7.1 Paving Systems: Rigid, Flexible & Unit Pavers

Core Focus: The Design Development subdomain represents 22% of scored items on LARE Section 2 (Planning and Design). Candidates must demonstrate comprehensive mastery of paving systems (rigid, flexible, and unit), retaining wall engineering, lateral earth pressure dynamics, subsurface drainage, and structural failure modes.

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1. Rigid Pavement Systems: Cast-in-Place Concrete

Cast-in-place (CIP) concrete is a rigid pavement system characterized by its high compressive strength, durability, and capacity to bridge minor subgrade deficiencies through beam action. Unlike flexible pavements that transfer wheel loads directly to the base course via localized aggregate interlock, rigid slabs distribute concentrated wheel and pedestrian loads across a broad slab area.

Concrete Mix Design & Engineering Properties

• Compressive Strength ($f'c$): Specified in pounds per square inch (psi) cured at 28 days (ASTM C39). Pedestrian pathways and residential sidewalks typically specify 3,000 to 3,500 psi. Heavy urban pedestrian plazas, commercial driveways, and light vehicular fire lanes require 4,000 to 4,500 psi. Heavy municipal transit lanes, loading docks, and severe freeze-thaw applications require 5,000+ psi.
• Water-Cement Ratio (w/c): The ratio of water weight to cementitious materials weight. A lower w/c ratio (0.40 to 0.45) significantly increases final compressive strength, lowers permeability, and improves abrasion and freeze-thaw resistance. Excess water elevates workability temporarily but drastically increases drying shrinkage, porosity, cracking, and surface dusting.
• Air Entrainment: Concrete exposed to freeze-thaw cycles must incorporate microscopic, spherical air voids (ASTM C260) representing 4.0% to 7.0% of total concrete volume (depending on maximum coarse aggregate size). As absorbed water freezes within capillary pores, it expands by approximately 9%; entrained air voids act as pressure relief chambers, preventing internal hydraulic shattering. Air entrainment also improves workability without requiring additional water.
• Slump Test (ASTM C143): Measures the consistency and workability of freshly mixed batch concrete using a 12-inch hollow slump cone. Typical specification for flatwork is a 3- to 4-inch slump (or up to 5 inches for lightly reinforced slabs). A slump exceeding 6 inches without superplasticizers indicates excessive batch water, resulting in weak, segregation-prone concrete.

Slump Test Verification (ASTM C143)
    |--- 4" Top ---|
    |              |
    /              \   12" High Mold
   /                \  (Filled in 3 equal layers, rodded 25x each)
  /                  \
 /                    \
|------- 8" Base ------|
      === Lift Cone ===
      [   Concrete    ] <--- Measure slump (inches of drop)

Concrete Jointing Mechanics

Uncontrolled concrete cracking is caused by tensile stresses developed during drying shrinkage (concrete shrinks roughly 1/16 inch per 10 linear feet as free moisture evaporates) and temperature fluctuations. Three distinct joint types must be detailed on site development plans:

1. Control / Contraction Joints

• Function: Induce controlled cracking along a predetermined, weakened plane rather than allowing random, jagged shrinkage cracks across the slab surface.
• Depth: Must be cut or grooved to a depth of at least 1/4 of the slab thickness ($D/4$) or a minimum of 1 inch (e.g., a 4-inch slab requires a 1-inch joint; a 6-inch slab requires a 1.5-inch joint).
• Spacing Rule: Maximum joint spacing in feet must not exceed 2 to 3 times the slab thickness in inches (often expressed as 24 to 36 times slab thickness). For a 4-inch slab: $4 \times 2 = 8\text{ ft}$ to $4 \times 3 = 12\text{ ft}$. For a 6-inch slab: 12 to 18 ft maximum. Slabs should be divided into square or near-square panels (length-to-width aspect ratio should never exceed 1.25:1 to 1.5:1; L-shaped panels must be avoided).
• Timing: Tooled joints are formed with a groover during initial finishing. Saw-cut joints must be cut as soon as the concrete can support foot traffic without ravelling or dislodging aggregate—typically 4 to 12 hours after pour in warm weather, and never delayed beyond 24 hours.

2. Expansion / Isolation Joints

• Function: Provide complete physical separation through the full depth of the slab to accommodate thermal expansion and isolate the pavement from immovable vertical structures (foundations, columns, catch basins, steps, retaining walls).
• Detail: Full-depth, compressible filler material (such as asphalt-impregnated fiberboard, closed-cell polyethylene foam, or cork) typically 1/2 inch thick, recessed 1/2 inch below the surface and capped with an elastomeric polyurethane or polysulfide joint sealant.
• Placement: Placed at all junctions where flatwork abuts rigid walls or curbs, at right-angle walk intersections, and at maximum intervals of 30 to 50 feet in continuous linear sidewalks.

3. Construction Joints

• Function: Established where concrete placement operations conclude for the day or where adjoining pours are staged sequentially.
• Detail: Keyed joints or smooth, greased steel slip dowels that allow horizontal contraction and expansion while preventing vertical shear displacement (faulting) across the pour interface.

Architectural Concrete Finishes

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2. Flexible Pavement Systems: Asphaltic Concrete

Unlike rigid concrete slabs, asphalt is a flexible pavement system. It possesses minimal tensile strength, relying on layered aggregate shear resistance to distribute surface wheel loads downward to the natural subgrade soil.

