STEEL REINFORCEMENT
is used in the form of steel bars with circular cross-section
and deformations to increase the bond between steel and concrete.
The reinforcement fabrication is the process of assembling the
reinforcing steel bars into mats or cages prior to concreting
at site. Accuracy of fabrication and dimensional tolerances are
important and should be ensured in the site. Best of designs produced
using sophisticated computer programmes will not make the structure
behave well unless reinforcement fabrication is accurate and conforms
to specifications of bar placements.
Bar Schedule
Fabrication and bending are on site operations. Bar bending
schedules is a table giving information on different types of
bars to be cut and bent for fabrication. The schedule contains
all necessary details of size, shape, dimensions of reinforcing
bars to be cut bundled and stacked at site. The schedule eliminates
the need to search for dimensions while placing the reinforcement
in the forms. This avoids the possibility of errors arising out
of wrong length or shape being used especially in the case of
bent or cranked bars.
Handling and storing bars
Methods of unloading reinforcing bars vary with the type
of structure and access to the site. The bars should be unloaded
at a place where it is convenient to cut and bend or store them.
Bars are unloaded as near as possible to the storage area to avoid
carry the bars through long distances
The fabrication of bars tendons and meshes adds almost 20 to
30 per cent to the basic cost of steel. Fabrication of reinforcements
involves the following steps:
Studying the bar bending schedule and detailing
Straightening of bars from coils or U bend shapes.
Cutting the bars to sizes.
Bending the bars to the required shapes
Bundling marking and stacking
Transportation and handling at site
Coating and treating of bars, where special protective measures
for corrosion.
Splicing of bars or welding
Binding the bars to form a rigid cage
Positioning the cage into the formwork.
Surface Preparation
The main factors affecting the bond performance in concrete structures
are the presence of scales, rust, oil and mud on the surface.
Scales appear due to rolling operation and usually fall off during
handling. Mild rust improves bond as it increases the surface
roughness. It is considered harmful only when the area of cross
section or the weight of the bar is reduced.
Bars should be stored such that they are free from mud. Any mud
coating must be washed before using the bars. When steel bars
are stored for long periods in the open they can be protected
by a cement slurry wash.
Sand blasting or torching is resorted to remove oil and scales
on the surface. Finally, the reinforcement should be free from
all impurities before placement in forms.
The preparation of reinforcement starts with straightening and
cleaning and cutting. The coiled bars are run through the straightening
machine. The standard practice is to shear the bars in group to
a tolerance of 2.5 cm in length. Bars must be bent into required
shapes accurately in accordance with the dimensions given in the
Bar schedule.
Wire ties
The bars are tied together at intersections and at sufficiently
close intervals to provide a rigid framework. This prevents the
displacement of bars before and while concrete is being executed.
For general work, a 16 gauge black soft iron wire is suitable.
It is advisable to keep the ends of the ties reasonably short
so that they do not touch the formwork as this may lead to rust
spots on the face of concrete.
To protect a bar or other reinforcement from corrosion, and to
ensure that combined action of the concrete and steel is effective
the bars are embedded at a certain minimum distance from the outer
surface of concrete. This is known as cover.
