Cracking Moment Research Articles

Study on Crack Resistance of Steel Fiber Reinforced Self-Stressing Concrete in Old Bridge Reinforcement

Steel Fiber Reinforced Self-stressing Concrete (SFRSSC) is a new type of fiber reinforced composite material. It has various applications in civil engineering for its well known superior properties such as self-expansive performance and high tensile resistance. However, it is not widely accepted as an effective reinforcement in the rehabilitation of the old bridges at present. The primary goal of this research is to apply SFRSSC to improve the crack resistance in the negative bending moment areas of the old bridges. Firstly, a computer analysis on the internal force of the continuous T-beams with 5 spans is given in this paper. The results show that the expansive action of SFRSSC can effectively decrease the internal force in the negative bending moment area. Meanwhile, based on the experiments of 5 composite concrete inverted T-beams, the crack resistance of the beams reinforced with SFRSSC layers is investigated. The test results obviously indicated that the composite layers enhanced the cracking moments 44.9% more than conventional concrete layers, though its height is only 13.9% of the cross section height. It is concluded that the continuous beams strengthened by SFRSSC has greatly improved the crack resistance in negative bending moment areas compared with the continuous beams strengthened by conventional concrete. According to the existing theoretical models, a procedure how to determine the self-stress is supplied and a formula which evaluating the crack resistance of composite T-beams in negative moment area is deduced in order to supply references to the old bridge rehabilitation design.

Comparison of Strands and Bars for Positive Moment Connection of Continuous Precast Concrete Bulb-Tee Girders

There are a number of benefits associated with making single-span precast concrete girders behave as continuous members. A simple connection to cause this behavior uses the longitudinal steel in the deck to resist the negative moment in the girders at the support. Any positive moment at the support caused by secondary effects is resisted by either strand or bars extended from the ends of the girders and cast within a continuity diaphragm. This paper compares the performance of 2 different connections, used in the positive-moment region of full-depth, 45-inch-deep precast concrete bulb-tee (PCBT) girders. The 2 connections use extended, 1/2-in.-diameter prestressing strands bent at 90 deg. and extended No. 6 reinforcing bars bent at 180 deg., cast into a continuity diaphragm. A conventionally reinforced concrete deck was applied to the tops of the girders. Both 2-span systems were tested to determine their initial positive cracking moments, the integrity of the connections under cyclic loads, and their ultimate positive moment capacities. When subjected to similar loading conditions, the connection made with the mild reinforcing bars performed better than that made with prestressing strand.

Experimental study of the torsion of reinforced concrete members

This paper presents the results of an experimental investigation on the behaviour of 56 reinforced concrete beams subjected to pure torsion. The reported results include the behaviour curves, the failure modes and the values of the pre-cracking torsional stiffness, the cracking and ultimate torsional moments and the corresponding twists. The influence of the volume of stirrups, the height to width ratios and the arrangement of longitudinal bars on the torsional behaviour is discussed. In order to describe the entire torsional behaviour of the tested beams, the combination of two different analytical models is used. The prediction of the elastic till the first cracking part is achieved using a smeared crack analysis for plain concrete in torsion, whereas for the description of the post-cracking response the softened truss model is used. A simple modification to the softened truss model to include the effect of confinement is also attempted. Calculated torsional behaviour of the tested beams and 21 beams available in the literature are compared with the experimental ones and a very good agreement is observed.

Optimisation of flexural stiffness profiles to compensate for reduced ductility in hyperstatic reinforced concrete structures

Hyperstatic–or statically indeterminate–reinforced concrete (RC) structures of limited ductility may significantly under-achieve their intended ultimate loads if detailed to the statement of the lower bound theorem. This article presents a computational procedure which beneficially offsets that undesirable effect of limited ductility. The procedure works by iterating towards design moments which, in addition to satisfying equilibrium, are also consistent with the flexural stiffness profile of the cracked structure. A computer algorithm for the procedure is combined with finite element analysis (which is first verified) for application to FRP flexural strengthening of existing hyperstatic RC structures, and also to high steel-reinforcing of a new hyperstatic concrete structure. Typical influences–on the iterations–of erroneous hog-to-sag flexural stiffness ratios and erroneous locations of contraflexure are illustrated. It is shown that the iterations progressively improve the actual load capacity of the structure relative to the intended load capacity. Where some ductility exists before debonding of FRP from concrete, algebraic analysis based on the structure’s stiffness profile is used to help show that partial convergence between cracked and design moment distributions can lead to attainment of intended load capacity. It is concluded that these ideas permit two approaches, from reliance on ductility for conventional under-reinforced concrete structures, to this stiffness profile-optimisation approach for ductility-deficient RC structures.

Size Effect on Fatigue in Bending of Concrete

This paper presents a semianalytical method to predict fatigue behavior in flexure of concrete in the case of maximum flexural moment larger than that of the first crack moment based on force equilibrium in the critical cracked section. The model relies on the cyclic bridging law, the so-called stress–crack-width relation under cyclic tensile load as the fundamental constitutive relationship in tension. The structural size effect on fatigue in bending of concrete beams is studied by the present model. Eight series of beams, with heights from 50 to 800 mm, are analyzed. The model results show that the fatigue performance in bending, normally expressed in terms of the maximum flexural stress versus fatigue life diagram (S-N) is strongly dependent on the structural size, even when the same material parameters are used in the model. Under the same cyclic flexural stress levels, the smaller the beam height, the longer the fatigue life is. Besides this, the deformation characteristics, such as fatigue crack gro...

Design Procedure for Reinforced Concrete Beams with Large Web Openings

Based on test evidence, guidelines for the placement of large web openings in reinforced concrete beams are given, following which a simple design procedure is suggested. Generally, openings should be positioned so that chords have sufficient concrete area to develop the ultimate compression block in flexure and adequate depth to provide effective shear reinforcement. They should not be deeper than one-half the beam depth and should be located not closer than one-half the beam depth from supports or concentrated loads. For analyses for elastic bending moments and shear forces by conventional methods, the recommended procedure uses an equivalent shear stiffness incorporating an effective length for the opening and considers the applied shear to be carried in proportion to the flexural stiffness of the chords. The design of chords for strength follows ACI code provisions. Cracking at the opening is controlled by proper detailing, while deflections are calculated using the same analysis procedure but considering cracked moment of inertia and checking against code requirements.

A Modified Approach for Estimating the Cracking Moment of Reinforced Concrete Beams

A Modified Approach for Estimating the Cracking Moment of Reinforced Concrete Beams