Construction

Depth of, and type of material required for, the base/subbase will vary from site to site, depending on expected loadings, level of treatment and rate of infiltration required.

The base/subbase should be constructed to the specified thickness, compaction and depth below finished pavement surface and to the design grade and crossfalls of the finished surface.

Minimum base thickness for light vehicle applications should be 150 mm, while a minimum of 250 mm is applicable to heavier and more frequently used traffic situations.

Material should be compacted in 100 mm layers until stable. The topmost layer should be filled to approximately 50 mm above the target height and compacted until stable. Depending on the compacted height achieved, further fill may be necessary or compacted fill removed. In both situations, material should be compacted within ± 10 mm. The level of the finished base course should be the combined thickness (mm) of the paver and bedding layer below the design surface of the pavement.

The base/subbase should be compacted to not less than 98% of standard maximum dry density according to AS 1289.5.1.1. Because of the low fines requirement, the base/subbase will be harder to compact than normal road basecourse DGB20.

Only contractors with experience in compacting low fines basecourse should be engaged for installing trafficable permeable pavements.

The finished base/subbase should be inspected and approved by the Project Supervisor/Engineer.

Edge restraints should be constructed along the perimeter of a HydroSTON pavement in accordance with approved project drawings. There is a wide range of edge restraints that can be used with HydroSTON pavements. Stronger restraints are required with vehicle pavements than for pedestrian only pavements in order to prevent the spread of paving units over time due to vehicle movements and load pressures.

An effective edge restraint for both HydroSTON vehicle and pedestrian pavements is shown in Plate 1, where HydroSTON units are mortared to sub-surface concrete haunches at 90 degrees to the body of the pavement. HydroSTON units can then be effectively butted to adjacent pavements, including asphalt.

An alternative form of restraint constructed from treated pine, but more suitable for pedestrian pavements, is shown in Plate 2.

200mm x 4mm cold dip galvanised steel flat bar can be effective as an edge restraint where pavement abut grass or landscaped areas.

Further options may be found in Concrete Masonry Association of Australia (CMAA) Concrete Segmental Pavements – Detailing Guide T46 and Concrete Flag Pavements – Design & Construction Guide MA44.

Sand should not be used as bedding material for HydroSTON pavements due to the rounded shape of grains, which can lodge in the surface voids of the paving unit and impede water flow.

HydroCon recommends that 2-5 mm graded or 5 mm single size crushed low fines aggregate, preferably basalt, be used as bedding material. Due to the angular shape of grains, washed 4 mm recycled glass (GlassSand) has been successfully used in a number of applications.

A suitable 5 mm single size aggregate will have a particle size distribution corresponding to the following:

Bedding material should be spread in a single uniform layer over the base course and screeded in a loose condition to a depth necessary to achieve a uniformly thick layer after laying and compaction of the pavers. Final depth should be 40 mm for vehicle traffic pavements and 30 mm for pedestrian pavements.

The appropriate depth of screeded material may be determined by laying a trial area before construction. 5mm single size basalt layer will generally reduce by 10mm after compaction of pavers.

Surfaces of the base course and bedding layer should be uniform with no irregularities in levels. The bedding layer should not be used to compensate for irregularities in the base course, since any unevenness in the bedding layer may cause deformation of the pavement during compaction, which could later increase under traffic load.

Any screeded bedding material left exposed should be checked for depressions and levels and, where necessary, rescreeded before further pavers are laid.

Unwoven geotextile (Plate 1) may be laid between the base course and the bedding layer to improve structural stability and/or prevent migration of bedding and base course materials.

Main laying patterns are herringbone, stretcher and basketweave. Zig-zag running bond is a variation of stretcher bond. Each pattern may be laid at either 90 degrees or 45 degrees to the line of edge restraints.

HydroSTON 50 pavers should be laid in either stretcher or herringbone bond. Since both HydroSTON 50 Block and Flag are dimensionally exact, these pavers are normally laid without joints.

HydroSTON 80, however, should be laid with 4 – 6mm joints in standard herringbone pattern or variants such as ‘elbow interlock’ (see section below on Joints & Joint Filling).

Pavers should be placed uniformly on the screeded bedding to the nominated laying pattern.

As concrete pavers may exhibit minor variations in colour, it is recommended that pavers be selected from 3 pallets and from different levels of each pallet. This widely recommended procedure avoids concentrations of colour and gives a more harmonious appearance.

The first row should be located next to an edge restraint or an established straight line, and laid at a suitable angle to achieve the required orientation of pavers in the completed pavement.

Normal practice is to work off a ‘header’ row of pavers laid at a 90 degree angle to the edge restraint or header row acting as the edge restraint (Plate 2).

In each row, full units should be laid first. Closure units should be cut with a brick cutting power saw and fitted subsequently. Where possible, edge restraints should be adjusted to minimize cutting, as shown below.

Except where necessary to correct any minor variations occurring in the laying bond, paving units should not be hammered into position. Where adjustment of position is necessary, care should be taken to avoid premature compaction of the bedding material.

To prevent disturbance to the pavement, no construction traffic should be allowed prior to compaction and joint filling. Boards should be used for foot or barrow traffic and completed pavement protected from contamination.

HydroCon recommends that HydroSTON 80 vehicle pavers be laid with different coloured string lines as shown in Plate 1. While ‘stringing off’ the joints in this manner takes a bit longer, the method ensures that the gaps are consistent and straight.

