By Marc Zimmermann. Joints in building construction help to avoid constrictions in the building, accommodate movements and join individual components to each other in a tailor-made way. There are a number of reasons for joint movements. For example, these include subsidence, creep or shrinkage of the concrete, but external influences, such as temperature changes, humidity, wind, seismic movements or elastic deformations, also play a role. Design errors may also result in unexpected joint movements.
Joints are, depending on their structure and design, sealed using different sealing materials for reasons of heat and sound insulation, to seal against the penetration of water and to protect components. In some cases, designers and manufacturers must, considering regulations, always take into account the property-specific circumstances for this such as movements to be expected, widths, filling depths, backfilling, etc. Complying with the prevailing fire safety regulations is added to this.
In building construction, there is a distinction between movement joints and rigid joints. If these building joints are arranged in the area of fire protection classified components, this is a fire protection joint. This means the joints in these areas must at least equate to the fire-resistance duration of the respective components.
Such a fire protection joint must, in addition to fire resistance, at the same time meet all the other requirements in terms of structural physics. This must implicitly be considered in the design.
There is often uncertainty as to how a particular joint should be realised. Building supervisory guidelines and certificates of suitability often have no direct connection as regards content between the fire-resistance requirement and the specifications for the design of a building construction joint without fire protection. In European regulations, proof in accordance with ISO 11600  can be included in the scope of application in the European Technical Approval (ETA) for the first time. A direct reference to the design option as a building construction and fire protection joint for the respective product will consequently also be apparent to the designer or manufacturer.
The design of a classic building construction joint
The joint rating is dependent on the anticipated movement of the joint. Classic building joints are implemented in combination with a primer, backfilling and a sealant. When manufacturing, it must be ensured that the sealant can accommodate the movements of the building structure in the joint. A bonding agent (primer), which improves the adhesion between the sealant and the component, is used for optimal flank adhesion. In addition, the primer binds light impurities such as dust or loose particles on the concrete surface.
The backfill material is used as an even and defined limit for the sealant. Using round cords so that the sealant retains an arched, concave shape inwards on both sides is recommended.
It must be compatible with the sealant. The backfill material should always be wider than the joint in order to guarantee sufficient resistance when inserting the sealant. If you dispense with the backfill material, there is a risk that the joint sealing will only combine with the joint edges insufficiently. The forces of joint movement will be unevenly directed into the substrate via the set sealant and adhesion failures will result.
Designing a building construction expansion joint
When designing expansion joints, a joint width of 10 mm should be reached. On no account should the joint be narrower than 5 mm, as it may otherwise lead to an excessive strain of the sealant.
In the case of joint widths ≥ 10 mm, the filling depth will be half the joint width, however a maximum of 15 to 20 mm. In joints which are narrower than 10 mm, the filling depth should correspond to the joint width. If a joint is calculated as wider than 35 mm, testing the design is recommended.
Specifications for the design of a fire protection joint
If the adjacent components require a joint design with fire protection, the corresponding regulations must be observed in the design. The respective building regulations describe the requirements of the corresponding fire compartments and the fire resistance to be observed for wall and ceiling designs.
A European Technical Approval (ETA) has recently been used as proof of use for fire protection joints. Joint sealing systems may only be installed in fire protection rated walls and ceilings if the entire joint design was tested with regard to its fire resistance and there is corresponding proof. It is important in the case of fire protection joints that not just the sealant, but always the entire joint design must be tested and classified.
Due to the fact that the actual joint design and its requirements are the result of customized building planning, the scope of application described in the certificate of suitability for fire resistance should be set very widely if possible. A large joint construction design range is consequently available to the designer.
Particular attention should be paid to the fact that flame-resistant sealants with the classification B1 must not be installed in fire protection joints without separate evidence regarding the fire resistance of the joint design. The classification B1 only describes the flammability of a building material, not its fire resistance.
Structure of a joint seal with fire resistance
The structural design of a joint seal is of vital importance for fire resistance. The following parameters will therefore be recorded in detail within a fire resistance test in Europe:
- Type and location of the components into which the joint system may be installed
- Materials of which the walls and ceilings (joint edges and adhesive surfaces) consist
- Depth and width of the joint
- Material and dimensions of the backfill material
- Type of sealant and primer
- Type of joint (connection, glazing or façade joint)
Deviations from a structure recorded in the certificate of suitability may negatively affect its fire behaviour and thereby its fire resistance and are therefore not permitted. If there is a deviation from the prescribed structure, a new certificate will be required. The certification of the equivalence of another building material by the respective manufacturer is also invalid. Only if the use of alternative products is specified in the certificate of suitability may a replacement be made. Special provisions apply to the joint sealant used in fire safety, as in the case of fire this must solely ensure the reliable seal of the joint. It must not only have the usual properties for the filling of a joint, but also in terms of fire protection be so equipped that in the case of fire a sufficiently sturdy partition is formed.
Dipl.-Ing. Marc Zimmermann: Managing Director ZAPP-ZIMMERMANN GmbH, www.z-z.eu
 ISO 11600:2002 + Amd 1:2011: Building construction – Jointing products – Classification and requirements for sealants