Bedding, restraints, and square layout

Bedding Sand: ASTM C33 Specifications vs. Stone Dust Prohibition

The bedding sand layer serves two critical engineering functions in a flexible pavement system: it provides a smooth, uncompacted setting bed that absorbs minor dimensional height variations inherent in manufactured concrete pavers, and it forces angular sand upward into the bottom an allowance established by a mock-up of the paver joints during initial vibratory compaction, initiating bottom-up joint interlock.

Strict Material Specification: ASTM C33 Concrete Sand

Under CMHA technical specifications, bedding sand must conform strictly to ASTM C33 (or CSA A23.1) for concrete fine aggregate. Bedding sand must consist of clean, hard, naturally occurring silica sand or manufactured crushed stone sand characterized by sharp, angular particle geometry. It must be thoroughly washed to eliminate clay and organic loam, with less than 1% passing the No. 200 (0.075 mm) sieve.

The Prohibition of Stone Dust and Screenings

A critical failure mode in residential landscape contracting is the substitution of stone dust, quarry screenings, decomposed granite, or masonry sand for washed concrete sand.

Bedding thickness and screeding

For conventional sand-set concrete pavers, CMHA guidance commonly uses one inch of loose screeded bedding sand. It consolidates during seating; do not specify a universal half-inch reduction. A mock-up can establish the actual finished-height allowance for the paver, sand, and compactor combination. Bedding is a setting layer, not a tool for correcting low spots in the aggregate base.

Set screed rails to the designed plane, remove them, fill the rail grooves, and avoid walking on the finished bed. Keep bedding material consistent and protected from contamination and excessive moisture. Correct the foundation first when a stringline reveals a depression.

Edge Restraint Systems and Mechanical Fastening

Interlocking concrete pavements operate under the law of horizontal containment. When a dynamic wheel load applies braking or turning forces to a paver, that force is transferred horizontally through the joint sand to adjacent pavers until it reaches the pavement perimeter. If the perimeter is unrestrained, edge pavers shift laterally outward, widening joint gaps. Once joints open beyond 3/16 inch (5 mm), joint sand falls out, vertical shear transfer ceases, and the entire pavement unravels.

Types of Commercial and Masonry Edge Restraints

1. Engineered Plastic / Aluminum Commercial Edging: L-shaped commercial edging manufactured from heavy-gauge polyvinyl chloride (PVC) or structural aluminum alloy. Features an upright vertical support wall that rests against the paver face and a horizontal bottom flange with pre-drilled spike bosses that sits directly on the compacted aggregate base extension.
2. Poured Concrete Bond Beam (Troweled Concrete Toe): Cast-in-place concrete curb troweled against the outer edge pavers. Mixed with fiber reinforcement, the concrete is placed along the perimeter at a 45-degree angle starting 1/2 inch below the paver top chamfer down to the aggregate base shoulder. It must be buried beneath adjacent sod or mulch.
3. Monolithic Concrete Curb / Submerged Curb: Precast or cast-in-place concrete curbs set on a compacted base footing flush with or slightly above the paver surface, providing superior lateral resistance for commercial vehicular roadways.

Restraint attachment

Install the restraint on a stable supporting foundation at the specified elevation. Plastic, metal, concrete, and proprietary restraints have different fasteners and attachment rules. Follow the system's fastener type, spacing, and base requirements; do not declare all plastic spikes unacceptable or one spike spacing valid for every restraint.

Check curved edges, corners, and vehicular turning areas because spreading forces can be greater there. A restraint unsupported by the base or installed only in bedding sand can move with the pavement. Permeable-pavement foundations may need a different anchorage detail from dense-graded foundations.

Paver Layout, Geometry, and Precision Cutting Protocols

Establishing perfect baseline geometry ensures paver joints remain straight and square across large fields, eliminating unsightly tapering or wedge-shaped cut pieces at borders.

Establishing Square: The 3-4-5 Baseline Method

Never assume building foundations, garage walls, or property line fences are square. Contractors must establish a true perpendicular 90-degree reference line using the 3-4-5 Pythagorean theorem:

1. Establish a primary baseline along the longest straight edge of the project (e.g., parallel to the house wall).
2. Measure exactly 3 feet (or 6, 9, or 12 feet for larger projects) along the baseline from a corner point.
3. Measure exactly 4 feet (or 8, 12, or 16 feet) perpendicular from the same corner point.
4. Adjust the perpendicular line until the diagonal distance between the two points measures exactly 5 feet (or 10, 15, or 20 feet).
5. Erect high-tension braided nylon string lines along both perpendicular axes, elevated 1/4 inch above the target finished paver surface.