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Causes of Cracks in concrete structures:
The principal causes of occurrence of cracks in a building are as follows: 1. Permeability of concrete. As deterioration process in concrete begins with penetration of various aggressive agents, low permeability is the key to its durability. Concrete permeability is controlled by factors like watercement ratio, degree of hydration/curing, air voids due to deficient compaction, micro-cracks due to loading and cyclic exposure to thermal variations. The first three are allied to the concrete strength as well. The permeability of cement paste is a function of water-cement ratio given good quality materials, satisfactory proportioning and good construction practice; the permeability of the concrete is a direct function of the porosity and interconnection of pores of the cement paste. 2. Thermal movement: Thermal movement is one of the most potent causes of cracking in buildings. All materials more or less expand on heating and contract on cooling. The thermal movement in a component depends on a number of factors such as temperature variations, dimensions, coefficient of thermal expansion and some other physical properties of materials. The coefficient of thermal expansion of brickwork in the vertical direction is fifty percent greater than that in the horizontal direction, because there is no restraint to movement in the vertical direction. Thermal variations in the internal walls and intermediate floors are not much and thus do not cause cracking. It is mainly the external walls especially thin walls exposed to direct solar radiation and the roof which are subject to substantial thermal variation that are liable to cracking. Remedial Measures: Thermal ts can be avoided by introducing expansion ts, control ts and slip ts. In structures having rigid frames or shell roofs where provision of movement ts is not structurally feasible, thermal stresses have to be taken into in the structural design itself to enable the structure to withstand thermal stresses without developing any undesirable cracks. 3. Creep
Concrete when subjected to sustained loading exhibits a gradual and slow time dependant deformation known as creep. Creep increases with increase in water and cement content, water cement ratio and temperature. It decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading. Use of ixtures and pozzolonas in concrete increases creep. Amount of creep in steel increases with rise in temperature. 4. Corrosion of Reinforcement A properly designed and constructed concrete is initially water-tight and the reinforcement steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density. Concrete also gives steel within it a chemical protection. Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area. Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value. Concrete normally provides excellent protection to reinforcing steel. Notwithstanding this, there are large number of cases in which corrosion of reinforcement has caused damage to concrete structures within a few years from the time of construction. One of the most difficult problems in repairing a reinforced concrete element is to handle corrosion damage. Reinforcement corrosion caused by carbonation is arrested to a great extent through repairs executed in a sound manner. However, the treatment of chloride-induced corrosion is more difficult and more often the problem continues even after extensive repairs have been carried out. It invariably re-occurs in a short period of time. Repairing reinforcement corrosion involves a number of steps, namely,
removal of carbonated concrete, cleaning of reinforcement application of protection coat, making good the reduced steel area, applying bond coat and cover replacement. Each step has to be executed with utmost care. When chlorides are present in concrete, it is extremely difficult to protect reinforcing steel from chloride attack particularly in cases where chlorides have entered through materials used in construction and residing in the hardened concrete. This increase in volume causes high radial bursting stresses around reinforcing bars and result in local radial cracks. These splitting cracks results in the formation of longitudinal cracks parallel to the bar. Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place Remedial Measures: Reinforcement steel in concrete structures plays a very important role as concrete alone is not capable of resisting tensile forces to which it is often subjected. It is therefore important that a good physical and chemical bond must exist between reinforcement steel and concrete surrounding it. Due to inadequacy of structural design and /or construction, moisture and chemicals like chlorides penetrate concrete and attack steel. Steel oxidizes and rust is formed. This results in loss of bond between steel and concrete which ultimately weakens the structure. The best control measure against corrosion is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting. 5. Moisture Movement: Most of the building materials with pores in their structure in the form of intermolecular space expand on absorbing moisture and shrink on drying. These movements are cyclic in nature and are caused by increase or decrease in inter pore pressure with moisture changes. Initial shrinkage occurs in all building materials that are cement/lime based such as concrete, mortar, masonry and plasters. Generally heavy aggregate concrete shows less shrinkage than light weight aggregate concrete. Controlling shrinkage cracks. 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 undergone most of its initial shrinkage. In case of structural concrete shrinkage cracks are controlled by using temperature reinforcement. Plaster with coarse well graded sand or stone chip will suffer less from shrinkage cracks and is preferred for plastering for external face of walls. Considering the building as a whole, an effective method of controlling shrinkage cracks is the provision of movement ts. The work done in cold weather will be less liable to shrinkage cracks than that in hot weather since movement due to thermal expansion of materials will be opposite to that of drying shrinkage. 6. Poor Construction practices.
The construction industry has in general fallen prey to non-technical persons most of whom have little or no knowledge of correct construction practices. There is a general lack of good construction practices either due to ignorance, carelessness, greed or negligence. Or worse still, a combination of all of these. The building or structure during construction is in its formative period like a child in mother’s womb. It is very important that the child’s mother is well nourished and maintains good health during the pregnancy, so that her child is healthily formed. Similarly for a healthy building it is absolutely necessary for the construction agency and the owner to ensure good quality materials selection and good construction practices. All the way to building completion every step must be properly supervised and controlled without cutting corners.
Some of the main causes for poor construction practices and inadequate quality of buildings are given below:
Improper selection of materials. Selection of poor quality cheap materials. Inadequate and improper proportioning of mix constituents of concrete, mortar etc. Inadequate control on various steps of concrete production such as batching, mixing, transporting, placing, finishing and curing Inadequate quality control and supervision causing large voids (honey combs) and cracks resulting in leakages and ultimately causing faster deterioration of concrete. Improper construction ts between subsequent concrete pours or between concrete framework and masonry. Addition of excess water in concrete and mortar mixes. Poor quality of plumbing and sanitation materials and practices.
7. Poor structural design and specifications Very often, the building loses its durability on the blue print itself or at the time of preparation of specifications for concrete materials, concrete and various other related parameters. It is of crucial that the designer and specifier must first consider the environmental conditions existing around the building site. It is also equally important to do geotechnical (soil) investigations to determine the type of foundations, the type of concrete materials to be used in concrete and the grade of concrete depending on chemicals present in ground water and subsoil. It is critical for the structural designer and architect to know whether the agency proposed to carry out the construction has the requisite skills and experience to execute their designs. Often complicated designs with dense reinforcement steel in slender sections result in poor quality construction. In addition, inadequate skills and poor experience of the contractor, ultimately causes deterioration of the building. Closely spaced of reinforcement steel bars due to inadequate detailing and slender concrete shapes causes segregation. If concrete is placed carelessly into the formwork mould, concrete hits the reinforcement steel and segregates causing fine materials to stick to the steel, obstructing its placement and is lost from the concrete mix while the coarse material falls below causing large porosity (honeycombs). Slender structural like canopies (chajjas), fins and parapets often become the first target of aggressive environment because of dense reinforcement, poor detailing, less cover of concrete to the reinforcement steel. Added to all this, low grade of concrete and poor construction practices can make the things worse. It is necessary for the structural consultant to provide adequate reinforcement steel to prevent structural from developing large cracks when loaded. To sum up, the following precautions are required to be taken by the Architects, Structural Consultants and Specifiers:
Proper specification for concrete materials and concrete. Proper specifications to take care of environmental as well as sub – soil conditions. Constructable and adequate structural design. Proper quality and thickness of concrete cover around the reinforcement steel. Planning proper reinforcement layout and detailing the same in slender structures to facilitate proper placing of concrete without segregation. Selection of proper agency to construct their designs.
Architects and Engineers are parents of the buildings they plan and design and therefore their contribution to the health and life of the building is quite significant. Once the plans are drawn the structural designs and specifications are prepared, it is then the turn of the agency to construct the building and bring the blue print to reality. Special care must be taken in the design and detailing of structures and the structure should be inspected continuously during all phases of construction to supplement the careful design and detailing.
