图像分析技术在混凝土断裂面研究中的应用

本文将先进的数值图像分析技术应用于破坏后混凝土断裂面的分析中,提出表征断裂面上粗集料破坏方式的参数——粗集料拉断概率的定义,并以不同强度等级、不同粗集料粒径的混凝土试件断裂面为例,探讨了粗集料拉断概率与混凝土强度和粗集料粒径的关系。初步的研究结果表明：混凝土强度越高,粗集料拉断率越大;在相同的混凝土强度下,粗集料粒径越大,粗集料拉断概率越大。     

图像分析在混凝土断裂面研究中的应用

本文将先进的数值图像分析技术应用于破坏后混凝土断裂面的分析中,提出表征断裂面上粗集料破坏方式的参数－－粗集料拉概率的定义,并以不同强度等级、不同粗集料粒径的混凝土试件断裂面为例,探讨了粗集料拉断概率与混凝土强度和粗集料粒径的关系。初步的研究结果：混凝土强度越高,粗集料拉断率越大;在相同的混凝土强度下,粗集料粒径越大,粗集料拉断概率越大。     

自燃煤矸石粗集料取代率对混凝土断裂韧性的影响

通过跨度和初始缝高比恒定的三点弯曲梁试验,计算不同自燃煤矸石粗集料取代率下混凝土的断裂韧度和断裂能,用特征长度和延性指数来衡量其脆性与延性.基于图像处理技术,在水平投影方向对断裂面混合粗集料的分布及破坏方式进行分析.通过三维重构断裂面形貌计算相应分形维数,用以表征粗集料表面粗糙度.结果表明:自燃煤矸石粗集料混凝土的弯曲破坏特征与普通混凝土相似,但随着取代率的增大,裂缝发展路径由弯曲扩展向竖直扩展转变,贯穿截面所用时间变短;韧性参数皆随取代率增大而降低,但降幅不同,当取代率为25%时,特征长度及延性指数与普通混凝土持平,之后随取代率增大而下降,当取代率达到100%时,起裂断裂韧度、失稳断裂韧度和断裂能较普通混凝土分别下降11.69%、39.04%、47.86%.取代率变化对混合粗集料与断裂带净面积水平投影之比影响不明显,但对混合粗集料断裂破坏比例影响显著.随着取代率的增大,分形维数逐渐降低,断裂面形貌渐趋平整.自燃煤矸石粗集料对混凝土断裂性能产生明显不利影响,断裂特征较普通混凝土显著,在工程中应予以高度重视.     

冶金集料对高性能混凝土力学性能的影响

针对高性能混凝土存在着脆性较大的问题,采用冶金集料置换了部分普通集料进行了增强增韧。试验结果表明：混凝土的强度和断裂韧性随冶金集料的置换率的增加而提高;断裂能随冶金集料置换率的的增加而提高特别显著,采用延性指数描述了冶金集料对混凝土脆性的影响,混凝土的脆性随冶金集料置换率的增加而降低,特别对于水胶比较高的系列;从断裂面信息的分析表明,断裂面的分数维和粗糙度的增加与混凝土断裂能或延性指数增加的趋势相一致。     

混凝土材料的真实断裂能研究

在对混凝土断裂面进行测定的基础上提出了真实断裂能的概念,并试验研究了真实断裂能与材料组成之间的变化规律,试验结果表明：真实断裂能几乎不随集料径的变化而变化,随集料品种的变化较小,随小胶比的降低而增大。     

粗集料与硬化水泥浆体界面结合能力对混凝土强度和脆性的影响

粗集料与硬化水泥浆体的界面是混凝土内部结构的薄弱环节。本文通过测定粗集料与硬化水泥浆体界面的劈裂强度及断裂能、不同集料配制的混凝土的强度和脆性,以及对测定结果的分析,得出了改变粗集料与硬化水泥浆体的界面结合能力可以强化及韧化混凝土的结论。     

