Make sure you use products certified to British Standards: BS 3921 for clay bricks; BS 187 for calcium silicate bricks; and BS 6073-1 for concrete bricks and blocks. European standards will gradually replace national standards with six masonry product standards and more than 20 associated test standards. So far, however, no national product standards have been superseded and all remain current.
Attain optimum durability
To achieve maximum durability, it is essential to anticipate environmental conditions and specify the appropriate masonry – in accordance with its qualities – and mortar combinations.
- Clay bricks are designated by frost-resistance (F = frost-resistant, M = moderately frost-resistant and O = not frost-resistant), soluble salts content (L = low, N = normal) and by a combination of compressive strength and water absorption. The last two properties are principally the concern of engineering and damp-proof course class bricks.
- Calcium silicate bricks are classified by compressive strength (3, 4, 5, 6 and 7, from 20.5 to 48.5 N/mm2).
- Concrete bricks are certified by strength (compressive and tranverse) and drying shrinkage.
- Concrete blocks are certified by strength (compressive and tranverse) and drying shrinkage. Aggregate blocks may need to be made from dense aggregate to a specific British standard. Aerated blocks may be inappropriate for some locations.
- Mortar is designated (i, ii, iii, and iv) according to strength/ability to accommodate movement.
The principal factor to consider is the degree to which the masonry will be subjected to wetting. Chimneys, parapets and work below ground are vulnerable locations, particularly when combined with the risk of freezing. In the standard BS 5628-3: Code of practice for use of masonry: Materials and components, design and workmanship, table 13 gives comprehensive guidance on the use of masonry unit and mortar combinations in locations that are subject to a range of exposure conditions. Remember, the risk of water saturation can be reduced by use of protective detailing, such as projecting roof edges, copings, sills, and the avoidance of recessed mortar joints.
Define allowable tolerances
The standard BS 5628-3 (annex A), reproduces BS 8000-3 Workmanship on building sites – Code of practice for masonry, which sets out what is an allowable deviation for masonry. The values are intended to ensure satisfactory structural performance and do not determine acceptable appearance (for this, see point 4 below), fit of incorporated components or acceptable substrate for finishing assemblies. Particular requirements should be specified separately, supported by setting out information on drawings.
The Brick Development Association and masonry manufacturers produce tables of vertical and horizontal brick dimensions.
Achieve desired appearance
- Masonry units: Units should be assessed for their intended use, such as common or facing work, and the nature of the product, such as engineering and handmade bricks or polished aggregate and aerated concrete blocks. Generally, all products should be free of deep cracks and facing units should not have damaged arrises. BS 5628-3, Annex D provides a procedure to determine the acceptability of facing units delivered to site, using reference and sample panels.
- Masonry work: The BS 5628-3 reference panel system deals with workmanship for aggregate block masonry, but not brick masonry. However, the general procedure can be followed to provide a control mechanism for both. Reference panels used to establish an acceptable level of workmanship should include critical features, such as plinth detail and window opening. Where appropriate, incorporate other components such as windows.
Joints should be provided in masonry to accommodate expansion or contraction caused by changes in temperature or moisture content. Careful consideration needs to be given to joints in fire or acoustically rated walling. BS 5628-3 gives detailed guidance on the design and provision of movement joints.
- Clay masonry: Unrestrained and lightly restrained unreinforced clay walling, such as parapets and non-loadbearing spandrel panels expand at about 1 mm per metre over the life of the building. As a rough guide, the width of the joint in millimetres needs to be 30% more than the numerical distance in metres between joints, but the exact dimension will be determined by the performance of the filler and sealant occupying the joint. Expansion joint spacing in unreinforced walls should never exceed 16 m. Any filling material needs to be capable of being compressed to 50% of its original thickness; fibreboard should not be used. Advice from sealant manufacturers should also be sought.
- Concrete masonry: Should be designed as a series of panels separated by movement joints to accommodate contraction. The usual spacing is between 6 m and 9 m.
Protect against adverse weather
Masonry should only be laid when there is no risk of frost damage to newly laid mortar. Cement-gauged mortar should not be used when the air temperature is at below 3°C and falling unless it is at least 1°C and rising. Hydraulic lime–sand mortars should not be used when at below 5°C and falling unless it is at least 3°C and rising. Newly laid, curing masonry should not be allowed to freeze and should be protected from rain, snow and excessive temperatures or drying winds.
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