PLACING TOLERANCES
Placing tolerances recognize the imprecise nature of the placing operation
and allow deviation criteria. ACI 117 Indicates tolerances on clear distance to side forms
and resulting concrete surfaces, and on clear distance to formed concrete soffits in the
direction of the tolerance. These tolerances are:
| Member Size |
|
Tolerance |
 4 in. |
|
+ 1/4 in., - 3/8 in. |
| > 4 in.,
12 in. |
|
3/8 in. |
| > 12 in.,
24 in. |
|
1/2 in. |
| > 24 in. |
|
1 in. |
The ACI 117 tolerances for concrete cover measured at right
angles to the concrete surface in the direction of the tolerances are:
| Member Size |
|
Tolerance |
| 12 in. |
|
-3/8 in. |
| > 12 in. |
|
-1/2 in. |
Reduction in concrete cover is permitted
by ACI 117. Generally the reduction should not exceed one-third of the specified cover.
For formed soffits, the reduction in cover is limited to 1/4 in.
Uniform spacing or center-to-center distance between rebars in slabs
and walls have a tolerance of 3 inches from a theoretical position. Stirrup and tie
spacing in beams and columns is plus or minus 1 inch. The tolerance in the longitudinal
location of bends and ends of bars in general is plus or minus 2 inches, but at a
discontinuous end of a member is reduced to plus or minus 1 inch. The length of lap
splices has a tolerance of minus 11/2 inches, obviously a longer splice might be
considered safer, hence there is no plus tolerance. Finally the tolerance for embedded
length is minus 1 inch. In terms of percentages, these tolerances may be considered
liberal, and normally are easily met.
The tolerance for beam and column forms is plus 1/2 inch, minus 3/8
inch for form dimensions larger than 12 inches. A potential problem occurs if the stirrups
or ties are fabricated to the plus tolerance of 1/2 inch when the forms are made to the
minus tolerance of 3/8 inch. This has the effect of reducing the clearance to 7/16 inch
each side of the stirrup or tie. The inspector must determine whether or not this
encroachment on cover is critical. As stated above, ACI 117 permits a reduction in
concrete cover of 3/8 inch when the member size is 12 inches or less, and 1/2 inch if the
member is over 12 inches in the direction of tolerance. This type of problem should be
resolved with the assistance of the design engineer.
Another example of cover encroachment is a formed footing; the form
tolerance is plus 2 inches, minus 1/2 inch, while the rebar tolerance, either straight or
bent is plus or minus 1 inch. Again, if the forms are 'minus' and the rebars are 'plus'
the cover at each end of the bars is reduced from 3 inches to 21/4 inches. This would be
acceptable since the allowed reduction in cover is 1 inch for member sizes in excess of 2
feet.
For example, the length of a beam which is discontinuous at each end
may create a problem if the formwork is 'minus' when at the same time the rebar is 'plus'.
Each trade, carpenter and ironworker, can claim that their work is within tolerance, thus
squarely putting it into the inspector's lap to make a decision. He must determine whether
the encroachment is detrimental to the safety of the structure, or within the allowable
limits of reduction in cover.
Inspection of rebar placement in walls and slabs is usually straight
forward and normally no misplacement will be found. Small openings, pipe sleeves,
electrical outlets, and similar items may interfere with the exact placement of the rebar,
but the ironworker usually will shift the bar to one side or the other to avoid the
obstruction. ACI 117 allows a maximum deviation from the specified location of 3 inches.
This normally is sufficient provided that the total number of bars in a wall or slab panel
is not reduced. Recommendations for spacing of bars in slabs or walls (Appendix C in
CRSI's Manual of Standard Practice) should be used to determine first bar spacing if that
information is not shown in the contract documents.
Spacing tolerance for stirrups and ties in beams and columns is also
quite flexible. ACI 117 allows a deviation from the specified position equal to the least
dimension of the member divided by 12, thus stirrup or tie placement in a 18-inch square
beam or column could vary from the specified location by 11/2 inches.
IN-SITU BENDING AND REBENDING
One of the more controversial construction procedures
is the practice of bending and rebending installed rebar. The reasons are many. As an
example, the horizontal leg of bent rebars projecting from a wall to become the top bars
of a beam may have been improperly placed too low or too high, well in excess of cover and
placing tolerances. These bars will require 'straightening' and then 'rebending' to the
proper height horizontally. The inspector, after consultation with the engineer, should
approve the procedure used to accomplish the task. CRSI's EDR No. 12 can be used as a
guide to recommended practice for straightening and rebending embedded rebar.
In some instances, the use of planned prebent dowels is requested by
the contractor. The engineer should approve the procedure and notify the inspector.
Similarly, a straight dowel may be planned to be field bent into place, such as an outside
face wall vertical bent horizontally to become the slab end top bar. This procedure is
intended to facilitate slab formwork erection. Again, the engineer should approve the
procedure and notify the inspector. The inspector should discuss the bending procedure
with the placing foreman to ensure that the bends conform to the ACI 315 Standard. Where
large diameter bars are involved, some amount of heating may be recommended by the design
engineer to avoid brittle failure.
LAP SPLICES, MECHANICAL
SPLICES, AND WELDED SPLICES
The location and length of lap splices has always been
a concern of the engineer, estimator, detailer, placer, and inspector. The ACI 318
Building Code states that the design drawings shall show the location and length of lap
splices, and the type and location of mechanical splices and welded splices. ACI 315
repeats the above requirement and further instructs the detailer to follow the engineer's
details, thus both the placing drawing and the structural drawing should show the same
location and length of lap splices. Chapter 12 of ACI 318 contains provisions for tension
lap splice lengths. Lengths will vary due to concrete compressive strength, steel grade,
bar spacing, epoxy coating and concrete cover. The inspector should make certain that the
tabulated lengths are for the strength criteria of the material furnished to the job site,
and that the placer follows them.
Mechanical splices are not difficult to inspect. The engineer should
approve the use of the splice furnished, and his drawings should show the location of all
splices. If a full mechanical splice is required, the inspector should make certain that a
compression only mechanical splice is not furnished in error. In all cases, inspection
should verify that the manufacturer's installation procedures, and any project
specification instructions are followed. Dowel bar mechanical splices and lap-splice
connector systems are used to replace prebent or other types of dowels which connect two
separate pours of a concrete structure. The type and location should be approved by the
engineer.
