Causes and Prevention of Building Cracks - Report Example

Causes and Prevention of Building Cracks Name Institution Table of Contents 1.0 Introduction 3 2.0 Classification of Building Cracks 4 3.0 Principal Causes of Cracks 4 3.1 Elastic Deformation 5 3.2 Moisture Change 5 3.2.1 Reversible Movement 6 3.2.2 Initial Shrinkage 6 3.2.3 Plastic Shrinkage 7 3.3 Chemical Reaction 7 3.3.1 Attack by Sulphate 8 3.3.2 Carbonation of Cemented Materials 8 3.3.3 Corrosion of reinforcement in brick and brickwork 8 3.3.4 Alkali Aggregate Reaction 9 3.4 Settlement of Soil and Foundation Movement 9 3.5 Vegetation Growth 10 3.6 Thermal Movement 10 4.0 Prevention of Cracks 11 4.1 Building Design 11 4.2 Practices and Techniques of Construction 12 4.2.1 Filling in Plinth 12 4.2.2 Joint Movements 12 4.2.3 Masonry Work 13 4.2.4 Plastering 13 4.2.5 Construction pace 13 4.3 Environment 13 4.3.1 Construction under Cold Weather 13 4.3.2 Construction under Dry and Hot Weather 14 4.3.3 Vegetation 14 5.0 Conclusion 14 References 15 1.0 Introduction Occurrence of cracks in buildings during construction and after construction is a phenomenon that is very common in concrete structures. A building structure may develop crack when its strength is exceeded by the stress in the building components. Stress in buildings can be because of internal or external forces (Bakhoum, 2010). External forces includes, wind, foundation settlement and seismic loads. Internally applied forces that may cause stress include moisture changes, thermal movements, chemical action or elastic deformation. Cracks in buildings can be either structural or non-structural (Holland, 2012). This report therefore looks into the causes and preventive measures of non- structural cracks in concrete structures. 2.0 Classification of Building Cracks Building cracks can be classified as: Structural cracks: can be as a result of faulty of construction, incorrect design and overloading. These structural cracks may make the building unsafe (Holland, 2012). Non-Structural cracks: Non-structural cracks occurs due to stresses induced in buildings internally. These cracks may indicate that the building is unstable or faulty (Holland, 2012). These cracks may also affect internal finishes in a building thus raising the cost of maintenance, or corroding the reinforcement materials. In the long, these can affect structure stability, for instance a vertical crack in a long compound wall due to thermal movement or shrinkage. Cracks can either be wide crack, medium crack, thin crack and crazing. Cracks in concrete structures can also be narrow at one side and then may become wide with time at another end or may have a uniform width in the whole structure (Holland, 2012). Cracks may also be random, stepped, toothed, map pattern or straight and it may be horizontal, diagonal or vertical (Holland, 2012). This report therefore looks into the causes and preventive measures of non- structural cracks in concrete structures. 3.0 Principal Causes of Cracks Some of the causes of non- structural cracks include: Thermal movement Chemical Reaction Moisture change Settlement of soil and foundation movement Growth of vegetation Elastic deformation 3.1 Elastic Deformation According to Hook’s law building structures undergo elastic deformation as a result of imposed live and dead loads and live loads (Bakhoum, 2010). According to Holland (2012), the strength of deformation relies on magnitude of loading, elastic modulus and the dimension of the structure. Under certain instances, elastic deformation can result into building cracks, for instance: Uneven loading of the walls, which creates excess shear stress on the walls thus causing, cracks on the building. Building side-by-side using materials with two different elastic properties. If this happens, shear stress develops at the interface of the two materials causing cracks at the junction of the two materials When a slab or beam of a large span experiences deflection. If this happens and there were no load to offer support as for roof slab, the end of slab or beam curl resulting into cracks in the building. 3.2 Moisture Change Moisture change can take place in some buildings. Since some materials are porous in nature, for instance timber, mortar, plywood, concrete and burnt clay brick , they are able to absorb atmospheric moisture expand and later shrink after they have dried (Bakhoum, 2010). This process is reversible and is due to decline or increase in pressure in the inter-pore with change in the moisture. The extent of these movements relies on porosity of the building material and the molecular structure. Irreversible movements can also take place in some materials. This happens when the change in their moisture especially after construction or manufacture (Holland, 2012. Some of the materials (Holland, 2012) in which irreversible movements may take place involve shrinkage of lime and cement based materials during their initial drying, that is initial expansion and initial shrinkage. 3.2.1 Reversible Movement Various materials are classified as either moderate movement, small movement or large movement. Some of the materials under small movements include limestone, igneous rocks, and marble and burnt clay bricks (Bakhoum, 2010). Examples of moderate movement materials include sand stones, concrete, lime mortar, cement and sand-lime brick. On the other hand, large movement materials include plywood, timber, cement products, wood, block boards and asbestos cement sheets. 3.2.2 Initial Shrinkage Initial shrinkage takes place in building materials that are lime or cement based for instance mortar, concrete, plaster and masonry units. Initial shrinkage is one of those factors that mainly cause shrinkage on concrete structures (Holland, 2012). The strength of initial shrinkage on cement mortar and cement concrete relies on, content of water; cement content, curing, and chemical composition of the cement, availability of fines on aggregates, maximum grading, quality and size of aggregate and fresh concrete temperature relative to the surroundings humidity (Holland, 2012). 