At Lesa Systems Ltd, we want to help engineers, designers, developers and contractors to mitigate against any risk in their concrete construction project.
Lower the evaporation from the slab. (See Concrete Curing Tip) Reduce the restraints to slab movement/ contraction e.g. Lower bay widths and pour dimension. Correct location of saw cuts and construction joints. Properly positioned and use of free movement joints. Once Cracked (Assess what the crack will do) Leave as they are, better not to fill. 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.
Crucial in achieving this is aligning your business with a single source partner. One that is trusted, reputable, and has extensive experience with concrete and the construction process across all types of projects, as well as having extensive knowledge of the inevitable issues encountered with poorly designed and constructed slabs on grade.
That's why in 2017, Canzac Group acquired Lesa Systems Ltd, to not only broaden an already extensive product offering but more importantly, to give you one point of contact.
Lesa Systems Ltd and Canzac Limited have both been in business for 30 years, offering 60 years of combined industry experience. We are the 'go to' company for engineers and contractors as we know what works and we know what doesn't.
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).
- Experience: Benefit from a highly experienced team with extensive and specialised skills in all aspects of design, construction and performance of slabs on grade across all types of projects.
- Timely advice: Access to the right information at the right time.
- Specialised knowledge:Utilise the team's industry experience in load transfer systems (dowels), appropriate to the project and other aspects of construction including vapour barriers, control joint locations, edge protection, joint filling, curing, densification etc.
- High-level technical support: Gain access to expert consultants, when required, for geotechnical considerations, design loads, minimum slab thickness etc.
- Certification: Provision of producer statements for the slabs (PS1, PS2 and PS4).
- Responsibility: Accountability for all aspects of the slab performance.
- Commitment: Work with a company/supplier that understands what clients want and what they expect with their slab on grade floor (not just a product supplier).
Harder Than Concrete - As told to Hugh de Lacy
Harder Than Concrete tells of the struggles founders Lance and Carolyn Canute (CC) encounter in their pursuit to establish and expand Canzac.
It recounts the unconventional path of a young, budding kiwi entrepreneur, who leaves school at 15 and his home at 18 to travel abroad. Along the way, an unlikely friendship ignites a lifelong passion within the construction industry and just as everything seems to be going to plan, life takes an unexpected turn.
Returning to New Zealand in a bleak job market, he tries his hand as a fast-food business owner, only to find that even the world of fast food has its critics. With little interest in improving his cooking, he decides to shut the business, but not before realising that he had won over one customer, a shy, unassuming girl who eventually shows what she’s made of when she asks him out. It’s not long before her loyalty and quiet determination is put to the test as they set up house, start some questionable sideline businesses and spend their entire savings on some Super-Tie formwork systems. Supported only by her medical practice wages they anxiously await for the orders to roll in. From product lines, upsizing and company takeovers, would these risks pay off and just how long would it take this concrete rebel with a cause?
An insightful story, full of witty, down-to-earth anecdotes that have been cleverly crafted and interwoven into the political and historical landscape of the time, by award-winning Christchurch journalist Hugh de Lacy.
Soft Copy: $17.50 (inc P&P) - Hard Copy: $22.50 (inc P&P)
Email: [email protected]
Email: [email protected]
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?
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
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.
Concrete Slab Crack Control Joints
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Build-138-24-Build-Right-Concrete-slabs-and-control-joints.pdf
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Concrete Slab Crack Inducer Nzxt
Articles are correct at the time of publication but may have since become outdated.