Dampening bricks before laying versus using dry bricks
Dampening bricks before laying them will give a stronger structure than using dry bricks.
Should you speak to any bricklayer worth their salt, they will tell you about the benefits of dampening any absorbent bricks before laying them!
When they talk about dampening the bricks, they don’t just mean a quick spray, they mean submerging the entire brick in a bucket of water. The practice of doing this will make the building project much stronger than if the bricks were laid dry.
What factors affect the density of a brick?
To determine how dense a brick is and therefore how much water it will absorb, one has to look to the raw material the brick is made from as well as the manufacturing process.
The earlier pre-industrial revolution bricks were made from the topmost brickearth and clays that were hand moulded and fired, frequently using wood as the fuel at a relatively low temperatures. After the Industrial Revolution, there was a major impact on brickmaking and the types of bricks made. This era saw the arrival of steam powered machines that could excavate the clay to greater depths than ever possible by hand. An assortment of machines were developed that could either press or extrude and wirecut bricks to the requisite size and shape, and the majority of these bricks were fired with industrial grades of coal within very sophisticated kilns that produced much harder bricks, of much greater density and lower water absorption than ever before. This is typical of most of the bricks that are manufactured in the United Kingdom today.
How to determine the porosity and water absorption of bricks
We are fortunate in the United Kingdom as we have the British Standards to look to to determine the quality of many products manufactured, the porosity and water absorption of bricks is no exception either. BS EN 771-1 requires an average of ten sample bricks to be tested when checking for water absorbency, and the brick manufacturer should declare it. Under 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 vast majority of dense wirecut bricks tend to fall into the first category of less than 7%, whereas typical handmade and stock bricks are within the third category, along with the vast majority of handmade bricks that were often found to have an average porosity of around 35%.
It should be stressed that there should be no confusion made between brick porosity and brick permeability, as they are different concepts. Porosity is a measure of the available pore space within a brick. Permeability is a measure of the extent to which air and water can pass through the brick, and depends upon the pore structure as a means of transporting the moisture from the face to the rear of the brick. In other words, a brick can be very porous, yet impermeable to water, as its pores are not interconnected, therefore no water falling on its face can pass through them to the back of the brick.
So why should you wet bricks before laying them?
One of the main problems of having a very porous brick is that there is a greater risk that they might rapidly absorb moisture from the bedding mortar, even quicker should the bricklaying be done on a warm day, causing the mortar to stiffen far too quickly. This would result in it losing the vital plasticity, thus preventing correct and accurate positioning of the bricks and the provision of a secure bedding. This leads to poor adhesion and a subsequantly weak walling structure. On top of this there is the additional problem from the loss of moisture out of the mortar into the porous bricks that was necessary to complete the full chemical setting action of the lime or OPC binder, this will also result in a final mortar performance that is far weaker than it should be.
Some bricklayers will use a mortar with a higher moisture content in an attempt to avoid the scenario detailed above. A better solution and one that tends to be adopted on some of the better building sites, particularly when dealing with highly porous bricks, is to dampen, or thoroughly wet the bricks in clean water when laying them. On small areas of brickwork, placing individual bricks into a container of water ready for laying into position can be used.
Bricklayers should be very careful and ensure they wear rubber gloves to prevent their skin from softening from the repeated immersion and becoming sore from recurrent chaffing against the surfaces of the rough and dampened bricks. A far more efficient and safer procedure would be to make use of a hosepipe to evenly wet 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 before the bricks are loaded into the hod for bricklaying, which must commence shortly afterwards, and before the bricks can begin to dry out.
The amount of water one needs to sufficiently dampen the bricks and reduce their absorbency to a level ideal for bricklaying is difficult to gauge and tends to come with years of experience in the trade. A brick that has been properly dampened should not leave the bricklayers hand dripping wet when held.
Soaked v. saturated bricks
The best way to explain just how much water is required can be determined by differentiating between a brick that is soaked and a brick that is saturated. A brick that has been soaked in water will clearly have a high percentage of moisture, but it will retain sufficient pore space to still provide the essential water suction necessary for it to be properly bedded into fresh bedding mortar. A brick that has been saturated has had all available pore space filled with the water. In this case there is no ability for further water uptake, with seriously reduced adhesion, so the brick only floats on the mortar; and it can even begin to shed its excess moisture into the mortar resulting in it leaking out of the joint and staining the facework below. With some newer bricks, a further problem is that saturation can liberate any integral soluble salts into solution resulting in the unsightly efflorescence as they emerge and crystallise on the face of the bricks. This powdery white coating often ruins the whole appearance of a property and can be difficult to remove permanently.
One should remember that not all bricks need to be dampened before laying, in actual fact, pressed or extruded bricks of low porosity should never be dampened before the bricklaying process as they naturally have a reduced water uptake that, if dampened, would result in the brick retaining a thin film of water on all its surfaces and this would cause it to float on the bedding mortar; and this leads to it both sliding out of face line and sinking out of level. In such a case, it is best to adjust the water content of the mortar so that it is used as stiff as possible. With this in mind, as they have greater plasticity and a lot more workability with the reduced water content of a stiff mortar, the more traditional lime based mortars have the advantage over mortars based on OPC.
Remember! The rules change during the winter months.
The only time you don’t really need to worry about wetting bricks before the laying process is in the colder winter months. This is owing to the atmosphere being sufficient to raise the bricks moisture content. Another thing to watch out for in winter is the need to keep porous bricks dry to protect brickwork from the damaging effects of frost.