Improved Lattice Model for Concrete Fracture

An improved regular lattice network model is developed and validated for fracture of concrete structures. The improvements pertain to the inclusion of the tension softening response of the matrix phase, and of the modelling of structural response by incrementing the deformation rather than the load. A further development is the use of a regular square rather than a triangular lattice network to speed up the computations. These improvements eliminate the need for the introduction of arbitrary scaling parameters in the beam element failure criteria. Validation checks on notched three-point bend concrete specimens confirm the predictive capabilities of the improved lattice model with regard to both the load-deformation behavior and the energy dissipation mechanisms.     

Experimental Investigation and Fracture Analysis of Debonding between Concrete and FRP Sheets

The last few years have witnessed a wide use of externally bonded fiber reinforced polymer (FRP) sheets for strengthening existing reinforced and prestressed concrete structures. The success of this strengthening method relies on the effectiveness of the load-transfer between the concrete and the FRP. Understanding the stress transfer and the failure of the concrete–FRP interface is essential for assessing the structural performance of strengthened beams and for evaluating the strength gain. This paper describes an experimental investigation of the interfacial bond behavior between concrete and FRP. The strain distributions in concrete and FRP are determined using an optical technique known as digital image correlation. The results confirm that the debonding process can be described in terms of crack propagation through the interface between concrete and FRP. The data obtained from the analysis of digital images was used to determine the interfacial material behavior for the concrete–FRP interface (stress versus relative displacement response) and the fracture parameter GF (fracture energy). The instability in the test response at failure is shown to be the result of snapback, which corresponds with the elastic unloading of the FRP as the load carrying ability of the interface decreases with increasing slip.     

Assessment of Construction Joint Effect in Full-Scale Concrete Beams by Acoustic Emission Activity

In the present paper the mechanical and acoustic emission (AE) behaviors of full-scale reinforced concrete beams are evaluated. One of the beams was constructed in two parts, which were assembled later in order to evaluate the effect of the joints in the structural behavior. The load was applied by means of a four-point-bending configuration. It is revealed that at initial stages of loading, the conventional measurements of strain and deflection, as well as pulse velocity, do not show any discrepancy, although the structural performance of the two beams is eventually proven to be quite different. On the contrary, AE parameters, even from early load steps, indicate that the damage accumulation is much faster in the assembled beam. This is confirmed by the calculated sources of AE events which are close to the construction joints. The results show that the AE technique is suitable to monitor the deterioration process of full-scale structures and yields valuable information that cannot be obtained at the early stages of damage by any other way.     

Microstructural Mechanisms of Early Age Cracking Behavior of Concrete: Fracture Energy Approach

This paper presents findings from a study directed at identifying key properties of ingredient materials that are influential on the early age cracking behavior of concrete, with an emphasis on the effects of aggregate size, aggregate morphologies, and water-cement ratio (w/c). Fracture energy (GF) was determined using a wedge-split test configuration for concrete samples at the age of 12?h. Based on image analysis, three signature morphologies of aggregate particles, i.e., the angularity, surface texture, and surface area, were quantitatively determined in terms of developed angularity index (AI), surface texture (ST) index, and surface area (SA) measurement, respectively. The high consistency between GF and aggregate SA of the concrete samples suggests that the interfacial transition zone (ITZ) at the cement paste-aggregate interface is the critical location that primarily accommodates the 12?h cracking of concrete. The critical role of ITZ in the early age cracking of concrete was further confirmed by its microstructural and chemical features under scanning electron microscopy/energy dispersive X-ray spectroscopy.     

Energy Release in Fiber-Reinforced Plastic Reinforced Concrete Beams

A set of 30 concrete beams reinforced with carbon/epoxy FRP (fiber-reinforced plastic) and four reinforced with comparable size steel rebars were subjected to static bending tests. Adequate bond between the FRP and the concrete was obtained, due to the use of carbon fiber overwrap on the smooth pultruded FRP rods. With adequate bond, the large strain to failure (>2%) of the FRP determines the ductility and failure mode of the FRP reinforced beams. An analytical evaluation of the fracture energy in these experiments shows that there is ductility due to the large fraction of the total strain energy that is absorbed in the concrete, because of the formation of distributed cracking. Variations in overwrap configuration, addition of steel stirrups, addition of polypropylene fibers, and comparison with four beams reinforced with equivalent steel reinforcement were also analyzed.     

