ARTICLE Anya Kussler
If you're about to invest in a driveway or pathway upgrade/installation, start off on the right track by checking out our driveway Q&A guide. We cover all the essentials, from the ideal minimum driveway thickness through to how long concrete will take to set.
Q. Can I pour concrete directly onto the ground?
- Concrete Crack Inducers. Concrete crack inducers allow you to put the crack where YOU want it. Don't risk cutting cables or pipes again. Crack inducers are available in 35mm and 47mm sizes and are supplied in 2700mm or 1150mm lengths. Connect them with a.
- Use CEMIX CONCRETE CURE WB to cure concrete, or just keep the surface damp for the first 7 days. Note: Fibers in this mix will resist cracking and thus not prevent cracking caused by overloading the slab with weight or due to movement in the substrate. For commercial heavy-duty slabs please follow the engineers design guidelines.
THE JOINT FREE SLAB CRACK INDUCER GRID The crack inducer grid used in New Zealand is a one metre square grid comprising extruded plastic tubes and plastic four-way junctions. The plastic tubes are supplied cut to length for the one metre square grid, and the grid is assembled on site with the tubes fitting snugly onto the junctions. Our concrete fibre slabs have been around longer than our competitors have been in business. Some of the many benefits of having Lesa Systems/Canzac as a single source supplier: Design assistance: Involvement in the design and documentation from the very beginning of the project - before the working drawings are commenced (EDI).
A: Not a good idea! In fact, it’s really important to prep your surface well before you pour the concrete to prevent it from weakening and cracking. To create an even base and separate the concrete from the ground or clay, use compacted material like GAP20 with fine material (such as GAP7) to finish.
Q How thick do I pour it?
A: Residential driveway (used for light traffic) or path should have a thickness of at least 10cms to minimise the risk of shrinkage and cracking.
Q. What is the difference between concrete and cement?
A: Cement is an ingredient of concrete. Concrete is actually a mixture of sand, gravel or crushed stone aggregates and a paste made up of water and cement.
Q. How can heat and wind cause the concrete to crack, and what can I do about it?
A: Laying concrete in summer, especially a dry hot one, can be challenging because a lack of moisture in the air can cause the concrete to tear. There are numerous ways to prevent cracks from ruining your handiwork:
- Never pour concrete during the hottest time of day.
- Use mesh for driveways (not necessarily for paths) – it helps minimise the width of surface cracks. Position the concrete a third of the way below the surface by placing it on bar chairs with each sheet tied together.
- Dampen the base down properly the night prior to pouring the concrete (that’s if you’re not laying it on polythene (a common plastic used as a base). If you are using polythene, it needs to be a hundred per cent flat, otherwise it can cause the concrete to crack.
- Fog the concrete surface by holding a water blaster a few metres away so a soft mist can settle on it.
- Using a tooled joint, where the cement is compressed into a V shape with a trowel and induces a crack whilst wet, is effective in keeping cracks at bay in heavy-duty areas such as long, narrow driveways or internal corners.
- Specially designed crack inducers can also be used in high-risk areas.
- Use the largest aggregate concrete size available, rather than fine aggregate. Fine aggregate contains more water, which makes it more prone to shrinking and cracking.
Q. How does cold weather affect pouring concrete?
A: Don’t pour concrete in temperatures lower than 5 degrees Celsius as the concrete may freeze and you’ll have trouble setting it, especially when placed on polythene or floor slabs. In colder climates, consider adding a set accelerator to the concrete to speed up the time it takes to harden.
Q. How long do I have to pour the concrete before it sets?
A: You have one hour (for a truckload worth) before it starts to harden, so it’s crucial that you act quickly (you have about an hour and a half from batching to pouring the concrete). Basically, what happens is that once water is added to cement, it begins to form crystals that interlock with one another and reinforce the concrete. So the quicker you have your concrete in place the stronger it is likely to be once set. If your property is a long distance away from where you get the concrete from, consider adding a set retarder to the mix to delay the hardening process.
Q. Do I need to vibrate the concrete?
A: You sure do – vibrating the concrete in order to release trapped air is essential for it to become strong and durable (except in the case of aggregate or polished concrete where the vibrating can change the look of the finish).
Q. What does cutting concrete mean?
A: Using a circular saw, you literally cut evenly spaced, square or rectangular patterns into the concrete – this will generate joints and prevent shrinkage and ultimately, cracking. It is highly recommended to employ a professional do this job
Q. How long after laying the concrete do I have to wait before I cut it and how deep down do I go?
A: In summer, you need to wait 24 hours before you cut the concrete and in winter 48 hours. You might want to leave exposed aggregate a tad longer; otherwise it can chip. Cut down one-quarter of the thickness of the concrete (i.e. 3cm for a concrete path or driveway that’s 12cm thick).
Q. How do I cure my concrete path or driveway?
A: Concrete becomes hard due to a chemical reaction between cement and water, so without the essential hydration the cement won’t set properly, plus it’ll be weak and look white and dusty. It’s important to keep the concrete consistently wet as soon as possible after pouring it without damaging its surface, for seven consecutive days. Using sprinklers, drip hoses or fogging (see the question on how to prevent cracking) are good options for keeping your driveway and path ‘well hydrated’.
Q. How can I cure my cement if I don’t have access to water?
A: You can spray a curing compound onto the concrete surface of the concrete after you’ve finished laying it. A protective film will develop, which locks the moisture into the concrete.
Q. What are permeable pavers?
Permeable paving is a great option for anyone wanting to put less pressure on their drainage systems. Permeable paving is designed to sustainably reduce rainfall runoff. Firth's durable FlowPave range is designed with driveways in mind and filters both water and pollutants. This driveway option requires minimal maintenance; only the occasional sweep and/or leaf-blowing.
