Quick answer
Interlocking concrete pavers are solid, factory-made concrete units that are laid in a pattern on a bedding-sand layer over a compacted aggregate base, with sand-filled joints and an edge restraint holding the field together.
The Interlocking Concrete Pavement Institute (ICPI) publishes the construction guidelines most specifications cite. In the U.S., units are expected to meet ASTM C936 for strength, absorption and freeze-thaw resistance. This page explains the unit and its numbers, not the buried layers.
Quick facts
The key facts about Interlocking Concrete Pavers, with sources:
- What it is
- Solid concrete paving units set in sand over a compacted aggregate base 1
- U.S. unit standard
- ASTM C936: average compressive strength at least 8,000 psi, absorption no more than 5% 1
- Freeze-thaw
- At least 50 cycles with average material loss not above 1% (ASTM C936) 1
- Thickness
- 2 3/8 in. (60 mm) for pedestrian areas and driveways; at least 3 1/8 in. (80 mm) for streets 1
- Joint width
- 1/16 to 3/16 in. (2 to 5 mm), kept consistent 1
- Minimum base
- 4 in. under patios and walkways on well-drained soil; 6 in. for a residential driveway 1
- Surface slope
- A minimum of 1.5% 1
Key takeaways
- An interlocking concrete paver is a unit and a system. The unit alone does not make a patio. The base, bedding, joints and edges do.
- ASTM C936 sets the U.S. unit standard: 8,000 psi average strength, 5% absorption limit and 50 freeze-thaw cycles.
- Thickness follows use: 2 3/8 inches for patios and walks, at least 3 1/8 inches for streets, per ICPI.
- Joints are narrow, 1/16 to 3/16 inch, and filled with sand. The sand is what locks the units.
- Ask a supplier for proof of ASTM C936 or CSA A231.2 conformance, not just a color sample.
What an interlocking concrete paver is
An interlocking concrete paver is a precast concrete unit made in a factory.
The Interlocking Concrete Pavement Institute (ICPI) describes the pavement as units laid on an aggregate base and bedding sand, with the structure commonly used for pedestrian and vehicular areas 1.
The Concrete Masonry and Hardscapes Association (CMHA), which has absorbed ICPI, says municipalities have used interlocking concrete pavements for decades in sidewalks, plazas and streetscapes 3.
The word “interlocking” refers to the way sand-filled joints and the edge restraint let the field act as one surface. ICPI’s Tech Spec 2 notes that herringbone patterns are recommended for all street applications because they give the maximum load-bearing support and resist creep from starting, braking and turning tires 1.
It is not the same as a poured concrete slab, a stamped surface or a natural stone flagstone. Each unit can be lifted and reset. ICPI’s Tech Spec 18 adds that paving units and base materials can be removed and reinstated 2.
For the finished feature, see the patios and walkways service page. For the buried layers, the hardscape base specification owns the detail, so this page does not repeat it.
How the system holds together
A paved area has five working parts, from top to bottom: the paver, the joint sand, the bedding sand, the compacted base and the soil beneath. The edge restraint wraps the field. ICPI says a minimum of 1 inch of vertical restraining surface should touch the side of the paver to hold it 1.
Compaction does the seating. ICPI says that after an area is laid, a vibrating plate compactor capable of at least 5,000 pounds of force and running at 75 to 90 hertz should make at least two passes to seat the pavers in the bedding sand and force sand into the joints.
Dry joint sand is then spread and the pavers are compacted again until the joints are full, which may take two or three passes 1.
The sand fill is the locking step. ICPI says joint sand may be finer than bedding sand to fill the joints, and names ASTM C144 and CSA A179 requirements 1. The paver joint sand page covers the sand choices.
