11. Quality Assurance

11.4.1 Selection of Sampling Methods and Sample Frequency

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Sampling Procedures

Methods and locations of sampling can have a significant influence on the outcome of the parameters being measured for QC. Sampling, therefore, is an important aspect of the overall QC program.

The methods and frequencies selected for sampling are not always the same for QC as they are for acceptance. The sampling methods used to determine the aggregate gradation are one example. Some contractors use cold-feed belt sampling methods for testing aggregate gradation for QC purposes. These samples are taken at the asphalt mixing plant and can provide test results within hours of sampling. These QC results allow any necessary adjustments to be made throughout the day.

Acceptance results should be communicated to the contractor in time to make any necessary adjustments, typically within 24 hours. But sometimes they may not be received until days after placement, potentially causing several days of mix to be placed out of specification. The amount of time needed to get results is an important factor in the selection of sampling methods during production.

11.4.2 Selection of Testing Methods

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Common QA Lab Tests

Traditionally, control of mix production was almost exclusively driven by binder content and aggregate gradation. As QC testing during mixture production improved, and as the technical understanding of the volumetric factors affecting longevity of asphalt pavements progressed, there has been an increased emphasis on controlling these properties in the mix. Most agencies now include mixture volumetric tests in their requirements for both QC and acceptance.

To predict long-term durability, some agencies are now including performance tests in their QA or acceptance criteria. The Balanced Mix Design philosophy (as discussed in Chapter 3) combines performance testing and traditional mix testing to enhance the mixture’s ability to perform over time.

Based on NCHRP Synthesis 346 “State Construction Quality Assurance Programs,” most State DOTs in the United States select specific sampling and testing methods that are to be used at minimum for QC on their projects. These testing requirements are typically identified in the standard specifications. Many agencies include QC line items in bids to reimburse contractors for costs associated with providing the tests. The synthesis report identified the following parameters that may be required for QC testing programs:

• Asphalt binder content.
• Aggregate gradation.
• Compaction.
• Volumetric properties (air voids, VMA, VFA).
• Ride quality.
• Aggregate characteristics (course aggregate angularity, fine aggregate angularity, flat and elongated particles, sand equivalent, etc.).
• Pavement thickness.

Many of the test values are compared with the JMF. When that is the case, the same tests should be used for QC as were used in the development of the JMF. It would not be advisable, for example, to use Marshall density values to evaluate air voids on mixes that were designed using Superpave.

For tests that are not compared with the JMF, the test methods selected for QC purposes may be different from the test methods used for acceptance of the same parameter. For example, QC testing often uses nuclear or non-nuclear density gauges to give an indication of roadway density during rolling operations. Agencies commonly require cores to be cut from the finished pavement for final acceptance of density. Test results from cores come too late to be used for adjusting the compaction operation in real time because the asphalt has already cooled. Density gauge readings can be taken while the mix is still hot and the compaction operation is ongoing. Thus, gauge test results can be an important part of the overall QC program for asphalt compaction.

Since QC methods must emphasize timeliness and convenience, the results can be less accurate and more variable. If the QC test method is different than the acceptance method, a minor shift in the data should be anticipated. It would be unreasonable, for example, to stop compaction operations when a reading of 92.6 percent compaction was read on a nuclear density gauge if the minimum required compaction was 93 percent using core densities. A higher target could be established to account for the likely variation and the possible shift of compaction values from the nuclear density method to the core density method. It is also a prudent QC measure to correlate QC test methods and laboratories to the acceptance methods performed by the acceptance laboratory. This can be accomplished by comparing gauge readings to core densities obtained at the same locations from a test strip or at the beginning of mix production.

If a gauge is being used to control density, an accurate correlation to core densities should be established. For each density gauge, it is reasonable to expect a consistent offset value from a gauge reading and that obtained from a core. Most agencies have established methods for correlating gauge values to those from cores.

11.4.3 Written Quality Control Plan

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Complete Lot Report

Most agencies that specify QC sampling and testing on their projects also require the contractor to submit a “Quality Control Plan” before beginning paving operations. The requirements for these plans can be extensive and usually involve more than just sampling and testing plans. They will generally require documentation verifying compliance with certain project requirements related to the QC program. NCHRP Synthesis 346 indicates the following items are usually required as part of written QC plans for plant mix production:

• Responsibility matrix showing who is responsible for reviewing QC results and directing any corrective action.
• Sampling and testing plan identifying specific tests and frequencies to be used for the project.
• Documentation demonstrating that the plant meets the required qualifications, certifications, and inspection frequencies.
• Documentation demonstrating that the laboratory that will provide the QC testing meets the required qualifications and accreditations.
• Documentation of the equipment contained within the laboratory and any calibration certifications that may be necessary.
• Documentation demonstrating that the technicians who will provide the sampling and testing meet the required qualifications and certifications.
• Example report forms, including control charts that summarize certain key data on plots that can rapidly show trends that need attention.
• Written procedures describing criteria that will be used to identify “out of control” production, what actions will be taken when materials fall outside of those criteria, and what the timing/deadlines of these actions will be.

An example sampling and testing plan that would be a part of the QC plan for mixture production is presented in Table 10, showing both the ASTM and the AASHTO methods. The methods are not interchangeable because they are not always the same. Therefore, standard specifications should designate the method to be followed.

Table 10. Example QC Sampling and Testing Plan

After the project is awarded, the written QC plan is generally submitted to the owner for review and approval. The plan is usually submitted with other project documents related to the mix production, like the asphalt mix designs for all the mixtures to be used on the project, aggregate source qualification testing, certifications for commercially provided materials that will be incorporated into the mix, designation of what plant or plants will be used for production, and the paving plans. After the review and approval process is completed, the contractor is generally authorized to begin production, unless the project specifications call for production of a trial batch and/or test strip.