强度等级和养护龄期对素混凝土脆性的影响

使用作者提出的一种评价混凝土脆性的方法——断裂石子面积分数法,研究了强度等级和养护龄期对混凝土脆性的影响。断裂石子面积分数法是以混凝土试样断面上断裂石子的投影面积占断面总投影面积的百分数来表示混凝土的脆性,该面积分数越大,表示混凝土的脆性越大。研究结果表明:随着强度等级的提高和养护龄期的发展,混凝土中裂纹的扩展通道由沿砂浆和粗集料之间的界面过渡区进行逐渐转变为穿过粗集料进行;随着混凝土强度等级的提高,混凝土的脆性增大,同时,随着养护龄期的延长,混凝土的脆性也增大。     

集料对混凝土断裂性能影响的研究

研究了粗集料粒径、集料体积率和粗集料的种类对混凝土断裂能和断裂韧性的影响;并从细观结构上分析集料对混凝土断裂性能作用机理。     

粗集料用量对混杂纤维混凝土性能的影响

为了探究粗集料体积分数对混杂纤维混凝土工作性、力学性能的影响,采用2种聚合物纤维制备了不同粗集料体积分数的混杂纤维混凝土,研究了粗集料体积分数对混凝土坍落度、抗压强度、抗弯拉强度以及弯曲韧性的影响,并对其机理进行了分析。结果表明,当粗集料体积分数从30%增大到42%后,坍落度降低了19mm,混凝土工作性显著降低,拌合过程中甚至出现露石和成型困难等情况;当粗集料体积分数控制在34%~38%时,粗细纤维分散性良好,混杂纤维混凝土力学性能达到最佳。     

煤矸石粗集料混凝土抗碳化性能研究

探究了水灰比(质量比)、煤矸石粗集料掺量(质量分数)、碳化时间以及煤矸石粗集料是否煅烧等对煤矸石粗集料混凝土抗碳化性能的影响规律,借助扫描电镜分析了煅烧与未煅烧煤矸石粗集料混凝土微观结构对其抗碳化性能的影响机理,建立了适用于不同水灰比、煤矸石粗集料掺量以及碳化时间下的多参数混凝土碳化模型,并将碳化模型在隧道工程结构的耐久性设计中进行了应用.结果显示:煅烧与未煅烧煤矸石粗集料混凝土的碳化深度均随煤矸石粗集料掺量增加而增大,且均与煤矸石粗集料掺量和碳化时间的平方根呈线性正相关;在低水灰比(0.35)时,煅烧煤矸石粗集料混凝土抗碳化性能明显优于未煅烧煤矸石粗集料混凝土.扫描电镜显示,煅烧煤矸石粗集料混凝土内部结构比未煅烧煤矸石粗集料混凝土更密实,二氧化碳的渗入通道减少;在进行隧道工程结构耐久性设计时泵送混凝土中未煅烧煤矸石粗集料的掺量不应大于25%.研究结果为煤矸石粗集料混凝土的抗碳化性能分析及其工程应用提供了理论依据.     

冻融作用混凝土断裂面的分形特征研究

对冻融前后混凝土断裂面进行分形特征研究,将分形理论应用于混凝土冻融损伤研究中。混凝土断裂面具有良好的分形特点,冻融后混凝土断裂面断裂表面进行分维计算表明,其分形维数的方差非常小,线性回归的相关系数也非常好,断裂韧度与分维具有良好的相关性,可以将分形理论与混凝土的断裂理论结合起来,评价混凝土在冻融作用下的断裂行为。     

分形理论在岩石力学中的应用研究动态

分形几何用于定量描述复杂的自然现象和不规则事物已有许多成功范例,它较好地描述了岩石力学中的不规则性和不确定性,并在岩石力学中得到较好的应用。综述分形几何在岩石损伤、岩石微破裂过程、岩石断裂分布、岩体裂隙分布及岩石受力分析等多方面的最新研究成果。     

Freeze–thaw and thermal degradation influence on the fracture properties of carbon and glass fiber reinforced polymer concrete

