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USDA Forest Service Products |
Improving Engineered Wood Fiber Surfaces for Accessible Playgrounds
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Abstract
Some engineered wood fiber surfaces are uneven, tend to
shift, and have low density. The goal of our research was to
develop a playground surface material that cushions impact
and is accessible to people with disabilities. In the initial
screening phase, we evaluated a variety of in situ surface
treatments and mixtures of wood particles combined with
various binders. Engineered wood fiber (EWF) was prepared
from three species, red maple, ponderosa pine, and one-seed
juniper, which have a wide range of densities and bonding
properties. In the scale-up phase, we evaluated commercially
available EWF and several promising binding systems from
the screening phase trials. Seventeen test configurations
were formed in plywood boxes, using different levels of
EWF compaction, fiber moisture content, surface layer
thickness, and types of binders. Binder systems that show
promise for surface stabilization and satisfactory impact
behavior are polyurethane, latex, and silicone. These binders
were chosen on the basis of processing ease, flexibility
(elongation to failure), cost, and safety in application and
use. In this report, we identify the strengths and weaknesses
of the surface treatments, review the viability of the systems
and the testing concepts we have developed, and identify
further research needs.
Keywords: surfacing, impact, accessibility, ADA, compos-
ite, polyurethane, cushioning, engineered wood fiber, latex,
silicone
March 2003
Laufenberg, Theodore; Krzysik, Andrzej, M.; Winandy, Jerrold, E. 2003.
Improving engineered wood fiber surfaces for accessible playgrounds. Gen.
Tech. Rep. FPL-GTR-135. Madison, WI: U.S. Department of Agriculture,
Forest Service, Forest Products Laboratory. 15 p.
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Acknowledgments
This project was conducted cooperatively with and funded
by a grant from the United States Architectural and Trans-
portation Accessibility Compliance Board (Access Board).
We are grateful to Zeager Brothers, Inc. (Middletown, Penn-
sylvania), and Beneficial Designs, Inc. (Minden, Nevada),
for the loan of specialized test equipment and training in its
operation.
Improving Engineered Wood Fiber
Surfaces for Accessible Playgrounds
Theodore Laufenberg, General Engineer
Andrzej M. Krzysik, Forest Products Technologist
Jerrold E. Winandy, Supervisory Research Forest Products Technologist
Forest Products Laboratory, Madison, Wisconsin
Background
Engineered wood fiber (EWF) products are the first choice
of playground designers who wish to provide a natural or
rustic-looking environment or for whom cost is a primary
concern. Such products cost only 10% to 15% of the cost of
synthetic rubber surfaces for full-depth playground system
installations. When installed with proper drainage and well
maintained, a wood fiber surface can last for 8 years or
more. It can be designed to exceed Consumer Product and
Safety Commission guidelines for the safety of playground
surfaces.
For physically challenged children and adults who must use
a wheelchair, crutches, or a walker, play areas and trails
surfaced with EWF are generally suitable for short distances.
However, recent research by Axelson and Chesney (1999)
indicates that some EWF installations, especially when wet,
might only be marginal for use on .accessible. traffic routes.
As a consequence, the U.S. Architectural and Transportation
Accessibility Compliance Board (known as the Access
Board1) has been examining the appropriateness and usabil-
ity of wood-based playground surface materials for outdoor
environments designed to be accessible to children with
disabilities (U.S. Architectural and Transportation Barriers
Compliance Board 1991, 1994).
The baseline of safety performance is ASTM F1292, the
Standard Specification for Impact Attenuation of Surface
Systems Under and Around Playground Equipment (ASTM
1999a). The F1292 specification establishes impact attenua-
tion requirements, when tested in accordance with Test
Method F355 (ASTM 1995). The F1292 specification ap-
plies to all types of material that can be used under play-
ground equipment. It establishes a direct means of compari-
son and does not imply that an injury cannot be incurred if
the surface system complies with the specification.
The requirements of ASTM F1292 (and F355, Procedure C)
are as follows:
______________________
1 The Access Board is a Federal steering group created
under the Americans with Disabilities Act of 1992.
- The surface does not impart peak acceleration in excess of
1,961 m/s2 (200 g) to an instrumented 4.54-kg (10-lb)
head form dropped on a surface from the maximum fall
height (critical height). - The surface must meet the head injury criterion (HIC)
established in F1292 of less than 1,000 when properly in-
stalled.
The critical height of a surface material is established and
published by the U.S. Consumer Product Safety Commission
(1998) based on typical performance. The head injury crite-
rion is an energy absorption measure for the entire impact
event.
Axelson and Chesney (1999) researched the capability of a
wide array of commonly used accessible surfaces. Accessi-
ble test courses, in compliance with the Americans With
Disabilities Act (ADA) accessibility guidelines, were de-
signed and built using three types of exterior wood-based
surfaces. The firmness and stability of the test surfaces were
measured using the wheelchair work measurement method
and a portable surface measurement device, a rotational
penetrometer. To determine energy required, persons with
and without disabilities walked or wheeled across each
surface.
Wheelchair work per meter values for forward movement
and turning were determined for all test course surfaces
under dry conditions using the wheelchair work measure-
ment method in accordance with ASTM F1951 (formerly
PS 83.97) (ASTM 1997, 1999b). The test courses were
measured using a rotational penetrometer under both wet and
dry conditions. The rotational penetrometer is a portable
device developed by Axelson and Chesney (1999) that pro-
vides quantitative (ASTM F1951) measurements of firmness
and stability on a wide variety of surfaces.
Standardized tests of physical fitness and community ambu-
lation indicate that measures of mechanical work expended
during ambulation would be a more consistent measure of
surface acceptability than are subjective user assessments. In
the study by Axelson and Chesney (1999), the work required
to propel a wheelchair in a straight path was correlated to the
firmness of the surface. The work required to propel a
wheelchair through a 90° turn was correlated to the stability
of the surface. The results indicated that the chipped brush
surface and two EWF surfaces had higher work per meter
values compared with that of the other surfaces tested. All
exterior surfaces, except chipped brush, became less stable
when wet. Axelson and Chesney recommended that these
surfaces be considered moderately firm and stable and that
each be rated as suitable for use in level areas for limited
distances on trails and playgrounds.
Scope of Study
The study objective was to develop an EWF-based compos-
ite material, composed of a highly resilient bonded EWF
surface and an unbonded EWF core, that would improve
accessibility to playgrounds and other recreational surfaces
for people with disabilities. In the initial screening phase, we
identified several fiber processing options and evaluated
several promising bonding systems. In the scale-up-phase,
we performed impact cushioning tests and simulated acces-
sibility testing using laboratory-sized full-depth surface
specimens.
