106.3.2.41 TM-41, Density Testing of Bituminous Mixtures with Nuclear Gauges

This test method establishes the procedures for density testing of bituminous mixtures with nuclear gauges. These procedures apply to the backscatter nuclear test mode and direct transmission mode. Backscatter nuclear test mode should be used for asphaltic concrete mixtures 3-inches or less. Paving lifts greater than 3-inches shall be cored.

106.3.2.41.1 Apparatus

a) Nuclear gauges with supporting equipment meeting the apparatus and precision requirements of AASHTO T 355. Other non-nuclear gauges will be allowed if acceptable correlations are obtained during the test strip.
b) Fine sand to fill all voids if applicable.
c) Equipment required to determine specific gravity of compacted bituminous mixtures.
d) Drilling equipment capable of obtaining 4 in. diameter undisturbed cores from the pavement.

106.3.2.41.2 Preliminary Procedures for Standardization

Tests shall be performed in accordance with AASHTO T 355.

Standardization of the nuclear gauge on a reference standard block is required at the start of each day’s use and when test measurements are suspect. The following steps shall be conducted:

1) Warm up the nuclear gauge in accordance with the manufacturer’s recommendations.

2) Properly seat the nuclear gauge on the standard block and take a four-minute reading on the reference standard. The density shall be within 1% of the density for the previous day’s reading. If not, or on the first day, repeat the four-minute reading. The two numbers shall be within 1% of each other and within 2% of the density of the previous day’s reading. If the readings do not compare, check the machine for malfunction.

106.3.2.41.3 Correction Factor Determination

Correction factors for nuclear gauges shall be determined during the trial section in accordance with Section 405.4.8 (provided below). All nuclear gauges shall collect density readings at the five Evaluation Locations on the test strip along with density cores.

At each Evaluation Location, 5 one-minute readings shall be taken with the nuclear gauge in the backscatter mode position and mark the outline of the gauge in these locations. Record the wet density values, discard the single and highest and lowest values, average the three remaining values, and record the average value.

Note: Material within one foot of an edge or joint shall not be tested. Check tip of source rod to assure it is free of any foreign substances.

Two to three pavement cores shall be cut at each Evaluation Location corresponding to the nuclear gauge location. The Gmb shall be determined for each core in accordance with AASHTO T 166. The average bulk density of the cores shall be calculated by the Gmb x 62.245 lb/ft³ at each Evaluation Location.

The correction factor for each gauge shall be determined by the five Evaluation Locations within the test strip. The two data sets shall be evaluated for outliers by the “T” Statistic Test at 95% confidence level. The following provides an example.

N = Number of Evaluation Locations

di = Difference between Core Density and Nuclear Gauge Density

CF = Correction Factor is the average of the di

S = Standard Deviation

“T” Test = Finds Outliers

Rejection Quotients N T0.95 3 1.15 4 1.46 5 1.67 6 1.82 7 1.94 8 2.03 9 2.11 10 2.18

Evaluation Location Avg. GmbCores (AASHTO T166) Bulk Density of Cores (lb/ft3) Avg. Nuclear Gauge Reading (%) Difference (di) (di – CF) (di – CF)2 1 2.400 149.4 152.6 -3.2 -2.7 7.29 2 2.432 151.4 151.2 0.2 0.6 0.36 3 2.421 150.7 150.7 1.8 2.3 5.29 4 2.411 150.1 150.1 0.4 0.64 6.25 5 2.388 148.6 148.6 -1.5 -1.0 1.00 N = 5 Gmm = 2.555 Correction Factor (CF) -0.5

Correction Factor (CF) = Average of Differences = -0.5

Standard Deviation (S) = ${\sqrt {\frac {{\mathit {\Sigma }}\ (di-CF)^{2}}{\ n-1}}}=1.9$; Is S < 2.5?

Standard Deviation shall be less than 2.5; if not – redo test strip

Calibration of Nuclear Gauge

Max. Absolute Value of ǀ di – CF ǀ = 2.7

“T” Test = Max. Abs. of (ǀ di – CF ǀ) / S = 2.7 / 2.0 = 1.41

Is “T Test” value < T_0.95 @ N = 5

If “T” Test (1.41) < T_0.95 @ N = 5 (1.67); All results acceptable

If “T” Test value greater than 1.67; Discard results at the evaluation location, recalculate with N=4 for the remaining locations and re-test. If these results fail; then nuclear correlation does not pass and more cores shall be taken for correlation or only use cores for density pay factors.

A minimum of four data points must pass the T-test in part 106.3.2.41.3 of this test method.

The standard deviation shall be less than or equal to 2.5. If it is larger than 2.5, then there is too much variation in the densities of the test strip and the test strip would need to be redone.

The “T” Test value shall be less than “T_0.95” at the critical n value for an acceptable calibration of the nuclear gauge device within the test strip.

The correction factor (CF) is applied to each nuclear density reading on the project to obtain a corrected nuclear density.

When calibrating multiple gauges, if it becomes necessary to discard a location because it failed the standard deviation (S) or the “T” test, then the same location must be discarded for all gauges being calibrated using these set of cored locations.

106.3.2.41.4 QC/QA Sampling

For QC testing random test locations shall be determined by the anticipated amount of paving and the Quality Control plan in accordance with Sec 403. The nuclear gauge shall be in backscatter mode and properly seated in accordance with AASHTO T355 and record the following information:

Take 5 one-minute readings with the nuclear density gauge and mark the outline of the gauge at the random location.

Record the 5 nuclear density values in lb/ft³,

Discard the single highest and lowest values,

Average the three remaining values.

106.3.2.41.5 Calculations

Correction Factor (CF) – The CF established during the test strip shall be reported.

Gmm – The Gmm from the most recent sublot shall be reported.

Density of Water @ 77 F = 62.245 lb/ft³

The Average Nuclear Density (ND) from the random sampling location shall be reported and the CF from the test strip applied to report the ND (corrected) value.

ND_Corr = ND + (CF Value from Test Strip); where either a positive or negative CF is added to ND.

Calculate Bulk Specific Gravity of Nuclear Density (G_ND);

G_ND = ND_Corr / 62.245

Calculate % of Theoretical Maximum Density (% Density);

% Density = G_ND / Gmm * 100

Report % Density to the nearest 0.1

106.3.2.41.6 Results

If either QC or QA are questioning the values of nuclear gauge readings on a sublot; coring shall be conducted to determine the pay factors for that sublot.

QA may not have the means of using a nuclear gauge methods; therefore, QA sampling by cores will continue and compared to the QC nuclear gauge readings.

Nuclear density measurements may also be used for random unconfined longitudinal joint testing following the same sampling procedures described above. The source rod of the gauge shall be within 8 to 12 inches of the mid-vertical edge of the mat.