In M-25 concrete the m is used to represent mix which is the ration of the components in the cement. There are three different types of concrete mixes including designed mixes, nominal mixes and standard mixes.
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The M25 motorway, London Ring Motorway, or London Orbital motorway is a 117-mile (188 km) motorway that almost encircles Greater London, England, in the United Kingdom. The motorway was first mooted early in the 20th century. A few sections, based on the now abandoned London Ringways plan, were constructed in the early 1970s and it was completed in 1986. At 188 kilometres (117 mi) (including the A282), the M25 is Europe's second longest orbital road after the Berliner Ring (196 kilometres (122 mi)) and one of the busiest and most congested parts of the British motorway network. 196,000 vehicles were recorded in a single day near London Heathrow Airport. Plans to widen additional sections to eight lanes (four each way) were scaled back in 2009 in response to rising costs