饱和钢纤维再生混凝土氯离子扩散特性研究

利用废弃骨料制作再生混凝土是建筑固废弃物的再生资源利用途径之一,其在氯盐侵蚀环境条件下的耐久性规律与普通混凝土不同。本文以模拟海洋侵蚀环境,通过改变水灰比及NaCl溶液浓度,分析各因素对钢纤维再生混凝土氯离子扩散行为的影响规律。结果表明:浸泡液浓度越大,进入混凝土内部的氯离子越多,随着深度的增加氯离子含量逐渐减少;较低的水灰比与适量钢纤维的掺入均可以有效降低氯离子扩散系数,提升钢纤维再生混凝土的抗氯盐侵蚀能力;氯离子扩散系数随着NaCl溶液浓度的增加而增大。     

纤维增强混凝土氯离子扩散系数的多尺度预测模型

纤维增强混凝土材料属于多相非均质复合材料,其宏观耐久性能由微观和细观的组分占比和夹杂关系共同决定。为考虑纤维增强混凝土材料不同尺度下的非均质性对整体氯离子扩散系数的影响,本文基于从微观到宏观的多尺度方法选取了不同层级代表单元,建立了纤维增强混凝土氯离子扩散系数多尺度预测模型。模型在充分考虑微观水泥水化过程和阈值效应的基础上,分析了细观尺度下纤维、骨料及其与水泥浆体的结合界面对混凝土宏观扩散性能的共同影响,并探究了纤维尺寸、纤维-浆体界面过渡区厚度等因素与扩散系数之间的影响关系,且通过第三方试验对模型的可靠性进行了验证。参数化分析的结果表明,当水灰比大于一定限值(约为0.45),水泥浆体的氯离子扩散系数与水灰比呈指数增长,而在细观层级上,纤维-浆体界面过渡区是影响混凝土整体扩散性能的主要因素：纤维掺量的增加和纤维直径的减小都会增大界面过渡区的体积,而较高的纤维过渡区体积占比和纤维界面扩散系数都会增大纤维增强混凝土的宏观氯离子扩散系数。结果还表明混凝土扩散系数与纤维掺量之间并无直接关系,而需要综合考虑纤维直径、界面过渡区厚度等各种因素的影响。本文所提模型能够有效预测纤维增强混凝土的扩散系数...     

基于多尺度分析的再生混凝土有效氯离子扩散系数预测

将再生混凝土视为由砂浆相、再生骨料相和二者砂浆之间的界面过渡区相(ITZ-2)组成的非均质复合材料,其中砂浆相由细骨料、细骨料-新砂浆界面过渡区(ITZ-1)、硬化水泥浆体三相组成,而再生骨料相由旧骨料、附着砂浆及旧骨料与旧砂浆的界面(ITZ-3)三相组成。基于N层球夹杂理论,考虑微观相的影响,建立了再生混凝土有效氯离子扩散系数预测的五相多尺度模型,通过硬化水泥浆体、砂浆和再生混凝土的稳态扩散系数实测值与模型预测值对比分析,验证其模型的准确性和有效性;最后,进一步讨论了氯离子侵蚀时间、再生骨料体积分数和附着砂浆含量等关键参数对其有效扩散系数的影响。结果表明：有效扩散系数预测值与试验值吻合较好,说明模型对预测再生混凝土的有效氯离子扩散系数具有普适性,为氯盐环境下再生混凝土耐久性评估与寿命预测提供理论依据。     

Ag/AgCl氯离子传感器在混凝土中的应用

尝试将试验室制作的Ag/AgCl氯离子传感器用于混凝土中测试,验证该传感器应用于混凝土结构中的可行性。测试了传感器在混凝土试件中的长期稳定性,建立了传感器在混凝土中的线性响应方程,并通过加速氯离子渗透的方式对建立的线性方程进行了验证。试验结果表明,传感器在混凝土试件中可以长期稳定有效地工作,85d电位波动25mV,传感器在氯离子掺量相同的试件中电位响应一致性良好;通过建立的线性方程计算得到的电位响应值与实测电位响应值符合得很好,所建立的方程与试件水胶比水平无关,与氯离子的引入方式无关,是适用于混凝土结构的比较准确的线性方程。通过该方程的建立即可实现使用该传感器监测混凝土结构中的氯离子分布,达到钢筋锈蚀提前预警的目的。     

