U.S. Department 

of Transportation 

Federal Aviation 

Administration 

Advisory 

Circular 

Subject: CHANGE 1 TO AIRPORT PAVEMENT Date: 1130196 AC NO: 150/5320-6D 

DESIGN AND EVALUATION Initiated by: AAS-200 Change: 1 

1. PURPOSE. The purpose of this change is to correct an error in the graph for Figure 2-4, Effcct of Subbase 

on Modulus of Subgrade Reaction, and to change a typographical error in paragraph 339b, which was incorrect 

in the initial printing of AC 150/5320-6D. 

The change number and date of change is shown at the top of each page. 

PAGECONTROLCHART 

Remove Pages Dated ll Insert Pagcs Dated 

15 and 16 

55 and 56 

7/7/95 

7/7/95 

15 

16 

55 

56 

~~ ~ 

1/30/96 

7/7/95 

7/7/95 

1 /3 0196 

DAVID L. BENNETT 

Director, Office of Airport Safety and Standards 
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1/30/96 AC 150/5320-60 CHG 1 

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FIGURE 2-4. EFFECT OF SUBBASE ON MODULUS OF SUBGRADE REACTION 

15 
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AC 15015320-61) 7/7/95 

207. 

conditions exist: poor drainage, adverse surface drainage, frost, or need for a stable working platform. Subgrade 

stabilization can be accomplished through the addition of chemical agents or by mechanical methods. 

SUBGRADE STABILIZATION. Subgrade stabilization should be considered if one or more of the following 

a. Chemical Stabilization. Different soil types require different stabilizing agents for best results. The 

following puSlications are recommended to determine the appropriate type and amount of chemical stabilization for 

subgrade soils. US Army, Corps of Engineers, Technical Manual TM 5-81 8-UAFM 88-6 Chapter 6 ; Technical Manual 

5-825.UAFM 88-6 Chapter 2; Technical Manual 5-824-3/AFh4 88-6, Chapter 3; Soil Cement Handbook. Portland 

Cement Association; and The Asphalt Institute Manual Series MS-19, A Basic Asphalt Emulsion Manual. 

b. Mechanical Stabilization. In some instances subgrades cannot be adequately stabilized through the 

use of chemical additives. The underlying soils may be so soft that stabilized materials cannot be mixed and compacted 

over the underlying soils without failing the soft soils. Extremely soft soils may require bridging in order to construct the 

pavement section. Bridging can be accomplished with the use of thick layers of shot rock or cobbles. Thick layers of 

lean, porous concrete have also been used to bridge extremely soft soils. Geotextiles should be considered as mechanical 

stabilization over soft, fine-grained soils. Geotextiles can facilitate site access over soft soils and aid in reducing 

subgrade soil disturbance due to construction traffic. The geotextile will also function as a separation material to limit 

long-term weakening of pavement aggregate associated with contamination of the aggregate with underlying fine-grained 

soils. More information regarding construction over soft soils using geotextiles is provided in FHWA-KI-90-001 (see 

Appendix 4). 

208. SEASONAL FROST. The design of pavements in areas subject to seasonal frost action requires special 

consideration. The detrimental effects of frost action may be manifested by nonuniform heave and in loss of soil strength 

during frost melting. Other related detrimental effects include: possible loss of compaction, development of pavement 

roughness, restriction of drainage, and cracking and deterioration of the pavement surface. Detrimental frost action 

requires three conditions be met simultaneously: first, the soil must be frost susceptible; secondly, freezing temperatures 

must penetrate into the frost susceptible soil; thirdly, free moisture must be available in sufficient quantities to form ice 

lenses. 

a. Frost Susceptibility. The frost susceptibility of soils is dependent to a large extent on the size and 

distribution of voids in the soil mass. Voids must be of a certain critical size for the development of ice lenses. Empirical 

relationships have been developed correlating the degree of frost susceptibility with the soil classification and the amount 

of material finer than 0.02 mm by weight. Soils are categorized into four groups for frost design purposes, Frost Group 1 

(FG-I), FG-2, FG-3, and FG-4. The higher the frost group number the more susceptible the soil, i.e., soils in frost group 

4 ze more frost susceptible than soils in frost groups 1, 2, or 3. Table 2-4 defines the frost groups. 

b. Depth of Frost Penetration. The depth of frost penetration is a function of the thermal properties of 

the pavement and soil mass, the surface temperature, and the temperature of the pavement and soil mass at the start of the 

freezing season. Several methods are available to calculate the depth of frost penetration and subsurface temperatures. 

