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.

The Testing Strength Curves of Lightweight Aggregate Concrete by Rebound Method and Ultrasonic-rebound Combined Method

The strength curves of lightweight aggregate concrete （LWAC） were tested based on detecting LWAC with density of 1 400-1 900 kg/m3 and LWAC with strength grade of LC15-LC50 by rebound method and ultrasonic-rebound combined method.The results show that the common measured strength curves tested by above two methods can not satisfy the required accuracy of LWAC strength test.In addition,specified compressive strength curves of testing LWAC by rebound method and ultrasonic-rebound combined method are obtained,respectively.

Developing Sustainable Ultra High Strength Concrete Mixtures Using Spent Foundry Sand

This study presents the development of ultra high strength concrete(UHSC)that has been made more sustainable by using both local materials from central Texas and spent foundry sand(FS)from the metal casting industry,which has also been obtained locally.This study first describes various trial mixtures tested as well as the specimen preparation techniques investigated that led to the final UHSC-FS mixtures.The developed mixtures were proportioned with local constituents to increase the sustainable impact of the material by reducing emissions due to shipping as well as making UHSC more affordable to a wider variety of applications.The final mixture design constituents were:river sand,locally available type I/II cement,silica fume,and spent FS,which was obtained from a local steel casting company.Multiple variables were investigated,such as the aggregate type and size,concrete age(7,14,and 28-days),the curing regimen,and the water-to-cement ratio(w/cm)to optimize a UHSC mixture that used local materials and FS.This systematic development revealed that heat curing the specimens in a water bath at 50 oC(122 oF)after demolding and then dry curing at 200 oC(392 oF)two days before testing with a w/cm of 0.20 at 28-days produced the highest compressive strengths.Once an optimum UHSC mixture was identified a partial replacement of the fine aggregate with FS was completed at 10%,20%,and 30%.The results showed an increase of compressive strength performance at 10%replacement,followed by no change at 20%,and finally a slight decrease at 30%.Developing this innovative material with local materials and FS ultimately produces a novel sustainable construction material,reduces the costs,and produces mechanical performance similar to prepackaged,commercially,available construction building materials.

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.

Developing Sustainable High Strength Concrete Mixtures Using Local Materials and Recycled Concrete

This study presents the development of high strength concrete (HSC) that has been made more sustainable by using both local materials from central Texas and recycled concrete aggregate (RCA), which has also been obtained locally. The developed mixtures were proportioned with local constituents to increase the sustainable impact of the material by reducing emissions due to shipping as well as to make HSC more affordable to a wider variety of applications. The specific constituents were: limestone, dolomite, manufactured sand (limestone), locally available Type I/II cement, silica fume, and recycled concrete aggregate, which was obtained from a local recycler which obtains their product from local demolition. Multiple variables were investigated, such as the aggregate type and size, concrete age (7, 14, and 28-days), the curing regimen, and the water-to-cement ratio (w/c) to optimize a HSC mixture that used local materials. This systematic development revealed that heat curing the specimens in a water bath at 50℃ (122oF) after demolding and then dry curing at 200℃ (392oF) two days before testing with a w/c of 0.28 at 28-days produced the highest compressive strengths. Once an optimum HSC mixture was identified a partial replacement of the coarse aggregate with RCA was completed at 10%, 20%, and 30%. The results showed a loss in compressive strength with an increase in RCA replacement percentages, with the highest strength being approximately 93.0 MPa (13,484 psi) at 28-days for the 10% RCA replacement. The lowest strength obtained from an RCA-HSC mixture was approximately 72.9 (MPa) (10,576 psi) at 7-days. The compressive strengths obtained from the HSC mixtures containing RCA developed in this study are comparable to HSC strengths presented in the literature. Developing this innovative material with local materials and RCA ultimately produces a novel sustainable construction material, reduces the costs, and produces mechanical performance similar to prepackaged, commercially, available construction building materials.

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.

A Research on the Usage of Corn Cob in Producing Lightweight Concrete

In this study, the possibility of using corn cobs as an organic aggregate in producing lightweight concrete has been investigated. First, some important physical properties of corn cob have been determined in the laboratory. These properties are as follows: weight to volume ratio (unit weight), water absorption rate and granulometric analysis. Later on, 4 concrete mixtures have been prepared according to the workability of concrete and standards specified in Turkey. After that, unit weight, heat transmissibility coefficient and 28-day pressure strength of these 4 concrete samples have been determined using machines measuring these properties. The 28-day pressure endurance value has been found between 1.4 - 56.25 kgf/cm2, heat transmissibility coefficient has been found between 0.19 - 0.35 Kcal/m∙h∙˚C and unit weight of samples have been found between 800 - 1520 kg/m3. Lastly, these properties of concrete samples have been compared with other lightweight materials being used in the construction of buildings.