8. Poor Maintenance A structure needs to be maintained after a lapse of certain period from its construction completion. Some structures may need a very early look into their deterioration problems, while others can sustain themselves very well for many years depending on the quality of design and construction. Regular external painting of the building to some extent helps in protecting the building against moisture and other chemical attacks. Water-proofing and protective coating on reinforcement steel or concrete are all second line of defence and the success of their protection will greatly depend on the quality of concrete. Leakages should be attended to at the earliest possible before corrosion of steel inside concrete starts and spalling of concrete takes place. Spalled concrete will lose its strength and stiffness, besides; it will increase the rate of corrosion as rusted steel bars are now fully exposed to aggressive environment. It is not only essential to repair the deteriorated concrete but it is equally important to prevent the moisture and aggressive chemicals to enter concrete and prevent further deterioration. 9. Movement due to Chemical reactions. The concrete may crack as a result of expansive reactions between aggregate containing active silica and alkalines derived from cement hydrations. The alkali silica reaction results in the formation of swelling gel, which tends to draw water form other portions of concrete. This causes local expansion results in cracks in the structure. To control Cracks due to alkali-silica reactions, low alkali cement, pozzolona and proper aggregates should be used. 10. Indiscriminate addition and alterations. There have been some building collapses in our country due to indiscriminate additions and alterations done by interior decorators at the instance of their clients. Generally, the first target of modifications is the balcony. Due to the requirement to occupy more floor area, balconies are generally enclosed and modified for different usages. Balconies and canopies are generally cantilever RCC slabs. Due to additional loading they deflect and develop cracks. As the steel reinforcement in these slabs have less concrete cover and the balcony and canopy slab is exposed to more aggressive external environment, corrosion of steel reinforcement takes place and repairs become necessary. The loft tanks are generally installed in toilets or kitchens, which are humid areas of the buildings. The structure in addition to being overloaded is also more prone to corrosion of reinforcement steel in these areas and therefore deteriorates and if not repaired, part of the building can even collapse. EVALUATION OF CRACKS IN CONCRETE TO FIND LOCATION AND EXTENT OF CRACKING When anticipating repair of cracks in concrete, it is important to first evaluate cracks in concrete to identify the location and extent of cracking. It should be determined whether the observed cracks are indicative of current or future structural problems, taking into consideration the present and anticipated future loading conditions.
The cause of the cracking should be established before repairs are specified. Drawings, specifications, and construction and maintenance records should be reviewed. If these documents, along with field observations, do not provide the needed information, a field investigation and structural analysis should be completed before proceeding with repairs. The causes of cracks are discussed here.
A detailed evaluation of observed cracking can determine which of those causes applies in a particular situation. Cracks need to be repaired if they reduce the strength, stiffness, or durability of the structure to an unacceptable level, or if the function of the structure is seriously impaired. In some cases, such as cracking in water-retaining structures, the function of the structure will dictate the need for repair, even if strength, stiffness, or appearance are not significantly affected. Cracks in pavements and slabs-on-grade may require repair to prevent edge spalls, migration of water to the sub grade, or to transmit loads. In addition, repairs that improve the appearance of the surface of a concrete structure may be desired.
Determination of location and extent of Cracks in Concrete Location and extent of cracking, as well as information on the general condition of concrete in a structure, can be determined by both direct and indirect observations, nondestructive and destructive testing, and tests of cores taken from the structure. Information may also be obtained from drawings and construction and maintenance records. Direct and indirect observation of Concrete Cracks The locations and widths of cracks should be noted on a sketch of the structure. A grid marked on the surface of the structure can be useful to accurately locate cracks on the sketch. Crack widths can be measured to an accuracy of about 0.001 in. (0.025 mm) using a crack comparator, which is a small, hand-held microscope with a scale on the lens closest to the surface being viewed (Fig. 1).
Fig.1: Comparator for Measuring Width of Cracks in Concrete Crack movement can be monitored with mechanical movement indicators of the types shown in Fig. 2.2. The indicator, or crack monitor, shown in Fig. 2.2 (a) gives a direct reading of crack displacement and rotation. The indicator in Fig. 2.2 (b) (Stratton et al. 1978) amplifies the crack movement (in this case, 50 times) and indicates the maximum range of movement during the measurement period.
Fig.2: Monitoring Crack Movement in Concrete Sketches can be supplemented by photographs documenting the condition of the structure at the time of investigation. Guidance for making a condition survey of concrete in service is given in ACI 201.1R, ACI 201.3R, ACI 207.3R, ACI 345.1R, and ACI 546.1R. Nondestructive testing of to Determine Concrete Cracks Nondestructive tests can be made to determine the presence of internal cracks and voids and the depth of penetration of cracks visible at the surface. Tapping the surface with a hammer or using a chain drag are simple techniques to identify laminar cracking near the surface. A hollow sound indicates one or more cracks below and parallel to the surface. The presence of reinforcement can be determined using a pachometer (Fig. 3) (Malhotra 1976). A number of pachometers are available that range in capability from merely indicating the
presence of steel to those that may be calibrated to allow the experienced a closer determination of depth and the size of reinforcing steel. In some cases, however, it may be necessary to remove the concrete cover (often by drilling or chipping) to identify the bar sizes or to cerebrate cover measurements, especially in areas of congested reinforcement.
Fig.3: Pachometer – Reinforcement Bar Locator in Concrete If corrosion is a suspected cause of cracking, the easiest approach to investigate for corrosion entails the removal of a portion of the concrete to directly observe the steel. Corrosion potential can be detected by electrical potential measurements using a suitable reference half cell. The most commonly used is a copper-copper sulfate half cell (ASTM C 876; Clear and Hay 1973); its use also requires access to a portion of the reinforcing steel. With properly trained personnel and careful evaluation, it is possible to detect cracks using ultrasonic nondestructive test equipment (ASTM C 597). The most common technique is through-transmission testing using commercially available equipment (Malhotra and Carino 1991; Knab et al. 1983). A mechanical pulse is transmitted to one face of the concrete member and received at the opposite face, as shown Fig. 4. The time taken for the pulse to through the member is measured electronically. If the distance between the transmitting and receiving transducers is known, the pulse velocity can be calculated. When access is not available to opposite faces, transducers may be located on the same face [Fig. 4(a)]. While this technique is possible, the interpretation of results is not straightforward. A significant change in measured pulse velocity can occur if an internal discontinuity results in an increase in path length for the signal. Generally, the higher the pulse velocity, the higher the quality of the concrete. The interpretation of pulse velocity test results is significantly improved with the use of an oscilloscope that
provides a visual representation of the received signal [Fig. 4(b)].
Fig.4 : Ultrasonic Testing of Concrete Cracks Tests on Concrete Cores to Evaluate Cracks in Concrete Significant information can be obtained from cores taken from selected locations within the structure. Cores and core holes afford the opportunity to accurately measure the width and depth of cracks. In addition, an indication of concrete quality can be obtained from compressive strength tests; however, cores that contain cracks should not be used to determine concrete strength. Ultrasonic equipment should be operated by a trained person, and the results should be evaluated cautiously by an experienced person, because moisture, reinforcing steel, and embedded items may affect the results. For example, with fully saturated cracks, ultrasonic testing will generally be ineffective. In some cases, it is difficult to discern between a group of close cracks and a single large crack. An alternative to through-transmission testing is the pulse-echo technique in which a simple transducer is used to send and receive ultrasonic waves. It has been difficult to develop a practical pulse-echo test for concrete. Petrographic examinations of cracked concrete can identify material causes of cracking, such as alkali reactivates, cyclic freezing damage, “D” cracking, expansive aggregate particles, fire-related damage, shrinkage, and corrosion. Petrography can also identify other factors that may be related to cracking such as the water-tocement ratio, relative paste volume, and distribution of concrete components. Petrography can
frequently determine the relative age of cracks and can identify secondary deposits on fracture surfaces, which have an influence on repair schemes. Chemical tests for the presence of excessive chlorides indicate the potential for corrosion of embedded reinforcement. Review of Drawings and Construction Data The original structural design and reinforcement placing or other shop drawings should be reviewed to confirm that the concrete thickness and quality, along with installed reinforcing, meets or exceeds strength and serviceability requirements noted in the governing building code(s). A detailed review of actual applied loading compared to echo technique. design loads should get.
Selection of Repair Procedures of Cracks in Concrete Based on the careful evaluation of the extent and cause of cracking, procedures can be selected to accomplish one or more of the following objectives:
Restore and increase strength Restore and increase stiffness Improve functional performance Provide water tightness Improve appearance of the concrete surface Improve durability Prevent development of corrosive environment at reinforcement
Depending on the nature of the damage, one or more repair methods may be selected For example, tensile strength may be restored across a crack by injecting it with epoxy or other high strength bonding agent. However, it may be necessary to provide additional strength by adding reinforcement or using post-tensioning. Epoxy injection alone can be used to restore flexural stiffness if further cracking is not anticipated (ACI 503R). Cracks causing leaks in water-retaining or other storage structures should be repaired unless the leakage is considered minor or there is an indication that the crack is being sealed by autogenous healing. Repairs to stop leaks may be complicated by a need to make the repairs while the structures are in service. Cosmetic considerations may require the repair of cracks in concrete. However, the crack locations may still be visible and it is likely that some form of coating over the entire surface may be required. To minimize future deterioration due to the corrosion of reinforcement, cracks exposed to a moist or corrosive environment should be sealed. The key methods of crack repair available to accomplish the objectives outlined are described in METHODS OF CONCRETE CRACK REPAIR.
