CCIL Hot Mix Asphalt Field Density Testing 5 — Questions and Answers
Question 1: What is the primary reason for performing a thin-lift correction when using a nuclear density gauge on HMA lifts less than 75 mm (3 in) thick?
- Thin lifts have lower compaction requirements
- Gamma radiation penetrates beyond the lift into the underlying layer, affecting the reading (Correct answer)
- The gauge cannot detect moisture in thin lifts
- Thin lifts require a different isotope source
Correct answer: Gamma radiation penetrates beyond the lift into the underlying layer, affecting the reading
In thin lifts, gamma rays penetrate into the underlying layer(s), causing the gauge to measure a composite density rather than the density of only the lift being tested.
Question 2: A field density test on HMA yields a percent compaction of 91.5% where the specification requires a minimum of 92%. The theoretical maximum specific gravity (Gmm) used was from a mix design produced six months ago. What should the technician consider?
- The test result is valid and the pavement is deficient
- The Gmm should be verified with a current mix sample, as mix properties can change over time (Correct answer)
- The test should be repeated with a different gauge model
- The 0.5% deficiency is within normal gauge accuracy and no action is needed
Correct answer: The Gmm should be verified with a current mix sample, as mix properties can change over time
Gmm can vary if mix materials or proportions have changed; using an outdated Gmm can introduce error in percent compaction calculations, so a current Gmm should be confirmed.
Question 3: During HMA paving in warm weather, a technician observes the mat temperature is still 155°C (311°F) when the first breakdown roller pass begins. What is the concern?
- The mix is too hot and may shoving or rutting under the roller (Correct answer)
- The mix has already cooled below the compaction window
- Nuclear gauge readings are inaccurate above 150°C
- High temperature reduces the effectiveness of vibratory compaction
Correct answer: The mix is too hot and may shoving or rutting under the roller
Rolling HMA that is too hot can cause shoving, displacement, and surface distortion because the binder viscosity is too low to resist roller-induced shear forces.
Question 4: What does the 'percent of theoretical maximum density' (%Gmm) express in HMA compaction testing?
- The ratio of the field core density to the laboratory-compacted specimen density
- The ratio of the bulk specific gravity of the compacted mix to its theoretical maximum specific gravity, expressed as a percentage (Correct answer)
- The percentage of air voids removed during compaction
- The compaction efficiency of the roller expressed as a percentage
Correct answer: The ratio of the bulk specific gravity of the compacted mix to its theoretical maximum specific gravity, expressed as a percentage
%Gmm = (Gmb / Gmm) × 100, representing how close the compacted field density is to a theoretically void-free density.
Question 5: Which ASTM standard governs the use of the nuclear method for determining in-place density of bituminous paving mixtures?
- ASTM D2950
- ASTM D1556
- ASTM D6938 (Correct answer)
- ASTM D4791
Correct answer: ASTM D6938
ASTM D6938 covers the use of nuclear methods (both transmission and backscatter) for determining in-place density and moisture content of soil and paving mixtures.
Question 6: A density test result is reported as 2.285 Mg/m³. The mix Gmm is 2.498 Mg/m³. What is the percent compaction to the nearest tenth?
- 91.5% (Correct answer)
- 92.5%
- 93.5%
- 91.5%
Correct answer: 91.5%
(2.285 / 2.498) × 100 = 91.5%, which represents the percent of theoretical maximum density achieved in the field.
Question 7: When extracting a density core from HMA pavement, what is the recommended procedure for handling the core hole to maintain pavement integrity?
- Leave the hole open to allow pavement drainage
- Fill the hole immediately with cold-mix patch and tamp flush with the surface
- Fill the hole with hot mix asphalt and compact it with a hand tamper (Correct answer)
- Mark the hole with paint for future reference only
Correct answer: Fill the hole with hot mix asphalt and compact it with a hand tamper
Core holes should be filled with hot mix asphalt and compacted flush with the surface to restore structural integrity and prevent water infiltration.
CCIL Hot Mix Asphalt Field Density Testing 5
What is the primary reason for performing a thin-lift correction when using a nuclear density gauge on HMA lifts less than 75 mm (3 in) thick?