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It must also be considered that footprints discovered at a crime scene remain fixed, while the wearer of the footwear continues on likely changing the wear of the shoe. Thus, unless the print is immediately matched its potential value may be lost. Also of concern is the lack of science and standards demonstrating that footwear marks are unique.
Concrete has a total embodied energy of 1.69 GJ/tonne, lower per unit mass than most common building materials besides wood. However, concrete structures often have high masses, so this comparison is not always directly relevant to decision making. Additionally, this value is based only on mix proportions of up to 20% fly ash.
This means it packs together really well, and the absence of any wind on the surface — we've been over this, there's no atmosphere for wind — means those footprints won't degrade anytime soon ...
Concrete typically comprises sand, aggregate, water and cement and it is the cement part of the concrete that has the largest single CO 2 emission impact. Approximately 14 billion m 3 of concrete is poured globally each year, [ 7 ] this equates to more than 32 billion tonnes or 1,070 tonnes per second.
Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep. The density of concrete varies, but is around 2,400 kilograms per cubic metre (150 lb/cu ft). [1]
Fossil footprints show humans in North America more than 21,000 years ago, the earliest firm evidence for humans in the Americas and show people must have arrived here before the last Ice Age.
Serafini, F.L. Corrosion protection of concrete using a Controlled Permeability Formwork (CPF) system; Proceedings of the International conference Corrosion and Corrosion Protection of Steel in Concrete (Ed R. Narayan Swamy), Sheffield, Vol. 2, 24–28 July 1994, pp. 1114–1131.
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