The screening phase included evaluation of a variety of in
situ surface treatments and mixtures of wood particles with
binder (silicone, urethane polymer resin, synthetic latex, or
low molecular weight butylene co-polymer). Trials were
made with several application techniques and binders to
assess process and performance attributes.
A variety of potential binding agents were identified for
mixing with EWF to stabilize the upper surface of a play-
ground surface. This adhesive, filler, or matrix binding
material was intended to bond or encapsulate the top surface
of the EWF composite, thereby imparting a degree of resis-
tance to wheelchair casters from penetrating the surface. A
related objective was to impart upper layer stiffness so that
the wheel or caster did not depress the surface. Somewhat at
odds with these objectives was our desire to produce a stabi-
lizing material that provided cushioning from falls.
Processing procedures needed to be developed for each
stabilizing binder system. In each instance, we sought a
system of materials with certain performance and processing
attributes. The stabilizing binders were intended to be ap-
plied on site or mixed with the EWF no longer than 1 hour
prior to installation. The method for mixing depended on the
speed of curing, viscosity, tack, and similar attributes of the
resin or binder. Costs for each type of operation was a con-
cern as were such factors as worker safety, quality control,
and performance assurance of the finished system.
Our expectations for good processing performance of a
stabilizing binder were subjective but hinged on practical
considerations. We sought a system that would provide
impact safety and good access behavior. Impact safety (that
is, energy absorption characteristics) is quantifiable by
ASTM F1292 (ASTM 1999a) and preliminary tests using a
portable impact test provided indication of stabilizing binder
potential. Accessible surfaces are defined (by ADA) as
stable, firm, and slip resistant. Accessibility of the potential
surfaces was quantified using a rotational penetrometer
(Axelson and Chesney 1999), which has good correlation to
the wheelchair work method in ASTM F1951 (ASTM
1999b). Two cooperators provided portable test apparatuses
and training in their use.
Wood Fiber Processing Trials
Wood fiber material consisted of three underutilized small-
diameter species: red maple (Acer rubrum), one-seed juniper
(Juniperus monosperma), and ponderosa pine (Pinus pon-
derosa). These species were selected to represent a range of
density, natural durability, and geographic location to ap-
proximate the wood material used for playgrounds and trails.
In addition, species were selected to achieve USDA Forest
Service objectives of reducing forest fire fuel loading, improving
timber quality for increased growth and forest health, and
removing invasive species to restore the native
ecosystem.
The fiber was processed and analyzed at the Forest Products
Laboratory. Approximately 90 to 136 kg (200 to 300 lb) of
roundwood was obtained for each species evaluated. The
wood was maintained in the .green. condition. The process-
ing steps were as follows:
- Produce approximately 19-mm (approximately 0.75-in.) "pulp quality" chips.
- Screen out oversize chips, bark, and fines larger than 3.1 mm (1.25 in.) and smaller than 0.6 mm (0.25 in.).
- Hammermill fiber through screens to obtain an EWF mix of fiber sizes.
- Perform sieve analysis of resulting EWF.
- Dry to 8% to 10% moisture content.
Standard samples of EWF from two industrial suppliers were
used to develop the test protocol. Visual assessment was
used in the initial stages of the process. A sieve analysis was
later used to generate comparative data to the EWF draft
standard, which was being developed by the ASTM F08.63
subcommittee. Each sieve analysis was completed with EWF
in the dry condition (approximately 8% to 10% moisture
content) and repeated three times, as called for in earlier
versions of the draft standard. The eventual EWF standard,
ASTM F2075.01a, does not require three sample repetitions
of the sieve analysis.
There are many methods for preparing EWF to meet the
standard but even more ways to prepare sub-standard mixes.
During our trials we found several ways to successfully
prepare the ponderosa pine and red maple constituents.
However, we could not find a method for preparing juniper.
When chipped and hammermilled, juniper fractured into
needle-like elements that could be forced to meet the EWF
standard. However, in our opinion, the use of such needle-
like elements on a playground might create secondary safety
hazards in the form of splinters, punctures, and other inju-
ries.
A large sample (2 to 3 m3 (2 to 3 yd3)) of primarily oak
industry-supplied EWF material was used to prepare full-
depth specimens for impact testing and evaluation with the
rotational penetrometer in the scale-up-phase. This material
was donated by a manufacturer with a license for production
of EWF from our cooperator, Zeager Brothers, Inc.
Bonding System Evaluations
Trials were made with several application techniques and
binders to assess process and performance attributes. The
techniques included several in situ surface treatments and
mixtures of wood particles with binder. Binders were a
butylene co-polymer, synthetic latex emulsion, silicone, and
polyurethane polymer resin.
Process details were developed for each stabilizing binder
system. In each instance, we evaluated a system of materials
and sought a balance of several attributes.
Application
We sought a stabilizing binder that could be mixed on site or
mixed with EWF no longer than one hour prior to applica-
tion on the ground surface. The method for mixing depended
on the speed of curing, viscosity, tack, and similar attributes.
Options for application were as follows:
- Spray mixing in a blender (concrete mixer or augured
adhesive blender) - In-place spraying on compacted EWF with a tank or back-
pack sprayer - Flood coating on compacted EWF
- Spray or flood coating with mechanical mixing onto top
layer of EWF
Costs for each type of operation was a concern as well as
worker safety, quality control, and performance assurance of
finished system.
Curing Conditions and Behavior
Expectations for a successful stabilizing binder are not stan-
dardized. Thus, our assessment was partially subjective, but
it was based on relevant practical considerations, including
(1) cure or set time prior to surface use, (2) range of mois-
ture and temperature conditions acceptable for use, and (3)
fume, odor, toxicity, outgas, exotherm, or other chemical
release from the binder/EWF mixture.
Preliminary screening phase evaluations were completed for
26 small specimens. The primary variables were binder type,
binder content, and EWF type. Other variables included
concepts of layering a surface, compaction, and surface
coating. From these evaluations we selected several binders
and surfacing concepts that were usable from a processing
standpoint and showed potential for meeting the perform-
ance needs of an accessible playground surface.
