The first step in the placing of concrete is to prepare the area by removing the sod and all organic debris.  Tree roots and all other organic matter must be removed, because they will inevitably decompose and leave a void that can lead to cracking or caving.  For the placement of concrete alone with no substrate (not recommended), this will require the excavation of a minimum of 4 inches with the depth of the gravel substrate requiring additional excavation.  The area to be filled with concrete then needs to be moistened, and the earth and gravel substrate compacted to decrease future settling.

Forming and Filling

Once the area is prepared, forms (wood and, perhaps, steel) will be fabricated and placed.  Forms contain and guide the poured concrete while it sets (becomes hard).  For skateparks, the concrete will be reinforced with steel rods (rebar).  Diesel fuel is sprayed onto wooden forms before the concrete is placed to act as a release agent to facilitate clean removal of the forms from the concrete.  It is best to keep the ground moist when concrete is placed.  Dry ground will draw moisture from the concrete too quickly and will inhibit proper curing.  Concrete should not be placed on a hot, dry day.  The rapid evaporation of the surface moisture makes finishing the concrete a nightmare, will not allow proper curing and will ultimately decrease the strength of the concrete.

When concrete is added to the forms, the forms are slightly overfilled and the material worked down and into the corners.  Spreading the concrete is done with a square head shovel and a muck rake.  This step is critical and must be done quickly.  Avoid overworking the concrete and trapping air bubbles.  Work the concrete only until the forms are filled completely with enough material to being screeding (see below).  Don’t worry if the concrete does not look completely smooth, because screeding, floating and finishing is yet to come.  Overworking will cause an excess of concrete to rise to the surface.  This can cause scaling (see concrete repairs in this chapter). 

Screeding

Once placed and spread the concrete is now ready for screeding.  Screeds are guides that make contact with and move across the edges of the forms to shape the surface of the concrete.  Typically screeds are flat, but for skatepark construction screeds come in a variety of shapes.  The screed is used to remove excess concrete and low spots will be immediately evident.  Low spots should be filled immediately and the section then rescreed.  Screed about 3 to 4 feet at a time.  There is no difference in procedure between screeding flat or curved areas.

Floating the concrete is next.  Floating will begin to smooth the surface of the concrete and work a slight amount of water to the top.  Use a magnesium bull float.  A bull float has a large rectangular blade that pivots at the end of a long aluminum pole.  A bull float helps to level the high spots, fill in any voids and forces the aggregate slightly below the surface of the concrete.  The bull float is pushed away from you with the front edge raised, so as not to dig into the concrete.  The blade is then brought back toward you almost flat.  Again, do not overwork the concrete.

Finishing the Concrete

Next, you will begin to finish the concrete.  Finishing results in a smooth, even surface.  The first step in finishing the concrete is to re-float the surface using a magnesium hand float.  This process is very precise, because you basically go over every section by hand.  Magnesium (mag) floats are very lightweight and will pass easily over the concrete.  Floats with an equal radial transition can be used for curved surfaces (see the section on 8-Feet Radius Floats).  Finishing the curing concrete will require the use of knee boards.  Knee boards are rectangular plastic boards that that are placed on the curing concrete for the finisher to kneel on.  The knee boards minimize the disturbance of the concrete underneath by displacing the weight of the finisher over a greater surface area. 

Continued finishing includes rounding the outside edge of the concrete.  Rounding the edges will prevent chipping when the forms are removed or from heavy impact after curing.  Rounding is done with an edging tool.  Like the bull float, the edging tool should be slightly tilted upward on the forward motion to prevent gouging the concrete, while the rear of the edging tool should be slightly elevated upon return.

Troweling is the last step of the finishing process.  Quality skateparks have a hard troweled finish.  Hand trowels are made in various sizes.  Generally they have a rectangular blade.  Pool trowels are rounded at the corners and, as the name implies, are designed for working the radial transitions of swimming pools.  Use pool trowels on radial transitions and whatever type of trowel you prefer on the flat surfaces.  During the first troweling the blade should be nearly flat on the surface of the concrete.  It is best to use a broad, sweeping arc motion, overlapping each previous stroke by ½-inch or more.  Additional troweling, called “burning” is needed for the smooth durable surface of a skatepark.  Tilting the trowel slightly during additional troweling will result in an increasingly smooth surface.  Always allow some time in between trowelings for the concrete to set up a bit more.  Gasoline powered trowels can be rented from equipment rental stores and used for the finishing of large flat slabs.  Power trowels cannot be used on curved elements.  The same basic principles are used with a power trowel as with a hand trowel.  After a few days of curing the forms can be carefully removed.  A pointing trowel is used to separate the concrete from the forms.  Slide the trowel along the inside surface of the form board.

Curing Concrete

The concrete should be kept damp and free of traffic for at least 28 days.  With the anticipation that surrounds the construction of a skatepark this may be difficult, but the slow undisturbed cure process is critical to keep cracking to a minimum.  Although hardened concrete appears to be cured within a few days the appearance is deceptive as the concrete is still quite fragile.  At a minimum the concrete should be allowed to cure for 7 days before being used, but we strongly recommend a full 28 day cure daily saturating the area with water in order to keep random cracking to a minimum.  We also find that having lawn sprinklers saturating the park during daylight hours actively discourages unauthorized use.  Your patience will pay off in the long run.

RETURN TO CONTENTS