LITERATURE REVIEW
1.1 Cracks in Building; An overview
Poor and improper building maintenance will definitely will cause more damages and costly repair work if left unattended. Building defects are inevitable aspects of building construction. Defects occur in various forms and to different extent in all types of building irrespective of their age. Cracks can be structural or nonstructural depending upon the location of the crack in the building. Non Structural cracks occur mostly due to internally induced stresses in building material and generally do not affect the safety of structure but develop an anaesthetic appearance and create an impression of faulty construction work. These defects can be seen on the walls of structure in various forms like dampness, paint peeling, cracks. plaster rendering etc. Whereas structural cracks are the result of incorrect structural designing or insufficient survey of site and statistics of the location or even both in the worse scenario and are seen on structural elements i.e. Beam, column, slab, footing and structural cracks are the one which may cause the failure of the structure during its life period.
2.2 Causes of Cracks on Building
Causes of the cracks can be listed as:-
1) Poor workmanship
Poor mixing of building materials, lack of curing, change in water-cement ratio, proper compaction will cause cracks in the walls, beams, slabs etc. Normally poor workmanship is as a result of ignorance, carelessness, negligence, lack of proper supervision or many others.
2) Faulty design.
Poor structural design and specifications are another cause of the cracks in concrete works. It’s important the most important factor in the failure of a building. Design should be in accordance of all the environmental surveys that include soil (Geotechnical) investigations. Buildings are designed for particular uses, and also to withstand a given load conditions for example a building designed as residence will have different structural specifications from the one designed to operate machinery.
3) Structural overloading
( Overloading of the ground
( Overloading due to its dead load
( Overloading due to live loads present result in cracks
4) Due to moisture.
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.
5) Chemical reaction cement is an alkaline material and will react with acidic compounds present in moisture and will result in the weakening of the internal bonds. Certain limestone aggregate forms alkali -silica product, this reaction is known as alkali carbonation reaction. These reactions of aggregate, cement paste with the surrounding often causes cracks in building.
6) Creep Concrete when subjected to sustain 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 admixtures and pozzolonas in concrete increases creep. Amount of creep in steel increases with rise in temperature..
7) 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.
8) Thermal movement.
Various building materials are used for the construction of a building and all the materials have different coefficient of expansion. Due to changes in the temperature, the expansion and contraction of the building components takes place which result in the changes in the size and shape of the components. Smaller buildings are less affected. In larger buildings, the change in size of one part causes cracks although not in expanded part.
9) For example; Crack below the slab/beam in RCC frame Brick pin buildings. These cracks can close up completely as a result of changes of temperature.
10) Corrosion of reinforcement.
This primarily causes structural failure or structural crack in building. Oxidation of the steel due to the presence of oxidants like o2 in the atmosphere causes the change in volume of steel reinforcement which develops a radial bursting stress in the surrounding area and resulting in cracks. Corrosion of the reinforcement cannot be eliminated but can be reduced to lower extent by using various techniques during construction
11) Foundation settlement.
Foundation may settle due to land slips, earthquakes, moisture changes due to clay shrinkable soils (for example, Black cotton soil). cracks occur because a part of the building is displaced from its position without any change in the size of material
12) Poor maintenance it’s always important to take good care of your house, by doing maintenance works after a lapse of certain periods. This will keep the building intact and also extend their life span.
2.3 Types of Cracks
The magnitude of the risk caused due to a crack can be characterized in terms of its direction, and dimensions. Cracks can be horizontal, vertical, diagonal or random.
1) Horizontal crack horizontal crack or crack which runs zigzag 45-degree angle, reason for this zigzag form might be severe such as foundation shifting or water damage. Severe cracks usually require immediate attention and might include some reconstruction to prevent further damage.
2) Vertical crack whereas vertical crack starting near the junction where the wall and ceiling meet, it indicates that it developed when the foundation settled after construction. Vertical cracks run the same direction as drywall.
3) Stair-step crack A stair-step crack looks like a flight of stairs and runs in both vertical and horizontal directions across the wall. The continuous pattern usually follows the brick line or the stone block and can be seen in unfinished basements due to the result of soil settling beneath the centre of the wall. For the rehabilitation of such cracks the soil test and the core test is recommended to encounter the probable damage to the building.
