Overview

Concrete is a versatile material that has been widely used since the 19th century, contributing to the construction of many structures in modern times. While concrete offers various benefits, it also comes with some challenges, especially the occurrence of cracks in structures. Understanding the causes of these cracks and how to address them is important to maintain the integrity of concrete structures.

Cracking in any concrete structure is very dangerous and poses a threat to the stability of the structure. Cracks in concrete structures cannot be ignored or reduced to zero from their initial microscopic level. However, effective measures can be taken to repair these cracks and make the structures stable and safe for occupancy, while ensuring the usefulness of buildings and other structures.

In IS: 456-2000, it is given that limit state of serviceability covers below two main parameters

1. Deflection
2. Cracking

Learning Outcome

After reading this article, you will gain insights into

Cracks in Concrete and Their Classification: Explore the types of cracks that can manifest in concrete structures, each indicating different issues and potential risks.

Core Mechanism of Cracks in Concrete: Delve into the fundamental mechanisms that lead to the formation of cracks in concrete, offering a deeper understanding of the underlying processes.

Causes of Cracks in Concrete: Identify the various factors contributing to cracks in concrete, ranging from shrinkage and settling to structural overload and environmental conditions.

After carefully reading an article, you will understand the topics and also ensure that cracks do not arise in buildings or any concrete structures. Also, you will get an Idea how to repair them if cracks are present. Cracks in any concrete structure can be different, but in a general manner, concrete structures are only classified into two types as per IS Code 456-2000 or SP 25 (1984).

A. Structural cracks.

Structural Cracks are due to

Faulty construction: It is very obvious that poor construction quality, negligence in choosing materials, and not adhering to proper guidelines, such as inadequate compaction, improper curing, or insufficient reinforcement, can lead to structural weaknesses and eventual cracking.

Improper design: Flaws in structural design, such as miscalculations of loads, inadequate sizing of structural elements, or insufficient consideration of environmental factors, can contribute to the development of cracks.

Exceeding load capacity: Applying loads that exceed the designed capacity of a structure can cause structural overstrain, resulting in cracks. This may be due to increased live loads, seismic activity, or other external forces.

Example

Extensive cracking of RCC beam: Extensive cracking in reinforced concrete (RCC) beams is a clear example of structural cracking. This could be a sign of various issues, such as:

• Overloading: A beam may be loaded beyond its design capacity, leading to stress-induced cracks.
• Inadequate reinforcement: Insufficient or improperly placed reinforcement can compromise the load-handling capacity of the beam, resulting in cracks.
• Design flaws: Errors in structural design, such as underestimating loads or ignoring important design considerations, can contribute to cracking

B. Non-Structural cracks as per the IS Code 456-2000 or SP 25 (1984).

Non-structural cracks (as per the IS Code 456-2000 or SP 25 (1984) Are those that develop internally due to stress-induced factors. Although these cracks may not directly impact the structural strength of a building, their long-term effects can be seen through moisture ingress affecting the steel reinforcement. The weakening of steel induced by moisture ingress can ultimately compromise the structural integrity of the building.

Over time, non-structural cracks caused by factors such as weathering or shrinkage can cause corrosion of the reinforcement. In turn, this corrosion has the potential to render the structure unsafe. An example of non-structural cracking is the development of vertical cracks in a long composite wall due to shrinkage or thermal movement.

Non-structural cracks do not usually pose an immediate threat to the safety of a building. However, they may be aesthetically displeasing, create the perception of faulty workmanship, or give a feeling of instability in certain situations. Additionally, these cracks, through moisture ingress, (as earlier mentioned on the above) can damage interior finishes, increasing maintenance costs.

A deep understanding of the cracks and their type or size is essential. Cracks can exhibit different characteristics, such as being of uniform width or narrowing or widening at different ends. They may appear straight, jagged, stepped, forming map patterns, or haphazardly. Additionally, cracks may be vertical, horizontal, or diagonal, (Tension crack, Compression crack, Share) and they may be confined to the surface or extend through multiple layers of material. The occurrence of closely spaced fine cracks on the surface is sometimes called “crazing”.

Cracks caused by different reasons have different characteristics. Careful observation of these characteristics is important to accurately diagnose the cause or causes of cracking and determine appropriate remedial measures.

Shrinkage cracks can vary in width and distance, depending on certain properties of the construction material. They may be wider but more spaced apart, or thinner but closely spaced. As a general rule, thin cracks, even if closely spaced and large in number, are less damaging to the structure and not as objectionable from aesthetic and other considerations as a small number of wide cracks.