The method presented here is a simplification of a method based on the modified Berggren equation. This method 

requires the use of the air freezing index and the dry unit weight of the soil. 

(1) Air Freezing Index. The air freezing index is a measure of the combined duration and 

magnitude of below freezing temperatures occurring during any given freezing season. the average daily temperature is 

used in the calculation of freezing index. For example, assume the average daily temperature is 10 degrees below 

freezing for 10 days. The freezing index would be calculated as follows, 10 degrees X 10 days = I00 degree days. 

Ideally, air freezing indices should be based on actual data obtained from a meteorological station located in close 

proximity to the construction site. The air freezing index used for design (design air freezing index) should be based on 

the average of the 3 coldest winters in a 30 year period. if available, or the coldest winter observed in a 10 year period. 

Figures 2-6 and 2-7 show the approximate design air freezing indices for the lower United States and Alaska, 

respectively. The values shown in Figures 2-6 and 2-7 do not show local variation which may be substantial, especially 

in mountainous areas. 

16 
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Y 

7/7/95 AC 15015320-6D 

SECTION 3. RIGID PAVEMENT DESIGN 

324. GENERAL. Rigid pavements for airports are composed of portland cement concrete placed on a granular or 

treated subbase course that is supported on a compacted subgrade. Under certain conditions, a subbase is not required, 

see paragraph 326. 

325. 

surface water, and provide structural support to the Item P-501. Cement Concrete Pavement. 

CONCRETE PAVEMENT. The concrete surface must provide a nonskid surface, prevent the infiltration of 

326. 

pavement slabs. A minimum thickness of 4 inches (100 mm) of subbase is required under all rigid pavements, except as 

shown in Table 3-10 below: 

SUBBASE. The purpose of a subbase under a rigid pavement is to provide uniform stable support for the 

TABLE 3-10. CONDITIONS WHERE NO 

Soil Good Drainage 

Classification 

No Frost Frost 

GW 

GP 

GM 

GC 

SW 

X X 

X X 

X 

X 

X 

Note: X indicates conditions where no 

SUBBASE IS REQUIRED 

Poor Drainage 

No Frost Frost 

X X 

X 

subbase is required. 

327. 

Subbase Course. In some instances it may be desirable to use higher quality materials or thicknesses of P-154 greater 

than 4 inches (100 mm). The following materials are acceptable for use as subbase under rigid pavements: 

SUBBASE QUALITY. The standard FAA subbase for rigid pavements is 4 inches (100 mm) of Item P-154, 

Item P-154 - 

ItemP-208 - 

Item P-209 - 

ItemP-211 - 

Item P-304 - 

Item P-306 - 

ItemP-401 - 

Subbase Course 

Aggregate Base Course 

Crushed Aggregate Base Course 

Lime Rock Base Course 

Cement Treated Base Course 

Econocrete Subbase Course 

Plant Mix Bituminous Pavements 

Materials of higher quality than P-154 and/or greater thicknesses of subbase are considered in the design process through 

the foundation modulus (k value). The costs of providing the additional thickness or higher quality subbase should be 

weighed against the savings in concrete thickness. 

328. 

accommodate aircraft weighing 100,OOO pounds (45 400 kg) or more. Stabilized subbases are as follows: 

STABILIZED SUBBASE. Stabilized subbase is required for all new rigid pavements designed to 

Item P-304 - Cement Treated Base Course 

Item P-306 - Econocrete Subbase Course' 

Item P-401 - Plant Mix Bituminous Pavements 

The structural benefit imparted to a pavement section by a stabilized subbase is reflected in the modulus of subgrade 

reaction assigned to the foundation. Exceptions to the policy of using stabilized subbase are the same as given in 

paragraph 320. 

