Cracks in Building

Building repairs and maintenance services mainly includes works undertaken for maintaining proper condition of buildings, its services and works in ordinary use. The use for which buildings are designed is the main factor in determining the required standard of maintenance.

Excessive building maintenance should be avoided. At the same time, building maintenance should ensure safety to the occupant or the public and should comply with the statutory requirements. The need also depends upon intensity of usage.

Cracks can occur due to chemical reactions in construction materials, changes in temperature and climate, foundation movements and settling of buildings, environmental stresses like nearby trains, earth quakes etc. Faulty design, bad quality materials, wrong method of construction, weather effects and lots of wear and tear can create cracks in walls, floors and ceilings. Here are given various reasons of cracks and their prevention techniques.

Cracks may appear in the walls bricks/block mortar plaster, beam-wall joints, columns –wall joints, slabs, beams, columns, foundations and in many other places. Much as it’s not a simple task, it’s essential you take any cracks seriously and get assessment done in order to specify their significance. Most buildings will develop cracks at some point in time during their service life due to the following causes.

• Poor workmanship – Poor mixing of building materials, like cement sand and aggregate will cause cracks in the walls, beams, slabs etc. Normally poor workmanship is as a result of ignorance, carelessness, negligence, lack of proper supervision and many others or worth still a combination of all these.
• Temperature variations – It’s important to know that most materials tend to expand when subjected to direct sunlight and contract in cooler temperatures this process, and therefore end up causing cracks. The cracks will be severe, especially in thin walls that are exposed to direct sunlight.
• Structural design – Poor structural design and specifications are another cause of the cracks in concrete works. It’s important that the designer considers all the environmental aspects that include soil (Geotechnical) investigations that will enable the designer to come up with a proper design of the foundation.
• Normally, buildings are designed for particular uses, and also to withstand a given measure of weight (loading) for example a building designed as residence will have different structural specifications from the one designed to operate machinery.
• Lack of (or) poor maintenance – It’s always important to take good care of your house, by doing maintenance works after a lapse of certain periods. This will keep the building intact and also extend their life span.
• Natural forces – Earthquakes, tremors like the ones that were experienced in Kampala recently, winds, rains, flooding and many others may cause cracking in the buildings. There are many other causes and types of cracks in buildings therefore always look out for them and take the necessary actions.

Other causes and their prevention

Elastic Deformation – When the walls are unevenly loaded, due to variation in stresses in different parts of wall the cracks are formed in walls. When two materials having wide different elastic properties are built together under the effect of load, different shear stresses in these materials create cracks at the junction. Dead and live loads cause elastic deformation in structural components of a building.

Prevention – Create slip joints under the support of RCC slab on walls. Masonry work on RCC slabs and beams should not be started before drying RCC slab and beam. Provide horizontal movement joints between the top of brick panel and RCC beam/slab.

 Thermal Movement – All materials expand on heat and contract on cool. Thermal movement in components of structure creates cracks due to tensile of shear stresses. It is one of the most potent causes of cracking in buildings and needs attention.

Prevention – Construct joints such as construction joints, expansion joints, control joints and slip joints. The joints should be planned at the time of design and be constructed carefully.

Chemical Reaction – Chemical reactions in building materials increase their volume and internal stress causes cracks. The components of structure also weaken due to chemical reactions. Some common instances of chemical reactions are following.

• Sulphate attack on cement products
• Carbonation in cement based materials
• Corrosion of reinforcement in concrete
• Alkali aggregate reaction

Prevention – Use dense and good quality concrete i.e. richer mix of cement concrete 1:1.5:3 to prevent cracks. Repair corrosive cement concrete surface by ‘guniting’/ injecting technique after removing all loose and damaged concrete and cleaning reinforcement from all rust also.

Shrinkage – Most building materials expend when they absorb moisture from atmosphere and shrink when they are dry. Cement made materials shrink due to drying up of the moisture used in their construction. The factors causing shrinkage in cement concrete and cement mortar and their preventions are following.

Excessive Water – The quantity of water used in the mortar mix can cause shrinkage. Vibrated concrete has less quantity of water and lesser shrinkage than manually compacted concrete.

Prevention – Use minimum quantity of water required for mixing cement concrete or cements mortar according to water cement ratio. Never allow cement concrete work without mechanical mix and vibrator.

Quantity of Cement – As a general rule, the richer the mix is, the greater the shrinkage/drying will be.

Prevention – Do not use excessive cement in the mortar mix.

Un-graded Aggregate – Aggregate can cause shrinkage also. If un-graded and fine material/aggregate is used in cement concrete and cement mortar which requires more water and can cause greater shrinkage.

