when a pipe is running horozontally in the bottom of concrete, and into the concrete, it will crack and weaken the concrete. best pratice is to go under the concrete, then come up vert. in a sleeve. how about a drain line for a wash sink, toilet ect? not a bad idea to run one extra 3" or 4" pvc with a long sweep 90 in the back wall slab area, in the slab. this is handy in the future to pull in what ever. i,m a gen. contr. myself. don,t forget plenty of elec. for that welder!