For those who are not well versed with the strength of concrete, it is significant to note that concrete is strong in compression but weak in tension. Despite its good compressive strength, it is comparatively weak in tension. The classification of non-structural cracks and their sizes can be found in the table below.

Concrete Failure Mechanism

Well, we are aware about the Concrete is weaker in tension and stronger in compression. To overcome this disadvantage, generally we use steel bars during concrete construction in beams and columns etc. The steel bars provide strength in compression, preventing failure in tension.

As per the recent Studies have shown that crack formations are closely linked to tensile and compressive loading on the concrete. Cracks occur when there is restraint to movement due to dimensional changes caused by internal stresses. These internal stresses can be tensile, compressive, or shear.(as discussed earlier)

In the case of compressive loading, before loading starts volumetric changes occur in cement, resulting in cracks at mortar and aggregate boundaries. Until the load applied is under 30% of the compressive strength of the concrete, these boundary cracks do not extend beyond the boundaries. However, when the load exceeds this limit, cracks form throughout the concrete. With further increases in compressive load above 70%, these cracks travel deeper in the concrete until it eventually fails and collapses. In the case of tensile load, this upper limit is 60% of the tensile strength of concrete.

Microcracking is generally not very dangerous for concrete, but these microcracks may accumulate and travel deeper, and later it can create problems for the structure. Some researchers suggest that if microcracking occurs before loading initiation, it may not affect the strength. However, this applies only to cases with the least water-cement ratios, as prior loading cracks may increase when met with shrinkage cracks.

Studies of stress-strain graphs have shown that the beginning of major cracks is related to the Poisson’s ratio of concrete. Cracks increase with an increase in the Poisson’s ratio.

Causes of Cracking in Concrete

• High water cement ratio
• Loss from concrete surface
• Rapid setting
• Poor maintenance
• Structure settlement
• Corrosion in steel
• Improper concrete mix
• Weathering
• Improper structure usage
• Design defects
• Increase in loading
• Bad quality of materials used
• Bad placing techniques or inadequate vibration
• Vegetation
• Unskilled labour
• Thermal stresses generation
• Concrete movement
• Chemical attacks

Prevention Of Cracks in Structures or HOW to Fix the concrete cracks,

• Choice of material
• Specifications for Mortar and concrete
• Architectural design of buildings
• structural Design
• Cnstruction practice and techniques and
• Environments

The general cause of cracks in concrete structures includes moisture changes, thermal variations, stress, and chemical actions.

Choice of Materials

When constructing buildings, certain material properties significantly influence cracking. It is crucial for engineers and architects to possess proper knowledge of these properties to avoid or minimize cracking. Properties that influence cracking are drying shrinkage, moisture movement, thermal expansion, modulus of elasticity, porosity, creep, thermal conductivity, thermal insulation, thermal capacity, reflectivity, and chemical composition.

Masonry Units

• It is essential not to use burnt clay bricks and other burnt clay products in masonry for at least two weeks in summer and three weeks in winter after unloading from kilns. These products should be kept exposed to the atmosphere during this period.
• It is vital to use well-burnt bricks in masonry.
• Avoid using burnt clay bricks containing excessive quantities of soluble sulphates. If their use cannot be avoided, take suitable precautions against sulphate attack.

Important Note

In the case of structural concrete in foundations, if the sulphate content in soil exceeds 0.2 percent or in groundwater exceeds 300 ppm, it is advisable to use very dense concrete. You can either increase the richness of the mix to 1:1 1/2:3 or use Sulphate Resisting Portland cement/super sulphate resisting Portland cement or a combination of the two methods, depending on the sulphate content of the soil.

Similarly, for mortar in masonry, increase the richness of the mix (1:1/2:4 – 1/2) or (1:1/4:3) or use special cement, or a combination of the two methods.