3.2.3 Plastic Shrinkage Plastic shrinkage is usually witnessed when cracks occurs on the surface of a freshly formed concrete. When the rate at which concrete losses water happens to be higher to be higher than the rate at which bleeding action raises water onto the concrete surface, the top layer of the concrete shrinks (Bakhoum, 2010). At this point, the concrete cannot resist any tension since it is in plastic state thus resulting into the development of cracks (Holland, 2012). Cracks due to plastic shrinkage are mainly common in slabs. The rate of plastic shrinkage is a factor of heat exposure due to radiation from the sun, concrete temperature, velocity wind and ambient air’s relative humidity (Holland, 2012). 3.3 Chemical Reaction Chemical reactions can take place in buildings resulting into an increase in materials. This result into creation of internal stress that may cause outward thrust creating cracks in the buildings (Holland, 2012. This may also weaken the materials used in building. Some of the chemical reactions include (Holland, 2012: Carbonation of cemented materials Attack by sulphate Alkali aggregate reactions Corrosion of reinforcement in brickwork and concrete 3.3.1 Attack by Sulphate Soluble sulphates present in clay bricks, ground water and soil may react with hydraulic lime and tri-calcium aluminate within the cement in the presence of moisture forming compounds, which are more complex and larger than the former compounds. This reaction weakens the concrete, masonry, plaster thus creating cracks in the building structures (Holland, 2012). The reaction may take more time before it is felt in the structures, for instance two years. The strength of the sulphate reaction depends on permeability of mortar and concrete, amount of soluble sulphates, elements of tri-calcium-aluminate present in the cement and the time a building remains unattended to. A severe reaction of sulphate structures weakens the foundation of a building (Holland, 2012). 3.3.2 Carbonation of Cemented Materials During hydration of cement in the process of concrete hardening, production of calcium hydroxide takes time. This prevents steel corrosion due to alkaline medium created (Revie, & Uhlig, 2011). After a given period, calcium carbonate is formed as a result of reaction of atmospheric carbon (iv) oxide and calcium hydroxide. Calcium carbonate occupies small space compared to calcium hydroxide therefore causing shrinkage of cracks (Revie, & Uhlig, 2011) This is known as carbonation effect that creates cracks in buildings especially in industrial locality. 3.3.3 Corrosion of reinforcement in brick and brickwork Concrete normally protects steel embedded in it. The ability of concrete to protect steel depends on its electrical resistivity and its alkalinity, depth of concrete cover and concrete quality (Revie, & Uhlig, 2011). However, when corrosion occurs in the steel reinforcement, its volume increases thus creating internal stress within the concrete (Bakhoum, 2010). This stress after some time will create cracks within the building structure, especially on the direction of steel reinforcement. This later dislodges the reinforcement cover from the concrete body thus damaging the building structure. Some of the factors that accelerates corrosion include, concrete permeability, presence of moisture, impurities, improper covering of the reinforcement, soluble sulphates and seawater (Revie, & Uhlig, 2011) 3.3.4 Alkali Aggregate Reaction Ordinarily, cement contains potassium oxide and sodium oxide. These compounds may react with some chemicals in the concrete resulting into expansion, disintegration and cracking of the building structures (Revie, & Uhlig, 2011). These chemicals (Revie, & Uhlig, 2011) may also cause reinforcement corrosion. Cracks due to these chemical reactions takes a long period before it can be noticed and it creates map patterns on the building structures. 3.4 Settlement of Soil and Foundation Movement Foundation movement may take due to various factors thus causing shear cracks in building structures. Some of the causes of foundation movements include (Holland, 2012): Inappropriate designing of the building foundations Various parts of the building structure having unequal bearing pressure High bearing pressure as compared to soil strength Nature of the soil supporting the building especially when it cannot support the building design 3.5 Vegetation Growth Trees located near building can lead to building cracks especially in cases where the tree roots can expand and grow within the foundation. If not removed these roots may spread into the walls of the building and continuously develop thus causing cracks. Some tree roots can cause dehydration of the soil within which the building is located leading to foundation movement due to shrinkage (Holland, 2012). 3.6 Thermal Movement Thermal movement is one of the main causes of building cracks. Most building materials, expand on heating and on cooling, they contract (Holland, 2012). According to Holland, (2012), this heating and cooling creates internal stresses on the concrete structures and may cause cracks in due to shear or tensile strengths. Thermal movement strength relies on: Thermal expansion coefficient Components dimensions Ambient temperature variation Cracks from thermal movement can be due to internal heat or external heat. External heat occurs when there is variation in the ambient temperatures while internal heat takes place when there is heat hydration from the mass concrete especially during construction (Ward-Harvey, 2009). Cracking due to thermal movement usually takes place on the external walls or roofs since they are highly exposed to direct radiation or sunlight (Dutta, 2012). 