TheStructuralEngineer24

Technical Guidance Note

Technical

June 2013

Note 27 Level 1

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Guide to masonry

The three forms of masonry are: brick, concrete block and stone. Primarily, brick and block will be covered in this text as they are the most commonly used. Bricks are made from a clay material with, in many cases, the addition of sand to reduce the shrinkage during the drying process. Once the bricks have dried they are ‘fi red’ in a kiln at temperatures in excess of 900ºC.

Concrete blocks are formed from a particular concrete mix that has within it aggregate that is no bigger than 5mm. There is also a large proportion of sand within the mix in order to reduce the amount of shrinkage during the curing process. Lightweight concrete blocks contain aggregate in the form of sand and are made from a mix containing expanded clay, shale or slate. These materials expand during the curing process, resulting in a lighter block that has similar dimensions to medium and/or dense concrete blocks.

Modern bricks (Figure 2) are produced in standard sizes and thus dictate the setting out of elements that interact with them in some way. Traditionally, concrete blocks are 440mm x 215mm. However, modern blocks are available in a wider range of sizes. This

Introduction to masonryIntroduction

The use of masonry dates back to antiquity with evidence of the use of some form of stone masonry originating over 10,000 years ago. Over the past 100 years, however, the use of masonry has taken on a much less prominent role as a structural material. This is due mainly to the adoption of steel and concrete frame construction that creates structures that are much lighter and quicker to construct.

In response to this change, masonry is being used as a form of cladding, at least for concrete and steel framed structures. In the UK, the amount of wind-driven rain has led to the general adoption of cavity wall construction. This means that each leaf will be fairly slender and this needs to be considered in design together with the load path for actions due to wind.

Load bearing structural masonry is of course still in use and is employed in the construction of low rise buildings and soil retaining structures, as it has been for millennia. Its durability is demonstrated amply by the resilience of the Pont du Gard; the Roman aqueduct built entirely from stonemasonry that still exists almost 2,000 years after it was constructed (Figure 1).

W Guide to masonry

W Applied practice

W Further reading

W Web resources

ICON LEGEND

S Figure 2Dimensions for bricks and concrete blocks

N Figure 1Pont du Gard, Vers-Pont-du-Gard, France

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has mainly been driven by the requirement to limit the weight of units that need to be manually handled.

Clay bricks have varying levels of absorption, depending on the way they have been created and the type of mix within them. They are divided into three categories, based on their capacity to absorb moisture:

• Less than 7%• Between 7% and 12%• More than 12%

These have an impact on the fl exural strength of masonry, with the more absorbent bricks being the weakest.

Concrete blocks also come with cavities that are created using void formers that are installed during the curing process. Those with cavities that do not extend throughout the depth of the block are called cellular blocks. With them being much lighter it enables the use of much larger block sizes as they are easier to lift into place. Blocks with voids that extend throughout their depth are called hollow blocks and it is not uncommon to have steel reinforcing bars being threaded through them (Figure 3).

MortarMortar is the glue that binds masonry together and is typically 10mm thick,

will, over time, take up moisture from the atmosphere; causing it to expand. Concrete blocks will, like all concrete, initially shrink as the moisture content reduces to be in equilibrium with the atmosphere. After the initial movement, the moisture content of the masonry will follow the seasons leading to a seasonal variation in size. Both brick and concrete masonry will undergo changes in size due to changes in temperature. The coeffi cient of linear thermal expansion for clay masonry will tend to be slightly lower than for concrete masonry.

For clay bricks, the longest horizontal length of wall that can be constructed without the need of a vertical movement joint is 15m, although typically joints are placed at 12m. Concrete blocks undergo an initial shrinkage and have a slighter higher temperature coeffi cient. Walls constructed from such blocks require vertical movement joints at a maximum spacing of 9m, although 6m is more common. Stone masonry has a far smaller initial movement and therefore only seasonal moisture and thermal movements need to be considered, hence it only requires joints at every 20m.

A vertical movement joint typically consists of a gap measuring 10-20mm with a compressible material within it and a mastic sealant to prevent water ingress (Figure 4). In order for it to be eff ective there is no form of continuity across the joint, hence it is unrestrained.

Joint spacing can be infl uenced by the proportions of the panel and the presence of openings. Both should be considered when determining the position of joints.

BondingMasonry can be laid in a variety of patterns according to the levels of robustness and complexity required. The most common are the Stretcher bond and Flemish bond variants (Figure 5). If a wall is constructed in two layers and the joints are parallel to the face of the wall then this is described

Hollow block

Compressible material

Mastic sealantS Figure 4Vertical expansion

joint in masonry

Cellular block

S Figure 3Blocks with voids

although it is possible to have mortar as thin as 0.5mm. Originally it was a clay based mud that eventually became a lime and sand based mixture that remained in use in some form until the early 20th century, when cement based mortar became prevalent.

