Influence Mechanism of Curing Regimes on Interfacial Transition Zone of Lightweight Ultra-High Performance Concrete

This study aims to clarify the effects of curing regimes and lightweight aggregate(LWA)on the morphology, width and mechanical properties of the interfacial transition zone(ITZ) of ultra-high performance concrete(UHPC), and provide reference for the selection of lightweight ultra-high performance concrete(L-UHPC) curing regimes and the pre-wetting degree LWA. The results show that, under the three curing regimes(standard curing, steam curing and autoclaved curing), LWA is tightly bound to the matrix without obvious boundaries. ITZ width increases with the water absorption of LWA and decreases with increasing curing temperature. The ITZ microhardness is the highest when water absorption is 3%, and the microhardness value is more stable with the distance from LWA. Steam and autoclaved curing increase ITZ microhardness compared to standard curing. As LWA pre-wetting and curing temperatures increase, the degree of hydration at the ITZ increases, generating high-density CSH(HD CSH) and ultra-high-density CSH(UHD CSH), and reducing unhydrated particles in ITZ. ITZ micro-mechanical properties are optimized due to hydration products being denser.

Autogenous Shrinkage of High Strength Lightweight Aggregate Concrete

The characteristic of autogenous shrinkage（AS） and its effect on high strength lightweight aggregate concrete（HSLAC） were studied.The experimental results show that the main shrinkage of high strength concrete is AS and the amount of cement can affect the AS of HSLAC remarkably,At the early stage the AS of HSLAC is lower than that of high strength normal concrete,but it has a large growth at the later stage.The AS of high strength normal concrete becomes stable at 90d age,but HSLAC still has a high AS growth .It is found that adjusting the volume rate of lightweight aggregate,mixing with a proper dosage of fly ash and raising the water saturation degree of lightweight aggregate can markedly reduce the AS rate of HSLAC.

Brittleness Generation Mechanism and Failure Model of High Strength Lightweight Aggregate Concrete

The brittleness generation mechanism of high strength lightweight aggregate con-crete(HSLWAC) was presented, and it was indicated that lightweight aggregate was the vulnerable spot, initiating brittleness. Based on the analysis of the brittleness failure by the load-deflection curve, the brittleness presented by HSLWAC was more prominent compared with ordinary lightweight aggregate concrete of the same strength grade. The model of brittleness failure was also established.

Different Curing Systems on Mechanical Properties of Ultra-High Performance Concrete with Coarse Aggregate

High durability and high tensile strength makes ultra-high performance concrete( UHPC) an ideal material for bridges,while its early shrinkage in the construction of cast-in-situ mass concrete leads structure crack-easily,which restricts the application of UHPC in deck system. Whether reasonable amount of coarse aggregate can influence the strength of UHPC and improve the shrinkage performance or reduce the cost is still in doubt. Besides,in order to improve its constructability and workability, whether autoclaved curing system of UHPC can be changed remains to be further researched. In response to these circumstances, a systematic experimental study on the strength of UHPC mixed with coarse aggregate in different ratios has been presented in this paper. The three curing systems,namely standard curing,180-200 ℃/1. 1 MPa autoclaved curing,and hot water curing were tested to reveal the relationship between UHPC's properties and curing systems,and the UHPC ' s microstructure was also preliminarily studied by scanning electron microscope( SEM). The experimental research can draw the following conclusions. Under the condition of the same mix ratio, autoclaved curing guarantees the highest compressive strength,followed by hot water curing and standard curing. The compressive strength of concrete increases with the temperature in the range of 25 to 90 ℃ hot water curing,and high temperature in precuring period can speed up the strength development of UHPC,but the sequence of precuring period does not obviously affect the results. In 90 ℃ hot water and autoclaved curing,the strength is over 150 MPa,and it has little relation with gravel ratio. While the value increases first and then decreases in a lower temperature curing with the increasing of gravel amount,even only about 80 MPa at room temperature. The strength increases moderately along with the increase of the curing age by standard curing,especially in the initial stage.

Bond Behavior between BFRP Bars and Hybrid Fiber Recycled Aggregate Concrete after High Temperature

The aim of this study is to improve the bond performance of basalt fiber reinforced polymer(BFRP)bars and recycled aggregate concrete(RAC)after being exposed to high temperatures.The bond behavior(failure modes,bond strength,bond stress-slip curves)between BFRP bars and hybrid fiber recycled aggregate concrete(HFRAC)after being exposed to temperatures ranging from 20℃up to 500℃was studied by using pull-out tests.The effect of high temperatures on mechanical properties of concrete(compressive strength,splitting tensile strength)and tensile strength of BFRP bars was also investigated.The bond strength decreased as the temperature increased and the drop of bond strength between RAC and BFRP bar was larger than that between HFRAC and BFRP bar.As the temperature rises,the key factor affecting the bond strength was gradually transformed from concrete strength to BFRP bar strength.The relationship between bond stress and slip in the dimensionless bond stress-slip ascending section was established,which was in good agreement with the experimental results.

