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OCR for page 267
CHAPTER 12 Field Adjustments and Quality Assurance of HMA Mixtures Chapter 12 of the Manual is made up of two distinct parts. The first part of this chapter deals with how a typical HMA mix design, as developed in the laboratory must be adjusted during field production. The most important of these adjustments involves accounting for the increase in mineral dust that typically occurs during field production. The second part of the chapter deals with quality assurance (QA) of HMA. Although not strictly part of the HMA mix design process, it was thought that QA is such an important part of a typical HMA laboratory's work and is so closely related to the mix design process that the Manual would not be complete without ad- dressing this topic. Information for this part of the chapter comes from various sources, but by far the most important are Hot Mix Asphalt Construction, Instructor Manual, Part A: Lecture Notes and the NHI course manual Highway Materials Engineering, Module I: Materials Control and Acceptance--Quality Assurance (49, 50). Table 23 (Table 12-1 in the Manual) shows typical amounts of mineral dust generated during HMA plant production. This table is based in part on data from the NCAT test track; Figure 14 shows the increase in the passing 0.075-mm fraction of aggregate in QA data compared to the JMF gradation as a function of % passing the 2.36-mm sieve (51). For gradations having more than 35% passing the 2.36-mm sieve, the increase appears to mostly fall in the range of from 1.0 to 3.0%. However, as the % passing the 2.36-mm sieve decreases, the amount of dust generated during plant production appears to increase. A second factor affecting the amount of dust gen- erated during plant production is aggregate hardness. Unfortunately the nature of the NCAT data does not allow development of such a relationship. The values shown in Table 23 are there- fore based on engineering judgment. The L.A. abrasion values for different aggregate types are Table 23. Typical amounts of mineral dust generated during HMA plant production for different aggregates and gradations (Table 12-1 in the Mix Design Manual). 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, quartzites, slags, and crushed gravels Poor > 35% Soft limestones, sandstones, 4.5 3.5 3.0 graywackes and granite 267

OCR for page 267
268 A Manual for Design of Hot Mix Asphalt with Commentary 5 Increase in % Passing 4 0.075 mm 3 NCAT 12.5-mm SP NCAT 12.5-mm SMA 2 NCAT 19-mm SP 1 0 0 10 20 30 40 50 60 % Passing 2.36 mm Figure 14. Increase in % passing the 0.075-mm Sieve during HMA Plant production as a function of % passing the 2.36-mm Sieve, for mixes placed in the first cycle of the NCAT test track. based on a number of sources, including NCHRP Report 405 and NCHRP Report 557. It should be emphasized that Table 23 is meant to provide approximate guidelines concerning the amount of dust generated during HMA production. Although such values cannot be predicted exactly, using estimated values in the mix design process will provide more reliable HMA mix designs than ignoring the amount of dust likely to be generated during plant production. Table 24 (Table 12-2 in the Manual) gives precision values for commonly used tests on aggre- gates and HMA mixtures. These values are taken directly from AASHTO or ASTM standards for the various tests. Equations 25 and 26 (Equations 12-2 and 12-3, respectively, in the Manual) for calculating the upper and lower control limits for an X-bar control chart are taken from Hot Mix Asphalt Construc- tion, Instructor Manual, Part A: Lecture Notes (49). Similar equations appear in other documents dis- Table 24. Single-operator and multi-laboratory precision for test results commonly used in HMA quality assurance and acceptance plans (Table 12-2 in the Mix Design Manual). Single Operator Multi-laboratory Test Procedure Std. Dev. D2S Std. Dev. D2S Aggregate gradation, percent passing Coarse aggregate (CA)* 0.27 to 0.8 to 6.4 0.35 to 1.0 to 8.0 2.25 2.82 Fine aggregate (FA)* 0.14 to 0.4 o 2.4 0.23 to 0.6 to 4.0 0.83 1.41 Mineral Filler (in CA/ in FA) 0.10/0.15 0.28/0.43 0.22/0.29 0.62/0.82 Asphalt content, weight % Ignition oven 0.04 0.11 0.06 0.17 Quantitative extraction** 0.19 to 0.54 to 0.29 to 0.82 to 0.30 0.85 0.37 1.05 Maximum theoretical specific gravity 0.0040 0.011 0.0064 0.019 Bulk specific gravity, SSD 0.0124 0.035 0.0269 0.076 Bulk specific gravity, Paraffin-coated 0.028 0.079 0.034 0.095 Air void content, Vol. %*** 0.5 1.5 1.1 3.0 Effective asphalt content, Vol. %*** 0.3 0.9 0.6 1.6 Voids in mineral aggregate, Vol. %*** 0.5 1.5 1.1 3.1 Voids filled with asphalt, Vol. %*** 2.2 6.2 4.5 12.8 Dust/asphalt ratio, by weight*** 0.05 0.13 0.09 0.25 * Lower values are for very high, very low, or both percent passing; higher values are for percent passing values close to 50%. ** Value depends on method used. *** Typical values, estimated from data on aggregate gradation, aggregate and mixture specific gravity, and asphalt content using ignition oven. Values estimated using standard deviations for quantitative extraction vary slightly from these values.

