Study on mechanism of strength distribution development in vacuum-dewatered concrete based on the consolidation theory

The strength and hardness of a concrete slab surface are considered to be significantly affected by concrete bleeding. Vacuum dewatering is reported to be quite effective in imparting high density and strength. However, in Japan, in contrast with concrete work in civil engineering applications, concrete work in the field of building construction has not been successfully treated by this method. In an earlier report, the authors pointed out the strong relationship between strength distribution and density distribution in vacuum-dewatered concrete, both of which gradually decrease from the top surface to a depth of about 15 cm. The main purpose of the present study is to discuss the mechanism of the occurrence of such distribution of strength and density, based on consolidation theory. In an experiment, pore water pressure distribution in concrete is measured by means of an original measuring system. The results of the experiment confirm that the consolidation theory is quite effective in explaining the internal properties of vacuum-dewatered concrete as well as those of press-dewatered concrete. A prediction method for the strength improvement of concrete by vacuum dewatering is also discussed. It was considered likely that pore water pressure distribution generated by vacuum dewatering could be attributable to the influences of capillary tension and viscous resistance. This mechanism was verified by model experiment.     

角点支承双向密肋楼盖试验与极限平衡荷载计算方法

为了了解角点支承方形区格钢筋混凝土双向密肋楼盖的受力性能与破坏形态,寻求极限平衡荷载计算方法,进行过一块尺寸为5.46m×10.92m角点支承的两跨连续方形区格双向密肋楼盖的短期荷载试验,根据试验结果,提出了此种楼盖结构的破坏机构,并由此推导出角点支承方形区格钢筋混凝土双向密肋楼盖的极限平衡荷载计算公式。     

Algorithm for automatic data interpretation software for concrete slab in impact echo method

The commercially available impact echo assessment is effective in assessing flaws in concrete but the evaluation is manual, thus, to evaluate large area by this method is neither practical nor cost effective. In order to enhance the method in application to reinforced concrete slab such as bridge decks, where duration of investigation plays substantial influence on traffic activity, data analysis and interpretation should not only be rapid and objective but also should provide automatic documentation of assessment results straight away at the end of the investigation. This research paper describes a new approach for impact echo method with automatic interpretation of signal through an algorithm. The algorithm detects the defect; calculates its depth and the assessment result is presented in 2D view instantaneously. The algorithm is verified by means of an experimental study, which is conducted with four reinforced concrete slabs incorporating induced delamination, void and honeycombing. A software named Imco@slab is developed based on this algorithm. Finally, a field study over reinforced concrete slab using this software has been conducted and its applicability in practical sense is verified. The field study shows that the discrepancy in assessment by Imco@slab interpretation with hammer tapping method and conventional manual impact echo method is negligible. This software is applicable in assessing delamination and void defects in reinforced concrete slabs.     

矩形钢筋混凝土双向板板底塑性弯矩比设计取值研究

提出了通过调整板底塑性弯矩比控制板底裂缝宽度的概念;基于薄板理论的Navier解建立了提高钢筋混凝土板板底抗裂性能的板底塑性弯矩比值公式;基于塑性铰线理论,给出了保证配筋量最少的板底塑性弯矩比值公式。通过对比,给出了工程中板底弯矩取值的建议公式。     

考虑徐变效应的施工期钢筋混凝土结构荷载传递规律

施工期钢筋混凝土结构是由材料性能不断变化的混凝土构件以及临时支撑组成的时变结构。在楼板的养护过程中,各层楼板承担的荷载会发生重新分配现象,但理论上对其产生的原因一直没有合理的解释。在结构特征参数方法和徐变理论的基础上分析了考虑混凝土徐变效应的的荷载传递规律,探讨了徐变造成荷载重分配的机理并证明了徐变是产生施工期混凝土结构荷载重分配现象的原因之一。同时讨论了在常用施工方案下,徐变效应对结构最大荷载的影响程度以及结构最大荷载的分布范围。通过比较发现徐变能够降低结构的最大荷载,有利于施工期结构安全。施工期混凝土结构的安全性验算可以参照本文的结果考虑徐变的有利影响。     

