Accounting for Movement in Concrete Bricks
An increasing demand and availability of construction materials has resulted in concrete bricks becoming more popular.
All building materials move in one way or other. Movement can be as a result of drying shrinkage, moisture movement (absorption and drying out), thermal effects or structural movement. Concrete bricks are no exception.
This article looks at what you need to consider when accounting for movement in concrete bricks.
Types of movement in concrete bricks
Movement in a structure can arise from the effects of:
- Thermal influences
- Shrinkage and moisture movement characteristics of the external masonry
- Changes in the orientation or shape of a building
- Site practice
- Type and grade of mortar
- Incorrect storage and protection of materials
The tendency for all concrete products is to shrink slightly over time and when drying out, although they may revert back to near their original size when subject to moisture.
Like all materials they are also subject to thermal movement. Consequently, the location of movement joints will define the most appropriate position to accommodate this movement whilst considering the aesthetic, practical and structural factors.
Shrinkage in concrete
Concrete shrinkage is due primarily to the shrinkage of the hardened cement paste. The type and amount of aggregate can affect the amount of shrinkage in a concrete bricks performance.
Typical 1 year shrinkage @ 0.116%
Typical 1 year shrinkage @ 0.047%
Typical 1 year shrinkage @ 0.032%
With the high differential of movement between the various available raw materials used in concrete brick manufacture, it is imperative that the design for movement in a wall panel is specific to the shrinkage capacity of the concrete brick used. The manufacturer’s requirements should be followed and in all cases site-specific advice should be obtained from the manufacturer before work begins.
Concrete brick materials
Concrete bricks must display a CE marking. The use of non-CE concrete bricks will not be acceptable for Warranty purposes.
Concrete bricks must be adequately cured at the factory before they are delivered to site.
Excess moisture introduced into the brick will result in a higher shrinkage rate. Bricks must be kept dry whilst storing on site and stacking out.
Bricks should not be wetted before laying and incomplete brickwork should be protected from the rain and snow as this will minimise the risk of shrinkage and efflorescence leaching from around the mortar joints.
South-facing walls, particularly those built of dark coloured bricks, are more susceptible to thermal movement than other elevations. Whereas a simple contraction joint may suffice in more sheltered elevations, joints for southern facing elevations should have movement joints which are capable of responding to both expansion and contraction.
The mortar should be suitable for use and the specific strength of the mortar used on site and should be accounted for in the design.
Stronger mortars have higher shrinkage values and care should be taken to ensure the correct grade of mortar is specified and used.
Due to safety factors incorporated by mortar suppliers, the onsite mix has the potential to have an increased strength. Therefore, the mortar manufacturer’s supply documents must be available to verify that the mortar is the required specification and ensure the mortar mix is not a stronger mix due to safety factors.
In certain instances, different masonry materials may be combined within the same elevations. In the case of clay bricks, which have expansive properties, and concrete bricks, which may shrink slightly, it is important to make provision for this differential movement.
Where, for example, a clay brick is used up to dpc level and a concrete brick built as the superstructure, the dpc itself may act as a slip plane and allow the differential movement to occur. This can be dependent on the dead load on the dpc and advice from a structural engineer should be sought. In all cases provision should be made to ensure structural stability.
If two dissimilar materials are mixed on one elevation, then slip planes should be introduced or bed joint reinforcement incorporated to dissipate the areas of tensile stress. Again, a structural engineer’s advice should be sought and provision must be made to ensure that structural stability is not compromised.
Panel ratios and movement joints
External wall leafs of concrete brickwork should be designed as a series of panels separated by movement joints to control contraction.
The degree of movement is dependent upon unit type. As a rule, vertical joints to accommodate horizontal movement should be provided at intervals of 6m. The ratio of length to height of the panels should generally not exceed 3:1 (BS 5628).
