APAI Article Archive

(APAI Web Site, April 2006)

APAI Recommends New Strength Coefficient Values

By: John Bellizzi, P.E.

Hey, Iowa local system paving designers. Today’s Hot Mix Asphalt (HMA), developed under SuperPave criteria and tested under quality management asphalt (QM-A), is stronger and more durable than ever before. Properly designed HMA is not expected to require major maintenance for at least 20 years when a thin (1.5”) mill and fill can rehabilitate the surface with the balance of the structure considered perpetual. Design of modern HMA requires adjustment of past strength determinations last made in the 1970s.

Your local paving association of Iowa contractor and the Asphalt Paving Association of Iowa (APAI) personnel will assist you in determining equivalent full-depth asphalt paving for various thickness of Portland Cement Concrete (PCC).

When evaluating strength, HMA and PCC are not equal on an inch-to-inch basis. The American Association of State Highways and Transportation Officials (AASHTO 1972 Interim Guide) included after test results structural coefficients per inch of pavement for use in design of roadway thicknesses. The structural coefficient for PCC was determined to be 0.50. Type A asphaltic concrete 0.44 and Type B asphaltic concrete 0.40.


	60% Crushed, Type “A” Aggregate

	60% Crushed, Type “A” Aggregate

APAI recommends changes in the asphalt strength coefficients based on the following: HMA utilized for intermediate and surface layers using aggregate crushed to 60% from a Type A aggregate source will continue to be a structural coefficient of 0.44. HMA for surface and intermediate courses using 75% crushed aggregate from a Type A aggregate source is recommended to be assigned a structural coefficient of 0.46. HMA base courses using 45% crushed aggregate from a Type B aggregate source is recommended to be a structural coefficient of 0.42.

The AASHTO tests performed in the early 1970s recommended a structural coefficient of 0.44 for HMA with 65% crushed material. APAI continues to recommend a structural coefficient of 0.44 for HMA with 60% crushed materials from a Type A aggregate source. The improvements in the sheer strength of binder and in construction and testing methods compensates for the reduction of 5% crushed material.

HMA with 75% crushed aggregate from a Type A aggregate source increases the internal friction of the HMA mix and it is recommended that the structural coefficient be increased to 0.46.

HMA base courses with 45% crushed Type B aggregate exceeds by 15% in crushed content over which AASHTO tested for Type B asphaltic concrete base courses containing 30% crushed aggregate. Therefore the increase in structural coefficient is recommended from 0.40 to 0.42.

As most of the pavement designers are well aware, Iowa’s local system’s Statewide Urban Design & Specifications (SUDAS) will be the standard specifications for urban and county road design. In the new SUDAS specifications and design standards, the HMA mixture design criteria requires 60% crushed A quality aggregate in all intermediate and surface courses for all traffic loadings up to 300,000 ESALs. Also the SUDAS standards require 45% crushed B quality aggregate for all base courses for all traffic loading under 300,000 ESALs. For traffic loading above 300,000 ESALs SUDAS requires HMA surface courses to be 75% crushed A quality aggregate. These categories for traffic loading include most of the roadway pavement designs for local systems in Iowa. When comparing PCC and HMA the above APAI strength coefficient changes, outlined above, align with the SUDAS HMA mixture selection design requirements.

The structural coefficient changes recommended by APAI have been studied and reviewed by Dr. Brian Coree, P.E., former ISU assistant professor of Civil Engineering. Who has issued the following report in July of 2005:

“The traditional HMA layer coefficient derived from the AASHTO Road Test (1972) was based on pre-SuperPave mixtures that would not meet today’s higher standards. Evaluating modern mixtures with modern techniques, including the mechanistic-empirical evaluation of resilient modulus and seasonal temperatures profiles, results in the conclusion that the values below are highly conservative for use in Iowa.”

75% crushed, Type A aggregate Layer coefficient = 0.46
60% crushed, Type A aggregate Layer coefficient = 0.44
45% crushed, Type B aggregate Layer coefficient = 0.42

Today’s HMA performs in terms of strength and longevity with PCC when equivalent designs are determined. Slight modifications of the 1970s AASHTO strength coefficients for HMA are recommended by APAI due to the modern mixtures, increased crushed aggregate content and the sheer strength of the aggregate binder. SUDAS established mixture selection criteria will require knowledge of the layer strength coefficient to determine equivalent designs. Although HMA and PCC are not equivalent on an inch-to-inch basis, the above recommendation and the next example illustrate how to compare PCC and HMA.

The following example of urban residential equivalent paving (<300K ESALS) utilizing the recommended structural coefficients (SC) determines structural numbers (SN).

Portland Cement Concrete
6” PCC	@	SC 0.50	=	SN 3.00

Local system’s survey results indicate 6” non-reinforced PCC with 6” integral curb is the predominate cross section design for paving of low volume (less than 300K ESALs) streets and roadways. PCC paving is typically supported by 4-6” of compacted modified aggregate sub-base.

Hot Mix Asphalt
4.25” base	@	SC 0.42	=	SN 1.78
3” inter./surface	@	SC 0.44	=	SN 1.32
Total				SN 3.10

Equivalent HMA paving cross-section design consists of 7” thick 2’ wide PCC curb and gutter (C&G) section with 6” integral curb and 7.25” of HMA paving. The advantages of the 2’ wide, 6” thick C&G compared to 2’-6” wide, 7” thick C&G are that the sub-base or sub-grade can be prepared uniformly curb-to-curb and the tendency for the C&G to tilt is minimized by the shift center of gravity to the curbline.