Tension softening and cracking in drying concrete

Tension softening and cracking due to non-uniform shrinkage is investigated in a parametric study and the relation between shrinkage as a material property and the dimensional change of drying specimens is analysed. It is found that regions with tensile strains causing softening or cracking are developed in the early stages of drying. The shrinkage of a drying concrete specimen is always less than or equal to the true material shrinkage, but for normal concrete and drying situations which occur in practice the difference is small compared with the final shrinkage.     

Strategic reinforcement for controlling volume-change cracking in base-restrained concrete walls

In this work the control of volume-change cracking in concrete walls with continuous base restrains was considered. Investigators and design codes have proposed several different procedures for the calculation of the amount and distribution of steel reinforcement required for the control of this type of cracking. All these methods provide amounts of reinforcement which are in excess to that really required. This is either due to simplifying the design procedure or neglecting the effect of variable restraint in the wall. Since crack formation and its width depends on the amount of total restrained movement in th concrete member, steel reinforcement is necessary only in positions in which wide cracks are expected to form. Using finite-element analysis to obtain the distribution of restraint in the wall, degree-of-restraint contour diagrams in walls with different length/height ratios of 1, 2, 3, 4, 6 and 8 were prepared. These diagrams can be used to determine the amount and distribution of steel reinsforcement in positions in which it is really effective in controlling cracking. At positions of low restraint in the walls in which no or only narrow cracks will form, only minimum reinforcement will be provided.     

Factors controlling cracking of concrete affected by reinforcement corrosion

The present paper tries to contribute to quantifying the relationship between the amount of corrosion and cover cracking. The variables studied were: cover/diameter (c/ø), proportions of cement, w/c, cast position of the bar, transverse reinforcement and corrosion rate. The corrosion is accelerated by applying constant currents causing the rebar to act as an anode. The results indicate that the cracking process develops in two steps: generation and propagation. Radius losses of about 15–50 μm are necessary to generate the first visible crack (<0.1 mm width). The propagation follows a behaviour of the type: w (crack width in mm)=a+bx (radius loss in μm).     

基于XFEM的强震区砼重力坝开裂与配筋抗震措施研究

已有震害表明,混凝土坝遭遇强烈地震将不可避免地产生开裂。扩展有限元法（XFEM）通过在相关节点的影响域上富集非连续位移模式,使得对非连续位移场的表征独立于单元边界,可以有效描述混凝土中的裂纹扩展。基于扩展有限元模型,采用合理的地震波动模型对国内某混凝土重力坝强震下的动力破坏过程进行了分析;针对大坝破坏情况,应用嵌入式滑移模型模拟了混凝土重力坝配筋前后的地震响应和破坏状况,据此评价局部配筋的抗震效果。研究表明,局部配置抗震钢筋虽无法防止裂缝的发生,但可有效限制坝体裂缝的开裂扩展范围及深度,减少裂缝的开度,有效改善坝体的抗震性能。     

Shakedown with softening in reinforced concrete beams

Critical softening parameters for first-formed highes at any location in a two-span beam are derived, and the relationship between positive load increments and softening parameter less than critical for midspan or interior support highes are also determined. The effect of non-critical softening on the well known static collapse and shakedown loads for an elastic-plastic beam is found. It is shown that the combined effect of softening and residual moments (such as caused by differential settlements) on the static collapse and shakedown loads may be dramatic.     

Lattice modelling of corrosion induced cracking and bond in reinforced concrete

A lattice approach is used to describe the mechanical interaction of a corroding reinforcement bar, the surrounding concrete and the interface between steel reinforcement and concrete. The cross-section of the ribbed reinforcement bar is taken to be circular, assuming that the interaction of the ribs and the surrounding concrete can be captured by a cap-plasticity interface model. The expansive corrosion process is represented by an Eigenstrain in the lattice elements forming the interface between concrete and reinforcement. Several pull-out tests with varying degree of corrosion are analysed. The numerical results are compared with experiments reported in the literature. The influence of the properties of concrete are studied. The proposed lattice approach offers insight into corrosion induced cracking and its influence on bond strength.     