In recent years, it has been found that some concrete structures, even for some high performance concrete and ready mixed concrete under good quality control, start to deteriorate long before reaching their designed service life. Cracks are found after the structure has been completed for a few years, which results in the shortening of service life and lowering in durability. In this paper, the influence on the fracture properties of fiber reinforced polymer concrete, FRPC, subjected to freeze–thaw and thermal degradation, is discussed. The study on the damage influence was conducted using a climate chamber to perform the temperature changes on the single edge notched beams. FRPC specimens were subjected to freeze–thaw and higher temperature cycles and then tested according to RILEM standards. To determine the stress intensity factor, K_Ic, critical crack tip opening displacement CTODc, the Two Parameter Method was used and the fracture energy, G_f of the specimens was calculated to evaluate the fracture properties of FRPC. Three-point bending tests were carried out on notched beams with a clip gauge attached to measure the crack mouth opening displacement CMOD. Polymer concrete was reinforced with short glass and carbon fiber with 1% and 2% in mass respectively, to determine whether the reinforcement increases the performance of the material. The fracture properties of FRPC are reported in such conditions.     

水泥路面开裂破坏的非线性机制

根据混凝土材料组成的微观颗粒结构 ,利用分形几何学的基本原理对水泥混凝土路面裂缝分布进行描述 ,对混凝土回弹模量、配合比和分维数对水泥混凝土路面性能的影响 ,进行了研究和分析 ,然后建立水泥混凝土路面裂缝形成机制的键渗流模型 ,应用重正化群理论计算渗流阀值 ,确定混凝土路面的破坏条件。对分形、渗流阀值和水泥混凝土路面性能及其破坏条件之间的关系进行了综合分析     

Numerical simulation on shear fracture process of concrete using mesoscopic mechanical model

The numerical simulation of the damage and fracture processes of concrete structures has evolved considerably in the past years. In this contribution, a newly proposed mechanical model is used to simulate the fracture behavior of double-edge notched (DEN) and double central notched (DCN) concrete specimens loaded in shear. In this numerical model, the concrete is assumed to be a three-phase composite composed of matrices, aggregates and matrix–aggregate interfaces. An elastic finite element program is employed as the basic stress analysis tool while the elastic damage mechanics is used to describe the constitutive law of meso-level element. The maximum tensile strain criterion and Mohr–Coulomb criterion are utilized as damage thresholds. The heterogeneous stress field is obtained from numerical simulation, thus it is found that heterogeneity of mechanical properties has significant effect on the stress distribution in concrete. The crack propagation processes simulated with this model shows good agreement with those of experimental observations. It has been found that the shear fracture of concrete observed at the macroscopic level is predominantly caused by tensile damage at the mesoscopic level.     

高掺量钢纤维混凝土表面损伤特征的定量研究

对高掺量钢纤维混凝土（SIFCON）在不同纤维掺量（体积分数）下的力学性能及破坏形式进行了数字图像分析。并用分数维对SIFCON的表面裂缝进行了定量描述，结果发现；不同纤维掺量下的SIFCON表面裂缝存在分形特征，分数维可以对表面裂缝进行定量描述，SIFCON的力学性能与分数维之间存在较好的关系。     

透水混凝土的表面纹理和抗滑性能

在路面防滑性方面,表面纹理曾长期地被认为是主要因素,但没有定量计算不同表面参数是透水混凝土路面的防滑性。引入了分形几何,通过分形插值可精确模拟透水混凝土断面轮廓曲线。结合轮胎/路面接触模型,表面纹理的优化方法被应用于设计和建造具有防滑性和低噪音的混凝土路面中。结果表明,相比于传统的槽式或暴露粗骨料的混凝土路面,透水混凝土路面的表面纹理非常耐用。     

Modeling in-plane and out-of-plane displacement fields in pull-off tests on FRP strips

