. >> this is called unbonded, because of this plastic sheeting. it is not bond to the concrete. >> why do you want to do that? >> because there are two methods of decreasing stress. one is posttensioning. the other is pretensioning. this method that we are using is a very conventional for on-site stressing of the slab. what it does is it does not bond to the concrete. you put the form up, he placed the attendance. >> this will be or is not all. inside the floor. -- this will be horizontal, inside the floor. >> this is the anchored and of it. there is a parable -- parabolic profile tendons. their lowest in the mid span and they are the highest above. what happens is once that you place these inside of the form work and you have the other reinforcements, you pour the concrete. at a certain point where you have adequate curing of the concrete, which is usually about two-thirds of the strength of the concrete, then you jack these and pull them. >> you pull them with hydraulics. you attention it. >> it does two things. it puts large. compression into t