Study of the Behavior of Concrete under Triaxial Compression

An experimental study of the confined compression behavior of concrete has been performed using 150×300?mm cylindrical specimens subjected to hydrostatic pressure in a triaxial cell and axial loading through a servo-hydraulic testing machine. A confining stress range of 0 to 60 MPa (about twice the uniaxial compressive strength) was employed to obtain the brittle-ductile transition behavior of the material. The increase in confining pressure leads to a change in the mode of failure and an increase in the maximum axial load-carrying capacity. It is seen that, at zero or low confinement, there is distributed microcracking and several macrocracks, and the response exhibits a well-defined peak and subsequent softening. At high confinements, relatively large axial and transversal strains of over 10% have been obtained, with monotonically increasing loads leading to horizontal plateaus. There is no distributed cracking and failure occurs with the propagation of few macrocracks. In general, the observed trends confirm and extend previous results reported in the literature. Optical microscopy shows extensive microcracking, especially in the aggregates, and pore collapse at high confinement. A preliminary interpretation of the results based on the theory of elastoplasticity is also presented.     

Global Monitoring of Large Concrete Structures Using Acoustic Emission and Ultrasonic Techniques: Case Study

Global monitoring of civil structures is a demanding challenge for engineers. Acoustic emission (AE) is one of the techniques that have the potential to inspect large volumes with transducers placed in strategic locations of the structure. In this paper, the AE technique is used to characterize the structural condition of a concrete bridge. The evaluation of AE activity leads to information about any specific part of the structure that requires attention. Consequently, more detailed examinations can be conducted once the target area is selected. In this case, wave propagation velocity was used as a means to evaluate, in more detail, the condition of the region indicated by the AE analysis.     

钢筋混凝土结构裂缝分析与防治

多年来,在工程建设领域一直存在着钢筋混凝土结构的裂缝问题,这是个相当普遍的质量问题也是个迫切需要解决的技术难题,本人从设计、施工、材料等方面结合自己的实践经验分析了成因并提出防治办法。     

Velocity Distribution and Energy Dissipation along Stepped Chutes Lined with Wedge-Shaped Concrete Blocks

Stepped channels lined with wedge-shaped concrete blocks may constitute a low-cost alternative to provide overtopping protection of embankment dams if the discharge capacity of existing spillways is not adequate or even to be used as the main spillway of newly built embankment dams. This paper addresses the velocity distribution and the energy dissipation, downstream of the inception point, on stepped chutes lined with wedge-shaped concrete blocks. An experimental setup was developed with two flumes designed with a relative scale of 1∶2.5. Air concentration was measured with an optical probe in several cross sections of both flumes. The velocity profiles along chutes lined with wedge-shaped blocks with the upper face sloping downstream were analyzed. The measurements’ accuracy was checked by comparing discharges indicated by a facility flowmeter and obtained by the integration of velocity and air concentration profiles. The effect of the steps-slope in the energy dissipation is studied. Values of the Darcy-Weissbach friction factor are proposed for this type of chute lining, for transition flows, and for skimming flows.     

Stiffness Degradation and Time to Cracking of Cover Concrete in Reinforced Concrete Structures Subject to Corrosion

Corrosion-induced cracks in reinforced concrete (RC) structures degrade the stiffness of the cover concrete. The stiffness degradation is mainly caused by the softening in the stress-strain relation in the cracked concrete. Limited efforts have been made to model the cracking and the corresponding effects on the cover concrete, despite of its importance in assessing and modeling the behavior of RC structures. This paper proposes a stiffness degradation factor to model the stiffness degradation of the cover concrete subject to cracking. The proposed factor is computed in terms of the cracking strain corresponding to the maximum opening of the concrete cracks based on an energy principle applied to a fractured RC structure. The time to cracking of the cover concrete is then determined as the time from the corrosion initiation needed by the crack front to reach the outer surface of the cover concrete. The proposed stiffness degradation factor and the method to compute the time to cracking are illustrated through two numerical examples. The times to cracking of the cover concrete that are predicted using the proposed method are in agreement with the measured values from laboratory experiments.     

