Shrinkage caused by the drying out of moisture content in building materials is a common culprit behind cracks in structures. Typically, these cracks impact the appearance and finish of the building, but do not compromise its structural stability. The majority of these unsightly cracks tend to emerge during the initial dry spell that occurs after construction is completed.
Mechanism of Cracks due to Shrinkage
Tensile stress can occur in a material when it undergoes shrinkage and is restrained from movement due to expansion or compression. If the induced stress exceeds the strength of the material, it can lead to cracking, which helps to relieve the stress. In internal walls, cracks tend to occur at weak sections such as window and door openings, or staircase walls. On the other hand, in external walls of buildings, shrinkage cracks typically run downward from the window sill to the plinth level, or from the window sill on an upper storey to the lintel of a lower storey. In this article, we will discuss strategies for controlling cracks caused by shrinkage induced by various factors.
1. Shrinkage Cracks in Masonry
One way to minimize cracks caused by shrinkage in masonry walls is to avoid using rich cement mortar in the masonry. Additionally, delaying plasterwork until the masonry has dried after proper curing can also help reduce cracks, as the masonry undergoes most of its initial shrinkage during this period.
Using composite cement-lime-sand mortars with weaker proportions, such as 1:1:6, 1:2:9, or 1:3:12, in masonry work can also help reduce the tendency to develop cracks. This is because weak mortar allows for accommodation of shrinkage in the individual masonry units, helping to minimize cracks.
2. Shrinkage Cracks in Concrete Works
Controlling shrinkage cracks in concrete works requires taking necessary precautions to limit drying shrinkage. One effective approach is to use precast components in construction, which offers distinct advantages over in-situ concrete jobs. With precast components, initial shrinkage can occur without any restraint before incorporating the elements into a building, thereby preventing subsequent shrinkage issues.
An example of shrinkage preventive measures is the use of precast tiles in terrazzo flooring. In contrast to in-situ/terrazzo flooring, where cracks can be problematic, precast tiles can help control shrinkage cracks. This can be achieved by laying the floor in small alternate panels or by introducing strips of glass, aluminum, or plastic material at close intervals in a grid pattern. This approach effectively renders the shrinkage cracks imperceptibly small, minimizing their impact on the overall flooring appearance.
3. Shrinkage Cracks in Structural Concrete
In order to control shrinkage cracks in structural concrete, reinforcement is commonly used, referred to as “temperature reinforcement.” For plain concrete walls that are not designed to take any forces due to loading, it is recommended to use a minimum reinforcement of 0.25% to 0.20% in the horizontal direction and 0.15% to 0.12% in the vertical direction, using plain or deformed bars. This reinforcement is intended to mitigate the effects of shrinkage and temperature changes in the concrete. It is more effective when smaller diameter bars are used and closely spaced, which helps to make any cracks that may occur less noticeable and thin in nature.
For basement floors that are subject to water pressure and where laying floors in panels is not feasible due to the risk of water seepage from joints, reinforcement is necessary to control shrinkage cracking in the concrete.
General Precautionary Measures to Control Shrinkage Cracks
One effective method for controlling shrinkage cracks in buildings or structures is by providing movement joints, such as expansion, control, and slip joints. These joints allow for movement caused by drying shrinkage and thermal expansion, reducing the likelihood of shrinkage cracking. In particular, work carried out in cold weather is less prone to shrinkage cracking compared to hot weather, as the movement due to thermal expansion of materials will be opposite to drying shrinkage.
To protect external walls of buildings from shrinkage cracks, a finish of rich cement-based material such as terrazzo, pebble dash, or artificial stone can be installed. To further minimize shrinkage cracks, the finish can be divided into small panels with dimensions ranging from 0.5 to 1.0 meters, and grooves of 8 to 10 mm width can be provided in both directions.
For brick masonry, precautions should be taken to avoid cracking due to initial expansion. Burnt clay bricks should be exposed to the atmosphere for a minimum period of 2 weeks in summer and 3 weeks in winter after unloading from kilns before use, to allow for acclimatization and reduce the risk of shrinkage cracks.
