Shrinkage due to drying out of moisture content in building materials is one of the main causes of cracks in structures. Cracking due to shrinkage mainly affects the appearance and finish; the structural stability stays intact. Most of the unsightly cracks usually develop in the first dry spell after the construction of a building.
Mechanism of Cracks due to Shrinkage
Tensile stress is induced due to shrinkage in the material when there is a restraint to movement due to expansion or compression. When the induced stress exceeds the strength, cracking occurs, thus relieving the stress.
Cracks in internal walls generally get localized at weak sections, such as window and door openings or staircase walls.
In external walls of buildings, shrinkage cracks generally run downward from window sill to plinth level and from window sill on an upper storey to the lintel of a lower storey.
In this article, we discuss how to control cracks due to shrinkage induced by various factors.
1. Shrinkage Cracks in Masonry
Cracks caused due to shrinkage in masonry walls can be minimized by avoiding the use of rich cement mortar in masonry. It can also be reduced by delaying plasterwork until masonry has dried after proper curing has undergone most of its initial shrinkage.
Masonry work carried out with composite cement-lime-sand mortars (1:1:6, 1:2:9, or 1:3:12), which are weak, will have a lesser tendency to develop cracks. It is due to accommodation of shrinkage in weak mortar of individual masonry unit.
2. Shrinkage Cracks in Concrete Works
Shrinkage cracks in concrete works can be controlled by taking some necessary precautions to limit the drying shrinkage.
Construction based on the use of precast components has a distinct advantage over in-situ concrete jobs since initial shrinkage is allowed without any restraint before incorporating the elements in a building, thus preventing subsequent shrinkage.
The use of precast tiles in terrazzo flooring is an example of shrinkage preventive measures. In the case of in-situ/terrazzo flooring, cracks are controlled by laying the floor in small alternate panels or by introducing strips of glass, aluminum, or some plastic material at close intervals in a grid pattern to render the shrinkage cracks imperceptibly small.
3. Shrinkage Cracks in Structural Concrete
In structural concrete, shrinkage cracks are controlled by the use of reinforcement, commonly termed as ‘temperature reinforcement.’ For plain concrete walls (i.e., walls that are not reinforced to take any forces due to loading), it is recommended a minimum reinforcement of 0.25% to 0.20% in the horizontal direction and 0.15% to 0.12% in the vertical direction when using plain/deformed bars.
This reinforcement is intended to control the shrinkage and temperature effect in concrete. It is more effective if bars are small in diameter and are closely spaced so that the cracks are less noticeable and thin in nature.
For basement floors subject to water pressure, since laying of floors in panels is not feasible as water would seep out from joints, shrinkage cracking in concrete has to be controlled with the help of reinforcement.
General Precautionary Measures to Control Shrinkage Cracks
In a building or a structure as a whole, an effective method of controlling shrinkage cracks is by providing movement joints i.e., expansion, control, and slip joints.
Work carried out in cold weather will be less liable to shrinkage cracking than that done in hot weather since movement due to thermal expansion of materials will be the opposite of drying shrinkage.
For architectural considerations, the external walls of buildings are protected by shrinkage cracks by installing a finish of rich cement-based material such as terrazzo, pebble dash, or artificial stone. In such cases, to avoid shrinkage cracks, the finish is divided into small panels of dimensions varying from 0.5 to 1.0 meters by providing grooves of 8 to 10 mm width in both directions.
To avoid cracking in brick masonry due to initial expansion, burnt clay bricks should be exposed to the atmosphere after unloading from kilns for a minimum period of 2 weeks in summer and three weeks in winter before use.
Some general precautions that must be taken to minimize shrinkage cracks in the case of materials that are commonly used in buildings are summarized in the table below:
|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.|
Due to shrinkage in material, tensile stress is induced when there is a restraint to movement due to expansion or compression. When the induced stress exceeds the strength, cracking occurs, thus relieving the stress.
Cracks caused due to shrinkage in masonry walls can be minimized by avoiding the use of rich cement mortar in masonry and by delaying plasterwork until masonry has dried after proper curing has undergone most of its initial shrinkage.