Analyses of the multiple cracking behaviour of brittle hollow cylinders under internal pressure

In order to elucidate the multiple cracking behaviour of brittle hollow cylinders under static internal pressure, two-dimensional dynamic finite element analyses have been performed firstly for graphite hollow cylinders with inner and outer diameter of 16 and 22 mm, respectively, under internal gas pressure. In the analyses, propagation speed of the primary crack was set to be extremely high by instantaneously releasing the nodes that defined the path of the primary crack, and internal pressure was preserved after the primary cracking. The analyses showed that the stress was enhanced due to stress waves generated by the primary cracking. The initial stress enhancement was observed at the side position of the cylinder, which was located at approximately ±90° with respect to the primary cracking site. This implied that secondary cracking could occur at the side positions. Fracture modes of the cylinders might depend on the following parameters: (1) propagation speed of the primary crack, (2) pressure drop rate after the primary cracking, (3) medium to generate internal pressure, (4) geometry of a cylinder, (5) mechanical properties of brittle materials, and (6) presence of a notch. Thus, the effect of the above parameters on the behaviour of the multiple cracking was also analysed. It was found that secondary cracking would still occur at the side positions if (i) the crack propagation speed was between 70% and 100% of the theoretical crack propagation speed, (ii) the pressure drop rate was below 10⁷ Pa/s, (iii) wall thickness of the cylinder was changed, and (iv) other brittle materials were employed. Also, it was found that multiple cracking would not be observed if liquid pressure was employed instead of gas pressure, because of fluid-structure interaction. In addition, the position of the secondary cracking would be shifted by introducing a notch on the outer surface of the cylinder. These results were in good accordance with experiments formerly reported.     

Cover cracking as a function of bar corrosion: Part I-Experimental test

The appraisal of concrete structures suffering rebar corrosion is one of the most urgent needs regarding the selection of the technical and economical optimum time for repair. Up to now this appraisal has been mainly based on empirical and subjective considerations. Among the different distressing consequences of rebar corrosion the best known is the cracking of concrete cover. However, very few data have been reported in the literature on the amount of corrosion needed to induce this cracking. In the present paper, some preliminary experiments are reported in which small reinforced beams are artificially corroded by an impressed current, and the amount of current (and loss of bar cross-section) needed to induce the crack at the surface are monitored, together with the evolution of crack width, by the use of strain gauges applied to the surface of the specimens. In a companion paper, a numerical model to relate the decrease in rebar cross-section to the cover cracking will be developed. That model is based on the orderly imposition of corrosion to finite elements of the rebar by a fictitious temperature increment that produces analogous effects, while concrete cracking is introduced by a standard smeared-crack model. The experimental results indicate that only a few micrometres of loss in rebar cross-section are needed to induce visible cover cracks (0.1 mm width) in the conditions of the test.     

The efficiency of self-healing concrete using alternative manufacturing procedures and more realistic crack patterns

During the last years, more and more research has been devoted to self-healing in cementitious materials. While most research is still done on carefully prepared small-scale mortar samples with predefined cracks, the healing efficiency should be investigated after exposure of the capsules to the concrete mixing and casting process and for random appearing cracks. In the current study, the resistance of brittle encapsulation materials, containing polyurethane, against the mixing and manufacturing process of concrete was studied. Different methods to protect the capsules were proposed and evaluated. In addition, realistic crack patterns were created in beams with embedded capsules. Non-destructive testing techniques such as digital image correlation, acoustic emission analysis and X-ray radiography were used to evaluate the survivability of the capsules upon mixing and the breakability of the capsules upon crack formation. Evaluation of the crack repair efficiency by performing water permeability tests showed some improvement in water tightness due to self-healing, but the water ingress into the cracks was not completely prevented.     

