What are the cause of huge amount of cracks in plaster surface? How to repair it?
- Thread starter Rakesh391
- Start date
er kiran b h
Newbie
There are many reasons for cracking in plaster
major is due to differential settlement of footings,
low cement content, high silt content in plaster sand.
major is due to differential settlement of footings,
low cement content, high silt content in plaster sand.
Generally, the most obvious reason for cracks is settlement. Settlement does not only mean that foundation gets settled and it leads to cracks in building. Settlement is a very large term and is not only limited to the foundation. It can also occur because of material you have used for brick masonry. If curing of brick masonry not done proper before plastering, then it also leads to cracks in walls. Every material has specific curing time. Walls should be cured minimum 7-days before any other activity. Areas where temperature is high, need curing of exposed areas directly to sun light after every two hours. Make sure water in binding material only used for bond formation and not become a part of evaporation. If water gets evaporate then ultimately bond formation will become poor and as time passes, material will fail to carry load and starts deterioration of binding material. In such case, particles gets separate and above brick layer settled down and ultimately displace the entire layers above that weak point.
Second most obvious reason is vibration. Sources of vibrations are natural disasters like earth quake, some road construction nears your property premises, installed heavy duty equipment at your home without vibration absorber pads and sometime nail you are fixing with wrong tool.
Third most obvious reason of cracks is atmospheric conditions. Some materials lack properties that able to survive in winter as well as in summers. So what happen is that some materials gets shrink in winter and expand in summers. Sometimes the door or window frames not properly fixed and available room for free movement. In such case, every time door you closed the door have a direct impact on the door jamb. This also propagate cracks in walls.
CRACKS REPAIR
1. Use blade or putty knife to enlarge and undercut opening to provide grip for filler. If filler material used in very small proportion then strong bonding will not form and if it is too much, then also not recommendable. In second option, go for plastering first.
2. Prepare the patching material to fill the crack; mix to a thick, buttery consistency for immediate use. Ready made putty is available almost everywhere. You just need to mix it with water.
3. Dampen the surface with the help of brush or spray to the full depth of the opened crack. Make sure water reaches the far end of crack. The better you dampen the surface the more complete bond will form.
4. Fill the opening completely with patching material (putty) by using flexible putty knife. At the end remove excess and flatten it on the wall with the help of paint scrapper.
5. Smooth the patch surface by using fine grade sandpaper. Make sure it gets level with the surrounding surface. If there is still gap between crack layer and surrounding, then apply second filling coat and let it dry. Once it get dry, again apply sand paper. Dust the surface and observe for any discrepancies.
6. Once repaired patch dried, apply prime coat over the filled-in crack and surface surrounding area. This will hide the repaired area and gives a fine smooth finishing.
7. If you are patching a large crack, then undercut it, remove all the loose plaster and dampen the surface like similar to small crack.
8. Mix a batch of plaster and fill the cavity. Allow a drying time of 24hours. Shrinkage may occur during drying.
9. In case shrinkage occurs, dampen again the patch area and fill cavity with fresh filler. Let dry. Sand smooth and at the end apply prime coat.
Second most obvious reason is vibration. Sources of vibrations are natural disasters like earth quake, some road construction nears your property premises, installed heavy duty equipment at your home without vibration absorber pads and sometime nail you are fixing with wrong tool.
Third most obvious reason of cracks is atmospheric conditions. Some materials lack properties that able to survive in winter as well as in summers. So what happen is that some materials gets shrink in winter and expand in summers. Sometimes the door or window frames not properly fixed and available room for free movement. In such case, every time door you closed the door have a direct impact on the door jamb. This also propagate cracks in walls.
CRACKS REPAIR
1. Use blade or putty knife to enlarge and undercut opening to provide grip for filler. If filler material used in very small proportion then strong bonding will not form and if it is too much, then also not recommendable. In second option, go for plastering first.
2. Prepare the patching material to fill the crack; mix to a thick, buttery consistency for immediate use. Ready made putty is available almost everywhere. You just need to mix it with water.
3. Dampen the surface with the help of brush or spray to the full depth of the opened crack. Make sure water reaches the far end of crack. The better you dampen the surface the more complete bond will form.
4. Fill the opening completely with patching material (putty) by using flexible putty knife. At the end remove excess and flatten it on the wall with the help of paint scrapper.
5. Smooth the patch surface by using fine grade sandpaper. Make sure it gets level with the surrounding surface. If there is still gap between crack layer and surrounding, then apply second filling coat and let it dry. Once it get dry, again apply sand paper. Dust the surface and observe for any discrepancies.
