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Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit

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Page 1: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Choosing the Right

Asphalt Mix

For the Right Road

Clark Morrison

Pavement Management Unit

Page 2: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

What is the Difference

Between Asphalt Surface Mixes?

SF9.5A

S9.5B

S9.5C

S9.5D

Page 3: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

As you go from A to B to C to D:

• Liquid AC gets stiffer

• AC content generally decreases

As a result:

• Resistance to rutting increases

• Resistance to cracking decreases

Page 4: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Q: How do we compensate for the

decreased resistance to cracking in

higher level mixes?

Page 5: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

A: Make the pavement thicker.

Page 6: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Q:

What happens if we

put a high level mix on a

thin pavement?

Page 7: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

A:

Compaction difficulties

Increased likelihood of cracking

Page 8: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Q: How do we choose

the right mix?

Page 9: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

18 kips

Page 10: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Mix Type 20 Year Loading

(Million ESALS)

Liquid AC

SF9.5A Less than 0.3 PG 64-22

S9.5B Less than 3 PG 64-22

S9.5C 3 to 30 PG 70-22

S9.5D Over 30 PG 76-22

Page 11: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Checking the Mix Type

Page 12: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

I’m supposed to overlay this road

with 1.5” S9.5C, but the existing

pavement is only 2 inches thick,

and I haven’t seen a truck out here

all day.

Can that be right?

Page 13: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Short-term traffic counts

• Count tractor-trailers and single unit

trucks on the road for one hour.

• Try to pick a “representative” hour

• “Hourly ESALs” =

(tractor trailers)+(single units)/3

Page 14: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Mix Level for Hourly ESALs

Hourly ESALs Mix Level

Less than 4 A

4 to 40 B

More than 40 C

Page 15: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 1:

• Resurfacing project will place 1.5”

S9.5C.

• In one “representative” hour you count

2 tractor trailers and 4 single unit trucks.

• Is the mix type appropriate?

Page 16: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 1

• “hourly ESALs” = (2 tractor trailers) +

(4 single units)/3 = 3.3

• Check the Chart

Page 17: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Mix Level for Hourly ESALs

Hourly ESALs Mix Level

Less than 4 A

4 to 40 B

More than 40 C

Page 18: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 1

• “hourly ESALs” = (2 tractor trailers) +

(4 single units)/3 = 3.3

• From Chart, mix type should be A.

• Mix type “C” is probably not appropriate.

Page 19: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Remember!

• This is a very rough guideline.

• It should not be used to make changes to

the plans immediately.

• It should be used to know when to raise

the question.

Page 20: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

But there’s still more to it. The

thickness of the existing

pavement matters too.

Page 21: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Existing Pavement Thickness

• Higher level mixes are stiffer, and require more effort to compact.

• The stiffness increases rut resistance, but makes it more likely to crack.

• Higher level mixes need a thicker “base” to get adequate compaction.

• Stiffer mixes need a thicker “base” to prevent cracking under traffic.

Page 22: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

A very rough guideline

Existing Pavement

Thickness* Surface Mix Level

Any A

More than 4” B

More than 7" C

* Each inch of ABC counts as ½ inch of asphalt.

Page 23: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 2:

• Resurfacing project will place 1.5” S9.5C.

• In one “representative” hour you count 30

tractor trailers and 45 single unit trucks.

• The existing pavement is 5 inches thick and

has moderate alligator cracking.

• Is the mix type appropriate?

Page 24: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 2

• “hourly ESALs” = (30 tractor trailers) +

(45 single units)/3 = 45

• Check the Hourly ESAL-Mix Level

Chart.

Page 25: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Mix Level for Hourly ESALs

Hourly ESALs Mix Level

Less than 4 A

4 to 40 B

More than 40 C

Page 26: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 2

• “hourly ESALs” = (30 tractor trailers) +

(45 single units)/3 = 45

• From Chart, mix type should be C.

Page 27: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 2

• “hourly ESALs” = (30 tractor trailers) +

(45 single units)/3 = 45

• From Chart, mix type should be C.

• Look at the thickness chart!

Page 28: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

A very rough guideline

Existing Pavement

Thickness* Surface Mix Level

Any A

More than 4” B

More than 7" C

* Each inch of ABC counts as ½ inch of asphalt.

Page 29: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Example 2

• “hourly ESALs” = (30 tractor trailers) +

(45 single units)/3 = 45

• From Chart, mix type should be C.

• Thickness is less than 7, so it may be

better to use a B-level mix.

Page 30: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

One More Example

• An engineer knows he has B-level traffic

on a pavement that is only 3 inches thick.

• He only has money to place one lift of

surface course.

• To compensate for the pavement being

too thin, he decides to go with a higher

level mix, placing 1.5” S9.5C.

Page 31: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

One More Example

• Is this a good idea?

• Why or why not?

Page 32: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Remember!

• These are very rough guidelines.

• They should not be used to make changes

to the plans immediately.

• They should be used to know when to

raise the question.

Page 33: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Why are the Guidelines Rough?

• We are projecting traffic over the life of the pavement from a one hour count using assumed ESAL coefficients.

• The existing structure depends on more than the thickness of the pavement. The condition of the pavement and the quality of the subgrade matter too.

Page 34: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

Summary

Checking the Mix Type

• Count trucks for a “representative” hour and calculate hourly ESALs.

• Check the hourly ESALs chart.

• Determine the thickness of the existing pavement.

• Check the pavement thickness chart.

• Raise the question if needed.

Page 35: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt

The End

Page 36: Choosing the Right Asphalt Mix For the Right Road · Choosing the Right Asphalt Mix For the Right Road Clark Morrison Pavement Management Unit . What is the Difference Between Asphalt