The work'of storing, cutting, bending and fixing reinforcement becomes easier with proper planning and careful selection of storage and working area. The requirement will differ from job to job depending on the quantum of work and whether or not all above activities are to be executed at site.
a) Storage
- The points to be borne in mind regarding storage are as follows:
- The bars must be laid so that the correct type, size and length are located easily without double handling.
- The bars must not get contaminated by materials like mud, oil and grease.
- The bats should be stored on a raised platform or wooden sleeper precast concrete supports so that they do not get contaminated with mud, water, moisture or humidity and start rusting. The supports should be close enough so that bars do not develop a bend on account of improper storage. In addition, a suitable all round protective cover of polythene sheeting is preferred and it remarkably improves performance of reinforcement steel as it prevents rusting to a great extent during storage.
- Long storage period specially in high humidity conditions and monsoon period should be avoided. Alternatively steel reinforcement must be properly covered and protected from rain and moisture.
Generally, in our country, cutting and bending of steel bars is done manually. Even coiled steel is straightened manually at site. Equipment are now easily available for cutting, bending and straightening reinforcement steel. The job can be done much faster, giving better quality and economy.
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| Equipment for Cutting of Reinforcement Steel |
The cutting area should be as close to the storing area as far as possible, so that handling is simple and does not require long leads to transport. Handling of the bars has to be done properly and carefully during each and every operation. Avoid throwing the bars as bends and kinks will develop at undesired locations which will be difficult to straighten at a later stage.
Before cutting. the bar should be correctly selected. The type of steel diameter and length should be properly checked and verified. Proper planning of cutting schedule is necessary. This can prevent necessary wastage and scrap formation.
It is recommended to use steel tapes for measuring bars as there are less chances of an error when steel tapes are used.
The cutting lengths must be properly established from the bar bending schedules. This is extremely necessary when hard required for stirrups are cut. If the length cut is short then the stirrups may not properly or may create problems of-increased cover or decreased effective depth. Similarly if the length cut is more, than the stirrup will be fixed in a skew manner and result in less cover or congestion of reinforcement at certain locations.
Bending should be carried out with proper machine whether hand operated or powered. Use of manual makeshift Methuselah will produce poor results and are dangerous.
Some precautions required for bending are listed below:
- Before bending a large complicated bar shape, prepare a full scale drawling which can be used for checking or alternately prepare a jig or gauge for checking bar diameters and shapes before cutting and bending. Visual identification of bar sizes should be avoided.
- Bending machines or jigs may cause variations, so before bending a number of bars with several bends especially with critical dimension, the first finished bar should be properly checked 10 see that is of correct dimensions.
- Check the diameter of the mandrel before bending. The inside radius of the bend should not exceed '2d' for mild steel bars and '3d' for high-yield bars ('d' being bar diameter). Too tight a radius will Windbreaker the bar.
- Always bend high-yield has add. Mild steel bars of large diameter may be heated to a cherry red heat but must not be quenched in water to cool them.
- During cold weather the speed of bending be reduced for high-yield bars as steel becomes more brittle below 5°C. This reduced speed of bending also applies to dowel hard which have been bent already and require straightening. Bending of high-yield steel bars by 90" for the &meter above 10 mm for the dowels is generally not recommended as at later stage while straightening it is likely to break.
- Make sure that latest revisions of drawings and schedules are available and referred. Superseded drawings and schedules must be destroyed once revisions are received at site.
- The bar to be bent must be properly gripped while being bent. If support is not firm than bar may bend in the wrong place.
- Cut and bend bars must be bundled and labelled before stacking so that filters can be easily spot them and fix them at a correct location. Each bundle should not weigh more than 75 kg, it should have bars of identical shape and size, and have label which indicates length, size, shape and type of bar used.
Reinforced concrete structure is a composite mass whose performance greatly depends on the bond between the steel bars and the concrete placed and compacted around them. The quality of bond will greatly depend on the cleanliness of the steel bar surface and the quality of concrete around it. It is therefore absolutely necessary that bar surfaces and free from grease. oil, mud, mould coil, loose mill scales, excessive rust, loose concrete andor ice.
It is also recommended not to leave the steel bars exposed for a long period of time (more than two weeks) after they are fixed. This will cause rusting of
steel and subsequent staining of concrete.
The effect of rust on the bond between steel and concrete is often a debatable issue on site. A little rust cor brownish tinge on the bar is not harmfulness for reinforced concrete. However, lexer mill scales or excess flaky rust must be removed by firm tapping or scraping. Reinforcement steel, before it is placed and embedded in concrete, reals with oxygen and moisture in the atmosphere while it is being transported and stored. This causes formation of thin film of rust which is reddish brown in colour. This reddish brown film, so long as it is skin deep, is considered as desirable.
Thus brownish coating is said to improve bond quality and is not easily removable even after wire brushing or scraping. The brownish tinge can remain and it need not be removed.
Besides improved bond quality, this thin brownish till on the surface of the reinforcement also offer passivation to steel surface against corrosion.
Steel which is stored for a long period of time also needs to be checked for cleanliness and excessive rusting before cutting and bending is done.
d) Cover and Spacers
To protect the reinforcement from corrosion and for the composite action of the reinforcement and concrete to be effective, it is absolutely necessary tor the steel bars to be surrounded by sufficiently impermeable strong concrete. If moisture and chlorides penetrate into concrete the steel bars will rust and due to formation of ferrous oxide or ferrous hydroxide there will be expansion in the volume of steel nearly 7 to 8 times its original size. This increase in volume of steel creates tensile forces in concrete specially in the cover portion and causes spelling of concrete and exposes the concrete to further attack.
All reinforcement steel has to be embedded to a certain minimum distance from the surface. This distance is called 'cover' and is specified by the Structural Engineering, in the drawings, depending on following actors:
- Quality and grade of concrete
- Environment in which concrete is located
- Thickness of structure and its location
- Special provision to resist fire.
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