Stabilized Engineered Wood Fiber for Accessible Playground Surfaces

Playground Study Site

An Access Board solicitation for potential study sites yielded numerous responses. Fortuitously, an accessibility coordinator for the Wisconsin State Parks offered a site close to the Forest Products Laboratory-a sand-surfaced playground at Governor Nelson State Park in Waunakee, Wisconsin.

Design

The playground was originally designed with some structural provisions for accessibility. A transfer point/platform was incorporated in the climbing structure; however, the surface leading to it was fine beach sand. Total fall height was determined to be 3.1 m (10 ft). Discussions with the park staff provided insight to the usage of this area. In response, the staff decided to retain sand on a portion of an adjacent (but not conjoining) playground. The remaining area of approximately 190 m² (2,020 ft² ) was converted to a full-depth EWF surface (Fig. 1).

Figure 1-Schematic plan of playground site at Governor Nelson State Park

Preparation of Playground Subsurface

Our efforts began by removing the existing sand surface to a depth of 0.38 m (15 in.) (Fig. 2). All roots, stones, and vegetation were removed. Much of the tonnage of sand was moved by two skid-steer loaders, but significant amount required handwork by a dedicated and hardworking volunteer crew from the Waunakee Rotary and a local chapter of Telephone Pioneers of America. The work crew also included employees of the park, the Wisconsin Department of Natural Resources, and the Forest Service. The majority of the clean sand was used to replenish the adjacent beach at the park and the remainder was piled in a wooded site nearby. Approximately 12 h of equipment time and 48 h of personnel time were required to remove the sand.

Figure 2-Removal of sand from existing playground surface. 

Installation of Drainage Base

Following industry standard EWF installation practices, we ensured that the excavated surface had a minimum of 1% slope for drainage. A lightweight landscaping geotextile fabric was placed on the surface, followed by a 0.08-m (3-in.) layer of 18-mm (3/4-in.) washed, angular drainage rock (Fig. 3). Half the rock was placed using a skid-steer loader and the other half was placed manually using wheelbarrows. All the rock was shoveled and raked by hand to a uniform depth. Another layer of geotextile fabric was laid on top of the rock layer (Fig. 4). Handfuls of rock were thrown on the fabric to keep the wind from blowing it out of place. The layers of geotextile fabric kept soil and fiber from clogging the rock and thus preserved the drainage quality of the rock layer. Approximately 25 metric tons (28 tons) of rock was used. Placing the rock and geotextile required 25 h of manual labor and 3 h of skid-loader use.

Figure 3-Placement of drainage fabric and rock on playground subsurface. 

Figure 4-Completion of drainage system; second layer of fabric laid over drainage rock. 

EWF Application

Fifty cubic meters (66 yd³ ) of EWF, donated by a cooperator (Zeager Bros. Inc.), was obtained from BNB Bedding of Oskaloosa, Iowa, and delivered in a 75-m³ (100-yd³ ) livebottom trailer (Fig. 5). The EWF was manually applied to a thickness of 0.3 m (12 in.). One week later, after the surface had been further compacted by usage, approximately 40 m³ (53 yd³ ) of EWF was added and compacted to return the surface to the full depth of 0.3 m (12 in.).

Figure 5-Application of engineered wood fiber (EWF). 

Bonded Surface Installation

Two weeks after applying the EWF, we returned to stabilize the upper surface. Considering that children had used the playground in the meantime, we had hoped the EWF was adequately compacted to support the stabilized layer. Our plan was to treat approximately 30% of the playground with the two binding systems and to leave the remainder as the untreated control (Fig. 1). The two binder systems used to fabricate these systems were

1. an acrylic and polyvinyl acetate polymer emulsion, SoilSement (Midwest Industrial, Canton, Ohio), mixed 30% by dry weight of solids to unit weight of dry EWF and applied 63 mm (2.5 in.) thick, and

2. 2. a non-foaming polyurethane (Vitricon), Vitri-Turf (Polymer Plastics Corp., Commack, New York), mixed 30% by weight to unit weight of dry EWF and applied 37 mm (1.5 in.) thick. 

If the EWF has 20% moisture content, that weight should be subtracted from the EWF weight prior to calculating the weight addition of the binder. The same procedure should be followed for the binder that does not contain 100% solids. The weight percentage should be calculated only on the solids content of the binder. Because the EWF was installed in the fall, we monitored the air temperature; both stabilizing binders required 4°C (40°F) for proper curing. On the date of installation, the overnight temperature had dipped to −2°C (28°F). The crew waited for the temperature to rise before mixing the EWF with the binders, which had been stored at room temperature. When the EWF was mixed with the binders, the temperature of the resultant mixture was well above 10°C (50°F).

A portion of EWF was removed from the play area for stabilization. For the polyurethane binder, 38 mm (1-1/2 in.) of EWF was removed and for the emulsion polymer, 64 mm (2-1/2 in.) of EWF. The EWF was placed in a 160-L (40-gal) portable mortar mixer (Fig. 6). The amount of binder added was determined as a proportion (30%) of EWF dry weight (volumetrically equivalent to 0.041 m³ , 1.45 ft³ ) to 5.3 L (1.25 gal) of Vitri-Turf or 10.6 L (2.5 gal) of SoilSement. Weight proportion was 77:23. The EWF and binder were mixed for approximately 3 min. The mixture was transported by polyethylene tray wheelbarrows to the target pad and spread with hand tools to an even thickness (Fig. 7).

Figure 6-Mixing of binder and EWF in mortar mixer. 

Figure 7-Leveling and compaction of binder-EWF mixture. 

The area was then compacted and flattened with a 1.2-m by 1.2-m by 16-mm (4-ft by 4-ft by 5/8-in.) piece of plywood covered with a polyethylene release sheet. To compact the cushioning pad to the full 0.3-m (12-in.) depth required for unbonded EWF, a 90-kg (198-lb) person slowly stepped on the plywood in each quadrant, applying firm pressure.

The two SEWF surfaces were allowed to cure or bond for 6 days prior to usage. The entire surface was covered with polyethylene sheeting for 3 days to protect it from rain. Within 2 h of placing the Vitri-Turf, the surface was somewhat rigid to slight hand pressure. The Soil-Sement surface did not begin to cure or cross-link until more than 48 h had passed; when the polyethylene sheeting was removed, the surface was still slightly tacky. The area was left open to the air for another 3 days prior to opening the play surface for use. Figure 8 shows the completed surface, with little notable differences between the three surface materials.

Figure 8-Completed playground looking north: left, Vitri-Turf SEWF; right, Soil-Sement SEWF; top, EWF. Line of demarcation is below wheelchair footrest.