Effect of Pre-wetted Light-weight Aggregate on Internal RelativeHumidity and Autogenous Shrinkage of Concrete

This research indicates that the gradient of internal relative humidity （IRH） decreases rapidly within 7-day curing age in HPC. The amount of water imported by pre-wetted light-weight aggregate can regulate IRH of concrete. By importing a proper amount of water, the process of the decline of IRH can be delayed and the autogenous shrinkage can be reduced. The relationship among the amount of water imported by pre- wetted lightweight aggregate, IRH and AS was established. The result provides a new method of reducing early AS and enhancing early cracking resistance of HPC.

Fracture analyses of different pre-holed concrete specimens under compression

Crack propagation processes in specially prepared concrete discs and rectangular specimens containing a single cylindrical hole or multiple holes of varying diameters have been studied both experimentally and numerically. In this research, the cracks coalescence paths in Brazilian disc and rectangular specimens made from rock-like material containing multi-holes are investigated. These concrete specimens are specially prepared from an appropriate mixture of Portland Pozzolana Cement （PPC）, fine sands, and water. The pre-holed Brazilian discs and rectangular specimens are experimentally tested under compression. The breakage load in the ring type disc specimens containing an axial hole with varying diameters is measured and the distribution of the induced lateral stress is obtained. The mechanism of cracks propagation in the wall of the ring type specimens is also stud- ied. In the case of multi-hole Brazilian disc and rectangular specimens, the cracks propagation and cracks coalescence are also investigated. These experiments are numerically modeled by a modified higher order displacement discontinuity method. It has been shown that the corresponding experimental and numerical results are in good agreement with each other. The results presented in this research vali date the accuracy and applicability of these crack analyses procedures.

Design and Preparation of High Elastic Modulus Self-compacting Concrete for Pre-stressed Mass Concrete Structures

Requirements of self-compacting concrete （SCC） applied in pre-stressed mass concrete structures include high fluidity, high elastic modulus, low adiabatic temperature rise and low drying shrinkage, which cannot be satisfied by ordinary SCC. In this study, in order to solve the problem, a few principles of SCC design were proposed and the effects of binder amount, fly ash （FA） substitution, aggregate content and gradation on the workability, temperature rise, drying shrinkage and elastic modulus of SCC were investigated. The results and analysis indicate that the primary factor influencing the fluidity was paste content, and the main methods improving the elastic modulusof SCC were a lower sand ratio and an optimized coarse aggregate gradation. Lower adiabatic temperature rise and drying shrinkage were beneficial for decreasing the cement content. Further, based on the optimization of mixture, a C50 grade SCC （with binder amount of only 480 kg/ m3, fly ash substitution of 40%, sand ratio of 51% and proper coarse aggregate gradation （Vs.~0 mm： V10-16 ram： V16.20 mm= 30%： 30%：40%）） with superior workability was successfully prepared. The temperature rise and drying shrinkage of the prepared SCC were significantly reduced, and the elastic modulus reached 37.6 GPa at 28 d.

Effect of concrete creep on pre-camber of continuous rigid-frame bridge

The effect of concrete creep on the pre-camber of a long-span pre-stressed concrete continuous rigid-frame bridge constructed by cantilever casting method was investigated.The difference of creep coefficients calculated with two Chinese codes was discussed.Based on the calculations,the pre-camber of a pre-stressed concrete continuous rigid-frame box bridge was computed for construction control purpose.The results show that the short-term creep coefficient and long-term creep coefficient calculated with the CC-1985 are larger than those calculated with the CC-2004,while the medium-term creep coefficient calculated with the CC-1985 is smaller than that calculated with the CC-2004.The difference of creep deformation calculated with these two codes is small,and the influences of concrete creep on the pre-camber for most of the segments are negligible.The deflections and stresses of the box girder measured during the construction stages agree very well with the predictions.

