Tender Mixes

“Tender” (slow-setting) mixes, also known as “tender” pavements, comprise a major problem faced by the asphalt paving industry. Many factors contribute to pavement tenderness, and in a pavement exhibiting tender characteristics several of them will be present at the same time.

A tender or slow-setting asphalt pavement is defined as one that scuffs under horizontally-applied shearing loads after compaction has been completed such as when power steering turns are made by a stationary vehicle.

Interestingly, complaints about tender or slow-setting asphalt pavements in the United States always arise at about the same time of year, regardless of location, i.e., from about the first part of July through the middle of September. At this time of year ambient temperatures are high. Because one rarely encounters a tender pavement in cool weather, it seems obvious that one of the conditions that must be obtained for this type of distress is hot weather. A paper presented in 1963 by Hveem, Zube & Skog notes that no tenderness problems occurred when ambient temperatures were below 80°F.

A second condition almost invariably noted in tests of tender pavements is relatively low mix density. Rarely will a mix that has been compacted to high density in a pavement exhibit tenderness. On the other hand, some mixes compacted to relatively low density may not exhibit tender characteristics either.

A third condition usually noted is that tenderness in a mix is likely to be more severe when the asphalt used is of lower viscosity. In other words, a harder or more viscous asphalt when present in a given mix will tend to result in a reduction of tenderness. The viscosity of the asphalt alone may not be the only consideration; the amount and kind of material passing the No. 200 sieve must be taken into account in conjunction with the asphalt viscosity. The combination probably contributes to the “mass viscosity” of the mix and its cohesion, which affect resistance to both punching shear loads and horizontal shear forces. AR8000, instead of AR4000, is often used in the summer months due to its increased hardness in high temperatures.

A fourth condition that reveals pavement tenderness as a problem is a critical load. Normally the average rolling load of a vehicle wheel will not be critical. But high-intensity unit stress such as that developed under the steel wheel of a truck-trailer parking dolly, a woman’s high-heel shoe with a small cross-sectional area, or the legs of tables or benches having small contact area, will punch holes in the pavement. Severe horizontal shear forces developed under standing tires by power steering turns, severe braking of a rolling wheel, or rapid acceleration or spinning of a wheel may cause scuffing of the pavement surface.

A phenomenon of the tender pavement is that its tenderness shows up during or immediately after construction and normally disappears as a problem within six months. Rarely will tenderness in a pavement continue after this period of time. Similarly, a tenderness complaint is practically never heard for the first time about a pavement that is six months old or older. The reasons given for this also vary, but probably the principal one is that the pavement toughens with age and traffic use. The toughness may increase because of some oxidation of the asphalt on the pavement surface. The kneading action of pneumatic tires tends to knit the surface particles of the pavement more closely, resulting in greater tensile strength at the surface and increased resistance to horizontal shearing forces. Usually after a couple of months when weather conditions have moderated and ambient temperatures are lower, one of the major contributing factors to the problem disappears.