329. 

stability and uniform support as with flexible pavement; however, the compaction requirements for rigid pavements are 

not as stringent as flexible pavement due to the relatively lower subgrade stress. For cohesive soils used in fill sections, 

SUBGRADE. The subgrade materials under a rigid pavement should be compacted to provide adequate 

55 
G 

AC 150/5320-6D CHG 1 1/30/96 

the entire fill shall be compacted to 90 percent maximum density. For cohesive soils in cut sections, the top 6 inches 

(1 50 mm) of the subgrade shall be compacted to 90 percent maximum density. For noncohesive soils used in fill 

sections, the top 6 inches (150 mm) of fill shall be compacted to 100 percent maximum density, and the remainder of 

the fill shall be compacted to 95 percent maximum density. For cut sections in noncohesive soils, the top 6 inches 

(1 50 mm) of subgrade shall be compacted to 100 percent maximum density and the next 18 inches (460 mm) of 

subgrade shall be compacted to 95 percent maximum density. Swelling soils will require special considerations. 

Paragraph 3 14 contains guidance on the identification and treatment of swelling soils. 

a. Contamination. In rigid pavement systems, repeated loading may cause intermixing of soft subgrade 

soils and aggregate base or subbase. This mixing may create voids below the pavement in which moisture can 

accumulate causing a pumping situation to occur. Chemical and mechanical stabilization of the subbase or subgrade 

can be effectively used to reduce aggregate contamination (refer to Section 207). Geotextiles have been found to be 

effective at providing separation between fine-grained subgrade soils and pavement aggregates (FHWA-90-00 1) (see 

Appendix 4). Geotextiles should be considered for separation between fine-grained soils and overlying pavement 

aggregates. In this application, the geotextile is not considered to act as a structural element within the pavement. 

Therefore, the modulus of the base or subbase is not considered to be increased when a geotextile is used for 

stabilization. For separation applications, the geotextile is designed based on survivability properties. Refer to 

FHWA-90-00 1 (see Appendix 4) for additional information regarding design and construction using separation 

geotextiles. 

330. 

addition to the soils survey and analysis and classification of subgrade conditions, the determination of the foundation 

modulus is required for rigid pavement design. The foundation modulus (k value) should be assigned to the material 

directly beneath the concrete pavement. However, it is recommended that a k value be established for the subgrade 

and then corrected to account for the effects of subbase. 

DETERMINATION OF FOUNDATION MODULUS (k VALUE) FOR RIGID PAVEMENT. In 

a. Determination of k Value for Subgrade. The preferred method of determining the subgrade 

modulus is by testing a limited section of embankment which has been constructed to the required specifications. The 

plate bearing test procedures are given in AASHTO T 222, Nonrepetitive Static Plate Load Test of Soils and Flexible 

Pavement Components, for Use in Evaluation and Design of Airport and Highway Pavements. If the construction and 

testing of a test section of embankment is impractical, the values listed in Table 2-3 may be used. The designer is 

cautioned that the values in Table 2-3 are approximate and engineering judgment should be used in selecting a design 

value. Fortunately, rigid pavement is not too sensitive to k value and an error in estimating k will not have a large 

impact on rigid pavement thickness. 

b. Determination of k Value for Granular Subbase. The determination of a foundation modulus on 

top of a subbase by testing is usually not practical, at least in the design phase. Usually, the embankment and subbase 

will not be in place in time to perform any field tests. The assignment of a k value will have to be done without the 

benefit of testing. The probable increase in k value associated with various thicknesses of different subbase materials 

is shown in Figure 2-4. The upper graph in Figure 2-4 is intended for use when the subbase is composed of well 

graded crushed aggregate such as P-209. The lower graph in Figure 2-4 applies to bank-run sand and gravel such as 

P-154. These curves apply to unstabilized granular materials. Values shown in Figure 2-4 are considered guides and 

* 

may be tempered by local experience. * 

C. Determination of k Value for Stabilized Subbase. As with granular subbase, the effect of 

stabilized subbase is reflected in the foundation modulus. Figure 3-16 shows the probable increase in k value with 

various thicknesses of stabilized subbase located on subgrades of varying moduli. Figure 3-16 is applicable to cement 

stabilized (P-304) Econocrete (P-306), and bituminous stabilized (P-40 1) layers. Figure 3-16 was developed by 

assuming a stabilized layer is twice as effective as well-graded crushed aggregate in increasing the subgrade modulus. 

Stabilized layers of lesser quality than P-304, P-306 or P-401 should be assigned somewhat lower k values. After a k 

value is assigned to the stabilized subbase, the design procedure is the same as described in paragraph 33 1. 

56 * US. GOVERNMENT PRINTING OFFICE: 1996 - 715-000 - 1302/20038