A tender mix is an internally unstable mix that tends to displace laterally and shove rather than compact under roller loads. Internal mix stability is a function of asphalt binder viscosity and aggregate gradation and shape. Mix tenderness is usually one of two varieties (TRB, 2000):
- High temperature tenderness. At high temperatures some mixes may not be viscous enough to support roller loads without laterally displacing. Since this occurs at high temperatures, it is most often encountered by the breakdown rollers when near the paver. Mats exhibiting high temperature tenderness are typically compacted by waiting for the mat to cool to a point such that its viscosity is high enough to support roller loads without lateral displacement. Usually this is enough to solve the problem but in rare instances the wait period may be so long that the mix cannot be adequately compacted before cessation temperature is reached. In these cases, a new mix design may be warranted. Mixes that exhibit high temperature tenderness are often susceptible to rutting later in life.
- Mid-range temperature tenderness. Some mixes are fine at high temperatures but are unable to support roller loads without laterally displacing at mid-range temperatures (typically between 115°C (240°F) and 90°C (193°F)). The “tender zone” is a term generally associated with this mid-range temperature tenderness. The most commonly described mechanism for this type of tenderness is described as follows: when HMA is placed, its temperature is relatively hot and uniform throughout. During compaction, the top and bottom layers cool more rapidly due to their higher rate of heat transfer to the environment. The middle layer, which remains relatively hot, is less stiff than the surrounding top and bottom layers. When rolled in this condition, a steel wheeled roller has a tendency to push the top portion of the mat laterally past the bottom portion of the mat using the middle portion as a lubricating layer. Additionally, the HMA is still fluid enough in the middle portion of the mat to allow the roller to sink into the mat, thus forming a small wave in front of the roller. Several different techniques can be used to compact a mat exhibiting mid-range temperature tenderness. First, if done quickly and efficiently, breakdown rolling can be completed before the mat reaches the tender zone. A pneumatic tire roller can then be used with some effectiveness while the mat is in the tender zone. Finally, the static steel wheel finish roller should be kept off the mat until it has passed through the tender zone.
No matter the temperature at which they occur, tender mixes exhibit excessive lateral displacement and shoving when subject to roller loads. Several compaction options have been developed to deal with tender mixes although the best solution is typically a mix redesign.
Tender mixes are most typically caused by (Brown et al., 2000):
- Excessive moisture content. Excess moisture can come from inadequately dried aggregate or, in the case of an overlay, moisture on the existing pavement surface. This moisture decreases the internal mix strength by (1) increasing the liquid content of the mix and (2) being converted to steam, which greatly increases the volume of moisture and tends to push aggregate particles apart as the mix is being compacted.
- Excessive light ends in the asphalt cement. Some asphalt cements have light ends that may vaporize and be driven from the mix at about 150 – 175°C (300 – 350°F). These light ends will have much the same effect as moisture at high temperatures on compaction of the mix.
- Excess asphalt binder content. Asphalt binder lubricates the aggregate during compaction and therefore, mixes with high asphalt content will compact easily but may shove under roller loads (TRB, 2000).
- Rounded aggregate particles. Rounded particles, sometimes found in sands and gravels, tend to slip by one another during compaction causing distortion and shoving during compaction.
- Excess midsize fine aggregate (between the 0.60 and 0.30-mm (No. 30 and No. 50) sieves). This can be a result of mix design or HMA manufacturing.
- Insufficient fines (aggregate passing the 0.075-mm (No. 200) sieve). In HMA, fines become mixed in with the asphalt binder and provide a certain amount of stiffness to the asphalt binder. Therefore, insufficient fines can result in a binder that is too fluid. Generally, mixes with a more uniform gradation are more likely to be tender than those with a more dense gradation.
- Poor bonding to the existing pavement (for overlays). If an overlay is poorly bonded to the existing surface it may act tender as it displaces laterally rather than compacts under roller loads. A poor bond can result from (1) an extremely smooth, oxidized underlying pavement, (2) a raveled underlying pavement, (3) not enough tack coat or (4) too much tack coat.
- Excessive mix temperature. At excessively high temperatures, the asphalt binder may not be viscous enough to support compaction. In some cases, even though the surface has cooled the center of the mix can still be too hot to support rollers. Mixes should generally be placed at an appropriate compaction temperature so rolling can begin immediately behind the paver.
- Compaction techniques. Poor compaction techniques can exacerbate tenderness problems. Quick stops and starts by a steel wheeled roller tend to cause lateral displacement. Operating a steel wheel roller with the drive wheel on the rear of the roller tends to increase the potential for lateral mix movement. Generally, mixes that appear tender under a steel wheel roller will appear less tender under a pneumatic tire roller.
- Contamination. Mixes contaminated with diesel fuel or other petroleum products may be tender. These petroleum products tend to thin the asphalt binder and make it less viscous.
- Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook 2000. Transportation Research Board, National Research Council. Washington, D.C.↵