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OCR for page 112
112 A Manual for Design of Hot Mix Asphalt with Commentary Table 8-4. Recommended aggregate nominal maximum aggregate sizes for dense-graded HMA mixtures. Application Recommended Recommended Lift Thickness, mm NMAS, mm Fine-Graded Coarse-Graded Mixtures Mixtures 4.75 15 to 25 20 to 25 Leveling course mixtures 9.5 30 to 50 40 to 50 4.75 15 to 25 20 to 25 Wearing course mixtures 9.5 30 to 50 40 to 50 12.5 40 to 65 50 to 65 19.0 60 to 100 75 to 100 Intermediate course mixtures 25.0 75 to 125 100 to 125 19.0 60 to 100 75 to 100 Base course mixtures 25.0 75 to 125 100 to 125 37.5 115 to 150 150 9.5 30 to 50 40 to 50 Rich base course mixtures 12.5 40 to 65 50 to 65 Step 5. Determine Target VMA and Design Air Void Content One of the unique features of the design method described in this manual for dense-graded mixtures is that the target VMA and air void content--and the resulting target binder content-- are determined early on and maintained throughout the mix design procedure. In this way, the proper binder content is ensured, and effort is not wasted evaluating mixtures that do not have the proper VMA and binder content. This also reduces the chances that an error in volumetric calculations or laboratory testing will result in a mix design that does not meet the specified requirements. The allowable VMA range depends only on the aggregate NMAS; as in the Superpave system, minimum and maximum VMA values increase with decreasing aggregate NMAS-- 1% for each decrease in standard aggregate size. Limits for VMA are given in Table 8-5. A 2% range is specified for allowable VMA. When selecting VMA for a mix design, the target value is in the center of this allowable range. These target values should be used for the initial devel- opment of a mix design--for determining the composition of trial mixtures to be evaluated and refined in the laboratory during the mix design process. The design VMA value can be adjusted during the later stages of the mix design process or during construction, in order to further refine the mix or to adjust for field production. Using a target VMA value in the Table 8-5. VMA requirements for standard dense-graded mixtures. Aggregate Minimum Maximum NMAS VMAa VMAa (mm) (%) (%) 4.75 16 18 9.5 15 17 12.5 14 16 19.0 13 15 25.0 12 14 37.5 11 13 a The specifying agency may increase the minimum and maximum values for VMA by up to 1% to obtain mixtures with increased asphalt binder content, which can improve field compaction, fatigue resistance, and general durability. Care should be taken to ensure that the resulting HMA mixtures maintain adequate rut resistance for their intended application.

OCR for page 112
Design of Dense-Graded HMA Mixtures 113 center of the allowable range ensures that such adjustments can be made. If a mix design is started at the minimum allowable VMA, adjustments needed later in the mix design process or during field production can be difficult or impossible without lowering the VMA below the specified minimum. As noted in Table 8-5, the specifying agency can increase the minimum and maximum (and resulting target) VMA values by up to 1% if desired. This will provide additional binder content in the resulting HMA mixtures, which can have several desirable effects--it will tend to produce a mixture that is easier to compact in the field, more fatigue resistant, and, in general, more durable. However, increasing VMA can also decrease rut resistance, so care is needed when increasing minimum VMA requirements. As discussed below, the required dust/binder ratio of 0.8 to 1.6 should not be lowered if VMA requirements are increased beyond those given in Table 8-5, or the resulting mixtures might at times exhibit poor rut resistance. Agencies should in general be wary of simultaneously changing mix design requirements that all tend to reduce rut resistance--these include increasing VMA, decreasing dust/binder ratio, decreasing Ndesign, reducing requirements for FAA, or lower requirements for CAFF. Agencies should also be aware that the only foolproof way of increasing binder content in HMA mixtures is to increase minimum VMA requirements. Reducing Ndesign values will make it easier to design mixtures with higher VMA, but producers will find it easy to adjust their aggregate proportions after such a change in order to maintain binder content at the lowest possible level when economic incentives make such an approach desirable. When considering increasing VMA requirements, it should be remembered that many HMA performance problems are the result of construction problems, especially poor field compaction, rather than improper mix design. If high in-place air void content is the cause of poor durability--raveling and surface cracking--increasing VMA or decreasing Ndesign will not improve field performance unless these changes result in significant improvement in field compaction. For most surface course and intermediate (binder) course mixes, a design air void content of 4.0% is recommended. However, the design air void content for these mixtures is allowed to vary from 3.5% to 4.5%. Specifying a lower design air void content of 3.5% will result in an increase in binder content of a few tenths of a percent and a mixture that is slightly easier to compact. It will, however, also tend to decrease rut resistance. Increasing the design air void content by 0.5% will have the opposite effect--it will slightly decrease the design binder content and produce a mix that is more difficult to compact, while increasing rut resistance. Rich bottom or base course mixes, as now sometimes used in the design and construction of perpetual pavements, should be designed at a slightly lower air void content of 3.0 to 4.0%. This helps ensure that these mixes have the binder needed for exceptional fatigue life and are also easy to compact to a very low air void content in the field. Because base course mixtures are located deep within the pavement structure, the decrease in rut resistance caused by a lower design air void content is not normally a major concern for these applications. In HMA Tools, the initial target values for VMA and air void content are selected in the worksheet "General." This worksheet lists the minimum and maximum values for VMA along with the suggested target--the midpoint between the VMA limits. Target VMA and air void content can also be refined in the worksheet "Trial_Blends." As discussed above, it is recommended that (1) such adjustments be made only after evaluating several trial batches and (2) they be kept small--about 0.5% or less. Changing target VMA and air void content during the initial stages of the mix design process can make it difficult to evaluate the effect that changes in aggre- gate gradation have on these values, making the mix design process longer and more complicated than it needs to be. As discussed above, designing a mix near the minimum or maximum allowable VMA and/or air void content can also make adjustments during field production more difficult.