Determination of masonry crack evolution due to differential displacements: a numerical study*

Brick walls of ceramics without any mortar covering or paint are used extensively in building façades in Spain. One of the most used masonry wall systems is based on non‐bearing panels partially supported, about two‐thirds of the brick width, over the edge beams of the structural skeleton. The edge beam is veneered with special thinner bricks to achieve the visual continuity of the façade. A considerable number of these walls show cracking. In a previous work, finite element simulations were performed in order to gain insight into the causes of cracking. A special finite element, based on the strong discontinuity analysis and the cohesive crack theory, is used in the numerical simulations. The results agree with the overall cracking patterns observed but if an imposed displacement is applied in the range allowed by the standards, extensive cracking occurs. This implies that the design displacements are not the actual ones. In this work, an elastic study using the principle of superposition is used to determine the effective deflections under service loading. Then, these deflections are applied to the structure and the evolution of cracking is studied. This study shows that the masonry panels of the first and last store have the major probability of cracking. Another parametric study is carried out changing the elastic and tensile properties of the masonry. This study shows that although the cracking of the masonry panels starts at different loads for different tensile properties, the crack patterns are similar for a given panel geometry and loading. This numerical study provides a method of design to determine the crack width for different geometries, loadings and fracture properties.     

X-ray tomographic observations of microcracking patterns in fibre-reinforced mortar during tension stiffening tests

For the last decades, new reparation or fabrication processes have been studied to replace traditional rebar by roving of different mineral or organic fibres to avoid corrosion issues. Such materials refer to the family of cementitious composite. Their tensile strength would directly depend on the proportion of reinforcement and strongly on the interfacial mechanical properties between fibres and cementitious matrix. From now, evaluation of interfacial properties was mostly limited to the use of force–displacement curves obtained from mechanical experiments. This work presents a new methodology using micromechanical tension stiffening tests combined with X-ray computed tomography (XRCT) observations, performed at the Anatomix beamline at Synchrotron SOLEIL, and specific image processing procedures. Multi-XRCT acquisitions with suitable scanning strategy are used to image the whole fibre-matrix interface along centimetric samples at four to five different levels of loading magnitude. Intensive image processing is then performed on tomographic images including digital volume correlation (DVC), image subtraction and Hessian-based filtering. This experiment allows to study damage mechanisms at small scale. The proposed methodology shows great potential to provide both qualitative and quantitative elements on interfacial mechanical behaviour such as crack growth and crack orientation. The interface between mortar and sufficiently small multi-fibre yarn used in this paper is shown to behave in certain condition as traditional rebar interface producing conical cracks in the surrounding matrix rather than debonding in mode 2, permitting a much higher energy dissipation during debonding. According to this study, conical cracks repartition and geometry are mostly influenced by the cementitious matrix. The spacing between cracks goes from 50 to 100 μm, and the angle between crack normal vector and yarn orientation goes from 35° to 50°.     

Corrosion resistance of self-consolidating concrete in full-scale reinforced beams

The corrosion of steel reinforcement embedded in full-scale self-consolidating concrete (SCC) beams was investigated compared to normal concrete (NC). 400 mm width × 363 mm depth × 2340 mm length beams containing epoxy- and non-epoxy-coated stirrups were monitored under an accelerated corrosion test. The corrosion performance of NC/SCC beams was evaluated based on the results of current measurement, half-cell potential tests, chloride ion content, mass loss and bar diameter degradation. The investigation also included the effect of admixture type and the size of specimen on corrosion performance.In general, SCC beams showed superior performance compared to their NC counterparts in terms of corrosion cracking, corrosion development rate, half-cell potential values, rebar mass loss and rebar diameter reduction. However, SCC beams showed localized corrosion with concrete spalling due to non-uniform concrete properties along the length, which was a result of the casting technique. The results also showed that the difference between SCC and NC mixes in terms of corrosion was more pronounced in large-scale beams, and that types of admixture used in SCC have no influence on corrosion performance.     

