International Concrete Abstracts Portal

Failure in beams reinforced with a small amount of transverse reinforcement is brittle due to reinforcement rupture after critical shear cracking occurs. To eliminate this problem, standards recommend formulas to calculate the minimum amount of transverse reinforcement in reinforced concrete structures. Reinforcement can resist loads after the first crack’s appearance, preventing beam rupture from being brittle but making it somewhat ductile. This paper presents a theoretical experimental analysis to determine the minimum transverse reinforcement ratio in beams of high-strength ordinary portland cement concrete (BHSOPCC), low-strength ordinary portland cement concrete (BLSOPCC), and low-strength geopolymeric concrete (BLSGC). The beam dimensions were 150 x 450 x 4500 mm. They were subjected to a four-point bending test to assess shear failure. The transverse reinforcement ρsw,minfyk ranged from 0 to 1.16 MPa, in the ranges provided by ACI 318-19, AASHTO LRFD, fib Model Code, and ABNT NBR 6118:2014. This paper investigates the minimum shear reinforcement ratio for various types of concretes with different strengths and attempts to reevaluate the associated standards that have already been established. The parameter τwy*/τwcr proposed in this paper to define whether or not a beam has minimum transverse reinforcement is more appropriate.

ASTM C618 and AASHTO M 295 specifications for fly ash represent the primary documents used by U.S. state and federal agencies to determine the suitability of a fly ash source for use in concrete. Other countries have broadly similar specifications for fly ash. The article compares specifications from the United States, Canada, Europe, Australia, and New Zealand, noting similarities and differences. Despite its common use, several criticisms of the ASTM C618 specification exist and are discussed in this document. Specifically, concerns exist regarding its dependence on strength activity index testing for determination of fly ash reactivity and strength generation potential, and loss on ignition for quantification of unburnt carbon content, as these tests relate somewhat poorly to performance of the fly ash in concrete. Recently developed test methods that could improve some of the most problematic components of the ASTM C618 specification are discussed.

This paper presents the results from a combination experimental and analytical study that investigated the use of the strutandtie model for analyzing posttensioned anchorage zones. The portion of the study described in the paper deals with concentric anchorage zones. The paper describes 17 specimens used to develop code provisions adopted by the AASHTO LRFD Bridge Specification. The strutandtie model successfully and conservatively estimated the capacity of the specimens. An example demonstrates how to analyze an anchorage zone using the strut-and-tie model.

One of the main reasons for setting permissible stresses in prestressed concrete flexural members is to insure that serviceability criteria are met. However, the allowable compressive stress at service seems to be unjustified. The issue of allowable compressive stress has not been emphasized in the past, since it seldom controlled in design. But now the compressive stress limit has become a critical factor and more restrictive in modern designs, such as spliced girder bridges, long-span beams in office buildings, and segmental box girders. This paper discusses the proposed ACI 318-95 Building Code and AASHTO Bridge Specifications allowable compressive stress changes. It is suggested that the ACI allowable compressive stress be 0.60 due to load combinations of transient nature, plus the corresponding prestress forces, and 0.45 due to effective prestress plus permanent (dead) loads. The reasons for the proposed changes and several examples are presented. It is shown that these changes do not adversely affect the acceptable safety margin against compression failure or deflection and fatigue limits.

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.