- Concrete paver2 3/8 in. for pedestrian areas and driveways 1
- Joint sand1/16 to 3/16 in. joints, filled and compacted 1
- Bedding sandAbout 1 in. uncompacted when screeded 1
- Compacted aggregate baseAt least 4 in. under patios and walks on well-drained soil 1
- Geotextile (optional)Where soil stays saturated 1
- Compacted soil subgradeAt least 98% of standard Proctor density 1
Edge restraint is not shown. Base thickness is a minimum for a patio, not a design. Source: [1]
A spec sheet decoder: the numbers and standards you will see
| On the spec sheet | What it means | The published figure |
|---|---|---|
| ASTM C936 | The U.S. standard for solid interlocking concrete paving units | Average compressive strength at least 8,000 psi (55 MPa); average absorption no more than 5%; resistance to at least 50 freeze-thaw cycles with average material loss not above 1%; abrasion tests 1 |
| CSA A231.2 | The Canadian standard for precast concrete pavers | Average cube strength of 7,250 psi (50 MPa) or 5,800 psi (40 MPa) at delivery; no more than 500 g/m² lost after 50 freeze-thaw cycles in a 3% saline solution 1 |
| Thickness 60 mm or 2 3/8 in. | Unit for pedestrian areas, driveways and limited vehicular use | Same figure in Tech Spec 18 for pedestrian use 12 |
| Thickness 80 mm or 3 1/8 in. | Unit for streets and industrial pavements | At least 3 1/8 in. 12 |
| Spacer bars | Small lugs on the sides that hold the joint open | Maintain a minimum joint width and let sand enter 1 |
| Joint width | The gap between units | Between 1/16 and 3/16 in. (2 to 5 mm); gaps over 3/8 in. are filled with cut pavers 1 |
A standard is voluntary until a contract or code cites it
ASTM C936 and ICPI Tech Specs are voluntary industry documents. ICPI says its Tech Specs are guidelines, not an industry standard, certification or specification, and that professional help should be sought for design and construction 1. A permit or contract can make them binding.
Example: checking a supplier’s pavers against the published figures
The supplier answers below are invented to show the method. The thresholds are ICPI Tech Spec 2’s, and a real unit’s test report is what you should ask for 1.
| Item on the quote (example) | Published figure | Verdict |
|---|---|---|
| Paver listed as 60 mm thick for a patio | 2 3/8 in. (60 mm) for pedestrian areas 1 | Matches the pedestrian figure |
| Same paver proposed for a driveway | 2 3/8 in. is listed for driveways; streets need at least 3 1/8 in. 1 | Fits a residential driveway; base is the bigger question, with 6 in. as the ICPI minimum on well-drained soil 1 |
| Sheet says “average strength 7,000 psi” | ASTM C936 requires an average of at least 8,000 psi 1 | Below the U.S. standard; ask for the test report |
| Joint gap measured at 1/4 in. | 1/16 to 3/16 in. 1 | Wider than ICPI’s range; ask why |
| No test report offered | Conformance to ASTM C936 or CSA A231.2 is the published expectation 1 | Ask for it before ordering |
Use the paver calculator to turn a patio size into units, base and sand, and read the landscaping estimates guide to compare quotes.
Where you meet the term: product pages, bids and city specs
On product pages and sample boards. Pavers are sold by shape, color, texture and thickness. Standards and strengths often sit in the fine print. The table above helps you find them.
On a landscape bid. A bid for a paver patio should name the paver, the base depth, the bedding, the edge restraint and the joint material. The guide to choosing a landscaping company explains what to compare.
In municipal specifications. City landscape specs, such as the City of Brampton’s unit paver section, point to ICPI for the bedding type and set joint widths and base compaction at fixed values 4. That is Ontario, and your city’s numbers may differ.
In stormwater rules. A special version, permeable interlocking concrete pavement, has its own rules; see permeable pavers.
What interlocking pavers do well, and where they fall short
Strengths shown in the sources
- ICPI’s permeable-pavement bulletin says paving units and base can be removed and reinstated, and utility cuts do not leave unsightly patches 2. It states this for permeable systems; the same unit-by-unit idea applies to standard pavers.
