New Jersey

Department of Transportation

Standard Specifications

for Road and Bridge Construction

2007

NJDOT B-8 – Determining Job Mix Formula for
Modified Open-Graded Friction Course Mixes

1. Scope. This test method is used to determine gradation and the percent asphalt in a MOGFC mixture. The gradation is verified to ensure stone-on-stone contact, and the impact resistance of the final JMF is verified. The optimum asphalt content (AC) is determined from: (1) aggregate surface area, (2) relative Voids in Mineral Aggregate (VMA), and (3) visual drain-down determination of asphalt content. A simple average of these 3 criteria is used to determine the JMF asphalt content.
   
   
2. Apparatus. Use the following apparatus:
   
   
   1. Equipment as needed for AASHTO T 19.
   2. Equipment as needed for Superpave mix design as specified in AASHTO T 312.
   3. Equipment as needed for AASHTO T 209.
   4. Ovens capable of maintaining temperatures as specified in this method.
   5. Clear, glass (Pyrex) 9-inch diameter pie pans.
   6. L.A. Abrasion Machine conforming to AASHTO T 96.
   7. Equipment as needed for AASHTO T 85.
   8. No. 4 sieve according to AASHTO M 92.
   
   
3. Procedure. Perform the following steps:
   
   
   1. Verification of Stone-On-Stone Contact. Choose the design gradation to meet the air void requirements and to ensure that the aggregate skeleton exhibits stone-on-stone contact. Sieve aggregates proportioned to meet the selected JMF gradation over a No. 4 sieve. Keep the material retained on the No. 4 sieve as the coarse aggregate fraction. Determine the unit weight G_uwca of the coarse aggregate fraction of the aggregate using the dry rodding technique according to AASHTO T 19 For the selected JMF, determine the voids in the coarse aggregate of the mix (VCA_mix) based on specimens compacted with 50 gyrations according to AASHTO T 312. Calculate:
   
   
   

   	VCAdrc =	100 ( Gsbca − Guwca )
   		Gsbca

   Where:
   VCAdrc =	The voids in the coarse aggregate fraction of the JMF aggregate skeleton.
   Gsbca =	The bulk specific gravity of the coarse aggregate fraction as determined by AASHTO T 85.
   Guwca =	The unit weight of the coarse aggregate fraction (expressed in kilograms per cubic meter) as determined by AASHTO T 19.

   
   

   	VCAmix = 100 −	Pca − Gmb
   		Gsbca

   Where:
   VCAmix =	The voids in the coarse aggregate fraction of the mix. The coarse aggregate fraction of the aggregate is that portion of the JMF aggregate skeleton not passing the No. 4 sieve.
   Pca =	The percent of the coarse aggregate fraction by weight of total mix.
   Gmb =	The bulk specific gravity of the mix at the design AC content as determined by Section 3.4.
   Gsbca =	The bulk specific gravity of the coarse aggregate fraction as determined by AASHTO T 85.

   
   For stone-on-stone contact VCA_mix must be less than VCA_drc
   
   
   2. Surface Area Asphalt Content. Determine “surface area” asphalt content according to NJDOT B-9.
      
      
   3. Relative VMA Asphalt Content. Mix aggregate graded to proposed JMF with asphalt and fiber at 5 asphalt contents (1 at the estimated JMF asphalt content, 1 each at + and −0.5 percent and 1 each at + and –1.0 percent of the estimated JMF asphalt content) according to AASHTO T 312. After mixing, return sample to the oven if necessary, and when at the recommended compaction temperature, compact the specimens. Compact 3 specimens at each asphalt content using 50 gyrations of a Superpave Gyratory Compactor according to AASHTO T 312. Determine the bulk specific gravity, G_mb from each specimen according to NJDOT B-6 or AASHTO TP 69.
      
      Determine maximum specific gravity, G_mm, at each asphalt content according to AASHTO T 209. From G_mb, G_mm, and each known asphalt content, calculate volumetric information as follows:
      
      Plot asphalt content versus “relative VMA” and select the asphalt content at the lowest point on the curve.
   
   
   

   	Va = 100 x	1 − Gmb
   		Gmm

   	Vb  = b x Gmb

   	VMAR = Va x Vb

   	Where:
   b =	Percent AC by weight of total mix
   Gmb =	the bulk specific gravity of the specimen as determined by NJDOT B-6 or AASHTO TP 69.
   Gmm =	the maximum specific gravity at each asphalt content as determined by AASHTO T 209.
   Va =	Volume of air is equal to the percent air voids.
   Vb =	Percent by volume of asphalt cement.
   VMAR =	Relative VMA

   
   The volume of the fiber, absorbed asphalt, and specific gravity of asphalt binder are not accounted for in this procedure. This procedure measures “relative VMA” not the true VMA.
   
   
   4. Visual Draindown Asphalt Content. Prepare 1000-gram samples of the uncompacted mix for each of the asphalt contents. Place each sample into a clean, clear glass (Pyrex) 9-inch pie pans. Place samples in oven for 1 hour at the binder manufacturer’s recommended mixing temperature. Remove and let cool for 1 hour at room temperature. Visually observe the amount of liquid asphalt on the bottom of each pan. Select asphalt content where ample bonding is evident, without having excessive drainage as evidenced by an appearance of unconnected pools of asphalt binder around aggregate points of contact.
      
      
   5. Select Optimum Asphalt Content for JMF. Determine the JMF asphalt content by averaging the results from the 3 methods (surface area, relative VMA, and draindown).
   
   
   

   	ACjmf =	ACsc + ACvma + ACdd
   		3

   Where:
   ACjmf =	the design JMF
   ACsc =	the asphalt content determined by the surface area in Step 2.
   ACvma =	the asphalt content determined by relative VMA in Step 3.
   ACdd =	the asphalt content determined by draindown in Step 4.

   
   
   6. Verification of Abrasion and Impact Resistance of JMF (Cantabro Test). Compact 2 specimens using the JMF at the optimum asphalt content as specified in Step 3. Age the specimens for 7 days ± 8 hours in an oven at 140 ± 5 °F. Weigh each sample. Utilizing a Los Angeles Machine conforming to AASHTO T 96, without the charge of steel balls, subject the aged samples to 300 revolutions at 30 to 33 revolutions per minute. After the 300 revolutions reweigh the samples.
      
      Calculate the Percent Loss:
   
   
   

   	Ploss =	A − B
   			x 100
   		A

   Where:
   Ploss =	The loss expressed as percent of aged sample before L.A. Abrasion Machine treatment.
   A =	The weight of the sample before modified L.A. Abrasion test.
   B =	The weight of the sample after modified L.A. Abrasion test.

   
   If the loss is less than 30 percent, the JMF is acceptable. If the JMF is determined to be unacceptable, redesign the mix.
   
   
4. Report. Report all test results on ME provided forms.