Chloride induced corrosion of reinforcement in volcanic ash and pumice based blended concrete

This paper reports the results of experiments evaluating the corrosion resistance of plain, volcanic ash (VA) and volcanic pumice powder (VPP) concrete mixes. Variables were VA and VPP additions of 0–20% as cement replacement and cement contents. X-ray diffraction (XRD) analysis, electrochemical and electromechanical measurements and physical tests were used to monitor the corrosive behaviour of embedded steel bars in concretes. Results showed that additions of VA and VPP are effective in inhibiting corrosion of reinforcing bars. The superior performance in inhibiting corrosion in reinforcing steel is attributable to the densification of the cement-paste matrix due to pozzolanic action in the VA and VPP concrete mixes.     

Snap-back softening instability in high-strength concrete beams

Three-point bending tests on pre-cracked slabs of high-strength concrete are interpreted on the basis of a virtual crack propagation model. As theoretically shown by the model, a snap-back softening instability appears only for initial crack lengths smaller than 0.3 times the beam depth. As a limit-case, when the material is sufficiently brittle or the specimen size is sufficiently large, such an instability can be predicted by the LEFM condition K₁=K_IC.     

Mechanical reinforcement of concrete with bi-component fibers

For some applications the reinforcement of concrete with fibers is an economical alternative to conventional steel bar reinforcement. Steel fibers have been the first choice for many years because of their high tensile strength and high elastic modulus. Low modulus fibers, such as polyolefin based fibers generally are thought to be less suitable for this purpose.However, it is shown that polyolefin fibers with sufficient tensile strength can, applying a novel bi-component approach, successfully enhance the mechanical properties of concrete. The effect of the introduction of nanoparticles into the fiber polymers and of a fiber surface structuring on the fiber pull-out characteristics and the fiber–matrix bond strength is presented.The performance of bi-component fiber reinforced concrete is studied in 4-point bending and square slab tests. Ductile post-peak behavior of such fiber reinforced concrete is achieved, making this new fiber technology interesting for applications in pre-cast elements, industrial floors and earth quake protecting systems.     

Stability of sulphur concrete beams with steel reinforcement

Tests are reported on the structural behaviour of unmodified and modified sulphur concrete beams with steel reinforcement and subjected to air and water curing. The sulphur concrete contained fly ash as a filler and both dicyclopentadiene and dipentene were used as modifiers. The tests were conducted at three months and one year. Unmodified concrete beams showed enhanced strength, stiffness and ductility with ageing when cured in a dry environment. Modified concrete beams showed improved behaviour compared to unmodified beams, but even when dry, they showed strength loss with ageing and their long term stability is open to question. Ageing in a wet environment has a destructive effect on the structural behaviour of all sulphur concrete beams. Loss of strength and softening of sulphur concrete lead to shear failures, and severe corrosion and cracking occur. Stability of sulphur concrete beams can be guaranteed only when they are unmodified and kept dry. In a wet regime sulphur concrete cannot have stability and durability.

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Résumé On rend compte d’essais du comportement structural de poutres de béton de soufre avec armatures d’acier, les unes traitées les autres non, conservées dans l’air et dans l’eau. Des cendres volantes ont été utilisées comme charge et les dicyclopentadiène et dipentène comme agents modificateurs. Les essais ont été conduits sur 3 mois et 1 an. On a constaté une amélioration des propriétés de résistance, de rigidité et de ductilité pour des poutres de béton non traitées dans le vieillissement en atmosphère sèche. Le comportement des poutres de béton traitées a été supérieur à celui des poutres non traitées, mais, même en milieu sec, elles ont accusé une perte de résistance avec le vieillissement et on peut s’interroger sur leur stabilité à long terme. Le vieillissement en milieu humide a un effet destructif sur le comportement structural de toutes les poutres de béton de soufre. La perte de résistance et l’adoucissement du béton soufré conduisent à la rupture par cisaillement; d’autre part une corrosion et une fissuration importantes se produisent. On ne peut garantir la stabilité des poutres de béton de soufre que lorsqu’elles sont non traitées et conservées à sec. En régime humide, le béton de soufre ne peut être ni stable ni durable.

