| 1. Why
is Proper Placement
important? |
| Improper
placement of
reinforcing steel can greatly reduce the strength and life of a
structure
requiring premature costly repairs and replacement of structures. It
may also
to a structural failure. The failure of a structure does not
necessarily
mean the collapse of a structure. A structure fails when it
can no longer
be used in the manner in which it was intended. |
| 2. Different
aspects of
reinforcement placement |
The
important aspects
of reinforcement placement are:-
- Concrete
Cover
- Lap
Length
- Development
Length
- Bar
Spacing and Bar Size
|
| 3. Concrete
Cover |
|
Cover
less than the design cover: The life of the reinforcing
steel can be shortened
due to corrosion from increased exposure to deicing materials and/or
the
elements. Corrosion of the reinforcing steel will cause an increase in
the
diameter of the steel, which will cause the concrete outside the steel
to
sometimes spall off and the concrete around the reinforcing steel to
become
debonded from the reinforcing steel. This debonding of the
concrete from
the reinforcing steel can reduce the strength of the structure by
preventing
the necessary interaction between the concrete and the reinforcing
steel.
Cover more than the design
cover: will reduce the strength of the
structure. Placing the reinforcing steel with more than the
design cover
causes the neutral axis to be shifted higher in the section, which
reduces the
area of concrete that is in compression and increases the cracked area
of
concrete in tension. This decrease in useful concrete (over Neutral
Axis) and
increase in useless concrete (below Neutral Axis) greatly
reduces the strength
of the structure.
|
| 4. Lap
Length |
|
It
would be ideal to
make reinforcing bars in structural members (like slabs, columns, beams
etc)
one continuous bar, this however would be impractical due to
difficulties in
transporting and handling the steel.
In
order to achieve
the same effect as having one continuous bar, shorter bars with minimum
(over)
lap lengths are used.
The
design
lap length
is usually a minimum length required to transfer stress from one bar to
another. If the actual lap length is less than the
required lap length, the
stress may not be transferred to the other bar, which could cause a
failure in
the structure at that lap location.
|
| 5. Development
Length |
|
The
development length is
often shown on the plans as a minimum embedment length. The purpose of the
development length is to anchor the reinforcing bars beyond the area
where the
strength of the bars is needed.
Without the required development
length, the
reinforcing bar would pull out of the concrete surrounding it and the
structure
could fail.
A typical example of this situation would be the top transverse
reinforcing bars in the cantilevered section of a bridge slab outside
the
exterior beam of a bridge. The critical section of the slab is just
outside the
outside edge of the beam supporting the slab. The reinforcing bars must
extend
into the slab beyond the critical section for a required length. If the
reinforcing bars do not extend beyond the critical section
sufficiently, they
will be pulled out and the structure will fail. |
| 6. Bar
Spacing and Bar Size |
| Both
the spacing and the size of the reinforcing
bars control the amount of steel in the tension area. The
amount of reinforcing
steel in the tension area of the structure has a large impact
on the
strength of the structure. |
| 7. Photos
showing proper placement
of Renforcing bars |
 |
| Template
used in a pier
footing to assure proper spacing & alignment of column steel |
 |
| Hoop
bars in pier diaphragm
are properly and adequately supported |
 |
| Two
separate mats of
reinforcing steel are independently supported in this slab for the RCB |
 |
| Proper
spacing of vertical
column bars |
 |
| Vertical
bars in a culvert
wall are properly and adequately supported |
 |
| The
vertical wall steel is
properly secured to prevent it from drifting inward |