Full-Depth Surface Specimens
Screening Phase
Bonded and unbonded specimens for the screening phase are
described in Table 1. The specimens were formed in
450- by 450- by 300-mm (18- by 18- by 12-in.) test boxes
filled with compacted EWF as defined in ASTM Standard
F2075 (ASTM 2001). Bonded specimens were made from
red maple chips, processed through a 38-mm (1.5-in.) ham-
mermill screen. This material was compacted into the test
box, and the filled box was weighed. Two silicone-based
bonding systems were used for the screening phase prelimi-
nary tests: (1) AllGuard (Dow Corning Corp., Midland,
Michigan), a masonry waterproofing formulation, and
(2) 3.5000 (Dow Corning Corp.), a roof coating
formulation.
Table 1. -- Specimens for screening phase
|
Specimen |
Species |
Chip and |
Adhesive matrix |
Top surface |
Densityb |
|
A |
Red maple |
Pulp, |
NA |
NA |
0.29 (18.3) |
|
B |
Red maple |
Pulp, large |
NA |
NA |
0.29 (17.9) |
|
C |
Red maple |
Large, small |
NA |
NA |
0.20 (12.4) |
|
D |
Ponderosa pine |
Pulp, large |
NA |
NA |
0.26 (16.4) |
|
E |
Ponderosa pine |
Large, small |
NA |
NA |
0.16 (9.8) |
|
F |
Juniper |
Large, small |
NA |
NA |
0.15 (9.3) |
|
G |
Juniperc |
Large, small |
NA |
NA |
0.10 (6.3) |
|
SA |
Red maple |
Pulp, large |
AllGuard |
25 |
0.29 (18.1) |
|
SB |
Red maple |
Pulp, large |
AllGuard |
50 |
0.28 (18.5) |
|
SC |
Red maple |
Pulp, large |
AllGuard |
75 |
0.27 (16.7) |
|
SD |
Red maple |
Pulp, large |
3-5000 |
25 |
0.29 (17.8) |
|
SE |
Red maple |
Pulp, large |
3-5000 |
50 |
0.29 (18.3) |
a Top surface composed of target thickness of EWF removed from specimen,
then weighed and mixed with 40% (dry weight) adhesive matrix binder.
b Density of EWF specimen was calculated prior to addition of surface layer.
c Uncompacted.
All specimens were compacted to simulate the finished
surface of a play area. It is standard practice to compact
EWF for testing and actual installations. Given the lack of an
industry, governmental, or association accepted standard for
compacting EWF, we established a procedure for this study.
Our intent was to simulate the amount of compaction pro-
duced by an average 10-year-old child while playing on the
surface.
Table 2. -- Bonded specimens for scale-up phase
|
Specimen ID |
Adhesive matrix |
Top Surfacea |
Densityb (g/cm3 (lb/ft3)) |
|
J |
Polyurethane Vitriturf |
25 mm (1 in.) thick, 30% adhesive, perforationsc |
0.27 (16.8) |
|
K |
Polyurethane Vitriturf |
37 mm (1.5 in.) thick, 30% adhesive, perforationsc |
0.26 (16.5) |
|
L |
Polyurethane ReacTITE 8143 |
25 mm (1 in.) thick, 30% adhesive, geotextived |
0.25 (15.5) |
|
M |
Polyurethane ReacTITE 8143 |
37 mm (1.5 in.) thick, 30% adhesive, geotextiled |
0.27 (17.0) |
|
N |
Latex Soil-Sement |
63 mm (2.5 in.) thick, 35% adhesive, perforationsc |
0.28 (17.4) |
|
O |
Latex Soil-Sement |
63 mm (2.5 in.) thick, 25% adhesive, perforationsc |
0.30 (18.6) |
|
P |
Latex Soil-Sement |
63 mm (2.5 in.) thick, 25% adhesive, geotextiled |
0.27 (17.0) |
|
Q |
Latex Soil-Sement |
63 mm (2.5 in.) thick, 30% adhesive, geotextiled |
0.28 (17.5) |
|
R |
Siliconef D-C AllGuard |
37 mm (1.5 in.) thick, 40% adhesive, perforationsc |
0.28 (17.2) |
|
S |
Siliconef D-C AllGuard |
50 mm (2 in.) thick, 35% adhesive, perforationsc, plus 5% top coatinge |
0.29 (17.9) |
|
T |
Siliconef D-C 3-5000 |
37 mm (1.5 in.) thick, 40% adhesive, perforationsc |
0.27 (17.1) |
|
U |
Siliconef D-C 3-5000 |
50 mm (2 in.) thick, 35% adhesive, perforationsc, plus 5% top coatinge |
0.28 (17.4) |
a Top surface composed of target thickness of compacted EWF; proportion of
compacted EWF removed from top surface, weighed, and mixed with measured
percentage (dry weight EWF) of adhesive matrix binder
b Density of EWF specimen was calculated prior to addition of surface layer.
c Specimen perforated on 75- by 75-mm (3- by 3-in.) grid to depth of 200 mm (8 in.).
d Application of polyolefin geotextile under surface layer.
e Topcoating of adhesive matrix brushed on top of surface after cure; adhesive matrix
consisted of controlled percentage of oven-dry weight of surface layer.
f Surface layer removed and dried to 7% moisture content prior to mixing with
adhesive matrix.
To compact the fiber, EWF was uniformly and evenly
tamped into a 450- by 450-mm (18- by 18-in.) box using a
7.25-kg (16-lb), 57-mm- (2.25-in.-) diameter, 375-mm-
(15-in.-) long steel rod. The surface was tamped 50 times.
Each tamp used only the dead weight of the rod applied in
evenly spaced locations over the box.
This was accomplished in four stages:
- Fill box to 300-mm (12-in.) depth with loose EWF, level,
and tamp uniformly with 9 strokes of rod. - Refill box to 300-mm (12-in.) depth, level, and tamp
16 times. - Refill to 300-mm (12-in.) depth, level, and tamp 25 times.
- Refill to 300-mm (12-in.) depth, level, and roll rod over
surface once in both directions.
This technique made the test EWF 25% to 47% more dense
than uncompacted EWF. The densification values were
influenced by species, EWF particle configuration, and EWF
moisture content.
A proportion of the compacted EWF, depending on surface
thickness, was mixed with the matrix binder. Matrix binder
surface was 40% (by dry weight) of EWF. This mixture was
returned to the test box and allowed to cure for 5 days prior
to F1292 testing (ASTM 1999a).