核电厂预应力混凝土安全壳中氯离子扩散系数的预测方法EI北大核心CSCD

氯离子扩散系数是评价沿海环境中核电厂预应力混凝土安全壳耐久性的重要参数。该文基于两尺度方法,通过分析双轴等压下水泥浆基体、界面和混凝土细观结构,建立了水泥浆基体和界面毛细孔隙率与预应力之间的定量关系。为了量化预应力对微裂纹闭合、产生和扩展的影响,提出了临界毛细孔隙率与预应力之间的经验公式。将混凝土模拟成由骨料、界面和水泥浆基体组成的三相复合材料,获得预应力混凝土氯离子扩散系数比。通过与试验结果比较,校正了经验公式中的两个参数。用三组试验数据初步验证了该预测方法的有效性。     

多壁碳纳米管增强混凝土的抗氯离子渗透性能

采用快速氯离子迁移系数(RCM)法和自然浸泡法对多壁碳纳米管(MWCNTs)增强混凝土的抗氯离子渗透性能进行研究,测量混凝土试件纵向断面上的氯离子扩散深度,据此计算氯离子扩散系数。试验结果表明：当MWCNTs掺量为0.15wt%时,混凝土28天的氯离子扩散深度、氯离子扩散系数分别降低了25.7%、19.1%;在4种不同侵蚀龄期的自然浸泡下,掺入MWCNTs的混凝土,内部氯离子浓度始终低于对照组。结合两种方法分析得出：混凝土内部各深度的自由氯离子浓度随着MWCNTs掺量的增加而降低,致使氯离子扩散系数随着MWCNTs掺量的增加而变小,MWCNTs的掺入提高了混凝土的抗氯离子渗透性能。此外,通过SEM和压汞(MIP)测试进一步探究MWCNTs对混凝土抗氯离子渗透性能的微观增强机制,分析结果表明,MWCNTs具有一定的桥接和填充效应,这可能使混凝土裂缝扩展受到抑制、孔隙更加细化,从而改善混凝土的微观结构,提高混凝土的抗氯离子渗透性能。     

基于RCM法的砼氯离子扩散系数测定仪的检测

氯离子引起的腐蚀严重影响到混凝土结构的持久性,为了提高钢筋混凝土构筑物的持久性,需掌握和了解氯离子在混凝土中的扩散行为及其规律,以便根据这些规律进行混凝土结构耐久性设计和服役寿命预测。混凝土氯离子扩散系数测定仪可定量评价混凝土抵抗氯离子扩散的能力．为氯离子侵蚀环境中的混凝土结构耐久性设计与施工以及使用寿命的评估与预测提供基本参数。     

裂缝对混凝土内氯离子扩散和钢筋锈蚀的影响

为了研究受力裂缝对混凝土内氯离子扩散的影响,对8根开裂(预制裂缝和受弯裂缝)的钢筋混凝土梁试件进行了盐溶液干湿循环试验;并采用半电池电位法和RCT(快速氯离子含量检测)法,分别对梁内钢筋的锈蚀状态以及各裂缝处氯离子含量进行了检测;最后采用ADINA有限元软件对开裂混凝土内氯离子的二维扩散行为进行模拟分析。试验和模拟结果表明:1)裂缝加速了氯离子的侵入,导致钢筋过早的开始锈蚀,且为局部氯离子的二维扩散提供了条件;2)裂缝宽度越大或间距越小,其对氯离子侵入引起的钢筋锈蚀影响程度越大;3)有限元模型的计算与试验结果吻合较好。     

Damage model for simulating chloride concentration in reinforced concrete with internal cracks