TYPES OF CRACKS IN CONCRETE BEAMS 1. Cracks in concrete beams due to increased shear stress
2. Cracks in concrete beams due to corrosion or insufficient concrete cover
3. Cracks parallel to main steel in case of corrosion in beams
4. Cracks due to increased bending stress in beams
5. Cracks due to compression failure in beams
Methods of Repairing Active Cracks in concrete structures: 1. Drilling and Plugging through Crack: One of the approximate methods would be to drill holes normal to cracks, fill them with a suitable epoxy or epoxy-mortar formulation and then place reinforcement bars (of predetermined sizes and lengths) in them to stitch across the cracks. The bars may be placed in the clean holes prior to filling the epoxy (so as to save loss of epoxy) but then great care is needed not to entrap any air. 2. Stitching of Concrete Cracks: Stitching involves drilling holes on both sides of the crack and grouting in Ushaped metal units with short legs (staples or stitching dogs) that span the crack as shown in figure below:
Stitching should be used when tensile strength has to be restored back across major cracks. Stitching a crack tends to stiffen the structure and the stiffening may increase the overall structural restrain, causing the concrete to crack elsewhere. Therefore, i t is necessary that proper investigation is done and if required, adjacent section or sections are strengthened using technically designed reinforcing methods. Because stresses are often concentrated, using this method in conjunction with other methods maybe necessary. The procedure consists of drilling holes on both sides of the crack, cleaning the holes and anchoring the legs of the staples in the holes, with either a nonshrink cement grout or any epoxy resin-based bonding system. The staples should be variable in length, orientation, or both and they should be located so that the tension transmitted across the crack is not applied to a single plane within the section but is spread over an area. 3. External Prestressing: The flexural cracks in reinforced concrete can be arrested and even corrected by the ‘Post –tensioning’ method. It closes the cracks by providing compression force to compensate for tensions and adds a residual compression force. This method requires anchorage of the tierods (or wires) to the anchoring device (the guide – bracket- angles) attached to the beam (Fig. 2).
Fig. 2: Post Tensioning Cracked Beam The rods or wires are then tensioned by tightening the end-nuts or by turning of turnbuckles in the rods against the anchoring devices. However, it may become necessary in certain critical case to run at least an approximate stress-check to guard against any possible adverse effects. 4. Drilling and Plugging: When cracks run in reasonable straight lines and are accessible at one end, drilling down the length of the crack and grouting it to form a key as shown in Fig. 3 could repair them. Form key with precast concrete or mortar plugs set in bitumen. The bitumen is to break the bond between plugs and hole so that plugs will not be cracked by subsequent movement of the opening. If a particularly good seal is required, drill a second hole and plug with bitumen alone, using the first hole as a key and the second as a seal.
Fig. 3: Drilling and Plugging A hole of 50 to 75mm dia depending on width of crack should be drilled, centered on and following the crack. The hole must be large enough to intersect the crack along its full length and provide enough repair material to structurally take the loads exerted on the key. The drilled hole should then be cleaned, made tight and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. If water tightness is essential and structural load transfer i s not, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. If the key effect is essential, the resilient material can be placed in a second hole, the first being grouted.
TYPES OF CRACKS IN REINFORCED CONCRETE SLABS
1. Cracks in reinforced concrete columns due to Steel Corrosion (cracks parallel to steel bars)
2. Cracks in reinforced concrete columns due to shrinkage of concrete
3. Cracks in reinforced concrete columns due to increased load on the slab
4. Cracks in reinforced concrete columns due to sulphate attack
5. Cracks in reinforced concrete columns due to alkaline aggregates
CAUSES AND TYPES OF CRACKS IN MASONRY BUILDINGS AND THEIR REPAIR METHODS There are various causes for various types of cracks in masonry buildings such as in walls, foundations, slabs, columns. Repair methods of such cracks in masonry buildings is discussed.
There are certain problems in structures that arise suddenly. Some problems like crack formation or settlement of foundation won’t give a caution before it appears. Most of these problems arise due to improper construction method and carelessness during initial construction. So, care during initial stages can help avoid such problems that require huge maintenance.
Causes and Types of Cracks in Masonry Buildings and their Repair Methods The cracks appear in the masonry structure, at a certain period of time. Most commonly caused cracks with their respective causes and precaution, are explained below: Cracks in Brick Mortar ts Vertical or horizontal cracks are seen at the brick mortar ts. One of the main reason is the sulfate attack, that weakens the mortar. These cracks mainly appear after 2 to 3 years of construction. These cracks can be avoided by:
Checking the sulfate content of bricks used in construction The damping of brick wall has to avoided, as these are more prone to sulfate attack when it is damp
Fig.1: Cracks Formed in the Brick Mortar ts Crack Formation Below the Load Bearing Walls Cracks are observed below the load bearing walls, mainly those that s R.C.C slabs. Now the temperature variation makes the reinforced concrete slab to expand or contract, but both in the horizontal direction. These are observed in the Top most story that is more exposed to the temperature changes. There no smooth between the wall and the slab. Hence the frictional forces are developed at the place of the wall and the slab. This creates cracking in the walls. The precaution that can be suggested is to provide a bearing plaster over the brick wall, which helps in having a smooth with the floor over it. If required a bituminous coating can be applied over the plaster applied.
Fig.2: Cracks in Masonry Walls of Multi-Storey Building at Higher Floors
Main Wall and Cross Walls t Cracks Improper bonding between the cross wall and the main wall creates cracks between the ts. This suggests us to have proper and quality bonding between the two walls. These are properly done by toothing.
Fig.3: Shear Cracks between Cross Wall and the Main Long Wall of Masonry Building
Fig.4: Tooth connection between the Walls Cracks Found in R.C.C Columns and Masonry One of the main reason behind this is the differential movement of the columns and the masonry because of temperature variation. This variation can be either expansion or contraction depending upon the temperature. These cracks can be hidden by making a groove in the reinforced concrete column and masonry junction. The provision of chicken wire alternatively at the plaster between the junction of columns and masonry can also help in this variation. The Horizontal cracks between R.C.C slab and the brick parapet The non-projecting slab is mainly subjected to such cracks. This too is due to the temperature variation and the drying shrinkage. Small micro cracks formed he propagated with the increase in expansion or contraction. These cracks can be hidden by making a groove at the masonry junction will help in hiding the cracks. The provision of chicken wire alternatively at the plaster junction can also help. Cracks in Roof Slab The exposure of roof slab to higher temperature variation cause cracks numerously. This can be reduced by providing a weather proof course. New treatment methods and compounds are available as weather course, that is applied over the terrace.
Repair Methods for Cracks in Masonry Building Structural Measures to be followed for already appeared cracks are: 1. Application of grouting or uniting for cracks that are appeared in the main structural , that cannot be compromised at any cost. The material mainly used for this is either cement or
epoxy mixture. The epoxy has the ability to fill even small and thin cracks, say as fine as 0.1mm. These epoxy gain high strength and adhesion. 2. The flexible sealant can be used for cracks that are appeared on the non-structural . This helps in having a control over the differential movement (expansion or contraction) of the member under temperature changes. 3. Epoxy putty, polymer filler or lime cement mortar can be used for filling the cracks seen in plain cement concrete.
Measures for Foundation Settlement The unequal settlement of foundation due to the variation of bearing capacity at different points of the building result in the formation of cracks in the building. The Certain preventive measure is: 1. The foundation is planned to lay or hard soil 2. Gradual raising of foundation and wall has to be made, for letting the structure to have an allowable settlement. 3. The settlement value of should not go beyond allowable, under any combination of loads. 4. The foundation designed should facilitate uniformly distributed pressure on the soil.
Plinth Protection The unequal settlement of plinth is avoided by removing expansive soils like black soils (black cotton soil), nearby plinth. This barrier is kept with the help of sand harries. Providing drains and flagging concrete help in avoiding rainwater away from the plinth. The penetration of roots into the plinth has to avoided. This can be avoided by stopping the construction of trees that has lateral growing roots nearby. METHODS OF TESTING COMPRESSIVE STRENGTH OF MASONRY Testing compressive strength of masonry before construction and for every 464.5 m2 of masonry work during construction is required as per Building Code Requirements and Specification for Masonry Structure (ACI 530.1- 11) and under its quality assurance program, certification of compliance.