Rock fall impact on reinforced concrete slab: an experimental approach

This paper concerns the protective structures against rock fall. We propose a new concept of rock-shed protection. The proposed rock-shed protection is made of reinforced concrete slabs held up by specially designed supports that act as a type of expendable fuse to absorb high rock-fall energy especially when shock occurs on the side of the slab. Thus, by a simple change of damaged supports and local restoration of concrete in the impact area, the structure can continue to be used. We present also an experimental method to characterize and analyze shock absorption effected by this type of reinforced concrete slab. The analyzed slab is a 1/3 scale reproduction of an actual structure.     

飞机滑行作用下水泥混凝土道面板动响应分析

摘 要：研究了飞机滑行作用下机场水泥混凝土道面板动响应问题。将道面结构视为粘弹性层状地基上单块四边自由的矩形板,采用半解析法,建立力学模型和计算方法。飞机滑行作用在道面板上的荷载是由自身重量和滑行产生的升力决定的;并计算了飞机滑行作用下道面板的响应。采用在道面内部钻孔安装位移传器的方法,对H-6飞机以不同滑行速度通过道面板时,道面板产生的动挠度进行了实测。计算结果与实测结果对比表明,两者误差在3%以内。说明本文所建立的计算方法是正确的。该方法可用来进行机场水泥混凝土道面板在飞机滑行作用下动响应的计算。     

模板支撑薄弱层对现浇钢筋混凝土建筑结构施工荷载分布的影响

对现浇钢筋混凝土建筑结构施工期间,模板支撑系统架设中可能存在的薄弱层对施工荷载分布的影响,提出了简化分析方法,并用工程实测结果进行了验证,表明模型计算结果的正确性.分析表明,模板支撑薄弱层将导致模板支撑薄弱层的上部楼板承担的施工荷载增大,而模板支撑薄弱层下部楼板承担的施工荷载减小;实测表明,模板支撑薄弱层上的楼板承担的施工荷载同比增大约4.3%,薄弱层下的楼板承担的施工荷载减小约3.9%.施工期间应避免模板支撑薄弱层的出现,以防模板支撑薄弱层可能带来的风险.     

混凝土翼板完全断开的钢-混凝土组合梁纯钢梁段的受压翼缘局部稳定性分析

为解决实际工程中遇到的混凝土翼板完全断开的钢-混凝土组合梁受压翼缘局部稳定性问题,基于薄板理论,对组合梁混凝土翼板开洞处纯钢梁段受压翼缘的临界屈曲应力计算公式进行了推导,考虑了弯矩梯度的影响,并用通用有限元软件ANSYS验证了公式的准确性。根据临界屈曲应力计算公式得出了合理的受压翼缘宽厚比限值,与现行《钢结构设计规范》中的规定进行了对比。对比结果表明:将《钢结构设计规范》中的宽厚比限值直接用于组合梁开洞处的纯钢梁段,显得过于保守。最后通过算例分析,进一步验证了该文的结论。     

楼板系统对去柱后钢框架梁的轴向约束刚度研究

钢梁两端约束刚度对其悬链线效应影响很大,因此钢梁悬链线效应中约束刚度的研究是一个重要的内容。该文研究“双跨钢梁”两端的钢梁、楼板以及栓钉连接件对钢梁的轴向约束作用。首先,提出提供约束的钢梁、楼板以及栓钉连接件的非线性位移和变形函数。其次,利用最小势能原理,推导楼板系统对去柱钢框架梁轴向约束刚度的计算公式。考虑提供约束的楼板总是局限在一定范围内,提出并使用楼板“临界长度”对理论公式进行了修正。最后,采用20 个有限元算例对理论公式进行算例验证,证明了该文研究结论的可靠性。     

