Increasing In-Place Densities to Improve the Performance of Asphalt Pavements

SHRP2 Performance Specifications for Rapid Renewal (R07)

Ray Brown, National Center for Asphalt Technology

September 20, 2016

Density essential for good performance

Some say that density is the most important factor that affects performance

Density may not be the most important factor but it is clearly an important factor

National density variability

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Cumulative Frequency of Construction Densities

0102030405060708090

100

84 85 86 87 88 89 90 91 92 93 94 95 96

Percent Maximum Density, %

Cum

ulat

ive

Freq

uenc

y, %

55%

78%

Low density causes unacceptable permeability

FHWA funded project to improve density

NCAT prepared report on expected savings due to increased density

Ten states funded to investigate improved density For each state:

Best practices presentation by Asphalt Institute Preconstruction meeting to discuss plans Construction of control section and test section(s)

documented by NCAT

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NCAT study funded by FHWA

NCAT report showed that 1% increase in density conservatively resulted in at least 10% increase in life

NCAT Report 16-02 (can be downloaded from ncat.us)

ENHANCED COMPACTION TO IMPROVE DURABILITY AND EXTEND PAVEMENT SERVICE LIFE: A LITERATURE REVIEW

By Nam Tran, Pamela Turner, and James Shambley

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FHWA funded project to improve density

Ten states funded to evaluate methods for improving density NCAT selected to monitor these projects Construction of projects were to be completed in 2016 Seven of these projects have been completed Each project had to have a control section and at least one test

section For control section the contractor followed his normal compaction

procedures For test section, contractor looked for ways to improve compaction

without changing mix or without adding additional rollers Some states elected to construct 2 test sections

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Some items being usedto improve density

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Increase number of passes Apply more passes with vibrator on Reduce speed of rollers Some states elected to construct a second test section

where additional an additional roller was used

Target density

Percent of Laboratory Not used much today In-place density compared to laboratory density

Percent of TMD---Generally preferred method Most common method used today In-place density compared to theoretical maximum

density Percent of Control Strip

Still used by some today but must use caution In-place density compared to density obtained in

control strip9

What density level do we need

How about 92% of TMD? How about 94% of TMD? How about 96% of TMD?

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Type of mix

Fine-graded mixes tend to be less permeable for equal density than coarse-graded mixes

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Mix consistency

Mix variability results in increased variability in density

Use of material transfer vehicle will help to reduce variability

Use of infrared camera and IR bar will help to ensure reduced temperature variability

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Sampling for QC testing

Good procedures should be used for sampling whether samples taken from truck or behind paver or from some other location.

Material transfer vehicle

Some devices remix asphalt and these are preferred

Even if they dont remix they do keep paver and trucks separated resulting in smoother pavement

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Mix temperature

As mix cools it generally becomes more difficult to compact

Some have used WMA as a compaction aid Infrared gun measures the surface temp but not internal

temp Thin layers cool quicker so must be rolled quicker

especially in cool weather---this may mean more rollers

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Keep rollers close up to paver

Typically good to keep rollers close behind paver

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Laboratory air voids

Some have reduced laboratory air voids to help ensure specs are met for density

Must be careful when adding asphalt cement to lower air voids since this may result in rutting

This resulted in major rutting prior to Superpave Superpave placed emphasis on controlling volumetrics

during construction and reduced the amount of projects adding additional asphalt to reduce in-place air voids

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Layer thickness and NMAS

The thickness to NMAS should be at least 3 for fine-graded mixtures and at least 4 for coarse-graded mixtures.

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

2.0 3.0 4.0 5.0t/NMAS

Incr

ease

in A

ir Vo

ids,

%

All Data

Fine-graded

Coarse-graded

Chart1

4.0818181818

1.5

0.2363636364

0.4181818182

t/NMAS

Increase in Air Voids, %

Chart2

4.08181818182.654.4

1.50.71.6777777778

0.23636363640.050.2777777778

0.41818181820.70.3555555556

All Data

Fine-graded

Coarse-graded

t/NMAS

Increase in Air Voids, %

Sheet1

2.20.80.10.12.20.80.10.1

3.10.601.33.10.601.3

2.510.50ave2.70.70.10.7

1.70.400.8

4.71.70.20.22.510.50

1.10.200.51.70.400.8

10.33.60.30.84.71.70.20.2

6.53.20.501.10.200.5

1.80.60.10.110.33.60.30.8

4.81.400.86.53.20.50

6.230.901.80.60.10.1

ave4.11.50.20.44.81.400.8

6.230.90

ave4.41.70.30.4

Increase in Air Voids, %

2.04.12.74.4

3.01.50.71.7

4.00.20.10.3

5.00.40.70.4

Sheet2

Sheet3

Roller weight and type of roller

Typically 10-12 ton vibratory rollers used. Weight and tire pressure important for rubber tire rollers.

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Oscillating roller

Oscillating roller has shown some promise to obtain good density but more research needed.

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Quality of bond between layers

Clean underlying surface Consistent application of

tack coat should be applied

Too much tack sometimes a problem

Allow to cure before placing asphalt mixture

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Ensuring satisfactory bond

Bond test has been developed but not implemented by many

Bond test would be helpful

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Roller pattern

What pattern works on one project may not work on the next project

Use density gauge to help set rolling pattern

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Watch out for tender zone

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Testing location is important

Some areas receive more compaction than other areas

Density tests should be at random locations

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Summary

Increased effort to obtain density can result in at least 1 to 2 percent higher density

1 to 2% higher density will provide a significant increase in pavement life, generally believed to be 10-20%

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