Structural & Non Structural Cracks All concrete is susceptible to cracking, both in the plastic state and in the hardened state. All concrete has a natural tendency to crack due to either internal or external factors, generally influenced by materials, design, construction, service loads and exposure conditions either individually or in combination. Suitable measures including good material selection, design detailing and proper construction practices, need to be undertaken to prevent or at least minimize the occurrence of cracking.

Structural & Non Structural Cracks When they occur, it is important to determine whether the cracks will adversely affect the durability, load bearing capacity and the serviceability of the structure.

Structural Cracks Structural cracks are caused by applied loads Flexural Cracks and Tensile Shear Cracks Internal Micro-Cracks (Compressive cracks) Creep Accidental Overload

Flexural Cracks Cracking in reinforced concrete flexural members subjected to bending starts in the tensile zone, e.g: at the soffit of beams. Generally beams and slabs may be subjected to significant loads and deflection under these loads, with the steel reinforcement and the surrounding concrete subject to tension and stretching. When the tension exceeds the tensile strength of the concrete, a transverse or flexural crack is formed (Fig. 1). Although in the short term the width of flexural cracks narrows from the surface to the steel, in the long-term under sustained loading, the crack width increases and becomes more uniform across the member.

Fig 1 - Flexural cracks that self sealed through autogenous healing

Shear Cracks These are caused by structural loading or movement after the concrete has hardened. Shear cracks are better described as diagonal tension cracks due to the combined effects of bending and shearing action. Beams and columns are generally prone to such cracking.

Internal Micro-Cracks Micro cracking can occur in severe stress zones, due to large differential cooling rates, or due to compressive loading. These are discontinuous microscopic cracks which can become continuous and become a visible sign of impending structural problems.

Non Structural Cracks Non-structural cracks include pre-hardening cracks, cracks in hardened concrete, and cracking due to chemical effects. Non-structural cracks are influenced by properties of concrete and its constituent materials, design practices , construction practices and. where in-service conditions, restraint effects of either internal or external nature and other factors such as ambient temperature, humidity, overall exposure conditions, Due to cumulative nature, the inherent effects of one type of crack can be further worsened by the effects of another type. Some crack types allow penetration of aggressive chemical agents to the steel reinforcement, leading to corrosion of the steel and possible cracking and spalling. Inherent effects of cracking can usually be minimized and controlled by careful attention to both design details such as distribution and positioning of reinforcement and construction techniques.

Pre-hardening (Plastic) Cracks These cracks occur within a few hours after the placement and compaction of concrete, but before the concrete has fully hardened. Plastic Shrinkage Cracks Plastic Settlement Cracks Cracks Caused by Formwork movement

Plastic Shrinkage Cracks Caused by rapid drying of the concrete surface Within the first six hours (even within minutes) after placement, As a result of large moisture losses from the surface (Fig. 2). Cause of rapid drying or moisture loss (plastic srinkage cracking) Strong winds, high air or concrete temperatures and low humidity, alone or in combination, because they promote evaporation of water which exceeds the rate of bleeding of water to the surface.

Plastic shrinkage cracks can form large map patterns or they may appear as diagonal or parallel cracks of various depths. Any drying cracks which appear before or during finishing operations should be immediately closed with either a wooden or steel float and curing should commence immediately following the progressive completion of final finishing operations. If not prevented or minimised initially, plastic shrinkage cracking can be further exacerbated by subsequent drying shrinkage and thermal contraction (movement).

Fig 2 - Plastic shrinkage cracks

Plastic Settlement Cracks Caused by concrete settling under its own weight, especially when there is excessive bleeding and the settlement is impeded by a local restraint. The cracks occur in the hardening mass over restraints such as steel reinforcement, deep sections and steps in formwork. The cracks can be further exacerbated by inadequate compaction and the presence of voids under reinforcing bars.

Plastic settlement cracks can be prevented by ensuring that Plastic settlement cracks can be enlarged by subsequent drying shrinkage and become more obvious. These cracks tend to form longitudinally over the steel reinforcement and can be a cause of serious corrosion. Plastic settlement cracks can be prevented by ensuring that the concrete is a well graded, well balanced mix at appropriate water content which enables good compaction, and the formwork is rigid and not subject to movement

Cracks Caused by Formwork Movement Movement of formwork after the concrete has started to stiffen but before it has gained enough strength to support its own weight, can cause cracking. Formwork must be left in place until the concrete has gained sufficient strength to support itself. Formwork must also be sufficiently strong to avoid excessive deflections.