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.

掺轻质砂超高性能混凝土的轴拉力学性能

为研究轻质砂对不同试件尺寸下超高性能混凝土(ultra high performance concrete,UHPC)拉伸应变强化性能的影响,采用轻质砂对原黄砂进行等体积替代,完成9组不同轻质砂体积率(0~35%)和不同试件厚度(30~100 mm)的单轴拉伸试验,并同步进行声发射实时探伤测试。结果表明:轻质砂体积率对UHPC弹性极限点对应应力和对应应变的影响较小,但轻质砂体积率由0增加到35%时,UHPC的极限抗拉强度和极限拉应变分别由10.6 MPa和2.35×10^(-3)提高到了19.4 MPa和4.3×10^(-3);轻质砂体积率大于15%时,UHPC的应变强化程度得到显著提升,试件内部产生的损伤点数更多且分布更均匀,展现出良好的裂缝控制能力;在相同轻质砂体积率下,UHPC的应变强化程度随试件厚度的增加而降低,且试件内部的损伤点趋于集中,表现出明显的尺寸效应。

粗骨料超高性能混凝土应用研究

为满足桥梁工程中桥面板材料轻质高强的市场需求,在活性粉末混凝土中掺入粒径不大于8 mm的粗骨料,制成含粗骨料超高性能混凝土。这种混凝土具有力学性能佳、材料自重轻、早期收缩小和造价较低等特点。首先阐述含粗骨料超高性能混凝土的配制原理及各掺合料对其性能的影响,其次分析其力学性能、疲劳性能和高温性能,最后展示了其在南京江心洲长江大桥的工程应用状态,并提出了其尚待深入研究的问题和发展方向。

废弃黏土砖骨料轻质混凝土正交试验研究

将废弃黏土砖骨料应用于轻质混凝土中,可实现建筑垃圾再利用,同步节省建筑材料,改善轻质混凝土性能。以废弃黏土砖颗粒为骨料,开展轻质混凝土制备试验,研究不同水灰比、泡沫掺量、纤维掺量对废弃黏土砖骨料轻质混凝土的物理力学性能影响。结果表明:水灰比对吸水率影响显著,随着水灰比的增大,吸水率增加了27.03%,泡沫掺量对干密度、抗压强度和劈裂抗拉强度影响显著,随泡沫掺量的增加,干密度、抗压强度和劈裂抗拉强度减小了17.78%、52.52%和34.83%,并建立了抗压强度与干密度、劈裂抗拉强度之间的对应关系式。

废黏土砖骨料轻质混凝土物理力学性能试验研究

将废弃黏土砖破碎成骨料用于制备轻质混凝土,可同步达到资源再利用、降低环境污染、节省天然骨料的目的。为揭示废弃黏土砖骨料轻质混凝土的性能,通过室内试验对废砖骨料级配、废砖骨料体积率、水灰比、泡沫掺量等因素影响轻质混凝土的干密度、吸水率、抗压强度性能进行试验研究,结果表明:泡沫掺量对干密度和抗压强度影响显著,泡沫掺量由2%增加到5%时,干密度由1407kg/m^(3)减小到1128kg/m^(3),抗压强度由13.39MPa减小到4.66MPa,废砖骨料级配对吸水率影响显著,骨料级配从A组到C组级配时,吸水率由11.3%减小到5.7%。

轻骨料混凝土的研究进展与应用前景

介绍了轻骨料混凝土的特性,分析了轻骨料配合比设计方法,综述了国内外近年来不同轻骨料混凝土受各因素影响的性能研究进展,探究了轻骨料混凝土的微观表象,指出了轻骨料混凝土存在的如工作性较差、可泵送性易受阻等缺陷不足,给出了合理建议,对轻骨料混凝土的未来发展方向和应用前景做出了展望。