Short and long-term cracking behaviour of GFRP reinforced concrete beams

A total of ten simply supported beams reinforced with different amounts of GFRP and steel bars were subjected to two consecutive test phases in order to evaluate their short and long-term cracking behaviour. The beams were initially tested up to service load and subjected to two additional load cycles. Subsequently, the specimens were subjected to two different levels of sustained load for 250 days. The effect of cyclic load during short-term tests resulted in an increase in crack width up to 25% more than the initial value. The sustained load led to an increase in crack width up to 2.9 times larger than that measured under the corresponding short-term load. A similar cracking behaviour was observed when reinforcing solutions with similar stiffness (GFRP or steel bars) were used.Existing models to estimate crack spacing and crack width for FRP and steel reinforced concrete elements, including ACI 440.1R-06, Eurocode 2 and Model Code 2010 are discussed and their performance is assessed against the experimental results. Model Code 2010 was found to yield more accurate predictions of the cracking behaviour of the test specimens under both short-term and long-term loading.     

A radially cracked,cylindrical fracture toughness specimen

A new fracture toughness specimen in the shape of a hollow circular cylinder has been characterized, allowing more economical and representative use of as-formed cylinders in toughness tests. Several configurations of a cylinder with radial cracks parallel to the axis of revolution are described with respect to crack mouth compliance and stress intensity factor. Specimen geometries useful for fracture toughness tests are analyzed in detail, based on results derived from finite-element codes. Results of experiments using 707S-T6 aluminum and uranium alloys of known toughness showed good agreement with the analytical results.     

Workability, mechanical properties, and chemical stability of a recycled tyre rubber-filled cementitious composite

The workability and mechanical properties of mortar containing shredded automobile and truck tyres were evaluated. Two different shapes of rubber particles were used as constituents of mortar: (1) granules about 2 mm in diameter, and (2) shreds having two sizes which were, nominally, 5.5 mm×1.2 mm and 10.8 mm×1.8 mm (length×diameter). As expected, the geometry of the rubber particles influenced the fracture behaviour of rubber-containing mortar. The addition of rubber led to a decrease in flexural strength and plastic shrinkage cracking of mortar. The crack width and crack length due to plastic shrinkage were reduced for mortar containing the 10.8×1.8 mm rubber shreds compared with a mortar without shreds. The rheological properties of the mortar containing rubber shreds were comparable to those of a mortar without rubber and yielded lower plastic viscosity than a mortar containing 25.4 mm×15 m (length×diameter) polypropylene fibres. The alkaline stability of rubber in mortar was also evaluated by immersing rubber shreds in NaOH and Ca(OH)2 solutions for 4 mon and the results showed that there is less than 20% change in stress and strain value. The findings of the research suggest that automobile and truck tyres can be recycled by shredding and incorporating them into mortar and probably concrete for certain infrastructural applications. © 1998 Chapman & Hall     

Plasticity approach to shear design

The paper presents some plastic models for shear design of reinforced concrete beams. Distinction is made between two shear failure modes, namely web crushing and crack sliding. The first mentioned mode is met in beams with large shear reinforcement degrees. The mode of crack sliding is met in non-shear reinforced beams as well as in lightly shear reinforced beams. For such beams the shear strength is determined by the recently developed crack sliding model. This model is based upon the hypothesis that cracks can be transformed into yield lines, which have lower sliding resistance than yield lines formed in uncracked concrete. Good agreement between theory and tests has been found.     

Evaluation of fracture in mortar subject to tension loading using phase field model and three point bending test

The Classic Fracture Mechanics (CFM) is the most widely used approach to study the initiation and propagation of cracks, which has the limitations in numerical simulations due to complex topological changes and possible singularity. To overcome its limitations, the Phase-Field Model (PFM) is presented to model the cracking failure in mortar subject to pure tension loading (Mode I) with different water/cement ratio and thickness. The mortar cracking surfaces are described using a phase-field variable which assumes one in the intact region and negative one in the crack region. The new white noise term is added into the classical Phase-Field Model to reflect the quasi-brittle cracking behavior of mortar. The non-conserved Allen–Cahn dynamics is then employed to simulate the growth of cracks. To verify the simulation results, three point bending test is conducted. It is discovered that the crack initiation and propagation in our simulation agrees very well with the experimental results.     

