Fun boxes and similar obstacles are some of the easiest to form and pour.  These obstacles are basically combinations of squares and rectangles stacked one on top of another with the occasional flat bank thrown in for variety.  Rails can be configured and attached in any way desired.  The strength of the forming is most critical.  The walls must be strong in order to withstand the substantial pressure of the poured concrete.  It is better to go overboard on the bracing than to have a “blow out” during the pour.  If desired, the obstacles can be poured solid, but this will use a lot of concrete.  It is more economical to build up the substrate with compacted earth and gravel.  Some individuals may try to tell you to use hay bales in the center of the obstacle and to pour the concrete around them.  Although we have never tried it, we do not recommend this procedure. 

Build the forms for the fun box or similar obstacle from ¾-inch plywood.  Since you will be pouring the obstacle before the slab, add 4 inches for the depth of the concrete.  Position the forms with the good face of the plywood inward.  Assemble the forms with screws so that they can be removed easily after the concrete has set.  Reinforce the plywood with 2 x 4 stiffeners at least every 2 feet.  Place the horizontal stiffeners first and then fasten vertical stiffeners across them.  Brace the vertical stiffeners with 2 x 4 strong-backs secured to the ground with stakes.  Drive stakes on the outside of each form at least every 12 inches.  Be sure the forms are at right angles and plumb, with the tops level.  Any number of subsequent levels can be formed upon the first level in a manner similar to building stairs.  Simply run sections of 2 x 6 across the top of each lower level to support the forming above it.  Hang sections of rebar tied on 1-foot centers 2 inches below the top of each level of the obstacle. Suspend these sheets of rebar from the corners of the forms with wire.  Coat the inside surfaces with diesel fuel so that the forms will release cleanly. 

When pouring the obstacle, pour concrete into the forms up to the height of the first level.  Work the concrete slightly with a shovel or muck rake and tap the sides of the form to eliminate air bubbles.  Wait until the first level sets a bit before pouring the next one to make sure that it can sustain the weight of the concrete above it.  If there are to be any embedded handrail anchors add them now.  Follow the manufacturers instructions.  When the concrete has set enough to hold its shape, finish the concrete as usual.  Remove the forms, smooth the faces, trim up the edges with an edging tool and round the junction where the different levels join. 

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