4) Doors and window it is a way to test the severeness of a wall cracks in wall by checking the swing of the doors and windows while opening and closing the internal doors and evaluate whether the door is obstructed. If so, ensure that the obstruction is not due to the recent paint work, faulty material if you determine there's nothing obstructing the swinging motion of the door, it might be a sign of a moderate or severe foundation settlement, and may result in wall cracks. According to Real Estate, sticky doors could indicate that the frame has been twisted by a shifting house. If you notice a visible gap at the top of a sticky door where it meets the door frame and you see light shining through, that might also signal a serious settlement problem, often resulting in jagged, horizontal cracks on nearby walls.
5) Visible nails inspect the area surrounding the crack on wall and look for nail heads or screw heads that might be visible on the surface of the wall. The nail or screw might not have damaged the drywall, but it has likely pulled away from the wooden stud beneath. This phenomenon is often known as "nail pops" or "nail popping" and might be a sign of structural problems. Nail pops are frequently associated with more serious wall cracks and often signal significant drywall shear movement.
TREATMENT MEASURES
The aim of crack repair has to be established a prior and achieved by proper selection of repair material and methodology. The goal of all crack repairs is to achieve one or more objectives such as:
( Restore and increase the strength of cracked components
( Restore and increase the stiffness of cracked components
( Improve functional performance of the structural members
( Prevent liquid penetration
( Improve the appearance of the concrete surface
( Improve durability; and prevent development of a corrosive environment at the reinforcement. Materials for nonstructural crack repair of dormant nature should be a rigid material. Cementitious, polymer modified cementitious grouts of acrylic, styrene-acrylic and styrene-butadiene should be used for wider cracks. However polyester and epoxy resins should be used for injection of dormant cracks. For live cracks flexible material of polysulphide or polyurethane should be used. Before repair of any non-structural cracks the factors have to be considered are:
( Whether the crack is dormant or live;
( The width and depth of the crack;
( Whether or not sealing against pressure is required, and, if so, from which side of the crack will the pressure be exerted and
( Whether or not appearance is a factor.
A. Repair of Dormant cracks
Dormant cracks may range in width from 0.05 mm or less (crazing) to 6 mm or more. The width of the crack has a considerable influence on the materials and methods to be chosen for its repair. The fine cracks are repaired by low viscous epoxy resin and other synthetic resin by injecting. Wide cracks on a vertical surface are also repaired by injection methods. Cracks on horizontal surface can be repaired by injection or by crack filling by gravity. Dormant cracks, where the repair does not have to perform a structural role, can be repaired by enlarging the crack along the external face and filling and sealing it with a suitable joint sealer. This method is commonly used to prevent water penetration to cracked areas. The method is suitable for sealing both fine pattern cracks and larger isolated defects. Various materials are used such as including epoxies, urethanes, silicones, polysulphides, asphaltic materials and polymer mortars. Polymer mortars are used for wider cracks. The crack is routed out, cleaned and flushed out before the sealant is placed. It should be ensured that the crack is filled completely. Where ever a cementitious material is being used, dry or moist crack edges must be wetted thoroughly.
B. Cementitious Grouts
It is used for repair of cracks that are greater in width. It is a mixture of cementitious material and water that is proportioned to produce a pourable consistency. Cement-based grouts are available in a wide range of consistencies; therefore, the methods of application are diverse. These materials are the most economical of the choices available for repair. They do not require unusual skill or special equipment to apply and are reasonably safe to handle. These materials tend to have similar properties to the parent concrete & mortar and have the ability to undergo autogenous healing due to subsequent hydration of cementitious materials at fracture surfaces. Shrinkage is a concern in such type of grouts. These are not suitable for structural repairs of active cracks. For application of cementitious grouts generally, some form of routing and surface preparation, such as removal of loose debris are needed. Pre-wetting should be done to achieve a Saturated-Surface-Dry (SSD) condition. Grouts are generally to be mixed to a pourable consistency by using a drill and paddle mixer, and the consistency may be adjusted thereafter. Application should be done by hand troweling or dry packing into vertical and overhead cracks to fill all pores and voids. Finally, a suitable coating to be applied on the repaired surfaces.