Prevention – Use largest possible aggregate and ensure good grading of materials. The use of water according to required workability has less shrinkage because of reduction in the porosity of hardened concrete.

Curing – After laying cement concrete mix, the hardening of cement takes place, causes reduction in moisture and creates shrinkage. This causes cracks in concrete work.

Prevention – Proper curing should be started as soon as initial setting has taken place and be continued for at least seven to ten days. When hardening of concrete takes place under moist environment, the shrinkage due to drying is comparatively less.

Excessive Fine materials – Fine materials take more surface area and require more water for mix. The use of excessive fine materials i.e. silts; clay and dust in aggregate create more shrinkage.

Prevention – Do not use fine materials containing silt, clay and dust. Use coarse sand/fine aggregate in cement concrete and cement mortar mix which has silt and clay less than 4%. Use coarse aggregate and fine aggregate after washing to reduce silt contents.

Foundation Movement and Settlement of Soil – Shear cracks occur in buildings when there is large differential settlement of foundation due to any of following causes.

• Unequal bearing pressure under different parts of the structure
• Bearing pressure being in excess of safe bearing strength of the soil
• Low factor of safety in the design of foundations
• Local variation in the nature of supporting soil

Prevention – The design of foundation must be based on sound engineering principles and good practice.

Earth Quake – Crack may occur due to sudden shift in lower layer of the earth. The voids in the earth might have suddenly collapsed and be filled with soil from the above. Many geological events can trigger earth movements but is continuous movement. This results in cracks.

Prevention – Construct the foundation of buildings on firm ground while doing construction. Tie up the building with connecting beams at foundation level, door level and roof level.

Vegetation – The roots of trees located in the vicinity of a wall can create cracks in walls due to growth of roots under foundation. The cracks occur in clay soil due to moisture contained by roots.

Prevention – Do not let trees grow too close to the buildings, compound walls etc. Remove any saplings of trees as soon as possible if they start growing in or near of walls etc.

Repairing Cracks

Cracks must be repaired immediately when observed otherwise the life of structure will reduce causing structural failure. They not only weaken the building but spoil the beauty of walls also. This write up tells you about repair of cracks in plaster, RCC member, Brick walls, Floor and Fine Cracks.

Repair Cracks in Plaster

Two types of cracks are found on surface of plaster i.e. A. Crack with hollow sound in plaster. B. Crack with no hollow sound in plaster. Hollow sound can be checked by striking the plaster.

Repair cracks giving hollow sound

• Remove plaster applying proper cut – Defective plaster should be removed and the edges should be slightly cut in square or rectangular shape to provide a neat joint.
• Prepare the surface thoroughly – Clean the surface with wire brushes and wash it. Wet the surface 24 hours before application of cement mortar.
• Apply fresh cement mortar and finish the surface – Defective portion should be filled in with cement mortar. Keep the surface wet at least for 3 days. When surface is dry, finish it according to adjoining area on the wall.

Repair cracks if it is solid without hollow sound – Clean cracked portion of all loose materials and fill crack seal sealants in cracked portion. Finally finish the surface according to adjoining area.

Repair Cracks on RCC Surface

• Clean cracked portion and apply guniting – The cracked portion should be widen gently and be cleaned of all loose materials. Fill the cracked portion by pressure grouting or ‘guniting’. The material for such grouting can be epoxy or cement (with suitable admixture).
• Apply epoxy and finish the surface – Epoxy has more strength and adhesion than cement and can go into very thin cracks as fine as 0.1 mm. The surface should be finished according to adjacent area on the wall.

The cracks in RCC members/ structural members should be repaired immediately with consultation form structural engineer/professional.

Repair Cracks on face of plaster – When two components having different elastic properties such as RCC slab and brick work abating each other, the crack on face of plaster is formed between these components due to their thermal expansion.

• Remove plaster and old material – The plaster should be removed from cracked portion. The joints should be widened at least 20 mm to 25 mm in thickness. Clean old material by washing and brushing.
• Apply cut and prepare the surface – Apply slight cut on edges in straight line to provide neat joint. Make the surface damp 24 hours before application of new plaster.
• Apply cement mortar – Cement mortar should be applied on defective portion. Make 20 mm wide and 25 mm thick groove at the junction of two materials/components.
• Do curing and finishing – Keep the surface wet at least for 3 days. When surface is dry, finish it according to adjacent area on wall.

Repair Cracks in Masonry Walls

Two types of cracks are seen in masonry walls i.e. Crack in mortar joints of walls and crack in bricks/Diagonal Cracks

Repair cracks in mortar joints of walls

• Remove mortar – The mortar should be removed from cracked joint with the help of iron tools at least up to 25 mm depth. All loose material should be cleaned from cracked portion.
• Wet the surface and apply cement mortar – Cracked portion must be kept moist at least for 24 hours before doing repair. Fill in the joints with cement mortar of coarse sand in 1:4.
• Do curing and finishing – After filling the joints with cement mortar, keep the repaired surface wet at least for 3 days. After drying the surface, finish the repaired area according to the adjoining area.