We present a method to simulate, in three dimensions, the concentration of chloride ions that penetrate into concrete with internal cracks. The method comprises the crack-propagation analysis of concrete and the diffusion analysis of chloride ions. A finite-element model with a damage model that is based on fracture mechanics for concrete was applied in the crack-propagation analysis, and we were able to reproduce the three-dimensional geometry of the internal cracks. Chloride-ion transfer through internal cracks was simulated by diffusion analysis with the simultaneous consideration of damage, and a diffusion coefficient that was expressed as a function of the damage variable obtained from crack-propagation analysis. We present a formulation of crack-propagation analysis by using the damage model and unsteady-diffusion analysis in consideration of damage. We also present a verification analysis of internal cracking in concrete to demonstrate that the crack width and the chloride concentration can be evaluated without mesh dependency. This is followed by a validation analysis. A comparison between the numerical and experimental results shows that the proposed method enables the high-accuracy simulation of chloride penetration into concrete with internal cracks.     

Prediction of the chloride diffusion coefficient of concrete

It has been experimentally verified that the structure of the interfacial transition zone (ITZ) in concrete differs from that of bulk cement paste. As such, concrete should be modeled as a three-phase material at a mesoscopic level. This paper presents a three-phase composite model for predicting the chloride diffusion coefficient of concrete. Taking the inclusion as aggregate and the matrix as cement paste, the composite circle model is established by adding an ITZ layer in between the inclusion and the matrix. Solving the asymmetrical problem analytically, a closed-form solution for the chloride diffusion coefficient of concrete is derived. After verifying this model with experimental results, the effects of the aggregate area fraction, the chloride diffusion coefficient of ITZ, the ITZ thickness, the maximum aggregate diameter and the aggregate gradation on the chloride diffusion coefficient of concrete are evaluated in a quantitative manner. It is found that the chloride diffusion coefficient of concrete decreases with the increase of the aggregate area fraction and the maximum aggregate diameter, but increases with the increase of the chloride diffusion coefficient and thickness of ITZ. It is also found that the aggregate gradation has a significant influence on the chloride diffusion coefficient of concrete.     

Determination of diffusion coefficient of chloride in concrete: an electrochemical impedance spectroscopic approach

For predicting the service life of concrete structures in marine environment, diffusion of chloride (D) is an important parameter. Electro-migration tests and ponding tests are two techniques conventionally adopted, however they are destructive in nature. EIS (Electrochemical impedance spectroscopy) being non-destructive appears a promising technique to arrive at ‘D_R’ (D from EIS) in situ in structures. The D_R of ordinary Portland cement concrete (OPC) was compared with that of Portland pozzolana cement concrete (PPC). The effect of curing on D_R was analyzed. The splash zone condition was created by subjecting the specimens to alternate wetting and drying cycles. At the end of 28 days of curing, the D_R of PPC concrete is only 66.7% of that obtained in OPC concrete. A linear correlation was established between D_R and the porosity of the concrete_. Due to pozzolanic reaction, the rate of pore refinement is faster in PPC concrete compared to OPC concrete. In M25-PPC concrete at the end of 28 days of curing, the pore size is decreased to 14.6% of that obtained at the end of 3 days of curing. The reduction of pore size by densification of pore structure due to pozzolanic reaction reduces the D_R value in PPC concrete. In 30 MPa concrete the D_R under wet cycle is 3 times higher than in dry cycle, which implied that corrosion is initiated 3 times faster in concrete exposed to the splash zone condition.     

