PAVING THE WAY – CONSTRUCTION (PART 1)

The basic function of residential pavements, such as driveways, paths and patios, is to provide safe and easy access onto or around a property. Concrete is typically used to not only provide a durable paving surface (sloped, to provide surface water run-off) but one which can also incorporate a wide range of decorative finishes to complement the design and landscaping of the residence.

While creating a concrete pavement is relatively straightforward, there are a number of design and construction aspects that need to be considered in order to produce a finished product that satisfies the functional and aesthetic requirements of the homeowners.

Subgrade preparation

The preparation of the subgrade (supporting ground) for residential pavements is a relatively simple process, as typically the loads are light. All topsoil (which may settle) should be removed and the area leveled to provide a uniform platform. For heavy loads, the ability of the subgrade to adequately support the loads without excessive deflection and cracking of the pavement needs to be considered.

A reasonably uniform foundation is essential to the long-term performance of a concrete pavement. An assumption that a concrete pavement will bridge over a poor subgrade could lead to a false sense of security. Thus, any poor, saturated, highly compressible or otherwise weak areas should be removed and replaced with suitable subgrade or roadbase material. This should be adequately compacted with a plate compactor or small roller, to avoid subsequent settlement. The same applies to the backfilling of service trenches.

A level area also minimises friction between the pavement and ground. This reduces the restraint of the pavement and, in turn, the risk of unplanned cracking.

Subbase

The function of the subbase of granular material is to provide a more uniform slab support by equalising minor subgrade defects (see Figure 1) and to prevent water wicking up through the concrete. The granular fill should have a good particle size distribution – aggregate meeting the Transit New Zealand grading requirements for AP20 and AP40 are suitable. 100 mm is a typical sub-base thickness. For clay subgrades, the thickness should be increased. 

Reducing friction under the slab

Providing polyethylene sheeting, 0.25 mm thick, is optional as an effective way of reducing the friction between the slab and subgrade/sub-base. The sheet edges should be taped. Compared to placing the concrete directly on the subbase, the frictional force (and hence restraint of the slab) can be more than halved. This allows the concrete slab to move more freely due to drying shrinkage and thermal volume changes, thereby reducing the risk of cracking.

Reinforcement fixing

The reinforcement (usually mesh) should be located within the top third of the pavement and have 30 mm of concrete cover to the top of the slab. It should be fixed in position using purpose-made supports or mesh chairs, spaced at maximum 800 mm centres to prevent sagging of the reinforcement during concrete placement.

Note: for light meshes, the spacing may need to be reduced to prevent sagging.

Mesh should not be walked into position or lifted to height using a hook during concrete placement. Nor should the concrete truck be driven over it to gain access for concrete discharge. Where sheets of mesh are joined, they should have a minimum overlap of two cross wires (ie, the two outermost cross wires of each sheet are overlapped).

Formwork

Formwork should be secured accurately in position to maintain lines and/or shape during concrete placement. It should also be set to the correct levels, so as to provide a guide for the finished surface levels and tolerances. Formwork should be pegged at around every 1 m, so that it is ridged enough to take the load of the concrete during placing and finishing without distortion.

Concrete ordering

When concrete is ordered, the following information should be supplied:

• The specified compressive strength.

• The nominal maximum aggregate size.

• The slump.

• The method of placement.

• Any additional requirements associated with Special Concrete (if applicable).

Typically for driveways, the specified strength should be 20 MPa, the nominal maximum aggregate size 20 mm and the recommended slump should be 100 mm if using hand held mechanical screed vibrators. For hand screeding without mechanical vibration, the slump should be higher at 120 – 130mm; for vibrating beam screeds, the slump should be lower at 80 mm.

About CCANZ

This article contains information from CCANZ’s Information Bulletin 80 Residential Concrete Driveways and Paths, which provides guidance on the planning, design, construction, maintenance and specification aspects that need to be considered to ensure a successful concrete paving project. Content from New Zealand Standards in this bulletin has been reproduced by CCANZ with permission from Standards New Zealand under licence 000997. Any New Zealand Standard referred to in this bulletin can be purchased from Standards New Zealand by telephoning 0800 782 632 or visiting www.standards.co.nz

CCANZ would also like to thank Cement Concrete Aggregates Australia (CCAA) for their assistance in producing the bulletin. To download the full Bulletin visit the CCANZ website – www.ccanz.org.nz