C. Epoxy Resin Grouts
This is most common polymer material used for gravity feed crack repairs. It should be formulated to have a very thin consistency (low viscosity) and low surface tension to enable the resin to easily penetrate fine cracks by gravity alone. Viscosities below 200 centipoise (cps) should be a minimum requirement. The horizontal concrete elements such as bridge and parking decks, floor slabs, plaza decks, and similar surfaces can be repaired with gravity feed resin. The cracks should be cleaned and free from dust. If required some routing may be required to facilitate pouring of resin. The surface should be cleaned with a compressed air. If water is used during cleaning then it should be dried for 24hr because the moisture present inside the crack may obstruct the flow of resin. The resin has to be mixed in a bucket with a paddle mixer. Small cans or squeeze bottles can be used for pouring into individual cracks. Before pouring of resin the underside of cracks should be sealed temporarily to avoid any leakage. The pouring should continue till the cracks go on absorbing after which the excess resin should be removed with a flat rubber squeegee.
D. Application of Epoxy Injection
1) Surface preparation
The cracks should be cut and cleaned properly. Any contamination should be removed by flushing with water or some especially effective solvent. Then the solvent should be blown out with compressed air, or adequate time should be given for air drying. The surfaces should be sealed. This keeps the epoxy from leaking out before it gelled. A surface can be sealed by brushing an epoxy over the surface of the crack and allowing it to harden. If extremely high injection pressures are needed, then the crack should be cut into a V-shape, filled with an epoxy, and should be stroke off flush with the surface. The entry ports should be installed thereafter.
2) Fixing of injection ports/nozzles
There are three ways to do this. Fitting of nozzles to be inserted in drilled holes should be made by drilling a hole into the crack for 8 mm dia injection packers @ 200 to 300 mm c/c, penetrating below the bottom of the V-grooved section. A fitting such as a pipe nipple should be inserted or tire valve stem should be inserted into the hole and bonded with an epoxy adhesive. A vacuum chuck and bit will help to keep the cracks from being plugged with drilling dust. The second method is by bonded flush fitting. When the cracks are not V-grooved, a common method of providing an entry port is to bond a fitting flush with the concrete face over the crack. Last method is by interruption in seal. Another way to allow entry is to omit the seal from part of the crack. This method uses special gasket devices that cover the unsealed portion of the crack and allow injection of the adhesive directly into the crack.
3) Mixing
Mixing the two components of epoxy injection grout of base and hardener should be done in a suitable container with heavy duty slow speed drilling machine with paddle attachment. Mixing should be made for 2 to 3 minutes to obtain a uniform color.
4) Injection of Epoxy
For smaller area or isolated crack a hand pump may be used for injection. Hydraulic pumps, paint pressure pots, or air-actuated caulking guns can be used for larger cracked areas. The pressure should be selected carefully, because too much pressure can extend the existing cracks and cause more damage. If cracks are clearly visible, injection ports can be installed at appropriate interval by drilling directly into the crack surface. The surface of the crack between ports is allowed to cure. For vertical cracks, pumping of epoxy into the entry port should start at the lowest elevation until the epoxy level reaches the entry port above. Then the lower injection port is caped and the process is repeated at successively higher ports until the crack has been completely filled in. For horizontal cracks, injection starts from one end of the crack to the other in the same way. When the pressure is maintained, the crack is filled completely. For injection from underside of ceiling of flat roof a lot of pressure is being exerted. Hence care should be taken while injecting from underside. (Hosein, 1980)
5) Removal of the Surface Seal
After the injected epoxy has cured, the surface seal is being removed by grinding or some other appropriate means.
CONTROLLING MEASURES
Measures for controlling cracks due to shrinkage
( To avoid cracks in brickwork on account of initial expansion, a minimum period varying from 1 week to 2 weeks is recommended by authorities for storage of bricks after these are removed from Kilns.
( Shrinkage cracks in masonry could be minimized by avoiding use of rich cement mortar in masonry and by delaying plaster work till masonry has dried after proper curing and has undergone most of its initial shrinkage.
( Use of precast tiles in case of terrazo flooring is an example of this measure. In case of in-situ/terrazo flooring, cracks are controlled by laying the floor in small alternate panels or by introducing strips of glass, aluminum or some plastic material at close intervals in a grid pattern, so as to render the shrinkage cracks imperceptibly small.
