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This sounds like a fun undergraduate project but after living for the last 30 years in West Africa, I wouldn’t put my money on it replacing concrete bricks in the long term or solving the housing crisis

Three things I might like to consider in addition to the engineering aspects:

• Availability of raw material: empty PET bottles are reused especially by women who make and sell drinks. Most commonly they want the small size. Some will even buy them so they may have a small cash value (after a recent marathon there were kids out collecting up the empty water bottles left on the track). Don’t assume they will actually go to landfill until they are old or cracked. Check what proportion might be reasonably available. Then consider where are they available and how much would it cost to get the bottles to where the housing need is?

• Aspirations/alternatives: people dream of living in concrete houses like the ones they see in the Brazilian soap operas on TV. Or at the very least they aspire to putting a corrugated iron roof on their adobe (mud brick) house. Bottle construction would need to have significant advantages (cost, durability etc.) on current construction techniques and you would still need an excellent aspirational marketing campaign to get the majority to accept them. Within a concrete frame you could fill in the walls with sand filled bottles and people wouldn’t see them once you plastered over but you can also use adobe brinks or what is called in French “banco stabilisé” a mud/concrete mix (and even though its much cheaper, still relatively few people do this).

• Building regulations: these may not allow construction in urban areas in anything other than concrete. To go to scale you’d have to get the norms agreed and the regulations changed

Finally – yes you can stop PET bottles from going to landfill in the short term by using them for building but I believe we should be reducing demand for non-biodegradable plastics while recycling as much as possible of the ones we do use. At which point there would be no “waste” PET bottles & people would have to go back to adobe or concrete anyway