It's being called a significant milestone in the construction of the airport tunnel, as concrete pouring is more than half complete.  More than 22,000 cubic metres of concrete has been poured since the first tunnel section was done just before March.  26 of the 50 sections have now been poured, with each section taking 80 trucks working in a 12-hour period to complete.  Mayor Naheed Nenshi says this will avoid the city having to pay millions of dollars in compensation should the airport's runway be delayed because of the tunnel.  He says they're not interested in slowing down the airport authority's project.  The airport runway project is scheduled for completion in 2014.