If possible, the whole stringed section should be laid without walking on the pavers. Once the section is finished, strings should be lifted and joint filler brushed in as shown in Plate 2. The filler secures the pavers in place enabling them to be walked upon without their movement while the next section is being prepared.

Joint filling material should be clean dry low fines crushed aggregate having a particle size of around 1–3 mm. HydroCon supplies a specially graded -3.5 +1.0 mm virgin basalt aggregate suitable for 4 – 6 mm joints. HydroCon Joint Filler is available in 1 tonne and 0.5 tonne bulk bags. Rate of application is around 10 kg per m2 of pavement.

Care should be taken to ensure that joints between individual paving units, and between paving units and edge restraints, are fully filled with joint material.

Careful attention to joint materials and clearances is important in allowing deflections of pavers under traffic load. Deflections are typically less than 2 mm for loads less than 2 kN. As deflections increase, jointing material is progressively drained by gravity during successive cycles of loading and unloading. This action can destabilize the pavement and cause deformity.

The pavement should be compacted only after the entire pavement, including edges, has been completed.

The pavement should then be compacted to achieve consolidation of the bedding layer (approximately 10 mm settlement). The surface should be brought to design levels and surface profiles by not less than two passes of a high frequency, low amplitude mechanical flat plate vibrator fitted with a rubber or plastic apron to protect the paved area.

Joints should be refilled after compaction to fully close gaps in the joints.

Paving units, which are structurally damaged during compaction, should be removed immediately and replaced. The pavement should then be recompacted for at least 1 metre surrounding each replacement unit.

When compaction is complete, the pavement should be swept to remove excess filling material.

While fully porous HydroSTON pavers provide maximum opportunity for air outflow at the surface, pavement systems designed for stormwater harvesting may need to be vented to avoid variable infiltration rate performance.

Infiltration of rainfall involves water inflow and air outflow, and possibly air compression. Laboratory studies have shown that soil air compression can lead to a substantial decrease in the rate of infiltration. When air pressure is sufficiently high, air will escape from the soil surface, thereby causing a sharp decrease in air pressure and a major increase in the rate of water infiltration.

Infiltration rate is always equal to, and controlled by, the rate of air outflow from the soil (or pavement) surface. The infiltration rate varies inversely with the air pressure ahead of the wetting front and with the ponding depth over the pavement. A minor decrease in surface water head can cause a substantial increase in infiltration rate.

Air outflow dynamics has implications for the specification of system infiltration rates and for construction, particularly when permeable pavements are to be used for stormwater harvesting.

Where drainage pipes are required, it is recommended they be laid at a gradient of min 0.5% (a minimum fall of 5 mm per metre) towards the discharge point. Slotted drainage pipes should be wrapped in unwoven geotextile.

Subbase or base course material should be placed evenly around each side of the drainage pipes. A light compactor should be used to ensure that the material under the pipes is stable and provides an appropriate support angle.

Cover above the drainage pipe should be at least 50 mm.

Joints of impermeable liners and geofabrics should have 300 mm overlaps and be sealed with double sided tape.

Woven geofabric should be placed over impermeable liners to prevent damage from base/subbase material. Some subgrades may necessitate placement of woven geofabric under the liner.

Pavement construction should be carried out following completion of general site works. During construction, the pavement area should be protected from any materials eg sediments, site debris and ingress of foreign matter from areas adjacent to the construction site, that could contaminate the pavement structure.

Pavement under construction should be fully protected from machinery and traffic of all kinds until joint filling and compaction of the pavement cover has been completed. Boards should be used for foot or barrow traffic during the construction stage.

Permeable pavements should be installed by experienced pavement contractors, who understand the processes of stormwater treatment and infiltration and the importance of adhering strictly to design and material specifications.

Installation of the base/sub-base is a critical aspect of trafficable pavement construction. A sub-contractor should be selected with experience in permeable sub-structure installation.

Installation should be closely supervised by a representative of the commissioning body or client. The representative (Project Supervisor) should be responsible for certifying that construction has been carried out in accordance with specifications in this document. Failure of permeable pavements is invariably due to changes in recommended substructure materials and inappropriate substructure installation.

The Project Supervisor should ensure that contractors supply for approval samples and details of bedding, joint filling, base/subbase materials proposed for use at least 28 days prior to delivery of materials or commencement of construction of the pavement. Samples of 20 kg mass should be provided unless otherwise advised by the Project Supervisor. Samples should be supported with test results from a NATA registered laboratory, confirming that the constituents comply with the requirements of this Specification. Materials used should conform to approved samples.

The Project Supervisor should certify the appropriateness of samples of base/subbase, bedding and joint filling materials submitted by the Contractor.

The Contractor should take digital photographs of each stage of construction – subgrade, base/subbase and bedding layer. Photographs should be transferred to computer disk and provided to the Project Supervisor prior to commissioning. The photographs will provide a visual record of construction, helping to verify that construction was in keeping with relevant specifications.

As soon as practicable after compaction, light vehicle use of the pavement should be permitted to assist in “lock-up”. The effectiveness of lock-up will depend largely on workmanship in laying of the pavement cover, particularly the filling of joints.

Pavements should be inspected by contractors at regular intervals up until the expiration of the Defects Liability Period to ensure that all joints remain completely filled. Any joints not completely filled should be topped up as necessary.