论锰铁合金企业如何节能减排

从锰铁合金行业的工艺技术的改进、节能技术的应用和企业节能管理等方面进行了阐述,提出了锰铁合金企业节能减排的一些措施。     

Mode II Fracture Localization in Concrete Loaded in Compression

As an alternative to a series-coupling model for localization of deformations under uniaxial and confined compression, an approach based on Mode II crack growth is proposed. Such a model appears to be in closer agreement to experimental observations than the presently used series-coupling model, and has the advantage that both material and structural aspects of softening are incorporated directly. Similarities to tensile fracture exist that would make the approach universal.     

非均质岩石破裂及声发射演化数值模拟

在应变软化本构模型的基础上,考虑岩石材料非均质性和损伤过程中力学性质的弱化特性,建立了非均质岩石损伤软化本构模型,推导了损伤软化本构模型的差分格式,在VC++环境下实现了损伤软化本构模型在FLAC3D中的二次开发。研究了不同均质度对岩石变形强度等力学特性的影响,以及岩石破坏过程中的声发射演化特性。研究表明：随着岩石均质度的增高,岩石的破坏过程由延性向脆性转化,岩石峰值强度和峰值应变不断增大,而残余强度降低;当岩石均质度较低时,岩石破坏剪切带的形成会发生滞后,随着均质度的增加,单轴加载条件下岩石声发射体现出由强度低、频率高向强度高、频率低转化的特性,并表现出群震型、前震—主震—余震型和主震型3种典型模式。     

Fatigue Fracture of Concrete Subjected to Biaxial Stresses in the Tensile C–T Region

In this paper cyclic quasi-static and constant amplitude fatigue responses of concrete subjected tensile compression–tension (C–T) biaxial stress are presented. In the tensile C–T region within the biaxial stress space, magnitude of the principal tensile stress is larger than or equal to that of the principal compressive stress. An experimental program consisted of subjecting hollow, cylindrical concrete specimens to torsional loading. Failure in both quasi-static and fatigue is due to crack propagation. It is shown that the crack propagation resulting from the biaxial loading can be predicted using Mode I fracture parameters. The fatigue crack growth is observed to be a two-phase process: an acceleration stage that follows a deceleration stage. The crack length where the rate of crack growth changes from deceleration to acceleration is shown to be equal to the crack length at the quasi-static peak load. Analytical expressions for crack growth in the deceleration and acceleration stages are developed in terms of the mechanisms that influence quasi-static crack growth. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. Finally, a fracture-based fatigue-failure criterion is proposed, wherein the fatigue failure can be predicted using the critical Mode I stress intensity factor.     

Regularization of Inverse Problems in Reinforced Concrete Fracture

Reinforced concrete beams with flexural cracks are simulated by the bridged crack model. The weight function method of determining stress intensity factors has been followed to derive a transformation between the crack bridging force (the rebar force) and the crack opening displacements (CODs). The matrix of the transformation is then approximated by its finite difference equivalent within finite dimensional vector spaces. Direct problem of the transformation solves for CODs, which require a known rebar force. Alternatively, the inverse problem works out the rebar force from known CODs. However, the inverse transformations of such convolution type integral equations become ill-posed if input CODs are perturbed. The Tikhonov regularization method is followed in its numerical form to regularize the linear ill-posed inverse problem. Restoration of mathematical stability and consistency are demonstrated by specific examples, where the results of the direct and the corresponding inverse problem are cross checked. Results of the direct problem (i.e., the analytical CODs) are deliberately perturbed by adding machine generated random numbers of a certain width. The inverse problems are solved with these CODs to simulate practical situations, where measured CODs data will inevitably be noisy. Computations reveal that the inverse analysis of CODs satisfactorily determines the rebar force without cross-section information.     

