What is a Base Course in Pavement Structures?

Definition

A base course is the pavement layer under the surface course that spreads wheel loads onto the subbase or subgrade. AC 150/5320-6G names four parts of a pavement structure: the surface course, the base course, the subbase course and the subgrade (1.6).

The function depends on the pavement type:

• Flexible pavement. The base is the principal structural layer. It distributes the wheel load to the subbase or subgrade (AC 150/5380-6C, 2.2.1).
• Rigid pavement. The base gives uniform, stable support to the concrete panels (AC 150/5320-6G, 3.16.3.1). It also controls frost action, helps drainage, controls swelling of the subgrade and prevents pumping of fine-grained soils (AC 150/5380-6C, 2.2.2).

Pavement Interactive describes the same layer for highways. It says the base course is immediately beneath the surface course. It adds load distribution and helps drainage and frost resistance.

The base is not the subbase. The subbase lies between the base and the subgrade. AC 150/5380-6C says it works like the base, but its material requirements are less strict because it carries lower stresses. It serves where frost action is severe or the subgrade is weak. In a flexible pavement, AC 150/5320-6G requires a subbase on a subgrade with a CBR below 20 (3.15.5.1). The glossary entry on the subbase course gives the details.

Types of base

The FAA sorts base courses into two classes (AC 150/5320-6G, 1.6 and Table 1-1).

• Unstabilized base. Crushed and uncrushed aggregate, for example items P-208 and P-209.
• Stabilized base. Aggregate stabilized with cement or asphalt. The items are P-401 and P-403 (asphalt), P-304 (cement-treated aggregate), P-306 (lean concrete), P-307 (cement-treated permeable base) and P-407 (asphalt-treated permeable base).

The AC says to use P-304, P-306 and P-307 with caution, because all three are susceptible to reflective cracking. It requires a stabilized base when the design traffic mix includes aircraft of 100,000 lb (45,360 kg) or more (3.5).

For a flexible pavement, P-209 is the standard aggregate base. As a base, P-208 serves only pavements designed for 60,000 lb (27,200 kg) or less (Table 1-1, note 5). Items P-210, P-212 and P-213 carry the same limit (3.15.4.4). Items P-207 and P-219 need laboratory tests. If the CBR is above 80, P-207 or P-219 may replace P-209. If it is above 60, either may replace P-208 (Table 3-3, note 6, and 3.15.4.6).

Aggregate base materials

An aggregate base is clean, sound, durable particles of crushed stone or crushed gravel with fine aggregate. P-208 can also include other blended material such as sand and screenings. It must be free from clay lumps, organic matter and other harmful coatings (AC 150/5370-10H, 208-2.1 and 209-2.1).

The P-209 gradation has these limits. 100 percent passes the 2 in (50 mm) sieve. 55 to 85 percent passes the 3/4 in (19 mm) sieve. 30 to 60 percent passes the No. 4 (4.75 mm) sieve. Both items limit the fines. The No. 200 (75 µm) fraction must not exceed two thirds of the No. 40 (425 µm) fraction. For non-frost-susceptible material, the maximum passing the No. 200 sieve falls to 0 to 5 percent. Where frost goes deep, the specification recommends a separation filter layer or a geosynthetic separation layer.

The same word does not mean the same material on a gravel road. The Gravel Roads Maintenance and Design Manual says that base gravel has larger top-size stone and very few plastic fines. This gives strength and drainage. Surface gravel needs more fines to bind the surface. In the manual’s sample state specification, base gravel has 3 to 12 percent fines and a plasticity index of 0 to 6. The manual says that one gravel is usually good for one purpose or the other, but not for both.

Stabilized and asphalt bases

A cement-treated base (Item P-304) is aggregate mixed with cement and water. The mix design uses a cement content that gives a 7-day compressive strength of 300 to 600 psi (2,068 to 3,447 kPa) by ASTM D1633. The FAA says to avoid higher strengths because they cause shrinkage and reflective cracks (304-3.2). The coarse aggregate has a maximum Los Angeles abrasion loss of 40 percent. The fine aggregate has a liquid limit of 25 or less and a plasticity index of not more than 6 (304-2.1). Item P-306, lean concrete, has a 7-day compressive strength of 500 to 800 psi (3,445 to 5,516 kPa). The FAA does not recommend P-306 in flexible pavement structures.

