The anchorage device may be connected to the ground by chemical means, mechanical expansion or concreting. In the first case, polyester resin cartridges are placed in a perforation to fill the ring space around the end part of the bolt. The main advantage of this type of anchorage lies in its simplicity and in the speed of installation. The main disadvantage is in its limited strength. In the second case, the anchorage is composed of steel wedges driven into the sides of the hole. The advantage of this type of anchorage lies in the speed of installation and in the fact that the tensioning can be achieved immediately. The main disadvantage with this type of anchorage is that it can only be used with hard rock, and the maximum traction force is limited. In the third case, the anchorage is achieved by concreting the whole metal bar. This is the most-used method since the materials are cheap and installation is simple. Injected concrete mixes can be used in many different rocks and grounds, and the concrete sheath protects the bar from corrosion. The concrete mixture is generally made up of water and cement in the ratio "W"/"C" = 0.40-0.45, producing a sufficiently fluid mixture to allow pumping into the hole, while at the same time, providing high mechanical strength when set.