Reinforcement Congestion in Cast-in-Place Concrete
Allowances for construction tolerances and for adequate placement and consolidation
by James Klinger, Oscar R. Antommattei, Aron Csont, Trevor Prater, Michael Damme, and Bruce A. Suprenant
Since 1983 edition,1 the ACI 318 Code Commentary has cautioned designers to avoid reinforcement congestion in earthquake-resistant structures. And
since its 1999 edition,2 the ACI 318 Code has required designers to consider fabrication and placement tolerances at anchorage zones for post-tensioning tendons. The relevant sections of ACI 318-193 state:
“R18.2.2 Analysis and proportioning of structural members In selecting member sizes for earthquake-resistant
structures, it is important to consider constructability problems related to congestion of reinforcement. The design should be such that all reinforcement can be assembled and placed in the proper location and that concrete can be cast and consolidated properly. Using the upper limits of permitted reinforcement ratios may lead to construction problems.”
“25.9.5 Reinforcement detailing
25.9.5.1 Selection of reinforcement size, spacing, cover, and other details for anchorage zones shall make allowances for tolerances on fabrication and placement of reinforcement; for the size of aggregate; and for adequate placement and consolidation of the concrete.”
While the engineer is responsible for detailing reinforcement, we’ve been unable to find Commentary guidance on reasonable detailing practice or acceptability of details. Compliance is, therefore, open to subjective interpretation. However, it’s clear that many designers are struggling to meet either the spirit of the Commentary or the letter of the Code.
In addition, constructability requirements and recommendations should not be limited only to earthquake- resistant structures or post-tensioning tendon anchorage zones. Engineers should provide an allowance for construction tolerances and consider the need for adequate placement and consolidation of concrete for all designs.
This article provides information on design and detailing prerequisites, reinforcement congestion economics, allowance recommendations related to congestion of reinforcement, and proposed Code and Commentary language with respect to constructability.