The change to cement based mortars occurred because they are less weather dependent during construction than lime based mortars. They also gain early strength rapidly, speeding up the building process. Cement based mortars do not self-heal as well as lime based ones, which means that greater attention needs to be paid to movement. Additionally, cement based mortars will force any moisture in the wall to evaporate from the face of the brick, and not from the mortar, which can lead to damage of the brick surface.

When working on historic structures, it is important to ensure that appropriate mortar is used. In the UK, this may be a requirement of gaining statutory approval from the local authority's Historic Buildings department.

The UK National Annex to BS EN 1996-1-1 Design of Masonry Structures divides mortar into four classes: M2, M4, M6 and M12. The lower the class number, the weaker it is, which is inversely proportionate to its fl exibility. Class M12 mortar has a compressive strength of 12 N/mm2 and is quite brittle when compared to Class M2 mortar, which has a compressive strength of 2 N/mm2, yet is the most fl exible of the cement based mortars. The most commonly used mortar is Class M4 as it off ers suffi cient fl exibility without sacrifi cing too much in the way of compressive strength (4 N/mm2).

MovementMasonry will expand or contract to refl ect changes in both moisture content and temperature. Brick masonry is very dry when it leaves the manufacturer and

"Masonry can be laid in a variety of patterns according to the levels of robustness and complexity required"

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TheStructuralEngineer26

Technical Guidance Note

Technical

June 2013

Note 27 Level 1

›

Eurocode 0.Web resources

Brick Development Association: www.brick.org.uk

Concrete Block Association: www.cba-blocks.org.uk

The Institution of Structural Engineers library: www.istructe.org/resources-centre/library

as a collar jointed wall, which does have an impact on the wall’s strength. Cavity walls are constructed as a single skin with a gap between them. Provided the two walls are connected via wall ties, both leaves can be considered to act compositely.

Exposure conditions and moisture contentThe projected exposure conditions of new masonry play an important part in their durability. Water ingress impacts on the strength of the material and must therefore be considered during the design. BS EN

Table 1: Categories of exposure of masonry

Class Exposure condition Example

MX1 Dry environment

Interior of buildings and external masonry that has

been rendered and is not exposed to driving rain

MX2

Exposed to moisture or wetting but not freeze/thaw

action, especially below ground sulphates and other

aggressive materials

Interior of buildings with high humidity, masonry covered with overhanging roofs and external walls not exposed to driving rain. External walls that are not exposed

to chemicals and with cappings that provides some protection

MX3 Exposed to wetting and freeze/thaw action

Similar to MX2 only with freeze thaw action

MX4 Exposed to salt water and/or saturated salt air

Masonry within coastal areas and buildings close to roads that

are salted during the winter

MX5 Exposed to aggressive chemicals

Masonry subject to aggressive chemicals e.g. below ground masonry,

retaining walls in contact with sulphate bearing soils and masonry

near some industrial buildings

1996-2 Design of Masonry Structures - Design considerations, selection of materials and execution of masonry, allows for this and categorises fi ve exposure conditions: MX1, MX2, MX3, MX4 and MX5. MX1 is the most inert environment whereas MX5 is the most extreme. Table 1 defi nes how these exposure conditions are classifi ed.

Guidance on the specifi cation of masonry for a particular exposure class is given in Table 15 of PD 6697: Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2.

S Figure 5Common bonds in masonry

Stretcher bondFlemish stretcher bond English bond

Eurocode 0.Applied practice

BS EN 1996-1-1 Eurocode 6: Design of Masonry Structures – Part 1-1: General Rules for Buildings

BS EN 1996-1-1 UK National Annex to

Eurocode 6: Design of Masonry Structures – Part 1-1: General Rules for Buildings

BS EN 1996-2: Design of Masonry Structures - Design considerations, selection of materials and execution of masonry

PD 6697: Recommendations for the design of masonry structures to BS EN 1996-1-1 and BS EN 1996-2

Glossary and further reading

Bond – The pattern in which masonry is laid.

Lime mortar – A mix of lime, sand and water that creates a bonding agent between masonry units. In the UK, lime based mortars tend to be used only on conservation type projects due to the limited current experience in the materials and the limited periods during the year when the weather is suitable.

Further Reading The Institution of Structural Engineers (2008) Manual for the design of plain masonry in building structures to Eurocode 6 London: The Institution of Structural Engineers

Morton J. (2011) Designers’ Guide to Eurocode 6: Design of Masonry Structures:EN 1996-1-1 London: Thomas Telford Ltd

The International Masonry Society (1996) Eurocode for Masonry EN 1996-1-1 and EN 1996-2 Guidance and worked examples Surrey, UK: The International Masonry Society

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