Scale-Up Phase
Bonded specimens were made with several new application
techniques and binders to assess process and performance
attributes (Table 2). Unbonded specimens provided an EWF
baseline for tests in the scale-up-phase (Table 3). These new
techniques and binders expanded upon the options identified
in the screening phase and assisted in quantifying the impact
and accessibility of the novel surfaces.
Table 3. -- Unbonded specimens for scale-up phase
|
Specimen ID |
Compaction |
Target moisture |
Densitya (g/cm3 (lb/ft3)) |
MC at testb (%) |
|
W |
Yes |
Less than 10%, |
0.20 (12.7) |
7.9 |
|
X |
Yes |
Greater than 30%, |
0.25 (15.9) |
27.0 |
|
Y |
No |
Less than 10%, |
0.16 (10.2) |
7.7 |
|
Z |
No |
Greater than 30%, |
0.19 (12.0) |
28.3 |
|
XX |
Yes |
As for X + 5% |
0.26 (16.1) |
28.0 |
a Density of entire specimen calculated on basis of weight and volume of EWF in specimen test box.
b Moisture content (MC) based on small sample removed from test box immediately after impact test.
The scale-up series included a variety of in situ bonding
treatments and interfacial treatments, and several thicknesses
and quantities of binder/EWF mixtures. Binders were syn-
thetic latex emulsions, silicones, and foaming and resilient
polyurethanes. Twelve modified surface test specimens were
formed in 450- by 450- by 300-mm (18- by 18- by 12-in.)
test boxes filled with EWF, as defined by ASTM F2075
(ASTM 2001) unless otherwise noted.
All specimens were commercial EWF obtained from a pro-
ducer in Oskaloosa, Iowa (a licensee of our cooperator,
Zeager Brothers, Inc.). A sieve test was completed according
to the ASTM F.08.63 draft standard. The test material was
placed in the test box and compacted. The weight and bulk
density of the filled box was then determined.
Three bonding systems were used:
- Silicone-based (Dow Corning Corp., Midland, Michigan)
a. AllGuard, a waterproofing coating
b. 3.5000, a roof coating/sealant - Synthetic latex, Soil-Sement (Midwest Industrial
Supply, Canton, Ohio) - Polyurethane
a. Vitriturf (Polmer Plastics Corp., Hauppauge
New York)
b. ReacTITE 8143 (Franklin International,
Columbus, Ohio)
The full-depth surface samples were formed in 300-mm-
(12-in.-) deep boxes in a manner similar to that described for
screening phase specimens. The percentage of matrix binder
added to the surface layer was a prescribed quantity (by dry
weight) of EWF.
Two interfacial treatments, perforation and geotextile rein-
forcement, were used to improve the adhesion of the bonded
surface to the remainder of the cushioning surface:
Perforation.After the surface layer was applied to EWF,
the surface was penetrated by a 12-mm (0.5-in.) steel rod to
a depth of 200 mm (8 in.) to achieve a .rough. interface
between the bonded and unbonded portions of the specimen.
This treatment improved surface drainage for binders that
formed a non-draining film on the surface. Perforations were
100 mm (4 in.) from the edge, 125 mm (5 in.) on center, and
were placed in a 3 by 3 pattern through the surface.
Geotextile reinforcement.A 450- by 450-mm (18- by
18-in.) single ply of lightweight polyolefin landscaping
geotextile (100 g/m2) was placed between the bonded layer
and unbonded base of the specimen. The geotextile was
intended to provide continuity for the bonded surface layer
in the event that it fractured through its entire thickness. By
bonding this membrane to the top layer, the fractured seg-
ments of the layer were less likely to be ejected from their
original position and to pose a hazard on the remaining
bonded surface.
Four identical unbonded test specimens (W through Z) were
made with only EWF. These baseline EWF specimens were
used to preliminarily assess variability and the effects of
moisture content and compaction on cushioning. Thus, the
test matrix had two levels of compaction and two levels of
moisture content. To assess the effectiveness of a simple top
coating on cushioning and accessibility behavior, specimen
XX had only a sprayed-on coating of Soil-Sement (5% by
weight of top 25 mm (1 in.) of EWF).
Test Procedures
Impact Behavior
Test specifications in ASTM F1292 (ASTM 1999a) and
F355.95 test procedure C (ASTM 1995) were used at a
constant test drop height of 3.0 m (10 ft) (Fig. 1). Specimens
were preconditioned for a minimum of 4 days at ambient
conditions (approximately 50% relative humidity and 23°C
(74°F)) in a dry storage building during late summer of
2001. At least three impact tests were run in sequence on
each specimen per ASTM F1292. Several specimens were
dropped 10 times to assess the effect of multiple drops on
impact parameters.
Figure 1. -- Impact testing set-up for 3-m (10-ft) drop
using free-fall test method of ASTM F1292.99.
The instrumented hemispherical impactor (Fig. 2) was
dropped by a magnetic release over the drop site. The impact
site was a hardened zone with a mass of approximately
4,500 kg (10,000 lb). A minimum mass of 454 kg (1,000 lb)
is dictated by the standard. Results of the second and third
impact tests were averaged to compare to playground surface
specifications. Immediately after impact testing, samples of
each species were obtained from representative boxes for
moisture content determination.
The performance requirements for a tested surface to meet
ASTM F1292 and F355 Procedure C are as follows:
- Test does not impart a peak deceleration in excess of
1,961 m/s2 (200 G) to an instrumented 4.5-kg (10-lb)
ANSI head form (minimum requirement) dropped on a
surface from maximum fall height (critical height). - Surface must meet the head HIC of less than 1,000 when
properly installed. - HIC is the summation of energy absorption for the entire
impact event; maximum allowed fall is 3.0 m (10 ft).
Accessibility
Figure 2. -- Impact head caught on rebound from
EWF surface.
We used a relative measure of accessibility because of the
small size of our specimens. The 450- by 450-mm (18- by
18-in.) test surfaces were objectively measured using a
portable rotational penetrometer supplied by Beneficial
Designs Inc. (Minden, Nevada), who also provided original
carpet samples (C1 and C3 without pads) for ongoing cali-
bration of the device to the original study.
The rotational penetrometer was used to measure compacted
or stabilized surface test specimens from the scale-up phase.
The rotational penetrometer was mounted atop 450- by 450-
by 300-mm (18- by 18- by 12-in.) plywood boxes. All tests
were performed on specimens previously subjected to impact
testing. Special care was taken to avoid, to the greatest ex-
tent possible, evaluating areas damaged by the impactor or
fractures resulting from the impact tests. Contact between
the footpads and the surface was assured. The tests were
conducted as specified by the Beneficial Designs protocol
(Axelson and Chesney 1999).
