[填空题]
A concrete highway is bu7a 6h + kn-yg/rh)jpjiilt33,d51qsnoy 1 ( e ttuno6k7d1aetn iw of slabs 12 m long (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