6. Once repaired patch dried, apply prime coat over the filled-in crack and surface surrounding area. This will hide the repaired area and gives a fine smooth finishing.
7. If you are patching a large crack, then undercut it, remove all the loose plaster and dampen the surface like similar to small crack.
8. Mix a batch of plaster and fill the cavity. Allow a drying time of 24hours. Shrinkage may occur during drying.
9. In case shrinkage occurs, dampen again the patch area and fill cavity with fresh filler. Let dry. Sand smooth and at the end apply prime coat.
Dear Rakesh,
Please find below link for download the Technical Research Paper for myself
Modern structures are comparatively tall and slender, have thin walls, are designed for higher stresses and are built at a fast pace. These structures are therefore, more crack-prone as compared with old structures which used to be low, had thick walls, were lightly stressed and were built at a slow pace. Moreover, moisture from rain can easily reach the inside and spoil the finish of a modern building which has thin walls. Thus measures for control of cracks in buildings have assumed much greater importance on account of the present trends in construction.
Cracks in buildings are of common occurrence. A building component develops cracks whenever stress in the component exceeds its strength. Stress in a building component could be caused by externally applied forces, such as dead, live, wind or seismic loads, or foundation settlement or it could be induced internally due to thermal variations, moisture changes, chemical action, etc.
Cracking in concrete is a phenomenon which is recognized world-wide. Some cracks in some situations do no harm and are entirely acceptable. In other concrete, cracks are serious defects, in that they adversely strength, function or appearance. There is considerable attention paid to the problems of cracking but the current reaction to cracking is often dissociated from the significance of the cracks in the situation in which it occurs. This reaction ranges from the extreme of concern about the presence of a single hair cracks to the blase view that cracks are part of the nature of concrete and can be safely ignored wherever they occur and however wide they are.
Some of the main causes held responsible can be listed below as:
1) Permeability of Concrete.
2) Thermal Movement.
3) Corrosion of Reinforcement.
4) Moisture movement.
5) Creep.
6) Poor construction practices.
7) Poor structural design and specifications
8) Poor Maintenance.
9) Movement due to chemical reaction.
10) Indiscriminate additions and alterations.
11) Foundation Settlement.
12) Movement due to Elastic Deformation
13) Bad quality of Materials used.
14) Improper Concrete Mix Proportions
15) Thermal Stresses Generations
16) High Water Cement Ratio
17) Richer Mix
Measures for controlling cracks due to shrinkage
- To avoid cracks in brickwork on account of initial expansion, a minimum period varying from 1 week to 2 weeks is recommended by authorities for storage of bricks after these are removed from Kilns.
- Shrinkage cracks in masonry could be minimized by avoiding use of rich cement mortar in masonry and by delaying plaster work till masonry has dried after proper curing and has undergone most of its initial shrinkage.
- Use of precast tiles in case of terrazzo flooring is an example of this measure. In 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, so as to render the shrinkage cracks imperceptibly small.
- In case of structural concrete, shrinkage cracks are controlled by use of reinforcement, commonly termed as 'temperature reinforcement'. This reinforcement is intended to control shrinkage as well as temperature effect in concrete and is more effective if bars are small in diameter and are thus closely spaced, so that, only thin cracks which are less perceptible, occur.
- To minimize shrinkage cracks in rendering/plastering, mortar for plaster should not be richer than what is necessary from consideration of resistance to abrasion and durability.
- Wherever feasible, provision should be made in the design and construction of structures for unrestrained movement of parts, by introducing movement joints of various types, namely, expansion joints, control joints and slip joints.
- Even when joints for movement are provided in various parts of a structure, some amount of restraint to movement due to bond, friction and shear is unavoidable. Concrete, being strong in compression, can stand expansion but, being weak in tension, it tends to develop cracks due to contraction and shrinkage, unless it is provided with adequate reinforcement for this purpose. . Members in question could thus develop cracks on account of contraction and shrinkage in the latter direction. It is, therefore, necessary to provide some reinforcement called 'temperature reinforcement" in that direction.
- Over flat roof slabs, a layer of some insulating material or some other material having good heat insulation capacity, preferably along with a high reflectivity finish, should be provided so as to reduce heat load on the roof slab.
- In case of massive concrete structures, rise in temperature due to heat of hydration of cement should be controlled.
- Provision of joints in structure.
