Mortar mix ratio i.e. proportioning of cement sand in mortar provides consistency in the performance and appearance of masonry construction. Proper proportioning of mortar ingredients helps in having the following advantages:
- Uniformity of strength
- Uniform workability
- Uniform color
- Uniformity of proportions and yields
Mainly the proportioning of the cement and sand for mortar are conducted by volume proportioning compared to weight proportioning.
The table-1 below shows the amount of lime, sand and ordinary Portland cement for different mortar type as per ASTM C270 – Standard Specification for Mortar for Unit Masonry.
Table: 1: Mix Proportions of Mortar as per ASTM C270.
|Mortar Type||Proportions by Volume|
Considerations for Mortar Mix Proportion
Special Care must be taken during the measurement of sand, as variations occur with the presence of moisture content in it. The moisture present in the sand will result in the bulking of the sand.
Compared with dry sand, the wet sand will have more volume which will bring error in measurement. This change in mixture amount will affect the strength and the bond characteristics of the mortar.
A mortar mix with sand in large amount (more than required) will result in a harsh and an unworkable mix creating a weak bond. These types of mortar will perform badly during freeze and thaw conditions.
It is a good practice to check the volume measurement twice a day, where the batching of a sand is done by volume in a box that is made of plywood or a lumber. The person in charge of this process can record on how many shovels of sand will fill the box. Any change in volume of the sand, therefore, is easily understood.
Workability of mortar, when compared with concrete, is high. This is because the masonry units absorb some amount of water, this will reduce the water-cement ratio in the mortar mix that will affect the strength and the bond.
Therefore, it is recommended to dip the masonry units in water for some minutes before the masonry construction. Provision of excess water will meet this need for absorption.
Mortar mix is not designated by slump or water-cement ratio. It is the optimum moisture content determined by the mason. Mortar that is too dry won’t spread properly which will result in poor bond and incomplete hydration of the cement. Too wet mortar will settle quickly and not easy to trowel.
Features of a Good Mortar Mix
A good mortar mix must have the following features:
- Must possess good workability
- Mortar have to spread easily
- Mortar must easily extrude into the joints
- It must stick to the vertical surfaces easily
- Permit easy positioning of the unit to line, plumb and the level
Types of Mortar Mix Proportions
Mainly there are three types of mortar mixes used in the masonry construction. They are the:
- Cement – Lime mortar mix
- Masonry Cements mortar mix
- Mortar Cements mix
Cement Lime Mortar Mix
This mortar mix is produced by blending lime-sand mortar with ordinary Portland cement. This mix will gain a well uniform physical property. These mortar mix have high excellent workability, high capability of water retention, increased setting time and provision of additional strength.
Masonry Cement Mortar
This mortar mix was developed to reduce the mortar mixing process. This is produced by blending the masonry cement and sand. The ingredients in the mix will differ depending on the manufacturer.
The constituents of masonry cement are:
- For higher strength and increased setting time, Portland cement is used
- For increasing workability, the plasticizers are used
- For higher durability and workability, air-entraining additives are used
Mortar Cement Mix
This mortar mix is the next generation masonry cement. The mixture of mortar cement is similar to masonry cement. The only difference is that the mix is prepared and optimized to reduce the air content in it.
Masonry cements are prepacked and they are mixed with water and sand at the construction site. These provide increase properties and greater flexural bond strengths.