- That bulletin says installation can continue in cold weather if the subgrade and aggregates remain unfrozen, and the surface is ready for traffic with no curing time 2.
- Settling is expected and correctable: ICPI notes a small amount of settling is typical of all flexible pavements 1.
Limits shown in the sources
- Performance depends on compaction and base, which you cannot see once the surface is laid 1.
- Frozen or saturated sand should not be installed, and frozen material should not be compacted 1.
- Joint sand can wash or blow out, which is why some owners use stabilized sand; see the joint sand page 1.
- Saw-cutting produces dust containing respirable crystalline silica 1.
Limits of the evidence
The ICPI Tech Spec 2 we read is a member-hosted copy of a document revised in February 2020, and ICPI’s own site now redirects to CMHA. Figures are ICPI’s guidelines and, as the document itself says, not an industry standard, certification or specification 1.
We did not load the text of ASTM C936 itself. The strength, absorption and freeze-thaw figures are as ICPI states them. ASTM revises its standards, so confirm the current edition with the supplier.
Base thickness is the weak point of any quick answer. The 4-inch and 6-inch figures are ICPI minimums over well-drained soils.
ICPI says continually wet or weak soils in colder climates need bases at least 2 to 4 inches thicker 1.
Local frost and soil conditions can change the number, and an engineer sets it for streets and heavy loads.
Paver shape and pattern effects on performance are not ranked here. ICPI says it takes a conservative approach and does not recognize differences among paver shapes for structural or functional performance 1.
What drives the price, and how long a paved area lasts
We do not publish prices. A bid moves with the area, the paver, the pattern and number of cuts, the base thickness, how much soil must be dug out and hauled, access for machines, drainage work, edge restraint and any retaining or step work.
A thicker base for clay or cold climates adds material and labor.
ICPI states a 50-year design life for permeable systems based on field performance 2. That is a manufacturer-association claim for a specific system, not a promise for a yard. The life of any paved area depends on the soil, drainage, compaction and use. We give no number of years.
To see what sets a total, use the cost hub.
How a paver installation runs, in order
A homeowner can build a small patio, and many hire it out. ICPI recommends ICPI Certified Concrete Paver Installers for construction, and says ICPI keeps a list 1.
- Plan and check utilities. ICPI says to check with local utility companies before excavating, and many places have one number to call at least two days ahead 1. In the U.S. that number is 811.
- Lay out and excavate. Stake the area, set the finished slope to at least 1.5%, remove organic material and compact the soil subgrade 1.
- Build the base. Spread and compact aggregate in lifts. ICPI gives 4 to 6 inch lifts with a plate compactor 1.
- Set the edge restraint, then screed the bedding sand to about 1 inch 1.
- Lay the pavers. Mix units from several bundles to blend color; keep joints consistent 1.
- Compact, fill and compact again. Seat the pavers, sweep in dry joint sand and compact until the joints are full 1.
- Check tolerances and finish. ICPI’s surface tolerance is plus or minus 3/8 inch under a 10-foot straightedge, and lippage between pavers at most 1/8 inch on a pedestrian route 1.
Call 811 first, and protect yourself from dust
Call 811 before any excavation so buried lines are marked. Saw-cutting pavers throws dust that contains respirable crystalline silica, per ICPI, so cut with dust control and wear suitable protection 1. A tall or terraced patio edge may be a retaining wall and need a permit; see retaining wall permit thresholds.
Keeping a paved surface in shape
Upkeep tasks are listed here without fixed dates. How often depends on use, climate and soil.
- Top up joint sand when it drops. ICPI says stabilized sand may help in areas prone to joint sand loss 1.
- Keep water moving off. The minimum slope is 1.5%; a puddle means a low spot 1.
- Fix low units early. Units can be lifted and reset, and the base re-compacted 2.
