Paving the way – Design (Part 2)

The basic function of residential pavements such as driveways, paths and patios is to provide safe, 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 providing a concrete pavement on the ground 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.

Reinforcement

The use of steel reinforcement or mesh does not increase the load capacity of the pavement, or affect its thickness. Its function is mainly to hold tightly closed any shrinkage cracks that may form. In general, the amount of reinforcement required to provide the appropriate degree of crack control will depend on the thickness of the slab, the joint spacing and the resistance to the slab shrinkage between joints.

For a 75 mm-thick footpath, which typically would have closely spaced joints (up to 3 m) – and hence little risk of cracking – either no reinforcement or a 668 mesh is generally appropriate.

For 100 mm-thick pavements with joints spaced further apart (up to 5 m), 665 mesh would typically be used.

For 150 mm thick pavements, use 663 mesh.

The construction of driveway pavements without reinforcing mesh is not recommended.

Mesh is typically based on a 150 mm square grid; however, mesh based on a 300 x 300 mm grid is also available, allowing the placers to walk between the mesh rods rather than on the mesh itself.

Note: slab construction requiring building consent requires Grade 500E (ductile) reinforcing mesh.

In some cases, increasing the minimum reinforcement would be recommended. For example, where decorative finishes are required, increasing the mesh size by one or even two sizes will provide better control of cracking that may occur, so that it does not significantly affect the appearance.

Crack control

All standard concrete shrinks during the drying out phase. This starts at the end of the curing period and will continue until the moisture content stabilises with the ambient conditions, which may take many months. While the key elements that determine the amount of shrinkage are the type and quantities of materials used in the concrete mix – particularly water, cement and aggregate – it is not possible to design a mix that will avoid slab shrinkage altogether.

Cracks observed within one to two days are not caused by the long-term drying of hardened concrete; they are caused by premature drying of plastic concrete or by ‘early age’ thermal movements. These are primarily issues that need control during the construction phase.

When shrinkage strain causes stresses to exceed the tensile strength of the concrete slab, then the slab will
crack. In practice, there is likely to be a combination of linear shrinkage between free joints and curling effects.

Curling is caused by the differential shrinkage within the depth of the slab, ie the top dries out faster than the bottom of the slab. With the top shortening more than the bottom, the slab will try to curl upwards at any edges or joints. At free joints, concrete on each side of the joint can move relative to one another.

The classic design for shrinkage in a concrete ground slab relates to allowing the shrinkage movements to take place and controlling the positions of where the movement is allowed to occur. This process is influenced by the following:

(a) Ground friction. If the interface between the concrete slab was frictionless, then the phenomenon of the concrete shrinkage would not cause any stress build up. Friction can be reduced by placing polyethylene sheeting or a 20 mm thick layer of sand under the slab.

(b) Construction features that cause a constraint to movement. For example: slabs tied to foundations internal or external, or changes in direction of the slab. Such construction features need to be isolated to allow the slab to move independently of them.

Joints typically form a weakened plane at which the concrete cracks. Without them, drying shrinkage will result in random cracking. Joints should be provided at maximum 5 m centres, depending on the reinforcement, and at any changes in shape (eg a narrow path attached to a driveway), at any changes in direction (eg around corners, especially where a re-entrant corner may be formed), and at any rigid structures (eg access holes, pits, columns) that may prevent movement and increase the risk of cracking.

Free movement joints should be placed at maximum 15 m centres to allow the pavement on each side to move independently of each other.

Double car driveways require a longitudinal joint splitting the driveway.

Wherever possible, the location of joints should be planned. A typical layout of joints in the paving to a residence is shown in Figure 1. Note that one aim should be to make the concrete panels defined by joints roughly square in shape. The ratio of length to width should not exceed 1.7:1. For decorative work, joints should, if possible, be located to suit the proposed decorative pattern or finish.

With all joint types, the angles formed at edges and intersections of joints should not be too acute, as this increases the risk
of cracking and/or breaking off the tapered section of concrete. A good detail in these situations is to keep at least 500 mm of the joint at more than 75° (and preferably at right angles) to the slab edge.

Surface tolerances

Pavements generally have to meet two independent tolerance criteria: the ‘flatness’ of the surface and ‘levelness’ or variation from the designed elevation. Both of these should be specified.

The flatness tolerance gives the permitted variation from a 3 m straightedge placed on the surface. For hand-placed pavements, a reasonable flatness tolerance is a maximum deviation or gap of 5 mm under a 3 m straightedge placed anywhere on the pavement, including on slopes. The maximum abrupt deviation anywhere on the surface shall be 5 mm.

The levelness tolerance gives the permitted variation of the slab surface from a fixed external reference point or datum. A reasonable tolerance for the surface of a newly-constructed pavement would be ±10 mm from the designed level or elevation. Note that the nominal slab thickness should not be compromised by tolerance variations.

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 license 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