2017 CalAPA Fall Asphalt Pavement Conference presentation: What are Voids in the Mineral Aggregate (VMA)? How can designers and producers design for VMA and manage in production.

1. October 26, 2017
Pascal Mascarenhas

2.  VMA –
◦ Intergranular void space between the aggregate
particles in a compacted paving mixture.

Why do we need VMA:
◦ To increase the durability of asphalt concrete mixes
◦ Kandhal – VMA allow enough asphalt to be
incorporated into the mix to obtain durability
without flushing.
◦ Minimize the severity of thermal and reflection
cracking.

3.  Lower susceptibility to variations in asphalt
and fines during production.
 Have adequate VMA without weakening the
aggregate skeleton.

4.  In 1948 Marshall mix design did not have a
VMA requirement.
 Marshall believed “no limits can be
established for VMA for universal application,
because of the versatile application of
bituminous materials to many types and
gradations of aggregate.
 1955 - 1962 - McLeod presented Marshall
mix design methodology listed VMA as a
requirement.

5.  NMAS – Nominal Maximum Aggregate size –
One size larger than the first sieve to retain
more than 10%.
 No VMA requirements for Hveem mix designs
which are still being used today.

6. MS-2
Sixth
Edition
Size, NMAS Minimum VMA %
Design Air
Voids 3.0 4.0 5.0
3/8" 14 15 16
1/2" 13 14 15
3/4" 12 13 14
MS_2
Seventh
Edition
Size, NMAS Minimum VMA %
3/8" 15
1/2" 14
3/4" 13
Size, NMAS Caltrans Superpave
Min VMA Max VMA
3/8" 15.5 18.5
1/2" 14.5 17.5
3/4" 13.5 16.5
RHMA-G 18 23

7.  We went from no VMA requirements
 Tighter requirements in mix design
 Has to be tested in production
◦ 1 per 10,000 tons or 2 per project

8.  After implementation nationwide of
Superpave researchers discussed problems
with volumetric design:
◦ Achieving minimum VMA is the most difficult and
time-consuming step in the mix design process.
◦ Increased compactive effort of the Superpave
Gyratory compactor.

9.  Gradation Effect
 Type and Amount of Compactive Effort
 Shape
 Surface Texture
 Strength
 Mineral filler (minus 200)

10.  Bailey Method
 Use the 0.45 chart.
 Maximum Density Line – a straight line from
the zero to the maximum aggregate size.
 Some FAA specs draw MDL from the percent
passing the largest sieve to the percent
passing the P200
 Increase fine portion

11.  Determined by Specifications for HMA
 Adjust pressure, Squaring time and number
of gyrations for RHMA mixes

12.  Aggregate particles will pack according to the
shape of the particles.
 Cubical particles will not pack as tightly.
 Flat particles do not help for Superpave mixes

13.  Rougher textured particles do not move as
much during compaction.
 More crushed faces result in greater texture.

14.  Higher strengths means loss LAR loss.
 Less breakdown and generation of fines
during production and compaction.

15.  Mineral filler fills voids and takes place of
asphalt.
 This is noticeable in mixes that mandate the
use of minimum lime content.
 Same aggregate structure without lime can
result in 0.5% increase in AC.

16.  VMA = 100 – (Gmb Ps)/Gsb
 Gmb – Bulk specific gravity of compacted mix
 Ps - Percent of aggregate by TWM
 Gsb - Bulk specific gravity of aggregate

17. -0.10
0.10
0.30
0.50
0.70
0.90
1.10
1.30
1.50
12 13 14 15 16 17 18
NCHRP 567 Effect of Design VMA & Air Voids on Rut Resistance
3% AV Design / 7% in-place 4% AV Design / 7% in-place
5% AV Design / 7% in-place

18. -0.10
0.10
0.30
0.50
0.70
0.90
1.10
1.30
1.50
12 13 14 15 16 17 18
NCHRP 567 Effect of VMA & in-Place Air Voids at Constant Air Voids on Rut
Resistance
4% AV Design / 8% in-place 4% AV Design / 7% in-place
4% AV Design / 6% in-place

19. Comments?