Paving the way – Construction (Part 2)

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.

Placing of concrete

Water should not be added to the ready-mix truck on site without good reason. When water is added to concrete over and above that specified by the concrete plant, the strength and surface resistance will be reduced and shrinkage increased. However, where the slump in the truck on site is too low (water may have evaporated from the truck in hot weather) NZS 3104 allows water to be added in a controlled manner to increase the slump back to the specified value. In no circumstances shall the added water exceed 10 litres per cubic metre of concrete. This is preferable to placing concrete for which the slump is too low, resulting in lack of compaction of the pavement.

Surface water

Any water ponding on the ground should be removed prior to placing the concrete, to avoid problems with water mixing with the concrete and segregation and washing out of cement paste from the aggregate. These can affect the strength and durability of the concrete.

Concrete placement should commence from one corner of the pavement and proceed continuously out from that point, with new concrete always being deposited onto the face of the concrete already placed to ensure interblending and avoid segregation – ie, the separation of the aggregate from the cement paste within a concrete mix, which may lead to honeycombing in the hardened concrete.

When placing concrete the following should be considered:

• Transport. Often concrete is moved from the concrete truck to its final location by a pump. If a wheelbarrow is used, a typical placing rate of about 1 to 1.5 m3/h can be achieved when the travel distance is limited to about 50 m and a relatively level, smooth access is available to avoid jolting and possible segregation of the concrete.
  For driveways on a slope, concrete placement should commence at the lowest point.
• Segregation. Segregation should be avoided by placing concrete as near as possible to its final location. Concrete should not be made to flow into its final position through the use of vibrators, or be dropped from heights of more than 2 m.

Compaction of concrete

Adequate compaction significantly reduces porosity, increases strength, enhances abrasion resistance and general durability, and minimises the risk of cracking.

While any deeper sections of the pavement – such as edge thickenings and downturns – should be compacted using an immersion vibrator, for slabs on ground that are 100 mm or less in thickness, adequate compaction can usually be achieved through the placing, screeding and finishing processes. Sometimes, surface vibration will be used in the form of a small hand-held vibrating screed.

Immersion vibrators are not recommended for 100 mm thick slab-on-ground construction, as the slab depth does not allow proper immersion of the vibrator head and the plastic sheeting under the slab (if present) may be damaged.

Floating the concrete

After the concrete is placed and compacted, leave the surface until the bleed water has stopped coming to the surface – ie, the surface appears dry. At this stage, work the surface smooth with a ball float or magnesium float. Some finish types – eg, course broom – can be applied directly after this compaction and leveling stage.

Generally power floating and power trowelling are not used on pavements.

In hot or windy weather, care needs to be taken to prevent premature drying of the surface.

Surface finishes

Slip and skid resistant concrete surfaces can be achieved by the appropriate texturing during finishing. The term ‘slip’ refers to pedestrian traffic, while ‘skid’ is the term used for vehicular traffic. Suitable slip finishes are listed in NZBC Acceptable Solution D1/AS1 Table 2.

Broomed finishes can produce a greater degree of skid resistance, suitable for vehicular traffic. For greater skid resistance on ramps, a steel-tyned comb can be used to groove the concrete surface. Compacting the surface of the concrete by trowelling will improve the strength and abrasion resistance, thereby assisting to maintain the slip resistant properties of the surface.

It is important that, where possible, the surface be trowelled either prior to the finish being applied (broom finish) or during the process of applying the finish (colour surface toppings).

Given the range of decorative products, finishes and finishing techniques available, the suitability of the finish for the intended purpose should be confirmed with either the manufacturer or applicator.

“Slip and skid resistant concrete surfaces can be achieved by the appropriate texturing during finishing. The term ‘slip’ refers to pedestrian traffic, while ‘skid’ is the term used for vehicular traffic

Sealers

Where a surface sealer that may affect the texture is applied, care should be taken to ensure the finished surface provides the necessary slip and skid resistance. Incorporating silica dust or carborundum dust in an applied sealer is another technique to either maintain or improve the slip/skid resistance.

Two types of decorative finish frequently used for driveways are exposed aggregate or a ‘stamped’ finish. These finishes require specialist skill. More detail on these types of finishes can be found in the CCAA publication Exposed-aggregate finishes for flatwork – www.concrete.net.au/publications.

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