Accounting for movement in concrete bricks

Figure 4: Incorrect positioning of movement joints

Types of movement

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 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.

Aggregate Type

Shrinkage Amount

Sandstone Aggregate

Typical 1 year shrinkage @ 0.116%

Granite Aggregate

Typical 1 year shrinkage @ 0.047%

Limestone Aggregate

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 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 must be referred back to the technical services team for consideration.

*CE marking is acceptable until 1st January 2022.

From 1st January 2022 UKCA marking in accordance with UK Construction Product Regulation and design standards will be required.

Concrete bricks must be adequately cured before delivery 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.

Thermal movement: warranty position

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.

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 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.

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 damp proof course (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, 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. 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

Movement in concrete bricks - Fig1

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

Movement in concrete bricks - Fig2

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). 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.

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 4).

Vertical movement joints should therefore be located in sections of full height masonry between the openings (Figure 3).

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.

Figure 3: Correct placement of movement joint and reinforcement

Movement in concrete bricks - Fig3

Note: the movement joint should be in full height masonry between the window and door openings.

Figure 4: Incorrect positioning of movement joints

Movement in concrete bricks - Fig4Note: 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.

Figure 5: Avoiding openings

Movement in concrete bricks - Fig5

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.

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
  • Where possible 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

References

  1. Technical manual Section 6 External Walls
  2. BS 5628 – 3:2001 codes of practice for design of masonry — Part 3: Materials and components, design and workmanship
  3. Aggregated concrete blocks a guide to movement control
  4. National Concrete Masonry Association an information series from the national authority on concrete masonry technology CRACK CONTROL FOR CONCRETE BRICK AND OTHER CONCRETE MASONRY VENEERS

*CE marking is acceptable until 1st January 2022.

Accounting for movement in concrete bricks

This technical document provides additional guidance relating to how the Functional Requirements in the Technical Manual may be satisfied. This article covers the following:

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. Concrete bricks are not unique in this respect.

download a pdf version of this article

Download a PDF version of this article

Every care was taken to ensure information in this article was correct at the time of writing (December 2021). 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. 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.

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