Fracture Mechanics Model for Analysis of Plain and Reinforced High-Performance Concrete Beams

In developing a one-dimensional analysis and design procedure for reinforced concrete structures, research is generally based on yield phenomena and the plastic flow of steel in tension and concrete in compression. The ability of concrete to resist tension is considered in the form of tension stiffening or is completely disregarded. This procedure does not account for the influence of structural size in changing the failure mode and the stress distribution across the uncracked or cracked ligament. The key factor affecting this stress distribution is found to be the strain-softening modulus. This paper presents an improved model that is based on the fundamental equilibrium equation for the progressive failure of plain concrete beams. The concrete stress-strain relationship in tension is derived by calculating the peak tensile stress and softening modulus for different depths of beams on the basis of the fracture parameters obtained with the size effect law. Thus, the proposed model uses the peak tensile stress and the softening modulus, which vary depending on the size of the beam. To study the effect of the strength of high-performance concrete (HPC) on the concrete tensile stress-strain relationship, the experimental load-deflection plots of different-sized beams are compared with those obtained by using the proposed analytical model for eight different mixes made with locally available fly ash and slag. The model is also extended for lightly reinforced concrete beams, and the results are compared with those in the literature and are found to be in good agreement.     

Evaluation of Fracture Energy of Composite-Concrete Bonded Interfaces Using Three-Point Bend Tests

In this paper, a conventional test method using a notched three-point bending beam (3PBB) specimen is adapted to characterize Mode I fracture of composite-concrete bonded interfaces, and the interface fracture energy is evaluated based on a fictitious crack model. Two types of fiber fabrics—E-glass and carbon—are used, and a common epoxy resin is applied to bond the composite fabri?s to concrete. Mode I fracture tests of the 3PBB specimens for carbon fiber reinforced polymer (CFRP)- and glass fiber reinforced polymer (GFRP)-concrete bonded interfaces are performed to determine the applied load and load point displacement relationship from which the interface fracture energy is computed. The effects of loading rates, types of fiber fabrics, and curing time on the fracture energy of FRP-concrete bonded interfaces are studied and discussed. It is expected that the proposed experimental method can be used effectively to obtain fracture data for performing delamination studies under various environmental exposures and service loading.     

混凝土材料声发射过程及其自相似性的研究

将对材料声发射的研究同对力学过程的研究、材料断裂损伤等破坏过程的研究相结合,提出了声发射过程的概念。在此基础上,进一步给出了定量考察声发射过程自相似性的自相似特征函数.测试结果分析表明,混凝土试块在单轴压缩过程中,声发射过程的自相似程度随着应力状态的变化而变化.     

Microplane Model M5f for Multiaxial Behavior and Fracture of Fiber-Reinforced Concrete

Despite impressive advances, the existing constitutive and fracture models for fiber-reinforced concrete (FRC) are essentially limited to uniaxial loading. The microplane modeling approach, which has already been successful for concrete, rock, clay, sand, and foam, is shown capable of describing the nonlinear hardening–softening behavior and fracturing of FRC under not only uniaxial but also general multiaxial loading. The present work generalizes model M5 for concrete without fibers, the distinguishing feature of which is a series coupling of kinematically and statically constrained microplane systems. This feature allows simulating the evolution of dense narrow cracks of many orientations into wide cracks of one distinct orientation. The crack opening on a statically constrained microplane is used to determine the resistance of fibers normal to the microplane. An effective iterative algorithm suitable for each loading step of finite element analysis is developed, and a simple sequential procedure for identifying the model parameters from test data is formulated. The model allows a close match of published test data on uniaxial and multiaxial stress–strain curves, and on multiaxial failure envelopes.     

Development of Energy-Efficient Concrete Buildings Using Recycled Plastic Aggregates

Recycled plastics (high-density polyethylene, polyvinyl chloride, and polypropylene) were used as coarse aggregates in concrete mixtures to alter and improve the thermal properties of buildings. Two similar retail buildings were designed and constructed in Lansing, Mich., one with normal concrete (control) and the other with high content of recycled mixed plastics. The thermal and energy performance of the two buildings were investigated and analyzed. Short-term (air tightness and infiltration, co-heating, lighting) and long-term monitoring were performed. The building simulation program SUNREL developed by the National Renewable Energy Laboratory (NREL) was employed to simulate the energy performance of the two buildings and to validate the experimental data. Both experimental and SUNREL program results showed that the recycled plastic concrete building exhibited higher levels of energy efficiency and comfort when compared with the normal concrete (control) building. Recycled plastic concrete in combination with energy-efficient building design techniques proved to be of tremendous value in lowering the cooling and heating loads of the buildings and also in enhancing the comfort level of the buildings.