- Clean with care. Pressure washing can displace joint sand; follow the maker’s guidance.
- Keep edges tight. A missing or failed edge lets the field spread.
Warning signs, and when to call a pro
- Dips or ruts that grow. Settlement points to base or subgrade compaction trouble 1.
- Pavers rocking or edges opening. The edge restraint may have failed or moved.
- Uneven bedding showing through. ICPI says bedding sand should not be used to make up for an uneven base, because unevenness reflects through over time 1.
- Ponding after rain. Slope below 1.5% or a low spot. See the drainage diagnostic.
- Heaving or cracking in cold or clay soil. Ask a paving contractor or engineer about base depth and frost.
Rules and standards that apply
Unit standards. ASTM C936 in the U.S. and CSA A231.2 in Canada, as cited by ICPI 1. These are voluntary until a contract, spec or code cites them.
Construction guidelines. ICPI Tech Spec 2, revised February 2020, covers construction of interlocking concrete pavements 1. ICPI’s Tech Specs are guidelines, not law.
Permits. Patios on grade often do not need a building permit, but local rules vary, and walls, steps and drainage changes may. We did not load a general rule, so check your building department. See retaining wall permit thresholds for the wall side.
HOA and stormwater. Some neighborhoods and cities limit hard surface area. Your HOA and your city stormwater office are the places to ask.
Interlocking pavers compared with nearby terms
| Term | What it is | How it differs |
|---|---|---|
| Interlocking concrete pavers | Factory units set in sand on an aggregate base 1 | This page: the unit and its numbers |
| Paver patio | The finished outdoor surface | The service and the whole job, not just the unit |
| Paver base and bedding | The buried aggregate and sand layers | Covered in the base specification guide linked above |
| Permeable pavers | Interlocking units with open joints over a stone reservoir 2 | Built to pass water through open joints and a stone reservoir |
| Poured concrete | A single cast slab | Rigid; cracks rather than settles; not unit-based |
Common paver myths
- Myth A thicker paver means you can use less base.
- Reality ICPI sets paver thickness by use and base thickness by traffic, soil and climate. They are separate numbers 1.
- Myth Bedding sand can level out a bumpy base.
- Reality ICPI says bedding sand should not be used to compensate for an uneven base, because the unevenness reflects through over time 1.
- Myth Any sand will do for bedding.
- Reality ICPI says masonry sand for mortar should never be used for bedding, nor should limestone screenings or stone dust 1.
- Myth Pavers are permeable.
- Reality Standard interlocking pavers shed water into the joints and off the edges. Permeable systems use special open joints and a stone reservoir 2. Fairfax County notes that even in permeable systems, the pavers are not permeable themselves; water moves through the joints 5.
- Myth A guideline is a code.
- Reality ICPI says its Tech Specs are not an industry standard, certification or specification 1.
The local variable: frost, soil and climate
Where you live changes the base. ICPI says that in colder climates, continually wet or weak soils need bases at least 2 to 4 inches thicker, and that greater thicknesses are often used in regions with many freeze-thaw cycles, expansive soils or very cold climates 1.
The frost depth is set locally and expansive-soil regions are mapped, so see our expansive soils page, and ask your building department what applies to your lot.
When to choose them, and your next step
Interlocking concrete pavers suit patios, walks and driveways where you want a surface that can be repaired unit by unit. Poured concrete or gravel may fit a site better if the budget, soil or style points that way.
Next, ask each bidder for the paver’s thickness, its ASTM C936 or CSA A231.2 conformance, the base depth, the edge restraint and the joint material, in writing. Then compare the answers with the decoder table above.
To start, size the area with the patio size planner, and read the gravel path vs. paver path comparison if a walkway is the goal.
Questions about Interlocking Concrete Pavers
What are interlocking concrete pavers?
