Strength of naturally aged glass units determined by experimental testing

Abstract

The reuse of insulating glass plays a crucial role in promoting sustainability and circularity within the construction sector. CO2 emissions must be reduced according to the Climate Agreement. Reusing materials can help achieve this goal. Reusing glass without reheating it can provide environmental and cost benefits, making it a suitable option. In this study, 36-year-old insulating glass panels (double-glazing) from an apartment building in Amstelveen have been tested for strength through destructive testing. This is done to see what the strength of the glass is after the glass has been in the building for 36 years and has been exposed to opportunities for damage. Various patterns of damage on both the interior and exterior side of the glass panels from an insulated glass unit affects the strength that can eventually be assigned to the glass. The aim of this thesis is to be able to show whether and how insulated glass units can be reused and what the strength is of naturally aged glass after a certain lifetime. From six insulated glass units obtained, specimens measuring 150mm x 150mm are extracted and tested for strength using a Coaxial Double Ring test. Prior to the destructive tests, a number of specimens were viewed under the Keyence VHX 7000 digital microscope to see how the damages are and whether they vary between the four sides of an insulated glass unit. This revealed that the outer side of the outer panel (side #1) contains the most and homogeneous damage. Weathering, especially rain and wind, which can bring sand particles, for example, create homogeneous damage on the glass surface. The entire surface is exposed to this type of weathering. The outer side of the inner panel (side #4) also contained quite a lot of damage from probably cleaning and (human) touching. Despite the fact that the inner sides of both panels (the sides in the cavity, side #2 and #3) theoretically had little potential for weathering, damage could also be seen on these, although it was more localised, meaning it probably occurred during the production process or preparation of the specimens for this experiment. During the Coaxial Double Ring tests, 406 specimens were tested for strength. The strengths of the specimens ranged from 16.8 MPa to 243.7 MPa, with an average strength of 67.5 MPa. The average strengths of side #2 and #3 (sides in the cavity) are slightly higher than those of side #1 and #4 (outer sides), between 9% and 17%. A tin-tester was used to determine which side of each specimen was the tin-side prior to the strength tests, to include this in the analysis of the results. The specimens tested on the tin-side were found to be on average much weaker than the specimens tested on air-sides. The average measured strength of the air-side is 31% higher than that of the tin-side. In a number of tests carried out with new glass, this large difference is already present, while literature states that the difference in strength between the air- and tin-sides can be considered marginal. Fracture statistics were used to analyse the data and find a strength for each series tested. This was done using Weibull theory. The two-parameter Weibull distribution has proven to be very conservative at low failure probabilities, resulting in a very low design strength (failure probabilities of 0.8% and 0.12%) for a series of specimens when there is a large spread in data. The spread in data was often of greater influence than the values of the measured strengths. As a result, the design strengths for series tested on the air-side were usually lower than specimens tested on the tin-side, while the average strength measurements were actually higher. The greater variation in strengths of specimens coming vi from the inner side #2 and #3 results in lower design strengths than the outer sides (#1 and #4), which contain more (homogeneous) damage. The values of design strengths of the naturally aged glass tested were almost always lower than the design strength of new glass according to current standards. The design strengths of new glass are 22.5 MPa according to NEN2608 and 25.0 MPa according to EN16612. When looking at how many specimens failed at stresses lower than these values, there are only three and six respectively. So, there are very few spots in 36-year-old glass that are weaker than the design strength that can be assumed according to NEN2608 or EN16612. These lowest values are not consistently found in specimens tested on either the air-side or the tin-side, so based on this study, the influence of this can be ruled out. By this means, a more realistic and less conservative option seems to be to take the strength of the weakest specimen in a test series and then assign it to the entire panel from which that test series comes. Reuse options are discussed in the final section of this thesis. Using a number of common glass structures, it explains the ways in which weathered glass can or cannot be reused and in what function. How the results obtained can be handled and used is a point of discussion, as the values are not always representative of the strength of the entire glass panel. Further research will have to show whether the current design standards and the parameters used in them are well chosen or whether they should be different for used glass. For example, use a lower material factor of 1.35 instead of 1.6 – 1.8 when calculating the design strength or use a higher failure probability than 0.0012 to determine design strength in a Weibull distribution. The efficiency and effectiveness of the method used can also be optimised in further research.