All statistically-oriented specifications must address the sample size and frequency of sampling clearly in order to determine compliance with the specifications. First, Sample Size: 1. The size of the sample depends on the variability of the population or lot. The greater the variability, the larger the sample size. 2. The higher the desired level of confidence, the larger the required sample size. 3. The smaller the error that can be tolerated, the larger the required sample size. 4. The size of the sample also depends on how important the material property being tested affects overall performance. The more important or critical the property, the larger the sample size.
Second, the Frequency of Sampling: 1.Materials that are highly variable should be sampled more frequently. Critical materials or materials properties that have a significant effect on performance should be sampled more frequently Materials or production procedures that do not have previous experience, should be sampled more frequently. When results and/or inspection suggest non-compliance with specifications, the sampling frequency should be increased.
Remember, sampling and testing programs are intended to provide an overall estimate of quality. They are not intended to replace proper inspection of materials and workmanship to identify mistakes.
Guides that can be used for selecting the size and number of samples are given. AASHTO R9 is also a general guide.
The size of field samples is determined by the requirements of the test procedure and the nominal maximum size of the aggregate in the mixture.
Always sample the center of the load. Note the sample spigot is located in the center of the back of this truck. Asphalt binders are shipped HOT. Personal safety protection equipment such as face and eye shields, long sleeves, and gloves are a must.
Always draw and discard at least 1 gallon of asphalt binder before taking the sample to be either retained or tested. This prevents the possibility of contamination and old material in the line from being sampled rather than the fresh shipment.
Once the line has been flushed, take at least a quart sample.
Immediately cover and seal the containers. Note that the cans are not completely filled.
Once the samples are covered, make sure that they are stored so that they are protected from freezing. This is especially important if the sample is an emulsion which is a water-based product. Avoid pouring from one container to another. This is to minimize the potential for contamination.
The plant tank can be sampled using the preceding directions for discarding, sampling, sealing, labeling, and storing.
Mix that is obtained by this method is used to determine the asphalt binder content, density, and aggregate gradation for a given lot of mix. All of these tests are used to adjust the contractor’s pay based on how close the results are to the specified values. In order to obtain accurate results, it is very important to obtain a representative sample to test.
If you are taking a sample from the roadway – it is essential that the entire lift be sampled. The steps outlined above can be used.
Also samples can be taken directly from the paver.
Sampling location must be considered when specification writing! Sample the MAT
Block 24 Obtaining Samples 13
Senior/Graduate HMA Course Quality Control / Quality Assurance Obtaining a representative sampleConstruction QC/QA Representative Samples 1
Sample Size• Mixture Component Variability• Confidence Level• Error or Bias From True Value• Effect on PerformanceConstruction QC/QA Representative Samples 2
Frequency of Sampling/Testing• Mixture Production Variability• Effect on Performance• Previous History• Compliance With SpecificationsConstruction QC/QA Representative Samples 3
SAMPLING• Preliminary Investigation of the Potential Source• Control of Product at the Source of Supply• Control of Operations at the Site of Use• Acceptance or Rejection of Material Construction QC/QA Representative Samples 4
SIZE AND NUMBER OF SAMPLESD 3665 Random Sampling of Construction MaterialsE 105 Probability Sampling of MaterialsE 122 Choice of Sample Size to Estimate the AverageQuality of a Lot or ProcessE 141 Acceptance of Evidence Based on the Result ofProbability of Sampling Construction QC/QA Representative Samples 5
Types of samples to be taken • Aggregate • Asphalt binder • HMA mixtureConstruction QC/QA Representative Samples 7
Sampling at Point of Shipment Delivery• By bleeder valve or tap in transfer line or at back of truck – During unloading, center 1/3 of loadSafety equipment is a must Construction QC/QA Representative Samples 8
Sampling Truck • Draw and discard at least a gallon of binder to clear lineConstruction QC/QA Representative Samples 9
Sampling Truck • Immediately take at least a quart sampleConstruction QC/QA Representative Samples 10
Protection and Preservation • Take care not to contaminate containers • Fill and immediately seal – Don’t immerse in solventConstruction QC/QA Representative Samples 11
Protection and Preservation• Store so that emulsions are protected from freezing• Avoid transferring from one container to another• Retain samples not tested (point of delivery)• Take precautions to avoid contamination during sampling, handling, shipping and storage Construction QC/QA Representative Samples 12
Sampling at Plant Tank• Same sampling directions apply to sampling the plant tank Construction QC/QA Representative Samples 13
Sampling Bituminous Paving Mixtures• Accurate test results depend upon a uniform, representative sample being collected.Construction QC/QA Representative Samples 14
Sample Location• At the plant – From the truck – most common – From plant discharge – From cold feed (gradation only)• At the roadway – Behind the paver (11 states) – From the windrowConstruction QC/QA Representative Samples 15
Which is the best location? WHY?Construction QC/QA Representative Samples 16
Sampling – Roadway • Approximately equal increments • Make vertical face with shovel 750 mm (30 in.) parallel to centerline • Pull material back 300 mm (12 in.)Construction QC/QA Representative Samples 20
Sampling – Roadway (continued) • Place scoop on pavement next to vertical face and fill scoop Obtain material to full depth • Close lid and remove scoop when full • Repeat to obtain required sample sizeConstruction QC/QA Representative Samples 21
Sampling from the roadwayConstruction QC/QA Representative Samples 22
Sampling – Paver • Obtain samples at end of auger using square shovel • Place shovel in front of auger extension with blade flat on surface • Allow front face of auger stream to cover shovelConstruction QC/QA Representative Samples 23
Sampling – Paver (continued) • Remove shovel before auger reaches it Lift shovel vertically • Repeat until sample size reachedConstruction QC/QA Representative Samples 24
Sampling from PaverWHAT ARE THE PROBLEMS WITH THIS?Construction QC/QA Representative Samples 25
“Sampling The Windrow” SolutionConstruction QC/QA Representative Samples 26
Sampling Loose HMA? See any problems with this?Construction QC/QA Representative Samples 27