Q. What should I look for in a landscape designer?
A: When seeking out a landscape designer to carry out your driveway/pathway installation, working with a company who can take care of both the design and installation can make a world of difference. Having the same team on board throughout the project will help to significantly keep costs down and the project on schedule. It goes without saying but, before you sign any contracts, make sure you have checked out your designers' past projects and testimonials.
Q. Why Zones?
A: Zones provides an end-to-end project management process which takes care of bespoke designs, builds and maintenance. We can advise you on the best materials for your driveway based on your environment, house style and the amount of traffic it will experience.
Find out more
To learn more about the costs and material options involved in driveway installations, you might like to read: Driveway Costs in NZ. For more information on what Zones has to offer, check out our customer testimonials and completed projects.
Get in touch with Zones to discuss your landscaping project
If you would like to discuss options and ideas for your outdoor landscaping project, please use the enquiry form on this page to provide us with your contact details. We will get in touch with you at a time that suits you to discuss your landscape design and build. If you would like to provide us with more information about your project, we have a more comprehensive enquiry form on our 'Get in touch' page too.
*All information is believed to be true at the time of publishing and is subject to change.
As a result of the Canterbury earthquakes, there have been changes to the requirements for concrete floor slabs. Are you up to date?
BEFORE THE CANTERBURY EARTHQUAKES, concrete floor slabs for single-storey dwellings could be unreinforced or reinforced with polypropylene fibres.
Due to the poor performance of concrete slabs in the earthquakes, the New Zealand Building Code compliance document B1 Structure was amended to require concrete slab-on-ground floors to be reinforced with grade 500E ductile reinforcing mesh in accordance with AS/NZS 4671:2001.
Reinforcing mesh
Although the requirement for reinforcing all concrete floor slabs initially only applied to the Canterbury region, since 1 February 2012, it has been mandatory for concrete slabs on ground throughout the country.
The 500E reinforcing must be a minimum 2.27 kg/m² (or 1.15 kg/m² in each direction) welded mesh sheets. The reinforcing mesh sheets must be lapped at sheet joints by the greater of a minimum 225 mm lap or in accordance with the manufacturer’s specifications. It must extend to within 75 mm of the outside edge of the floor slab (including foundation wall) and be tied to foundation wall reinforcing according to NZS 3604:2011 Figures 7.13, 7.14, 7.15 and 7.16 with R10 starters at 600 mm centres and lapped with the slab mesh.
When are free joints needed?
Concrete Slab Crack
In NZS 3604:2011 paragraph 7.5.1, the size of a reinforced concrete slab on ground is limited to a maximum of 24 m in any direction. Where concrete floor slab dimensions exceed 24 m in one or both directions, a free joint must be installed.
If a slab exceeds the 24 m maximum dimension without the inclusion of free joints, it must be specifically designed.
A free joint is defined as a construction joint ‘where no reinforcing mesh passes through the joint [to link] both sides of the concrete slab, and the vertical faces of the joint are not in bonded contact with each other’.
Bonding of concrete at the free joint is prevented by inserting building paper in the joint or by applying a bituminous coating to one face of the joint.
Reinforcement of the free joint consists of 600 mm long R12 dowel bars installed at 300 mm centres along the joint and lapped 300 mm with the slab reinforcement on both sides of the joint (see Figure 1). All dowel bars on one side of the joint must have a bond breaker applied, for example, petrolatum tape wrapped around the dowel bars for 300 mm. Dowel bars must be aligned and parallel with the reinforcing mesh.
Shrinkage control joints
Concrete shrinks as it cures, and unless controlled, this can result in unsightly cracking across the slab. Shrinkage control joints, defined in NZS 3604:2011 as lines ‘along which the horizontal strength of the slab is deliberately reduced so that any shrinkage in the slab will result in a crack forming along that line’, can contain cracking to locations where they have minimal impact or visibility.
Shrinkage control joints should extend into the slab for one-quarter of the slab’s depth (see Figure 2) and must not damage the DPM underneath. They may be formed either by saw cutting the slab after it has hardened or by casting a crack inducer into the slab when the concrete is poured. Control joints that are cut into the slab should be formed within 24–48 hours after pouring, depending on the ambient temperature.
Cracking is most likely to occur at major changes of plan, so NZS 3604:2011 paragraph 7.5.8.6.4 requires that shrinkage control joints are created to coincide with these locations (see Figure 3).
Shrinkage control joints must be at a maximum of 6 m spacings to create bays. The length to width ratio of bays between shrinkage control joints, or between shrinkage control joints and a free joint, should be between 2:1 and 1:1, so no bay should be more that 6 × 6 m.
Supplementary steel may be placed in irregularly shaped concrete floor slabs in positions shown in NZS 3604:2011 Figure 7.18 but must not be installed across shrinkage control joints.
Laying flooring over top
Flooring, particularly ceramic tiles, should not be laid across the free joint or shrinkage control joints, as any movement in the slab is very likely to cause cracking or damage to the flooring. Instead, create movement control joints in the tiling or other flooring to coincide with the shrinkage control joints in the concrete slab.
Referenced standards
Concrete Slab Crack Control Joints
The changes to concrete slabs on ground in clause B1 Structure reference NZS 3604:2011 Timber-framed buildings, NZS 4299:1998 Earth buildings not requiring specific design and NZS 4229:1999 Concrete masonry buildings not requiring specific engineering design.
Although Standards New Zealand published NZS 4229:2013 Concrete masonry buildings not requiring specific engineering design earlier this year, this latest standard is not yet referenced in clause B1, so the 1999 standard still applies.
Download the PDF
Build-138-24-Build-Right-Concrete-slabs-and-control-joints.pdf
Concrete Slab Crack Inducer
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Articles are correct at the time of publication but may have since become outdated.
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