碳纤维轻骨料混凝土高温后动态压缩性能研究

目前针对碳纤维轻骨料混凝土在高温作用后的力学性能研究匮乏,考虑到火灾伴随的冲击和爆炸荷载对结构的破坏,采用直径74 mm的分离式霍普金森压杆(SHPB)装置,对轻骨料混凝土(LWAC)进行了动态压缩试验,研究了温度和碳纤维含量对LWAC动态压缩性能的影响,并通过SEM扫描电镜揭示了高温对LWAC微观结构的影响。结果表明:碳纤维掺量的增加提高了LWAC的静态抗压强度,而高温对其静态抗压强度具有劣化作用,纤维的掺入能够在一定程度上阻止高温强度劣化。LWAC的应力-应变曲线随着温度的升高趋于扁平,动态抗压强度降低而峰值应变和极限应变增大,动态抗压强度、峰值应变和极限应变随着碳纤维掺量的增加均出现先增大后减小的趋势。不同温度作用和碳纤维掺量下LWAC试样的DIF随应变率的增加呈线性增加,普通LWAC试件比碳纤维LWAC试样的速率敏感性更高。高温最初改善了LWAC的内部微观结构,但随着温度持续升高,试样内部微观结构严重劣化。

U型组合再生混凝土-高强混凝土梁抗火性能

为突破单一混凝土材料在受力和抗火两方面的性能难以协同优化的难题,该文创新性地提出U型组合再生混凝土-高强混凝土梁(简称为RAC-HSC组合梁),系统地开展了火灾试验和常温、火后受弯试验,综合分析了不同工况下组合梁的性能表现。研究表明:火灾下,RAC-HSC组合梁的耐火性能显著优于高强混凝土整浇梁,其梁内温度较低,跨中挠度较小,跨中未出现明显裂缝,且未观察到爆裂现象;常温及高温后的受弯试验中,RAC-HSC组合梁的屈服荷载、极限承载力及初始刚度均显著优于再生混凝土梁,基本达到或超过普通混凝土整浇梁和高强混凝土整浇梁的水平。该文所提出的U型组合再生混凝土-高强混凝土梁是混凝土构件“抗火-受力一体化设计”的有益尝试。

桥梁动力效应对粗骨料UHPC的影响

为解决粗骨料超高性能混凝土(UHPC)在桥梁工程可能遭受的桥梁动力效应问题,通过小型振动台进行模拟桥梁振动,以振动幅度和频率为振动参数,研究了桥梁振动对粗骨料UHPC的力学性能、超声波波速及粗骨料沉降系数的影响规律.试验结果表明:1 mm的振动幅度可以有效改善孔结构,对粗骨料UHPC的力学性能和超声波波速具有促进作用;当振动幅度大于1 mm时振动会引入内部缺陷,并且粗骨料离析程度增大,刚性骨架被破坏,抗压强度、弹性模量和超声波波速降低;当振动频率增大时,能够有效改善钢纤维与基体的黏结性能,增强粗骨料UHPC的弯曲韧性;在实际粗骨料UHPC用于桥梁施工过程中应注意控制桥梁振动幅度,不应超过3 mm。

针片状颗粒对高强混凝土性能的影响研究

从粗骨料针片状颗粒角度出发,研究其含量和粒径对高强混凝土性能的影响。选取掺加5%、15%和25%针片状颗粒含量的粗骨料配制混凝土,对混凝土的工作性能、力学性能和耐久性能进行试验研究;选取5~10 mm、10~16 mm和16~20 mm共三种粒径区间的针片状颗粒,探究粒径大小对混凝土性能的影响规律。结果表明:针片状颗粒含量增加,高强混凝土和易性和泵送性能变差,混凝土抗压强度降低,混凝土收缩增大,且对高强混凝土耐久性能不利;混凝土拌合物坍落度和扩展度、硬化混凝土各龄期抗压强度均随着粗骨料中针片状颗粒粒径增大而降低;综合考虑针片状颗粒对高强混凝土性能影响规律,建议控制针片状颗粒含量不超过5%;同时,在计算针片状颗粒含量时,需在标准计算方法基础上按不同粒径针片状颗粒分别乘以其对应的影响系数进行修正,以实现对粗骨料粒形更好的控制。

含粗骨料超高性能混凝土的单轴受拉力学性能

考虑粗骨料掺量、钢纤维掺量及长径比等因素,研究了含粗骨料超高性能混凝土(UHPC-CA)的单轴受拉力学性能,揭示了纤维增强的机理和粗骨料的作用机制.结果表明:随着粗骨料掺量的增加,UHPC-CA的单轴抗拉强度与变形性能均下降;钢纤维掺量增加、长径比增大对UHPC-CA的受拉力学性能有较大的提升作用;钢纤维与基体间具有良好的握裹力,在基体内出现裂缝后,主要依靠握裹作用承受拉应力;粗骨料的掺入削弱了钢纤维空间分布的均匀性,同时引入了薄弱的粗骨料-基体界面过渡区,对UHPC-CA的受拉性能产生了不利的影响.

