Good mix design and workmanship keys to minimising concrete shrinkage


A study by BRANZ has 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.
Researchers set out to establish why floor slabs crack as a result of concrete shrinkage. If the effects of 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, BRANZ senior structural engineer Roger Shelton says following 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 floor slab remained intact, the damage to the building was relatively light,” Mr Shelton says.


“However, if the slab failed due to ground movement, repair costs were so high that demolition was often the only option.


“This highlighted the conflict between maintaining the continuity 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 effects of concrete shrinkage.”


The tests were carried out on slabs constructed by a building contractor at the BRANZ yard near Wellington, following typical slab details from NZS 3604:2011.


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.


Slabs for these were variously laid — on a 0.25mm polythene membrane directly onto the base course, with a sand blinding layer, with a double polythene membrane, and on a 50mm layer of polystyrene insulation. Concrete weights were used to simulate imposed gravity load.


The study also investigated whether including a drainage layer under a slab could reduce curling stresses, but found this provided no benefits. This was because analyses of the restraint provided by typical New Zealand foundation details had shown that concrete stresses due to curling are considerably higher than those due to direct shrinkage.


Mr Shelton says that, while concrete shrinkage can also be minimised by certain proprietary products or systems, these were outside the scope of the project, as contracted by the Building Research Levy.


“While products and systems such as shrinkage compensating cements and shrinkage reducing admixtures, and post-tensioning the slab are used in the commercial field in New Zealand, they are not currently routinely used in residential construction.”


The full report and conclusions are available at SR340 (2015) Revisiting concrete ground floor slabs.