There are two ways of specifying compressive strength of clay and concrete masonry which are unit strength methods and masonry prism method. These testing methods are provided by ACI 530.1-11. Either engineers or architects specify one of the two methods. In the case when the method of testing is not determined, the contractor is permitted to select method of determining compressive strength of masonry. Due to costs related to the making prisms and laboratory tests, the prism testing method is more costly compare with the unit strength method. Nevertheless, results of unit strength method are more conservative than prism strength method.
Methods of Testing Compressive Strength of Masonry The testing methods for compressive strength of masonry are:
Unit strength method Prism test method
Unit Strength Method of Testing Compressive Strength of Masonry In unit strength method, masonry units are needed to be tested before and during construction to guarantee their sufficient strength. The values of specified compressive strength of masonry (f’m) depend on not only the compressive strength of masonry units but also on the mortar. Both clay and concrete masonry units should conform to their related ASTM specifications. Clay masonry units need to conform applicable ASTM specifications namely ASTM C 62-05: standard specification for building bricks (solid masonry units made from clay or shale), ASTM C 216-05a: standard specification for facing brick (solid masonry units made from clay or shale), and ASTM C 652-05: standard specification for hollow brick (hollow masonry units made from clay or shale). Clay masonry samples and testing must be as per Test Methods for Sampling and Testing Brick and Structural clay tile. Similarly, concrete masonry unit should conform to the applicable ASTM specifications which are ASTM C55-03: Specification for Concrete Brick and ASTM C90-06: Specification for Load Bearing Concrete Masonry Units in addition to follow either ASTM C 55-03a or ASTM C 90-06 for sampling and testing. In case of grouted masonry, the grout employed for concrete and clay masonry units should conform to ASTM C 476 and grout compressive strength should equal or greater than the compressive strength of masonry and no smaller than 13.79 MPa. Additionally, bed t thickness for both concrete and clay masonry must be equal or smaller than 15.87 mm.
Prism Method of Testing Compressive Strength of Masonry Masonry prisms as shown in Figure-1 and Figure-2 are an assemblage of masonry units, mortar, and grout if applicable which are constructed and tested for their compressive strength as per the ASTM C 1314-03b: Standard Test Method for Constructing and Testing Masonry Prisms which is employed for determine compliance with specified compressive strength of masonry. The ASTM C 1314-03b specification deals with masonry prism construction procedures, testing, and procedures for determining masonry compressive strength (f’m). This test method can check whether masonry materials used produce masonry which meet the specified compressive strength or not. The masonry units utilized for building masonry prisms are an exemplary of those units that will be used construction. Moreover, in the prism test method, minimum three prisms are required to be constructed and the same material should be used. At least two masonry units should be employed to build prisms and they should be tested at the same age based on determined procedure. The aspect ratio or in other word the height to smallest lateral dimension of the prisms (hp/(tp) need to be between 1.3 and 5. All masonry units must be laid in stack bond in stretcher position and oriented as in corresponding construction and prepared with adequate mortar. The length and width of masonry prism is equal to masonry unit length and width. Not only does the mortar and thickness of t but also unit positioning and aligning method while prisms are prepared mortar need to be representative of the construction; the prisms might be constructed as solid or hollow ungrouted or solid or hollow grouted depend on the real situation of the structure. The procedure of grouting, its consolidation, and reconsolidation are required to be similar to the corresponding construction. Two series of prisms are required to be grouted; one group is grouted and another group is ungrouted. In the case of grouting prisms solidly, the timing of grouting prisms needs to be no less than twenty four hours and no more than forty eight hours. Added to that, they should be saved in air proof bags and not be disturbed before forty eight hours after that, the stored prisms have to be kept at 24 oC. Tests of prisms are carried out after 28 day or any other specified period but the prism must have taken out of the airtight backs two days before the test is began. .
Fig.1: Masonry Prisms: (A) Hollow Ungrouted Unit, (B) Hollow Grouted Unit
Fig.2: Solid Unit Prism The compressive strength of masonry is computed in by three steps from prism tests. Firstly, estimate masonry prism strength which is equal to sustained compressive load of prisms by net cross sectional area of that prism. Secondly, calculate masonry compressive strength depend on the aspect ratio (the height to smallest lateral dimension of the prisms) of masonry prisms tests. Thirdly, the compressive strength of masonry is considered to be equal to that of masonry prism provided that the masonry prisms have aspect ratio of 2. When the aspect ratio masonry prism is ranging from 1.3 to 5, the compressive strength of masonry is achieved by correction factor which provided in Table-1 multiply by masonry prism strength. The compressive strength of masonry is computed by averaging the achieved values. Table-1: Aspect Ratio Correction Factors Compressive Strength of Masonry Prism tp
Correction factor
1.3
0.75
1.5
0.86
2
1
2.5
1.04
3
1.07
4
1.15
5
1.22
Interpolation might be employed for obtain correction factor for those aspect ratio that is not included in the table TESTING OF CONCRETE MASONRY BLOCKS FOR COMPRESSIVE STRENGTH AND DENSITY Compressive strength of concrete Blocks or concrete masonry units are required to know the suitability of these in construction works for various purposes.
Concrete masonry blocks are generally made of cement, aggregate and water. Which are usually rectangular and are used in construction of masonry structure. They are available in solid and hollow forms. The nominal dimensions of concrete masonry block vary as follows. Length: 400 or 500 or 600mm Width: 200 or 100mm Width: 50, 75, 100, 150, 200, 250 or 300mm.
Tests on Concrete Masonry Block Units Different tests are conducted on concrete masonry unit to satisfy the all requirements. But now we are discussing about three tests conducted on concrete masonry block which are as follows. Blocks of same mix shall be taken and divide them as follows to conduct the following tests.
Dimension measurement (All blocks) Density of block (3 blocks) Compressive strength of block (8 blocks)
Dimension Measurement All blocks should be checked in this step. The length, width and height are measured with steel scale. If it is a hollow block, then the web thickness and face shell are measured with caliper ruler. And prepare a report of average length, width and height of block and average minimum face shell and minimum web thickness using recorded dimensions.
Density of Concrete Masonry Block As said above, 3 blocks shall be taken to conduct this test. To determine the density of block, first heat the block in the oven to 100oc and then cooled it to room temperature. Now take the dimensions of block and from that find out the volume and weigh the block. The density of block is determined from the below relation and the average density of 3 blocks will be the final block density. Density of block = mass/volume (kg/m3) Density values of different grades of blocks should be as follows. Type of unit
Hollow type unit
Solid type unit
Grade
Density of block (kg/m2)
A(3.5)
>/= 1500
A(4.5)
>/=1500
A(5.5)
>/=1500
A(7.0)
>/=1500
A(8.5)
>/=1500
A(10.0)
>/=1500
A(12.5)
>/=1500
A(15.0)
>/=1500
B(3.5)
1100-1500
B(5.0)
1100-1500
C(5.0)
>/=1800
C(4.0)
>/=1800
Compressive Strength Tests on Concrete Masonry Blocks Eight blocks are taken to determine the average compressive strength of concrete masonry block. The blocks should be tested with in 3days after collected in lab. The age of each block shall be 28 days. The compressive strength testing machine consist of two steel bearing blocks, one is in rigid position on which the masonry unit is placed and another one is movable which transmit the load to the masonry unit when applied.
If the bearing area of masonry unit is more than the bearing area of steel blocks, then separate steel plates are used. The plates are arranged on steel blocks in such a way that the centroid of masonry unit coincide with the center of thrust of blocks.
