Cracking
Cracking can occur in the fabric of a structure either shortly after construction or later. Generally, the cracking is superficial. If this is the case, it is easily repaired and unlikely to happen again. In some cases however, cracking can indicate subsidence or structural failure. Cracking can occur for any of a number of reasons including, settlement of foundations, proximity to trees, and spreading of roof structures.
Either way, cracking can be greatly minimised by allowing for movement in design. The following information deals with the issue of cracking as a result expansion and/or shrinkage.
Diagram C72 - Typical cracking patterns - front elevation
Diagram C73 - Typical cracking patterns - gable elevation
Cracking at the weakest areas of a house such as over windows etc. can occur due to blocks not being cured sufficiently. The problem occurs because concrete blocks manufactured in the open air have high moisture content. If these blocks are used too quickly for construction, they tend to shrink, causing cracks. The use of properly cured blocks reduces the risk of cracking. Check that blocks have been stored for at least 4 weeks prior to delivery to minimise the risk.
Movement Joints
Movement joints should be included to avoid cracking wherever long lengths of wall exist in order to account for expansion and contraction of blockwork/brickwork that occurs due to heating, cooling, and drying. The joints in concrete blockwork are contraction joints while the joints in clay brickwork are expansion joints.
It is the job of the building designer to specify the spacings of movement joints. Generally however, movement joints are not required in semi-detached houses and in terraces of 3 or more houses, expansion joints are to be placed every 2 houses, unless the designer specifies differently.
Diagram C74 - Typical movement joint locations
Please Note: Movement joints must be included at front and back of terraces. Generally, you should try and conceal the joints behind down pipes.
Movement Joints In Clay Brickwork
Movement joints in clay brickwork should be at least 18 mm and ties should be provided at 225 mm centres vertically alongside the joint.
The joint should consist of a backing of suitable material such as closed cell polythene in cord or sheet form pointed with either one part low modulus silicone or one part polysulphide.
The diagrams below detail how a joint should be constructed, it is important to note that this detail is only relevant for for a short terrace of less than 4 houses.
Diagram C75 - Typical clay brickwork outer leaf movement joints
Diagram C76 -Typical clay brickwork outer leaf movement joints - plan view
In terraces of more than 5 houses, it may be advisable to provide control joints to both leaves.
Diagram C77 - Typical clay brickwork outer leaf, blockwork inner leaf movement joints - terrace of 5 or more houses
Diagram C78 -Typical clay brickwork outer leaf, blockwork inner leaf movement joints - terrace of 5 or more houses - plan view
Diagram C79 -Typical internal corner detail finished with preformed galvanised or stainless steel plaster stops - plan view
An alternative to installing a movement joint for the inner leaf is to construct the party wall as a cavity wall with a 50 mm cavity.
In a situation where the party wall is a cavity wall, ensure double triangle wall-ties are used across cavity.
Diagram C80 -Typical clay brickwork outer leaf movement joints - cavity party wall
Diagram C81 - Typical clay brickwork outer leaf movement joints - cavity party wall - plan view
Movement Joints In Stepped Blocks/Terraces
Here, the optimum position for movement joints is the internal corner of returns. The diagrams below demonstrates the appropriate position.
Diagram C82 -Typical movement joint location in a stepped terrace
Diagram C83 - Typical movement joint location in a stepped terrace - plan view
Movement Joints In Concrete Blockwork
Similar to clay brickwork, concrete blockwork is prone to movement, mainly shrinkage. For this reason, in blocks of terraces that contain more than 3 houses, expansion/movement joints in the outer leaves should be located at 12 m centres, unless otherwise outlined by the designer.
Diagram C84 - Typical blockwork outer leaf movement joints
Diagram C85 - Typical blockwork outer leaf movement joints - plan view
In situations where a terrace consists of 5 or more houses, providing movement joints to the inner leaf, while not technically necessary should be considered during the design stage.
Diagram C86 - Typical movement joints where both leaves are concrete block wall
Diagram C87 - Typical movement joints where both leaves are concrete block wall - plan view
Diagram C88 - Typical internal corner detail finished with preformed galvanised or stainless steel plaster stops - plan view
Movement Joints In Hollow Block Walls
Movement joints in hollow block walls should be placed at no more than 12m centres in terraces with more than 3 houses, unless specified otherwise, similar to cavity wall construction.
The following diagrams detail the method used to create a movement joint in a hollow block wall.
Diagram C89 - Typical movement joint in hollow block external wall
Diagram C90 - Typical movement joint in hollow block external wall - plan view
In situations such as this, where the party wall is of cavity wall construction, double triangle wall ties should be used across the cavity. Never use longitudinal type hollow blocks in an external wall
Hollow Block Walls – Alternate
Detailed below is an alternative movement joint to that shown above. Similar to the movement joint discussed above, this type can be used in terraces of 3 or more houses also.
It is necessary to provide double triangle wall ties at every horizontal bed joint in order to securely join the party wall with the external wall. Steps 1 to 4 as detailed below depict the sequence of construction.
Diagram C91 - Movement joint in external hollow block - order of construction - Stage 1
Diagram C92 - Movement joint in external hollow block - order of construction - Stage 2
Diagram C93 - Movement joint in external hollow block - order of construction - Stage 3
Diagram C94 - Movement joint in external hollow block - order of construction - Stage 4
Movement Joints In External Render
Aligned With Background Joint
Detailed below is a render joint that should be formed in a situation where the render joint is aligned over the background movement joint.
Diagram C95 - Typical movement joint in external render - aligned with movement joint behind
Proprietary corrosion resistant bead should be fixed to the background masonry as shown opposite either using plaster/render dabs, or mechanical fixings at 600 mm centres either side of the joint.
If required to maintain the joint gap, timber slips tied to the beads with tying wire can be used to keep the gap. On removal, use a suitable sealant to fill the gap left. Beads are available in a range of materials and careful consideration should be given to which bead is chosen. It is important to limit the chance of corrosion between the constituents of the render mix and the bead.
Not Aligned With Background Joint
The diagram below shows how a movement joint can be formed in external render.
Diagram C96 - Typical movement joint in external render
Similar to before, fix the proprietary render movement joint beads to the background masonry. Incorporated in the bead is a PVC extrusion that spans the joint, allowing movement of +/-3 mm. Usually, the PVC extrusions incorporate tape that protects the joint during construction, It is then removed after. Only use the joint detail illustrated above outdoors.
Bed Joint Reinforcement
Bed joint reinforcement is incorporated to control cracking in areas of high stress. Reinforcement is laid in horizontal bed joints in these areas. It is important that the reinforcement is at least 600 mm either side of the ope in order to distribute the load completely from the area of high stress to an area of low stress. Cover of 20 mm at least should be given to the reinforcement from the outer face and it is also important that the reinforcement is made from a corrosion resistant material such as stainless steel.
Diagram C97 - Bed joint reinforcement detail at openings
Note: I.S. 845-3 Specification for ancillary components for masonry – Part 3: Bed joint reinforcement of steel meshwork outlines the requirements of bed joint reinforcement.
Wall Panel Provisions
A lot of times, cracks are caused more due to the shape and layout of opes than their size. Generally, there is an increased chance of cracking if the length of blockwork is more than twice the height. The image below details the typical cracking pattern that arises due to the proportions of the panel being as described.
To limit the risk of cracking, movement joints should be used in bungalows wherever the length of an uninterrupted wall is more than twice the height.
Diagram C98 - Cracking patterns typical caused by incorrect panel proportions
