Bricks are the only man-made building materials that testify to their use since the early human civilization. With their attractive appearances and superior properties such as high compressive strength and durability, excellent fire and weather resistance, good thermal and sound insulation, bricks are widely used for building, civil engineering work, and landscape design.
Mortars have direct influence on the strength, durability and impermeability of brickwork. Mortars also affect the speed of work, hence the laying cost of brickwork. It is therefore important to select and use correctly the right type mortar for any particular application to save labour cost and achieve better quality of brickwork.
Although mortar forms only a small proportion of brickwork as a whole, its functional performance has a significant effect, in particular, on the movement that affects the structure within the wall itself and with the adjacent parts of the buildings. The function of mortar, besides providing the necessary bonding between bricks, should give sufficient flexural tensile strength. In turn, this will help to relieve the stress and reduce the risk of cracking the brickwork. Therefore, strict quality control in batching and mixing of mortar plays a vital in brickwork construction.
A good mortar renders the following properties and functions :-
Good workability to ease laying work and improve quality of the joints
Good water retentivity to prevent excessive suction of water by bricks that have high initial rate of absorption and retain sufficient water required for hydration of cement
Development of suitable early and final strength
Offers water-tightness and durability to a wall
Good adhesion or bond for bricks
1. Types of Mortar
Mortar Grade
Cement : Sand
1 : 3
Cement : Lime : Sand
Masonry Cement : Sand
Cement : Sand & Plasticizer
Typical Compressive Strength, N/mm2
7 days
8 days
1
-
1 : 1/4 : 3
-
-
11.0
16.0
2
-
1 : 1/2 : 4½
1 : 3
1 : 4
5.5
8.0
3
-
1 : 1 : 5~6
1 : 4½
1 : 5~6
2.5
4.0
4
-
1 : 2 : 8~9
1 : 6
1 : 7~8
1.0
1.5
5
-
1 : 3 : 10~13
1 : 7
1 : 8
-
-
Note 1: "Lime" refers to non-hydraulic or semi-hydraulic lime
Note 2: Mixes are measured by ratio in volume
2. Selection fo Mortars
Types of Brickwork
Exposure Conditions
Mortar Designation
External Wall
All
1
Retaining Wall
All
1, 2, or 3
Parapet, Free Standing Wall, or
Below Damp-Proof Course
Severe
3
Between eaves and damp-proof course
Sheltered or Moderate
3 or 4
Internal Wall and Partitions
-
4 or 5
3. Workmanship Factors That Affect Quality of Brickwork
In practice, brickwork is liable to suffer certain degree of defects due to the following human factors:-
Incorrect selection of mortar: As a general rule for brickwork, do not use mortar with strength higher than necessary. Investigations show that compressive strength is not a crucial factor in the selection of correct mortar. Instead, the workability, water retentivity and bonding are the more important factors to consider.
Incorrect cement to water ratio of mortar mix: Workers at job site tend to mix mortar with a highr cement to water ratio in order to ease work. This may have an adverse effect on the final strength and drying shrinkage of mortar. The use of right cement water ratio required for optimizing the degree of cement hydration will thus enhance the strength and impermeability of mortar.
Incorrect batching and mixing of mortar: Too often mortar ingredients are not batched properly and not mixed uniformly at job site. This will eventually affect the workability and quality of mortar.
Incorrect jointing procedure: The most commonly seen defects of brickwork arise from incomplete or excessive thick filling of joints. Thick bed joint of 16-19 mm may reduce the strength of brickwork by 30% as compared to the normal 10 mm thickness.
Disturbance of bricks after laying: Any disturbance of bricks after laying for a while will result in breaking the bond between bricks and mortar and give rise to possible adverse effect on the strength of brickwork.
Failure to protect work from weather: Newly completed brickwork can be adversely affected by exposure to unfavorable weather conditions such as curing under hot sun and damage by rain. Polyurethane sheets can serve as a cheap mean for whether protection.
4. Materials
4.1 Cement
Portland Cement
There are five different types of Portland cement :-
Type A - ordinary Portland cement
Type B - fast setting Portland cement
Type C - low heat Portland cement
Type D - sulphate resistant Portland cement
White Portland cement
The strength and durability of mortar depend to a great extent on the type and quality of cement used. The choice of cement is based on the usage requirements and conditions of use. Type A cement is adequate for normal usage. In cases where brickwork is exposed to severe contaminated conditions or under seawater where brickwork is liable to salt attack, Type D sulphate resistant Portland cement is preferred. White Portland cement is only used for making white mortar.
Masonry Cement
Masonry cement is premixed combinations of Portland cement and other fillings and plasticizing compounds. They are intended to replaced the cement and lime in the conventional mortar. The composition of masonry cement varies with the manufacturer.
4.2 Lime
The function of lime is to give good workability and reduce cracks when used correctly. Only hydrated lime is used in masonry mortar.
4.3 Sand
The sand used for mortar must be clean, free from salt and organic contents. Most natural sand contains a small quantity of clay, which will improve the mortar workability. However, excessive clay content causes high drying shrinkage, low compressive strength and poor bandage in brickwork. Practical limit of clay content in sand should not exceed 10%
The grain size of sand has great effects on the properties of mortar. BS 1200 prescribes the limits of particle size distribution. In practice, a 50-50 mix of coarse sand (mean value of particle size at 600 microns) and fine sand (mean value of particle size at 300 microns) gives satisfactory result. Owing to the variation of natural sand and its property, local experience and experiment to optimize the property of mortar is necessary.
4.4 Admixtures
Admixtures are used in mortar to achieve the following purposes :-
To improve workability
To modify setting time
To improve water retentivity
To improve water-proofing property
There are many commercial cement admixtures that claim to achieve the above improvements. These admixtures contain synthesized polymers that produce amazing improvement in the properties of cement mortar when added in a small quantity. However, these admixtures should be used with care and to avoid negative results due to overdosing and overmixing of mortar.
