measuring microtexture of aggregates using the dynamic

2020 Transportation Short Course October 13, 2020

2020 Virtual Transportation Short CourseRichard Izzo, P.E.Soils & Aggregates SectionMaterials and Tests Division

October 13th, 2020

Measuring Microtexture of Aggregates Using the Dynamic Friction Tester (DFT)


Why measure the texture of aggregate?

2

Improve safety & maintain skid resistance of

hot mix asphalt pavements.

Vision – Texas envisions a future with zero

traffic fatalities and serious injuries.



3

Texas Transportation Commission set goals of cutting fatal crashes in half by 2035 and zero

fatalities by 2050.

Road to Zero

TxDOT #EndTheStreakTX Campaign

November 7th, 2000 is the last deathless day on Texas roadways.

One person has died on Texas roadways every single day since then.

Focus is on ‘Engineering: Roadway Factors.’

Wet Surface Crash Reduction Program (formerly Wet Weather Accident Reduction Program)

2020 Transportation Short Course October 13, 2020 4

Stone-Matrix Asphalt (SMA) ~ 1 Year Old

Improve skid resistance. Safety hazard, loss of skid resistance

& prone to wet surface crashes



Frictional Properties of Aggregate

(<0.5mm)(0.5mm - 5cm)

Macrotexture

HighLow

Microtexture


Frictional Properties of Aggregate

Tex-499-A, Aggregate Quality Monitoring ProgramTable 1 – Classification Criteria

Property Test Method SAC A SAC B SAC C

Acid Insoluble Residue,% Minimum Tex-612-J 55 ---- ----

5-Cycle Mg Sulfate Soundness,% Maximum

Tex-411-A 25 30 35

Surface Aggregate Classification (SAC) is determined for each aggregate source for texture.


Dynamic Friction Test (DFT) measures a friction value on wet pavement surface (ASTM E1911).

Spinning disk with 3 rubber sliders that contact the pavement surface as disk rotates.

Dynamic Friction Tester (DFT)


Spinning disk with 3 rubber sliders

L ¾”, W ⅝”, H ¼”

Water is applied to pavement

surface.

Disk contacts the pavement surface

at 50 mph.



DFT value is an average of data

collected at 37, 25, and 12 mph

(60, 40, and 20 kph).

37 1225



Maryland DOT Testing

Passing ½” and retained 3/8”

~ 24 hours with curing.

5-3/4”

1”

14”

Laboratory Implementation of DFT


Maryland DOT Testing

Circular Track Polishing Machine (3-Wheel Polisher)

100,000 revolutions (~36 hours)

No water is used during polishing.



Maryland DOT Specifications

DYNAMIC FRICTION VALUE (DFV)

Category DFV, minimum

HDFV I

High DFV

50

HDFV II 45

HDFV III 40

SDFV IVStandard DFV

30

SDFV V 25

LDFV VI Low DFV 20

Blended DFT values based on aggregate bin %’s and gradations from HMA mixture designs.

High DFV includes SMA, Open Graded Friction Course (PFC), & Surface Courses.

• DFV greater than 40

Standard DFV includes Surface Courses only.

• DFV greater than 25

RAP assigned a DFV of 30



13


TxDOT Implementation

Simulate loss of texture by polishing aggregate using the Micro-Deval testing machine.

10,500 revolutions for 1-¾ hours.


Laboratory testing of aggregate passing 3/8” and retained on ¼” sieves (same as polish value).



Polyester resin (bondo) is used to hold the ¼” aggregate in place.

~30 minutes to prepare aggregate ring, tedious process to place and align aggregate, and a minimum of 4 hours for curing before testing.

1”½” Thickness

12”




We try to spread and lay the aggregate with the flat face up

as much as possible, so good surface area is available for

contact with DFT rubber sliders.


DFT values of individual aggregate

sources, not blended values.

All material codes combined – Dolomite,

Gravel, Igneous, Lightweight, Limestone,

and Sandstone.

Poor correlation/trend of Soundness to

DFT Value for SAC A sources.

Soundness not a strong indicator of

friction for SAC A sources.

Laboratory DFT Test Results



All material codes combined – Dolomite,

Gravel, Igneous, Lightweight, Limestone,

and Sandstone.

Poor correlation/trend of Soundness to

DFT Value for SAC B sources.

Soundness not a strong indicator of

friction for SAC B sources.


DFT value compared with acid insoluble

for SAC A and SAC B sources.

All material codes combined –

Dolomite, Gravel, Igneous, Lightweight,

Limestone, and Sandstone.

Generally, a high AI > 55 will produce a

higher DFT Value.

DFT does a good job differentiating

aggregate sources with lower AI values.




DFT differentiates the aggregate sources as

well as the acid insoluble.

DFT does a good job differentiating

aggregate sources with lower AI values.

Gravel sources identified as ‘Limestone &

Gravel’ are predominately comprised of

limestone, calcium carbonate.

Gravel sources identified as ‘Gravel &

Limestone’ are predominately comprised of

siliceous gravel, silica.


Hot Mix Asphalt (HMA) Mix Design Blended DFT Value

DFT values used to calculate a

blended value for HMA mix.

Blended DFT value will be based on

gradation and % retained on the #4

or #8 sieve.

Fine aggregates will be assigned

coarse aggregate DFT value.

DFT value of 30 assigned for RAP.

DFT blended value assigned to the mix design.


Hot Mix Asphalt (HMA) Mix Design Blended DFT Value


HMA Mix Design Blended DFT Value



+#8 +#8


Skid resistance on State-maintained highways are usually made every 0.5 mile.

Skid measurements are taken while traveling 50 miles per hour with a smooth tire.

29

Field Skid Information



Factors/variables to consider when comparing and looking at roadway skid information.

• Traffic, Annual Average Daily Traffic (AADT).

• Service life, age of pavement at the time of skid testing.

• Roadbed, such as main lanes, frontage roads, different lanes.

• Hot mix asphalt (HMA) mixture type, coarse vs dense-graded mixtures.

• Surface Aggregate Classification (SAC) of HMA.

• Pavement condition score, roadway could be flushing at the time of skid testing.


Acknowledgments

Started designing and building the prototype testing mold in September, 2016

Performed over +800 laboratory tests since October, 2016.

People involved in this effort.

• Edward Morgan, P.G. - Soils & Aggregates Section (SA) /Materials & Tests Division (MTD)

• Jeffrey Perabo, G.I.T. – SA/MTD

• Andre Smit, P.E. – Quality Assurance/MTD

• Victor Vogt, E.I.T. – SA/MTD

• Blake Lawrence, E.I.T. – SA/MTD


Acknowledgments

People involved in this effort.

• Adam Sherrill – Contract Employee

• Chizitelu Modebelu – TxDOT Summer Hire

• Jackie Rambo – Contract Student Employee

• Kenley Ulveling – TxDOT Summer Hire

• Michael Ramos-Yeo – TxDOT Summer Hire

• Roy De La Rosa – Contract Employee

• SA/MTD Aggregate Laboratory Technicians

measuring microtexture of aggregates using the dynamic

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