不同纤维对高强轻骨料混凝土力学性能影响及微观机理研究

为改善高强轻骨料混凝土的力学性能和弯曲韧性,研究了玄武岩纤维、细聚乙烯醇纤维、仿钢纤维、聚丙烯纤维和粗聚乙烯醇纤维的掺入对高强轻骨料混凝土抗压强度、劈裂抗拉强度和抗弯强度的影响规律。通过4点弯曲试验获取荷载-挠度曲线,基于韧性指标对比分析了5种纤维对高强轻骨料混凝土弯曲韧性的影响。通过扫描电子显微镜观察纤维破坏前后的表面微观形貌,阐释了不同纤维的增强增韧机理。研究结果表明:掺入纤维对高强轻骨料混凝土的力学性能均有提升,主要提升高强轻骨料混凝土的抗拉性能和弯曲韧性。粗聚乙烯醇纤维提升劈裂抗拉强度和抗弯强度程度最大,分别提高了83%和220%,使高强轻骨料混凝土破坏由脆性转为高延性。纤维对高强轻骨料混凝土抗拉性能和韧性的提升与纤维表面损伤程度成正比。

粗集料对超高强混凝土徐变的影响

徐变是混凝土固有的时变特性,对混凝土结构和构件的受力及长期服役性能有着重大影响。以粗集料为切入点,研究粗集料的掺量、粗集料最大粒径以及粗集料与基体性质协调性等因素对超高强混凝土的力学性能和徐变性能的影响。结果表明:掺粗集料的超高强混凝土徐变明显小于不掺粗集料的混凝土,掺粗集料可有效降低超高强混凝土的徐变,但减小粗集料的最大粒径对徐变无明显抑制效果。此外,超高强混凝土并非强度越高徐变越小,其徐变的大小主要受超高强混凝土强度与弹性模量的协同发展程度的影响。通过修正FIB MC2010模型,建立了适用于超高强混凝土的徐变预测模型。

基于声发射技术的高强全珊瑚钢筋混凝土板破坏特性研究

为了探究全珊瑚钢筋混凝土板破坏特性,设计了3种不同配筋率(0.85%、1.13%和1.41%)的高强全珊瑚钢筋混凝土板构件.结合声发射技术,开展了全珊瑚混凝土板构件四点弯曲加载试验,揭示了配筋率对高强全珊瑚钢筋混凝土板力学性能的影响.基于声发射参数速率过程理论,建立了损伤定量评估模型,并进一步提出了可用于工程实践的损伤评估准则.结果表明:随配筋率增大,高强全珊瑚混凝土板构件起裂荷载、起裂时间与峰值荷载均增大,但中心挠度降低了11.29%;单位时间声发射参数变化均存在2个峰值区,可作为板构件的起裂评判和破坏预兆;根据声发射参数累计值变化可将高强全珊瑚混凝土板构件破坏过程划分为轻度(<0.35)、中度(0.35~0.66)、高度(0.66~0.84)和重度(>0.84)损伤4个程度.所建立模型适用于不同配筋率的全珊瑚混凝土板构件损伤评估.

双掺陶粒轻质高强混凝土的力学及耐久性能试验研究

通过在混凝土中掺入黏土陶粒和页岩陶粒,使其具备较高强度的同时具有较低的干表观密度和较好的耐久性能,旨在探索制备28 d干表观密度不大于1950 kg/m3、28 d强度不小于50 MPa的轻质高强混凝土(Lightweight High-Strength Concrete,缩写:LWHSC)的方法。同时研究了黏土陶粒和页岩陶粒双掺对轻质高强混凝土抗压强度、干表观密度和耐久性的影响。结果表明,所制备的LWHSC的抗压强度试件破坏形态无异于普通混凝土,但使用黏土陶粒部分替代页岩陶粒可有效降低LWHSC干表观密度;适当增加胶凝材料用量并辅以优化陶粒的双掺比例,对除抗冻性以外的其它耐久性能具有一定的改善作用。研究成果可为认识、优化和开发页岩陶粒和黏土陶粒双掺LWHSC及其在工程中的应用提供参考。

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。

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。

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。

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。

基于GA-BP神经网络的粗骨料UHPC的抗压强度预测

为实现对粗骨料UHPC的抗压强度的预测和配合比设计方法的优化,搜集了国内外文献中168组粗骨料UHPC配合比和标准养护28 d抗压强度实测值,给出了各材料组分和抗压强度频数分布,并基于灰色关联分析法分析了各材料组分与抗压强度的关联关系,通过神经网络参数分析,建立了基于遗传算法的前馈神经网络,相比普通的BP神经网络具有更好的预测精度和泛化能力。最后基于建立的GA-BP神经网络给出了不同强度等级粗骨料UHPC配合比设计中粗骨料/胶凝材料、钢纤维体积掺量、砂胶比的建议取值范围。

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。

混杂纤维高强轻骨料混凝土单轴受压试验研究

为了改善高强轻骨料混凝土(HSLC)的脆性破坏特征,采用钢纤维(SF)和聚丙烯纤维(PF)对HSLC进行增强,通过单轴受压试验研究两种纤维对HSLC单轴受压性能的影响。结果表明:SF和PF的体积掺量分别为2.0%和0.2%时具有最优的混杂效应,应力-应变曲线拐点处应力和收敛点处应变较单掺SF时分别提高了10.0%和56.5%,较单掺PF时分别提高了99.1%和150%;混杂纤维使HSLC的峰值应力、峰值应变和弹性模量最高分别提升了19.0%、1.20%和16.9%。最后,基于试验数据,提出了考虑两种纤维混杂效应的峰值应力、峰值应变和弹性模量的计算公式,建立了双参数混杂纤维增强HSLC的单轴受压本构模型,并且其与试验曲线吻合较好。该研究结果可为纤维增强HSLC的结构设计提供理论参考。