用Poly MAX方法进行弹性地基板的实验模态分析

对弹性地基上自由板进行了脉冲锤击法模态试验研究。针对传统模态识别方法难以准确识别重频、高阻尼结构的模态参数的特点，利用LMSTest．Lab软件中Poly MAX方法对地基板的传递函数进行模态参数估计和识别。实测及分析表明实际地基上的自由板不存在纯刚体模态。利用识别的模态频率分别对Winkler地基，双参数地基的地基参数进行了识别。利用蒙特卡罗模拟计算地基参数以标准差5％变化时的频率值，发现地基的不均对弹性地基板的频率值有着重要的影响。为道路工程和岩土工程中混凝土板的地基参数识别和损伤诊断提供了手段和方法。     

Experimental Study on Deicing Performance of Carbon Fiber Reinforced Conductive Concrete

Carbon fiber reinforced concrete (CFRC) is a kind of good electrothermal material. When connected to an external power supply, stable and uniform heat suitable for deicing application is generated in the CFRC slab. Electric heating and deicing experiments of carbon fiber reinforced concrete slab were carried out in laboratory, and the effect of the temperature and thickness of ice, the thermal conductivity of CFRC, and power output on deicing performance and energy consumption were investigated. The experimental results indicate that it is an effective method to utilize the thermal energy produced by CFRC slab to deice. The time to melt the ice completely decreases with increasing power output and ice temperature, and increases with increasing thickness of the ice. The energy consumption to melt 2 mm thickness of ice varies approximately linearly from 0.556 to 0.846 kW·h/m2 as the initial temperature ranges from -3℃ to - 18℃. CFRC with good thermal conduction can reduce temperature difference in CFRC     

混凝土现浇楼板的裂缝成因及预防控制

混凝土裂缝已成为建筑工程质量通病,其中最常见也是最多的是现浇楼板裂缝。如何防治混凝土裂缝是工程技术人员迫切需要解决的技术难题。混凝土开裂后,严重影响结构的整体性和耐久性。本文分析了混凝土现浇楼板裂缝产生的机理,从各个环节提出了预防现浇楼板裂缝的综合措施,以减少现浇楼板裂缝发生的机会。     

Composite concrete/GFRP slabs for footbridge deck systems

In recent years the use of advanced composite materials has gained wider space in the civil engineering sector, due to some favorable characteristics such as lightweight, high specific strength, resistance to corrosion and fatigue. Innovative systems that combine concrete with advanced composite materials have proved to be a viable and efficient solution as compared to conventional systems. In this work, a new slab system composed of a fiber-reinforced concrete top laid on glass fiber reinforced polymeric (GFRP) wide-flange-section pultruded profiles, filled in with foam blocks, is presented. The material properties of the GFRP profiles were obtained both theoretically and experimentally. Experimental tests to choose the appropriate resin to bond the concrete to the GFRP profiles and to select the appropriate short fiber and volume fraction to be used in the concrete top have also been conducted. The slab was designed to sustain constructive loads and live pedestrian loads for footbridge deck applications. To investigate the slab flexural behavior up to failure, three specimens were tested under four-point bending, and theoretical and finite element analyses were also performed. Comparisons of theoretical, numerical and experimental results show good agreement. Studies under way to complete the development of the proposed slab are briefly described at the end of the work.     

空间立管真空泥水分离试验研究

设计一种空间立管真空抽吸泥水快速分离试验系统, 系统由模型箱、空间过滤系统、真空控制系统和真空泵四个模块组成。开展单个空间立管真空泥水分离试验和组合空间立管真空泥水分离试验, 对比研究空间立管真空泥水分离规律。试验结果表明真空度在85 kPa时, 单个空间立管排水最大水平影响范围在1.5~2 cm内, 且泥浆含水率随水平距离的增大而增大, 逐渐趋于定值;多个立管组合后, 不同空间位置的泥浆因受到组合空间立管、过滤板和泥浆自重的共同复杂作用, 泥浆的含水率与水平距离呈波浪形非线性关系, 与时间和深度呈拟线性关系。试验设计的空间立管真空泥水分离方法相比传统真空泥水分离的方法效率大大提高, 试验结果为其工程推广应用提供了重要的理论支撑。     