Standard Commercial Asphalt Pavement Cross-Section
=======================================================
| Surface Wear Course (1.5" - 2.0" Dense-Graded AC)   |
|-----------------------------------------------------|
| Asphalt Binder Course (2.0" - 3.0" Coarse AC)       |
|-----------------------------------------------------|
| Crushed Aggregate Base (CAB) (6.0" - 10.0" Crushed) |
|-----------------------------------------------------|
| Compacted Subgrade (95% Modified Proctor Density)   |
=======================================================

Layered Pavement Cross-Section

1. Subgrade Preparation: The native soil bed excavated and compacted to 95% Modified Proctor density (ASTM D1557). Soft, high-plasticity clay subgrades may require lime or Portland cement stabilization, or geotextile separation fabrics.
2. Crushed Aggregate Base (CAB): A dense-graded layer of crushed stone (typically 3/4-inch minus with balanced fines) compacted in lifts of 4 to 6 inches. Thickness ranges from 4 to 6 inches for light vehicular/parking stalls up to 8 to 12 inches for heavy truck lanes and bus turnouts.
3. Asphalt Binder Course (Base Course): Hot-mix asphalt (HMA) utilizing larger aggregates (3/4" to 1" nominal size) providing structural load-bearing capacity (typically 2 to 3 inches thick).
4. Asphalt Surface / Wear Course: A dense-graded, fine-aggregate HMA wearing layer (typically 1.5 to 2 inches thick) providing a smooth, skid-resistant, water-shedding riding surface.

Engineering Strengths & Vulnerabilities

• Strengths: Lower initial installation cost than concrete; seamless surface; flexible adaptation to minor subgrade settling; quick return to traffic (hours vs. 7–28 days for concrete).
• Vulnerabilities: Susceptible to chemical softening and dissolution from gasoline, oil, and diesel spills; subject to rutting and depression under prolonged static loads (such as parked dumpsters or bus idling stops); requires periodic sealcoating every 3 to 5 years and structural milling/resurfacing every 15 to 20 years.

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3. Unit Paving Systems & Setting Beds

Unit paving systems utilize modular manufactured units (concrete pavers, extruded or molded clay bricks, natural stone slabs, or granite setts) laid in geometric bonding patterns (herringbone, running bond, basketweave, stack bond).

Setting Bed Typologies

• Sand-Set (Flexible Assembly): Pavers installed over a 1-inch uniform layer of uncompacted, sharp, washed concrete sand (ASTM C33) resting on a compacted crushed aggregate base (ASTM D2940). Joints are filled with dry silica or polymeric sand swept into joints and vibrated into place with a plate compactor. Advantages: Accommodates minor freeze-thaw movement without cracking; 100% salvageable when accessing underground utilities.
• Bituminous-Set (Semi-Rigid Assembly): Pavers installed over a 3/4-inch layer of neoprene-modified asphalt adhesive screeded over a cured concrete base slab, primed with asphalt primer. Joints are filled with polymeric sand or stone dust. Advantages: Provides high shear strength and resistance to horizontal braking/turning forces; the standard for high-traffic municipal streetscapes and urban bus loading aprons.
• Mortar-Set (Rigid Assembly): Pavers installed over a 1/2- to 1-inch thick cementitious mortar bed (Type M or Type S) supported by a reinforced concrete slab, with joints grouted with mortar. Vulnerabilities: Rigid and unyielding; any subgrade settlement or freeze-thaw expansion cracks the joints; trapped moisture beneath units leads to severe efflorescence, spalling, and debonding in freezing climates.

       SAND-SET (FLEXIBLE)                     BITUMINOUS-SET (SEMI-RIGID)
+-------------------------------+       +-------------------------------+
| Modular Pavers (ASTM C936)    |       | Modular Pavers (ASTM C936)    |
+-------------------------------+       +-------------------------------+
| 1" Washed Sand (ASTM C33)     |       | 3/4" Neoprene-Modified Bitumen|
+-------------------------------+       +-------------------------------+
| Compacted Aggregate Base      |       | Reinforced Concrete Slab Base |
+-------------------------------+       +-------------------------------+
| Compacted Subgrade            |       | Compacted Aggregate Subbase   |
+-------------------------------+       +-------------------------------+

The Critical Role of Edge Restraints

Without rigid edge restraints, unit paver installations inevitably fail through lateral creep—traffic loads push the perimeter units outward, opening joint spaces, destroying aggregate interlock, and allowing water to infiltrate and wash away the bedding sand. Acceptable edge restraints include:

• Poured cast-in-place concrete curbs or flush concrete header ribbons.
• Troweled concrete toe encasements (submerged concrete haunches) extending down into the base course.
• Heavy-duty commercial steel or aluminum edging secured with 10- to 12-inch steel spikes driven directly into the compacted aggregate base (never driven into loose topsoil).

Permeable Interlocking Concrete Pavers (PICP)

PICP systems function as on-site stormwater infiltration systems. Paver units have specialized spacer lugs creating open joints (typically 8% to 12% surface void ratio). Sand bedding and joint sand are strictly prohibited because fine sand clogs and seals voids. Instead, the assembly uses washed, open-graded crushed aggregate:

• Joint fill and 2-inch setting bed: ASTM No. 8 or No. 89 stone (1/4" to 3/8" crushed chip stone).
• Base reservoir course: ASTM No. 57 crushed stone (4 inches thick).
• Subbase detention reservoir: ASTM No. 2 crushed stone (6 to 18 inches thick, providing 30% to 40% void storage space for runoff).

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