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CHAPTER 12 Field Adjustments and Quality Assurance of HMA Mixtures Previous chapters of this manual deal with various parts of the HMA mix design process, including laboratory tests, material specifications, and calculations. The goal of these activities is to produce high-quality HMA at the plant--HMA that meets all the requirements of the end user. As discussed below, mix designs developed in the laboratory almost always must be adjusted for field production. Some agencies use the term "mix verification" for this important action. Once a mix design has been adjusted and verified, quality control procedures performed by plant personnel help to ensure that the mix properties stay within established limits, so that the agency will accept all of the production at full price. This chapter describes procedures for adjusting laboratory mix designs for field production and typical quality control procedures for HMA production. The information presented here is only an introduction to HMA quality control; engineers and technicians responsible for quality control should further their knowledge through training classes offered by their local highway agency, the FHWA or by other organizations such as The Asphalt Institute, the National Center for Asphalt Technology (NCAT), and the National Asphalt Paving Association (NAPA). Adjusting Laboratory HMA Mix Designs for Plant Production Most of this manual deals with developing HMA designs in the laboratory. Eventually, such mix designs will be used in actual pavement construction and must be produced at a hot mix plant--a much different environment than a laboratory. Engineers and technicians responsible for the design and production of HMA mixtures must understand that HMA designs will almost always have to be adjusted during field production. These adjustments are necessary for several reasons. One of the most common differences between laboratory mix designs and HMA produced in a plant is that the large amount of aggregate handling--transport from stockpiles to the aggregate bins, batching from the bins to the aggregate belt, heating in the drum or hot bin, mixing, and so forth--generates significant amounts of mineral filler. Also, mineral dust content in coarse aggregates can be significant, but is often--but should not be--ignored during mix design, which will also cause an increase in mineral filler content in plant-produced HMA compared to the design aggregate blend. This increase in mineral filler typically results in a decrease in air void content and VMA during field production. These changes can be large enough to negatively affect the performance or result in the HMA composition falling outside of the specification limits. Aggregate Gradation and Mineral Filler Content in HMA Mix Designs and During Plant Production The increase in mineral filler from laboratory design to plant production varies considerably, depending on the type of aggregate used, the aggregate gradation, and the type of HMA plant. 207

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208 A Manual for Design of Hot Mix Asphalt with Commentary Very hard aggregates will tend to produce less dust during handling, and the increase in mineral filler may be less than 0.5%. Softer aggregates may produce an increase in mineral filler of 2% or more. Analysis of a large number of HMA mixes placed at the test track at the National Center for Asphalt Technology showed an average increase in mineral filler during production of 2.2% for 12.5-mm dense-graded HMA, 2.8% for 19-mm dense-graded HMA, and 3.5% for 12.5-mm SMA mixes. In the NCAT test track mixes, it appears that the increase in mineral filler during plant production increases with an increasing proportion of coarse aggregate in the HMA. Table 12-1 gives typical values for the amount of mineral dust generated during plant production of HMA mixtures. These are only estimates, based on a limited amount of data on HMA field production. However, this table should provide technicians and engineers with some idea of how much aggregate gradations will change during production. In general, softer aggregates and coarser aggregate gradations will result in larger amounts of mineral dust being generated during plant production. A related problem is the potential use at the plant of mineral fines recycled from the dust collection system. Unfortunately, the effect of fines recycling varies significantly from plant to plant and depends, in part, on the type of collection system used and also upon how the plant uses recycled fines, if at all. If a wet scrubber is used to collect fines, these are normally wasted and not returned to the mix. Therefore, the aggregate gradation in the mix produced in such a plant might have fewer fines than suggested by the aggregate stockpile gradations. If a baghouse is used to collect fines at an HMA plant, they may or may not be returned to the mix. To further complicate matters, the nature of fines collected in any given baghouse will vary depending upon the condition of the filter bags--clean bags are inefficient and allow significant amounts of very small aggregate particles to escape. As the bags become coated with dust and organic material, they will capture a much greater proportion of these very fine particles. Cyclone collectors and similar dry dust collection systems that do not use cloth filters will tend not to collect extremely fine material; material collected by these systems tends to be no finer than about 150 m. If a plant does use a significant amount of recycled fines in their mixes, the technician developing mix designs should include a typical amount of this material in laboratory trial mixes. Reasonable efforts should be made to ensure that the recycled fines used in mix design work are representative of what normally goes into the mix at the plant. The use of ignition ovens to determine asphalt content and aggregate gradation from plant produced mixes can also contribute to differences between mix design and apparent aggregate Table 12-1. Typical amounts of mineral dust generated during HMA plant production for different aggregates and gradations. Abrasion L.A. % Retained on 2.36-mm sieve in Resistance Abrasion theoretical aggregate blend: Level Loss Examples 35 Wt. % Good < 20% Dense igneous rocks such as basalt, 2.5 1.5 1.0 diabase and gabbro ("trap rock") Moderate 20 to 35% Good quality igneous rocks of 3.5 2.5 2.0 moderate density such as granite, syenite, diorite; good quality dolomites, limestones and dolomitic limestones; most sandstones, graywackes, slags, and crushed gravels Poor > 35% Soft limestones, sandstones, 4.5 3.5 3.0 graywackes and granite