Figure 3. -- Rotational penetrometer mounted on
full-depth playground surface specimen.
Surface firmness was measured by a rotational penetrometer,
which applied a standard force (approximately 15 kg (33 lb))
to a pneumatic wheelchair caster. This permitted measure-
ment of the downward displacement of the surface and the
caster (Fig. 3). After the force was applied, the penetrometer
was rotated 90° to the left and right for two sequences, for a
total 360°. The final depth of penetration was then measured,
representing the stability of the surface (Fig. 4).
The criteria for acceptable accessibility performance have
not been embodied in a consensus test standard or guideline.
However, the correlation of the rotational penetrometer
measurement to the wheelchair work measurement with
realistic subjects provides a guideline for firmness and
stability within definitions set by the ADA Accessibility
Guidelines.
Three recommended levels of performance for firmness and
stability (Axelson and Chesney 1999) were analyzed
(Table 4). According to these recommendations, .moderate.
ratings are acceptable for areas such as playgrounds where
the slope is less than 3% and the distance traveled is less
than 160 m (525 ft).
Figure 4.Disturbed EWF at contact point of rotational
penetrometer after caster performed 360° rotation
sequence.
Table 4. -- Firmness and stability levels recommended
for accessible surfacesa
| Property | Performance Level | Depth of Penetrationb (mm (in.)) |
| Firmness | Firm | £ 7.6 (£0.3) |
| Moderately Firm | >7.6 (>0.3), <12.7 (<0.5) | |
| Not Firm | ³12.7 (³0.5) | |
| Stability | Stable | £12.7 (£0.5) |
| Moderately Stable | >12.7 (>0.5), £25.4 (£1.0) | |
| Not Stable | >25.4 (>1.0) |
a Axelson and Chesney (1999).
b Penetration of rotational penetrometer into specimen surface.
Results and Discussion
Wood Fiber Processing
During preliminary evaluations, we studied many types of
EWF and several potential binders for accessible playground
surfaces. Many of those evaluations were qualitative and
reflected our knowledge at the time. The EWF production
process used in the industry has not been standardized and
could conceivably have many processing variants. Neverthe-
less, there is a prescriptive standard for EWF material that is
a tentative step toward a performance standard. We under-
took the systematic development of the appropriate process
for converting pulp-size chips to EWF. We found that the
range of particle sizes in EWF was appropriate for our con-
cept of a bonded playground surface.
Ponderosa pine and red maple did not present any bonding
problems or special concerns. Both of these species bonded
well and produced good EWF, as defined by sieve analysis.
On the other hand, we encountered significant and ultimately
insurmountable challenges in making EWF from one-seed
juniper. Hammermilling and chipping of juniper produced
needle-like particles that would be inadequate for cushioning
and would be a hazardous surface for a playground.
Commercially available EWF was used for scale-up tests.
Because this fiber is usually composed of mixed hardwoods,
we expected it to have moderate to good durability. The
EWF for our tests was primarily composed of hickory, red
and white oak, and slippery and American elm, with lesser
amounts of lower density species (aspen, silver maple, and
cottonwood). Upon delivery, EWF moisture content was
42%. Sieve analysis indicated that the EWF met the F2075
Standard Specification when tested oven dry. We decided to
conform to existing material practices rather than develop a
new or radically different wood-based particle or configura-
tion for the purpose of evaluating the stabilized playground
surface concept.
Bonding System
Given the preliminary nature of this conceptual study, we
could not justify the development of a new adhesive or
binder system. Instead, we considered many existing formu-
lations and adopted several without modification. Although
we limited the number of variables by using available bind-
ers, we were able to change the binder type and quantity,
surface layer thickness, EWF moisture content, and the
application method. Nearly all the binders, when cured and
fully reacted, were considered benign from a toxicological
standpoint. The exceptions are discussed in the following
text.
Four classes of stabilizing binders were considered; three
classes were taken to the scale-up phase. We excluded the
butylene co-polymer because its hydrophobic nature and low
matrix strength resulted in a mat with diminished inter-fiber
bonding and a slippery glaze. Considering the low coeffi-
cient of friction imparted to the fiber-to-fiber matrix, this
.hot melt. type of adhesive would certainly have reduced the
traction of the system and hence accessibility, with little
effect on cushioning behavior.
The polyurethane class of adhesives offered a wide range of
viscosities, foaming potential, and cured resin flexibility.
Vitriturf is presently used in the recreational surfaces indus-
try for bonding rubber particle surfaces. This class of adhe-
sives bonds well to wood and can fill interstitial spaces.
Even at low application rates, some mat bonding occurs. The
cost for polyurethane adhesives is in the middle of those we
investigated. A special formulation for stabilized EWF
surfaces would enhance the cost competitiveness of the
system. Before it is cured, the sprayed resin can cause aller-
gic reactions, so care must be taken to protect breathing and
skin contact during mixing and application. Once the adhe-
sive is cured, it is considered benign.
Silicone was initially considered a good candidate because
of its potential for highly elastic behavior and ability to
perform in high ultraviolet light and moist environments.
However, the hydrophobic nature of silicone produces poor
bonding to wood and requires a high application rate. We
found that the silicone matrix needed to be continuous to be
effective in holding the EWF together as a unit. In addition,
we expect the unit cost to be somewhat higher than that of
most resins. In the cured state, the 3.5000 roof coating
forms methyl ethyl ketoxime (MEKO) upon contact with
water. Recommendations are to minimize exposure to
MEKO because high levels of exposure have been shown to
result in liver cancer in rodents. No such concerns have been
noted for the AllGuard silicone used in our study.
Synthetic latex was considered because of its acceptance in
the soil stabilization industry for trails, embankments, and
other landscaping applications. It remains flexible after
application and becomes stiff only after an extended period
of exposure. One concern is that synthetic latex is tacky to
the touch for an extended period after application, depending
on local temperature and humidity. An application rate of
20% to 25% produces a moderately well-bonded EWF mat.
The low cost of this system and its present large-scale use in
the landscaping industry make synthetic latex a good candi-
date for further development. A drawback is its leachability
and biodeterioration, which would require rejuvenation of
the surface at regular intervals (6 to 24 months). Rejuvena-
tion of the latex binder could be part of regular playground
maintenance.
