[填空题]
A concrete highway is built of svvfz qmy-8uz4+;s u 5ylabs 12 m long uz3pf( 8faa cjgsq*+a 4c8t4)negh0r (20 $^{\circ} $ C). How wide should the expansion cracks between the slabs be (at 20 $^{\circ} $ C) to prevent buckling if the range of temperature is -30 $^{\circ} $ C to +50 $^{\circ} $ C? m