Bearing area of concrete masonry units are capped with the Sulphur and granular materials coating or gypsum plaster capping. After placing the unit in testing machine, one-half of the expected maximum load is applied at a constant rate, and the remaining load is applied in not less than 2 minutes. Note down the load at which masonry unit fails and the maximum load divided by gross sectional area of unit will give the compressive strength of block. Similarly, test the remaining 7 blocks and the average of 8 blocks strength is the final compressive strength of concrete masonry unit. Below table represents the value of minimum average compressive strength of individual units. Type of unit
Grade
Min average compressive strength of individual units (N/mm2)
Hollow type concrete masonry unit
A(3.5)
2.8
A(4.5)
3.6
A(5.5)
4.4
A(7.0)
5.6
A(8.5)
7.0
A(10.0)
8.0
A(12.5)
10.0
Solid type unit
A(15.0)
12.0
B(3.5)
2.8
B(5.0)
4.0
C(5.0)
4.0
C(4.0)
3.2
The principal causes of occurrence of cracks in a building are as follows: 1. Permeability of concrete. As deterioration process in concrete begins with penetration of various aggressive agents, low permeability is the key to its durability. Concrete permeability is controlled by factors like watercement ratio, degree of hydration/curing, air voids due to deficient compaction, micro-cracks due to loading and cyclic exposure to thermal variations. The first three are allied to the concrete strength as well. The permeability of cement paste is a function of water-cement ratio given good quality materials, satisfactory proportioning and good construction practice; the permeability of the concrete is a direct function of the porosity and interconnection of pores of the cement paste. 2. Thermal movement: Thermal movement is one of the most potent causes of cracking in buildings. All materials more or less expand on heating and contract on cooling. The thermal movement in a component depends on a number of factors such as temperature variations, dimensions, coefficient of thermal expansion and some other physical properties of materials. The coefficient of thermal expansion of brickwork in the vertical direction is fifty percent greater than that in the horizontal direction, because there is no restraint to movement in the vertical direction. Thermal variations in the internal walls and intermediate floors are not much and thus do not cause cracking. It is mainly the external walls especially thin walls exposed to direct solar radiation and the roof which are subject to substantial thermal variation that are liable to cracking. Remedial Measures: Thermal ts can be avoided by introducing expansion ts, control ts and slip ts. In structures having rigid frames or shell roofs where provision of movement ts is not structurally feasible, thermal stresses have to be taken into in the structural design itself to enable the structure to withstand thermal stresses without developing any undesirable cracks. 3. Creep
Concrete when subjected to sustained loading exhibits a gradual and slow time dependant deformation known as creep. Creep increases with increase in water and cement content, water cement ratio and temperature. It decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading. Use of ixtures and pozzolonas in concrete increases creep. Amount of creep in steel increases with rise in temperature. 4. Corrosion of Reinforcement A properly designed and constructed concrete is initially water-tight and the reinforcement steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density. Concrete also gives steel within it a chemical protection. Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area. Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value. Concrete normally provides excellent protection to reinforcing steel. Notwithstanding this, there are large number of cases in which corrosion of reinforcement has caused damage to concrete structures within a few years from the time of construction. One of the most difficult problems in repairing a reinforced concrete element is to handle corrosion damage. Reinforcement corrosion caused by carbonation is arrested to a great extent through repairs executed in a sound manner. However, the treatment of chloride-induced corrosion is more difficult and more often the problem continues even after extensive repairs have been carried out. It invariably re-occurs in a short period of time. Repairing reinforcement corrosion involves a number of steps, namely,
removal of carbonated concrete, cleaning of reinforcement application of protection coat, making good the reduced steel area, applying bond coat and cover replacement. Each step has to be executed with utmost care. When chlorides are present in concrete, it is extremely difficult to protect reinforcing steel from chloride attack particularly in cases where chlorides have entered through materials used in construction and residing in the hardened concrete. This increase in volume causes high radial bursting stresses around reinforcing bars and result in local radial cracks. These splitting cracks results in the formation of longitudinal cracks parallel to the bar. Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place Remedial Measures: Reinforcement steel in concrete structures plays a very important role as concrete alone is not capable of resisting tensile forces to which it is often subjected. It is therefore important that a good physical and chemical bond must exist between reinforcement steel and concrete surrounding it. Due to inadequacy of structural design and /or construction, moisture and chemicals like chlorides penetrate concrete and attack steel. Steel oxidizes and rust is formed. This results in loss of bond between steel and concrete which ultimately weakens the structure. The best control measure against corrosion is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting. 5. Moisture Movement: Most of the building materials with pores in their structure in the form of intermolecular space expand on absorbing moisture and shrink on drying. These movements are cyclic in nature and are caused by increase or decrease in inter pore pressure with moisture changes. Initial shrinkage occurs in all building materials that are cement/lime based such as concrete, mortar, masonry and plasters. Generally heavy aggregate concrete shows less shrinkage than light weight aggregate concrete. Controlling shrinkage cracks. 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 undergone most of its initial shrinkage. In case of structural concrete shrinkage cracks are controlled by using temperature reinforcement. Plaster with coarse well graded sand or stone chip will suffer less from shrinkage cracks and is preferred for plastering for external face of walls. Considering the building as a whole, an effective method of controlling shrinkage cracks is the provision of movement ts. The work done in cold weather will be less liable to shrinkage cracks than that in hot weather since movement due to thermal expansion of materials will be opposite to that of drying shrinkage. 6. Poor Construction practices.
The construction industry has in general fallen prey to non-technical persons most of whom have little or no knowledge of correct construction practices. There is a general lack of good construction practices either due to ignorance, carelessness, greed or negligence. Or worse still, a combination of all of these. The building or structure during construction is in its formative period like a child in mother’s womb. It is very important that the child’s mother is well nourished and maintains good health during the pregnancy, so that her child is healthily formed. Similarly for a healthy building it is absolutely necessary for the construction agency and the owner to ensure good quality materials selection and good construction practices. All the way to building completion every step must be properly supervised and controlled without cutting corners.
Some of the main causes for poor construction practices and inadequate quality of buildings are given below:
Improper selection of materials. Selection of poor quality cheap materials. Inadequate and improper proportioning of mix constituents of concrete, mortar etc. Inadequate control on various steps of concrete production such as batching, mixing, transporting, placing, finishing and curing Inadequate quality control and supervision causing large voids (honey combs) and cracks resulting in leakages and ultimately causing faster deterioration of concrete. Improper construction ts between subsequent concrete pours or between concrete framework and masonry. Addition of excess water in concrete and mortar mixes. Poor quality of plumbing and sanitation materials and practices.
7. Poor structural design and specifications Very often, the building loses its durability on the blue print itself or at the time of preparation of specifications for concrete materials, concrete and various other related parameters. It is of crucial that the designer and specifier must first consider the environmental conditions existing around the building site. It is also equally important to do geotechnical (soil) investigations to determine the type of foundations, the type of concrete materials to be used in concrete and the grade of concrete depending on chemicals present in ground water and subsoil. It is critical for the structural designer and architect to know whether the agency proposed to carry out the construction has the requisite skills and experience to execute their designs. Often complicated designs with dense reinforcement steel in slender sections result in poor quality construction. In addition, inadequate skills and poor experience of the contractor, ultimately causes deterioration of the building. Closely spaced of reinforcement steel bars due to inadequate detailing and slender concrete shapes causes segregation. If concrete is placed carelessly into the formwork mould, concrete hits the reinforcement steel and segregates causing fine materials to stick to the steel, obstructing its placement and is lost from the concrete mix while the coarse material falls below causing large porosity (honeycombs). Slender structural like canopies (chajjas), fins and parapets often become the first target of aggressive environment because of dense reinforcement, poor detailing, less cover of concrete to the reinforcement steel. Added to all this, low grade of concrete and poor construction practices can make the things worse. It is necessary for the structural consultant to provide adequate reinforcement steel to prevent structural from developing large cracks when loaded. To sum up, the following precautions are required to be taken by the Architects, Structural Consultants and Specifiers:
Proper specification for concrete materials and concrete. Proper specifications to take care of environmental as well as sub – soil conditions. Constructable and adequate structural design. Proper quality and thickness of concrete cover around the reinforcement steel. Planning proper reinforcement layout and detailing the same in slender structures to facilitate proper placing of concrete without segregation. Selection of proper agency to construct their designs.
Architects and Engineers are parents of the buildings they plan and design and therefore their contribution to the health and life of the building is quite significant. Once the plans are drawn the structural designs and specifications are prepared, it is then the turn of the agency to construct the building and bring the blue print to reality. Special care must be taken in the design and detailing of structures and the structure should be inspected continuously during all phases of construction to supplement the careful design and detailing.