Impact Behavior
Data gathered from impact testing (Tables 5 to 8) included
maximum deceleration and HIC according to ASTM F1292
(ASTM 1999a) and ASTM F355 (ASTM 1995). Specimen
weight, density, and moisture content were measured. The
impact criteria for playground surfaces require that decelera-
tion not exceed 200 and HIC not exceed 1000. In the F1292
Standard, the critical tests are the second and third drops
only. The first drop is primarily for compacting the impact
site. Using these criteria, all test configurations from the
screening and scale-up phases passed the requirements for
the 3-m (10-ft) height used in this assessment. Thus, critical
height for all configurations was in excess of 3 m (10 ft).
Table 5. -- Results of screening phase impact tests
for unsurfaced specimens
|
Specimen |
Drop |
Peak |
HIC |
|
A |
1 |
62 |
252 |
|
|
2 |
74 |
272 |
|
|
3 |
89 |
372 |
|
|
4 |
96 |
428 |
|
|
5 |
102 |
464 |
|
|
6 |
110 |
549 |
|
|
7 |
113 |
593 |
|
|
8 |
116 |
573 |
|
|
9 |
115 |
593 |
|
|
10 |
114 |
581 |
|
B |
1 |
77 |
321 |
|
|
2 |
94 |
415 |
|
|
3 |
99 |
448 |
|
C |
1 |
59 |
174 |
|
|
2 |
78 |
316 |
|
|
3 |
91 |
451 |
|
D |
1 |
56 |
228 |
|
|
2 |
75 |
329 |
|
|
3 |
91 |
393 |
|
E |
1 |
69 |
251 |
|
|
2 |
80 |
346 |
|
|
3 |
84 |
368 |
|
F |
1 |
70 |
258 |
|
|
2 |
88 |
411 |
|
|
3 |
98 |
520 |
|
|
4 |
110 |
620 |
|
|
5 |
117 |
678 |
|
|
6 |
131 |
811 |
|
|
7 |
131 |
809 |
|
|
8 |
145 |
917 |
|
|
9 |
153 |
1,015 |
|
|
10 |
156 |
997 |
|
G |
1 |
66 |
121 |
|
|
2 |
109 |
485 |
|
|
3 |
142 |
774 |
|
|
4 |
194 |
1,351 |
|
|
5 |
250 |
2,078 |
|
|
6 |
259 |
2,157 |
|
|
7 |
272 |
2,381 |
|
|
8 |
318 |
3,119 |
|
|
9 |
260 |
2,059 |
|
|
10 |
320 |
3,028 |
Table 6. -- Results of screening phase impact tests for
surfaced specimens
|
Specimen |
Drop |
Peak |
HIC |
|
SA |
1 |
64 |
294 |
|
|
2 |
72 |
322 |
|
|
3 |
79 |
399 |
|
SB |
1 |
58 |
231 |
|
|
2 |
60 |
294 |
|
|
3 |
69 |
362 |
|
SC |
1 |
72 |
241 |
|
|
2 |
57 |
252 |
|
|
3 |
52 |
246 |
|
SD |
1 |
55 |
229 |
|
|
2 |
67 |
306 |
|
|
3 |
64 |
248 |
|
SE |
1 |
63 |
277 |
|
|
2 |
53 |
241 |
|
|
3 |
53 |
212 |
EWF Density and Compaction
Cushioning capacity of all test specimens was reduced by
successive impacts. This effect was particularly evident for
specimens A, F, and G, which were subjected to 10 drops.
The level of compaction (density) had a marked effect on
cushioning effectiveness (Figs. 5 and 6). Specimens A and G
represented the extreme range of densities evaluated with
hammermilled fiber. Specimen A, red maple, was the most
dense (292.8 kg/m3, 18.3 lb/ft3); specimen F, compacted
juniper, in the middle density range (148.8 kg/m3, 9.3 lb/ft3);
and specimen G, uncompacted juniper, the least dense
(100.8 kg/m3, 6.3 lb/ft3).
Specimen F exceeded HIC performance criteria on drop 9.
Specimen G exceeded maximum allowed deceleration
in drops 5 through 10 and maximum allowed HIC in
drops 4 through 10. Successive impacts had the following
general effects on cushioning:
- The mat was successively compressed and EWF was
progressively forced from the impact area. - Cushioning behavior continued to change significantly,
especially after the third impact. - Cushioning behavior began to stabilize after about
10 impacts. - Initial drop impact parameters were rather insensitive to
level of compaction and became more sensitive with
succeeding drops.
Table 7. -- Results of scale-up phase impact tests for
surfaced specimens
|
Specimen |
Drop |
Peak |
HIC |
|
J |
1 |
50 |
248 |
|
|
2 |
63 |
298 |
|
|
3 |
72 |
347 |
|
K |
1 |
47 |
209 |
|
|
2 |
65 |
325 |
|
|
3 |
69 |
406 |
|
L |
1 |
67 |
304 |
|
|
2 |
68 |
350 |
|
|
3 |
63 |
357 |
|
M |
1 |
73 |
308 |
|
|
2 |
67 |
332 |
|
|
3 |
62 |
315 |
|
N |
1 |
46 |
171 |
|
|
2 |
48 |
227 |
|
|
3 |
60 |
319 |
|
0 |
1 |
55 |
239 |
|
|
2 |
56 |
312 |
|
|
3 |
60 |
324 |
|
P |
1 |
45 |
166 |
|
|
2 |
52 |
256 |
|
|
3 |
60 |
289 |
|
Q |
1 |
51 |
199 |
|
|
2 |
55 |
236 |
|
|
3 |
54 |
248 |
|
R |
1 |
46 |
211 |
|
|
2 |
53 |
213 |
|
|
3 |
62 |
251 |
|
S |
1 |
52 |
235 |
|
|
2 |
52 |
244 |
|
|
3 |
57 |
272 |
|
T |
1 |
48 |
208 |
|
|
2 |
62 |
268 |
|
|
3 |
72 |
326 |
|
U |
1 |
45 |
207 |
|
|
2 |
56 |
242 |
|
|
3 |
60 |
229 |
Table 8. -- Results of scale-up phase impact tests for
unsurfaced specimens
|
Specimen |
Drop |
Peak |
HIC |
|
W |
1 |
64 |
227 |
|
|
2 |
89 |
389 |
|
|
3 |
89 |
397 |
|
X |
1 |
48 |
195 |
|
|
2 |
68 |
274 |
|
|
3 |
76 |
307 |
|
Y |
1 |
39 |
60 |
|
|
2 |
100 |
509 |
|
|
3 |
119 |
652 |
|
Z |
1 |
44 |
51 |
|
|
2 |
83 |
334 |
|
|
3 |
96 |
448 |
|
XX |
1 |
51 |
235 |
|
|
2 |
69 |
310 |
|
|
3 |
75 |
322 |
Compaction of EWF is a major factor in impact testing. Our
overall impression was that the initial impact drop was gen-
erally better by the uncompacted specimen and successive
impacts reduced cushioning performance when compared to
that of compacted specimens. However, all tested configura-
tions passed the F1292 performance criteria on the second
and third drops (Tables 5 to 8). Moisture content of EWF
was found to have a mild effect on cushioning performance.