     

Thermally induced acoustic emission in glasses

We have measured acoustic emission (AE) in commercially available silicate and metallic glasses while heating through the glass temperature (T_g). In silicate glasses AE is emitted at T_g which may be due to free volume diffusing to the surface. In amorphous Fe₄₀Ni₄₀P₁₄B₆ tapes the AE and self twist (ST) show a strong correlation in three distinct regions. Region 1 (150–225°C) is probably caused by the production process, ie secondary cooling. Region 2 (225–325°C) is probably due to structural relaxation. Crystallization occurs in Region 3 (325–450°C). The AE and ST are both irreversible when the sample is reheated to any specific temperature. The effects of varying stress levels and magnetic field are also examined. Stress relief is observed by AE measurements during an isothermal anneal.     

Permeability of concrete with fiber reinforcement and service life predictions

In the context of its long-term durability, permeability of concrete to water remains one of the most important characteristics. In the study reported here, water permeability of plain and fiber reinforced concrete (FRC) was measured with and without an applied compressive stress. For the stressed specimens, two levels of the applied stress, 0.3f _u and 0.5f _u , where f _u is the ultimate strength of concrete in compression, were investigated. A collated cellulose fiber at volume fractions of 0.1, 0.3 and 0.5% was used. Results indicate that for the unstressed concrete, fiber reinforcement reduces the permeability. For the stressed concrete, initially as the applied stress was increased, a reduction in the permeability for both plain and fiber reinforced concrete was observed. This reduction, however, occurred only to a certain threshold value of stress. Beyond this threshold, a rapid increase in the permeability occurred for plain concrete. For fiber reinforced concrete, while an increase in the permeability was noticed beyond the threshold value of stress, the magnitude of the increase remained small and the permeability remained well below the unstressed level. In the later part of the paper, some Service Life Predictions were carried out by first relating the measured permeability coefficients to chloride diffusion coefficients and then using available mathematical models. Results from the modeling exercise indicate that at least in qualitative terms, fiber reinforced concrete will depict a better durability in service than plain concrete.     

Investigation on the cracking behavior of concrete cover induced by corner located rebar corrosion

Non-uniform corrosion of reinforcement causes concrete cracking in chloride contaminated RC structures. Due to the special boundary conditions, the concrete cover with corner located rebar is often subjected to the attack of chloride ions in a marine environment from two directions, and thus the corresponding non-uniform corrosion distribution should be different from the one for side-located rebar. The aim of the work is to explore the effect of corner located rebar corrosion on the cracking of cover concrete. For corner located rebar, an improved non-uniform corrosion distribution model was established based on the analysis results of two-dimensional chloride diffusion in concrete. Considering the heterogeneities of concrete, a meso-scale mechanical model and method for the study on the failure behavior of concrete cover was built. In the analysis model and method, the non-uniform radial displacement distribution was adopted to simulate the corrosion expansion behavior of the rebar. The cracking of concrete cover with corner located rebar was simulated and studied. The present approach was verified by the available experimental observations. The influences of concrete heterogeneity, corrosion distribution types, rebar diameter and concrete cover thickness on the failure patterns of concrete cover and the expansive pressure were investigated. The simulation results indicate that the developed approach can well describe the cracking behavior of cover concrete and the corrosion-expansion behavior of steel rebar.     