Figure 1: Front elevation of dwelling, divided into panels
Figure 1 above shows the superstructure viewed as a series of panels (the diagram does not demonstrate position of movement joints). For example, in elevations where window openings are wide in comparison to their height leaving long low areas of masonry (refer to Panels 1, 2 and 3 in Figure 1), or where those types of openings are stacked above each other, this may result in the brick panels in between the windows being less than 6m metres but exceeding the 3:1 ratio.
In these instances, inclusion of additional vertical movement joints may need to be considered, or alternatively bed joint reinforcement introduced to dissipate the stresses within the panel (Figure 2, below).
Figure 2: Front elevation of dwelling, reinforcement examples
Provision of reinforcement around openings
Particular care should be taken with openings greater than 1.5m in width, especially where they are placed directly above each other. In this case the panel may not exceed 6m or the 3:1 ratio but can be subject to stresses from the larger areas of brickwork adjacent to the openings.
In these cases, bed joint reinforcing should be introduced above and below the openings (Figure 2 and 3). The bed joint reinforcement should generally extend 600mm past the openings and must not extend through any movement joint. Care must be taken to ensure the bed reinforcement is installed in the correct brick courses as per the brick manufacturer’s recommendations.
The reinforcement should be of the ladder/lattice type as opposed to the expanded mesh version.
Whilst this bed joint reinforcement will assist in the prevention of potential cracking, it is not a complete alternative to movement joints and these should still be provided at the appropriate locations.
Figure 3: Example of bed joint reinforcement courtesy of Edenhall
Position of movement joints
The position of movement joints should take into account the need to maintain the structural integrity of the wall. Movement joints should not pass through structural members e.g. lintels.
Where possible, it is recommended that movement joints should not coincide with door or window openings due to the difficulty in continuing the movement joint between the frames and masonry and around the ends of the lintels (Figure 5).
Vertical movement joints should therefore be located in sections of full height masonry between the openings (Figure 4).
Figure 4: Correct placement of movement joints
Note: the movement joint should be in full height masonry between the window and door openings.
Figure 5: Incorrect positioning of movement joints
Note: It is recommended that movement joints should not pass through openings due to the difficulty in continuing the joint between the frame and masonry and around the end of lintels.
Vertical movement joints should not pass through bed joint reinforcement and should not be located in close proximity to the openings that may impair the structural integrity of the wall.
Where a full height masonry panel does not exist, the location and detailing of the movement joint should be designed by an engineer to avoid it passing around window and door frames.
Where there is no full height path within the masonry, the movement joint should be engineer-designed to avoid any door or window openings. The design may involve the introduction of a slip plane to link the staggered joint. Small piers created by the placement of movement joints should be fully justified by a structural engineer.
Figure 6: Movement joints that avoid openings
Note: Where there is no full height path within the masonry, the movement joint should be engineer-designed to avoid any door or window openings. The design may involve the introduction of a slip plane to link the staggered joint.
Formation of joints
Contraction joints in external walls should be formed with a compressible material such as a polyurethane foam and used to form the joint and a sealant to prevent water penetration. The width of these joints should be in accordance with the manufacturer’s recommendations.
Movement in concrete bricks: recommendations
- Only concrete bricks displaying a CE marking should be used and must be adequately cured before use on site
- The design for movement should be specific to the shrinkage capacity of the brick used and the requirements of the brick manufacturer should be followed. In all cases site-specific advice should be obtained from the manufacturer before work begins
- The bricks must be kept dry whilst storing and stacking out. Bricks should not be wetted before laying and incomplete brickwork should be protected from the rain and snow
- Ensure that the correct grade of mortar is specified and used
- Movement joints should be located at 6m centres. The length-to-height ratio of the panels should not exceed the 3:1 ratio
- Accommodation for movement should be provided in long/low areas above or below large openings
- Movement joints should not pass through openings due to the difficulty in continuing the joint between the frame and masonry and around the end of lintels
- Bed joint reinforcement or additional provision for movement should be used where openings exceed 1.5m
Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.