This AHRC-funded research builds on the knowledge in 'TTURA - Open Loop Solutions' to investigate a radical new application for glass/polymer composites in architecture. 

Throughout the twentieth century, architects have been striving for the transparent/translucent building - notably Pierre Chareau's Maison de Verre, Mies van der Rohe, with iconic buildings such as the Barcelona Pavilion and the Farnsworth House at Plano Illinois, I M Pei's Pyramid at the Louve and a series of works by Sir Norman Foster starting with the Faber Dumas Building in Ipswich completed in 1975. The advances, spearheaded by Pilkington's R&D department, in the manufacture of float glass for architectural purposes was significant in facilitating the ambitions of architects during the later half of the century. Whist the innovative step in the manufacture of architectural glasses and in lightweight curtain wall configurations has supported the development transparent structure the development of concretes or concrete-like materials has not supported the same formal opportunities.

This research builds on and develops work in the programme open-loop solutions for recycled glass from the consumer, construction and automotive streams and investigates the proposition 'Are translucent/transparent structures possible'. 

From the beginning of the research a mentor user group was established to monitor and advise the research team and included structural engineers and manufactures. The input from the group was particularly significant throughout the research programme in identifying standards testing and performance parameters.

Testing 

At the beginning of the research in consultation with the industry based evaluation/user group an appropriate mechanical value comparative needed to be established to test and validate the developing outcomes. The obvious values and standards criteria were those used in the testing of concretes.

The values notated below were deemed appropriate but the inclusion of VHS concrete, a USA military development, was only useful as an extreme comparator of values against test regimes.

(See Table 1 Values and standards criteria used in the testing of concretes*)

To facilitate the theoretical modelling, an initial series of propositions based on the current knowledge (AHRB award for 'Open Loop Solutions for recycled glass from the consumer, construction and automotive streams') were manufactured and submitted for testing. These tests established the fundamental model composites and values to compare, review, assimilate and develop subsequent iterations of the composite. Whilst this testing provided comparative mechanical values appropriate for the stage of the research, in pursuing the question 'Are translucent/transparent structures possible?' the first trials could not be considered translucent. Therefore the testing was curtailed after the 7 day values were established. The smaller glass particles (measured in microns) used as filler in the previous composite did not aid the quest for translucency but confirmed the fundamental viability of pursuing a non-translucent composite. These outcomes provided the information to develop iterations to the composite mix to pursue the notion of translucency.

In preparing the new range of trial composites for testing, several issues had to be taken into account when detailing the new mix ratios of glass a ready supply of the proposed aggregate size(s) from the glass waste streams. Review of standards testing protocols as the new mix ratios proposed an increase in aggregate size.

Whilst the waste glass supply stream proved robust, the proposed increase in aggregate size became a problem in relationship to the protocols of tensile strength testing nominated. It became necessary to contact the British Standards Committee (BSC) and following an exchange of correspondence outlining the research with Professor Tom Harrison of BSC we were able to identify a recently developed standard for determining the tensile strength of the composite; BS EN 12309 (part 6) Testing of hardened concrete - tensile splitting strength of specimens. A range of samples were then produced for standards testing and submitted for 7 and 28 day testing.

From the palette of composite samples tested, two iterations indicated significant incremental steps in terms of mechanical values (see Table 2 - Palette of composite samples tested*) and translucency and lead to the establishment of a new control composite. The control mix nominated did not display the greater translucent qualities of the two iterations but the mechanical values achieved more than compensated for its nomination as the control mix. With these mechanical values established and physical 1:1 examples, the formal modelling was undertaken. This modelling was done in tandem with further iterations to the composite to pursue greater translucency./p> 

In the search for transparency/translucency, mechanical values of the composite altered the new control achieving higher performance in tensile and compressive strength but lower values for flexural strength. Therefore as new propositions were developed for translucency, further iterations of the opaque mix were developed to attempt to unify performance across the proposed palette of options. 

