Anchor bolts can be grouped into three main types. A chemical anchor is an adhesive that permanently fastens a threaded rod or rebar into a drilled hole. Some types can also be used to repair cracks in concrete. Foundation anchors are installed in place before the concrete is poured. Mechanical anchor bolts are installed in drilled holes and are held in place by threads or an expansion mechanism. There are mechanical anchor designs for use in concrete, masonry, and gypsum.