RIBA Core Curriculum
- Design, construction and technology
Knowledge level
- General Awareness
CPD\
CPD /
RIBA Core Curriculum
- Design, construction and technology
Knowledge level
- General Awareness
This course provides an overview of the modern brick manufacturing processes, the common terminology and the characteristics of different products. It covers topics such as brick technical properties, British Standards including BS EN 771-1, the influence of design and detailing on brickwork performance and the principles of brickwork movement, expansion joints and mortar specification.
In this section
- Mortar specification and why it should be an important consideration
- An introductory look at movement in brickwork
- Thermal and moisture movements and calculations
- UK and European Standards
It is important that the appropriate mortar specification for different applications is considered.
The mortar specification will depend on a number of factors such as the application, location, level of exposure, other site specific conditions.
Mortar accounts for 17% of the visible brickwork (dependant on bond pattern) and the colour chosen can change the look of a wall considerably.
Mortar Specification
Mortar strength is an important consideration, the choice of which will be affected by such factors as weather, degrees of exposure, location and loadbearing requirements.
A suitable reference panel should be constructed prior to commencement of brickwork in order to provide an indication of the finished wall appearance.
The reference panel should set the standard for appearance and workmanship for all of the brickwork to be completed and should be built with the correct bond pattern, mortar colour and joint profile.
The initial rate of water absorption, also known as the suction rate, is of importance in the laying of bricks.
The figures included on the brick Declaration of Performance should allow the contractor/mortar supplier to tailor the mortar to suit the bricks.
Brick with a high suction rate may need to be wetted prior to laying, particularly in hot, dry weather. Bricks with a very low suction rate (e.g. blues) can float on the mortar if they become saturated or if the mortar is too wet.
Mortar Specification
Typically we would suggest a Designation (iii) / Class M4 mortar for general brickwork areas in the walls of buildings.
For more exposed details, for example below DPC level, parapets, chimneys, garden/retaining walls etc. a Designation (ii) / Class M6 mortar would be more appropriate.
Designation (i) / Class M12 mortar is the most durable but is more suited to engineering type bricks.
It is preferable to use pre-mixed mortars which have the advantage of accurate blending and minimal variation in colour.
We would recommend speaking to a mortar supplier for more specialist advice, taking into account the brick properties and the target strength required.
The mortar joint profile has a significant impact on the aesthetics of a wall. It also plays an important part in the overall weather resistance of brickwork so the choice of joint profile used should be based on technical performance requirements as well as appearance.
Correct proportioning of mortar constituents is required to achieve adequate strength, a good bond and to avoid colour variation.
Gauging by mass using batching boxes is the most accurate method and water content should be accurately controlled.
Proportioning by volume using shovels should be discouraged.
Cross and bed joints should be fully filled and joints should be well tooled.
A bucket handle joint is usually favoured as it provides an attractive appearance combined with good weather resistance.
Recessed joints are not recommended in locations liable to severe exposure to wind-driven rain or in external works e.g. boundary walls because they impede water run-off and the overall weather resistance of the brickwork is reduced.
This small, brick built hunting lodge was designed by Nikolaus Bienefeld of Buro Bienefeld.
Wide joints and flush pointing create an archaic simplicity.
Demonstrates how well brick architecture can fit into rural areas.
Now let’s take a look at movement in brickwork. This is a complex subject that cannot be completely covered here but we have a separate CPD seminar devoted to just this topic if further information is required.
We are not concerned in this presentation with structural, settlement or foundation issues – we are looking at the material characteristics of clay facing bricks and the points to be considered in the design and at the specification stage.
There are three basic elements to consider with clay bricks – the expansion and contraction of material as temperature rises and falls, the effect of wetting and drying (reversible) but there is a third element to be considered with clay units, namely a longer term moisture expansion.
Movement of brickwork should be considered at the design stage. The main sources of movement relate to moisture and temperature.
In general, reversible movements are caused by temperature changes. Irreversible expansion (caused by adsorption of water molecules by the fired clay brick) can be larger and continues, albeit at a reducing rate, for a period of years.
Why don’t we see movement joints in historic buildings?
The method of construction was rather different centuries ago, when walls were thicker, offering more self restraint. Movement did occur of course, and in some cases, cracking would have been evident.
The type of mortar historically though was not of the strong cement based compound we use today – lime and sand was used, which has a self healing effect following movement.