Though it may not be possible to eliminate cracking altogether, following measures will considerably help in minimization of cracks due to elastic strain, creep and shrinkage:
Use concrete which has low shrinkage and low slump.
Do not adopt a very fast pace of construction.
Do not provide brickwork over a flexural RCC member (beam or slab) before removal of centering, and allow a time interval of at least 2 weeks between removal of centering and construction of partition or panel wall over it.
When brick masonry is to be laid abutting an RCC column, defer brickwork as much as possible.
When RCC and brickwork occur in combination and are to be plastered over, allow sufficient time (at least one month) to RCC and- brickwork to undergo initial shrinkage and creep before taking up plaster work. Also, either provide a groove in the plaster at the junction or fix a 10 cm wide strip of metal mesh or lathing over the junction to act as reinforcement for the plaster. (Central Building Research Institute, 1984)
In case of RCC members which are liable to deflect appreciably under load, for example, cantilevered beams and slabs, removal of centering and imposition of load should be deferred as much as possible (at least one month) so that concrete attains-sufficient strength, before it bears the load.
General measures for chemical attack
In case of structural concrete in foundation, if sulphate content in soil exceeds 0.2 percent or in ground water exceed 300 ppm, use very dense concrete and either increase richness of mix to 1:1/5:3 or use sulphate resisting Portland cement/super-sulphated cement or adopt a combination of the two methods depending upon the sulphate content of the soil.
For superstructure masonry, avoid use of bricks containing too much of soluble sulphates (more than 1 percent in exposed situations, such as parapets, free standing walls and masonry in contact with damp soil as in foundation and retaining walls; and more than 3 percent in case of walls in less exposed locations) and if use of such bricks cannot be avoided, use rich cement mortar (1:1/2:4.5 or 1:1/4 :3) for masonry as well as plaster or use special cements mentioned earlier and take all possible precautions to prevent dampness in masonry.
To prevent cracking due to corrosion in reinforcement and premature deterioration, it is desirable to specify concrete of richer mix (say 1:1/5:3) for thin sections in exposed locations and to take special care about grading, slump, compaction and curing of concrete. (Chand, October 2008) (CBRI, Roorkee)
To Prevent Cracks Due to Moisture Movement
- Select materials having small moisture movement e.g. bricks, lime stones, marble etc.
- Plan for less rich cements content, larger size of aggregates and less water content.
- Porous aggregates (from sand stone, clinker etc.) prone for high shrinkage
- Plan for offsets in walls for length of more than 600 mm
- Use of composite cement-lime mortar of 1:1:6 mix or weaker for plastering work
- Plan for proper expansion/control/slip joints
- For brick work 2weeks time in summer and 3 weeks’ time in winter should be allowed before using from the date of removal from kilns
- Delay plastering work till masonry dried after proper curing
- Proper curing immediately on initial setting brings down drying shrinkage.
- When large spans cannot be avoided, deflection of slabs or beams could be reduced by increasing depth of slabs and beams so as to increase their stiffness.
- Adoption of bearing arrangement and provision of a groove in plaster at the junction of wall and ceiling will be of some help in mitigating the cracks.
- Allow adequate time lag between work of wall masonry and fixing of tiles.
To prevent cracks due to Foundation Movement and Settlement of Soil
- Plan for under-reamed piles in foundation for construction on shrinkable soils
- Plan for plinth protection around the building
- Slip / expansion joints to ensure that new construction is not bonded with the old construction and the two parts (Old and new) are separated right from bottom to the top. When plastering the new work a deep groove should be formed separating the new work from the old.
- For filling deep - say exceeding 1.0m, Soil used for filling should be free from organic matter, brick-bats and debris filling should be done in layers not exceeding 25 cm in thickness and each layer should be watered and well rammed.
- If filling is more than 1 meter in depth, process of flooding and compaction should be carried out after every meter of fill.
To Prevent Cracks Due to Cracking Due to Vegetation
- Do not let trees grow too close to buildings, compound-walls, garden walls, etc., taking extra care if soil under the foundation happens to be shrinkable soil/ clay. If any saplings of trees start growing in fissures of walls, etc. remove them at the earliest opportunity.
- If some large trees exist close to a building and these are not causing any problem, as far as possible, do not disturb these trees if soil under the foundation happens to be shrinkable clay.
- If, from any site intended for new construction, vegetation including trees is removed and the soil is shrinkable clay, do not commence construction activity on that soil until it has undergone expansion after absorbing moisture and has stabilized.
