Followers of the Endeavour blog may remember that last year we used an innovative new insulation based on Poraver expanded glass beads as our sub slab insulation for Canada’s Greenest Home. At the time, we were excited to find an insulation that can be reliably used below grade and uses no cement and no foam and offers a reasonable insulation value of around R2 per inch.
As we reported last year, the Poraver product is made from recycled glass that is “aerated,” giving it a cellular structure that is strong but containing enough air pockets to have a good insulation value. As a dry insulation, it can be used in many infill applications, and it is frequently mixed with cement to make lighter weight concrete. The “magic” for us, however, is that a by-product of making the Poraver balls is fired clay, called metakaolin. Metakaolin is a pozzolan, which means that it can be mixed with local hydrated lime to create a hydraulic lime. Hydraulic lime achieves a large percentage of its curing by consuming water, much like cement, meaning a much faster set time, much higher early strength and the ability to use lime in thick applications like a foundation wall. Suddenly, we have a locally produced material that is made from recycled content, and we also use the by-product of the process! Eliminating foam and cement from a foundation with side benefits!
Our positive experience with the material led us to use the Poraver material as the grade beam foundation for the Circle Organics building. With a good compressive strength rating of 0.5 N/mm² (72.5 psi) after seven days, we were keen to build a grade beam that combined structural properties and insulative properties within the same material.
Thanks to our ever-helpful structural engineer, Tim Krahn of Building Alternatives, we were able to build the world’s first expanded glass bead foundation.
We started with a thin, 3-1/2 inch concrete grade beam on top of our rubble trench. The grade beam is below the floor level of the building, and added the bending strength required for the foundation. It is possible that the Poraver could be strong enough without the concrete beam underneath it, but further testing would have been required to justify this, so we went with a thin beam. The concrete was poured in the same formwork we used for the Poraver, simplifying the construction process.
The Poraver grade beam was 16 inches wide (to match the width of the plastered bale walls it supports) and 24 inches tall. It will be partially below grade once the building has been backfilled and graded.
As reinforcement for the Poraver, we bent welded wire mesh (typically used as concrete slab reinforcement) into a square “cage” and laid that in the formwork.
The Poraver was mixed on site in a mortar mixer, using a recipe of 44kg Metapor (metakaolin) to 44kg Hydrated lime. These dry ingredients are mixed with water to make a very wet (cream-consistency) slurry, to which we add 1000l of Poraver of the 2-4mm size. For this project, the engineer suggested adding 5% portland cement to the mix. Hopefully, next time we do this, it can be completely portland cement free.
The mixed material was poured into a form, much like concrete. Once in the form, we spread it and compacted it to ensure it was fully distributed in the forms without any air pockets. The material was hard to the touch in 24 hours, and we removed the forms after 48 hours.
The Poraver material is friable (crushable), making the corners and edges of the foundation vulnerable to chipping when kicked or struck with any force. However, beyond the very edges the material is remarkably resilient. It is able to hold a screw (as long as it isn’t over-torqued). We embedded metal tie straps to anchor the sill plates to the foundation core.
The mixing process was quite quick, as the round balls tumble well in the mixer and get coated in the slurry. Moving the material is easy, as it is very light weight.
This is a material that holds a great deal of potential for future use. It can be mixed on a commercial scale, and potentially formed into blocks or panels, as well as being used for custom pours like this one. This Poraver mix could drastically reduce the amount of styrofoam and concrete used in buildings, replacing it with materials that are much more benign and based on recycled content. We hope to do more work with Poraver in the future.