Mount Structures Directly to Asphalt

 
The SP14 is a medium-duty anchor rated at 600 lb. pull resistance.  The galvanized steel anchor is installed flush to the surface of the asphalt; its female thread accepts 3/8" bolts.  The structure to be attached to the ground is mounted using 3/8" bolts.

To install, a 3/4" hole is drilled in the asphalt, to a depth of 14".  The hole is filled with a special grout (EPX2), and the anchor is dropped into the grout so that its top is flush with the asphalt surface.  (See full instructions.)  Within minutes the anchor can be used to mount the structure.

We are often asked about the engineering ratings for the SP14 asphalt anchors.   The users need to know the pull, and the sideways force, that the installed anchors can withstand.

We have run tests in a 30 year old, 3" asphalt parking lot with 4" compacted fine gravel underneath. With the criteria being that the anchors fails if it starts moving vertically, we found that the SP14 (installed in a 3/4" hole with EPX2 grout) can resist pull forces in excess of 700 lb.  When using a stone-cement cartridge adhesive, the pull force was limited to about 250 lbs.

When trying to specify the minimum forces, our concern is the variability in the asphalt surfaces and their foundation.  Asphalt contractors vary in the methods they use, as well as in the quality of the asphalt installation that is contracted for.  A typical installation starts with a level ground, then a layer of aggregate 3 - 8 inches is applied.  Some contractors compact the aggregate, some do not.  The size of the aggregate's gravel can vary dramatically. Likewise, the type of asphalt used, its depth and its age, as well as how well it was compressed are all variables that have a significant impact on the strength of the bond between the grout and the roadway.

For a comprehensive guide to asphalt paving, see this document.

There is one parameter that can be specified, and that is the breaking point of the bolts that is used to attach the devices to the anchors.  Our SP14 has a M8 or 3/8" thread.  3/8" steel bolts Grade 5 are rated to withhold safely 1,300 lb. (600 Kg) in a pull direction.  They will withstand a higher force in the shear direction.  (None of the industry standards specifies a shear force rating for bolts.)  

GROUT: To get the best holding force and durability, anchors installed in asphalt need to be embedded in a special cement mixture, referred to as "grout".  The grout must be cement-based and must expand as it cures.  Fast curing is a distinct advantage, as the installation can proceed within minutes of the sinking of the anchor in the ground.  The grout should be self-leveling (meaning that it pours easily and will fill all the crevices in the hole) immune to standing water.  AAG's EPX2 is a 14 oz prepackaged grout.  Just add 2 oz of water and knead, then pour right out of the re-lockable bag.

ADHESIVE:  To glue a plate to asphalt, you need a special adhesive that is strong yet flexible, will not self-level (meaning that it will not run outside the area where you apply it), can handle UV (sun) and is easy to apply.  AAG's EPX1 is such a material that has been tested and withstood the test of time.  Do not use the EPX1 as grout.

Anchoring any device to an asphalt pavement is quite different from anchoring to concrete surfaces.  Asphalt in made of small aggregate stones, held together by the bitumen blacktop.  Over time, or under continuous pressure, the bitumen flows, and the surface yields.

There are 4 ways to permanently attach devices to asphalt:

Adhesive:  This is the simplest way, but it may not work reliably on older surfaces; it will also offer a limited force resistance.  The failure mode of gluing metal plates to asphalt is that the asphalt layer close to the surface peels off and the plate comes away with about 5mm (1/4") of asphalt.  This does not happen overnight, and clearly depends on the quality of the surface as well as on the forces involved.  As a rule, using adhesive requires a large area of contact between the structure and the asphalt.

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