The pavement section is the layered structure between the subgrade soil and the driving surface. Getting it right means a parking lot or road that performs for its design life. Getting it wrong means cracking, rutting, and an expensive reconstruction within a few years. Pavement section design is not complicated, but it requires specific inputs, and shortcuts lead to premature failure.

This article covers the fundamentals of pavement section design for site civil work: parking lots, drive aisles, private roads, and fire access lanes.

The Layers

A typical flexible (asphalt) pavement section from bottom to top:

1. Subgrade — the native soil or compacted fill that supports the entire pavement structure. Compacted to a minimum of 95 percent relative compaction in the upper 6 to 12 inches. The subgrade strength (R-value or CBR) is the primary input to pavement design.
2. Aggregate base (AB) — crushed gravel meeting a specific gradation (typically Class 2 aggregate base per Caltrans specifications). Provides structural support and drainage. Typical thickness: 4 to 12 inches.
3. Asphalt concrete (AC) — the surface course. Hot-mix asphalt placed in one or two lifts and compacted. Typical total thickness: 2 to 4 inches for parking lots; 3 to 6 inches for truck routes.

A rigid (concrete) pavement section is simpler: subgrade, optionally a granular subbase, and a Portland cement concrete (PCC) slab. The concrete slab provides both the structural support and the wearing surface.

Design Inputs

Subgrade R-Value

The R-value (resistance value) measures the subgrade soil's resistance to deformation under load. It is determined by laboratory testing on a disturbed sample from the site (Caltrans Test 301). Values range from 5 (very weak clay) to 80+ (well-graded gravel). The lower the R-value, the thicker the pavement section needs to be.

A typical parking lot subgrade in the Bay Area might have an R-value of 20 to 40 for alluvial soils, 5 to 15 for fat clays, and 40 to 60 for sandy or gravelly soils. The geotechnical report provides this value. If it does not, request it before you design the pavement section.

Traffic Index (TI)

The Traffic Index is a measure of the total traffic loading the pavement will experience over its design life, converted to equivalent single-axle loads (ESALs). The Caltrans method uses TI directly in the design charts. The AASHTO method uses ESALs.

For site civil work:

Design Method

The Caltrans method uses the R-value and TI to enter a design chart (or formula) that outputs the required thickness of asphalt concrete over aggregate base. The equation relates the required structural section thickness to the gravel equivalent (GE), where each material contributes a specific structural capacity per inch.

For example, if the required GE is 2.0 feet (a TI of 6.0 over an R-value of 20), you could provide: 3 inches of AC (GE = 3 x 2.50/12 = 0.625) plus 8 inches of AB (GE = 8 x 1.10/12 = 0.733), for a total GE of 1.358. That is not enough. You would need 3 inches AC plus 12 inches AB (total GE = 0.625 + 1.10 = 1.725) or 4 inches AC plus 10 inches AB (total GE = 0.833 + 0.917 = 1.750). Both are still under 2.0 GE, so you might need 4 inches AC plus 12 inches AB (1.90 GE) — getting closer — or add cement-treated base to the section.

Asphalt vs. Concrete

When to Use Asphalt

• Standard parking lots and drive aisles with passenger car and light truck traffic
• When initial cost is the primary driver (asphalt is typically 20 to 30 percent less expensive to install than concrete for equivalent loading)
• When the surface will be saw-cut and repaired frequently for utility work
• When a smooth, quiet riding surface is desired

When to Use Concrete

• Truck dock areas and dumpster pads where point loads from jack stands and outriggers can rut asphalt
• Fuel islands and areas with chemical spills (concrete resists petroleum better than asphalt)
• Fire lanes where heavy apparatus loads are concentrated (many fire districts prefer or require concrete fire lanes)
• Intersections and turning areas where slow-moving truck tires apply high shear forces that deform asphalt
• When a longer design life with lower maintenance is worth the higher initial cost

Concrete Pavement Design

Concrete pavement design uses slab thickness as the primary structural element. A typical parking lot concrete slab is 5 to 6 inches of PCC over 4 to 6 inches of aggregate base. Truck areas typically require 6 to 8 inches of PCC. The slab is jointed at regular intervals (typically 12 to 15 feet in each direction for a 6-inch slab) to control cracking. Dowel bars or keyways transfer loads across joints.

Reinforcement in concrete parking lot pavement is not for structural capacity — it is for crack control. Welded wire reinforcement (WWR) or fiber reinforcement holds the slab together if it cracks, preventing the cracks from opening and allowing water to infiltrate the base. Post-tensioned concrete pavement is used in special applications but is not standard for site work.

Common Mistakes

• Using a default pavement section without site-specific R-value. The difference between an R-value of 15 and 40 can change the aggregate base thickness by 4 to 6 inches. At $30 to $50 per cubic yard of aggregate in place, that is real money on a large parking lot.
• Underestimating truck traffic. A TI of 5.0 is adequate for passenger cars. A grocery store with daily truck deliveries needs a TI of 6.0 to 7.0 in the truck lanes and dock areas. One year of truck traffic on an under-designed section can cause more damage than 20 years of cars.
• Not separating truck areas from car areas. Design the truck lanes and loading areas with a heavier section. Transition the section at a well-defined boundary. You do not need to build the entire parking lot to truck standards if only one lane sees trucks.
• Ignoring drainage. Water in the base course is the primary cause of premature pavement failure. Edge drains, proper subgrade slopes, and adequate surface drainage are as important as the pavement section thickness.

What Goes on the Plans

The grading and paving plan should show:

• Pavement section detail with layer thicknesses, material specifications, and compaction requirements
• Boundaries between different pavement sections (truck areas vs. car areas)
• Joint layout for concrete areas, including joint types (contraction, expansion, construction)
• Subgrade preparation requirements (compaction, proof rolling, moisture conditioning)
• Pavement transition details at building entries, curb lines, and changes in section

The geotechnical report should be referenced on the plans as the basis of design, and the specific R-value and recommended sections should be called out in the notes.