Steel reinforcement was a dramatic innovation of the 19th century. The steel bars add strength, allowing the creation of long, cantilevered structures and thinner, less-supported slabs. It speeds up construction times, because less concrete is required to pour such slabs.
These qualities, pushed by assertive and sometimes duplicitous promotion by the concrete industry in the early 20th century, led to its massive popularity.
Reinforced concrete competes against more durable building technologies, like steel frame or traditional bricks and mortar. Around the world, it has replaced environmentally sensitive, low-carbon options like mud brick and rammed earth – historical practices that may also be more durable.
Early 20th-century engineers thought reinforced concrete structures would last a very long time – perhaps 1,000 years. In reality, their life span is more like 50-100 years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.
Many engineers and architects point to the natural affinities between steel and concrete: they have similar thermal expansion characteristics, and concrete’s alkalinity can help to inhibit rust. But there is still a lack of knowledge about their composite qualities – for example, in regard to sun-exposure-related changes in temperature.
The manufacture of concrete also turns out to be a major source of carbon dioxide producing about five percent of total human-caused emissions.
Ken begins the Xerox Alto restoration - good stuff for those into historical hardware. The Alto is one of those very significant machines - first built with a GUI and mouse and introduced the laser printer and ethernet - all a decade before the first Macintosh.
The Alto was introduced in 1973. To understand this time in computer hardware, the primitive 4004 microprocessor had been introduced a couple years earlier. Practical microprocessors such as the 6502 and Z-80 were still a couple years in the future and the Apple II wouldn't be released until 1977. At the time, minicomputers such as the Data General Nova and PDP-11 built processors out of hundreds of simple but fast TTL integrated circuits, rather than using slow, unreliable MOS chips. The Alto was built similarly, and is a minicomputer, not a microcomputer.
The Alto has 13 circuit boards, crammed full of chips. Each board is a bit smaller than a page of paper, about 7-5/16" by 10", and holds roughly 100 chips (depending on the board). For the most part, the chips are bipolar TTL chips in the popular 7400 series. (The MOS memory chips are an exception.) The image below shows the Alto's card rack and some of the boards.