Interlocking concrete pavers are solid, factory-made concrete units laid in a pattern on a bedding-sand layer over a compacted aggregate base, with sand-filled joints and an edge restraint. ICPI publishes the construction guidelines most specifications cite, and the units are expected to meet ASTM C936 in the U.S. or CSA A231.2 in Canada.
What does ASTM C936 require?
According to ICPI Tech Spec 2, ASTM C936 requires an average compressive strength of at least 8,000 psi (55 MPa), average absorption of no more than 5%, resistance to at least 50 freeze-thaw cycles with average material loss not above 1%, and abrasion resistance.
We did not read the standard itself, so confirm the current edition with the supplier.
How thick should pavers be?
ICPI says pedestrian areas, driveways and areas of limited vehicular use are paved with units 2 3/8 inches (60 mm) thick, while streets and industrial pavements use units at least 3 1/8 inches (80 mm) thick. Thickness is set by use. Base thickness is a separate decision based on soil, traffic and climate.
How wide should the joints between pavers be?
ICPI says joints should be consistent and between 1/16 and 3/16 inch (2 to 5 mm). Most pavers have spacer bars on their sides that hold a minimum joint width so sand can enter. Gaps greater than 3/8 inch should be filled with cut pavers rather than wider joints.
How deep should the base be under a paver patio?
ICPI gives a minimum compacted base of 4 inches for patios, walkways and pedestrian areas over well-drained soil, and 6 inches for residential driveways. It says continually wet or weak soils in colder climates need bases 2 to 4 inches thicker. Your local soil and frost depth can change the number.
Do I need edge restraints on a paver patio?
Yes. ICPI says edge restraints must be set at the correct level, and that at least 1 inch of vertical restraining surface should be in contact with the side of the paver. They are typically installed before the bedding sand and pavers. Without them the field can spread and the joints can open.
What slope should a paved area have?
ICPI says slopes should be a minimum of 1.5%. That is about 1.5 inches of drop for every 100 inches of run.
Water that cannot leave the surface will pond and can undermine the base.
Check the slope with a level before laying pavers, and send runoff where it will not harm a neighbor or the house.
Can I install pavers myself?
A small patio is a common homeowner project, but base preparation and compaction decide how it performs. ICPI recommends ICPI Certified Concrete Paver Installers for construction. Call 811 before digging, rent a plate compactor and follow the manufacturer’s guide. For driveways or tall edges, hire a pro and check permits.
How do I know my pavers meet the standard?
Ask the supplier for the unit’s test report showing conformance to ASTM C936 or CSA A231.2. ICPI describes the strength, absorption and freeze-thaw limits for each. A color sample or a brochure is not proof. A reputable supplier can provide the report for the production lot you are buying.
Sources
- Interlocking Concrete Pavement Institute, Tech Spec 2: Construction of Interlocking Concrete Pavements (rev. February 2020; read as the PDF republished by a member distributor, text extracted with pdftotext), retrieved .
- Interlocking Concrete Pavement Institute, Tech Spec 18: Construction of Permeable Interlocking Concrete Pavement Systems (rev. March 2022; member-distributor PDF, text extracted with pdftotext), retrieved .
- Concrete Masonry & Hardscapes Association, Interlocking Concrete Pavement, retrieved .
- City of Brampton, Ontario, Landscape Specifications, Section 02785 Unit Pavers (PDF, text extracted with pdftotext; undated), retrieved .
- Fairfax County Public Works and Environmental Services, Permeable Pavement fact sheet, retrieved .
Expert review
Written by the HyreYard Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Strength, absorption, thickness, joint, base, bedding, slope and compaction figures are quoted from ICPI Tech Spec 2 (a member-hosted copy; icpi.org now redirects to CMHA) and agree with facts.js on this site.
The ASTM C936 and CSA A231.2 figures are ICPI’s description of them; the standards themselves were not read. The Brampton spec is a Canadian municipal example, not a U.S. rule.
Suggest a correction. We fix errors and say what changed.