Page 78 - Build 165

Departments/Research By Zhengwei Li, BRANZ Senior Corrosion Scientist
Position, position, position
BRANZ research into material corrosion rates in various positions on a building envelope has made a surprising discovery that will help in material speci cation and maintenance.
THERE ARE TWO micro-environments typically found around the building envelope, de ned by NZS 3604:2011 Timber-framed buildings as sheltered and exposed. A previous research article in Build 154, Nailing micro-environments, identi ed that these two environments di er both from each other and from that of the surrounding atmosphere.
As such, materials may perform di erently depending on their position on the building envelope.
Currently, there is no comprehensive comparative data on how materials perform and the factors contributing to material perfor- mance in sheltered and exposed micro-environments.
BRANZ addresses information gap
The BRANZ project, Positional material deterioration over building enve- lope, was undertaken to quantitatively monitor micro-environments and investigate positional material performance on building envelopes.
It aims to build an information catalogue to help specify the best materials and maintenance schemes according to the atmospheric corrosivity – micro-environment locations established by NZS 3604:2011.
Details of house and monitoring
Preliminary results are now available on a real building at the BRANZ Judgeford campus. This unoccupied rectangular-shaped house has a corrugated steel roof, light-green painted bre-cement weatherboard
wall cladding and a suspended timber oor. Its length lies along the east–west direction. The semi-rural site is approximately 5 km from the sea and separated from a tidal estuary by gently rolling hills. The atmospheric corrosivity of this site is on the border between NZS 3604:2011 zones B and C.
Micro-environmental conditions were monitored at three positions on the building envelope – sheltered (under eave), fully exposed and boundary of sheltered and exposed (see Figure 1). Material performance was evaluated by assessing the rst-year corrosion rate of mild steel coupons installed at three angles – 0°, 45° and 90° relative to the ground.
Unexpected nding – sheltered not always more corrosive
The common understanding of positional e ects on building enve- lopes is that materials in sheltered areas undergo faster degradation than those in exposed areas. Consistent with this, the 45° inclined coupons on the north wall showed a dependence as expected. The corrosion rate in the sheltered position (147 g/m2/year) is greater than the boundary position (120 g/m2/year), which is greater than the exposed position (107 g/m2/year) (see Table 1).
Unexpectedly, the monitoring results were reversed for the corro- sion of steel coupons installed at all three angles on the south wall and horizontally installed coupons on the north wall. For these, the corrosion rate in the sheltered position is less than in the boundary position, which is less than the exposed position.
76 — April/May 2018 — Build 165