考虑楼板影响的外环板式梁柱节点抗震性能:试验研究

该文对采用Q390钢材制作的4个T形节点构件进行了低周反复荷载试验,研究了钢梁和混凝土楼板的组合作用对采用高性能钢的外环板式梁柱节点的刚度、承载力以及耗能能力的影响。试验结果表明:由于楼板组合作用,钢梁上翼缘的稳定性得到显著提高;采用外环板式的梁柱节点连接方式,能够有效地将塑性铰外移,避免了在节点域附近的焊缝发生撕裂;但由于楼板组合作用的影响,此种节点形式也存在节点域附近外环板处发生断裂的可能性;该试验4个构件的滞回曲线都较为饱满,表现出较好的耗能能力和延性性能,在塑性点时,无楼板作用的试件的粘滞阻尼系数略高于有楼板作用的试件的粘滞阻尼系数;在层间位移角达到0.05 rad时,各试件的粘滞阻尼系数趋于一致。     

弹丸冲击贯穿有限厚混凝土材料靶板的背面成坑效应

弹丸冲击贯穿有限厚混凝土靶板后,靶板背面有大块的混凝土剥落,形成近似的锥形坑。为了研究背面弹坑半锥角θ这一作为混凝土靶板贯穿后破坏范围问题研究中的重要参量,将贯穿问题等效为轴对称条件下的冲切破坏问题。采用双剪应力三参数强度准则及刚塑性模型,得到了极限应力圆的包络线方程,给出了轴对称破坏机构,进而求得θ值的表达式。理论计算结果与数值模拟结果、实验数据三者之间吻合度较好。研究表明,θ值由混凝土材料抗压强度与抗拉强度比值决定。基于上述研究,提出在混凝土中掺加钢纤维以提高有限厚靶板抗贯穿能力的实际方法,并进行了弹道实验。实验结果表明,贯穿破坏后靶板碎片的数量及θ大幅降低,显示了高含量异型钢纤维混凝土在抗贯穿方面的适用性。     

真空技术在污水处理中的若干典型应用

介绍了真空技术在污水处理中的几种典型应用，真空结晶法在以分离污水中具有结晶性能的固体溶剂；真空过滤脱水用于初沉池和消化池污泥脱水，真空浮选法通过制造负压条件加速浮选过程；还有真空搅拌、真空脱气等等。这些足以说明真空技术在污水处理乃至环境保护中占有重要的一席之地。     

腹板开孔的钢-混凝土组合梁的挠度计算

描述了一种腹板开孔组合梁挠度的计算方法。提出了楼板参与开孔段上部截面抗弯的有效宽度的概念,并进行了拟合。将剪切变形和空腹桁架弦杆的弯曲变形合并为等效剪切变形,得到开孔段的等效剪切刚度。利用开孔段与未开孔段截面刚度不同的现象,建立分段梁单元模型,采用传统梁理论求解挠度计算公式。与采用实体单元和板壳单元的有限元分析结果的对比表明,该文公式具有很好的精度。     

Heated slabs under biaxial compressive loading: a test set-up for the assessment of concrete sensitivity to spalling

Explosive spalling of concrete members in fire is the violent expulsion of shards from the exposed face caused by the combined effect of pressure build-up in the pores due to water vaporization and of in-plane stress induced by both external loads and thermal gradients. Spalling progression leads to the reduction of bearing cross-section and often to the direct exposure of rebars to the flames. Since established predictive models are not available yet, experimental studies appear to be the most effective means of investigation on this phenomenon. To this purpose an experimental setup has been developed for the assessment of concrete sensitivity to spalling. The specimen is a concrete slab (800 × 800 mm) with a thickness comprised between 100 and 200 mm. The bottom face is heated via a horizontal furnace, in which a propane burner is actively controlled in order to follow the prescribed fire curve. During heating, a biaxial compressive load can be applied thanks to hydraulic jacks restrained by a steel frame. Load and slab thickness can be adjusted in order to represent the mechanical conditions achieved in the hottest region of thicker concrete members such as tunnel lining segments. The setup proved to be very effective in comparing spalling sensitivity among different concrete mixes, as is often required in initial material testing for strategic infrastructures such as tunnels.