for this question, assume a linear$-$elastic stress$-$strain relationship for all materials. it is proposed to construct an innovative prestressed$-$concrete airport runway measuring excatly $2\mathrm{.}5$km long and $50$m width. the $200$mm thick $[$lain concrete slab is to be prestressed using a novel procedure. square concrete tile measures $9900$mmx$9900$mm are to be cast on a frictionless membrane, leaving $100$mm gap between adjacent tiles. after casting all the tiles, a special high$-$early strength, expansive grout is placed in all the joints between tiles. this grout takes about $2$ days to harden to a compressive strenght of $55$MPa $,$ and then over the next two days it expands in length by $5\%.$ the grouthing occurs about $28$ days after casting the concrete tiles at which time fc$=55$MPa and Ec$=31500$MPa. for the grout at this time assume E$=50000$MPa. a rigid abutment is provided on each end and on each side of the runway. a$)$ find the compressive stress in the concrete tiles $4$ days after placing the grout