8. Poor Maintenance A structure needs to be maintained after a lapse of certain period from its construction completion. Some structures may need a very early look into their deterioration problems, while others can sustain themselves very well for many years depending on the quality of design and construction. Regular external painting of the building to some extent helps in protecting the building against moisture and other chemical attacks. Water-proofing and protective coating on reinforcement steel or concrete are all second line of defence and the success of their protection will greatly depend on the quality of concrete. Leakages should be attended to at the earliest possible before corrosion of steel inside concrete starts and spalling of concrete takes place. Spalled concrete will lose its strength and stiffness, besides; it will increase the rate of corrosion as rusted steel bars are now fully exposed to aggressive environment. It is not only essential to repair the deteriorated concrete but it is equally important to prevent the moisture and aggressive chemicals to enter concrete and prevent further deterioration. 9. Movement due to Chemical reactions. The concrete may crack as a result of expansive reactions between aggregate containing active silica and alkalines derived from cement hydrations. The alkali silica reaction results in the formation of swelling gel, which tends to draw water form other portions of concrete. This causes local expansion results in cracks in the structure. To control Cracks due to alkali-silica reactions, low alkali cement, pozzolona and proper aggregates should be used. 10. Indiscriminate addition and alterations. There have been some building collapses in our country due to indiscriminate additions and alterations done by interior decorators at the instance of their clients. Generally, the first target of modifications is the balcony. Due to the requirement to occupy more floor area, balconies are generally enclosed and modified for different usages. Balconies and canopies are generally cantilever RCC slabs. Due to additional loading they deflect and develop cracks. As the steel reinforcement in these slabs have less concrete cover and the balcony and canopy slab is exposed to more aggressive external environment, corrosion of steel reinforcement takes place and repairs become necessary. The loft tanks are generally installed in toilets or kitchens, which are humid areas of the buildings. The structure in addition to being overloaded is also more prone to corrosion of reinforcement steel in these areas and therefore deteriorates and if not repaired, part of the building can even collapse. EVALUATION OF CRACKS IN CONCRETE TO FIND LOCATION AND EXTENT OF CRACKING When anticipating repair of cracks in concrete, it is important to first evaluate cracks in concrete to identify the location and extent of cracking. It should be determined whether the observed cracks are indicative of current or future structural problems, taking into consideration the present and anticipated future loading conditions.
The cause of the cracking should be established before repairs are specified. Drawings, specifications, and construction and maintenance records should be reviewed. If these documents, along with field observations, do not provide the needed information, a field investigation and structural analysis should be completed before proceeding with repairs. The causes of cracks are discussed here.
A detailed evaluation of observed cracking can determine which of those causes applies in a particular situation. Cracks need to be repaired if they reduce the strength, stiffness, or durability of the structure to an unacceptable level, or if the function of the structure is seriously impaired. In some cases, such as cracking in water-retaining structures, the function of the structure will dictate the need for repair, even if strength, stiffness, or appearance are not significantly affected. Cracks in pavements and slabs-on-grade may require repair to prevent edge spalls, migration of water to the sub grade, or to transmit loads. In addition, repairs that improve the appearance of the surface of a concrete structure may be desired.
Determination of location and extent of Cracks in Concrete Location and extent of cracking, as well as information on the general condition of concrete in a structure, can be determined by both direct and indirect observations, nondestructive and destructive testing, and tests of cores taken from the structure. Information may also be obtained from drawings and construction and maintenance records. Direct and indirect observation of Concrete Cracks The locations and widths of cracks should be noted on a sketch of the structure. A grid marked on the surface of the structure can be useful to accurately locate cracks on the sketch. Crack widths can be measured to an accuracy of about 0.001 in. (0.025 mm) using a crack comparator, which is a small, hand-held microscope with a scale on the lens closest to the surface being viewed (Fig. 1).
Fig.1: Comparator for Measuring Width of Cracks in Concrete Crack movement can be monitored with mechanical movement indicators of the types shown in Fig. 2.2. The indicator, or crack monitor, shown in Fig. 2.2 (a) gives a direct reading of crack displacement and rotation. The indicator in Fig. 2.2 (b) (Stratton et al. 1978) amplifies the crack movement (in this case, 50 times) and indicates the maximum range of movement during the measurement period.
Fig.2: Monitoring Crack Movement in Concrete Sketches can be supplemented by photographs documenting the condition of the structure at the time of investigation. Guidance for making a condition survey of concrete in service is given in ACI 201.1R, ACI 201.3R, ACI 207.3R, ACI 345.1R, and ACI 546.1R. Nondestructive testing of to Determine Concrete Cracks Nondestructive tests can be made to determine the presence of internal cracks and voids and the depth of penetration of cracks visible at the surface. Tapping the surface with a hammer or using a chain drag are simple techniques to identify laminar cracking near the surface. A hollow sound indicates one or more cracks below and parallel to the surface. The presence of reinforcement can be determined using a pachometer (Fig. 3) (Malhotra 1976). A number of pachometers are available that range in capability from merely indicating the
presence of steel to those that may be calibrated to allow the experienced a closer determination of depth and the size of reinforcing steel. In some cases, however, it may be necessary to remove the concrete cover (often by drilling or chipping) to identify the bar sizes or to cerebrate cover measurements, especially in areas of congested reinforcement.
Fig.3: Pachometer – Reinforcement Bar Locator in Concrete If corrosion is a suspected cause of cracking, the easiest approach to investigate for corrosion entails the removal of a portion of the concrete to directly observe the steel. Corrosion potential can be detected by electrical potential measurements using a suitable reference half cell. The most commonly used is a copper-copper sulfate half cell (ASTM C 876; Clear and Hay 1973); its use also requires access to a portion of the reinforcing steel. With properly trained personnel and careful evaluation, it is possible to detect cracks using ultrasonic nondestructive test equipment (ASTM C 597). The most common technique is through-transmission testing using commercially available equipment (Malhotra and Carino 1991; Knab et al. 1983). A mechanical pulse is transmitted to one face of the concrete member and received at the opposite face, as shown Fig. 4. The time taken for the pulse to through the member is measured electronically. If the distance between the transmitting and receiving transducers is known, the pulse velocity can be calculated. When access is not available to opposite faces, transducers may be located on the same face [Fig. 4(a)]. While this technique is possible, the interpretation of results is not straightforward. A significant change in measured pulse velocity can occur if an internal discontinuity results in an increase in path length for the signal. Generally, the higher the pulse velocity, the higher the quality of the concrete. The interpretation of pulse velocity test results is significantly improved with the use of an oscilloscope that
provides a visual representation of the received signal [Fig. 4(b)].
Fig.4 : Ultrasonic Testing of Concrete Cracks Tests on Concrete Cores to Evaluate Cracks in Concrete Significant information can be obtained from cores taken from selected locations within the structure. Cores and core holes afford the opportunity to accurately measure the width and depth of cracks. In addition, an indication of concrete quality can be obtained from compressive strength tests; however, cores that contain cracks should not be used to determine concrete strength. Ultrasonic equipment should be operated by a trained person, and the results should be evaluated cautiously by an experienced person, because moisture, reinforcing steel, and embedded items may affect the results. For example, with fully saturated cracks, ultrasonic testing will generally be ineffective. In some cases, it is difficult to discern between a group of close cracks and a single large crack. An alternative to through-transmission testing is the pulse-echo technique in which a simple transducer is used to send and receive ultrasonic waves. It has been difficult to develop a practical pulse-echo test for concrete. Petrographic examinations of cracked concrete can identify material causes of cracking, such as alkali reactivates, cyclic freezing damage, “D” cracking, expansive aggregate particles, fire-related damage, shrinkage, and corrosion. Petrography can also identify other factors that may be related to cracking such as the water-tocement ratio, relative paste volume, and distribution of concrete components. Petrography can
frequently determine the relative age of cracks and can identify secondary deposits on fracture surfaces, which have an influence on repair schemes. Chemical tests for the presence of excessive chlorides indicate the potential for corrosion of embedded reinforcement. Review of Drawings and Construction Data The original structural design and reinforcement placing or other shop drawings should be reviewed to confirm that the concrete thickness and quality, along with installed reinforcing, meets or exceeds strength and serviceability requirements noted in the governing building code(s). A detailed review of actual applied loading compared to echo technique. design loads should get.
Selection of Repair Procedures of Cracks in Concrete Based on the careful evaluation of the extent and cause of cracking, procedures can be selected to accomplish one or more of the following objectives:
Restore and increase strength Restore and increase stiffness Improve functional performance Provide water tightness Improve appearance of the concrete surface Improve durability Prevent development of corrosive environment at reinforcement
Depending on the nature of the damage, one or more repair methods may be selected For example, tensile strength may be restored across a crack by injecting it with epoxy or other high strength bonding agent. However, it may be necessary to provide additional strength by adding reinforcement or using post-tensioning. Epoxy injection alone can be used to restore flexural stiffness if further cracking is not anticipated (ACI 503R). Cracks causing leaks in water-retaining or other storage structures should be repaired unless the leakage is considered minor or there is an indication that the crack is being sealed by autogenous healing. Repairs to stop leaks may be complicated by a need to make the repairs while the structures are in service. Cosmetic considerations may require the repair of cracks in concrete. However, the crack locations may still be visible and it is likely that some form of coating over the entire surface may be required. To minimize future deterioration due to the corrosion of reinforcement, cracks exposed to a moist or corrosive environment should be sealed. The key methods of crack repair available to accomplish the objectives outlined are described in METHODS OF CONCRETE CRACK REPAIR.
