by using Poraver® as a lightweight aggregate
By the replacement of conventional aggregates with Poraver® expanded glass as lightweight aggregate in dry mortar applications, it is possible to reduce the density of a product. In addition to many resulting advantages such as increased thermal insulation, improved workability and a higher yield, a mortar with almost the same level of strength can be achieved despite its lower density. This is due to the high crushing resistance of the Poraver® grains and their very good bonding with the matrix. At the same time, the use of Poraver® reduces the Young’s modulus. This has a positive effect on reduced cracking.
Definition of the Young’s modulus
The modulus of elasticity (Young’s modulus) of a construction material describes its reversible deformation due to an applied stress. If the deformation returns to its initial state when the stress is removed, this is described as linear or ideal elastic behaviour. The gradient of the lines in the stress-strain curve in the elastic deformation region gives these material properties.
Young’s modulus in construction materials
Depending on the formulation of the product, the Young’s modulus of individual construction materials can vary considerably. A Young’s modulus of 2,000 N/mm², as achieved for example in lime plaster or in a plaster modified with polymers, already displays a high elastic deformation at low stresses. Systems such as self-levelling underlayments or cement renders achieve a Young’s modulus up to 50,000 N/mm² and thus display a higher resistance to elastic deformation.
The use of Poraver® in construction materials can significantly reduce the Young’s modulus of most products and thus makes them more resistant to cracks.
As you can see in the simplified diagram, the Young’s modulus is shown as a secant modulus, statically measured in accordance with DIN EN 12390-13. For the same stress state at 1/3 of the material strength, the conventional mixture displayed an elongation of Δε1, while the mixture with Poraver® displayed an elongation of Δε2 for the system.
The use of Poraver® thus reduces the Young’s modulus. In case of materials with the same brittle fracture behaviour, such as dry mortar products, this results in a higher fracture strain. For a render used on brickwork for example, this means that it will withstand greater movement of the brickwork before it cracks.
For the tested mixtures, a volumetric exchange of the aggregates by Poraver® expanded glass was carried out. The proportions of binder and water were maintained at a constant level in the base and light variants in order to produce comparable test specimens with corresponding levels of strength. In all trials the workability of the materials were in the typical range for those products.
We compared the properties of different dry mortar formulations with conventional filler and filled with Poraver®:
|Self levelling underlayment (SLU)||SLU (conventional) |
36.4 vol% conventional aggregates based on the fresh mortar
16.8 vol% Poraver® 0.25-0.5 mm and 19.6 vol% conventional aggregates based on the fresh mortar
|Density after 28 days [kg/m³]||1,901.0||1,574.0|
|Flexural strength after 28 days [N/mm²]||9.5||6.9|
|Compressive strength after 28 days [N/mm²]||45.4||41.7|
|Young’s modulus after 28 days [GPa]||31.6||12.4|
Summary: The density is reduced by 17.2 % and the Young’s modulus by 60.7 % using Poraver®.
|Tile adhesive (CTA)||CTA (conventional)|
30.4 vol% conventional aggregates based on the fresh mortar
21.5 vol% Poraver® 0.1-0.3 mm and 8.9 vol% conventional aggregates based on the fresh mortar
|Density after 28 days [kg/m³]||1,482.0||1,143.0|
|Flexural strength after 28 days [N/mm²]||6.9||6.8|
|Compressive strength after 28 days [N/mm²]||23.0||20.9|
|Young’s modulus after 28 days [GPa]||13.5||7.9|
Summary: The density is reduced by 22.9 % and the Young’s modulus by 41.5 % using Poraver®.
47.2 vol% conventional aggregates based on the fresh mortar
37.4 vol% Poraver® 0.5-1 mm and 8.3 Vol% conventional aggregates based on the fresh mortar
|Density after 28 days [kg/m³]||1,335.0||651.0|
|Flexural strength after 28 days [N/mm²]||1.7||1.0|
|Compressive strength after 28 days [N/mm²]||2.5||2.4|
|Thermal conductivity after 28 days [W/mK]||0.65||0.18|
|Young’s modulus after 28 days [GPa]||4.4||2.3|
Summary: It was possible to reduce the density by 51.2 % and the E-modulus by 48.9 % using Poraver®.
By replacing volumetrically the fraction of conventional aggregates with Poraver® in the tested systems and keeping the same proportions of binders, it was possible to produce test specimens with almost the same level of strength. The mechanical properties of these different formulations were determined. This showed a significant reduction in the Young’s modulus through the use of Poraver®. This was irrespective of the grain sizes tested. Assuming that the tested systems exhibit a brittle fracture behaviour, it can thus be expected that the system will handle greater elongation before it cracks. This means that enforced deformations, such as those caused by temperature, vibrations or combining the product with soft construction materials, will generate lower stresses in the mortar and thus counteract the formation of cracks. Furthermore, the density of the system was reduced, which also results in other positive characteristics such as higher thermal insulation properties.
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