Cracks in Hardened Concrete Cracking in hardened concrete can be attributed to drying shrinkage (loss of moisture), early thermal contraction (movement) and structural and chemical effects. Craze Cracking Drying Shrinkage Cracks Early Thermal Contraction (Movement) Cracks

Craze Cracking Characterized by a series of very fine closely spaced map pattern cracks Caused by the shrinkage of the cementious material of the surface layer of concrete. The cracks are fairly shallow and affect the appearance more so than the structural integrity or durability. They are mainly caused by the use of wet concrete mixes, working the bleed water into the surface during finishing, and inadequate curing

Craze cracking can be prevented by ensuring that Final finishing of concrete surfaces is only carried out after all bleed water has been removed, power trowels are not overused, driers such as dry sand, cement or stone dust are not used to absorb free water, by avoiding the use of wet concrete and by adopting good curing practices.

Drying Shrinkage Cracks Occur when concrete reduces in volume as a result of moisture losses into the atmosphere in its hardened state. If the concrete is unrestrained and free to move and undergo shortening without a build up of shrinkage stresses, no shrinkage cracking will occur. However, the combination of shrinkage and sufficient restraint (for example, by another part of the structure) produces tensile stresses. When these stresses exceed the tensile strength of concrete, cracks (Fig. 4) will occur that, over time, can penetrate the full depth of the concrete.

Drying shrinkage can be reduced by A significant proportion of shrinkage generally occurs within the first few weeks, with the drying environment surrounding the concrete having a major effect. Shrinkage cracks generally appear after several weeks or even months after casting. Drying shrinkage can be reduced by increasing the amount of aggregate, particularly the larger coarse aggregate, and more importantly by reducing the total water content. Other factors which influence cracking in hardened concrete such as restraints, geometry and construction practices.

Generally drying shrinkage can range from Adequate and correctly positioned steel reinforcement can more evenly distribute shrinkage stresses within a reinforced concrete member and better control crack widths. Generally drying shrinkage can range from 450 to 750 micro-strain for high quality special class concrete and About 1000 micro-strain for normal class concrete.

Fig 4 - Drying shrinkage cracking

Early Thermal Contraction (Movement) Cracks All immature concrete elements are subject to thermal contraction or movement for up to 14 days after placement, due to temperature rise from the heat of hydration of the cementitious material. This is more pronounced in the case of higher quality special class concrete which contains higher amounts of cementitious material.

Thermal cracking may appear between one day and two weeks after construction. Larger and thicker members (i.e. columns, beams, footings, etc.) are more susceptible due to the greater heat and higher internal temperatures generated which can be as much as 45oC to 65oC. As the surface temperature falls to the ambient level, a concrete element (i.e. cooler concrete surface) is subjected to thermal contraction or movement due to the development of large temperature differentials (greater than 20oC) across the concrete element.

If this contraction is restrained by either an internal restraint such as the inner core or adjacent pervious pours, tensile stresses are induced which can cause cracking of the concrete once its low tensile strength capacity is exceeded.

Cracks due to Chemical Effects The expansive effects of chemical reaction products from corrosion of steel reinforcement on alkali-aggregate reaction can also cause cracking in hardered concrete. Corrosion of Steel Reinforcement Alkali -Silica Reaction Cracks

Corrosion of Steel Reinforcement Some cracks are induced by the expansive forces associated with corrosion of the steel reinforcement which crack and subsequently spall the concrete (Fig. 5). These cracks are mainly longitudinal in nature and are located directly above or below the reinforcement, run parallel with it and are often associated with shallow or porous cover concrete. Such cracking and spalling is noticeable at corners of columns and beams and usually show signs of rust stains. Cracking associated with corroded reinforcement usually takes a long time to become evident.

Fig 5 - Longitudinal crack at corners of crosshead associated with corrosion of steel reinforcement

Alkali -Silica Reaction Cracks The chemical reaction between the alkali hydroxide in the concrete and reactive aggregates produces an expansive gel, causing map cracking or directional cracking (pre-stressed members) in the structure. Other visible signs of damage may be aggregate pop out and discoloration.

REPAIR FOR CRACKS

repair of cracks usually does not involve strengthening repair of a structure showing spalling and disintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of the reinforcement

Repairing cracks In order to determine whether the cracks are active or dormant, periodic observations are done utilizing various types of telltales by placing a mark at the end of the crack a pin or a toothpick is lightly wedged into the crack and it falls out if there is any extension of the defect

A strip of notched tape works similarly : Movement is indicated by tearing of the tape The device using a typical vernier caliper is the most satisfactory of all. Both extension and compression are indicated If more accurate readings are desired, extensometers can be used Where extreme accuracy is required resistance strain gauges can be glued across the crack

2.1 Types of cracks active cracks and dormant cracks the proper differentiation between active and dormant cracks is one of magnitude of movement, and the telltales are a measure of the difference

If the magnitude of the movement, measured over a reasonable period of time (say 6 months or 1 year), is sufficient to displace or show significantly on the telltales, we can treat the crack as an active one. If the movements are smaller, the crack may be considered as dormant.

