APAI Article Archive

(APAI Newsletter, May 2004)

HMA Notes
By Bob Nady, P.E.

Resurfacing … HMA or UTW (Ultra Thin Whitetopping)? Let’s discuss the Engineering properties of each material first.

The tensile strength of Portland Cement Concrete (PCC) is in the range of 1/7 - 1/10 of the compressive strength of a typical concrete mix. In buildings, this lack of significant tensile strength is compensated for by using steel reinforcement in the tension areas of the building members; i.e., beams, columns, and floor slabs. A balanced design is thereby achieved, taking advantage of the tensile strength of steel to make up the deficiency of concrete in tension.

Performance in pavements is no different. PCC is still weak in tension. Tension is the enemy of paving slabs because it is difficult to provide tensile steel in normal paving slabs. It is virtually impossible to provide this for UTW slabs. Simple engineering mechanics analysis shows that for a typical UTW slab (4 feet by 4 feet with thickness ranging from 3 - 4 inches) can have tensile stresses high enough to cause failure in tension. A simply supported slab will fail under a wheel load applied by one ESAL of legal limit intensity.

Of course, UTW, according to current design suggestions, is placed on a milled asphalt surface. It is milled to provide mechanical bond to the asphalt, thus, preventing a simply supported condition. But, is it? Slab curling can produce at least a partial unsupported area under the slab. Curling, caused by daily heating and cooling of the slab can raise the edges at night and raise the interior portions by day. Wetting and drying during wet weather intervals can produce the same results. Now, if the slab is in complete intimate contact with the substrate, simple linear shrinkage can cause tensile stresses. Since the slab is restrained from moving (which will prevent the stresses from being relieved). Is UTW technology between a rock and a hard spot?

It is also well known that stresses can combine in slab members. Thus, curling, wheel loads, and shrinkage can all combine to raise tensile stresses in the layer where these stresses can cause failure. UTW mix design information suggests that synthetic fibers should be used in the mix. But fibers can only hold the slab pieces together after it has cracked. Fibers cannot mitigate tensile stresses to any significant degree.

It is interesting to note that curling from daily temperature cycles can also induce fatigue into the system. Fatigue is a factor in PCC pavement thickness design methods. However, once the slab has cracked, the fibers can only prevent the crack from opening farther; they cannot eliminate the crack. This is in sharp contrast to HMA, which has documented healing properties if a crack does occur. Upon adding up the facts, the following points are noted:

* Stresses caused by slab curling, traffic loading, and material shrinkage can induce significant tensile stresses in a rigid material used for UTW overlays.

* Combined stresses can exceed the strength of UTW slabs, causing failure observed as cracking

* Fibers can only hold the cracked slab together for a while; they cannot eliminate cracking. The cracks cannot heal.

* The American Concrete Paving Association (ACPA) published an instructional brochure devoted to the repair and replacement of UTW just a short few years after the very first UTW trial project was built.

* Literally thousands of lane-miles of HMA resurfacing exist in Iowa. Many of which have been in service for over 18 years or more (3-inch thickness and a few 4½ inch projects) none of which show any of the failure signs noted in the few UTW projects.

So, when it comes to resurfacing; HMA or UTW? It’s a no-brainer.

Hot Mix Asphalt, smooth, quiet, economic, and durable.