Tender Asphalt Mixes - Causes and Cures
Posted by Graniterock on Mar 18, 2015
A“tender” asphalt mix usually refers to a mix that is difficult to compact. It may have a tendency to shove under the roller wheels and/ or leave longitudinal cracks at the edge of the steel drums. Either a lack of friction between particles or a lack of shear strength in the mix essentially causes tenderness. Both these problems can be the result of many factors, the most common of which we will discuss.
1. Particle shape and texture
Aggregates that are round and/or smooth create less friction and slip under shear stress. The effect is similar to coating marbles with oil and then trying to compact them. To avoid this problem, utilize crushed aggregates whenever possible.
Sand size particles (even from some crushed aggregate sources) tend to be rounded and act as ball bearings. Sand size is loosely defined as material passing a #8 sieve. Rounded sand particles may allow larger crushed aggregates to roll within the mix structure. Likewise, mixes with natural sand almost ensure tenderness in that the small particles are naturally round. Not that natural sand cannot be used in asphalt mix, but a close eye needs to be on how much is used and the effect on the overall gradation. FHWA0.45 power Gradation Chart is a useful tool when establishing a mix blend.
While charting the blended gradation, a “hump” at or near the #30 sieve in the gradation line will be a good indicator of mix tenderness. Natural sand tends to have a “surge” of #50 material, which causes a high percent passing the #30. This high percent passing the #30 causes the “hump”. It is like throwing a handful of very small marbles in the mix. Limit natural sand use when possible, and utilize the fine aggregate angularity test to ensure your fine particles are not excessively rounded. Also, in terms of gradation, low percent passing the #200 will contribute to tenderness. Mineral fillers fill in the gaps in the mix. They combine with liquid asphalt to add shear strength to the mix as well as adhere to coarser particles to increase friction. When designing, ensure a good dust to asphalt ratio (% passing the #200 divided by the % asphalt content). Most agencies specify a dust to asphalt ratio in the area of 0.6 to 1.2.
Try not to blend too close to the lower limit.
Liquid Asphalt (Binder) Properties:
Viscosity is a material’s resistance to flow. A low viscosity binder in a mix will contribute to mix tenderness. The stiffer the binder higher viscosity), the more shear strength of the binder and mix. From a contractor’s perspective, binder grade is usually selected for you, particularly on DOT or County projects, but if possible, selection of a stiffer binder can help.
Too much binder in a mix contributes to excessive fluidity, causing the aggregates to float around. Low shear strength and low friction are by products of high binder contents. Proper selection of binder content during design, and attention to binder content during production, will alleviate this problem.
Sometimes design parameters are inflexible, or locally available materials limit the workability of these suggestions. A mix that has been designed inherently tender can still be properly worked in the field. Likewise, oftentimes a mix is labeled “tender” when in truth it is not. Improper field operations can make a stiff mix seem tender, and on the flip side, proper field operations can yield a stiff mix from an inherently tender mix. Next, we will examine field operations.
Field Construction Operations
1. Temperature Zones
Proper compaction is the single most important factor affecting longevity and performance of pavement. Our experience has yielded an idea of “Temperature Zones”. There are three critical zones: 1) The Hot Zone—above 250 degrees F. 2) The Tender Zone—between 190 and 250 degrees F. and 3) the Cool Zone— below 190 degrees F. A stable, stiff mix will seem tender if you try to achieve compaction in the Tender Zone.
A tenderly designed mix can be compacted if you avoid compaction efforts in the Tender Zone. The Hot Zone provides the greatest opportunity to achieve proper density. Just under the Cool Zone limit is the best place to perform finish rolling. In the Tender Zone, the mix tends to shove and push under the steel wheel. You can roll mix in the Tender Zone, but a pneumatic roller is necessary. The rubber tires do not shove the mix in the Tender Zone.
2. Roller Patterns
Your main focus should be compaction in the Hot Zone. The breakdown roller must stay tight behind the paver and the speed of the entire paving train should be critically examined to ensure proper coverage before the mix creeps into the Tender Zone. Consistency is key also—consistent mix temperature, paver speed, and roller pattern. As each project, mix, and project conditions are different, you need to establish a roller pattern for each situation. Any change in paving equipment, particularly rollers, requires a new roller pattern. Also any significant mix change justifies a new roller pattern. Here are a few suggestions you may find helpful:
• Get on the mix hot. The best chance of compacting mix is above the 240—250 degree F. range (in the Hot Zone).
• Pay attention to temperature. Rolling and specially vibrating in the Tender Zone can cause major damage to the mat.
• Maintain a smooth and consistent paving train. Avoid unnecessary stopping and starting.
• Establish not only a roller pattern specifying the number of coverages, but a temperature range for each roller in the train.
• Mount temperature reading devices on the rollers and inform operators of their target temperature range If you are on a project with little or no Quality Control support or requirements, simply paying attention to the three Zones will be greatly beneficial.
Although tender mixes are viewed as troublesome, the truth is that most mixes may have one or more tender factors discussed herein. Proper compaction of these mixes will provide high quality and long lasting pavements. The suggestions given here are no guarantee of success, but rather ‘pointers’ that we have learned through our experience with tender mixes.
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