The table below summarizes some general precautions that can be taken to minimize shrinkage cracks in commonly used building materials:
- Provide movement joints such as expansion, control, and slip joints.
- Carry out work in cold weather if possible.
- Use a finish of rich cement-based material and divide it into small panels with grooves.
- Expose burnt clay bricks to the atmosphere for a minimum period of 2 weeks in summer and 3 weeks in winter before use in brick masonry.
| Sl. No | Material | Precautions |
| 1 | Burnt claybricks, igneous rocks-limestone | Bricks should be well-burnt; do not use very strong mortars and plastering should be done where required, after proper curing and adequate drying of masonry. |
| 2 | Sandstones | When sandstone is used as a construction material with appreciable moisture movement, it is not recommended to use rich cement mortar and control joints to be provided at regular intervals. |
| 3 | Cement concrete and cement mortar | Construction joints in concrete are provided with care where cracks are likely to develop due to shrinkage. |
| 4 | Blocks of normal or lightweight concrete, sand-lime bricks | 1. The blocks should be allowed to mature and dry before use and protected from getting wet at the site due to rain. 2. During work, the blocks should be protected by wet weather. 3. The blocks shall be lightly wetted before use. Avoid using strong and rich mortars for joining blocks. The mortar should have high water retentivity; thus, cement-lime composite mortars should be preferred. 4. For masonry work carried out in summer, use a mortar ratio of 1 cement: 2 lime: 9 sand. For masonry work carried out in cold weather, use a mortar ratio of 1 cement: 1 lime: 6 sand. 5. If the block masonry wall exceeds 6 to 8 m in length, provide control joints at weak sections. 6. Curing of masonry should be done sparingly to avoid the body of the blocks from getting wet. 7. Masonry should be allowed to dry and undergo initial shrinkage before plastering. 8. Avoid excessive wetting of masonry at the time of plastering so that moisture does not reach the body of the blocks. |
| 5 | Wood-wool slabs | Avoid the use of wood-wool material in external panels and internal panels to reduce shrinkage. If required, the wood-wool must be concealed by suitable joint treatment. |
| 6 | Asbestos cement sheets | Both surfaced of asbestos cement sheets to be protected with paint. |
| 5 | Timber | 1. Timber should be seasoned to a moisture content that is appropriate to the conditions at which equilibrium will be reached in the building. 2. As far as possible, door and window frames should not be fitted flush with a wall surface. 3. In joinery work, avoid using planks in panels wider than 25 cm, where unavoidable use of plywood panel or block-board construction for internal work. 4. Protect all woodwork surfaces with paint, enamel, polish or varnish, etc. |
| 8 | Blackboards and plywood | Confine the use of blackboard and plywood to interval locations and dry situations. |
FAQs
How do shrinkage cracks occur?
When materials undergo shrinkage, such as during cooling or compression, they may experience tensile stress when their movement is restrained. This restraint can be caused by factors such as expansion or compression. As a result, the material can be subjected to stress that exceeds its strength, leading to the occurrence of cracks. These cracks act as a mechanism for relieving the built-up stress, allowing the material to release the tension caused by the shrinkage.
How to prevent shrinkage cracks in masonry?
To minimize cracks caused by shrinkage in masonry walls, it is important to avoid using rich cement mortar in masonry. Rich cement mortar, which contains a high proportion of cement, tends to shrink more during the drying process, leading to cracks in the masonry. Instead, it is recommended to use a mortar mix with a lower proportion of cement to reduce the potential for shrinkage.
Another effective strategy to minimize cracks in masonry walls is to delay plasterwork until the masonry has dried after proper curing. During the curing process, masonry undergoes initial shrinkage as the moisture evaporates from the materials. By allowing the masonry to fully dry and undergo most of its initial shrinkage before applying plaster, the risk of cracks forming due to further shrinkage is reduced. This helps to ensure that the plasterwork remains intact and minimizes the occurrence of cracks in the masonry walls.