Perspectives
A Quarterly Information Source from Benchmark, Inc.

Volume 59 September 2007

Crushed stone or gravel are primary components of any hot mix asphalt material. These materials are also commonly used as aggregate base courses installed prior to hot mix asphalt paving. Asphalt pavement cross-sectional designs commonly have both an aggregate base layer component and an asphalt layer component in which different thicknesses of each layer are evaluated to achieve a target structural capacity. Structural capacity of a pavement structure is dependent basically upon anticipated traffic over the life of the pavement structure and the longevity goals desired.

Many other factors can influence pavement design analysis, but a useful and common rule of thumb in the asphalt paving industry is to utilize a 3 to 1 Rule that can assist in evaluating the use of different materials both during and prior to construction. The Rule is simple:

3" of Crushed Aggregate Base Course = 1" of Hot Mix Asphalt Pavement as it relates to the structural number.

So what is the basis of this Rule? In pavement thickness design, the different layers that comprise the structure are assigned strength coefficients. The coefficients have been developed, utilized and refined by Federal and State transportation organizations for several decades. In this particular instance, the structural layer coefficients are:

Crushed Aggregate Base Course (CABC) = 0.15

Hot Mix Asphalt (HMA) Pavement = 0.44

These values vary in different geographics and are dependent upon the specific aggregate properties, but the strength value of the HMA is always approximately 3 times that of CABC.

An example of a situation where this Rule can be applied is to make a determination of the most cost effective pavement design for a given application.

Pavement Area: 10,000 square yards

Structural Capacity needed: SN = 3.08

Cost of excavation/disposal: $1.00 / inch

Cost of CABC: $0.90 / inch

Cost of HMA: $3.60 / inch

CABC 9.00" depth x 0.15 - 1.35

HMA 4.00" depth x 0.44 = 1.76

Total Structural Number 3.11 exceeds 3.08 required

Costs

Excavation 13.00" depth x $1.00 /sq yd x 10,000 sq yds = $130,000

CABC 9.00 depth x $0.90/sq yd x 10,000 sq yds = $81,000

HMA 4.00" depth x $3.60/sq yd x 10,000 sq yds = $144,000

Solution A Total = $355,000

CABC 6.00" depth x 0.15 = 0.90

HMA 5.00" depth x 0.44 = 2.20

Total Structural Number 3.10 exceeds 3.08 required

Costs

Excavation 11.00" depth x $1.00/sq yd x 10,000 sq yds = $110,000

CABC 6.00" depth x $0.90/sq yd x 10,000 sq yds = $54,000

HMA 5.00" depth x $3.60/sq yd x 10,000 sq yds = $180,000

Solution B Total = $344,000

This is obviously a very simple example that illustrates

how the values can be used in an analysis type situation.

Other factors and considerations will always exist on a

case by case basis.