The motorway was closed in both directions between Junctions 5 and 6 over the weekend to allow the bridges to be moved into place.
Specialist teams carried out the work, taking over 40 hours to complete the work.
Weighing 2,050 tonnes each, the bridges come as part of a £282 million upgrade to the network.
Drivers in the area faced delays of over 30 minutes as a result of the closure.
A National Highways spokesperson said: "We thank motorists for their patience while this complex work was delivered last weekend."