Mortar is a combination of sand, a binding agent like cement or lime and water, used in masonry buildings to bridge the space between building blocks. It is applied in the form of a paste which then hardens and binds the masonry units such as stones, bricks, or concrete used in the construction. This article deals with the importance of construction mortar, its preparation, mortar mixes used for the various work and important points to be remembered regarding the mortar mix.
What is the Importance of Mortar in Construction?
The uses of masonry mortar used in construction are:
Construction mortar is used as a bed to even out the irregularities of the individual masonry units. The more even the individual stone or bricks surface, the thinner will be the mortar bed.
The mortar should provide some adhesiveness between the individual stones or bricks.
The mortar used in construction must transfer the compressive, tensile, and shear stresses between adjacent units and it must be sufficiently durable to continue to do so.
How are the Mortar Mixes Gauged?
The process of preparation of mortar paste is as follows:
1. The masonry mortar should be mixed only on a pucca platform made either with bricks, tin, or steel sheets.
2. The volume of material in each batch should be accurately proportioned.
3. When all the volumes of the materials for the mix have been measured, they are blended dry. For thorough mixing by hand, the mix should be turned over three times dry. Turning overtakes the form of moving all the material to one side with a shovel, moving it back, and then moving it to one side again. The mix should be of uniform color with no lumps or pockets of aggregate or binder at this stage.
4. A hollow like a volcano crater is made at the top of the pile and the water is added.
5. The dry material is then mixed with the water, starting with the inside of the ring. When enough water has been incorporated to make the mortar mix of the right workable consistency, the process of turning over is again performed three times wet.
Important Points to be Considered While Gauging for Obtaining a Good Mortar Mix
Water should not be added until the sand-cement mix is uniform in colour.
If water is added to the sand-cement mix, then it should be consumed within an hour, because the initial setting time of cement is about 30 minutes.
The mortar or the concrete should be mixed only on a steel or timber platform. Otherwise, the cement will be washed away with water.
How Long can the Mortar be Kept Before Using?
Mortar should be used as soon as possible once water is added. In any case, it should not exceed 30 minutes.
Why Should the Cement Mortar be Weaker than the Brick or Stone?
It is a combination of the brick and the cement mortar that is important. For any particular strength of the unit, there is optimum mortar strength. A stronger mortar will not increase the strength of the brickwork.
If the mortar mix is too strong for the stones or bricks, the evaporation of moisture, instead of taking place over the whole wall, including the joints, can only take place through the external face of the brick or stone. The exposed faces will then tend to flake off, through the crystallization of salts, resulting in a wall in which the mortar stands proud.
A weak mortar will retain the cracking within the numerous bed and perpendicular joints between the bricks. A correct relationship between the mortar and the brick will result in the total effects of the movement being distributed amongst numerous fine cracks. Such cracks are largely concealed and can easily be pointed over without becoming unsightly.
Why is Weathering of Mortar Happens Over a Period of Time?
Weathering refers to the cracks or tear and wear that appears in the mortar over time. The reasons for the weathering of mortar over time are:
The scouring actions of wind-borne grit and rain.
The spalling action of salt crystallization because of the use of unsuitable materials such as sea sand etc.
Defects in design details such as inadequate overhangs to copings, absence of drips, etc.
Excessive dampness caused by defective rainwater pipes etc.
Vegetation growth penetrating the mortar joints.
Industrial pollution of the atmosphere by acidic gases carried by rainwater is a very evident cause of deterioration. Sulphur dioxide is very common. It forms sulphuric acid when dissolved in water and causes serious damage, particularly lime mortar and anything containing calcium carbonate, oxide, or hydroxide.
Cement Mortar And Concrete Mixes For Various Works
The cement mortar mixes used for various purposes may vary depending upon the final strength required. The more commonly used mixes are suggested.
1. The minimum recommended as per the ISI specifications is M20 Grade Concrete. You may have to use 1:1.5:3. But a leaner mix than 1:2:4 should not be used for any structural purposes.
2. In practice, more sand will have to be added for bulking of sand, etc.
Points to be Remembered Regarding Mortar Mix
Only the specified mix for each purpose should be used.
More cement does not necessarily give more strength.
The way the mortar is prepared and used determines the final strength achieved.
Construction mortar is one of the ancient building materials, which made it possible to build huge structures with small and easy-to-handle blocks or units. It can offset differences in sizes of masonry blocks to create an aesthetic and structurally solid building. It is normally durable. Various admixtures can also be added in the masonry mortar to increase its properties such as workability and also can be produced in a wide range of colours as per the requirement. It is important to consider the points discussed above in the article during the preparation and application of mortar to obtain a good mortar mix that satisfies the requirements for structurally sound construction.