Analysis of crack propagation due to rebar corrosion using RBSM

Cracking behavior due to rebar corrosion in concrete specimens having a single rebar is evaluated experimentally and analytically. In the experiments, in which corrosion was induced electronically, the propagation of cracks (including internal crack patterns and surface crack widths) was monitored. In addition, deformation of the specimen surface was measured using a laser displacement meter. In the analysis, a three-dimensional Rigid-Body-Spring Method (RBSM), combined with a three-phase material corrosion–expansion model, is proposed to simulate crack propagation due to rebar corrosion. The effects of the properties of corrosion products such as elastic modulus, penetration of corrosion products into cracks, and local corrosion after cracking of the concrete are investigated. Cracking behavior due to rebar corrosion is simulated reasonably well. The simulations using RBSM provide insight into the mechanisms of crack initiation and propagation due to rebar corrosion.     

超高韧性水泥基复合材料多缝开裂特性及其自生愈合

应变硬化水泥基复合材料(Strain hardening cement-based composites,SHCC)是超高性能水泥基材料的一种。通过三点弯曲加载试验,分别对普通砂浆和SHCC试件诱导开裂,对两种材料的开裂特性及其裂缝分布规律进行了研究。结果表明,SHCC材料试件与普通水泥砂浆试件相比具有更高的承载力和应变能力,初始开裂荷载约为普通砂浆试件的2.5倍。SHCC材料中的PVA纤维的桥接作用有效延迟了裂缝的产生与扩展,将普通砂浆试件中数量少而大的裂缝转化为多而细的微小裂缝,呈现多缝开裂特性,产生的裂缝宽度符合正态分布规律,且90%的微小裂缝小于30μm。这些微小裂缝在潮湿环境中产生一定程度的自生愈合,在水中的愈合程度和速度均高于在潮湿空气中。     

钢筋混凝土梁静态破裂试验与应变分析

为研究钢筋混凝土构件的静态破裂规律,采用预留钻孔和内部预埋应变砖的方法,进行了钢筋混凝土梁的静态破裂试验。通过内置12个应变砖测得的钢筋混凝土梁内部应变以及梁破裂面的裂缝宽度变化,分析了钢筋混凝土梁在静态破裂过程中的应变变化和破坏形态。试验结果表明:孔内膨胀应力作用下,裂缝优先由孔周边向最小抵抗线方向的自由面扩展;同一排药孔位置的应变大小以及变化趋势大致相同;逐排灌孔的静态破碎方式能够顺利破裂钢筋混凝土构件,构件呈梅花状破裂。研究结果为静态破裂技术在钢筋混凝土工程中的应用提供了试验参考依据。     

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load-displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.     

Side Near Surface Mounted (SNSM) technique for flexural enhancement of RC beams

The rehabilitation of existing Reinforced Concrete (RC) structures constitutes one of the leading challenges in civil engineering. The crucial reasons for the strengthening of RC structures comprise frequent increases in design loads, engineering errors in design or workmanship issues during construction, changes in code and functional requirements. This paper introduces an innovative approach comprising the Side-Near-Surface-Mounted (SNSM) technique, which incorporates Carbon Fiber Reinforced Polymer (CFRP) and steel bars as strengthening reinforcement. Experimental and analytical investigation was adopted to explore flexural strengthening of RC beams with them. Analytical models are presented to predict the ultimate load, crack spacing and deflection. Four-point bending tests were performed up to failure on the rectangular RC beams strengthened with different ratios of SNSM reinforcement. The failure characteristics, yield and ultimate capacities, deflection, cracking behavior, ductility and energy absorption capacities were evaluated. The SNSM technique significantly enhanced the flexural behavior of the beams. The yield and ultimate load carrying capacities of the beams increased by a factor of 2 and 2.38 times, respectively. The cracking loads improved more notably (3.17 times). Predicted results from the analytical models showed good agreement with the experimental results, which confirmed proficient implementation of the proposed SNSM technique.     

