Tag: concrete

Concrete manufacture and testing

Concrete manufacture and testing

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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.

Layout of the plant:

  • Silos storing cement (or Earth Friendly cement alternative)
  • Aggregate storage area
  • 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.

Creation of a simple beam bridge for road traffic

Creation of a simple beam bridge for road traffic

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A beam bridge is the simplest type of bridge, consisting of a deck resting on vertical columns. Despite their simplistic physics and appearance, the construction of such a structure in the modern world takes more than just resting a plank on a column. I’ve made a short video showing an example of the construction process.

The main materials of interest are:

Reinforced soil. This is a specialist technique whereby the nearby soil is made stronger by the addition of a grid of a different material – usually metal or composite layers. This is a vital process in weak soils and is usually carried out by a specialist subcontractor on a big project.

U beams. The deck consists of these concrete beams, whose cross section is a U shape. This has a large second moment of area (see below for calculation), making the deck very resistant to torsional and bending stresses. The beams tend to be large enough that construction workers are able to stand inside them to attach the next layers of material.

Temporary works scaffolding. Usually consisting of metal bars which can be assembled and disassembled quickly, temporary works structures are needed to prop the structure up before all structural elements are completely secure.

Rebar. Rebar is a type of steel shaped into rods of about 2cm diameter. When worked into a cage shape and inserted during the concrete pour, the resulting ‘reinforced’ concrete, is much stronger in tension that its raw counterpart. This is because concrete tends to be strong in compression but not tension; using pure steel would be way too heavy, not to mention way too expensive. Combining these properties creates an excellent new material which is used in all large modern infrastructure projects.

Permadec plastic panelling. This is a very strong overlay material that is manufactured by a specialist company. It contains a shell of fibreglass with steel strips inside and is placed on top of the U-beams.

Composite top layer, consisting of some of the materials described above. This diagram is an expansion of the diagram of the U-beams above. It is a zoomed in version of the green box.

Calculation of the U-beam second moment of area:

We have to split the cross section up, calculate the value of I about the centroid of each section, then use the parallel axis theorem to find the total I about the centre of the section.

Rural alpine natural water system

Rural alpine natural water system

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I visited a low-tech mountain hut during a walking tour in the Alps in France. At 1500m elevation, it can only be accessed via cable car and then a mountain walk, and it is isolated from both electricity and water networks. This means the only power comes from gas, used for cooking, and wood, burned for heating. Water must be obtained from a nearby stream, yet I found the system to be surprisingly sophisticated.

To check it out, I walked about a hundred metres from where the hut stands, to locate the stream. Basically, the system consists of two plastic tanks, each holding about 100 litres of water, sitting in the stream. Water enters the first tank directly from the stream – it is fresh as it has come straight from the glacier higher up. This tank contains a filter to catch any sediment; at present, this is improvised with an old stocking.

The water that passes through the filter is led via a plastic hose to the second tank, which is a couple of metres down into the valley and is a storage tank: when supply exceeds demand, some water can be held here until it is needed on drier days. An overspill pipe attached to the top allows water to run safely back into the stream if the tank is full.

The main pipe leads from the storage tank, downhill to the hut, where it can be used cold or heated with gas.

Some calculations for quantifying the system

^ situation 1
^ situation 2

We can examine two situations; the first, where the second is not in overspill so nothing flows through location 6; the second where it does. Bernoulli’s equation is useful here. It is an energy balance equation and states that [pressure + kinetic energy + gravitational potential energy] = constant in a flow system, as long as the mass flow rate is constant (there are a couple more technicalities, but I won’t dwell on them here). Bernoull’s equation allows us to consider only start and end point, ignoring what happens in between.

Where p is pressure, rho is density, v is velocity, g is gravitational acceleration and h is height from a fixed point.

Also, with a constant mass flow rate (and incompressible fluid) we can assume:

where A is pipe area.

Situation 1: At both 5 and 1, the density of the water is the same and the area of the pipe is the same. Therefore the velocity of the water entering the house is the same as the velocity of the water from the stream. To calculate the pressure change, we can cancel the second term in Bernoulli’s equation (since velocity and density don’t change), giving p1 + (rho)(g)(h1) = p2 + (rho)(g)(h2). Rearranging gives p2 = p1 – (rho)(g)(h1-h2).

Sticking into this that p1 = air pressure = 1 bar = 100kPa and h1-h2 = 30, p2 is 3.94 bar.

French alpine viaducts – the old and the new

French alpine viaducts – the old and the new

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The A40 motorway in southern France is a busy highway through the mountains that provides the most direct route from Geneva to the mountain resort of Chamonix and surrounding towns. Together with a railways along a similar route, they are used all year round by both French and international tourists and local people; the area is famous for skiing, mountain hiking, climbing and local French culture – so the roads and railways along this route are essential.

Two prominent and impressive viaducts caught my eye as I drove down the motorway. For each, I sketched a front-on, fine-lined shape and a more visual sketch of what the bridges actually look like when you’re driving on the road.

First, a modern, slender concrete structure – the Viaduc des Egratz de Passy. This one is part of the westbound A40 motorway.

^ see the column cross sections at the bottom of the drawing

Viaduc des Egratz de Passy, 1981 (road)

  • The (presumably reinforced) concrete posts are generally rectangular, with their short side aligned with the length of the motorway, with one exception. The column on the right of the drawing above is hexagonal instead – the reason is not clear, but it may be required because of the harsh bend at that point on the structure.
  • The deck does not appear to be simply fixed straight on to the columns – at a glance, it looks like it is levitating slightly. This is probably because of the damping system between the deck and the columns. Allowing some small damped rocking, rather than rigidly fixing the two together, helps the structure deal with the vibrations of the road without sudden plastic collapse or fast fracture of the joints.

Second, a more traditional, heavier-weight arch design – clearly from a much older era – the Viaduc de Saint-Marie.

Viaduc de Saint-Marie, opening date unclear (rail)

  • Straight, sturdy columns form the bottom section. Semi-circular arches have been utilised for structural stability in the top section.
  • The main material is masonry – probably stone masonry by observation.
  • Arch bridges are excellent at dealing with the continuous vibrations of railway traffic without the need for external damping systems (which were likley not developed at the time of construction).
  • The project would have been advanced for its time, fitting in with such an uncertain landscape – not to mention massively expensive as it would have been build by human power alone. Impressive!