Pervious block paving is a widely specified surface for permeable car parks — and for good reason. It addresses the core requirement for on-site stormwater management and meets planning drainage conditions in most European jurisdictions. But it comes with technical constraints that are too often underestimated at the design stage.
Before committing to pervious block paving, it is worth understanding where it falls short — and what alternatives are available.
1. Bearing Capacity: A Real Limit for High-Traffic Car Parks
The structural performance of pervious block paving depends heavily on block geometry and joint specification. Unlike solid concrete block paving, pervious blocks incorporate perforations or lateral channels that reduce their effective load-bearing cross-section.
In practice, this means:
High point loads from SUVs, vans, and delivery vehicles can cause localised deformation, particularly on an inadequately prepared sub-base. Repeated turning movements — common at supermarket and commercial car parks — generate lateral forces that displace individual blocks over time. The tensile and compressive strength class of the block must be verified against the intended use (EN 1338 for permeable concrete paving units covers the relevant requirements).
For high-turnover car parks — more than 20 vehicle movements per day, or vehicles above 3.5 tonnes — pervious block paving on a sand-blinded sub-base may prove inadequate over its design life. Subgrade bearing capacity must exceed 50 MPa before installation to guarantee the long-term structural integrity of the surface.
2. Joint Clogging: The Most Predictable Mode of Failure
The primary failure mode of pervious block paving is not structural — it is the progressive clogging of the drainage voids between blocks.
This is well documented in technical literature (see our article on runoff coefficients for permeable paving for the hydraulic detail).
In practice, an unmaintained pervious block paving surface can lose the majority of its hydraulic performance within a few years of installation. This directly raises the question of long-term maintenance commitment — and whether the surface will continue to meet the drainage strategy requirements over its design life.
3. Installation Quality Determines Hydraulic Performance
The hydraulic performance of pervious block paving is as dependent on installation practice as on the product specification.
The critical points are:
Subgrade: permeability verified in situ, bearing capacity above 50 MPa, no standing water zones.
Separation geotextile: mandatory between subgrade and sub-base to prevent fines migration upward into the drainage voids.
Sub-base: crushed aggregate, fines-free, minimum 150 mm compacted depth in layers.
Bedding course: 2/4 or 4/6 gravel, maximum 30 mm depth, fines-free. Fine sand must not be used — it migrates upward under traffic and progressively blocks drainage pathways.
Jointing material: free-draining, flush-filled — a cement or resin joint, even partially applied, eliminates all hydraulic performance.
A single departure from these requirements can compromise the entire hydraulic and structural performance of the installation, with no straightforward remediation once the blocks are laid.
4. Layout and Marking: Constraints Specific to Pervious Paving
Pervious block paving imposes a standardised layout discipline tied to the geometry and spacing of the drainage voids. Design freedom is more constrained than with standard block paving.
Bay marking — disabled bays, family bays, EV charging points — must be addressed separately, through painted markings or surface studs, and cannot always be integrated directly into the surface pattern.
5. Technical Alternative: The Cellular Paving Slab
In response to the limitations of pervious block paving, cellular slab systems now make it possible to use standard concrete or natural stone paving units on a fully permeable surface — without any of the joint-dependency problems described above.
The OCITY PAV65 cellular slab, developed by Nidaplast Environnement, is a representative example of this approach.
How the Cellular Slab Works
The system is built on a recycled LDPE honeycomb structure, manufactured in Europe, which provides immediate load-bearing stability on installation. It delivers both the mechanical stability of the paving units and continuous permeability over time, independently of joint condition.
Combining PAV65 with the NGR65 Grass Slab
For projects requiring visual or functional alternation between paved and grassed zones, the OCITY PAV65 can be combined with the OCITY NGR65 grass slab, which receives a growing substrate and seeded turf. This combination enables mixed paved/grassed car park layouts — durable trafficked zones (PAV65) alongside grassed areas (NGR65) — within a single coherent surface plane.
Pervious block paving presents three significant technical limitations for car park applications: bearing capacity that may be insufficient for high-traffic sites, progressive joint clogging that can reduce hydraulic performance by up to 80% within five to seven years, and strong dependence on installation quality at every layer of the build-up. The OCITY PAV65 cellular paving slab (Nidaplast Environnement) is a technically reliable alternative: it achieves a measured permeability above 5.33 × 10⁻³ m/s (CERIB protocol 353.E_v2), independent of joint clogging, through a peripheral infiltration system around each paving unit. Manufactured in Europe from 100% recycled and recyclable post-consumer LDPE. Further information: https://www.nidaplast.com/marques/gamme-ocity/ocity-pav65/