OCR for page 267
Commentary to the Mix Design Manual for Hot Mix Asphalt 269 cussing QA for HMA production and other industrial operations. These equations calculate con- trol limits for the average (X-bar or X) as a function of the overall range (R-bar or R): UCL = X + ( A2 R ) (25) LCL = X - ( A2 R ) (26) where UCL = upper control limit LCL = lower control limit X = overall average or X-bar R = overall range or R-bar A2 = a factor that depends on sample size n (given in Table 25) Table 25 (Table 12-4 in the Manual) gives factors for calculating control limits for both X-bar charts and R charts. These numbers were also taken from Hot Mix Asphalt Construction, Instruc- tor Manual, Part A: Lecture Notes (49), but can be found in other references on QA. Tables 26 and 27 (12-5 and 12-6, respectively, in the Manual) show ranges for typical overall standard deviation for aggregate% passing, asphalt content, air voids, voids in the mineral aggregate (VMA) and voids filled with asphalt (VFA). These values are taken from a National Highway Institute (NHI) course manual on QA (50). Equations 27 and 28 (Equations 12-4 and 12-5 in the Manual) are used to calculate lower and upper control limits for range charts (R charts) for quality assurance of HMA production: LCL = D3 R = 0.0 1.35 = 0.0 (27) UCL = D4 R = 2.12 1.35 = 2.86 (28) where LCL = lower control limit UCL = upper control limit D3, D4 = factors for computing control limits for standard deviation control charts; see Table 25 R = overall range Table 26. Typical overall standard deviation values for aggregate gradation Table 25. Factors for (Table 12-5 in the Mix Design Manual) (50). Table 27. Typical overall standard computing control limits deviation values for asphalt for control charts Typical Range for content, air voids, VMA, and VFA (Table 12-4 in the Mix Overall Standard Sieve Size Deviation (Table 12-6 in the Mix Design Design Manual) (49). 19 mm 1.5 to 4.5% Manual) (50). 12.5 mm 2.5 to 5.0% Sample 9.5 mm 2.5 to 5.0% Typical Range of Size n A2 D3 D4 4.75 mm 2.5 to 5.0% Value for Overall 2 1.88 0.00 3.27 2.36 mm 2.5 to 4.0% Property Standard Deviation 3 1.02 0.00 2.58 1.18 mm 2.5 to 4.0% Asphalt content 0.15 to 0.30% 4 0.73 0.00 2.28 Air voids, from field cores 1.3 to 1.5% 0.60 mm 2.0 to 3.5% 5 0.58 0.00 2.12 0.30 mm 1.0 to 2.0% Laboratory air voids 0.9% 6 0.48 0.00 2.00 0.15 mm 1.0 to 2.0% VMA 0.9% 7 0.42 0.08 1.92 0.075 mm 0.6 to 1.0% VFA 4.0%

OCR for page 267
270 A Manual for Design of Hot Mix Asphalt with Commentary These equations, like Equations 25 and 26 presented previously, are taken from Hot Mix Asphalt Construction Instructor Manual, Part A: Lecture Notes (49). As with much of the other information in Chapter 12, these equations can be found in numerous other references on QA. The rules for interpreting statistical control charts (page 12-21) are based on those in refer- ence 50. The plan for investigating possible production problems as indicated by a control chart is based on the authors' engineering judgment and experience, as is much of the discussion for the last part of Chapter 12. The example of a stratified random sampling plan is based in part on the acceptance plan described in the Pennsylvania Department of Transportation's specification "Section 409--Superpave Mixture Design, Standard and RPS Construction of Plant-Mixed HMA Courses," as given in Publication 408 (52). The example QA plan given toward the end of Chapter 12 is based on this same specification.