Science //

The Quadrangle is a trapezoid

Alexandros Tsathas gets geological wit it

the_main_quadrangle_of_the_university_of_sydney

The University of Sydney is one of the six so-called “sandstone universities’” Beyond conferring a certain cachet, club membership means that, well, the University has a few buildings made of sandstone. And what a fascinating stone it be.

Who are you, sandstone?

When the wind blows, and the rivers flow, piles of sand and mud build up. That sand at the bottom of the pile gets compressed under the weight of the sand above it, and becomes hard. Looked at under a microscope, sandstone’s structure is reminiscent of a stone wall, with larger “framework” grains of sand (which, chemically, is silicone dioxide with a few impurities) cemented together by softer compounds. The exact chemical composition of the framework grains largely determines the colour, texture and hardness of the sandstone.

Built on sand(stone)

The only two genuine sandstone buildings at USyd, which is to say the only two constructed entirely of sandstone, are the Quadrangle and the Anderson Stuart Building. Posers like Madsen are constructed of brick, with a 10cm thick sandstone façade, and this is reflected in their non-heritage status.

The Quad and Anderson Stuart buildings were exclusively constructed of “Yellow block” sandstone (so named for its distinct hue) cut from quarries at Pyrmont, at a site just west of where Star City is now. Yellow block was also used to construct Central Station, Town Hall and the Mitchell Library.

Drawing blood from (sand)stone

In another of nature’s great ironies, sandstone’s very progenitors – water, wind and time – are also its killers! Repeated wetting and drying causes the microscopic cement between grains to crumble, and the wind shears the larger framework grains right off.

Logically, those sandstone blocks more exposed to the elements erode faster. Next time you walk by Anderson Stuart, pay close attention to the exaggerated erosion of those sandstone blocks behind downpipes, which inevitably spring leaks.

A number of design features can help minimise erosion, and make sure that when it does happen, it doesn’t look that bad.

The ledges atop the Quad and Anderson Stuart buildings do not sit flush with their walls, but extend beyond them. This feature is called a ‘string course’, and ensures that water running off the roof is directed away from the walls, and does not stream down them.

Secondly, sandstone blocks should be laid “bedface horizontal”. In other words, their final orientation should imitate how they were found in the ground. Flipping sandstone on its side – “bedface vertical” – means that water runs along the grain, rather than across it, effectively exfoliating it and hastening erosion.

You can mitigate the aesthetic impact of erosion through your choice of stone finish. Honed (smooth) finishes look very nice for the first few years, but then every blemish – on a background of parfait – becomes very noticeable. Sparrow-picking, where a rough “wormy” pattern is deliberately carved into the stone, is much more forgiving.

As an interesting aside, USyd’s Heritage Architect, Chris Legge-Wilkinson, tells your correspondent that for millennia, architects have employed strategies to distract from structural misalignments: “Notre Dame isn’t straight. From the Romans to the 1500s, people weren’t building straight, but you can’t tell because of the decoration they used”. From this, I learn that the Quad is actually a trapezoid, splaying west-ward. ¡Loco!

Why has Madsen got psoriasis?

In equal parts due to reckless disregard of the above principles in its conception, and preventative maintenance by Campus Infrastructure Services (CIS).

Madsen has no string courses, and its blocks were laid “bedface vertical”, and finished smooth. Additionally, in their wisdom, prior maintenance teams sealed its blocks together with silicone, which, when erosion occurs, causes large chunks to fall off rather than bite-size pieces.

CIS has had to perform prophylactic etching due to all of the above so that great big blocks of sandstone don’t fall onto students’ heads.

The Uni’s load(stone)

The Quad and Anderson Stuart are heritage listed, coming under “State Agency” qualification, meaning the Uni has an obligation to keep them in good repair, more so than an owner of a heritage-classified home.

CIS maintains a list of everything heritage-listed, keeps tabs on the state of disrepair of items on this list, and then has another list where work orders are scheduled and prioritised.

Residences now sit atop the original Pyrmont quarry, so when replacement blocks are sought, the University has to look elsewhere. It has four options.

The University is one of the few institutions allowed to dip into the Government Architect’s Yellow block stash. The stash though, is dwindling. The alternatives are blocks from Rockhampton (in Queensland), Gosford or Marulan, near Goulburn.

Sandstone isn’t cheap.

A block of the size used in the quad, roughly a quarter of a cubic metre, costs $5000. To get a gargoyle replicated for the Great Hall costs $20,000. The estimated cost of the remaining maintenance work to be done to the Quad (at least for the meantime) is $30 million – $15 million for Anderson Stuart.

Thus concludes our fascinating, sporadic foray into USyd’s sandstone.