4.0 Prevention of Cracks Several factors can cause cracks in buildings as discussed above. There are various measures that can be taken to control or stop cracks in various structures (Holland, 2012. These measures can apply to various causes. Some of the measures include: a) Concrete and mortar specifications b) Materials choice c) Building designs i.e. foundation, structural and architectural d) Environment e) Practices and Techniques for construction 4.1 Building Design 4.1.1 Architectural design – some of the architectural factors that may lead to cracks in buildings includes: inclusion of short return walls in external elevation, creation of large windows on the external walls and creation of large span of rooms in buildings (Dutta, 2012). Flush having plastered surface should not be used in windows and doors. To avoid any shrinkage windows or door joints should be tied up with the molding strip (Dutta, 2012). 4.1.2 Structural design – building walls and other structures should have uniform stresses in order to reduce or eliminate shear stress, differential strain and cracking. In order to avoid defection in building structures, stiffness in beams and slabs should be enhanced In structures where joints movements are rigid such as rigid shells and frames, shrinkage and thermal stresses must be taken into consideration during the designing process (Ward-Harvey, 2009). 4.1.3 Foundation design – designers need to avoid differential settlement. Therefore, a foundation design should cater for the bearing pressure, which needs to be uniform (Holland, 2012). In clay soil, foundation movement due to alternate drying and wetting and resultant shrinkage and swelling can be eliminated or reduced by: Having a foundation that is more than 3m deep and using quarry dust, granular soil and moorum to fill foundation trenches. Building by use of under ream piles 4.2 Practices and Techniques of Construction Construction practices and techniques when followed can help reduce or eliminate non- structural cracks in buildings (Bakhoum, 2010). Some of the practices and techniques of construction include: 4.2.1 Filling in Plinth One of the construction practices and techniques is filling in plinth, which requires the use of debris, brickbats and organic matter. Filling in plinth should be compacted and well watered to reduce chances of cracking and subsidence of floors (Ward-Harvey, 2009). 4.2.2 Joint Movements Control joint, expansion and slip joint should be created in a structure in order to help in reducing magnitude of stresses due to of shrinkage and thermal movement (Ward-Harvey, 2009). 4.2.3 Masonry Work When building a structure, builders should ensure masonry work is in the uniform level with other parts of the structure. This will help to reduce settlement of foundation or differential loading thus reducing chances of building cracking (Holland, 2012). 4.2.4 Plastering Plastering of the structure should take place after curing of masonry and allowing it to dry. This would allow initial shrinkage to take place before plaster. In case of concrete background plastering, should take place immediately through application of neat cement slurry on the surface of the concrete in order to enhance its bond (Bakhoum, 2010). 4.2.5 Construction pace The pace of construction should be within the schedules, standards and should be highly regulated. Plastering should be done at an appropriate stage of construction in order to avoid cracks in building structures (Dutta, 2012). 4.3 Environment Some of the environmental consideration during construction includes: 4.3.1 Construction under Cold Weather Construction that has taken place during cold weather is less prone to shrinkage cracking as compared to those that has been done during hot weather (Holland, 2012). This is possible since movement because of thermal expansion materials will take place in an opposite direction due to drying shrinkage. In addition, the concrete has a strong compression thus is able to withstand thermal expansion that could occur (Holland, 2012). However, such structures have weak tension and may develop cracks due to contraction. 4.3.2 Construction under Dry and Hot Weather Construction that has taken place during hot and dry weather is prone to cracks due to high shrinkage. Constructors should therefore avoid construction works during hot and dry weather conditions. Construction that has taken place during dry or hot weather condition dries up quickly after laying and is therefore prone to plastic cracking (Ward-Harvey, 2009). To avoid plastic cracking, one should act appropriately in order to prevent the concrete structure from drying up quickly. 4.3.3 Vegetation Trees should be removed away from building in order to avoid cracks due to vegetation. Some of the measures include: Trees should not be allowed to grow close compound walls or buildings. Any tree branches or roots that grows close to the building or moves towards the building should be removed immediately (Dutta, 2012). 5.0 Conclusion Buildings may develop cracks during construction or after construction. Occurrence of cracks in buildings during construction and after construction is a phenomenon that is very common in concrete structures. A building structure may develop crack when its strength is exceeded by the stress in the building components. Stress in buildings can be because of internal or external forces. External forces includes, wind, foundation settlement and seismic loads. Internally applied forces that may cause stress include moisture changes, thermal movements, chemical action or elastic deformation. It is therefore important to employ measures like building design, practices and techniques of construction and vegetation in order to control building cracks. References Bakhoum, M. M., & International Association for Bridge and Structural Engineering. (2010). Case studies of rehabilitation, repair, retrofitting, and strengthening of structures. Zurich, Switzerland: IABSE. Dutta, S. C., Mukhopadhyay, P., & Energy and Resources Institute. (2012). Improving earthquake and cyclone resistance of structures Holland, M. (2012). Practical Guide to Diagnosing Structural Movement in Buildings. New York, NY: John Wiley & Sons. In Revie, R. W., & In Uhlig, H. H. (2011). Uhlig's corrosion handbook. Ward-Harvey, K. (2009). Fundamental building materials. Boca Raton, Fla: Universal- Publishers. Read More