干湿循环作用对混凝土抗氯离子渗透侵蚀性能的影响

为了加速模拟海洋潮差区环境对混凝土耐久性能的影响,掺入矿粉和纳米改性矿物掺合料,研究了氯盐干湿循环作用对混凝土抗氯离子侵蚀性能及微观结构的影响,并将干湿循环试验与常规浸泡试验进行了对比。结果发现,干湿循环作用粗化了混凝土试件表层孔结构,增大了孔径>50 nm的孔隙含量,显著提高了自由及总氯离子浓度;掺入纳米改性矿物掺合料能降低混凝土内部孔隙率,减少有害孔含量,提高混凝土内部氯离子结合能力;干湿循环60天后混凝土表层Ca（OH）₂逐渐被消耗,生成了Friedel盐和CaCO₃。      

The chloride ponding test and its correlation to the accelerated chloride migration test for concrete

Abstract

In this study, the total chloride content and penetration depth of concretes were measured by the 90‐day salt ponding test, and the flux of chloride ions passing through the concrete was measured by the accelerated chloride migration test (ACMT; the electrochemical technique is applied to accelerate chloride ion migration). Fick's second law was fitted to the data from the ponding test to determine the diffusion coefficient. The steady‐state and the non‐steady‐state migration coefficients were determined from the modified Fick's first and second laws, respectively. The steady‐state migration coefficient, the non‐steady‐state migration coefficient, and the diffusion coefficient were compared. The non‐steady‐state migration coefficient, steady‐state migration coefficient, and diffusion coefficient were linearly correlated. The non‐steady‐state migration coefficient gave the highest value, being about 1.5 times higher than the steady‐state migration coefficient, and up to about 4 times higher than the diffusion coefficient. Since the 90‐day ponding test is time‐consuming, the ACMT provides a time saving method to obtain the transport property of concrete.     

混凝土I型裂缝扩展准则比较研究

针对混凝土I型裂缝扩展问题,分别采用以起裂韧度为参数的裂缝扩展准则、最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则,数值模拟了强度等级C20、C40、C60、C80和C100的混凝土三点弯曲梁裂缝扩展全过程,获取了试件的荷载-裂缝口张开位移(P-CMOD)曲线并与试验结果进行了比较。结果表明,三种准则中以起裂韧度为参数的裂缝扩展准则计算得到的峰值荷载及P-CMOD全曲线与试验结果差别最小。随着混凝土强度等级的提高,最大拉应力准则以及裂尖处应力强度因子为零的裂缝扩展准则计算出的P-CMOD曲线与试验结果相比均有较为明显的偏离,但以起裂韧度为参数的裂缝扩展准则计算结果与试验曲线更为吻合。试验与计算结果表明,以起裂韧度为参数的裂缝扩展准则更适用于不同强度混凝土材料的断裂分析。     

Long-term chloride diffusion in silica fume concrete in harsh marine climates

Supplementary cementitious materials such as silica fume are typically necessary for producing high performance concrete for marine environments in hot regions, such as the Persian Gulf. Silica fume use generally improves the strength and/or durability properties of the concrete. This paper investigates the effects of silica fume on various properties of concrete specimens that were exposed to Persian Gulf conditions. Samples were taken at the ages of 3, 9 and 36 months and analyzed to determine the chloride diffusion coefficient. The results show that partial cement replacement with up to 7.5% silica fume reduces the diffusion coefficient, whereas for higher replacement rates the diffusion coefficient does not decrease significantly. Also time-dependent chloride diffusion and compressive strength of concrete containing silica fume are investigated.     

Chloride ingress in microsilica concrete

The paper presents experimental results on chloride ingress and water absorption of microsilica concrete. To investigate the interaction of cement, water and microsilica content on diffusion characteristics and porosity (water absorption), 36 concrete mixes with water/cement ratios ranging from 0·4 to 0·9 and cement contents between 250 and 450 kg/m³ were studied. The microsilica content varied between 5 and 15%. Curing was under two conditions, 20°C, 100% RH and 35°C, 25% RH.

The results show that a relationship exists between chloride concentration and W/C ratio, the relationship being different for the various microsilica contents. The effect of high W/C ratios is much more pronounced in low microsilica content mixes (0 and 5% addition). The chloride ingress is also sensitive to cement content in low microsilica contents mixes (0 and 5%), but not in high microsilica content mixes (10 and 15%).

The diffusion coefficient, D_c, is not significantly affected by microsilica content, for mixes made at a constant W/C ratio.