TYPES OF CRACKS IN CONCRETE BEAMS 1. Cracks in concrete beams due to increased shear stress
2. Cracks in concrete beams due to corrosion or insufficient concrete cover
3. Cracks parallel to main steel in case of corrosion in beams
4. Cracks due to increased bending stress in beams
5. Cracks due to compression failure in beams
Methods of Repairing Active Cracks in concrete structures: 1. Drilling and Plugging through Crack: One of the approximate methods would be to drill holes normal to cracks, fill them with a suitable epoxy or epoxy-mortar formulation and then place reinforcement bars (of predetermined sizes and lengths) in them to stitch across the cracks. The bars may be placed in the clean holes prior to filling the epoxy (so as to save loss of epoxy) but then great care is needed not to entrap any air. 2. Stitching of Concrete Cracks: Stitching involves drilling holes on both sides of the crack and grouting in Ushaped metal units with short legs (staples or stitching dogs) that span the crack as shown in figure below:
Stitching should be used when tensile strength has to be restored back across major cracks. Stitching a crack tends to stiffen the structure and the stiffening may increase the overall structural restrain, causing the concrete to crack elsewhere. Therefore, i t is necessary that proper investigation is done and if required, adjacent section or sections are strengthened using technically designed reinforcing methods. Because stresses are often concentrated, using this method in conjunction with other methods maybe necessary. The procedure consists of drilling holes on both sides of the crack, cleaning the holes and anchoring the legs of the staples in the holes, with either a nonshrink cement grout or any epoxy resin-based bonding system. The staples should be variable in length, orientation, or both and they should be located so that the tension transmitted across the crack is not applied to a single plane within the section but is spread over an area. 3. External Prestressing: The flexural cracks in reinforced concrete can be arrested and even corrected by the ‘Post –tensioning’ method. It closes the cracks by providing compression force to compensate for tensions and adds a residual compression force. This method requires anchorage of the tierods (or wires) to the anchoring device (the guide – bracket- angles) attached to the beam (Fig. 2).
Fig. 2: Post Tensioning Cracked Beam The rods or wires are then tensioned by tightening the end-nuts or by turning of turnbuckles in the rods against the anchoring devices. However, it may become necessary in certain critical case to run at least an approximate stress-check to guard against any possible adverse effects. 4. Drilling and Plugging: When cracks run in reasonable straight lines and are accessible at one end, drilling down the length of the crack and grouting it to form a key as shown in Fig. 3 could repair them. Form key with precast concrete or mortar plugs set in bitumen. The bitumen is to break the bond between plugs and hole so that plugs will not be cracked by subsequent movement of the opening. If a particularly good seal is required, drill a second hole and plug with bitumen alone, using the first hole as a key and the second as a seal.
Fig. 3: Drilling and Plugging A hole of 50 to 75mm dia depending on width of crack should be drilled, centered on and following the crack. The hole must be large enough to intersect the crack along its full length and provide enough repair material to structurally take the loads exerted on the key. The drilled hole should then be cleaned, made tight and filled with grout. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. If water tightness is essential and structural load transfer i s not, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. If the key effect is essential, the resilient material can be placed in a second hole, the first being grouted.
TYPES OF CRACKS IN REINFORCED CONCRETE SLABS
1. Cracks in reinforced concrete columns due to Steel Corrosion (cracks parallel to steel bars)
2. Cracks in reinforced concrete columns due to shrinkage of concrete
3. Cracks in reinforced concrete columns due to increased load on the slab
4. Cracks in reinforced concrete columns due to sulphate attack
5. Cracks in reinforced concrete columns due to alkaline aggregates
CAUSES AND TYPES OF CRACKS IN MASONRY BUILDINGS AND THEIR REPAIR METHODS There are various causes for various types of cracks in masonry buildings such as in walls, foundations, slabs, columns. Repair methods of such cracks in masonry buildings is discussed.
There are certain problems in structures that arise suddenly. Some problems like crack formation or settlement of foundation won’t give a caution before it appears. Most of these problems arise due to improper construction method and carelessness during initial construction. So, care during initial stages can help avoid such problems that require huge maintenance.
Causes and Types of Cracks in Masonry Buildings and their Repair Methods The cracks appear in the masonry structure, at a certain period of time. Most commonly caused cracks with their respective causes and precaution, are explained below: Cracks in Brick Mortar ts Vertical or horizontal cracks are seen at the brick mortar ts. One of the main reason is the sulfate attack, that weakens the mortar. These cracks mainly appear after 2 to 3 years of construction. These cracks can be avoided by:
Checking the sulfate content of bricks used in construction The damping of brick wall has to avoided, as these are more prone to sulfate attack when it is damp
Fig.1: Cracks Formed in the Brick Mortar ts Crack Formation Below the Load Bearing Walls Cracks are observed below the load bearing walls, mainly those that s R.C.C slabs. Now the temperature variation makes the reinforced concrete slab to expand or contract, but both in the horizontal direction. These are observed in the Top most story that is more exposed to the temperature changes. There no smooth between the wall and the slab. Hence the frictional forces are developed at the place of the wall and the slab. This creates cracking in the walls. The precaution that can be suggested is to provide a bearing plaster over the brick wall, which helps in having a smooth with the floor over it. If required a bituminous coating can be applied over the plaster applied.
Fig.2: Cracks in Masonry Walls of Multi-Storey Building at Higher Floors
Main Wall and Cross Walls t Cracks Improper bonding between the cross wall and the main wall creates cracks between the ts. This suggests us to have proper and quality bonding between the two walls. These are properly done by toothing.
Fig.3: Shear Cracks between Cross Wall and the Main Long Wall of Masonry Building
Fig.4: Tooth connection between the Walls Cracks Found in R.C.C Columns and Masonry One of the main reason behind this is the differential movement of the columns and the masonry because of temperature variation. This variation can be either expansion or contraction depending upon the temperature. These cracks can be hidden by making a groove in the reinforced concrete column and masonry junction. The provision of chicken wire alternatively at the plaster between the junction of columns and masonry can also help in this variation. The Horizontal cracks between R.C.C slab and the brick parapet The non-projecting slab is mainly subjected to such cracks. This too is due to the temperature variation and the drying shrinkage. Small micro cracks formed he propagated with the increase in expansion or contraction. These cracks can be hidden by making a groove at the masonry junction will help in hiding the cracks. The provision of chicken wire alternatively at the plaster junction can also help. Cracks in Roof Slab The exposure of roof slab to higher temperature variation cause cracks numerously. This can be reduced by providing a weather proof course. New treatment methods and compounds are available as weather course, that is applied over the terrace.
Repair Methods for Cracks in Masonry Building Structural Measures to be followed for already appeared cracks are: 1. Application of grouting or uniting for cracks that are appeared in the main structural , that cannot be compromised at any cost. The material mainly used for this is either cement or
epoxy mixture. The epoxy has the ability to fill even small and thin cracks, say as fine as 0.1mm. These epoxy gain high strength and adhesion. 2. The flexible sealant can be used for cracks that are appeared on the non-structural . This helps in having a control over the differential movement (expansion or contraction) of the member under temperature changes. 3. Epoxy putty, polymer filler or lime cement mortar can be used for filling the cracks seen in plain cement concrete.
Measures for Foundation Settlement The unequal settlement of foundation due to the variation of bearing capacity at different points of the building result in the formation of cracks in the building. The Certain preventive measure is: 1. The foundation is planned to lay or hard soil 2. Gradual raising of foundation and wall has to be made, for letting the structure to have an allowable settlement. 3. The settlement value of should not go beyond allowable, under any combination of loads. 4. The foundation designed should facilitate uniformly distributed pressure on the soil.
Plinth Protection The unequal settlement of plinth is avoided by removing expansive soils like black soils (black cotton soil), nearby plinth. This barrier is kept with the help of sand harries. Providing drains and flagging concrete help in avoiding rainwater away from the plinth. The penetration of roots into the plinth has to avoided. This can be avoided by stopping the construction of trees that has lateral growing roots nearby. METHODS OF TESTING COMPRESSIVE STRENGTH OF MASONRY Testing compressive strength of masonry before construction and for every 464.5 m2 of masonry work during construction is required as per Building Code Requirements and Specification for Masonry Structure (ACI 530.1- 11) and under its quality assurance program, certification of compliance.
