5  Deflection

Pavement surface deflection measurements are the primary means of evaluating a flexible pavement structure and rigid pavement load transfer.  Although other measurements can be made that reflect (to some degree) a pavement's structural condition, surface deflection 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 flexible pavement can be determined by measuring its deflection in response to load.  Furthermore, pavement deflection measurements are non-destructive. 

5.1  Measurement

Surface deflection 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 locations, which provides a more complete characterization of pavement deflection.  The area of pavement deflection under and near the load application is collectively known as the "deflection basin".

5.2 Measurement Techniques

There are three broad categories of nondestructive deflection testing equipment:

• Static deflections
• Steady state deflections
• Impact load deflections (FWD)

5.2.1  Static Deflection Equipment

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

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 Deflection Measurements
• ASTM D 4695: General Pavement Deflection Measurements

5.2.2  Steady State Deflection Equipment

Steady state deflection 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 deflection measuring devices (transducers, accelerometers, seismometers, etc.).  The main advantage that steady state deflection equipment offer over static deflection equipment is that they can measure a deflection basin.  The most common steady state deflection 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 deflection 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). 

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 deflection equipment.  It must also be stationary to start and operates in a similar fashion to the Dynaflect. 

Figure 9.21: Road Rater

Standard stead state deflection tests are described in:

• AASHTO T 256: Pavement Deflection 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 common type of equipment is the falling weight deflectometer (FWD) (see Figures 9.22 through 9.26).  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 location.  The sensors are then lowered to the pavement surface and the weight is dropped.  Multiple tests can 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

5.2.4  Correlations Between Deflection Measuring Equipment

In general, correlations between deflection 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 specific 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)

5.2.4.2  Benkelman Beam to Dynaflect

(based on Hoffman and Thompson, 1981)

BB = 20.63 (D)

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)

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)