Although we cannot establish the significance of moisture
content because of the small number of specimens tested,
specimens with high moisture content generally provided
better cushioning than did specimens with lower moisture
content.
Figure 5.HIC values for three densities of EWF.
Specimen A is red maple; specimen F, compacted
juniper; and specimen G, uncompacted juniper.
Figure 6.Maximum deceleration (G) values for three
densities of EWF. Specimen A is red maple; specimen F,
compacted juniper; and specimen, G uncompacted
juniper.
Stiffness and Elasticity
We observed different impact behavior for full-depth speci-
mens depending on the type of surfacing. Compared with
unsurfaced EWF, silicone-stabilized surfaces .bounced. or
rebounded after impact (Fig. 7). Latex produced similar
results, but to a lesser degree. Polyurethane produced a
brittle fractured surface like a fractured eggshell (Fig. 8). For
unsurfaced specimens and more elastic silicone binders, the
plot of deceleration as a function of time showed a smooth
curvilinear rise to maximum in 15 to 20 µs (Figs. 9 and 10).
For brittle binders, the plot showed two distinct peaks and
the rise to maximum deceleration was 5 to 10 µs (Fig. 11).
Silicone and latex binders retained resilient and flexible
behavior after curing, which produced the elastic rebound
effect. These resilient surface layers began to deteriorate
after several impacts. Because of weak interparticle bonding,
the surface layer did not absorb significant energy upon
failure. Conversely, the polyurethane binders absorbed
energy by way of two mechanisms, fracture of the bonded
surface and cushioning of the underlying EWF layer.
The bonded surface fractured progressively with each
succeeding impact.
Figure 7. -- Typical silicone stabilized specimen after
three impact events.
Figure 8. -- Polyurethane stabilized specimen after
three impact events.
The importance of these two behaviors (surface fracture and
underlayer cushioning) was also observed using the decel-
eration and HIC parameters. Specimens with an elastic
binder initially provided better cushioning. However, as the
surface deteriorated with subsequent drops, the behavior of
these specimens was similar to that of baseline unsurfaced
EWF. Initially, the brittle binders provided slightly poorer
cushioning (that is, were harder); with subsequent impacts,
their behavior was similar to that of unsurfaced EWF. In the
section on accessibility, we will discuss the implications of
reduced cushioning performance on accessibility evaluations
of these brittle surfaces.
Figure 9. -- Deceleration as a function of time for
unsurfaced EWF.
Figure 10. -- Cushioning behavior of silicone stabilized
EWF specimen (R).
Figure 11. -- Cushioning behavior of polyurethane
stabilized EWF specimen (M).
In preliminary tests, the two interfacial enhancements (perfo-
ration and geotextile reinforcement) had little effect on
cushioning performance of full-depth specimens. Nonethe-
less, the geotextile kept the surface layer contiguous during
the impact tests and effectively reduced the loss (movement)
of EWF from the impacted area. At the end of impact test-
ing, significant adhesion remained between the geotextile
and the bonded surface layer.
Accessibility
Table 9. -- Results of rotational penetrometer test
on surfaced and unsurfaced specimensa
|
Specimen ID |
Firmness |
|
Stability |
|
|
|
(in.) |
(%) |
(in.) |
(%) |
|
|
|
Surfaced |
|
|
|
C3 Carpet |
0.32 |
(0.03) |
0.37 |
(0.02) |
|
C3 Carpet |
0.18 |
(0.01) |
0.20 |
(0.01) |
|
J |
0.58 |
(0.06) |
0.77 |
(0.11) |
|
K |
0.46 |
(0.06) |
0.58 |
(0.06) |
|
L |
0.26 |
(0.02) |
0.31 |
(0.01) |
|
M |
0.26 |
(0.03) |
0.30 |
(0.03) |
|
N |
0.50 |
(0.05) |
0.57 |
(0.07) |
|
O |
0.49 |
(0.03) |
0.59 |
(0.04) |
|
P |
0.51 |
(0.01) |
0.57 |
(0.05) |
|
Q |
0.44 |
(0.04) |
0.47 |
(0.04) |
|
R |
0.44 |
(0.04) |
0.59 |
(0.05) |
|
S |
0.42 |
(0.08) |
0.51 |
(0.12) |
|
T |
0.55 |
(0.05) |
0.84 |
(0.12) |
|
U |
0.51 |
(0.06) |
0.59 |
(0.07) |
|
|
|
Unsurfaced |
|
|
|
W (Control) |
0.49 |
(0.02) |
1.00 |
(0.07) |
|
X |
0.44 |
(0.03) |
1.01 |
(0.12) |
|
XX |
0.69 |
(0.06) |
1.23 |
(0.18) |
a Present test criteria for stability and firmness are
expressed in inches. 1 in. = 25.4 mm. Values in
parentheses are standard deviation.
A rotational penetrometer was used to test carpet calibration
samples, surface-stabilized EWF specimens, and unsurfaced
EWF specimens (Table 9). Results from calibration samples
indicated that average C3 values were comparable to previ-
ously reported C3 carpet readings. Average C3 carpet values
were within 10% and average C1 values were approximately
20% lower than those reported by Axelson and Chesney
(1999). We are unable to explain or correct the difference in
measurements for these identical sets of carpet materials.
Unsurfaced Specimens
In accessibility tests, compacted EWF specimens were rated
as moderately firm and moderately stable (Table 4). These
results are generally parallel to the findings of Axelson and
Chesney (1999). The uncompacted EWF specimens, Y and
Z, could not be tested because their resistance to the rota-
tional pentrometer did not approach the lower limit of the
mechanical range of this device. Both of the noncompacted
specimens would therefore be considered not firm and not
stable.