The present paper deals with modeling FRP strips bonded to concrete blocks and tested in pull-off. The investigation starts from the experimental observations obtained by means of an optical image-correlation measurement system which is able to monitor the 3D displacement components of a fine mesh of points on the surface of both the FRP strip and concrete block. Thus, refined measurements of both in-plane and out-of-plane displacements of the FRP strips are available.A brief overview of the key contributions available in the scientific literature on modeling the bond behavior of FRP strips glued to a concrete substrate is firstly proposed. Then, a novel model based on simulating the behavior of the FRP strip as a Bernoulli beam on a layer of springs is formulated. It is aimed at determining the 2D displacement field of the FRP strip during a pull-off test up to debonding which actually occurs in a mixed fracture mode. The model is firstly formulated within the linear range by assuming elastic behavior for the above mentioned springs. The nonlinear behavior due to the cracking of concrete beneath the adhesive interface is then introduced for simulating the above mentioned experimental results. In particular, a bilinear relationship is assumed between interface slips and shear stresses, as is generally accepted within the scientific literature. Furthermore, a damage model is considered for reducing the stiffness of the transverse springs and simulating the crack propagation at the adhesive-concrete interface. Although this is a simplified way of modeling the nonlinear behavior of concrete in shear/tension, it results in rather accurate simulations of the available experimental results. In fact, it can simulate accurately the overall behavior observed in three experimental tests on specimens characterized by significantly different mechanical properties of the strip. Since the model assumes a small set of mechanical parameters for describing the mechanical behavior of the adhesive FRP-concrete interface and results in a reasonable small set of equations, it can be easily employed for identifying the above mentioned mechanical behavior indirectly. Other numerical models already available in the scientific literature (especially those based on the theory of finite elements) for simulating the 2D displacement field in the debonding stage are generally based on much more equations and require a much higher computational effort which makes impractical their use in an indirect identification procedure like the one presented in this paper. In fact, one of the main results of this study consists in determining the distribution of normal (peeling) stresses throughout the adhesive-to-concrete interface. This stress component (Mode I) can neither be directly measured during the tests nor determined by the theoretical models usually adopted for simulating the fracturing behavior of FRP-to-concrete joints in the so-called fracture process in “mode II”. Finally, it was found that the shear stresses are significantly higher than the peeling ones and control the crack propagation process.     

双参数模型法研究早龄期混凝土的断裂性能

本试验采用了RILEM推荐的“双参数模型”(TPM)(RILEM Technical Committee 89-FMT)研究了早龄期混凝土的临界应力集中因子KIC^3和临界裂缝尖端开口位移CTOD这两个断裂参数。试验同时采用普通混凝土和高强混凝土作为试验对象，取4根同一龄期的混凝土用于试验。试验龄期为12小时，1天，2天，3天，7天和28天。试验结果表明两参数值随水化的进行增大，相对于成熟混凝土来说，早龄期混凝土的这两个参数值比较低；在同一龄期，高强混凝土的这两个参数值要比普通混凝土的对应值高；从试验中还可以看出混凝土的断裂参数与强度有相类似的发展规律，可以作为材料的性能指标，用于表征混凝土的阻裂能力。     

A new method for evaluating the surface roughness of concrete cut for repair or strengthening

Deterioration to concrete structures usually starts at the surface and progresses into the structure. The deteriorated concrete has to be removed and replaced by new repair material, which leads to a composite concrete/repair material structure. In order to ensure full structural interaction between the old and new material, it is necessary to have a good bond between them. The bond itself depends on having a clean and rough surface. The importance of surface preparation and the effect of roughness have been emphasised by many researchers. In order to study the effect of surface roughness on the bond between a repair material and the concrete substrate, the roughness of the surface needs to be measured. No standard method for measuring surface roughness of concrete has been adopted. A range of methods are available and are in use for measuring the surface texture of concrete pavements in the highway field, but no method is accepted as standard. A new method of quantifying surface roughness is proposed and compared with the only method reported in the literature. In this new method (referred to as the roughness gradient method) a new method of describing the surface roughness is proposed. The surface of the cut concrete is defined as a series of irregular waves. The roughness parameter is dependent on the peak-to-trough depth and the respective wavelength. The method adopted using the ‘roughness gradient’ (R.G.) gave a better indication of the treated surfaces used in testing concrete repairs. Visual observation of the treated surfaces seemed to be in agreement with the R.G. parameters, while the method reported in the literature failed to define the roughness of the surface of the whole substrate in an accurate and comprehensive way.