粗骨料取代率对机制砂再生混凝土高温性能的影响

为探究不同粗骨料取代率对机制砂再生混凝土(RCM)高温性能的影响,以不同粗骨料取代率(0%、20%、40%、60%、80%、100%,质量分数)制备RCM并研究其单面受高温后的表面形貌、质量损失率、抗压强度、荷载变形曲线及微观形貌。结果表明,粗骨料取代率在40%~60%时,在低于600℃的高温试验下RCM表现良好。当温度在200℃时,各组试块表面形貌变化不大,质量损失率较小,抗压强度略微上升;当温度在400℃时,各组试块表面开始出现裂缝,质量损失率为4.35%~6.47%,抗压强度开始下降,此时粗骨料取代率影响不大;当温度达到600℃时,表面形貌变化明显,质量损失率上升到6.42%~8.70%,抗压强度最终降低到基准强度的80%左右,此时粗骨料取代率在40%~60%的试块表现良好;当温度达到800℃时,各组试块表面形貌变化进一步加剧,粗骨料取代率大于60%的试块受影响非常明显,同时各类性能参数表明混凝土已完全丧失工作性能。

预置轻骨料混凝土中灌浆料与轻骨料的匹配机制研究

本文制备了不同强度等级(60、75、90、100、110、120 MPa)的轻质高强灌浆料,通过与不同类型、强度的轻骨料(高强球形粉煤灰陶粒、低强球形粉煤灰陶粒、碎石形页岩陶粒)复合,研究了预置轻骨料混凝土中灌浆料与轻骨料之间的匹配机制。结果表明,预置轻骨料混凝土强度与灌浆料强度存在分段线性关系,拟合曲线斜率可近似反映对灌浆料强度的利用效率。随着灌浆料强度等级提高,所配制混凝土对灌浆料强度的利用率下降。骨料的筒压强度与粒形也会影响灌浆料强度的利用率。由于嵌锁效应,碎石形页岩陶粒制备的轻骨料混凝土强度显著高于相近筒压强度球形粉煤灰骨料制备的混凝土。采用高强球形粉煤灰陶粒与碎石形页岩陶粒可分别制备出抗压强度为81.2 MPa、干表观密度为1 735 kg/m^(3)与抗压强度为68.2 MPa、干表观密度为1 520 kg/m^(3)的预置轻骨料混凝土。

骨料种类对超高性能混凝土性能影响机理研究

骨料优选可以弥补超高性能混凝土(UHPC)成本高、收缩大的缺陷。本文先以不同体积比河砂和陶砂混掺制作UHPC,并进行测试,确定陶砂替代河砂体积率为12.5%~25.0%时UHPC性能优异。然后利用石英砂、河砂和陶砂单掺及陶砂替代河砂体积率12.5%混掺制备UHPC,采用宏细观测试手段分析骨料对UHPC性能影响机理。结果表明,轻骨料内养护作用不仅能降低UHPC收缩,还可以显著改善界面区孔隙结构,提升混凝土劈裂抗拉强度和抗压强度。河砂骨料相比石英砂骨料主要区别在表面结构特征更粗糙和不规则,由其制备的UHPC工作性能稍差,但界面区孔隙结构更优。采用熵权法以坍落度、抗压强度、劈裂抗拉强度和56 d收缩为综合评价指标,虽然河砂骨料UHPC抗压强度高于石英砂骨料UHPC,但在其他方面存在劣势。陶砂替代河砂体积率在12.5%~50.0%时,混掺骨料UHPC综合性能高于石英砂骨料UHPC。

粗骨料UHPC单轴拉伸性能及损伤本构模型

通过单轴拉伸试验,研究粗骨料用量和钢纤维的掺量、类型及混杂配比对粗骨料超高性能混凝土(CA-UHPC)受拉性能的影响,揭示了CA-UHPC破坏机理以及粗骨料与钢纤维的相互作用机制.结果表明:粗骨料引起的薄弱界面和空间阻隔作用削弱了钢纤维的增韧作用,CA-UHPC拉伸性能随粗骨料用量增大而降低,建议其用量不超过450 kg/m^(3);增大钢纤维掺量能改善CA-UHPC拉伸性能,且平直钢纤维的增韧作用更稳定;1.0%平直和1.0%端勾钢纤维混杂时,其与粗骨料匹配良好,钢纤维有效利用率达到25.4%,能够充分发挥混杂纤维的协同增韧作用.最后建立了考虑粗骨料和钢纤维影响的CA-UHPC受拉损伤本构模型.