Lightweight Concrete/ Hebel

Q. Does AAC Block/Hebel easily develop hairline cracks? If you use single skin 200mm external walls does it then need a water proof sealant and special paint?

A. Insulation levels are good (is it R1.5 or R1.97?) for 200 mm rendered blocks -
one of the few materials that BASIX doesn't require additional insulation to be
added. In my experience the bigger issue is poor(er) sound attenuation
through the material than has been suggested/implied by the manufacturer.
Being relatively lightweight it has little thermal mass to minimise sound
transfer or moderate the diurnal temperature range. NB

A. I inspected a house some years ago which was constructed of Hebel Blocks which exhibited cracking at all the window corners due to movement of inadequate footings. Owner succesfully sued design engineer and Hebel (who by negotiation supplied Hebel blocks to value of settlement - last thing the owner wanted). He won the (very protracted) court case but in the end the costs almost outweighed the settlement. Expert opinion to the court,in conjunction with a structural engineer was that the cost of underpinning perimeter and internally footings would exceed the cost of total demolition and rebuilding. The existing conc floor slab would need to be removed at a 3 m grid throughout the whole floor area and new footing piers constructed.
This meant extensive disruption of existing tiled floor finish. It made me vary wary of Hebel blocks. You may also experience a pallet of blocks with a
crack through the stacked blocks, due to careless handling - they are quite
brittle. JD

A. Things to watch out for: Structural cracking (right through) from lack of control joints or expansion of timber floor (ie. a 10mm gap must be left around
floorboards); Surface crazing of the render (eg. hairline cracks from differential
drying - 'growing' out of GPO's ) - most noticeable if strong colours
used; Adequacy of the external (acrylic-modified) render (eg. cracks at
joins, corners) - note that the product is dependent upon the adequacy of
this finish for waterproofing; Tie-down adequacy (harder with this kind of light-weight product); Fixings generally - specialised fixings are required to bond to the
cell-like structure and you need to check whether these have been
loosened over time; General matters of checking such as evidence of past water
penetration, deterioration/break-down of finishes (especially around
openings), movement. NB

A. I concur with the tie down, shelter, waterproofing comments of everyone else.
I have used it on a house in Berrima, and to my knowledge all is well (12 years down the track). It is surrounded by verandahs, but also a cavity with the walls bearing on an internal load bearing frame. The other thing Hebel does not have inherently is impact resistance, so kids riding their trikes will leave dents unless you have a render finish that provides the hardness. MF

A. I recently visited a factory which was owner-builder constructed. The suppliers had prepared shop drawings for a fire rated party wall system incorporating XXXXX panels supported on a steel frame, the councils building surveyor approved the system for construction and the owner built the walls. On completion the building surveyors would not issue the occupation certificate until the steel frame had been spray coated with
concrete at a cost of around $15,000. In this case the panel system was only as
good as its support structure and the advice given by the supplier and in the
trade literature was a bit brief. I would only use the system if I happened
to have extensive personal detailing experience with the product and I knew the
builder also had a life time of experience with the product, as is the case in central
Europe but not in Rural NSW. RF

A. I have used Hebel power panels but not blocks. In my limited experience they tend to crack at the joint and do need to be sealed with a special render. The Hebel render contains a waterproofing compound which does not have to be further coated (so I just coloured it with oxide). I have had no problems from a water penetration point of view. My brother-in-law built a studio out of it with a single block providing an internal and external wall surface (ie single skin). As far as I know he has had no problems... PN