Sustainable concrete: ultra-thin casting tests
10th Feb 2020
This research post is a record of our explorations in ultra-thin polished sustainable concrete structures. The aim was to develop a material that would reduce the mass of concrete required for both surfaces and volumes.
Concrete has been identified as producing 8% of the worlds CO2 emissions (BBC, 2016). However, this is mostly due to the sheer quantities consumed. Through reducing the mass of concrete required in addition to cement substitutes such as GGBS, this material development hopes to produce a composite that can benefit from the visual and technical qualities of concrete, without the harmful emissions.
The research was divided into the following three different subcategories:
- Ultra-thin sustainable concrete
- Concrete shell forms
- Recycled construction waste
These investigated forming large slab forms whilst testing extremes of thinness, creating large masses in concrete through shell structures and using local site waste as aggregate.
1. Ultra-thin Sustainable Concrete
The aim of this material exploration is to seek the limits of the material in thinness, whilst still maintaining the required strength for heavy use and achieving the visual finish of honed concrete.
Two different mould-types were used within the testing process with urethane rubber used for producing the initial material composition tests.
We sought to use materials which provide both good visual qualities, strength and low embodied energy.
For consistency, a limestone aggregate was used in all the tests. The aggregate is a by-product from quarrying which would otherwise be returned to landfill.
GGBS was used as a cement replacement to lower the embodied energy of the material. This was applied at a 50:50 ratio to cement. This resulted in a longer curing time, with 7 days at approximately 16 degrees for the material to cure to a hardness allowing honing the fair-face.
Polymer strand fibres were used in different ratios. At 2:100 fibre to cement weight ratio the wet concrete still maintained good plasticity. When the ratio is increased to 5:100 as recommended, the plasticity is lost resulting in a more porous surface consistency.
Plasticiser was applied to all mixes at a weight ratio of 1:100 to cement. This reduces the need for vibrating the concrete and works as a levelling compound, ensuring equal thickness throughout the slab. The use of plasticiser also allows on-site manufacturing, eliminating the need for specialist machinery.
The resulting sustainable concrete slabs achieved a self supporting span of 1200mm with a thickness of just 6mm (a ratio of 1:200).
Whilst the concrete cured on a level surface, after 7 days the slabs were stored in an upright position. When inspected several weeks later, the resulting material had warped by several millimetres.
It is not conclusive if the warping is a result of storing upright, or shrinkage. It is possible that different curing rates from the surfacecontaining fibres in comparison to the honed surface would cause bending over the curing time (which can take 1 month to reach 95% hardness).
As the cause for the bending in the material is inconclusive, further tests are needed. Leaving the curing material in the horizontal position for longer, until the material has reached a greater hardness, should indicate if incorrect storage was the cause.
If bending still occurs, then tests in laminating the stone onto more stable materials could provide an alternative of providing a polished concrete surface.
2. Sustainable Concrete Shell Forms
We aimed to create massing and forms that would normally be substantially heavy, with all the associated difficulties of handling such a mass. Through reducing the mass of concrete, the amount of cement used is accordingly reduced, lowering the embodied energy of the final object.
For the purpose of testing limits in material fluidity and form, a design was produced with the intention of creating moments of resistance regarding fluidity. The intended form would have (if successful) resulted in a seamless honed concrete block, with two inset joints.
Unlike the previous ultra-thin sustainable concrete tests, only one concrete mix would be used throughout the entire pour. The concrete needed to be both fluid and strong. This meant a low fibre content was applied to avoid the clay-like texture resulting from too much polymer additive. Therefore a 2:100 fibre to cement ratio (by weight) was used. As before a 50:50 mix of GGBS to cement was applied.
Mould making Process
The mould to make the monolithic form was composed of the hard outer shell and the soft inner shell. The outer shell is intended for multiple uses, which could be dismantled and reused. The inner shell was to be ‘lost’ within the concrete, completely encased.
The outer form is phenolic resin plywood, which is moisture-resistant and has an integrated release agent to easily strike the mould.
The inner form is Bluefoam sheet, cut to size and taped into the desired shape. This is then filled with expanding foam to avoid collapse from the weight of concrete. The overall weight of the inner form is minimal.
The inner form was fixed in place using steel wire ties to avoid movement once the concrete is poured.
Whilst the shell form was mostly complete, the test was a failure. At the bottom-centre of the mould, it seems part of the Bluefoam inner shell detached under the pressure of the concrete. This pushes the part to the bottom of the mould, preventing the concrete from closing the shell.
This also resulted in a significant loss of moisture in the surrounding areas, as is was absorbed into the break of the inner shell. This resulted in the dark colouration and rough texture to the surface.
It is presumed that the shell structure would have been a success had the adhesive not failed on the inner Bluefoam form. Greater lengths should be taken to ensure this doesn’t happen again.
3. Recycled Construction Waste
Using the previously described techniques, but utilising site-specific construction waste, producing sustainable concrete with a very specific sense of place.
For this experiment, a brick from 333 Old Street was combined with red granite, oxide pigment, cement and GGBS.
During the vibration of the mould, the crushed brick rose to the top of the mix. This meant that when polished, what would usually be the fair face of the concrete had no visible trace of the brick. The rear face (pictured) however had a good display of brick aggregate.
In future, rather than placing the brick fragments in the mould directly, the amount of recycled aggregate could be increased and pre-mixed into the concrete mix to try and overcome the floating effect.
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