Crack Width Control in Design of Reinforced Concrete Pipe and Box Sections


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Title: Crack Width Control in Design of Reinforced Concrete Pipe and Box Sections

Author(s): Frank J. Heger and Timothy McGrath

Publication: Journal Proceedings

Volume: 81

Issue: 2

Appears on pages(s): 149-157

Keywords: concrete pipes; concrete slabs; crack width and spacing; criteria; reinforced concrete pipes; reinforcing steels; research; structural design; welded wire fabric.

Date: 3/1/1984

New semiempirical equations for crack width control in the vicinity of 0.01 in. (0.25 mm) applicable to flexural members such as pipe, box sections, and structural slabs are presented with comparisions with test results and equations in various codes and standards. The most common type of reinforcement in pipe and box sections is welded smooth wire fabric with main circumferential reinforcement at 2 to 4 inches on center and cross wires spaced at 6 to 8 inches on center. Typical reinforcement ratios are between 0.002 and 0.015. Most test results referenced in the paper are for this construction; however, pipes reinforced with smooth wire, smooth bars, deformed wire, deformed bars, and welded deformed wire fabric are used, and test results for pipe with all of these reinforcements are included. Furthermore, the use of ties improves crack control as shown by tests included in the programs described. The recommended design equations for crack control include the influence of surface roughness of reinforcement, area of concrete surrounding each bar or wire, and reinforcement ratio as major variables. It is shown that the nominal reinforcement stress at the specified maximum crack width decreases significantly with increasing reinforcement ratio p, a variation not considered in crack width control criteria found in current American reinforced concrete design codes. Because of this, existing crack width criteria can be unconservative in the design of concrete pipe, box sections, and other structural members with similar reinforcement. The design equations proposed in the paper have been adopted by the AASHTO Bridge Committee for use in Section 1.15.4 - Design of Precast Concrete Pipe - in the 1983 AASHTO Bridge Specification.