An asphalt base course uses P-401 or P-403 hot mix asphalt. FAA Table 3-3 sets a minimum thickness of 5 in (125 mm) for a stabilized base under pavements for aircraft of 100,000 lb or more. The standard FAA stabilized base is P-401 or P-403, and FAARFIELD gives it a fixed modulus of 400,000 psi (2,760 MPa) (3.15.4.7). P-401 Table 5 sets a lower mat density limit for a base course than for a surface course. It is 92.0 percent against 92.8 percent. When P-403 serves as a stabilized base under a concrete pavement (P-501), the specification asks for a bond breaker. Pavement Interactive says that an HMA base mix usually has a larger maximum aggregate size than a surface mix. It is more open graded and has looser specifications.

The FAA allows the substitution of a superior material for a stabilized base in some cases. The material must show a remolded soaked CBR of 100 or more and a proven record under similar loads and climate. The minimum thickness of the substitute is 6 in (150 mm). A base under a stabilized base should have a remolded soaked CBR of at least 35 (3.5).

Thickness, compaction and strength

AC 150/5320-6G sets minimum layer thicknesses by the heaviest aircraft (Tables 3-3 and 3-4). For aircraft of 60,000 lb or more, a flexible pavement needs a crushed aggregate base of at least 6 in (150 mm). A stabilized base is at least 5 in (125 mm) for aircraft of 100,000 lb or more. FAARFIELD also computes the base thickness that protects a layer with a California bearing ratio (CBR) of 20. The design value is the larger of the two (3.15.4.9). The base may be built up to 12 in (300 mm) wider than the asphalt surface course (3.15.4.10).

Item P-208 builds the base in lifts of 4 to 12 in (100 to 300 mm) compacted thickness. A control strip sets the lift thickness (208-3.1, 208-3.4). The model specification text gives a field density of at least 100 percent of maximum density. Moisture stays within 2 percentage points of the optimum (208-3.5).

The AC says to follow ASTM D698 for areas designated for aircraft of 60,000 lb or less. For heavier aircraft, follow ASTM D1557 (3.15.4.8.5). Item P-304 has a density of at least 98 percent in the model text. Crews finish all compaction within 2 hours of the start of mixing (304-5.7).

The surface of the finished P-209 base must not vary more than 3/8 in (9 mm) under a 12 ft (3.7 m) straightedge (209-3.8). The crew places the material only when the air temperature is at least 40 °F (4 °C) and rising. The subgrade or subbase must not be wet or frozen (209-3.6).

Water and contamination

Fines from the subgrade can contaminate a base or subbase and reduce its structural capacity. A geosynthetic separation layer limits this (AC 150/5320-6G, 3.6).

The AC recommends drainage layers for pavements that serve aircraft above 60,000 lb. They suit areas with excess subsurface moisture and soil permeability below 20 ft/day (6 m/day). An effective layer drains 85 percent in 24 hours for runways and taxiways, and in 10 days for aprons. A permeability of 500 to 1,500 ft/day may be used without calculations (3.7).

Base and inspection

The base lies under the surface, so an inspector sees its effects. For flexible pavements, weakening of the base, subbase or subgrade by water is one of the main causes of failure (AC 150/5380-6C, 2.3). On concrete, pumping ejects water and underlying material through joints and cracks. This causes a progressive loss of pavement support (AC 150/5380-6C, 3.2.2 and PAVER manual, Pumping 69). AC 150/5380-6C ties corner breaks to loss of support, which pumping can cause (3.2.2).

To assess a base, engineers use these tests:

• FWD or HWD. Deflection data give the stiffness of each layer by back-calculation (AC 150/5320-6G, C.5.3).
• Cores or GPR. They give layer thickness. GPR also detects excess water and may locate voids (C.4.1, E.1).
• DCP. It estimates the stiffness of the base, subbase and subgrade (D.1).

The visual condition is usually rated by ASTM D5340, the airport PCI survey (C.4.1). AC 150/5320-6G says that deflection testing assesses the structural condition, and that other methods are needed for the visual condition. A drone survey can record the surface symptoms of a weak base, but it does not measure the base itself.

When the base is replaced

AC 150/5320-6G calls the removal and replacement of any structural layer below the surface course reconstruction. In a flexible pavement, this includes the subbase, the base and a stabilized base (4.4). A mill and overlay of the surface course is rehabilitation instead.