Why does steel rebars corrode? REASONS and REMEDIES
Concrete is the most used and popular construction material in the world. However, concrete is known for low tensile strength. Which is why reinforced bars plays a vital role in providing the concrete the needed tensile strength.
Contrary to the amount of usage of reinforced bars, it does not get the right amount of attention as it should get.
Despite being the backbone of a structure, TMT bars do not get the amount of detailed analysis and discussion before starting construction projects.
It is no surprise that the small and middle scale construction projects overlook the quality and testing of the reinforced bars.
A large part of India comes under earthquake zone IV and V, and still unfortunately the right quality and right grade of TMT is least discussed and considered criteria while selecting the construction material.
Well the low quality of reinforced bars can be the major reason behind the concrete corrosion and it is no less than a gamble game of life and security.
How does a Reinforced bar get corrosion?
We all know the amount of assurance we all get about Corrosion-resistant TMT bars. But in reality the low quality TMT bars becomes a suicidal decision in the longer run of the structure.
Here are few reasons why low quality TMT bars corrode-
1. CRACKS- 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.
Ambient temperature changes and loss of heat of hydration in portion of structure at different rate lead to temperature variations and subsequent thermal movement.
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.
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.
2. Creep Movement
Gradual and slow time dependent deformation of concrete structure under sustained loads is known as creep. It may generate excessive stress and lead to the crack development.
Creep increases with increase in water and cement content, water cement ratio and temperature.
Added to that, admixtures and pozzolans will increase creep. The increase of temperature in steel bars will increase creep as well.
However, it decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading.
3. Corrosion of Reinforcement
Reinforcement corrosion will produce iron oxide and hydroxide on steel bar surface, consequently its volume increases.
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.
Reinforcement corrosion will occur unless it is protected properly. Steel reinforcement can be protected by providing adequate impervious concrete cover. This will prevent the ingression of moisture and other aggressive elements.
Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value.
4. Moisture Movement
Most of the building materials with pores in their structure in the form of inter-molecular 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.
Shrinkage can be of plastic or dry. Factors that cause cement or mortar to experience shrinkage include excessive water, and cement quantity; rich cement mixtures suffer greater shrinkage.
Crack above window due shrinkage
5. Poor Construction practices
There are broad variety of construction practices that lead to concrete cracking. Normally, improper construction practices are due to ignorance, carelessness, greed or negligence.
Main causes for poor construction practices:
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
Construction overloads induced during construction can frequently be more serious than those imposed during service.
Inadequate quality control and supervision causing large voids (honey combs) and cracks resulting in leakages and ultimately causing faster deterioration of concrete.
Improper construction joints between subsequent concrete pours or between concrete framework and masonry.
Addition of excess water in concrete and mortar mixes.
Lastly, poor quality of plumbing and sanitation materials and practices.
6. Improper Structural Design and Specifications
Several problems can occur due to incorrect structural design, detailing, and specifications.
Errors that may occur at this stage include inadequate thickness, insufficient reinforcement, incorrect geometry, improper utilization of materials, and incorrect detailing.
Problems encountered due to those errors include cracking due to insufficient reinforcement, excessive differential movement due to improper foundation design, increased concentration of stresses as a result of poorly design re-entrant etc.…
Additionally, it is of crucial that the designer consider the environmental conditions existing around the building site.
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