Repair cracks in bricks/diagonal cracks

Remove broken bricks – Before doing any repair of cracks, make sure that the cracks are stabilized and are not widening further. Remove broken bricks by scratching cement mortar from the joints gently.

• Prepare the surface – All loose material should be cleaned from the portion by brushing and washing. Wet the surface 24 hours before application of new plaster.
• Fix new bricks thoroughly – Lay fresh cement mortar with coarse sand in 1:4 and fix new bricks. Fill all joints around bricks with mortar tightly with the help of iron tools.
• Do curing and finishing – The repaired surface should be kept moist up to 7 days. When surface is dry, it should be finished according to adjoining area.

Repair Cracks in Floor

• Remove loose material and apply mortar – Create V shape groove in cracked portion and remove all loose material by washing and brushing. Fill in the groove with polymer modified mortar (for wide cracks) or with polymer modified cement grout (for thin cracks).
• Remove cracked panel or material – If there are many cracks in the floor, remove cracked panels and old materials.
• Prepare the surface and lay new panel – Clean the panel by washing and brushing. Prepare floor material according to the existing floor and lay new panel.

Repair Hair Cracks in plaster

• Broaden the crack gently – Widen the crack about ¼ inches with chisel and remove all loose plaster from the cracked portion.
• Apply vinyl spackle compound – Use vinyl spackle compound to fill the cracked portion. Press the vinyl spackle into the crack and let it dry. Sand or sponge the surface smooth.
• Seal the patch and finish the surface – Seal the patch with proper priming coat and finish it with proper shade according to the adjoining area.

Repair Fine cracks

Apply surface water proof coating on the entire area having very fine cracks (especially on roof) where filling /sealing of each individual crack is not possible.

Do proper curing of repaired area at least for 3 days.

Crack Prevention

Cracks in building are a common occurrence and it is important to understand the causes and the measures to be taken for their prevention. Materials such as bricks, mortar and concrete, which contain considerable quantity of water at the time of construction, dry out subsequently and undergo major contraction. The measures for prevention of cracks could be broadly grouped under the following main sub heads

• Selection of materials:- Good quality of building materials according to the specification if used that will minimize the cracks in the building.
• Specifications for mortar and concrete:- The ratio of the mortar and concrete should be according to the prescribed design. Care should be taken that the coarse aggregate, fine aggregate should be free from silt suspended impurities and the mix should be homogeneous, well mixed and contain no excessive of water.
• Drawing and Design of the building:- While doing the construction work care should be taken that foundation of the building should not be laid on loose /muddy soil. The mortar, M S steel reinforcement should be according to the design.
• Good Construction practices:- It is necessary that work proceed uniformly in all part of building. The workmanship should be according to the prescribed norms and best practice in the building construction.
• Weather Effect:- The construction work should be avoided in very hot and dry weather, and during very low temperature also.
• In general quality of water, sand ,Bajri , mixing of mortar ,quantity of water in mix , socking of bricks and workmanship have great effect on construction work. If possible the subsequent items of construction should be done after the drying of work previously done.
• Before casting RCC slabs over brick wall , smooth bearing plaster should be done over brick walls coupled with white wash / bitumen coating . over it. It allowed slab to move freely over wall.
• Leave expansion/contraction joints at regular interval specially in long stresses of wall.
• Construction of masonry over the slab should be deferred as much as possible ( at least one month ) so that concrete under goes some drying shrinkage prior to construction of parapet. Mortar of parapet masonry should be rich and a good bond should be ensured between masonry and concrete. Plastering on masonry and RCC work should be deferred as much as possible and made discontinuous at the junction by Providing V groove in plaster. This way the cracks if occur , will get concealed behind the groove and not be conspicuous. In case of iron railing , cracks could be avoided by substituting the brick work with a RCC wall, sporting RCC railing.
• Main and cross walls should be properly bonded by proper toothing.
• Some gap should be left between top of panel wall and bottom of the beam/slab so that beam can deflect freely. This gap can be filled by weak mortar. If lateral support is required to be given to wall beam/slab then some special lateral connections can be made in such a way that free deflection of beam is not hindered in vertical direction.
• Walls should not be built tightly to the columns. For taking lateral supports from column, special connections should be made between wall and column which provide lateral supports to wall but does not tie it too tightly to the column .
• Too much disparity in stress in different walls or parts of a wall should be avoided.
• Take all measures to prevent contact of sulphates with cement or use sulphate resistant cement.