3.2 Moisture Change Moisture change can take place in some buildings. Since some materials are porous in nature, for instance timber, mortar, plywood, concrete and burnt clay brick , they are able to absorb atmospheric moisture expand and later shrink after they have dried (Bakhoum, 2010). This process is reversible and is due to decline or increase in pressure in the inter-pore with change in the moisture. The extent of these movements relies on porosity of the building material and the molecular structure.

Irreversible movements can also take place in some materials. This happens when the change in their moisture especially after construction or manufacture (Holland, 2012. Some of the materials (Holland, 2012) in which irreversible movements may take place involve shrinkage of lime and cement based materials during their initial drying, that is initial expansion and initial shrinkage. 3.2.1 Reversible Movement Various materials are classified as either moderate movement, small movement or large movement.

Some of the materials under small movements include limestone, igneous rocks, and marble and burnt clay bricks (Bakhoum, 2010). Examples of moderate movement materials include sand stones, concrete, lime mortar, cement and sand-lime brick. On the other hand, large movement materials include plywood, timber, cement products, wood, block boards and asbestos cement sheets. 3.2.2 Initial Shrinkage Initial shrinkage takes place in building materials that are lime or cement based for instance mortar, concrete, plaster and masonry units.

Initial shrinkage is one of those factors that mainly cause shrinkage on concrete structures (Holland, 2012). The strength of initial shrinkage on cement mortar and cement concrete relies on, content of water; cement content, curing, and chemical composition of the cement, availability of fines on aggregates, maximum grading, quality and size of aggregate and fresh concrete temperature relative to the surroundings humidity (Holland, 2012). 3.2.3 Plastic Shrinkage Plastic shrinkage is usually witnessed when cracks occurs on the surface of a freshly formed concrete.

When the rate at which concrete losses water happens to be higher to be higher than the rate at which bleeding action raises water onto the concrete surface, the top layer of the concrete shrinks (Bakhoum, 2010). At this point, the concrete cannot resist any tension since it is in plastic state thus resulting into the development of cracks (Holland, 2012). Cracks due to plastic shrinkage are mainly common in slabs. The rate of plastic shrinkage is a factor of heat exposure due to radiation from the sun, concrete temperature, velocity wind and ambient air’s relative humidity (Holland, 2012). 3.3 Chemical Reaction Chemical reactions can take place in buildings resulting into an increase in materials.

This result into creation of internal stress that may cause outward thrust creating cracks in the buildings (Holland, 2012. This may also weaken the materials used in building. Some of the chemical reactions include (Holland, 2012: Carbonation of cemented materials Attack by sulphate Alkali aggregate reactions Corrosion of reinforcement in brickwork and concrete 3.3.1 Attack by Sulphate Soluble sulphates present in clay bricks, ground water and soil may react with hydraulic lime and tri-calcium aluminate within the cement in the presence of moisture forming compounds, which are more complex and larger than the former compounds.

This reaction weakens the concrete, masonry, plaster thus creating cracks in the building structures (Holland, 2012). The reaction may take more time before it is felt in the structures, for instance two years. The strength of the sulphate reaction depends on permeability of mortar and concrete, amount of soluble sulphates, elements of tri-calcium-aluminate present in the cement and the time a building remains unattended to. A severe reaction of sulphate structures weakens the foundation of a building (Holland, 2012). 3.3.

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