You may have noticed, driving past long estate walls that although no vertical joints are visible, the walls have nevertheless moved, possibly by curving where the mortar has accommodated the movement.
Thermal Movement
Thermal Movement
Every material has a co-efficient of expansion – this varies with different clay types although the difference is insignificant.
The amount of movement will vary depending on the orientation of elevations – a South facing wall will experience a higher temperature profile than the opposite side.
Thermal Movement
Thermal expansion of the brick will occur due to the range of temperatures experienced, the orientation of walls and the colour of the brick
Reversible (+/-)
Moisture Movement
Moisture Movement
The wetting and drying process will have an effect on the overall movement, although this is negligible.
Irreversible expansion occurs following a reaction at a molecular level as the bricks are exposed to the ambient atmosphere.
Moisture Movement
A greater influence on the overall movement of clay bricks is the long term effect of moisture
Reversible (+/-)
Irreversible (+)
Overall Movement Calculation
To consider the effect of all three elements, a calculation is helpful. For a metre length, with an average co-efficient of expansion, and the likely temperature rise (experienced by a southerly facing wall), the thermal contribution will be approx. 0.03%.
The reversible (wetting and drying) effect is in the order of 0.02%, and taking the worst case irreversible moisture expansion (0.07%), we arrive at a total theoretical movement of just over a millimetre per metre run of brickwork.
It is highly unlikely that all three elements will be acting together but nevertheless it is useful to offer a guideline of 1mm/ metre for the provision of movement joints.
Different technical characteristics, e.g. strength, water absorption etc. do not affect the principles of movement joint provision.
Similarly, although there are different categories of mortar (with cement rich mixes being more rigid and less forgiving) which can influence masonry movement, sensible provision of movement joints will avoid any issues.
Overall Movement Calculation
Guidance
Movement of brickwork should be considered at the design stage.
As a general rule in buildings, brickwork expansion joints should be provided every 10-12m maximum.
This dimension reduces to 6m in boundary walls or brick faced parapets.
Movement in Brickwork
There are a number of building features which need to be considered carefully when determining joint positions.
This is a typical detail where a rebated (or pistol) brick is able to accommodate the shelf angle support, avoiding an excessive horizontal joint; brickwork below the angle can expand vertically compressing the filler material.
Movement in Brickwork
Movement Joint Materials
An indication for normal storey height walls is that the joint width (in mm) should be at least equal to the joint spacing (in m) plus an allowance of typically 30% to allow for the compressibility of the filler and the performance of appropriate sealants – Thus movement joints at 10m centres will need to be approximately 13mm wide.
Movement Joint Materials
Bed Joint Reinforcement
Masonry Reinforcement is a fabricated steel reinforcement system that is located in the bed joint to strengthen masonry panels without thickening the wall.
Bed joint reinforcement may be used for a variety of purposes and locations, including:
A structural engineer should be consulted to assess the spacing of control joints and bed joint reinforcement.
Bed Joint Reinforcement
Differential Movement
When brickwork is to be used to clad a reinforced concrete or timber frame, the design should make particular allowance for differential movement.
Vertical movement can be cumulative, and with external brickwork expanding, and the frame potentially contracting, it is often more practicable to deal with movement in smaller sections.
Differential Movement
Standards
Standards
There are a number of standards that relate to brickwork, these are available from BSI.
Clay bricks are manufactured and tested in accordance with the European Standard BS EN 771-1.
Eurocode 6 comprises the following parts:
Part 1–1, General rules for reinforced and unreinforced masonry structures.
Part 1–2, Structural fire design.
Part 2, Design considerations, selection of materials and execution of masonry.
Part 3, Simplified calculation methods for unreinforced masonry structures.
Each part also has a National Annex (NA) which provides the Nationally Determined Parameters (NDPs) to be used in the application of Eurocode 6 in the UK.
PD 6697 contains useful guidance complementary to Eurocode 6.
Standards
Specification for masonry units. Clay masonry units
Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2
Workmanship on Building Sites – Part 3: Code of Practice for Masonry
HD clay bricks. Guide to appearance and site measured dimensions and tolerance
Summary
Summary
Brick has developed from being a traditional, plain building material into an innovative, modern and adaptable material providing solutions for the entire building envelope.
By understanding the technical considerations and following the guidance on design and detailing you can ensure that clay brick will retain its quality and beauty.
Learning Outcomes
Learning Outcomes
We have looked at the key technical considerations to enable the successful specification and execution of brickwork:
Thank you
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