Can You Use Bituminous Beds Under Granite?

Q:   I’m specifying an exterior pavement with predominantly brick pavers, but including accent bands of granite. The granite is in sizes of 16” x 24” x 3¾” (400 x 600 x 95 mm). The bricks will be set on a bituminous bed. Can I use a bituminous bed under the granite as well?

A:  Bituminous beds actually work quite well under granite. There are a few concerns that you will need to address. I had always been concerned about staining due to the oil-based bedding, although I’ve never actually seen it, so that concern is perhaps unfounded. The asphaltic bed is not quite as rigid, but given the relatively small face size and stout thickness of your pavers, I would not be worried about breakage with loads up to and including passenger vehicles. Joint fillers can be a concern, as tight joints do not work as well as they do with some other materials. In other materials, chamfers can be applied more economically, but in granite, we have to cut or grind the chamfers to each edge, so it becomes a cost issue. Without the chamfers, grains of sand migrate their way into tight joints and frequently cause spalling due to slight vertical displacements of the pavers as vehicles traverse the pavement. So it’s best to leave a joint that is swept with some type of granular filler. These joints may collect water and result in some temporary discoloration at the stone perimeter after rainfall.

The last concern is the thickness variation, which you can control in your specification.  Bituminous beds do not accommodate varying thickness well, since the installing mechanic cannot squash the paver into the bed when setting it. For slab thickness greater than 2” (50 mm) the industry tolerance is ±¼” (±6 mm). Most fabricators are much more accurate than this, but unless you specify a tighter tolerance, the industry norms will govern, and you could have two stones that differ by ½” (13 mm) with no grounds for rejection. I would encourage you to ask the suppliers in your region as to what tolerance they can achieve, and specify that. If they can’t accommodate a tolerance that is reasonable for the project, an alternative would be to sort the stones by thickness prior to palletization, so that stones on a given pallet have minimal thickness variation.  Unfortunately this adds some cost and may create color control challenges, as you could be mixing stones from many different blocks in one pallet.