Guidelines Concerning Expansion Joint Selection And Design

An expansion joint (dilation joint, from Latin dilatare: to expand, to extend) is a purposefully created slit in a building or structure, the task of which is to allow individual components of the structure to work independently of the rest. Separate components, their fragments, transfer the foreseen loads, deformations and movements by themselves.
In the construction industry one can differentiate i. e. between thermal expansion joints and structural expansion joints. Thermal expansion joints counteract structural damage such as i. e. cracks of roof or terrace slabs or walls as a result of temperature variations. One assumes that reinforced concrete roofs and terraces should have circumferential expansion joints, preventing the bending of outside walls or cornice cracks, as well as intermediate expansion joints, reducing the deformations of roof section slabs and terrace slabs. In addition, they prevent damage to coverage and outer layers.
The individual building structural components, due to different thermal effects, are subject to contractions. In order to limit the values of tension caused by these influences, thermal expansion joints are used.
Thermal expansion joints dissect the building from the terrace surface or roof surface through walls or the structural load bearing framework all the way to the top surface of the foundations. These joints do not pass through foundations, however, the wall expansion slits are in line with the joints in the roof slabs. This is because it is assumed that the components embedded in the soil are not subject to thermal influences.



Where large vertical deformations are expected, structural expansion joints are made due to buildings setting unevenly. Such joints are run from the underside of foundations all the way to the roof covering or the terrace outer surface. The forming of expansion slits (their width, location, distances between them) and the choice of protective materials for the individual slits depend on the type of chosen solution for the terrace or ceiling-roof, its structure, material, etc.


A very important factor influencing the shape of expansion joints is contraction.
Contraction is caused as a result of concrete maturing processes, its hardening, mortar drying, etc.
The extent of contraction depends on such factors as:

  •   concrete amount and quality,
  •   aggregate quality and purity,
  •   hardening method,
  •   care,
  •   amount of reinforcement preventing contraction,
  •   mode of execution,
  •   concrete age.

The total contraction deformation is composed of two parts, the contraction deformation caused by drying and autogenous deformation (self-inflicted contraction deformation). Deformation caused by drying develops slowly, because it is a function of migration of water through the hardened concrete. The autogenous contraction deformation develops during concrete hardening, and that is why, in the majority, it commences in the first days after the concrete is poured.
Movement of a part of a concrete structure due to ambient temperature changes may be described by the following equation:

∆l = ɑ ·∆T · l

Consideration of contraction is given by the following formula (equation):
∆l = Ɛcs · l     
                         
Total movement - temperature change and contraction:
∆lT+S = (ɑ · ∆T + Ɛcs) · l   

Key:
∆T - temperature change
ɑ   - (linear) thermal expansion coefficient of concrete
Ɛcs- total concrete contraction deformation
l    - component length

Assuming a temperature difference of ∆T = 15 °C and total contraction deformation (Ɛcs =Ɛcd +Ɛca - sum of the contraction deformation caused by drying and autogenous contraction) 0,30 ‰ for a component length of 10 m, assuming that the contraction progression is 50%, thus one obtains:

                                                                         ∆lT+S = (1· 10-5 · 15 + 0,5 · 0,0003) · 10000 = 3,00 mm

It is assumed that the contraction of reinforced concrete for the most commonly used ordinary concrete classes with appropriate aggregate granularity amounts to 0,15 mm/m (i. e. 0,15/1000 = 0,00015) on average. The same reduction of a component takes place by temperature reduction by 15 °C, because the thermal expansion coefficient for concrete is ɑ = 0,00001/1 °C), i. e. 0,00001 x 15 = 0,00015. Considering the above, the influence of concrete contraction in monolithic structures is usually assumed to be equal to that in case of a temperature drop by 15 °C.
Apart from contraction, structure movements may also be influenced by setting and external loads.

 

Possible directions of movement of building parts connected by expansion joints

During the design of inclined surfaces of a roof or a terrace, one should avoid choosing an inclination direction that would cross the expansion joint when descending. For the location of internal inlets for drainage purposes, one should assume that they have to be appropriately distant from the expansion joint in a manner allowing safe water migration.



Below is presented an 'envelope' approach to water drainage, with the course of the expansion slit taken into account.




The expansion joint is a key component of the roof cladding. According to the assumptions, this spot needs to compensate various tensions existing in a structure. The expansion joint is also susceptible to higher movements, and that is why one should use materials with appropriate resistance and extension ability (movement compensation ability) and water tightness for the finishing work.
In order to allow the structure to work in an appropriate manner, the protection of structural expansion joints in a manner that would prevent any movement of the individual components to cause cracks and fractures is necessary. This requires the use of parts that will be able to compensate the assumed movements.
In order for the structure of the roof or ceiling roof not to influence the inside walls, expansion slits should be created allowing unhindered structure movements.

Expansion joints with supplementary construction  Expansion joint of a ceiling slab with a cornice, upon a pillar Expansion joint of a ceiling slab upon a pillar

Roof profiles require absolute seal tightness. The structure, if it is sealed on its surface, and the joint with the subsequent structural component must be appropriately precisely executed with adherence to strict process technical instructions.
Mutual movements and the width of the expansion joint are determined at the design stage. Of paramount importance is thus the designing entity, which makes the choice of an appropriate expansion joint construction system and selects the appropriate profile. The contractor should adhere to the remarks and instructions of the manufacturer of the expansion joint profile system.


The choice of roof expansion joint profiles is made based on the following data that should be included in the design documentation or provided by the architect and designer:

  •    point of installation of expansion joint,
  •    width of expansion slit,
  •    number and type of individual finishing layers,
  •    horizontal movement,
  •    vertical movement.

 

 

 

The workload and costs related to repairs are incomparably higher in relation to the value of prior proper selection and installation of profiles.
That is why appropriate selection of Deflex profile structures is very important. This catalogue should be helpful during design and selection of appropriate profiles for expansion slits.
The knowledge and experience of Betomax shall allow you to design the most optimum solutions. In case of questions, please contact our technical advisor.

 

Deflex roof expansion profiles
Expansion profile types Expansion slit width[mm] Material Movement direction
Aluminium
Nitriflex®
Elastoflex®
Levels Vertical Horizontal
DEFLEX WOD 550
       
120


. . . .
DEFLEX 51

25÷80

.
. . .
DEFLEX 521

50÷150
.
.
. . .