给水管座开裂原因分析

锅炉锅筒的给水管座与锅筒在焊接和消除应力热处理后经探伤检查发现沿焊缝有环向裂纹,并扩展至给水管座的内壁形成纵向裂纹。采用化学成分分析、宏观和微观检验以及力学性能分析等方法对给水管座的裂纹产生原因进行了分析。结果表明：由于焊接工艺规范控制不当,未及时进行消氢处理,焊件的碳当量提高具有明显的淬硬倾向,焊接结构和较大的焊件厚度引起拘束应力等综合作用产生了延迟裂纹。     

The role of hardwood pulp fibers in mitigation of early-age cracking

The availability, relative consistency, and renewability of hardwood pulp fibers have prompted interest in their use in fiber–cement composites, in which they may be used for a variety of purposes. This study clarifies the ability of hardwood pulp to reinforce mortar, its capacity to provide internal curing, and its role as early-age crack-control reinforcement through a coordinated series of restrained shrinkage, free shrinkage, and mechanical testing on mortar samples. It finds that hardwood pulp improves the restrained shrinkage behavior of mortar at an early age. That is, 0.5% and 0.75% (by volume) hardwood pulp-reinforced mortars exhibited a lower rate of stress development and lengthened time-to-cracking by about 1.6 times and 2.3 times, respectively, compared to a companion crack-prone ordinary mortar. The initial crack width also decreased by 88% in 0.75% hardwood pulp-reinforced mortar samples, which suggested an application aimed at assisting self-healing in cement-based materials with an appropriate binder composition. Hardwood pulp successfully provided internal curing to crack-prone mortar and thus reduced autogenous shrinkage. This reduction in shrinkage, together with a combination of increased early tensile capacity, reduced stiffness, and improved post-cracking toughness were identified as the key contributions of hardwood pulp in the improved resistance of mortar to early-age cracking.     

Monitoring Early Age Properties of Cementitious Material Using Ultrasonic Guided Waves in Embedded Rebar

The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a sensing method to use ultrasonic guided waves in a rebar for monitoring the early age properties of cementitious material. An EMAT sensor was used to excite the longitudinal mode L(0,1) wave in a rebar embedded in cement/mortar, and an ultrasonic transducer was used for receiving the echo signals. Guided wave dispersion curves were developed to select appropriate frequency range. The leakage attenuations of the L(0,1) mode wave from the rebar to the surrounding cement materials were continuously monitored for the first 10 h. The evolution of the shear wave velocity was also monitored simultaneously. The leakage attenuation from experimental measurements was compared with the theory-predicted attenuation in both time and frequency domains, and showed good agreement. Experiments were performed on three cement paste samples and three mortar samples. The results indicated that attenuation is nearly linearly related to the shear wave velocity, and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. These results suggest that mechanical properties and hardening process of cement materials can be monitored by using the ultrasonic guided waves in a rebar.     

Rebar pullout testing under dynamic Hopkinson bar induced impulsive loading

The dynamic behavior of the bond slip between a deformed reinforcing bar and plain concrete has been experimentally investigated by employing Hopkinson bar techniques. Pullout tests with various specimen types (unconfined, confined, cast-in-place, post-installed etc.) have been performed. Pullout of the steel rebar and splitting of the concrete cylinder have been the failure modes induced. Test results comprise peak pullout forces and complete bond stress–slip diagrams. They clearly show that the dynamic pullout forces and curves are well above the static ones, and that the pullout work of bond failure is considerably greater for the dynamic impact loading. Confinement, provided by a steel tube, leads to improved bonding; peak loads increase up to 2.5 times. The effects of bond length and concrete strength have also been put into evidence. Finally it has been verified that post-installed rebars, depending upon the particular adhesive employed, can achieve the same bond resistance as the cast-in-place ones.