Review and assimilation of both the values notated in the strength comparison table and their associated translucency narrowed the palette of propositions and a refined range of mixes were developed and tested. The resultant outcome (see Table 3 - The mechanical value performance*) established the mechanical value performance and provided the basis for further testing and a wider end user engagement.

For the virtual modelling it was decided to use an existing iconic building, Mies van der Rohe's Farnsworth House (utilising the new control composite to re-state the Farnsworth House would consume 103,387.47 kilos of waste glass).

The initial attempts at virtual modelling the formal qualities of the developed composite, even utilising high-end software programmes, was a difficult process of representation given the nature of the material. But the outcomes, coupled with the test results, provided significant images and information for instigating debate, dissemination and exhibition material.

3rd Life Cycle

The crushing of structural elements proved a greater problem than had been expected and significant resources went into crushing the structural elements for closed loop reconstitution and standards testing. Whilst the 'crushing' had been a significant problem the subsequent outcomes exceeded the theoretical modelling outcomes. The 3rd lifecycle material achieved mix ratios of 82%+ saturation whilst maintaining acceptable performance levels. (See Table 4 - Close loop and standards tests*).

Based on the results above and subsequent feedback from the user/mentor group, a further test was nominated and undertaken to validate the developing composite. This test was to ascertain the materials bond strength to any reinforcing embedded within the material. A comparative Pull Test was carried out: 10mm reinforcing bars were embedded in cylinders of the translucent composite measuring 150mm diameter by 300mm high. To make a comparison of bond strength cylinders of 50N concrete were cast with the reinforcing rod also embedded. The Pull Tests were carried out on both materials after 28 days curing. Cubes of both materials were also cast from the same mixes in order to confirm the compressive strengths of both materials. The testing laboratory manufactured the comparative concrete samples. 

The tests on both materials had to be aborted before the bond strength of the materials could be ascertained, as the reinforcing rod embedded in the materials failed. The test detailed above was then repeated using 16mm reinforcing rod. The average force applied, which instigated the bond to fail on the 50N concrete, was 103.033kN. The average force applied to the translucent material was 117.9kN however the bond between the reinforcing rod and the material did not fail. The test had to be aborted due to the failure of the steel-reinforcing rod.

The BS639-11 Creep Test (six month duration) was delayed for five weeks in order to be able to test a refined mix ratio which achieved the optimum translucency and mechanical performance, utilising glass from the nominated waste streams. 

Advanced climatic testing was carried out on the translucent mix and the 3rd lifecycle mix. The tests performed were as follows;

• Resistance to Humidity (freeze thaw cycle) 100 cycles of 40°C and -10°C with a relative humidity of 60% programmed at 4 cycles per day. 
• Resistance to artificial weathering (UV radiation and water) . After 500 hours of exposure the sample showed signs of yellowing. This was localised to the surface that had been exposed and did not extend through the full thickness of the sample. 
• Determination of resistance to liquids (water immersion method). Samples were immersed in demineralised water for a period of 500 hours. A visual inspection of the samples was carried out with no deterioration evident in either mix. 

A further evaluation test was carried out on these samples to determine the compressive strength of the material after climatic testing (See Table 5 - Strength of materials after climate testing.*)

With the exception of the surface yellowing of the composite, after advance climatics testing, iterations to the mix ratios, outcomes and subsequent lifecycle testing proved positive and, a structure has been developed for long term testing and observation, of dimensions 2230 mm X 2300mm x 200mm and weight 820 kilos.

The rationale behind the configuration and subsequent location is as follows. 

Whilst standards and the advance climatic testing establish validation parameters these are still 'lab based'. For example the freeze thaw regime is based on the whole sample undergoing test cycle where as in many situations a single element of a structure may be substantial in a 'freeze' situation but with sections above zero or vice versa. The configuration, location and loading on this structure will provide a model for more complex 'real life' observation and review.