by Dr Gerard Lynch
It has long been recognised, within the bricklayer’s craft, the benefits of pre-wetting, or dampening, absorbent bricks before laying, as advocated by J. Moxon in his ‘Mechanik Exercises or the Doctrine of Handy-Works applied to the Art of Bricklaying’ (1703, 259), who states:
‘If you lay bricks in hot dry weather, and be it some small piece of work that you would have very strong, dip every brick you lay, all over in a pale of water, which will make the wall much stronger than if the bricks were laid dry…’
Various types of bricks
We must, however, be mindful of the nature of historic bricks up until the first quarter of the nineteenth century. These were generally made from the topmost, younger geological brickearth and clays that were hand-moulded and fired – often using wood as the fuel – at relatively low temperatures; that typically averaged 850 to 950°C. After this time the Industrial Revolution dramatically impacted on brickmaking and brick types. Steam-powered machines excavated to greater depths than possible by hand the older geological materials of harder Shale, Marl, as well as Boulder and Lias clays; and, where necessary, utilised mechanical pumps to keep out invasive groundwater. A variety of patented machines were developed that could either press or extrude and wirecut bricks to the desired size and shape, and the vast majority of these were fired with industrial grades of coal within increasingly sophisticated kilns that produced harder bricks, of greater density and lower water absorption than ever seen previously; and these characteristics are typical of the majority of bricks manufactured in the UK today.
Porosity and water absorption of bricks
BS EN 771-1 requires the average of ten sample bricks to be tested when checking for water absorbency, and the brick manufacturer should declare it. Under the old masonry structural code BS 5628: Part 1: 1992 (replaced with EC6 and PAS 6697) the categories for Water Absorption - expressed as a percentage increase in weight - are given as:
- Less than 7%
- 7% to 12%
- Greater than 12%
The majority of dense wirecuts fall into this first category, whilst typical handmade and stock bricks are within the third, as would the vast majority of handmade historic bricks; many of which are often found to have an average porosity value of around 35%. It is important not to confuse ‘porosity’ with ‘permeability’, as they are not the same. Porosity is a measure of the available pore space within a brick. Permeability, however, is a measure of the extent to which air, water, or other fluid can pass through a brick, and depends on the pore structure and degree to which these pores constitute a means of transporting from the face to the rear of the brick. A brick can be highly porous, yet impermeable, because if its pores are not interconnected then no water falling on its face can pass through them to the back.
Why wet bricks prior to laying them?
With highly porous bricks, of greater than 12% water absorption, there is a danger that they might rapidly absorb moisture from the bedding mortar (particularly in warm weather) causing it to stiffen quickly. This would result in it losing the all-important characteristic of plasticity that would inhibit correct and accurate positioning to line and face-plane and the provision of a secure bedding, leading to poor adhesion with attendant negative consequences on aspects of compressive and flexural strengths of the overall walling. Further, there is the additional problem arising from the detrimental effects of the loss of moisture out of the mortar into the porous bricks that was necessary to successfully complete the full chemical setting action of its hydraulic lime or OPC binder, resulting in a final mortar performance much weaker than specified.
With some porous bricks this problem can be overcome by having a slight increase in the moisture content of the bedding mortar that is agreed with the designer. A more favoured procedure specified on good sites, particularly with highly porous bricks, is to dampen, or wet, the bricks in clean, ‘potable’, water when laying them. On small areas of brickwork, ‘docking’ individual bricks into an adjacent container of water ready for laying into position, can be adopted, but it is essential to wear rubber gloves to prevent the skin softening from repeated immersion and becoming sore and even cracked from recurrent chaffing against the surfaces of the dampened bricks. Better practice is to employ a hosepipe to evenly spray a stack of bricks, continually removing the sufficiently dampened topmost bricks to allow the water to be equally distributed and absorbed throughout the entire stack and prevent areas of saturation. Spraying should always commence immediately prior to loading-out for bricklaying, which must commence shortly afterwards, and before they can begin to dry-out.
The amount of water required to sufficiently dampen the bricks and reduce their absorbency to a level ideal for bricklaying comes with experience, but a brick that has been sufficiently dampened should not leave the hand wet when held. There is a big difference between a brick that is ‘soaked’ and one that is ‘saturated’. A brick that has been soaked has a high percentage of moisture content, but retains sufficient available pore space to still provide the all-essential water uptake, or suction, necessary for it to be properly bedded into, and adhering onto, the fresh bedding mortar. A brick that has been saturated, however, has had all available pore space filled with water. In such a case there is no longer an ability for water uptake, with seriously reduced adhesion, or suction, so the brick ‘floats’ on the mortar; and it can even begin to shed its excess moisture into the bedding mortar that can result in it leaking out of the joint and staining the facework immediately below. With some new bricks, a further possible problem is that saturation can liberate any integral soluble salts into solution resulting in aesthetically disfiguring efflorescence as they emerge and crystallise on the face of the bricks.
Should one wet all types of bricks prior to bricklaying?
Not all bricks types require dampening before laying them. The pressed or extruded bricks of low porosity should never be wetted prior to bricklaying as they naturally have a significantly reduced water uptake (and almost zero with a Class A engineering brick) that, if wetted, would result in the brick retaining a thin film of water on all its surfaces and this would cause it to ‘swim’ on the bedding mortar; and that invariably leads to it both sliding out of face line and sinking out of level. In such an instance it is best practice to adjust the water content of the mortar so that it is used as stiff as possible. In this respect, as they have greater plasticity and far more workability with the reduced water content of a stiff mortar, traditional lime-based mortars have the advantage over mortars based on OPC.
Best practice during the winter months
In the winter months the need to dampen porous bricks is no longer necessary as the ambient atmosphere is normally sufficient to raise their moisture content, and then the concern transfers to the need to actually keeping the bricks dry and to protect newly constructed brickwork from the damaging effects of frost.
All photographs by Gerard Lynch. From top:
- Soaking, or 'docking' a highly porous brick that has been cut out from a Victorian wall, in clean potable water prior to re-laying brick into the wall.
- Spraying the indent to ensure that the brickwork does not draw out the moisture from the bedding mortar prior to re-laying the brick.
- The dampened brick bring carefully positioned back into the wall surrounded by fresh mortar on all joints to ensure a secure bedding.
- The re-set brick, with joints finished to match surrounding brickwork.