Page 15 - Build 151

News
BRANZ delivers concrete proof
A BRANZ study confirmed the best approach to minimising the effects of drying shrinkage in concrete floor slabs for new homes is through good mix design and workmanship.
‘The residential market is humming, andIhaveabig smile on my face. The reason for my smile ... at the end of 2014, we had around 550 dwellings on our books targeting a Homestar rating. I’ve just been informed the current figure is 2,534. That’s a very concrete figure the market is at last shifting.’ –
Alex Cutler, CEO New Zealand Green Building Council.
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‘I’m yet to find a housing market anywhereinthe world where prices go up at over 20% a year without stopping and then starting to come down again.’ – BillEnglish,Minister of Finance.
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CONSENTS
year to 9/15
What’s new?
Slab-on- ground floors
BULLETIN 592 Concrete slab-on-ground floors explains the principles involved and the practical steps necessary to specify and construct residential concrete slab-on- ground concrete oors that are durable and minimise uncontrolled shrinkage cracks. It replaces Bulletin 372.
Details Available for $13.50 from the BRANZ Shop at www.branz.co.nz or call 0800 80 80 85.
Non-structural
elements fact
sheets
THE BRANZ SEISMICALLY RESILIENT NON- STRUCTURAL ELEMENTS fact sheets provide information on having a more consistent approach to the design and installation of non-structural elements, components and systems in New Zealand buildings.
They assist building services industries to achieve compliance with the New Zealand Building Code.
The six fact sheets include clear diagrams and explanatory tables and cover:
1. Compliance and standards
2. Design criteria
3. Restraint systems
4. Seismic clearance at penetrations 5. Project process
6. Suspended ceilings.
Details Available at www.branz.co.nz/ non-structural.
RESEARCHERS SET OUT TO ESTABLISH
why oor slabs crack as a result of concrete shrinkage. If shrinkage could be reduced or eliminated altogether, the cost of cutting control joints could be avoided, and the performance of the slab as a structural member could be improved.
However, Roger Shelton, BRANZ Senior Structural Engineer, said that extensive testing, measurement and analysis of test slabs showed the best way to minimise shrinkage is through the tried and tested approach of good concrete mix design and good workmanship.
‘It was noticeable during our investigations of residential buildings following the recent Canterbury earthquakes that, if the concrete oor slab remained intact, the damage to the building was relatively light,’ Roger said. ‘However, if the slab failed due to ground movement, repair costs were so high that demolition was often the only option.
‘This highlighted the con ict between maintaining the con-
tinuity of the slab as the main structural member of a timber- framed building and the need to cut it into smaller sections to relieve the e ects of concrete shrinkage.’
Friction tests were carried out on six test slabs built on a gravel car park and slab edging tests on four foundations built on a cleared site.
Roger said that, while concrete shrinkage can also be minimised by certain proprietary products or sys- tems, these were outside the scope of the project.
‘While products and systems such as shrinkage-compensating cements and shrinkage-reduc- ing admixtures and post- tensioning the slab are used in the commercial eld in New Zealand, they are not currently routinely used in residential construction.’
For more Study report SR340 Revisiting concrete ground oor slabs is available for free download from the BRANZ shop www.branz. nz/shop.
ISSUE592
CONCRETE SLAB-ON-GROUND FLOORS
This bulletin outlines the speci cations and levels of workmanship required for to minimise cracking in residential concrete slab-on-ground oors.
Using specialists in concrete placing and nishing for the construction of residential ground oor slabs is recommended.
This bulletin updates and replaces Bulletin 372 Durability and crack control in domestic concrete oor construction.
Build 151 — December 2015/January 2016 — 13
FROM THE HORSE’S MOUTH
December 2015