Hydration evolution of pre-cast concrete with steam and water curing

The hydration characteristics of pre-cast concrete considering the effects of effective initial steam-curing and water-curing duration were measured and analyzed with XRD, TG, X-ray CT, SEM-BSE and MIP techniques. The results show that the effective initial steam-curing duration for pre-cast concrete with lower water-binder ratio was 10 14 h at 50 °C and the initial water-curing duration was 7 14 d. And the hydration evolution of cement, fly ash and slag in pre-cast concrete was obtained respectively by combining the hydrochlorides and EDTA selecting dissolution methods, based on which the contents of hydrated and anhydrate in concrete were calculated and the corresponding dynamic capillary porosity was also determined. Moreover, the comparison between calculated results and experimental ones indicates that the proposed evolution models of microscopic characteristics corresponding to hydration kinetics of cemented materials could be adopted to predict the developing trend of capillary porosity and hydration-products content in pre-cast concrete with fly ash and slag under certain curing conditions.

Self-adjustment of Carrying Capacity of Concrete Embedded with CFRC

By heating up the embedded carbon fiber reinforced cement based material （CFRC）, the carrying capacity and deformation of concrete member could be adjusted. The relationship between temperature difference and expansion strain of CFRC was demonstrated, and the temperature-deformation-load effect of concrete embedded with CFRC was studied. Heating the CFRC up to different temperatures resulted in different degree of inner pre-stress in concrete. Thus, the load capacity of concrete could be regulated owing to counteracting the pre-stress.

书夹型钢-混组合结构在桥梁加宽中的应用研究

为避免桥梁加宽时下部结构拓宽,减少新旧桥面纵向裂缝及铺装层损伤,提出一种以钢结构立柱及护栏、书夹式钢板、高强度螺栓为主要构件,超高性能混凝土(UHPC)为铺装层的书夹型钢-混组合结构进行桥梁加宽。为评估该组合结构在运营条件下的安全性,以厦门官浔溪大桥加宽工程为背景,通过实桥现场静载试验研究水平荷载作用下书夹型钢-混组合结构的力学性能,并与数值仿真结果进行对比,验证试验结果的可靠性。结果表明:实测结果与数值仿真结果吻合良好;书夹型钢-混组合结构关键部位荷载~位移曲线、荷载~应力曲线呈线性变化,钢结构最大拉应力约为材料屈服强度的75%,整体结构处于弹性工作阶段;螺栓预紧力有效减少了高强度螺栓的直剪作用,降低了剪切疲劳破坏风险;采用书夹型钢-混组合结构进行桥梁加宽可有效避免下部结构拓宽,改善新旧桥面纵向裂缝及铺装层损伤,具有较好的加宽效果。

桥梁施工中钢抱箍滑移原因与防滑措施研究

为研究桥梁施工中钢抱箍滑移原因并提出针对性的应对措施,对钢抱箍受力进行理论分析。针对钢-混界面进水及螺栓施拧不足造成的滑移,进行钢-混界面摩擦系数和螺栓拧紧力矩系数测试及定量分析,根据测试结果并结合现场经验提出钢抱箍防滑措施。结果表明:实测干燥状态下钢-混界面的静摩擦系数为0.62,大于常用设计值(0.3~0.4),钢抱箍偏安全;界面润湿后,静摩擦系数有所降低,但仍高于常用设计值,说明钢-混界面进水会降低钢抱箍承载能力,但降低程度还不足以导致钢抱箍滑移。螺栓拧紧力矩系数取值不当是钢抱箍滑移的主要原因,拧紧力矩系数常用设计值仅为测试值的1/1.7~1/3.6,偏不安全;拧紧力矩系数取值跟螺栓轴线与承压板法线间的夹角有关,建议现场对其进行实测,以保证螺栓预紧力与设计相符。钢抱箍设计时,摩擦系数可偏安全地取0.45,螺栓拧紧力矩系数可在参考范围内取大值,并结合1.5~2.0的安全系数确定螺栓数量,以避免钢抱箍滑移。

