Definition of Mortar
As per IS 2250-1981 Mortar is a homogeneous mixture, produced by intimately mixing cementitious materials, water and inert materials, such as sand, to the required consistency for use in building together with masonry units.
Desirable Properties
Desirable properties of mortars as per CPWD Specifications 2019 Volume1 for use in masonry are:
- Workability – That property of freshly mixed mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted and finished. It is the amount of energy to overcome friction and cause full consolidation.
- Water Retentivity – The ability of mortar to retain water against suction and evaporation in general is known as water retentivity. It is indirectly a measure of the workability. It is measured by the flow of mortar when tested on a standard flow table before and after application of a specified suction.
- Rate of Stiffing
- Strength – The strength of masonry depends upon both the mortar and the building unit (brick, stone or block). A very strong mortar with weak building units will be of little use. Strong cement mortars should be avoided as long as possible since they are susceptible to shrinkage cracks. On the other hand the use of much weaker mortar is not satisfactory since reduction in cement content leads to less workability, less cohesion and will produce porous joints.
- Resistance to rain penetration
- Durability
Grade of Mortar
The ‘grade’ of a masonry mortar will be defined by its compressive strength in N/mm2 at the age of 28 days. As per CPWD Specifications 2019 Volume1 following are the grades:
| S.No. | Grade of Mortar | Compressive Strength at 28 days in N/mm2 | Cement | Pozzolana (Fly Ash) | Sand |
|---|---|---|---|---|---|
| 1 | MM 0.7 | 0.7 to 1.5 N/ | 1 | - | 8 |
| 2 | MM 0.7 | 0.7 to 1.5 | 1 | 0.4* | 10 |
| 3 | MM 1.5 | 1.5 to 2.0 | 1 | - | 7 |
| 4 | MM 1.5 | 1.5 to 2.0 | 1 | 0.4* | 8.75 |
| 5 | MM 3.0 | 3.0 to 5.0 | 1 | - | 6 |
| 6 | MM 3.0 | 3.0 to 5.0 | 1** | 0.21 | 4.2 |
| 7 | MM 3.0 | 3.0 to 5.0 | 1 | 0.4* | 7.5 |
| 8 | MM 5.0 | 5.0 to 7.5 | 1 | - | 5 |
| 9 | MM 5.0 | 5.0 to 7.5 | 1 | 0.4 | 6.25 |
| 10 | MM 5.0 | 5.0 to 7.5 | 1 | 0.4 | 5 |
| 11 | MM 7.5 | 7.5 & above | 1 | - | 4 |
| 12 | MM 7.5 | 7.5 & above | 1** | 0.2* | 2.1 |
| 13 | MM 7.5 | 7.5 & above | 1 | - | 3 |
| 14 | MM 7.5 | 7.5 & above | 1 | 0.4 | 3.75 |
* – Pozzolona of minimum lime reactivity of 4N/mm2
** – This ratio by volume correspondence approximately to cement pozzolana ratio of 0.8:0.2 by weight. In this case, only ordinary portland cement is to be used (see IS 269). Specifications for ordinary rapid hardening and low heat Portland Cement (Third revision).
References
- IS 2250-1981 Code of Practice for Preparation and Use of Masonry Mortars
- CPWD Specifications 2019 Volume1
- ‘Engineering Materials‘ by Rangwala, Charotar Publications
- ‘Building Materials‘ by S.K.Duggal, New Age International Publishers
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