Cracks can also be divided into solitary or isolated cracks and pattern cracks Generally, a solitary crack is due to a positive overstressing of the concrete either due to load or shrinkage Overload cracks are fairly easily identified because they follow the lines demonstrated in laboratory load tests

In a long retaining wall or long channel, the regular formation of cracks indicates faults in the design rather than the construction, but an irregular distribution of solitary cracks may indicate poor construction as well as poor design Regular patterns of cracks may occur in the surfacing of concrete and in thin slabs. These are called pattern cracks

Repair for cracks: i) Stitching ii) Routing and sealing iii) Resin injection iv) Dry packing v) Polymer impregnation vi) Vacuum impregnation vii) Autogenous healing viii) Flexible sealing ix) Drilling and plugging x) Bandaging

Methods of repairing cracks Bonding with epoxies Cracks in concrete may be bonded by the injection of epoxy bonding compounds under pressure Usual practice is to drill into the crack from the face of the concrete at several locations

inject water or a solvent to flush out the defect allow the surface to dry surface-seal the cracks between the injection points inject the epoxy until it flows out of the adjacent sections of the crack or begins to bulge out the surface seals

Usually the epoxy is injected through holes of Usually the epoxy is injected through holes of about ¾ inch in diameter and ¾ inch deep at 6 to 12 inches centers Smaller spacing is used for finer cracks The limitation of this method is that unless the crack is dormant or the cause of cracking is removed and thereby the crack is made dormant, it will probably reoccur, possibly somewhere else in the structure

Also, this technique is not applicable if the defects are actively leaking to the extent that they cannot be dried out, or where the cracks are numerous Routing and sealing This method involves enlarging the crack along its exposed face and filling and sealing it with a suitable material

The routing operation placing the sealant This is a method where thorough water tightness of the joint is not required and where appearance is not important

This is the simplest and most common method of crack repair. ROUTING AND SEALING This is the simplest and most common method of crack repair. It can be executed with relatively unskilled labor and can be used to seal both fine pattern cracks and larger isolated cracks. This involves enlarging the crack along its exposed face and sealing it with crack fillers. Care should be taken to ensure that the entire crack is routed and sealed.

RESIN INJECTION Epoxy resins are usually selected for crack injection because of their high mechanical strength and resistance to most chemical environments encountered by concrete. Epoxies are rigid and not suitable for active cracks. This method is used to restore structural soundness of members where cracks are dormant or can be prevented from further movements.

STITCHING In this technique, the crack is bridged with U-shaped metal units called stitching dogs before being repaired with a rigid resin material. A non- shrink grout or an epoxy resin based adhesive should be used to anchor the legs of the dogs. Stitching is suitable when tensile strength must be re established across major cracks. Stitching dogs should be of variable length and orientation.

Stitching Concrete can be stitched by iron or steel dogs A series of stitches of different lengths should be used bend bars into the shape of a broad flat bottomed letter U between 1 foot and 3 feet long and with ends about 6 inches long The stitching should be on the side, which is opening up first

if necessary, strengthen adjacent areas of the construction to take the additional stress the stitching dogs should be of variable length and/or orientation and so located that the tension transmitted across the crack does not devolve on a single plane of the section, but is spread out over an area In order to resist shear along the crack, it is necessary to use diagonal stitching The lengths of dogs are random so that the anchor points do not form a plane of weakness

BENEFITS OF CRACKED STITCHING Quick, simple, effective and permanent. The grout combination provides an excellent bond within the substrate. Masonry remains flexible enough to accommodate natural building movement. Non-disruptive structural stabilization with no additional stress

External stressing cracks can be closed by inducing a compressive force, sufficient to overcome the tension and to provide a residual compression The principle is very similar to stitching, except that the stitches are tensioned; rather than plain bar dogs which apply no closing force to the crack Some form of abutment is needed for providing an anchorage for the prestressing wires or rods

Grouting same manner as the injection of an epoxy cleaning the concrete along the crack installing built-up seats at intervals along the crack sealing the crack between the seats with a cement paint or grout flushing the crack to clean it and test the seal; and then grouting the whole

Blanketing similar to routing and sealing applicable for sealing active as well as dormant cracks Preparing the chase is the first step Usually the chase is cut square The bottom should be chipped as smooth to facilitate breaking the bond between sealant and concrete

The sides of the chase should be prepared to provide a good bond with the sealant material The first consideration in the selection of sealant materials is the amount of movement anticipated and the extremes of temperature at which such movements will occur elastic sealants mastic sealants mortar-plugged joints

Use of overlays Sealing of an active crack by use of an overlay requires that the overlay be extensible and not flexible alone Accordingly, an overlay which is flexible but not extensible, ie. can be bent but cannot be stretched, will not seal a crack that is active

Gravel is typically used for roofs concrete or brick are used where fill is to be placed against the overlay An asphalt block pavement also works well where the area is subjected to heavy traffic