Surface-Stabilized Specimens
Only specimens L and M were rated as firm. Specimens J,
N, P, T, U, and XX were rated as not firm, and the remaining
specimens were rated as moderately firm (Figs. 12 and 13).
Figure 12. -- Rotational penetrometer firmness of various bonding systems. Vertical bars indicate
standard deviation.
Figure 13. -- Rotational penetrometer stability of various bonding systems. Vertical bars indicate
standard deviation.
Note that unsurfaced specimens W (EWF control) and X
were also rated moderately firm. In stability measurements,
specimens L, M, Q, and S were rated as stable, XX as not
stable, and all others as moderately stable.
The stability of most stabilized EWF specimens was gener-
ally good. The only material that showed no improvement
was the surface-coated latex Soil-Sement specimen (XX).
The binder remained tacky and did not penetrate to bind the
deeper EWF particles. Thus, when the rotational pentrometer
was turned 360°, the caster created a shear plane, which
caused a large section of the top layer to pull away from the
underlying EWF.
The firmness and stability of the polyurethane-stabilized
specimens (L and M) made with ReacTITE were signifi-
cantly better than that of the other test materials. The Reac-
TITE binder typified the performance requirements sought
for the two incongruous performance tests of impact cush-
ioning and reduced work for accessibility. Impact cushioning
requires a soft surface and the accessibility test a hard sur-
face. Specimens L and M had a thin surface layer underlaid
by soft EWF. Upon impact, the thin shell was broken and
significant energy was absorbed in the process. After the
impactor had broken through the hard shell, the behavior of
the specimen reverted to that of a full-depth EWF specimen.
The optimal performance of a surface system requires a
balance between stiffness and elasticity. The surface layer
must have adequate stiffness to yield a .stiff plate on an
elastic medium. response. The system must be stiff enough
to resist flexural fracture as well as punch-through shear by a
wheelchair caster or wheel. The .brittle eggshell. surface
effect was not as readily apparent with the other polyure-
thane system (Vitriturf) used with specimens J and K.
Vitriturf did not produce a surface as strong and stiff as that
produced by ReacTITE. The fact that the particles were not
as tightly bound for specimens J and K was also shown by
the stability rating; particles became detached by the rotating
caster.
Top coating with the latex binder (Soil-Sement) moderately
improved the stability of specimens Q and S. The latex
binder and the silicone systems (AllGuard and 3.5000) were
not consistently distinguished by the two test procedures.
Compared to unbonded EWF, silicone- and latex-stabilized
surfaces were significantly more stable. However, their
firmness ratings could not be consistently or confidently
distinguished from that of unsurfaced EWF. If firmness is
less critical than stability, then we believe that both the
silicone and latex systems are promising.
Conclusions
Compaction of engineered wood fiber (EWF) is a major
factor in impact testing. The initial drop was better cush-
ioned in uncompacted specimens, and successive impacts
decreased cushioning performance when compared to that of
pre-compacted specimens. However, all configurations
(surfaced and unsurfaced) passed the standard impact per-
formance criteria on the second and third drops. The EWF
moisture content was shown to have a mild effect on cush-
ioning performance. Although we cannot establish the sig-
nificance of moisture content because of the small number of
specimens tested, the higher moisture content specimens
provided better cushioning behavior.
Binders generally stabilized EWF specimens. The two poly-
urethane stabilized surface binders significantly improved
firmness and stability. This binder seems capable of bridging
the two performance tests, one of which requires impact
cushioning and the other reduced work (as indicated by
stability and firmness) for accessibility. The thin surface of
polyurethane-stabilized specimen is capable of supporting a
wheelchair caster but it breaks through upon impact, absorb-
ing significant energy in the process. Both the latex binder
and the silicone systems were significantly more stable than
were unbonded EWF, but inferior, in our opinion, to the
polyurethanes. The firmness of latex and silicone systems
could not be distinguished from that of unsurfaced EWF.
Recommendations
We recommend further study of systems that utilize easy-to-
process EWF/binder combinations. In our study, many com-
binations yielded promising test results for surface energy
absorption and resiliency and appeared to improve accessi-
bility significantly, with continued optimization of perform-
ance likely.
A logical next phase, now underway at the Forest Products
Laboratory, is to develop larger scale and longer term proto-
type surfaces for outdoor testing using our preliminary re-
sults to select stabilizing binder systems.
We need to engage the EWF and adhesive resin industries
for assistance in developing further information on the stabi-
lized EWF concept. This information would include lower
cost binder systems, with mid-range elasticity and good
extensional behavior, for reducing biodeterioration.
We acknowledge the experimental nature of this investiga-
tion. The concept needs to be made more feasible and practi-
cal in terms of performance and costs. To ensure reliable
surfacing systems we need to improve our understanding of
the effects of EWF decay and durability on impact behavior,
accessibility, maintenance, and cost.
Literature Cited
ASTM. 1995. Standard test method for shock-absorbing
properties of playing surface systems and materials. ASTM
F355.95, Vol. 15.07. West Conshohocken, PA: American
Society for Testing and Materials.
ASTM. 1997. Provisional standard specification for deter-
mination of accessibility of surface systems under and
around playground equipment. ASTM PS 83.97, Vol. 15.07.
West Conshohocken, PA: American Society for Testing and
Materials,
ASTM. 1999a. Standard specification for impact attenuation
of surface systems under and around playground equipment.
ASTM F1292.99, Vol. 15.07. West Conshohocken, PA:
American Society for Testing and Materials.
ASTM. 1999b. Standard specification for determination of
accessibility of surface systems under and around play-
ground equipment. ASTM F1951, Vol. 15.07. West Con-
shohocken, PA: American Society for Testing and Materials.
ASTM. 2001. Standard specification for engineered wood
fiber. ASTM F2075.01, Vol. 15.07. West Conshohocken,
PA: American Society for Testing and Materials.
Axelson, P.W.; Chesney, D. 1999. Accessible exterior
surfaces technical report. Washington, DC: United States
Architectural and Transportation Barriers Compliance
Board.
U.S. Architectural and Transportation Barriers Compli-
ance Board. 1991. Americans With Disabilities Act (ADA):
Accessibility guidelines for buildings and facilities. 36 CFR,
Pt. 1191. Washington, DC: State and local government
facilities.
U.S. Architectural and Transportation Barriers Compli-
ance Board. 1994. Surfaces. Bull. 4. Washington DC: 1.8.
U.S. Consumer Product Safety Commission. 1998.
Handbook for public playground safety. Pub. 325.
Washington, DC.