This is a project I created to improve my sketching skills, analyse historical, resilient buildings and engage with the history of London architecture..
Christopher Wren may be most famous for his design of the new St Paul’s Cathedral after the old one was destroyed in London’s Great Fire of 1666, but the architect and engineer was already an experienced church designer by the time he took that challenge on. Twenty two Wren-designed buildings still stand in London today, of which fifteen are churches located mostly in the Square Mile.
I wanted to capture Wren’s genius with an ongoing project to locate, observe and draw his buildings. Having stood the test of time, I believe they really showcase what good civil engineering should look like – robust, safe, human-scale and adapted to the needs of those who use it.
Wren lived and designed in a different century, but took pride in using what resources were available in his time. Today’s civil engineers may be pioneering low-carbon concrete and solar-energy-collecting windows instead of stone and glass, but we should emulate Wren’s approaches, because city dwellers still ultimately want their buildings to be fit-for-purpose, future-proof and beautiful.
I observed the testing of concrete intended for the headhouses of London Power Tunnels 2. This took place at the batching plant Capital Concrete in east London.
There are two types of concrete batching: wet mixes and dry mixes. The former can generally produce a higher quality concrete because it can be mixed using the industrial-scale mixers at the batching plant, rather than smaller mixers available on construction sites. Today, a wet mix is being prepared for the concrete slab. The intended strength is C40/50 (meaning a failure strength of 50MPa on a 5cm x 5cm x 5cm cube).
Note on sustainability. Concrete production is one of the world’s largest sources of CO2 pollution, meaning civil engineers need to come up with alternatives which are less polluting. In normal concrete, which is based on Portland cement, about 1000kg of CO2 is produced per tonne of concrete. A new technology called Earth Friendly Concrete (EFC) is capable of reducing this to about 200kg CO2 per tonne of concrete – a significant improvement – by replacing the Portland cement with Ground Granualted Blast Furnace Slag (GGBS) and waste fly ash from industry. The concept is in its infancy so its properties can be unpredictable – previous batches have not reached the required strength because of low-quality fly ash supplied from Dunkirk in France; now that the supplier has been changed, today’s mix should pass the required tests. If the rollout of EFC on this large project is successful, it could be a significant turning point for the construction industry to reduce its emmisions from concrete production.
The batch for testing is a sacrificial batch – it will not be sent directly to the project, but if test mix passes the strength tests, the same mix will be used on the project.
Huge industrial concrete mixer, with a ‘tap’ to transfer the mix straigh into mixing lorries for transport to site
^ The giant, industrial concrete mixer at the batching plant.
Testing the batch:
1) Slump test. This is done every half an hour for three hours. Concrete is filled into a conical mould and then the mould is removed so the mixture spreads into a heap. The change in height of the heap is the slump.
2) Bleed test. A cyclindrical container is filled with the concrete mix in five layers – between each layer a vibrating tool is used to ‘tamp’ the mixture and reduce air bubbles. In general, the way the concrete firms up is based on the water separating from the heavier particles over time until enough stiffness is achieved. The amount of water that forms at the top of the cylinder is the bleed; a lid prevents evaporated water from escaping from the test. The use of high proportions of GGBS results in a longer bleed time that conventional concrete but this can be reduced with fine aggregates (<50 micrometres). When bleed occurs on site, either the bleed water is remixed into the concrete (which may result in a weak top surface) or one can wait for the bleed water to evaporate. If the rate of evaporation is faster than the rate of bleed (e.g. on a very hot day), the concrete may experience plastic shrinkage, which is undesirable and should be avoided.
3) Cube tests. 5cm cubes are filled with the mix. They are tested, usually with a hydraulic machine, on days 1-7, day 14, 28, 56 and 96 for failure strength. Concrete gets stronger over time and the 28-day strength is usually the quoted value; the material must reach its target strength (here, 50 MPa) by that date to be acceptable.
4) Weight test. This is just a way of obtaining the concrete density, which is a vital parameter to consider for safe structural concrete design.
5) Beam test. A 3-point test (simple supports at the ends, and a point force applied at the middle) is taken on an unreinforced beam of concrete at 28 days. This tests the bending strength of the mix.
This church was rebuilt after the Great Fire of London when it was severely damaged. For Wren, the Gothic style makes this one of his more unique designs. I was particularly intrigued by the tower, which retains the blockiness of many of Wren’s churches yet is mellowed out by the classic Gothic arches of the windows and octagonal turrets. I’ve chosen to sketch that here.
Exquisite stained glass is the centrepiece on entering. I appreciate the little circular windows towards the top of the facade; they make you feel like you’re on a ship. They let light in, but their height makes it impossible to see out – reminding worshippers of their smallness. In religious terms, this perhaps indicates the almighty power of God, and to non-religious observers like me, the power of the planet and the hugeness of the universe.
From a construction perspective, the main advantage of such a window placement is that the glass does not have to hold any weight – this structure would hold up just fine even if the glass were damaged – the stone walls and arched ceiling form the essential structural elements. Modern glass-facaded buildings use a ‘curtain wall technique’, where again the glass doesn’t hold any weight, but instead of a massive stone outer structure, the building is supported by internal columns, beams and braces made of steel and reinforced concrete. The disadvantage being, of course, the loss of an unbroken internal space, a key component in many of Wren’s churches.
^ stained glass visible on entry^ high-up window placement^ the tower from the outside; on the right, the facade with the little circular windows
Missable from the street – you’d easily wander past this one. But tucked away behind the buzz of the City, the people and their pints and their suits, stands a huge expanse of courtyard. Empty and flat, the pale stone transports us to southern Europe; it feels almost Milanese. Yet the roughness of the church bricks brings in that earthy British tone. There the church stands, spire up into the sky on this strange island of peace in inner London.
St Lawrence Jewry has a very familiar style – very much a main block with a sloping roof, with a tower about double the height, also rectangular, on one end. Unfortunately I was unable to go inside this church to examine the structure in more detail.
The church was actually rebuilt after extensive damage during the Blitz, but the architect Cecil Brown stuck to Wren’s initial design.
^ the view into the courtyard; the facade of St Lawrence Jewry is the building on the very right and in the centre is Guildhall^ quick sketch-up showing familiar Wren style
On the other side of the courtyard stands another church-like building, which is actually Guildhall, the HQ for the City of London Corporation.
Today, this complex acts as a retirement and care home, but it has housed a range of other groups including ware veterans in the past. It is generally regarded as a luxury home for the mostly upper classes, and only admitted women in 2009.
The courtyard reminded me most of an Oxford college – in particular the Queen’s college – with its distinctive symetrical windows and neatly kept grass.
I decided to use two-point perspective as a sketching technique for this because it felt appropriate to the building scale and shape.
^ Chelsea Royal Hospital in two-point perspective^ Queen’s College, Oxford, for reference^ Queen’s College, Oxford, for reference
In more detail, I did a close-up sketch of the entrance in the middle of the courtyard. The neoclassical influence is very distinctive here.
