[填空题]
A concrete highway is bui1y,an wy,5tc +jc/b ub)u(tfnq3ot2xrii;*to eq e-j3v/8bl lt 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