( In case of structural concrete, shrinkage cracks are controlled by use of reinforcement, commonly termed as 'temperature reinforcement'. This reinforcement is intended to control shrinkage as well as temperature effect in concrete and is more effective if bars are small in diameter and are thus closely spaced, so that, only thin cracks which are less perceptible, occur.
( To minimize shrinkage cracks in rendering/plastering, mortar for plaster should not be richer than what is necessary from consideration of resistance to abrasion and durability
B. Measures for controlling cracks due to thermal variations
( Wherever feasible, provision should be made in the design and construction of structures for unrestrained movement of parts, by introducing movement joints of various types, namely, expansion joints, control joints and slip joints.
( Even when joints for movement are provided in various parts of a structure, some amount of restraint to movement due to bond, friction and shear is unavoidable. Concrete, being strong in compression, can stand expansion but, being weak in tension, it tends to develop cracks due to contraction and shrinkage, unless it is provided with adequate reinforcement for this purpose. Members in question could thus develop cracks on account of contraction and shrinkage in the latter direction. It is, therefore, necessary to provide some reinforcement called 'temperature reinforcement" in that direction.
( Over flat roof slabs, a layer of some insulating material or some other material having good heat insulation capacity, preferably along with a high reflectivity finish, should be provided so as to reduce heat load on the roof slab.
( In case of massive concrete structures, rise in temperature due to heat of hydration of cement should be controlled.
( Provision of joints in structure.
Measures for prevention of cracks due to creep
Though it may not be possible to eliminate cracking altogether, following measures will considerably help in minimization of cracks due to elastic strain, creep and shrinkage:
( Use concrete which has low shrinkage and low slump.
( Do not adopt a very fast pace of construction.
( Do not provide brickwork over a flexural RCC member (beam or slab) before removal of centering, and allow a time interval of at least 2 weeks between removal of centering and construction of partition or panel wall over it.
( When brick masonry is to be laid abutting an RCC column, defer brickwork as much as possible.
( When RCC and brickwork occur in combination and are to be plastered over, allow sufficient time (at least one month) to RCC and- brickwork to undergo initial shrinkage and creep before taking up plaster work. Also, either provide a groove in the plaster at the junction or fix a 10 cm wide strip of metal mesh or lathing over the junction to act as reinforcement for the plaster. (Central Building Research Institute, 1984)
( In case of RCC members which are liable to deflect appreciably under load, for example, cantilevered beams and slabs, removal of centering and imposition of load should be deferred as much as possible (at least one month) so that concrete attains-sufficient strength, before it bears the load.
General measures for chemical attack
( In case of structural concrete in foundation, if sulphate content in soil exceeds 0.2 percent or in ground water exceed 300 ppm, use very dense concrete and either increase richness of mix to 1:1/5:3 or use sulphate resisting Portland cement/super-sulphated cement or adopt a combination of the two methods depending upon the sulphate content of the soil.
( For superstructure masonry, avoid use of bricks containing too much of soluble sulphates (more than 1 percent in exposed situations, such as parapets, free standing walls and masonry in contact with damp soil as in foundation and retaining walls; and more than 3 percent in case of walls in less exposed locations) and if use of such bricks cannot be avoided, use rich cement mortar (1:1/2:4.5 or 1:1/4 :3) for masonry as well as plaster or use special cements mentioned earlier and take all possible precautions to prevent dampness in masonry.
( To prevent cracking due to corrosion in reinforcement and premature deterioration, it is desirable to specify concrete of richer mix (say 1:1/5:3) for thin sections in exposed locations and to take special care about grading, slump, compaction and curing of concrete. (Chand, October 2008) (CBRI, Roorkee)
THEORETICAL FRAMEWORK
2.3.1 Cracking Prediction Model
In this study, the methodology used was to analyse several pavement cracking deterioration models available in the literature with the objective of its integration in the management of cracks. The models selected are the following ones: the Brazilian model; the PAVENET-R model; the HDM-4 model; the Ker Lee Wu (KLW) model; the Indian model; and the Austroads model.
