Compaction of Tender Mixes
TENDERNESS DURING CONSTRUCTION
Tenderness of asphaltic concrete mixture during construction usually refers to a mix that is difficult to compact to required density. The mix tends to shove under steel wheel vibratory rollers and/or leaves longitudinal cracks in the pavement at the edges of steel drums. Tenderness described by scuffing, shearing or distorting by traffic after construction is complete is discussed in the “Tender Asphalts—post construction” technical note.
Tenderness during construction is caused by a lack of interparticular friction or shear strength in the asphalt concrete mix. This lack of shear strength is a result of a combination of factors. All aspects of asphaltic concrete construction contribute to the “tenderness” of asphalt. The most common properties that affect asphalt concrete tenderness are listed below along with suggestions for decreasing tenderness.
Tenderness can be caused by a combination of a few of these properties not necessarily all, and cannot always be avoided. In fact tender mixes produce high quality pavements if compacted properly. Compaction of tender AC mixes will be discussed later.
- ShapeAggregates that are rounded have less interparticular friction and slip more easily under shear stress. Crushed Particles
- GradationSand size particles (those that are retained on the #8, #16, #30 sieve sizes) tend to be rounded and act as bearings for the coarse aggregate to roll on, especially when combined with a low mineral filler content. Less material held on #8 to #30 screens
- Mineral fillers (those materials passing #30 sieve) fill in the gaps in the asphalt concrete and add interparticle friction. They combine with the asphalt binder to adhere the large aggregate and add viscosity and shear strength to the mix. Aggregate gradations with low percentages passing the #200 screen are increasingly susceptible to tenderness. 4% + passing 200
- Mix Stability is a major aspect in pavement quality, as well as pavement tenderness issues. A mix with a small maximum aggregate size will be tender compared to a larger maximum aggregate size due to its lack or large aggregate contact, without this contact, asphalt will tend to move laterally under load. Large maximum Aggregate Size
ASPHALT BINDER PROPERTIES
Asphalt binders with low viscosities tend to be more tender due to their lack of shear strength at high temperatures. Use Stiffer Binders
Too much oil on the aggregates decreases the amount of interparticle friction in the mix and is unnecessary for shear strength. Asphalt concrete mixes with high oil contents lack the shear strength to resist tenderness. A well-designed asphalt concrete mix will do much to alleviate this problem. This issue is compounded by a lack of mineral filler, which causes more oil to be used in filling air voids. As well, the presence of moisture in the mix during construction adds to the fluids content and causes the mix to act more tender. Proper Asphalt contents.
Sometimes aggregate and mix design properties are inflexible, such as the Caltrans QC/QA specification, or when placing superpave mixes. Conventional wisdom or methodology may be inadequate to compact these high performance mixes. The successful practice take more work, knowledge and diligence but in the end produces higher quality, better performing pavements.
Proper compaction is the single most important factor that affects the ultimate performance of pavement under traffic. Properly compacting the mix in the field to an air void content of 7% or less is essential to pavement performance; it increases pavement life and performance, decreases deformation under traffic (rutting), stripping, oxidization, and increases internal stability. A perfect mix placed poorly will perform poorly while an imperfect mix placed properly will perform extremely well. As a guideline remember that for every % air voids over 5% in the field will subtract 1 year from the pavement life.
The tender zone appears during the paving process as an area that moves under the roller’s applied compactive effort. In order to properly compact a mix that acts this way a contractor must take advantage of its 3 temperature zones. Hot, Tender, & Cool. The tender zone occurs in asphaltic pavements any where in the temperature range from 250 to 190°F, (120 to 90° C). Above this temperature is the greatest opportunity to achieve mix density. Some density can be achieved below this temperature range, however the amount of compaction available is limited to roughly 2%.
To compact tender mixes the contractor has two choices. First you can compact the mix in the upper and lower tender zones where the mix is internally stable and does not move or shove. Second, you can roll the mix in the tender zone with a pneumatic roller as the mix will not shove in front of the rubber tire as it will with the steel. The Rubber tire roller does increase compaction.
The main object of focus should be to compact the mix in the hot zone. This may take an adjustment to the conventional practices currently used. The breakdown roller must stay tight behind the paver in order to make greatest use of the temperature, As well; the paver should be slowed down to increase the number of coverages of a certain area before the mix becomes tender. As well, a consistent roller pattern should be chosen and stuck with. Inconsistencies in paver speed and subsequently roller speed vary the amount of compactive effort available and Provide poor and inaccurate results.
Some Acceptable rolling patterns are:
- Two steel drum vibratory rollers operating before 240°F, 1 or 2 steel drum finish roller operating after 180°F.
- Two steel drum vibratory rollers operating in echelon, 1 or 2 finish rollers.
- Rubber tire pneumatic roller followed by a steel drum vibratory roller. 1 or 2 finish rollers.
- If operating 2 breakdown rollers is impossible, use a rubber tire roller in the tender zone to increase compaction.
Different variations of these roller patterns have been successful. There are a few general rules to follow. They do not guarantee success but will definitely give you a fighting chance.
- Get on the mix hot. Your best chance of compacting the mix is above 240°F. We have found that 3 full coverages will get you the compaction you need.
- Pay attention to temperature. Rolling and especially vibrating in the tender zone can cause major damage to the mat. It is very beneficial to have roller-mounted temperature reading devices on all rollers to aid in this.
- Be as smooth as possible. Stopping and starting ads many variables to mix compaction. Choose a well-informed pattern and stay with it. Consistent is the only way to ensure good results.
Although tender mixes are seen as troublesome and a mix problem to many industry operators, the truth of the matter is that QC/QA specifications, superpave, and all high performance mixes will more than likely have some tender properties. Compacting these pavements provide the highest quality and longest lasting pavements. With proper knowledge effort and information compacting these mixes can be made manageable and the rewards worthwhile.