ASPHALT MIXTURES Q: For an asphalt pavement in a container terminal, are there any rules of thumb as to what the maximum load could be without causing damage? A: No rule of thumb answers your question, but two issues should be considered: Is the pavement structure (subgrade, subbase, base, and all asphalt layers) adequate to support the loads? You need to purchase our MS-23 Manual, Thickness Design of Asphalt Pavements for Heavy Wheel Loads. Or you can purchase our SW-1 Asphalt Thickness Design Software Program, where the MS-23 calculation procedures are automated in one of the modules. Info on both can be found in the link below. Is the hot mix asphalt surface stiff enough to resist deformation (ruts or indentations)? This is dependent on many factors, such as stiffness of the original mixture, age of the mix (gets stiffer over time), temperature of the mix during loading, loading itself, duration of applied load, etc. While not usually a problem, when it occurs it can typically be resolved by placing some steel (or other rigid material) plates below the point load to distribute the load across a wider area. Q: Can the same paving equipment be used for Superpave mixes that was used for conventional mixes? A: Yes. However, since Superpave mixes tend to be coarser and contain modified binders than conventional mixes, good construction practices are more important than ever. Segregation is more likely to occur with coarser mixes if proper equipment and techniques are not used. Density can also be more difficult to achieve with Superpave mixes. Proper rolling techniques and adequate equipment are essential to achieve sufficient compaction. Breakdown rolling for Superpave mixes is normally done right behind the paver when the mix is hottest. Some contractors have found that additional and/or heavier rollers are sometimes needed. Pneumatic rubber-tired rollers work well, but tend to stick to the mat when polymer modified asphalt is used. Hand-working should be minimized. Sufficient well-graded (not segregated) material should be supplied by the paver augers to the joint to facilitate a low-void, low-permeability seam. Q. Is there a problem with milling up and recycling asphalt mixes that used polymer modified binders? A. Generally speaking, there are no unique problems with using polymer modified mixes as RAP. Some individuals express environmental concerns about running millings containing ground tire rubber (GTR) through a drum plant. Florida uses a small percentage of GTR on most of their highway surface mixes. California and Arizona also use GTR frequently. Q. What is the proper mix temperature? A. Mix temperature is dependent on the grade of asphalt used in the mix: Less viscous asphalt requires lower temperatures, while more viscous asphalt requiers higher temperatures. At the start of a mix design, target temperatures are specified for proper mixing and compaction. These temperatures should be adjusted for project conditions (weather, haul distances, etc.). If at all possible, avoid discrepancies from the mix design temperature of more than 25 degrees. Note: When working with modified binder, the binder supplier should provide mix temperature recommendations. Q. What is a minimum temperature for asphalt mixes? A. Mixes must be placed and compacted before they cool to 185o F, so the minimum temperature will depend on the temperature of the layer upon which it is being placed as well as ambient conditions. Temperature session charts are shown on Page 6-6, Fig. 6.03 of the new MS-22 and Page 234 of the old MS-22. Generally, agency specifications will spell out a minimum acceptable temperature for the mix. Some specifications will use 225o F, and others may use 250o F. Q. How do I ensure HMA is impervious to water? A. Conventional mixes should be impervious to water as long as the total in-place air void content is below 7 to 8%. Mixes with higher void contents can be pervious to air and water leading to premature aging and raveling. Q. Is there a limit on the percentage of RAP utilized in new installations? What about RAP use for resurfacing of old asphalt roads? Any limits? If there are limits on the use of RAP in new or resurfacing installations, who sets the limits? A. The Asphalt Institute strongly endorses the use of RAP in asphalt mixtures. RAP has a history of positive performance. The specifying agency or owner will set the limit for RAP content. Almost all state highway departments now allow the use of RAP. A few restrict its use in wearing courses; even fewer (one or two) prohibit its use completely. Most agencies have developed a means of accomodating the stiffness of the reclaimed asphalt from the RAP by the selection of the particular grade of the virgin binder. The FHWA Asphalt Mixture Expert Task Group developed recommendations that are being considered by the Association of State Highway and Transportation Officials (AASHTO) to provide guidance in asphalt binder grade selection when using RAP. These recommendations are summarized below. • When 15% or less RAP is used: The binder grade for the mixture is selected for the environment and traffic conditions the same as for a virgin mix. No grade adjustment is made to compensate for the stiffness of the asphalt in the RAP. • When 16 to 25% RAP is used: The selected binder grade for the new asphalt is one grade lower for both the high and low temperature stiffness than the binder grade required for a virgin asphalt. For example, if the specified binder grade for the virgin mix is a PG 64-22, the required grade for the recycled mix would be a PG 58-28. • When more than 25% RAP is used: The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperature. The low temperature grade is one grade lower than the binder grade required for a virgin asphalt. Normally, the above guidelines would be applied to both new and existing pavements. If a warranty was applied to a project, a more conservative approach - such as the use of blending charts - might be taken. It is suggested that you contact the local state highway agency and/or asphalt binder supplier for the prevailing local practices. Q: Is it acceptable to run Theoretical Maximum (Rice) Specific Gravity on material obtained from cores or saw cutting? A: Rice (Gmm) is typically not run on material from cores as it is not the preferred method of material collection for this test. In fact, ASTM D5361, Standard Practice for Sampling Compacted Bituminous Mixtures for Laboratory Testing, does not include Rice testing in its Significance and Use section. Note paragraph 3.1 from the standard reads: 3.1 Samples obtained in accordance with the procedure given in this practice may be used to measure pavement thickness, density, resilient or dynamic modulus, tensile strength, Marshall or Hveem stability, or for extraction testing, to determine asphalt content, asphalt properties and mix gradation. There are a couple of reasons for this. First, coring is naturally a destructive process which alters the gradation. The level to which the gradation shifts varies with the nature of the parent gradation and material. i.e., a half-inch SMA is likely to see a greater gradation shift then say a fine, dense-graded three-eighths mix. Secondly, and more importantly, by coring you are creating aggregate that is not coated with asphalt. This lack of coating can then allow for water absorption into these non-protected surfaces. Naturally, the more absorptive the aggregate the greater the potential issue with this situation. The AASHTO standard for Rice is T-209. It addresses absorption in part 15 of the standard entitled, “Supplemental Procedure for Mixtures Containing Porous Aggregate.” This is also known as the “dry-back procedure.” It is used on mixes produced with aggregate who’s water absorption is greater than 1.5%. However, while collection of Rice material via cores is not the preferred method, it is an acceptable method when more preferred alternatives (plant or lab produced samples) are not available. I am unaware of any state that does not allow for cores to be used for Gmm when no good alternative is an option. With the previous discussion in mind, one should do what they can to minimize any potential problems that may arise from field-cut specimens. What this leads to is a bigger is better mindset. A 6-inch core will have a smaller percentage of its aggregate affected by the coring than would a 4-inch core from the same road. Therefore, it is highly recommended that if alternative methods of producing materials for Rice are not an option, to use at least a 6-inch core. If a bigger specimen can be collected, such as saw-cutting, then it should be considered. Judgment, and locally acceptable practice, will certainly need to come into play.
Posted on: Tue, 18 Mar 2014 06:06:17 +0000
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