Following two in-depth University of Auckland doctoral studies, an array of designers and product manufacturers constructed a post-tensioned concrete masonry house for the Habitat for Humanity organisation. The house is an exciting first for New Zealand’s concrete industry and is now an effective showcase, exposing and demonstrating the technology to all New Zealanders, say Gavin Wight, Jason Ingham and Andrew Wilton.

Post-tensioned Masonry House - Auckland.jpgNew Zealand is a country that experiences a high number of earthquakes, which means it will benefit substantially from the improved seismic performance that post-tensioned walls provide. Therefore, the University of Auckland undertook research to successfully design and construct a house that incorporated post-tensioned concrete masonry.

In collaboration with Habitat for Humanity Manukau (a global non-profit Christian housing organisation that assists low-income families into their own homes), and numerous consulting and supply companies, the University of Auckland helped construct New Zealand’s first post-tensioned concrete masonry house, which is located in Auckland.
The single story house comprised a simple and clear floor plan. The exterior walls of the house were constructed of post-tensioned Formblock® mortarless concrete masonry blocks. This was one of the first projects in New Zealand to use this type of mortarless block, which had only recently become available on the local market. The use of mortarless blocks is currently outside the scope of the non-specific masonry design standard, and therefore a specific design was required for this project.
The walls had a thickness of 190mm and were fully grouted using a specially developed block fill concrete. This concrete exhibits low shrinkage and self-compaction properties and has a target 28 days compressive strength of 20 MPa. The use of mortarless blocks required full grouting of the walls. A plaster finish was applied to the exterior of the building, with insulation and lining installed on the interior face of the block walls.
The interior partitions of the new home were made of triboard, a high-density timber panel. The floor was a concrete raft design, typical of the floors used in residential masonry construction in the Auckland area. The existing site had a sloping profile and two public drains running through the proposed house footprint, resulting in a more complex floor design. The raft floor sat on the ground surface and comprised an edge beam, internal ribs with a depth of 285mm, and a 85mm floor thickness. Finally, a simple timber truss roof with iron cladding enclosed the home.
The prestressing tendons were 12mm diameter, 500 MPa threaded Reid bars, and their layout was determined by ensuring that all wall panels contained at least one tendon, and that the tendons were distributed evenly around the structure. The top tendon anchorage consisted simply of a steel plate and nut bearing on the masonry wall top. A short length of prestressing tendon was hooked around the foundation reinforcement and terminated with a coupler at floor level, providing the lower prestress anchorage. The walls were prestressed one week after grouting using simple stressing equipment, consisting of a small hollow core jack and jacking chair, and a hand pump.
The finished project illustrates that a post-tensioned wall system can be simply integrated into existing floor and foundation design with minimal effort. The use of mortarless blocks significantly enhanced the speed of wall construction. Overall, the success of the Manukau Habitat for Humanity and University of Auckland home proves that post-tensioned concrete masonry is a construction technique that holds great promise for the future.