6,300 gal./hr./sq. ft. of

the traditional solution for maintaining these

water to drain directly through its surface, per-

openings. Pedestrians can walk on metal grates.

meate into a compacted aggregate base, and

Wheelchairs, strollers, and bicycles can roll over

then slowly ﬁlter into the ground below.

them. The grates protect trees and the soil from

Porous Pave is most commonly used to estab-

trafﬁc while allowing water and air to reach the

lish areas of permeable pavement that inﬁltrate

tree roots.

stormwater on site; decrease the volume and

However, tree grates present problems on

slow the velocity of rainwater runoff into streets,

commercial, public, and institutional proper-

storm drains, and storm sewers; improve water

ties. Grates are expensive and are supposed to

terial's porosity, permeability, and slip resis-

the opposite happens. Over time, iron grates

Michigan city makes its
downtown sidewalks safer and
streetscapes more attractive
with paved tree surrounds.

pollutants; and recharge groundwater. The ma-

tion. Without routine maintenance, however,

Permeable Pavement
Replaces Tree Grates

quality by reducing erosion and ﬁltering out

suppress weed growth and limit trash accumula-

exteriors

tance make it a good option for tree surrounds.

rust. With freeze-thaw cycles, tree growth, and

The city had installed a limited number of

the expansion of root systems, grates buckle and

Porous Pave tree surrounds in 2011. According

break. Rusted, broken grates are unsightly and

to Joe Sulak, city forester, City of Grand Rapids,

an unsafe tripping hazard.

the material proved it could work for the fullscale replacement project. "Rainwater and air go

PREPARING FOR A CROWD

right through Porous Pave and down to the tree

Grand Rapids, MI, hosts the annual ArtPrize

roots," said Mark DeClercq, PE, city engineer.

event, an independently organized interna-

"With its high rubber content and textured sur-

tional art competition that transforms three

face, it is slip resistant and safer when wet than

square miles of the city's downtown area into an

traditional metal tree grates."

open-air arts scene with more than 150 venues.

According to Dave Ouwinga, Porous Pave

During its annual 19-day run, ArtPrize attracts

president and CEO, the material is ADA com-

as many as 440,000 visitors. With so many peo-

pliant. In addition to making paved permeable

ple strolling the sidewalks, the city wanted to

surfaces slip resistant, the recycled rubber gives

improve the safety of tree openings.

it ﬂexibility, helping it to withstand freeze-thaw

Grand Rapids had conventional square and

cycles without heaving, cracking, or breaking.

round metal tree grates with four sections con-

When trees grow, their girths increase. The

nected to each other with bolts and to a metal

ﬂexibility of Porous Pave enables the surface to

frame cemented into the sides of the sidewalk

expand. As tree girth requires more space, the

openings. In 2014, the city inspected the grates

opening in the paved surface at the base of trees

in the downtown area to locate those that were

can be widened. "Installers thin the material

rusted, broken, or had cracked or missing bolts.

around the base of trees," said Ouwinga. "That

The city identiﬁed and marked 750 such grates.

makes it easier to cut rings out as trees grow."

The count included 250 that rated greatest-trip-

Moreover, because the permeable surface is

ping-hazard status, deﬁned as those with miss-

solid, it is much easier to maintain. Trash and

ing sections or more than a 3/4-in. difference in

falling leaves stay on the surface and can be

height between the level of the sidewalk and

swept by maintenance workers, rather than get-

broken grate sections. It was decided to replace

ting stuck in the grates.

those 250 grates, before ArtPrize started in Sep-

The product is poured in place at thicknesses

tember 2014, with something other than new

of 1 to 2 in. atop a compacted aggregate base of

metal grates.

2, 4, or 6 in., depending on the application and
required compressive strength. The aggregate is

PERMEABLE-PAVEMENT SOLUTION

1/4- to 3/8-in. nominal size. For the Grand Rap-

The grate alternative they chose was Porous

ids tree surrounds, 2 in. of Porous Pave XL was

Pave XL tree surrounds, manufactured by Po-

poured on an aggregate base of 2 to 4 in., in-

Above. For the Grand Rapids tree surrounds, 2 in. of
Porous Pave XL was poured on an aggregate base of 2
to 4 in., installed atop the tree root balls.
Right. When trees grow, their girths increase. The
ﬂexibility of Porous Pave enables the surface to expand.
As tree girth requires more space, the opening in the
paved surface at the base of trees can be widened.

cbpmagazine.com

MAY 2015

COMMERCI A L BUIL DING P RODUC T S

http://www.cbpmagazine.com

Commercial Building Products - May 2015

Table of Contents for the Digital Edition of Commercial Building Products - May 2015