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Field Adjustments and Quality Assurance of HMA Mixtures 209 gradation. This can occur when aggregates break down during the excessive heating that occurs in the ignition oven procedure. Aggregates containing significant amounts of limestone are especially prone to this problem. However, it must be emphasized that discrepancies caused by the use of the ignition oven are not real--these differences are not present in the actual mix as produced, but only in the aggregate left over after the ignition oven procedure. Care must therefore be used in interpreting aggregate gradation information gathered using material that has been through the ignition oven. Asphalt Binder Hardening Another source of difference between plant and laboratory mixes is the consistency of the asphalt binder. When developing HMA mix designs in the laboratory, asphalt binder is often handled in small containers and kept at high temperatures for varying periods of time. This can lead to significant but highly variable amounts of age hardening in the binder. Asphalt binder tanks, pumps, and piping at a hot mix plant are enclosed systems that tend to minimize age hardening in the binder. On the other hand, mixing in the pugmill or drum and subsequent storage of the mix in a silo can cause significant amounts of age hardening in the mixture prior to transport and placement. Such differences in binder or mixture age hardening can cause differences in air void content and other volumetric properties when comparing laboratory mix designs with plant-produced HMA. Laboratory engineers and technicians should use care in handling asphalt to ensure that unintentional age hardening is minimized during the mix design process. Differences Between HMA Mixing in the Laboratory and in the Plant Another source of differences between laboratory mix designs and plant-produced mixtures is the vast difference in the type and size of mixers used. Any HMA mixer will collect asphalt binder and mineral fines on the surfaces of the mixing container and stirrer blades. Because of the small size of laboratory mixers, the surface area of the container and blades is much larger relative to the volume of HMA, compared to plant mixers. This problem can be addressed by "buttering" the laboratory mixer before performing actual mix design work. Buttering is done by mixing a preliminary batch of relatively rich HMA and discarding this material before doing any mix design work. This preliminary mixture should use the same aggregate and asphalt binder used in the HMA mix design. Another problem related to mixers is differences in aggregate breakdown between laboratory- and plant-produced HMA. Large aggregate particles can be fractured in mixers, by being caught between the mixing blades and the container, or simply by the vigorous nature of the mixing process. The degree of breakdown can vary dramatically among different types of laboratory mixers, among different HMA plants, and between laboratory- and plant-produced mix. There is little that can be done either to predict these differences or to minimize them. To maintain the consistency of HMA mix design and verification work, all laboratories within a given organization (or among cooperating organizations) should use identical laboratory mixers with the same mixing blades. The discussion above should make it clear that there are many different sources of potentially large differences in composition between laboratory mix designs and plant produced HMA. Therefore, engineers and technicians responsible for HMA mix designs and for taking these mix designs from the laboratory to the field should always remember that an HMA mix design is only a starting point for developing the plant job mix formula (JMF). Significant adjustments in the mix design will almost always be needed when producing HMA for the first time based on a laboratory mix design. Such adjustments are not only acceptable, they are usually necessary to producing a good-quality mix.

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210 A Manual for Design of Hot Mix Asphalt with Commentary Minimizing Differences Between HMA Mix Designs and Plant-Produced Mix Technicians can take several measures when developing an HMA mix design to ensure that the design can be easily adjusted for plant production. The number of aggregates used in an HMA design should be kept as low as possible, while still meeting all pertinent requirements. Also, a reasonable effort should be made to use aggregate gradations in the laboratory that are representative of average stockpile values. As discussed in Chapter 4, this is normally done by breaking down aggregates into fractions and recombining them to create blends closely matching average gradation values provided by the HMA plant. The design VMA for a laboratory mix design should fall near the middle of the allowable range. For example, if the specified range for VMA for the given mix type is 14.0 to 16.0%, the laboratory design should have a VMA value close to 15.0%. This allows some room for field adjustments without producing a mixture that is out of specification. For the same reason, the dust/binder ratio should be kept away from the allowable minimum and maximum values. As mentioned above, care should be taken in handling asphalt binder in the laboratory to minimize age hardening. Some technicians may wish to adjust the laboratory mix design to account for additional mineral filler generated during plant processing of the aggregate. The amount of fines generated during plant production is best estimated by plant personnel familiar with the materials being used in the mix design. If this information is not available from the plant, the information given in Table 12-1 (presented earlier) can be used to estimate the extra mineral dust generated during plant production of HMA mixes. To use the information in Table 12-1 to make adjustments in the aggregate gradation, the percent passing all sieves smaller than the NMAS, including the 75m sieve, should be increased by the same amount as the anticipated difference in mineral filler. This approach assumes all of the extra mineral dust is produced from aggregate abrasion between the NMAS and the next smaller sieve size. Although not entirely accurate, this approach is a good first estimate of what happens during HMA field production. Mix Composition Adjustments During Plant Production The discussion above focused on (1) developing mix designs that, as much as possible, reflect actual production at the plant and (2) designing mixes that will be relatively easy to adjust at the plant during production. As emphasized many times before in this manual, laboratory designs for HMA mixtures are only starting points for the actual plant mix design used during construction. The plant operator will need to adjust the laboratory mix design to provide an HMA mix that is workable and meets all requirements of the pertinent specification. Although this manual is intended for engineers and technicians responsible for laboratory mix designs and related activities, it is important that these professionals understand the adjustments usually made when taking a laboratory mix design into field production. By far the most common reason for adjusting a mix design during initial plant production is to adjust the air void content and VMA obtained during testing of laboratory-compacted specimens. Air void content and VMA are directly related at a given asphalt content, so if the air void content decreases, VMA will also decrease unless the asphalt content is changed. As discussed above, most often, an increase in the amount of mineral filler during plant production will cause a decrease in air void content and VMA. Many plant operators will address this problem by decreasing asphalt content slightly, which will tend to increase air void content. However, the end result of this adjustment will be a decrease in VMA. Furthermore, decreasing asphalt binder content below the laboratory design value can decrease pavement performance. It is therefore suggested that low air void content and VMA be corrected during field production by adjusting aggregate proportions. For fine/dense-graded HMA, low air void contents can be increased by increasing the proportion of fine aggregate. For coarse/dense-graded HMA, air void content can