Getting the right mix - Do’s and Don’ts when using mortar in masonry construction

Masonry is one of the oldest, most established building techniques. Evolutions in materials and engineering has allowed for the construction of even stronger masonry buildings – structures that will last for generations.

Since mortar plays such an important role in masonry construction it is important to understand the requirements surrounding its use.

Here is a quick overview of the general requirements when using mortar in the construction of masonry walls.

General Requirements:

  • Select masonry mortar according to exposure conditions and the specification of the masonry units.
  • DO NOT add calcium chloride, ethylene glycol or admixtures containing these materials to protect against freezing or for any other purpose.
  • DO NOT use air, entraining or other admixtures unless specified by the designer and/or manufacturer recommendations.
  • When mortar strength is required for structural reasons ensure that careful consideration is given to allow for movement.
  • Mortar must not be ‘knocked up’ when it has begun to set.

There are a number of factors that can affect the use of mortar so we have put together a quick guide to the do’s and don’ts of using mortar.

Do’s and Don’ts When Using Mortar

 Cold weather working

 DO:

  • Ensure newly built work is protected from rain and frost with suitable waterproof sheeting.

 DO NOT:

 Mix mortar when the air temperature is at or below 2°C and falling.

  • Use sand or semi-finished mortar containing ice particles.
  • Build masonry when the air temperature is at or below 3°C and falling, or until it is no less than 1°C and rising.
  • Lay mortar on frozen surfaces.
  • Use wet bricks or blocks when there is a danger of freezing.

 Hot weather working

 DO:

  • Make use of the highly absorbent nature of certain clay bricks by wetting them to assist in reducing suction during periods of persistent hot weather.

 DO NOT:

  • Use this ‘wetting’ method on any other type of brick, nor on completed walls. 

 Site batched mortar

 DO:

  • Use a carefully selected prescription mix and a suitable mechanical mixer when conducting site batched mortar mixing.

 DO NOT:

  • Rely on volumes by the shovel to give sufficiently consistent mix proportions, particularly materials (e.g. damp sand). This can result in variations between batches which may affect the strength and durability of the mortar.

 Additives

 DO:

  • Add plasticisers and other additives to the mortar mix to improve workability if needed. Only products that have been specifically designed as a mortar additive should be used.

 DO NOT:

  • Use washing up liquid or similar products.

 Pre-mixed mortar

 DO:

  • Use pre-mixed or factory made mortar to ensure consistency throughout the build element. It may be produced as complete or semi-finished mortar.

 Table of mortar mixes

 DO:

  • Follow the table below to work out the recommended mortar mixes for clay bricks:

Mortar Mix in Masonry Table

Mortar Mix in Masonry

 Recessed Mortar Joints

 DO NOT:

  • Use recessed mortar joints for cavity walls with full fill cavity insulation.

 

For more information on building with bricks and mortar check out how to ‘make walls that are built to last with our guide to laying bricks and blocks’.

Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty Technical Manual.

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