Sources of crack in concrete

We all know cracking is one of the major problems in concrete and it should be considered while mix design, but for that, we have to know the sources which can cause cracking in concrete. Here is a list of sources of cracking in concrete.

Sources Of Cracking In Concrete :

Use of low-grade materials:

One of the major sources is the use of lower quality materials which include both concreteand steel in reinforced concrete structures.

Shrinkage:

Shrinkage can cause cracking if not controlled during mix design and curing stage. Shrinkage can become critical in high strength concretebecause of low water/cement ratio and also because of use of Mineral Admixtures.

Quality and Type Of Aggregate:

The quality of aggregate used in concretedetermines the overall strength of concrete. If the aggregate is of poor quality it will not make a proper bond with cement.

Overloading of structure:

Overloading of structure especially at younger age is a common source of cracking. This can happen if formwork is removed before time or more construction load is present.

Mistakes at design stage in office:

If there are errors at design stage than it is obvious that problems will occur at site.concrete cracking is one of them.

Improper Curing:

Another major cause of concrete cracking. If curing is not done appropriately for given time span than one should expect cracking.

Early Formwork Removal:

If formwork is removed before concrete has achieved strength, there will be cracking.

Use of Congested Reinforcement in Lean Concrete:

If you use heavy reinforcement in average quality concrete than stress distributionbetween steel and concrete can become non-linear causing cracking.

Mistakes at Site or during erection:

Proper and trained labour and workmanship are necessary for any site work. Lack of it during concreting can cause cracking.

ROLE OF SITE ENGINEER

ROLE OF CONSTRUCTION SITE ENGINEER

Role of Construction Site Engineer depends on the type of work involved and experience of site engineer in a construction project.

The duties and responsibilities of a construction site engineer are typically as follows, many of these will be delegated to other engineers on the site according to their experience and ability:

# Setting out the works in accordance with the drawings and specification

# Liaising with the project planning engineer regarding construction programmes

# Checking materials and work in progress for compliance with the specified requirements

# Observance of safety requirements

# Resolving technical issues with employer’s representatives, suppliers, subcontractors and statutory authorities

# Quality control in accordance with CSIs/procedures method statements, quality plans and inspection and test plans, all prepared by the project management team and by subcontractors

# Liaising with company or project purchasing department to ensure that purchase orders adequately define the specified requirements

# Supervising and counselling junior or trainee engineers

# Measurement and valuation (in collaboration with the project quantity surveyor where appropriate)

# Providing data in respect of variation orders and site instructions.

# Preparing record drawings, technical reports, site diary

# Job review of subordinate staff

IS CODES FOR RCC STRUCTURAL DESIGN

This article describes the basic codes for RCC structural design as per Indian standard codes. The structural design of reinforced concrete structures should be carried so as to conform to the Indian codes for reinforced concrete design, published by Bureau of Indian standards, New Delhi.

Purpose of design codes:

National building codes have been formulated in different countries to lay down guidelines for the design and construction of structures. The codes have been evolved from the collective wisdom of expert structural engineers, gained over the years. These codes are periodically revised to bring them in line with current research, and often current trends.

Following are the functions of design codes:

Firstly, the design codes ensure adequate structural safety, by specifying certain essential minimum reinforcement for design.

Secondly, they render the task of the designer relatively simple, often the result of sophisticate analysis is made in the form of a simple formula or chart.

Thirdly, the codes ensure a measure of consistency among different designers.

Finally, they have some legal validity in that they protect the structural designer from any liability due to structural failures that are caused by inadequate supervision and/or faulty material and construction.

Following are the design codes in India:

(i) IS456: 2000 – plain and reinforced concrete – code of practice (fourth revision)

(ii) Loading standard codes

The loads to be considered for structural design are specified in the following loading standards:

IS 875 (Part 1 to 5) : 1987 – code of practice for design loads (other than earthquake) for buildings and structures (second revision).

Part – 1: Dead loads

Part – 2: Imposed (Live) loads

Part – 3: Wind loads

Part – 4: Snow loads

Part – 5: Special loads and load combinations

IS 1893: 2002 – criteria for earthquake resistant design of structure (fourth revision).

IS 13920: 1993 – ductile detailing of reinforced concrete structures subject to seismic forces.

Design Handbooks:

The bureau of Indian Standards have also published the following handbooks which serve as useful supplement to the 1978 version of the codes. Although the handbooks need to be updated to bring them in line with the recently revised (2000 version) of the code, many of the provisions continue to be valid (especially with regard to structural design provisions).

SP 16 – 1980 – Design Aids (for Reinforced Concrete) to IS456: 1978

SP 24: 1983 – Explanatory handbook on IS 456: 1978

SP34: 1987 – Handbook on Concrete Reinforced and Detailing.

LIST OF IS CODES FOR REINFORCEMENT

1.  IS:432 – Mild steel & medium tensile steel bars and hard drawn steel wires for concrete reinforcement : Part-II -Hard drawn steel wire.

2. IS:1786 -  Specification for High strength deformed steel bars and wires for concrete reinforcement.

3.  IS:2502 -  Code of practice for bending & fixing of bars for concrete reinforcement.

4.  IS:2751 -  Recommended practice for welding of mild steel plain  & deformed bars for reinforced construction.

5.  IS:5525 -  Recommendation for detailing of reinforcement in reinforced concrete works.

6.  IS:9077 -  Code of practice for corrosion protection of steel reinforcement in RB & RCC construction.

7.  SP:34 – Handbook on concrete reinforcement detailing.

Construction Error during concreting

We had show below the common error happens while doing concreting at site. These errors not only occur during new construction, but may also happen during repair or rehabilitation works.

