FWD tests produce a series of numbers that describes the deflection basin shape. Although these numbers are descriptive, it is often desired to use a single number as a index of the deflection basin shape. One such number, the AREA Parameter represents the normalized area of a vertical slice taken through a deflection basin between the center of the test load and 914 mm (3 feet) away from the test load. Dividing the slice area by the deflection measured at the center of the test “normalizes” the AREA Parameter. Thus the AREA Parameter is the length of one side of a rectangle where the other side of the rectangle is D_{0}. Figure 1 illustrates this concept.
Figure 1: The FWD AREA Parameter
The basic AREA Parameter equation is:
where:  AREA  =  the FWD AREA Parameter. Expressed in units of length (usually inches or mm).  
D_{0}  =  surface deflection at the test load center  
D_{1}  =  surface deflection at 12 inches from the test load center  
D_{2}  =  surface deflection at 24 inches from the test load center  
D_{3}  =  surface deflection at 36 inches from the test load center 
Maximum and Minimum AREA Values
The maximum AREA value is 36 inches (915 mm) and would occur when all four deflection measurements are equal, which is not likely to actually occur. For all four deflection measurements to be nearly equal the pavement would have to be extremely stiff (like rigid pavement or thick, fulldepth HMA). The equation for the maximum AREA value is as follows:
The minimum Area value should be no less than 11.1 in. (280 mm) and would occur when all pavement layers have the same elastic modulus. Or, put another way, the pavement structure does not contribute any additional stiffness to the underlying subgrade. This is highly unlikely for actual pavement structures. Low AREA values suggest that the pavement structure is not much different from the underlying subgrade material. This is not always a bad thing if the subgrade is extremely stiff, but this does not occur very often). When all elastic moduli are equal the ratio of D_{1}, D_{2} and D_{3} to D_{0} are as follows, which gives the minimum AREA value as shown below:
Typical AREA Values
Table 1 shows some typical AREA values.
Pavement  AREA Value  
inches  mm  
Rigid pavement  24 – 33  610 – 840 
Thick flexible pavement >= 100 mm (4 inches) 
21 – 30  530 – 760 
Thin flexible pavement < 100 mm (4 inches) 
16 – 21  410 – 530 
BST  15 – 17  380 – 430 
Weak BST  12 – 15  300 – 380 
FWD Area Program
WSDOT has developed a program for the determination of the AREA Value and subgrade resilient modulus using the Dynatest FWD.
NOTES:

The FWD Area program can also be downloaded from the Washington State DOT Materials Lab at: http://www.wsdot.wa.gov/biz/mats.