This is not the kind of tension you pop a pill for. We’re talking pre-stressed concrete and masonry here. Not exciting from an aesthetic sense, but quite desirable from a structural sense. Why pre-stressing? Concrete + Steel = Strong Structures. Cement-based building materials are superb when it comes to resisting compressive forces – the kind that crush them – but not so good at handling tensile forces – the kind that pull them apart.
The two methods of pre-stressing are pre-tensioning and post-tensioning. For the “pre-“ technique, the steel is stressed first, then the concrete is cast around this reinforcing. This process is performed at a pre-cast manufacturing facility, so the completed pre-stressed concrete members are then transported out to the job site and assembled. On the other hand, the “post-“ way involves installing the reinforcing on the job site after the contractor forms up the slabs / constructs the walls and columns. The steel is housed in a sheathing or duct that prevents it from bonding to the concrete; therefore, it can be stressed after the concrete hardens. So now you know.
See also: core drilling and rebar