(1) Adding water to concrete: Water is usually added to concrete in one or both of the following circumstances:

First, water is added to the concrete in a delivery truck to increase slump and decrease pouring or placement effort. This will lead to concrete with lowered strength and reduced durability. As the water/cement ratio of the concrete increases, the strength and durability will decrease.

In the second case, water is commonly added during finishing of structural member. This leads to scaling, crazing, and dusting of the concrete.

(2) Improper alignment of formwork: Improper alignment of the formwork will lead to discontinuities on the surface of the concrete. While these discontinuities are unsightly in all circumstances, their occurrence may be more critical in areas that are subjected to high velocity flow of water, where cavitation-erosion may be induced, or in lock chambers where the “rubbing” surfaces must be straight.

(3) Improper consolidation or compaction of concrete: Improper compaction of concrete may result in a variety of defects, the most common being bugholes, honeycombing, and cold joints.

Bugholes are formed when small pockets of air or water are trapped against the forms. A change in the mixture to make it less “sticky” or the use of small vibrators worked near the form has been used to help eliminate bugholes.

Honeycombing can be reduced by inserting the vibrator more frequently, inserting the vibrator as close as possible to the form face without touching the form, and slower withdrawal of the vibrator. Obviously, any or all of these defects make it much easier for any damage-causing mechanism to initiate deterioration of the concrete.

Frequently, a fear of overconsolidation is used to justify a lack of effort in consolidating concrete.

Overconsolidation is usually defined as a situation in which the consolidation effort causes all of the coarse aggregate to settle to the bottom while the paste rises to the surface. If this situation occurs, it is reasonable to conclude that there is a problem of a poorly proportioned concrete rather than too much consolidation.

(4) Improper curing: Curing is probably the most abused aspect of the concrete construction process. Unless concrete is given adequate time to cure at a proper humidity and temperature, it will not develop the characteristics that are expected and that are necessary to provide durability. Symptoms of improperly cured concrete can include various types of cracking and surface disintegration.

In extreme cases where poor curing leads to failure to achieve anticipated concrete strengths, structural cracking may occur.

(5) Improper location of reinforcing steel: This section refers to reinforcing steel that is improperly located or is not adequately secured in the proper location.

Either of these faults may lead to two general types of problems. First, the steel may not function structurally as intended, resulting in structural cracking or failure. A particularly prevalent example is the placement of welded wire mesh in floor slabs. In many cases, the mesh ends up on the bottom of the slab which will subsequently crack because the steel is not in the proper location. The second type of problem stemming from improperly located or tied reinforcing steel is one of durability. The tendency seems to be for the steel to end up near the surface of the concrete. As the concrete cover over the steel is reduced, it is much easier for corrosion to begin.

(6) Movement of formwork: Movement of formwork during the period while the concrete is going from a fluid to a rigid material may induce cracking and separation within the concrete. A crack open to the surface will allow access of water to the interior of the concrete. An internal void may give rise to freezing or corrosion problems if the void becomes saturated.

(7) Premature removal of shores or reshores: If shores or reshores are removed too soon, the concrete affected may become overstressed and cracked. In extreme cases there may be major failures.

(8) Settling of the concrete: During the period between placing and initial setting of the concrete, the heavier components of the concrete will settle under the influence of gravity. This situation may be aggravated by the use of highly fluid concretes. If any restraint tends to prevent this settling, cracking or separations may result. These cracks or separations may also develop problems of corrosion or freezing if saturated.

(9) Settling of the subgrade: If there is any settling of the subgrade during the period after the concrete begins to become rigid but before it gains enough strength to support its own weight, cracking may also occur.

(10) Vibration of freshly placed concrete: Most construction sites are subjected to vibration from various sources, such as blasting, pile driving, and from the operation of construction equipment. Freshly placed concrete is vulnerable to weakening of its properties if subjected to forces which disrupt the concrete matrix during setting.

(11) Improper finishing of flat concrete surface:The most common improper finishing procedures which are detrimental to the durability of flat concrete surface are discussed below:

Adding water to the surface: Evidence that water is being added to the surface is the presence of a large paint brush, along with other finishing tools. The brush is dipped in water and water is “slung” onto the surface being finished.

Timing of finishing: Final finishing operations must be done after the concrete has taken its initial set and bleeding has stopped. The waiting period depends on the amounts of water, cement, and admixtures in the mixture but primarily on the temperature of the concrete surface. On a partially shaded slab, the part in the sun will usually be ready to finish before the part in the shade.

Adding cement to the surface: This practice is often done to dry up bleed water to allow finishing to proceed and will result in a thin cement-rich coating which will craze or flake off easily.

Use of tamper: A tamper or “jitterbug” is unnecessarily used on many jobs. This tool forces the coarse aggregate away from the surface and can make finishing easier. This practice, however, creates a cement-rich mortar surface layer which can scale or craze. A jitterbug should not be allowed with a well designed mixture. If a harsh mixture must be finished, the judicious use of a jitterbug could be useful.

Jointing: The most frequent cause of cracking in flatwork is the incorrect spacing and location of joints.