For superstructure masonry, avoid using bricks containing more than 1% sulphate in exposed situations (e.g., parapets, freestanding walls) and more than 3% in less exposed locations. If unavoidable, use rich cement mortar (1:1:4 or 1:1:3) for masonry and plaster or use special cements mentioned earlier. Take precautions to prevent dampness in masonry, and in certain situations, introduce closer intervals for expansion joints.Gypsum Plaster: Gypsum plaster, due to its sulphate content, chemically reacts with Portland cement in the presence of moisture. Therefore, gypsum plaster should never be gauged with cement and should not be used in locations where the wall is likely to get damp, making it unsuitable for external work exposed to rain

When using units with high values of drying shrinkage, such as concrete blocks and sand-lime bricks, take precautions as mentioned in the table

Important Note

To avoid cracking in brick masonry due to minimal expansion, burnt clay bricks should be exposed to the atmosphere after unloading from kilns for a minimum period of 2 weeks in summer and 3 weeks in winter before use

TABLE-2: GENERAL PRECAUTIONS FTABLE-2: GENERAL PRECAUTIONS FOR AVOIDING SHRINKAGE CRACKS IN THE USE OF SOME COMMON BUILDING MATERIALSOR AVOIDING SHRINKAGE CRACKS IN THE USE OF SOME COMMON BUILDING MATERIALS

Fine Aggregates:

• Excessive fineness or a high content of clay or silt in concrete and mortar aggregates is undesirable.
• The percentage of clay and silt in fine aggregates, especially uncured, should not exceed 3%.

Coarse Aggregate Density:

• Coarse aggregates for concrete should be well-graded to achieve high density.
• The maximum size of coarse aggregates should be as large as possible, considering job requirements and concrete workability.
• Avoid porous stones with a high shrinkage coefficient in coarse aggregates.
• Using excessive soluble sulphate, especially with brick aggregate in the base structure, should be avoided.
• Coarse aggregates should not contain fine particles exceeding 3%.

Cement:

• When alkali-reactive aggregates are unavoidable, the alkali content of cement should not exceed 0.6%.
• If low-alkali cement is not economically viable, the use of pozzolanas should be considered to mitigate alkali-aggregate reactions.
• When bricks with excessive soluble sulphates are unavoidable, increase the cement content in mortar, or use special cements such as sulphate-resisting Portland cement or super-sulphated cement.
• In massive structures, to limit the heat of hydration, consider using low-heat cement unless alternative methods are adopted to prevent a rise in concrete temperature.

Timber and Timber Products

• Avoid using unseasoned timber in woodwork and joinery.
• For large joinery panels (width larger than 25 cm), prefer plywood or blockboard panels over plain wood panels for internal work due to their superior dimensional stability.

CALCIUM CHLORIDE

• Avoid the use of calcium chloride in concrete as an accelerator whenever possible. If unavoidable, limit its quantity to 2 % of the cement content

GYPSUM

• Gypsum plaster (CaSO4) should not be used for external or internal work in locations prone to moisture.
• Keep in mind that gypsum and Portland cement are incompatible, leading to harmful chemical reactions in the presence of moisture.

WOOD WOOL

• When using wood wool slabs in partitions, conceal moisture movement by providing cover strips at joints and surrounds (or Follow The above Table)

STEEL REINFORCEMENT

• Avoid using steel as reinforcement in exposed brick masonry situations unless special precautions are taken to prevent rusting, (corrosion is The Serious problem)

Specifications for Mortar and Concrete

• Components most prone to cracking are walls, floors, plasters, and concrete work.
• Specifications for mortar and concrete should prioritize strength, durability, and resistance to shrinkage cracks.
• Mortar with coarse sand is recommended for plaster to reduce the likelihood of cracking

CEMENT CONCRETE

• The concrete mix should not be richer than required for strength. Aim for strong and durable concrete through careful mix design, aggregate grading, control of water-cement ratio, thorough mixing, proper compaction, and adequate curing. Avoid an overhanded
• Use the minimum amount of water in concrete consistent with laying requirements and proper compaction to minimize shrinkage and consequent cracking

Architectural Design of Buildings

Factors in architectural design affecting cracking include large spans, provision of large windows, and placement of door and window frames. Flush placement of frames should be avoided, and joints should be concealed with moulding strips

Structural Design of Buildings

• Stress in masonry walls should be uniform to avoid differential strain and shearing stresses.
• Flexural members like slabs and beams should have adequate stiffness to limit deflection.
• Limit the width of flexural cracks in concrete for both internal and external members
• In rigid structures, account for thermal and shrinkage stresses in the design

Structural Design of Foundation

• Ensure a uniform bearing pressure on the foundation soil to avoid differential settlement Choose a safe bearing pressure that keeps overall settlement within reasonable limits for the type of structure
• On shrinkable clay soils, address moisture-induced soil movements by providing special foundations like under-reamed piles and waterproof aprons

Construction Practices and Techniques:

MOVEMENT JOINTS

• Provide movement joints in structures following the provisions of 3.11 and guidelines in Table 5.

