# Limiting Deflection – Asphalt Institute

The basic approach of the overlay design procedure is to identify continuous pavement sections of uniform performance, obtain “static” pavement surface deflections with the Benkelman Beam or other device and determine the expected traffic by use of ESALs. This section summarizes the approach described in the Asphalt Institute’s Asphalt Overlays for Highway and Street Rehabilitation (MS-17[1]).

## Deflection Analysis

The Asphalt Institute recommends that a minimum of 20 deflection measurements be taken each mile and randomly located in the outer wheelpath. From this data for each “uniform” pavement section, a “representative rebound deflection” is determined as follows:

RRD = (x + 2s)(f)(c)

 where: RRD equals representative rebound deflection (inches) x equals mean of the individual deflections (inches) s equals standard deviation of the deflections (in) f equals temperature adjustment factor c equals critical period adjustment factor (where c = 1 if deflection tests made during the most critical period (highest pavement deflections)).

This calculation of RRD represents an upper bound of about 97 percent of all deflections measured. The temperature adjustment factor adjusts the existing asphalt concrete surfacing to a standard temperature of 21°C (70°F) (refer to Figure 1).

A study for the U.S. Forest Service completed during 1994 (Uhlmeyer et al. [7.33][2]) reviewed deflection data from paved roads in the Willamette National Forest (Central Oregon), the Olympic National Forest (both aggregate and asphalt surfaced pavements) and the Kootenai National Forest (western Montana). The critical period adjustment factor (c) ranged from about 1.0 to 1.7 (with the exception of one forest road in the Kootenai which ranged from 3.0 to 8.0). Overall, a c = 1.5 appeared to be widely applicable. Further, if the deflection data are collected for different time periods, the following adjustment factors could be used:

Time Period When Deflection Data Obtained Deflection Data Adjustment Factor (c)
January - March 1.00
April - June 1.25
July - September 1.50
October - December 1.25

The above “c” values are generally representative for flexible pavements located in areas with modest annual freezing and thawing (a Freezing Index of say less than 700°F-days) or a wet climate. A higher “c” is likely needed in areas with severe winter freezing and thawing. The Roads and Transportation Association of Canada (RTAC) (RTAC, 1977[3]) uses a multiplier of 2.5 to convert Benkelman Beam measurements taken during the fall period (September 1 to October 15) to “maximum spring values.” This ratio of 2.5 is a bit higher than generally observed in the U.S., possibly reflecting the generally more severe winter and thaw periods in Canada.

The deflection measurements can be used to estimate the remaining life of the pavement or the needed thickness of asphalt concrete overlay. To determine the required overlay thickness, Figure 2 is used with the RRD and ESALs as the required input.

Example

A two-lane highway has the following characteristics and resulting overlay requirement (same data as for the component analysis example):

1. Traffic
• Average Daily Traffic = 4,000
• Percent trucks (total all units) = 10%
• Traffic growth rate = 4%
1. Existing pavement structure and condition
• Asphalt concrete = 3 in.
• Crushed stone base = 8 in.
• Subgrade design strength value: CBR = 8 or MR » 12,000 psi
• Overall, the pavement structure is in poor condition, with the HMA exhibiting well defined crack patterns

Determine overlay thickness for a 20-year design period.

1. 80 kN (18,000 lb.) ESALs
• number of trucks in the design lane per day = (4,000)(0.50)(0.10) = 200
• ESALs/day = 200(0.4 ESAL/truck) = 80 ESALs/day
• 18K ESAL for design period = (80 ESALs/day) (365 days/yr)(29.8) » 870,000 after adjustment for design period and traffic growth rate. Note:
1. Deflection data
• mean (x) = 0.061 in., standard deviation (s) = 0.004 in.
• f: If the mean asphalt concrete temperature during deflection = 15°C (60°F) then f = 1.10 (refer to Figure 1).
• c: If tests are made during the critical period, then c = 1.0.
1. Representative rebound deflection
• RRD = x + (2s)(f)(c)
• RRD = [0.061 + 2(0.004)](1.10)(1.0)
• RRD = 0.076 in.
1. Required asphalt concrete overlay thickness = 96.5 mm (3.8 in) (refer to Figure 2)

Footnotes    (↵ returns to text)
1. The Asphalt Institute.  (2000).  Asphalt Overlays for Highway and Street Pavement Rehabilitation, Third Edition.  Manual Series No. 17.  The Asphalt Institute.  College Park, MD.
2. Roads and Transportation Association of Canada (RTAC).  (1977).  Pavement Management Guide.  Roads and Transportation Association of Canada.  Ottawa, ON.