预弯钢-混组合跨座式单轨轨道梁力学性能分析及截面优化

[目的]传统跨座式单轨轨道梁大多采用宽为850 mm的截面形式,针对我国三四线城市的城市轨道交通建设要求,相关企业提出了宽为1250 mm的跨座式单轨列车截面形式。为解决新型跨座式单轨列车的特殊需求,有必要对其轨道梁的力学性能和截面形式进行研究。[方法]采用数值模拟方法,建立了预弯钢-混组合轨道梁模型;基于传统轨道梁截面形式,提出宽为1250 mm的预弯钢-混组合跨座式单轨轨道梁初步截面形式;分析不同荷载组合情况下的截面力学性能,并以用钢量最省为目标,对初步截面形式进行截面参数优化。[结果及结论]随着钢箱梁顶板厚度增加、钢箱梁上翼缘最大正应力呈减小的趋势;随着钢箱梁底板厚度的增加,钢箱梁下翼缘最大正应力值呈减小的趋势;随着腹板厚度的增加,钢箱梁腹板最大剪应力值呈减小的趋势。在最不利荷载工况组合下,所提轨道梁初步截面形式的刚度与强度均满足规范容许限值;优化后的顶板厚度、腹板厚度、底板厚度分别为16 mm、20 mm、16 mm,钢材表面积可节省约29.89%。

轨宽为1250 mm且跨度为25 m时跨座式单轨交通的预应力钢筋混凝土轨道梁截面跨高比

[目的]为适应新型跨座式单轨列车的构造特点,需对新型预应力钢筋混凝土轨道梁开展力学性能分析及界面优化研究。[方法]提出了适合该新型列车特点的预应力钢筋混凝土轨道梁,并采用经验证的数值分析模型,对8根具有不同跨高比的25 m跨轨道梁开展了静力分析。[结果及结论]提出的新型预应力钢筋混凝土轨道梁的力学性能满足规范要求;25 m跨度的新型轨道梁在跨高比为16.6~18.5时具有较好的力学性能。

预应力混凝土高层建筑稳定性研究

介绍了预应力混凝土结构的基本原理,以及预应力混凝土结构的施工工艺与质量控制措施,通过拟静力试验方法,评估了预应力混凝土在高层建筑墙体稳定性方面的应用效果。试验结果表明,预应力混凝土施工工艺显著提高了墙体的抗裂性和稳定性,减少了裂缝的产生,并在极端情况下仍能保持一定的承载能力。

大断面类矩形顶管注浆压力对管节损伤的影响分析

为探究大断面矩形顶管注浆压力波动及其控制边界难以确定的实际问题,依托南京某大断面类矩形顶管工程,借助理论分析、工程类比和数值模拟等方法,研究了顶板和侧壁局部憋压工况下,大断面类矩形管节结构的变形和损伤机制。通过分析矩形顶管管节受力特征,计算了大断面类矩形管节承受的外荷载,并采用ABAQUS有限元数值模拟软件内置的混凝土损伤塑性模型进行数值建模,研究不同注浆压力作用下钢筋混凝土管节损伤演化机制。结果表明:(1)注浆荷载作为可变荷载,对管节受力有明显影响,应将注浆压力纳入管节设计环节;(2)针对该工程,其理论注浆压力为0.311 MPa,考虑抑制背土效应的需要,应适当提高注浆压力,但必须控制在合理范围内;(3)从损伤演化的角度来看,注浆荷载达到0.6 MPa时,管节顶板的拉伸损伤已达到较高水平,因此,将0.6 MPa作为该工程的注浆压力预警指标,而0.9 MPa应作为注浆压力的控制指标。

预应力混凝土实心方桩在某泵站工程中的应用

在水工建筑物非岩石基础处理的诸多措施中,预应力混凝土实心方桩在工期、投资、质量控制、承载能力和沉降量控制等方面都有一定的优势。本文依托某泵站基础处理工程,系统比较分析了预应力混凝土实心方桩和混凝土灌注桩两种处理方案,并优选适合于本工程的实心方桩处理方案,有效解决了基础承载力和沉降量问题,可供其它类似工程的基础处理参考。

