Many large concrete structures are too big to be poured as one monolithic unit and therefore have a number of constructions joints.
Two different types of joints can be identified:
- The work-joint which has to be watertight without having to accommodate movement between the adjacent concrete pours.
- The expansion joint which has to accommodate water pressure as well as the possible movement between the individually poured concrete sections.
In a joint three types of waterstops can be utilized:
The external waterstop is mounted on the wet side of a concrete structure. This type of waterstop protects the joint against ingress of mud or any other substance and is also installed in structures where a “double tightness” system is desired, e.g. nuclear power plants or at ramps to motorway tunnels. This type of waterstop is only recommended in open structures such as tunnel roofs where air can be trapped within the joint.
The primary seal between two concrete structures is accomplished by the internal waterstop. This waterstop is always fitted roughly in the centre of the wall, and bridges the expansion/movement joint in the wall. When this type of waterstop is used to seal an expansion joint, it has a central sponge at its centre. The purpose of this central section is to form a free space in the concrete, which is necessary to allow movement. The central sponge may be formed with hard rubber with a central hole or with two rectangular strips of sponge rubber at the centre, one on each side. The sponge rubber strips, being flexible, facilitate the fixing of the waterstop into the shuttering and create at the same time a perfect seal between the sponge rubber and the shuttering. During the concrete pouring and casting process no cement slurry is able to drain off owing to shutter leakage thereby preventing honeycombing and the formation of cavities in the concrete. The concrete quality in the area around the waterstop is critical in ensuring water-tightness. The sponge rubber is available in different sizes, dependent on the joint width during pouring.
The expansion joint is installed once the concrete structure has hardened. It’s function is to keep the joint clean and open to prevent ingress of dust and dirt which could lead to dis-function of the joint. The expansion joint needs special dimensions to ensure that it fits the joint and accommodates the joint expansion.