The National Academies Press

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Pages 207-224

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From page 207... ... 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. Read the entire page →
From page 208... ... 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. Read the entire page →
From page 209... ... 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. Read the entire page →
From page 210... ... 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. Read the entire page →
From page 211... ... The rules given above are general and, in practice, there are many exceptions; plant operators should use their experience in adjusting aggregate proportions during field production to obtain the target air void content and VMA. The general mix design procedure described in Chapter 8 of this manual should also provide information on how aggregate gradation changes will affect air void content and VMA, which can be used as a guide when making adjustments during field production. Read the entire page →
From page 212... ... A quality characteristic is simply something about a product that is measured as an indication of its quality. Both HMA quality control and acceptance revolve around various quality characteristics: asphalt content, mineral filler content, aggregate gradation, in-place air void content, and so forth. Read the entire page →
From page 213... ... * 0.19 to 0.30 0.54 to 0.85 0.29 to 0.37 0.82 to 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. Read the entire page →
From page 214... ... An experienced technician or engineer can look at a set of control charts and determine if a plant is operating smoothly and producing acceptable material or if there is a production problem that requires investigation and perhaps adjustment. Control charts are not just used at HMA plants, but form an essential part of quality control plans in most manufacturing industries. Read the entire page →
From page 215... ... Figure 12-2 is a control chart of the same data, but here running averages are plotted instead of single measurements. The specification requirement shown in this plot is that the average asphalt content for the lot must be within ±0.4% of the target. Read the entire page →
From page 216... ... This will delay creation of the control chart and identification and correction of any problems that occur. Plotting the specification limits on control charts can be useful, but HMA production can usually be better controlled by using statistical control limits. Read the entire page →
From page 217... ... The advantage of using statistical control limits is that potential problems in production can often be identified and corrected before material exceeds specification limits. An important question in the construction of statistical control charts is what values to use for the overall average and range. Read the entire page →
From page 218... ... Typical overall standard deviation values for aggregate gradation. Property Typical Range of Value for Overall Standard Deviation Asphalt content 0.15 to 0.30% Air void content, from field cores 1.3 to 1.5% Laboratory air void content 0.9% VMA 0.9% VFA 4.0% Table 12-6. Read the entire page →
From page 219... ... Technicians responsible for plant quality control may also be required to keep control charts for in-place air void content and pavement thickness. For each set of test data, it is suggested that plots be made for individual measurements, running averages, running standard deviation, daily average, and daily standard deviation. Read the entire page →
From page 220... ... Tests included in acceptance plans might include asphalt content, percent of aggregate passing the 0.075-mm sieve, percent of aggregate passing the 2.36-mm sieve (or some other intermediate sieve size) , in-place air void content, pavement smoothness, and pavement thickness. Read the entire page →
From page 221... ... AASHTO publishes three standard methods for sampling aggregate, asphalt binders, and HMA: • T 2, Sampling of Aggregates • T 40, Sampling Bituminous Materials • T 168, Sampling Bituminous Paving Mixtures Engineers and technicians responsible for collecting samples for quality control and acceptance testing should follow these procedures or appropriate procedures provided by their state highway agency. Often large samples of material taken during production must be reduced in size for testing purposes. Read the entire page →
From page 222... ... Random number tables are often used to generate the times and locations of sampling in HMA quality control and acceptance sampling plans. Many HMA acceptance plans are stratified random sampling plans. Read the entire page →
From page 223... ... Bibliography AASHTO Standards R 9, Acceptance Sampling Plans for Highway Construction R 10, Definition of Terms for Specification and Procedures T 2, Sampling of Aggregates T 40, Sampling Bituminous Materials T 168, Sampling Bituminous Paving Mixtures Field Adjustments and Quality Assurance of HMA Mixtures 223 Read the entire page →
From page 224... ... 224 A Manual for Design of Hot Mix Asphalt with Commentary Read the entire page →

From page 207...

... 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.