Bituminous Compaction Uniformity

Assessment Using Non-Destructive

Testing GPR

Dr. Shongtao Dai

Office of Materials and Road Research,

Mn/DOT

Dr. Kyle Hoegh

University of Minnesota

Mn/DOT

Office of Materials and Road Research

What is GPR?

GPR is a noninvasive,

nondestructive testing

tool for mapping

subsurface conditions

Radio waves to detect

objects and determine

distance from the object

Wave propagation in solids

Each scan

GPR is an effective tool for thickness determination

(similar to x-ray)

GPR cannot identify material type or object

(dielectric only)!

Single frequency impulse GPR

1 pair antenna: emits only one

frequency

2.0 GHz, 1.0 GHz, etc

One transverse location in each

pass

Step Frequency GPR (3D-GPR)

Multiple pair antenna

11 pairs

Frequency range: 50 MHz - 3.0 GHz

Variable depth

5ft footprint in each pass

Traditional GPR and 3D GPR (Air Coupled)

Ground-coupled: 2.5 GHz, 1.5 GHz, 400 MHz and

100 MHz

Walking speed

Locate underground objects

Approximate penetration depth:

2.5 GHz: 1.0 ft

1.5 GHz: 1.5 ft

400 MHz: 10 ft

100 MHz: 40 ft

In general, the higher the frequency, the higher the

resolution, but less penetration depth

Pavement density has great effects on performance. Lack of density --- localized failure

1989 – “Effect of Compaction on Asphalt Concrete Performance” (Wash.DOT) Each 1% increase in air voids (over 7 percent)

tends to produce ~10 percent loss in pavement life.

Core used to determine density At a particular location, not represent the entire

pavement density.

Need a way to obtain full coverage of the surface

GPR is a potential good tool: Continuous profile

Locate relative high or low density areas based on

dielectric map

Compaction Uniformity Mapping

a : dielectric constant of the material near the surface.

a = [(1+A1/Am)/(1-(A1/Am)]²

TH13 (June 27-28, 2012) Overlay project

Surveyed 300ft

y = 0.0584x + 0.3046 R² = 0.9785

5.5

5.55

5.6

5.65

5.7

5.75

5.8

5.85

5.9

5.95

90 92 94 96

Die

lectr

ic

Density (%)

TH61 (White Bear, 2014) 4” mill and overlay

Surveyed 600ft section on NB 2 times: test repeatability

Surface Arrival Amplitude

3D-GPR

Dielectric Map 3” antenna spacing and 3” measurement spacing

Trial 1

Trail 2

TH494: South of Carlson Parkway

1. 5ft coverage in the transverse direction in each Pass.

2. 400 ft coverage in the longitudinal direction.

3. Each pass over joint between two lanes.

4. 3 passes total to ensure repeatability of data.

Core 2 Core 4 Core 6 Core 8

Core 1 Core 3 Core 5 Core 7

Lab Air Voids vs Dielectric of 3 Trials

y = 151.46e-0.66x

R² = 0.9436

y = 136.82e-0.637x

R² = 0.9176

y = 177.63e-0.69x

R² = 0.9512

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

11.00

4.00 4.50 5.00 5.50 6.00

Lab

Mea

sure

d A

ir V

oids

(%)

GPR Measured Dielectric

Lab Measured Air Voids vs. Dielectric

Trial 1

Trial 2

Trial 3

Expon. (Trial 1)

Expon. (Trial 2)

Expon. (Trial 3)

Air Void Map

The UofM SHRP2 Project: Rolling Density Meter (RDM)

Core 1: 4.5 in from joint on right lane

TH25: moving from left lane to right lane

(Air void:8.9%)

L. Joint Core

3

3.5

4

4.5

5

5.5

6

0.0000 1.0000 2.0000 3.0000

Die

lect

ric

Distance, ft.

Static

Transversal (003)

Longitudinal (002)

Core 4 Location

Averaged Time Scan (001)

LG_1

LG_2

Longitudinal Joint

Core L. Joint

(Air void:8.7%)

C01_6inAC1_Cl4

zcell14_114

Thank you