考虑破损泡沫前补偿设计的泡沫混凝土制备方法

为了研究不同因素对泡沫混凝土中泡沫破损的影响,优化泡沫混凝土配合比设计方法,进而提高最终产品质量,对搅拌速率、水灰比和设计密度对消泡率的影响进行了研究,提出了一种考虑破损泡沫前补偿,即对破损泡沫量提前补偿以及对多余水量提前去除的泡沫混凝土配合比设计方法。依据前/后补偿法分别制备了400~1000 kg/m^(3)的泡沫混凝土,并对比产品的物理性能和力学性能。结果表明:与后补偿法相比,利用前补偿法制备的泡沫混凝土在满足密度等级一定的情况下,抗压强度最高提高了39.6%。与实际工程中常用的需要数次“补泡-称量”的后补偿法相比,前补偿法工艺简便,降低了长距离作业中堵管风险,提高了泡沫混凝土以及最终的工程质量。

新疆阿拉尔地区C30负温混凝土受冻临界强度试验研究

研究了不同配合比下C30混凝土在-5~-20℃受冻温度下的受冻临界强度,同时采用快冻法对达到受冻临界强度的混凝土在-15℃温度下进行冻融循环试验。试验结果表明:未掺防冻剂混凝土在受冻时,其预养护时间和受冻临界强度具有规律性的变化。受冻温度愈低,混凝土预养护时间愈长,受冻临界强度愈大。但在相同受冻温度下,掺防冻剂负温混凝土的受冻临界强度呈不同程度的降低,且无规律性。掺与未掺防冻剂负温混凝土在达到受冻临界强度后,后期抗压强度均有所保障,但抗冻性较标准养护条件下的混凝土劣化严重。

某核电厂预应力安全壳钢衬里应力应变分析

根据《压水堆核电厂预应力混凝土安全壳设计规范》要求,考虑了混凝土中钢筋预拉伸作用,计算了核电厂反应堆厂房安全壳钢衬里在各工况(正常运行工况、设计基准事故工况、严重事故工况)的载荷组合作用下的应力应变,按照规范进行应变评定,以确保钢衬里结构设计满足规范要求。

隧道预切槽法合理支护时机及管片接头结构研究

预切槽法是隧道暗挖施工的一种超前支护工法,此工法可大幅度降低地表沉降量,对隧道围岩土体及地表建(构)筑物影响较小。采用室内试验和数值模拟的方法,针对浇筑预切槽管片所用混凝土的前期强度、合理支护时机以及预切槽管片接头处合理的接茬方式进行了研究。研究结果表明,按C30配比的混凝土养护1.5~2 h时即具有6 MPa以上的抗压强度,满足拆模要求;粘接介质对管片环向位移的约束效果明显,环氧树脂可作为优良的管片粘接剂;榫槽式接头榫头角度α的理想范围为55°~70°,综合考虑预切槽工艺的可操作性,榫槽式接头深度B的理想范围为6~8 cm。

高速公路桥梁建设中预应力混凝土连续T梁预制技术研究

为提升高速公路桥梁建设质量,以某地区的高速公路桥梁工程为例,研究高速公路桥梁建设中预应力混凝土连续T梁预制技术。结合工程需求选择施工的混凝土和预应力钢筋材料后,确定预应力混凝土连续T梁预制装配施工流程,并严格按照流程步骤完成预应力混凝土连续T梁施工后,采用加载装置测试其施工效果。试验结果表明,该预制装配施工技术的施工效果较好,能够满足桥梁工程的施工需求。

混凝土支柱支护技术在预掘回撤通道支护中的应用

为了评价混凝土支柱在预掘回撤通道支护中的应用效果,采用相似材料模拟试验的方法对混凝土支柱的力学性能进行了测试,并以此为基础,在何家塔煤矿50103工作面预掘回撤通道进行了工业性试验,同时对工作面回采过程中的矿压规律进行了监测。试验中主回撤通道共安装了2排直径800 mm的混凝土支柱,配合锚网梯梁组合支护系统对顶板进行支护。试验结果表明,混凝土支柱能够有效的保证工作面末采期间回撤通道的安全,显著提高搬家效率,并大幅降低搬家过程中的支护成本。