旋挖钻机施工钻孔灌注桩的工艺研究

泰州大桥夹江左汊桥主跨采用87.5m+3×125m+87.5m的五跨连续梁桥,水中主墩基础采用2.0m桩径的钻孔灌注桩,最大设计桩长为94m,采用旋挖钻机施工。文章结合泰州大桥的施工实例,阐述了旋挖钻机成孔的工艺原理,按照工艺流程,重点叙述了泥浆配置、安装钻孔、钻机钻进等工艺,并重点总结了旋挖钻机施工钻孔桩的施工特点,为今后类似工程施工提供了借鉴。     

基坑爆破预留层对围护桩的保护作用数值分析

基坑爆破开挖工程中爆破开挖部分与围护桩之间通常会预留一定厚度的岩土层,用以保护基坑围护结构的安全,免受爆破产生的巨大能量的损害。为了得到合理的预留层厚度,保证基坑开挖过程的安全,最大限度缩减施工周期,提高经济效益,采用LS-DYNA有限元分析软件,对基坑爆破开挖时对临近钻孔灌注桩的保护效果进行了数值计算分析。分析得出,预留1.5m厚中风化泥岩作为临近钻孔灌注桩的保护层,能显著降低爆炸荷载作用下钻孔灌注桩的压力峰值和最大主应力峰值,保护临近钻孔灌注桩免遭破坏;同时临近钻孔灌注桩的最后一排炮孔孔间采用25ms延时时间,可有效避免应力波峰值的叠加,进一步保护了钻孔灌注桩的安全。此结论可为类似工程提供参考。     

基坑爆破预留层对围护桩的保护作用数值分析

基坑爆破开挖工程中爆破开挖部分与围护桩之间通常会预留一定厚度的岩土层,用以保护基坑围护结构的安全,免受爆破产生的巨大能量的损害。为了得到合理的预留层厚度,保证基坑开挖过程的安全,最大限度缩减施工周期,提高经济效益,采用LS-DYNA有限元分析软件,对基坑爆破开挖时对临近钻孔灌注桩的保护效果进行了数值计算分析。分析得出,预留1.5m厚中风化泥岩作为临近钻孔灌注桩的保护层,能显著降低爆炸荷载作用下钻孔灌注桩的压力峰值和最大主应力峰值,保护临近钻孔灌注桩免遭破坏;同时临近钻孔灌注桩的最后一排炮孔孔间采用25ms延时时间,可有效避免应力波峰值的叠加,进一步保护了钻孔灌注桩的安全。此结论可为类似工程提供参考。     

Assessing relationships among properties of demolished concrete, recycled aggregate and recycled aggregate concrete using regression analysis

Recycled demolished concrete (DC) as recycled aggregate (RA) and recycled aggregate concrete (RAC) is generally suitable for most construction applications. Low-grade applications, including sub-base and roadwork, have been implemented in many countries; however, higher-grade activities are rarely considered. This paper examines relationships among DC characteristics, properties of their RA and strength of their RAC using regression analysis. Ten samples collected from demolition sites are examined. The results show strong correlation among the DC samples, properties of RA and RAC. It should be highlighted that inferior quality of DC will lower the quality of RA and thus their RAC. Prediction of RAC strength is also formulated from the DC characteristics and the RA properties. From that, the RAC performance from DC and RA can be estimated. In addition, RAC design requirements can also be developed at the initial stage of concrete demolition. Recommendations are also given to improve the future concreting practice.     

Optimization of normal and high strength recycled aggregate concrete mixtures by using packing model

This paper analyzes the possibility of applying the Compressible Packing Model (CPM) for the proportion of concrete mixtures produced with Recycled Concrete Aggregates (RCAs). As a matter of fact, the RCAs are composed of natural aggregates and attached mortar and, as a consequence, they generally present a higher porosity in comparison with ordinary natural aggregates. The higher porosity of RCAs can affect the resulting Recycled Aggregate Concretes (RACs) properties and, for this reason, the mix design procedure available in literature for ordinary concrete mixture cannot be applied as such in the case of RACs. In this context, the present work first presents a preliminary study in which the optimal mixing procedure for RACs is investigated and then, a possible extension of the CPM in the case of RACs is analyzed. Several structural RAC mixtures were designed for three strength classes (25, 45 and 65 MPa) by considering the variation of the aggregate replacement from 0 to 100%. Finally, the proposed procedure is experimentally validated by performing mechanical and durability tests on selected mixtures for the three strength classes with a RCAs content up to 60%. The results reported herein demonstrate the applicability of the CPM for recycled concrete mixtures and highlight as the rational use of RCAs lead to produce structural RAC without affecting its mechanical and the durability performance.     

Heterogeneous and anisotropic long-term concrete damage of the dez arch dam using thermal inverse analysis

In this paper, an assumption used in the recent work of the author and his contributors considers the long-term concrete damage of the Dez arch dam as a homogeneous and isotropic process, was investigated in more detail and was adjusted. To this end, the vertical dam sections were divided into nine and six subsections along the thickness and height directions of the dam, respectively. In each subsection, a transversely isotropic damaged elastic constitutive law was considered for diagnostic analyses. Following the previous authors’ mentioned work, an optimization procedure (minimization of a certain error function) accompanied with an inverse thermal analysis was carried out. That analysis was performed within the framework of finite element (FE) method. Mentioned error function was defined as the sum of the squared residuals. Residuals were set as difference between nodal recorded displacements with inverse pendulums of the dam and the corresponding computed ones with the proposed model. Parameters considered as unknowns in the present inverse analysis, which have contributions in that above-mentioned error function, were the five independent elastic moduli presented in the formulation of the earlier mentioned transversely isotropic damaged elastic constitutive law: two Young’s moduli: one in the planes parallel to the up and down stream surfaces of the dam, called in-plane and the other in the perpendicular planes to those planes, named out-of-plane Young’s modulus; two Poisson’s ratios (in-plane and out-of-plane); and one shear modulus (out-of-plane). These parameters were identified performing numerous thermal inverse analyses. Obtained results revealed that the long-term damage of concrete of the Dez dam is a heterogeneous and anisotropic phenomenon because that the magnitude of the mentioned error function was obtained smaller than the corresponding value in the previous study which had been performed before based on the homogeneous and isotropic damage evolution assumption.     

