Recently, the increasing demand and availability of construction materials has resulted in concrete bricks becoming more popular.
All building materials move in one way or other, either from drying shrinkage, moisture movement (absorption and drying out), thermal effects or structural movement – and the same applies for concrete bricks.
Types of Movement
Movement in a structure can be caused by:
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 is to define the most appropriate position to accommodate this movement whilst considering the aesthetic, practical and structural factors.
Shrinkage
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.
With the high differential of movement between the various available raw materials used in concrete brick manufacture, it’s important that the design for movement in a wall panel is specific to the shrinkage capacity of the concrete brick used and the requirements of the manufacture should be followed. In all cases site specific advice should be obtained from the manufacture before work begins.
Materials
Concrete bricks must display a CE marking. The use of non CE concrete bricks will always be referred back to the Premier Guarantee Technical Services team for their consideration.
It’s important that concrete bricks are 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 should be kept dry whilst storing on site and stacking out.
Bricks shouldn’t 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.
Thermal Movement
South facing walls, particularly those built from 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.
Mortar
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 manufactures 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.
Dissimilar Materials
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, then 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
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 and, 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 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 such as fig 1 panel 1, 2 and 3, 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).
Figure 2: Example of reinforcement above and below openings exceeded 1.5m
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 (fig 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 manufactures 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)
Positioning 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).
Figure 5: Incorrect positioning of movement joints
Vertical movement joints should therefore be located in sections of full height masonry between the openings (figure 4).
Figure 4
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.
Note: The movement joint should be in full height masonry between the window and door openings.
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.
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.
Note: Small piers created by the placement of movement joints should be fully justified by a structural engineer.
Figure 6: Avoiding openings
Formation of joints
Contraction joints in external walls should be formed with a compressible material, such as a polyurethane foam, and a sealant to prevent water penetration. The width of these joints should be in accordance with the manufactures recommendations.
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