FILLING IN PLINTH

• Use good soil free from organic matter, brick-bats, and debris for plinth filling. Lay it in 25cm thick layers, well-watered and compacted to prevent subsidence and cracking of floors. Special precautions are needed for deep filling or flooring bearing heavy loads

MASONRY WORK

• Masonry work should proceed uniformly to avoid differential loading on the foundation.
• Mortar for masonry should not contain excessive water. Curing for masonry work is recommended for a minimum of 7 to 10 as per the IS Codes,
• Masonry work on RCC (Known as Controlled Concrete) slabs and beams should not start until at least 2 weeks have passed after striking the

CONCRETE WORK

• Compact concrete by vibration, when possible, to enable the use of low-slump concrete
• Avoid concreting in very hot, dry, and windy conditions. If unavoidable, take precautions to control the temperature of fresh concrete and prevent quick drying
• Curing should be done for a minimum period of 7 to 10 days and terminated gradually to avoid quick drying
• For RCC members prone to large deflection under load, such as cantilevered beams and slabs, defer the removal of cantering and imposition of load to allow the concrete to attain sufficient strength
• Water used for mixing and curing concrete should not contain impurities beyond permissible limits

RCC FRAMED CONSTRUCTION:

• Complete framework before starting work on panel walls for cladding and partition walls.
• Defer the construction of panel walls and partitions as much as possible and proceed from top to bottom
• Provide horizontal movement joints between the top of panel walls and the soffit of the beam. When structurally necessary, provide lateral support to the walls at the top using telescopic anchorages or similar restraints
• When partition walls are to be supported on floor slabs or beams, provide upward camber in the slab/beam to prevent deflection
• Provide a horizontal expansion joint between the top of a partition wall and the soffit of the slab/beam, filling the gap with some compressible jointing material
• If a door opening is to be provided in a partition wall, a central opening is preferred over an off-centre opening
• Defer plaster work on panels and partitions as much as possible

PACE OF CONSTRUCTION

Construction pace can impact the occurrence of cracking. Masonry items should be properly cured and allowed to dry before plastering work begins [refer to Table 2 This conceals shrinkage cracks in masonry within plaster work.

Plaster work should undergo curing and drying before applying finishing coats. This allows plaster to undergo unavoidable shrinkage before finishing, concealing cracks.

In the case of concrete work, it’s essential that before construction of any masonry work either over it or by its side, drying shrinkage, creep, and elastic deformation should be allowed to take place. Construction schedules should consider these factors, and the pace of construction should be regulated to avoid rushing jobs unnecessarily [see 5.10.1].

EXTENSION TO AN EXISTING BUILDING

When making a horizontal extension, introduce a slip joint/expansion joint between old and new work to prevent cracks at the junction due to settlement of soil under the load of the new portion.

When making a vertical extension, work should proceed at a uniform level all around to avoid differential loading on the foundation. Despite precautions, cracks may appear in lower floors due to increased elastic deformation and creep in RCC columns. Renewal of finishing coats on walls of the old portion should be deferred to concerete

Important Links

Downlod IS Code 456-2000 Click here

Downlod handbook Sp 25: (1984) Click here

USE OF PRECAST COMPONENTS

Judicious use of precast components can help reduce the incidence of cracking since such components are pre-shrunk [refer to 2.5.4].

CONTROLLING HEAT OF HYDRATION

In massive concrete structures, the heat of hydration of cement should be controlled to prevent cracking. This can be achieved by using low-heat cement or adding pozzolanas in the concrete. Pre-cooling aggregates and mixing water or cooling freshly laid concrete with refrigerated water can also be effective

TREATMENT ON EXTERNAL WALLS WITH COMPOSITION RICH IN CEMENT

When giving treatment to external walls rich in cement (e.g., artificial stone finish, terrazzo), lay the finish in small panels with deep grooves in both directions

RCC ROOF SLAB

Provide adequate thermal insulation or protective cover on RCC roof slabs to check thermal movement and consequent cracks. Slab supports should permit unrestrained movement

ENVIRONMENTS

Consider precautions related to vegetation, especially for buildings founded on shrinkable soil. Fast-growing trees should not be grown within the expected height of the trees. When removing old trees close to a building, it should be done in stages if possible Construction on a site with shrinkable soil should not start until the soil has normalized in terms of moisture content after clearing trees and vegetation

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