强夯施工对塑性混凝土防渗墙的振动测试分析

提出强夯施工时塑性混凝土防渗墙的振动测试方案。试夯数据表明,加速度控制指标0.1g比速度控制指标5cm/s更加严格;给出5个不同能级对应第一、二遍点夯的安全保护范围。基于Hilbert-Huang Transform信号方法的时频分析可知,不同夯击能级引起塑性混凝土防渗墙墙体测点的振动主频均小于20Hz;强夯施工对埋深仅为5m的塑性混凝土防渗墙的振动影响,瑞利波占主导地位,表现为墙体上端率先振动,且振动强度最大;塑性混凝土防渗墙沿墙高的垂向应力分布比较均匀;瑞利波由于受地基分层的频散影响,当主频降低时,波速和波长增加,最大瞬时能量和主频对应能量小幅增加     

超高性能混凝土在埋置炸药下的抗爆试验及数值模拟

该文研究了超高性能混凝土在埋置炸药爆炸载荷作用下的动态力学性能和损伤规律。通过改变混凝土靶体的配合比和炸药放置深度,得到了不同试验条件下混凝土靶体的破坏数据,采用非线性有限元法对靶体的毁伤效果进行了数值模拟,模拟结果与实际情况吻合较好。研究结果表明混杂纤维增强的超高性能混凝土具备更加优异的抗爆炸性能,炸药在靶体中的放置深度对混凝土的抗爆性能有显著影响。     

Quantification of evolving moisture profiles in concrete samples subjected to temperature gradient by means of rapid neutron tomography: Influence of boundary conditions,hygro-thermal loading history and spalling mitigation additives

Concrete has a tendency to spall, that is, to eject layers when subjected to high temperatures. This is an erratic phenomenon, and our understanding of the underlying physical process is still limited. A driving process is moisture transfer, whose experimental investigation has so far mostly been limited to macroscopic or point-wise observations, limiting both our understanding and the validation of the proposed models. In this paper, a non-contact technique, neutron imaging, is used to extract a the full-field distribution of moisture in 3D and in real time, while the concrete is heated at high temperatures. This reveals a number of processes often underestimated or ignored in the traditional experimental approaches reported in the literature. Notably, the effect on the evolving moisture profiles of varying heating rates for multiple insulation techniques as well the strong influence of the addition of spalling-mitigating additives is presented. The first ever example of neutron tomography of a spalled sample is also reported, and some preliminary analyses of the effect that moisture clog formation and heating rates have on it are revealed.     

Influence of drapability by using stitching technology to reduce fabric deformation and shear during thermoforming

Forming of woven fabric into a complex shape involves shear deformation of the fabric elements (warp and weft). Various researchers investigated the shear deformation during the manufacturing of thermoplastic composites (TPC) based on the hemispherical geometry, where the highly deformed edges and corners are considered as a waste. In many applications the complete formed geometry including areas around the curvature has to be used. A new idea to influence the deformation behavior of woven fabric by incorporated seams at localized sections is presented in this paper. The study is based on the photographic analysis of dry sewn preforms and thermoplastic matrix (PA12) impregnated laminates, before and after the deformation. The seams applied at the highly deformable and predefined zones help to reduce fabric shearing or to transfer the shear forces into the unsheared areas during the thermoforming process. The intensity of reduction in the fabric shear depends on the type of the sewn geometry. The results obtained can be applied in the part design, the preform engineering, and the positioning of stitches.     

Stress analysis of powder beds using constant-volume and constant-load shear tests: Characterization by repeated shearing and application to quasi-steady load shearing

Two types of high-performance shear test methods were employed to analyze the mechanical properties of powders. Both tests accurately measured the normal and shear stresses acting on the shear plane. Alumina powders with mass median diameters of 0.67, 4.0, 9.9, and 49 µm were used as samples. The effects of repeated shearing on the void fraction, stress relaxation ratio, stress transmission ratio, powder yield locus (PYL), consolidation yield locus (CYL), and critical state line (CSL) were determined using a constant-volume shear tester. The PYL and CSL were also measured using a constant-load shear tester, and the results obtained were compared. This comparison demonstrated a reasonable concurrence between the results obtained with the two testers. Furthermore, quasi-steady load shear tests were conducted using a constant-volume shear tester with feedback control, which demonstrated that the PYL could be obtained in a single shear test within a few minutes.     

Damage mode analysis of MCrAlY overlay coatings subjected to isothermal stepwise tensile testing by using in situ video imaging and acoustic emission monitoring

The mechanical performance of gas turbine material systems consisting of a Ni‐base superalloy substrate with three different MCrAlY overlay coatings with various Al contents was assessed, close to service temperature spectrum, by means of stepwise tensile tests. Furthermore, acoustic emission (AE) activity was monitored during the tensile testing. Tensile tests were performed for all three types of coated systems at four different temperatures (300, 500, 700 and 900 °C). The AE data were analysed by means of a standardized k‐means algorithm with respect to nine parameters (such as amplitude, number of counts, duration, rise time, energy, etc.). In conjunction with in situ video imaging of the specimen surface and postmortem microstructural analysis of the samples, the classification of AE data has provided a means for damage identification and performance assessment of the substrate‐coating material systems.