Meso-scale numerical investigation on cracking of cover concrete induced by corrosion of reinforcing steel

Concrete cover cracking induced by corrosion of steel reinforcement is a major influencing factor for durability and serviceability of reinforced concrete structures. Here in this study, the influence of concrete meso-structure on the failure pattern of concrete cover is accounted for. The concrete is assumed to be a three-phase composite composed of aggregate, mortar matrix and the interfacial transition zone (ITZ). And a concrete random aggregate structure is established for the study on the mechanical behavior of radial corrosion expansion. In the present simulations, the plasticity damaged model is used to describe the mechanical behavior of the mortar matrix and the ITZ, and it is assumed that the corrosion of steel reinforcement is uniform. The cracking of concrete cover due to steel reinforcement corrosion is numerically simulated. The simulation results have a good agreement with the available test data and they are between the two analytical results. The failure patterns obtained from the macro-scale homogeneous model and the meso-scale heterogeneous model are compared. Furthermore, the influences of ratio of cover thickness and reinforcement diameter (i.e. c/d), the location of the steel reinforcement (i.e., placed at the middle and corner zones) and the concrete tensile strength on the steel corrosion rate when the concrete cover cracks are investigated. Finally, some useful conclusions are drawn.     

Flexural ductility of high-strength concrete columns with minimal confinement

The use of high-strength concrete (HSC) instead of normal-strength concrete (NSC) in columns has the advantage of allowing the column size to be reduced and is thus becoming popular. However, since HSC is more brittle than NSC, its use could result in undesirable brittle failure. To evaluate the ductility of columns, nonlinear moment–curvature analysis taking into account the stress-path dependence of the steel reinforcement is required. Based on such analysis, a parametric study has been conducted to investigate the effects of various factors on the ductility of columns. The results revealed that the effect of concrete strength is dependent on the axial stress level (axial load to area ratio) and axial load level (axial load to capacity ratio). At the same axial stress level, the use of HSC has little or basically no adverse effect on the ductility but if the same axial load level is maintained to reduce the column size, the use of HSC would significantly reduce the ductility. Finally, two formulas for direct evaluation of the ductility of columns are developed.     

Mechanical behaviour of self-compacting concrete with hybrid fibre reinforcement

In this study, fibre-reinforced self-compacting concretes were developed for precast building components, incorporating either adherent metal fibres or polymeric synthetic slipping fibres or a combination of both. To achieve the warranted workability, compressive and splitting tensile strengths, compositions were determined by preliminary tests on self-compacting materials with various proportions of metal fibres. Bending tests in controlled deflection confirmed the positive contribution of fibres in the mechanical behaviour of self-compacting concrete. The comparison between vibrated and self-compacting concretes of similar mechanical characteristics indicated a possible better fibre-matrix bond in the case of self-compacting types. The results also showed that the properties of the hybrid fibre-reinforced self-compacting concrete could be inferred from the properties of the individual single-fibre reinforcements and their respective proportions through simple mix-rules.     

Seismic performance of precast concrete frames with debonded reinforcement

This paper utilizes experimental and numerical studies to investigate the seismic behavior of precast concrete frames. The system is composed of monolithic columns and composite precast concrete beams with debonded reinforcement at the beam end, with the purpose of distributing plasticity over a larger rebar length to improve the seismic performance of traditional precast concrete frames. Two half scale precast concrete frames, with and without debonded rebar, were tested under quasi-static cyclic lateral load. The observations during the test, load–displacement curves, stiffness, energy dissipating capacity and rebar strain are discussed. The experimental findings demonstrate that rebar debonding lead to reduced strain in tensile reinforcement. The decrease in strain due to the debonded rebar was 40.2% at a drift ratio of 1%. The performance of the specimens was evaluated according to ACI 374, which demonstrates that this precast system is applicable to seismic regions. In the numerical simulation study, a macro-based finite element (FE) model was developed using fiber-section beam-column element with a modified rebar constitutive model to take into account the effect of rebar buckling. The feasibility of the FE model was verified by comparing with the experimental data.