It's a sign of the times: with technology infiltrating every aspect of human existence, marketers must push the creative envelope to be seen amidst the noise.
One of the trends has been a swing back to the physical, where an in-person presence is combined with a digital element to expand the engagement. Long used as part of sponsorship activations at events, cities are now getting in on the game to promote their locales.
First, Taiwan brought together a massive amount of people to form a giant QR code. Now, Rio de Janeiro has employed artisans to create in-ground QR codes to promote the city.
In order to maintain the city's aesthetic, specialist stonemasons were selected to install the QR code. Euronews quoted one mason, Gediao Jorge, talking about the intensive nature of the project:

“It was a challenge. I’ve built Portuguese pavements like these for 27 years now, but this was the biggest challenge of my career.”
The city eventually plans to install 30 codes throughout the city, making this perhaps the most integrated example of QR codes in a large urban area.
Finally, the QR code is closer to where it belongs: in the ground. The technology just has not proven itself to be the cure-all that many have been touting it as for years.
According to a comScore report, 1 in 3 QR scanners are in the $100k+ income bracket and yet only 6.2 percent of the mobile audience scanned a code. Granted, this data is not the freshest and is derived from the US and so may not reflect uptake in Brazil of these codes.
Nonetheless, this technology has a long way to go to be widely accepted by the public even though it's loved by marketers.
Mark Donavan, comScore SVP of mobile, noted in the same report:

“QR codes demonstrate just one of the ways in which mobile marketing can effectively be integrated into existing media and marketing campaigns to help reach desired consumer segments.
For marketers, understanding which consumer segments scan QR codes, the source and location of these scans, and the resulting information delivered, is crucial in developing and deploying campaigns that successfully utilize QR codes to further brand engagement."
It's unknown whether placing QR codes in high-traffic areas will encourage consumers to be more open to the QR code.
At the moment, while marketer interest at driving real-world traffic online is strong (see slides below), there is simply just not much evidence of widespread consumer acceptance of this marketing channel to justify its integration so visibly in a city like Rio.