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Major Topics on this Page

5.1 Measurement

5.2 Measurement Techniques

5 DEFLECTION

Pavement surface deflection measurements are the primary means of evaluating a flexible pavement

structure and rigid pavement load transfer. Although other measurements c an be made that reflect (to

some degree) a pavement's st ructural condition, surface deflect ion is an important pavement

evaluation method because the magnitude and shape of pavement deflection is a function of traffic

(type and volume), pavement structural section, temperature affecting the pavement structure and

moisture affecting the pavement structure. Deflection measurements can be used in backcalculation methods to determine pavement structural

layer stiffness and the subgrade resilient modulus. Thus, many characteristics of a f lexible pavement can be determined by measuring its

deflect ion in response to load. Furthermore, pavement deflect ion measurements are non-dest ructive.

5.1 Measurement

Surface deflec tion is measured as a pavement surface's vertical deflected distance as a result of an applied (either static or dynamic) load. The

more advanced measurement devices record this vertical deflection in multiple locat ions, which provides a more complete characterization of

pavement deflect ion. The area of pavement deflect ion under and near the load applicat ion is collectively known as the "deflect ion basin".

5.2 Measurement Techniques

There are three broad categories of nondestructive deflection testing equipment:

Static deflections

Steady state deflect ions

Impact load deflections (FWD)

The general principal is to apply a load of known magnitude to the pavement surface and analyze the shape and magnitude of the deflection

basin to assess the strength of the pavement structure (see Figure 9.15).

Figure 9.15: Deflection Measurement Schematic

WSDOT Deflection Measurement Method

WSDOT uses the FWD.

WSDOT FWD Trailer

5.2.1 Static Deflection Equipment

Static deflection equipment measure pavement deflection in response to a static load.

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5.2.1.1 Benkelman Beam

The Benkelman Beam (see Figure 9.16), developed at the Western Association of State Highway Organizations (WASHO) Road Test in 1952, is a

simple device that operates on the lever arm principle. The Benkelman Beam is used with a loaded truck - typically 80 kN (18,000 lb) on a single

axle with dual tires inflated to 480 to 550 kPa (70 to 80 psi). Measurement is made by placing the tip of the beam between the dual tires and

measuring the pavement surface rebound as the truck is moved away (see Figure 9.17). The Benkelman Beam is low cost but is also slow, labor

intensive and does not provide a deflection basin.

Figure 9.16: Benkelman Beam Schematic

Figure 9.17: Benkelman Beam in Use

Standard Benkelman Beam tests are described in:

AASHTO T 256: Pavement Deflect ion Measurements

ASTM D 4695: General Pavement Deflection Measurements

5.2.2 Steady State Deflection Equipment

Steady state deflect ion equipment measure the dynamic deflection of a pavement produced by an oscillating load. These devices consist of a

dynamic force generator (that produces the oscillating load), a motion measuring instrument (to measure the oscillating load), a calibration unit

and several deflect ion measuring devices (transducers, acc elerometers, seismometers, etc.). The main advantage that steady state deflect ion

equipment offer over static deflect ion equipment is that they can measure a deflection basin. The most common steady state deflec tion

equipment are the Dynaflect and the Road Rater.

The stead state deflection equipment (see Figure 9.18) is stationary when measurements are taken with force generator (counter rotating

weights) started and deflect ion sensors (transducers) lowered to the pavement surface. Figure 9.19 is a plot of a typical force output and

Figure 9.20 shows the location of the equipment's loading wheels and five transducers. The equipment is most suitable for use on thinner

pavements including low volume rural highways, county roads, municipal streets, and parking lots (IMS, 2001).

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Figure 9.18: Dynaflect

Figure 9.19: Dynaflect Force Output

Figure 9.20: Standard Location of Dynaflect Loading Wheels

and Transducers

The Road Rater (see Figure 9.21) is the other popular type of steady state deflect ion equipment. It must also be stat ionary to start and

operates in a similar fashion to the Dynaflect .

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Figure 9.21: Road Rater

Standard stead state deflection tests are described in:

AASHTO T 256: Pavement Deflect ion Measurements

ASTM D 4695: General Pavement Deflection Measurements

5.2.3 Impact (Impulse) Load Response

All impact load devices deliver a transient impulse load to the pavement surface. The subsequent pavement response (deflection basin) is

measured by a series of sensors. The most c ommon type of equipment is the falling weight deflectometer (FWD) (see Figures 9.22 through9.26). The FWD can either be mounted in a vehicle or on a trailer and is equipped with a weight and several velocity transducer sensors. To

perform a test , the vehicle is stopped and the loading plate (weight) is positioned over the desired locat ion. The sensors are then lowered to

the pavement surface and the weight is dropped. Multiple tests c an be performed on the same location using different weight drop heights

(ASTM, 2000). The advantage of an impact load response measuring device over a steady state deflection measuring device is that it is quicker,

the impact load can be easily varied and it more accurately simulates the transient loading of traffic . Results from FWD tests are often

communicated using the FWD AREA Parameter.

Figure 9.22: FWD Impulse Loading

Mechanism (foreground) and Sensors

(background)

Figure 9.23: FWD Figure 9.24: Dynatest 8000 FWD

Figure 9.25: KUAB FWD Figure 9.26: JILS FWD

The standard impact load response test method is:

ASTM D 4694: Standard Test Method for Deflections with a Falling Weight Type Impulse Load Device

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5.2.4 Correlations Between Deflection Measuring Equipment

In general, correlations between deflect ion devices should be used with caution. Too often, a correlation is developed for a specific set of

conditions that may not be present for those using the correlation. It appears that the best approach is to obtain pavement parameters (such

as layer moduli) from the spec ific device being used. However, that said, a few of many such correlations that have been developed follow.

5.2.4.1 Benkelman Beam to FWD

(based on unpublished data collected by the Washington State DOT Materials Laboratory in 1982-1983)

BB = 1.33269 + 0.93748 (FWD)

where: BB = Benkelman Beam deflection (inches x 10-3)

FWD = FWD center-of-load deflection (inches. x 10-3) corrected

to a 9,000 lb. load applied on a 11.8-inch diameter plate

R2 = 0.86 Standard Error = 3.20 mils Sample Size = 713

5.2.4.2 Benkelman Beam to Dynaflect

(based on Hoffman and Thompson, 1981)

BB = 20.63 (D)


D = Dynaflect center-of-load deflection (inches x 10-3)

R2 = 0.72

5.2.4.3 Benkelman Beam to Road Rater

(based on Hoffman and Thompson, 1981)

Comparing a Benkelman Beam load at 9,000 pounds on dual tires with 70-80 psi inflated tires and Road Rater at 8,000 pound peak-to-peak load

at 15 Hz on a 12 inch diameter plate on a stabilized pavement:

BB = 2.57 + 1.27(RR)


RR = Road Rater (Model 2008) center-of-load deflection at

8,000 pounds and 15 Hz (inches x 10-3)

R2 = 0.66

The Western Direct Federal Division, Federal Highway Administration, Vancouver, Washington provides the following correlation for the Benkelman

Beam to Road Rater Model 400:

BB = 8.0 + 9.1026 (D0)


RR = Maximum deflection from Road Rater Model 400

(deflection location between load pads) at a load of

1,300 pounds at 25 Hz

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