CRSI |
ENGINEERING
DATA REPORT NUMBER 40 |
CONSTRUCTION TOLERANCE CONFLICTS IN REINFORCED CONCRETE |
 A
SERVICE OF THE CONCRETE REINFORCING STEEL INSTITUTE
933 N. Plum Grove Rd., Schaumburg, Illinois 60173-4758 |
INTRODUCTION
The construction tolerances for cast-in-place reinforced
concrete buildings have evolved within the American Concrete Institute over the past 50
years into what is generally considered to be a reasonable set of criteria that can be
achieved using normal construction practices.
One of the early attempts to publish building tolerances appeared in
the ACI literature in 1940. The first presentation of forming tolerances in an ACI
standard appears to be in 1963 in "Recommended Practice for Concrete Formwork".
Subsequently, construction tolerances have been included in many ACI committee reports and
specifications. ACI tolerances for fabricated reinforcing bars have been developed by
CRSI. All ACI tolerances are now being consolidated under the auspices of ACI Committee
117, Tolerances. The committee has developed "Standard Tolerances for Concrete
Construction Materials". The current document is designated as ACI 117-90.
Reinforced concrete is the result of the work of several separate
trades or subcontractors who utilize various ACI tolerances applicable to their trade.
Tolerance conflicts in reinforced concrete construction are multiplied by having
potentially inconsistent tolerances within ACI with which to build and the requirement to
accommodate the tolerances of other building elements. Conflicts within the set of ACI
tolerances still exist, but most construction disputes arise from conflicts with adjoining
elements.
This report presents some common areas of conflict due to tolerances.
Recommendations of actions that may lessen or eliminate the conflicts are presented.
CONFLICTS BETWEEN ELEMENTS OF
REINFORCED CONCRETE FRAMES
Reinforcing Steel And Member Profile
The depth of a beam between 12 and 36 in., Figure 1, has a tolerance of
+1/2 in. and -3/8 in. The height of the stirrup and the position of the horizontal bar
positioned by the stirrup has a tolerance of +/- 1/2in. Since the bars are supported by
the 1-1/2 in. bar supports on the soffit, all of the tolerances affect the top cover. If
the stirrup is 1/2 in. taller and the beam is 3/8 in. shallower than theoretical, the top
cover could be reduced by 1/2 plus 3/8 or 7/8 in. An actual cover of 5/8 in. would be the
result. According to ACI 117, the tolerance for the 1-1/2 in. cover over the stirrup is
1/2 in., providing for a minimum cover of 1 in. Therefore, the cover will be reduced below
the allowable tolerance though all components of the assembly, the formwork, the rebar
fabrication, the rebar placement, and the concrete finishing, usually performed by
separate subcontractors, were within acceptable tolerances. Careful coordination of the
trades and field measurement before placing concrete is recommended to resolve this
conflict. Specifying additional cover where cover is critical may also be appropriate.
For a beam perpendicular to a sloping surface such as in a parking
garage, Figure 2, the decision on where the beam depth is to be measured also affects the
depth of the beam stirrup. Since the top slopes and the beam soffit is usually horizontal,
if the depth is measured on the downhill side, the stirrups will be the same as for
non-ramp beams. If, however, the decision is made to measure the beam depth at the
centerline, the stirrup will have to be roughly 3/4 in. less in height than for the
non-ramp beams. Making this decision also reduces the effective depth of the beam
reinforcement and therefore its capacity. The Architect/Engineer should specify the point
at which the depth is to be measured and consider the effects on the reinforcing steel.
© Copyright 1995 by the Concrete Reinforcing Steel Institute
