The composition of mortar is very often given insufficient consideration and in many instances it is left to site operatives to decide on the specification. Mortar forms an integral part of the wall, comprising between 6 and 18% of the total wall volume when general purpose mortar is being used. It is important, therefore, that the appropriate specification is determined at the design stage.
General purpose mortar
The nominal thickness of general purpose mortar, in both horizontal and vertical joints, is normally taken to be 10mm, exclusive of any key in the jointing surface of the masonry units.
Specification of general purpose mortar When specifying general purpose mortar, the following details of the project should be taken into account:
- The type of masonry unit to be used
- The structural requirements
- The degree of exposure of the site
- The level of workability required
- The location of the masonry, i.e. above or below ground level
When selecting a suitable mortar specification, it is important to ensure that the composition is compatible in strength with the blocks selected for the project.
Mortars are defined by designations (i) to (iv) in accordance with Table 13 of BS 5628: Part 3. Based on this table the following general use mortars are recommended for use with Hanson’s aggregate blocks:
Mortar designation (iii) for internal and external walls above d.p.c. (all proportions by volume)
| 1 : 1 : 6 |
cement : lime : sand |
| 1 : 6 |
cement : sand with a plasticiser |
| 1 : 5 |
masonry cement : sand |
Mortar designation (ii) for use below d.p.c. (all proportions by volume)
| 1 : 4 |
cement : sand |
| 1 : 1/2 : 4 |
cement : lime : sand |
These mortar specifications are suitable for use with all products in the aggregate block range and may also be used with other types of masonry. They may be used throughout the United Kingdom for the construction of internal and external loadbearing and non-loadbearing walls.
The use of mortar mixes other than those listed above may cause difficulties during construction and may lead to defects after completion.
Application notes for general purpose mortar Cement-rich mortar specifications are non-resilient and subject to high shrinkage. They are, therefore, less able to accommodate movement and may cause cracking in completed walls.
It is important that the sand should be well graded and comply with BS EN 13139, and that the water should come from a clean supply. The addition of either lime or an air entraining plasticiser to a lean mortar mix will improve frost resistance and greatly improve workability, thereby increasing productivity. Care should be taken, however, to avoid misuse of liquid plasticisers, as it is possible to produce a workable mix with an unacceptably high sand : cement ratio. Masonry cements are less prone to misuse than liquid plasticisers.
Cement : lime : sand mortars have better wet adhesion and early strength with both bricks and blocks than mixes incorporating plasticisers or masonry cements, and may have superior bond characteristics. They also allow small amounts of movement without cracking. Additionally, lime tends eventually to seal hairline cracks.
Tests carried out on behalf of the Mortar Producers Association have indicated that walls built with cement : lime : sand mortars have better resistance to driving rain than those built without lime.
Reduced adhesion between the masonry units and the mortar may be caused by inadequate cement content, excessive air content or the use of unspecified admixtures. Satisfactory adhesion can only be achieved by correct mix design and careful raw material selection.
Pre-mixed and ready-to-use general purpose mortars
The use of these mortars can be a means of ensuring consistent quality. Also, where large volumes of mortar are required, the storage of large quantities of raw materials and their potential deterioration can be eliminated.
Cold weather working BS 5628: Part 3 states that masonry should not be built when air temperature is at or below 3°C and falling, or unless it is at least 1°C and rising. Conditions should be regularly monitored and account should be taken of the wind chill factor. The use of covers will protect materials when not for immediate use. Frozen materials must not be used. It is essential to protect newly laid masonry from incidental conditions and should be insulated from frost.