Brazilian Model (1994)
This model was developed by Visser, Queiroz and Caroca [6] based on a longterm pavement monitoring program carried out in Brazil between 1975 and 1985. The road sections were unbound granular base flexible pavements in areas with tropical to subtropical climate with an average annual precipitation between 1200 and 1700 mm/year. Cracked area over time is predicted with Eqn. (1), developed with multiple regression and probabilistic time failure analysis, which depends on traffic volume, the pavement bearing capacity (load deflection) and age. For existing pavements with asphalt surfacing the model only applies to cracking progression prediction, while for asphalt overlays and slurry seals the model comprises two-phases, initiation time and progression prediction. Two-phase models give an extra opportunity to calibrate the model for cracking prediction over time. This model considers the Benkelman beam to measure pavement deflections, but this equipment is not used in Europe any more. However, some regressions relating the modified structural number with the Benkelman beam maximum deflection have been proposed by several authors such as Eqn. (2) by Paterson [7, 8]. The modified structural number is the evolution of the AASHTO structural number by considering the subgrade contribution.
Ct = (B×10−2)×log(N80ct ) (× 0.0456 + 0.00501×Yt )−18.53−C0
(1)
SNC=3.2×B−0.63
(2)
Where Ct is the pavement cracked area (class 2 cracking or worse, i.e. crack width larger than 1 mm) in year t (m2/100m2); Yt the age of pavement since original construction or since subsequent AC overlay (years); N80ct is the cumulative 80 kN equivalent single axle load (ESAL) at age t (ESAL/lane); B is the Benkelman beam maximum deflection for the existing pavement (mm); C0 is the cracking offset term calculated to ensure that predicted cracking conforms with the initial value at the start of analysis; SNC is the modified structural number.
PAVENET-R Model (1996)
The cracking prediction model defined by Eqn. (3) is used in the computer model PAVENET-R [9] aiming at the optimization of the maintenance-rehabilitation problem at the network level. The cracked area over time is predicted based on traffic and the pavement AASHTO structural number calculated using Eqn. (4). As for the previous model it is an only one phase model (just dealing with progression) and it does not include a variable that accounts for the existing cracked area at the beginning of the analysis.
Ct = 617.14×N80ct ×SN−SN
(3)
N SN =(Hn ×Cne ×Cnd
(4)
n=1
Where Ct is the total cracked area in year t (m2/100m2); N80ct is the cumulative equivalent standard axle load (ESAL) at age t (million ESAL/lane); SN is a structural number;Cne is the structural coefficient of layer n;Cnd is the drainage coefficient of layer n; and Hn is the thickness of layer n (mm).
INDIAN Model (1994)
The Indian model was derived from a pavement performance study carried out during the 90’s with extensive monitorisation of pavement sections (145) along national and state highways in four Indian states [10]. The cracking prediction model is a two-phase model, considering the time to cracking initiation calculated using Eqn. (5), and the cracking progression calculated using Eqn. (6). As for the previous models, just two variables were included (traffic and the pavement structural number). The model is applicable to pavements with asphalt surfacing (excluding surface dressing and slurry seal). The climate where the pavement data was gathered varies from arid to humid subtropical, being far from the Portuguese Mediterranean climateHDM-4 Model (2000)
The Highway Development and Management (HDM-4) system uses a cracking pavement performance model applied in two phases [6, 10-14]: the time to structural crack initiation and the structural crack progression. The HDM-4 is the successor of the World Bank Highway Design and Maintenance Standards Model HDM-III, which has been used by various road agencies all over the world for the last 20 years. Eqn. (7) is used to calculate the time to structural cracking initiation (years). Eqn. (8), (9), (10) and (11) are used to calculate the percentage of cracking over time (class 2 or worse), designed as the “all cracking” model. This formulation is valid for flexible pavements with asphalt or surface treatment as surface course and granular or asphalt base course
Thermal cracking is not considered to be an important source of cracking in Portuguese road pavements due to small number of days with subfreezing temperatures (Mediterranean climate). In a different position, reflexion cracking, which is the progression of cracks upwards to the surface from previously cracked asphalt pavements or cement stabilized materials, is important in every cracking deterioration model. It allows prediction of this pavement distress evolution after M&R actions have been taken, considering the common large concession periods. The HDM-4 model for reflexion cracking prediction.