There are two ways of specifying compressive strength of clay and concrete masonry which are unit strength methods and masonry prism method. These testing methods are provided by ACI 530.1-11. Either engineers or architects specify one of the two methods. In the case when the method of testing is not determined, the contractor is permitted to select method of determining compressive strength of masonry. Due to costs related to the making prisms and laboratory tests, the prism testing method is more costly compare with the unit strength method. Nevertheless, results of unit strength method are more conservative than prism strength method.
Methods of Testing Compressive Strength of Masonry The testing methods for compressive strength of masonry are:
Unit strength method Prism test method
Unit Strength Method of Testing Compressive Strength of Masonry In unit strength method, masonry units are needed to be tested before and during construction to guarantee their sufficient strength. The values of specified compressive strength of masonry (f’m) depend on not only the compressive strength of masonry units but also on the mortar. Both clay and concrete masonry units should conform to their related ASTM specifications. Clay masonry units need to conform applicable ASTM specifications namely ASTM C 62-05: standard specification for building bricks (solid masonry units made from clay or shale), ASTM C 216-05a: standard specification for facing brick (solid masonry units made from clay or shale), and ASTM C 652-05: standard specification for hollow brick (hollow masonry units made from clay or shale). Clay masonry samples and testing must be as per Test Methods for Sampling and Testing Brick and Structural clay tile. Similarly, concrete masonry unit should conform to the applicable ASTM specifications which are ASTM C55-03: Specification for Concrete Brick and ASTM C90-06: Specification for Load Bearing Concrete Masonry Units in addition to follow either ASTM C 55-03a or ASTM C 90-06 for sampling and testing. In case of grouted masonry, the grout employed for concrete and clay masonry units should conform to ASTM C 476 and grout compressive strength should equal or greater than the compressive strength of masonry and no smaller than 13.79 MPa. Additionally, bed t thickness for both concrete and clay masonry must be equal or smaller than 15.87 mm.
Prism Method of Testing Compressive Strength of Masonry Masonry prisms as shown in Figure-1 and Figure-2 are an assemblage of masonry units, mortar, and grout if applicable which are constructed and tested for their compressive strength as per the ASTM C 1314-03b: Standard Test Method for Constructing and Testing Masonry Prisms which is employed for determine compliance with specified compressive strength of masonry. The ASTM C 1314-03b specification deals with masonry prism construction procedures, testing, and procedures for determining masonry compressive strength (f’m). This test method can check whether masonry materials used produce masonry which meet the specified compressive strength or not. The masonry units utilized for building masonry prisms are an exemplary of those units that will be used construction. Moreover, in the prism test method, minimum three prisms are required to be constructed and the same material should be used. At least two masonry units should be employed to build prisms and they should be tested at the same age based on determined procedure. The aspect ratio or in other word the height to smallest lateral dimension of the prisms (hp/(tp) need to be between 1.3 and 5. All masonry units must be laid in stack bond in stretcher position and oriented as in corresponding construction and prepared with adequate mortar. The length and width of masonry prism is equal to masonry unit length and width. Not only does the mortar and thickness of t but also unit positioning and aligning method while prisms are prepared mortar need to be representative of the construction; the prisms might be constructed as solid or hollow ungrouted or solid or hollow grouted depend on the real situation of the structure. The procedure of grouting, its consolidation, and reconsolidation are required to be similar to the corresponding construction. Two series of prisms are required to be grouted; one group is grouted and another group is ungrouted. In the case of grouting prisms solidly, the timing of grouting prisms needs to be no less than twenty four hours and no more than forty eight hours. Added to that, they should be saved in air proof bags and not be disturbed before forty eight hours after that, the stored prisms have to be kept at 24 oC. Tests of prisms are carried out after 28 day or any other specified period but the prism must have taken out of the airtight backs two days before the test is began. .
Fig.1: Masonry Prisms: (A) Hollow Ungrouted Unit, (B) Hollow Grouted Unit
Fig.2: Solid Unit Prism The compressive strength of masonry is computed in by three steps from prism tests. Firstly, estimate masonry prism strength which is equal to sustained compressive load of prisms by net cross sectional area of that prism. Secondly, calculate masonry compressive strength depend on the aspect ratio (the height to smallest lateral dimension of the prisms) of masonry prisms tests. Thirdly, the compressive strength of masonry is considered to be equal to that of masonry prism provided that the masonry prisms have aspect ratio of 2. When the aspect ratio masonry prism is ranging from 1.3 to 5, the compressive strength of masonry is achieved by correction factor which provided in Table-1 multiply by masonry prism strength. The compressive strength of masonry is computed by averaging the achieved values. Table-1: Aspect Ratio Correction Factors Compressive Strength of Masonry Prism tp
Correction factor
1.3
0.75
1.5
0.86
2
1
2.5
1.04
3
1.07
4
1.15
5
1.22
Interpolation might be employed for obtain correction factor for those aspect ratio that is not included in the table TESTING OF CONCRETE MASONRY BLOCKS FOR COMPRESSIVE STRENGTH AND DENSITY Compressive strength of concrete Blocks or concrete masonry units are required to know the suitability of these in construction works for various purposes.
Concrete masonry blocks are generally made of cement, aggregate and water. Which are usually rectangular and are used in construction of masonry structure. They are available in solid and hollow forms. The nominal dimensions of concrete masonry block vary as follows. Length: 400 or 500 or 600mm Width: 200 or 100mm Width: 50, 75, 100, 150, 200, 250 or 300mm.
Tests on Concrete Masonry Block Units Different tests are conducted on concrete masonry unit to satisfy the all requirements. But now we are discussing about three tests conducted on concrete masonry block which are as follows. Blocks of same mix shall be taken and divide them as follows to conduct the following tests.
Dimension measurement (All blocks) Density of block (3 blocks) Compressive strength of block (8 blocks)
Dimension Measurement All blocks should be checked in this step. The length, width and height are measured with steel scale. If it is a hollow block, then the web thickness and face shell are measured with caliper ruler. And prepare a report of average length, width and height of block and average minimum face shell and minimum web thickness using recorded dimensions.
Density of Concrete Masonry Block As said above, 3 blocks shall be taken to conduct this test. To determine the density of block, first heat the block in the oven to 100oc and then cooled it to room temperature. Now take the dimensions of block and from that find out the volume and weigh the block. The density of block is determined from the below relation and the average density of 3 blocks will be the final block density. Density of block = mass/volume (kg/m3) Density values of different grades of blocks should be as follows. Type of unit
Hollow type unit
Solid type unit
Grade
Density of block (kg/m2)
A(3.5)
>/= 1500
A(4.5)
>/=1500
A(5.5)
>/=1500
A(7.0)
>/=1500
A(8.5)
>/=1500
A(10.0)
>/=1500
A(12.5)
>/=1500
A(15.0)
>/=1500
B(3.5)
1100-1500
B(5.0)
1100-1500
C(5.0)
>/=1800
C(4.0)
>/=1800
Compressive Strength Tests on Concrete Masonry Blocks Eight blocks are taken to determine the average compressive strength of concrete masonry block. The blocks should be tested with in 3days after collected in lab. The age of each block shall be 28 days. The compressive strength testing machine consist of two steel bearing blocks, one is in rigid position on which the masonry unit is placed and another one is movable which transmit the load to the masonry unit when applied.
If the bearing area of masonry unit is more than the bearing area of steel blocks, then separate steel plates are used. The plates are arranged on steel blocks in such a way that the centroid of masonry unit coincide with the center of thrust of blocks.
Bearing area of concrete masonry units are capped with the Sulphur and granular materials coating or gypsum plaster capping. After placing the unit in testing machine, one-half of the expected maximum load is applied at a constant rate, and the remaining load is applied in not less than 2 minutes. Note down the load at which masonry unit fails and the maximum load divided by gross sectional area of unit will give the compressive strength of block. Similarly, test the remaining 7 blocks and the average of 8 blocks strength is the final compressive strength of concrete masonry unit. Below table represents the value of minimum average compressive strength of individual units. Type of unit
Grade
Min average compressive strength of individual units (N/mm2)
Hollow type concrete masonry unit
A(3.5)
2.8
A(4.5)
3.6
A(5.5)
4.4
A(7.0)
5.6
A(8.5)
7.0
A(10.0)
8.0
A(12.5)
10.0
Solid type unit
A(15.0)
12.0
B(3.5)
2.8
B(5.0)
4.0
C(5.0)
4.0
C(4.0)
3.2
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