Pumpability of Manufactured Sand Self-compacting Concrete

By the addition of superplasticizer and air entraining agent,manufactured sand selfcompacting concrete(MS SCC)with slump flow varying from 500 to 700 mm and air content varying from 2.0%to 9.0%were prepared and the pumpability of MS SCC was studied by a sliding pipe rheometer(Sliper).According to the Kaplan’s model,the initial pump pressure and the pump resistance of MS SCC were obtained.Meanwhile,rheological properties including the yield stress and the plastic viscosity of MS SCC were measured by a rheometer.The experimental results show that the increase of slump flow contributes to a higher pumpability and a proper air content,i e,6%is beneficial for the pumpability of MS SCC.Due to the existence of stone powder and stronger angularity of MS,the initial pump pressure of MS SCC is only about 60%-88%that of river sand(RS)SCC with the same slump flow and air content,indicating that MS SCC possesses a higher pumpability than RS SCC.

Effect of Methylene Blue (MB)-value of Manufactured Sand on the Durability of Concretes

The relation between the methylene blue （MB） value of MS and its limestone powder content and clay content was investigated. The effects of MB values ranging from 0.35 to 2.5 on the workability of fresh concrete, the mechanical properties, the resistance to freezing as well as the resistance to chlorine ion permeation of the hardened concrete were all investigated. The experimental results showed that the MB value had no correlation with the limestone powder content of MS, while it was directly related to the clay content. With an increase of MB value, concrete workability decreased, as did the flexural and 7-day compressive strengths, however, the 28-day compressive strength was not affected. Furthermore, influence of MB value on concretes of different strength levels was different. For low-strength concretes, an increase of MB value could improve its impermeability, but this was not the case for high-strength concretes. Instead, their resistance to chloride ion permeability decreased slightly. However, even a slight increase in MB value remarkably accelerated freeze-thaw damage of MS concrete. It was thus concluded that the critical MB value of 1.4 would not cause significant deterioration in the performance of MS concretes.

Influence of MB-value of Manufactured Sand on the Shrinkage and Cracking of High Strength Concrete

The relation between methylene blue （MB） value of MS and its limestone dust content and clay content was investigated. The effects of MB value ranging from 0.35 to 2.5 on the workability of fresh concrete and crack propagation characteristics at the age of 24 hours, and effects on the mechanical properties, dry shrinkage of the harden concrete were tested. The experimental results show that the MB value is not related with the limestone dust content of MS, but in direct proportion to clay content. With the increase of MB value, the concrete workability decreases, and the flexural strength and 7 d compressive strength reduce markedly, whearas the 28 d compressive strength is not affected. When the MB-value is less than or equal to 1.35, the change of the MB-value has a little influence on early plastic cracking and dry shrinkage property of concrete, but when the MB-value is more than 1.35, the tendency of plastic cracking and dry shrinkage is remarkable.

Relationships between Modified Methylene Blue Value of Microfines in Manufactured Sand and Concrete Properties

To better understand and assess the effect of microfines on concrete properties, the synergetic effect of methylene blue value and content of microfines on properties of low and high strength concrete was studied and then the relationships between the index of modified methylene blue value （MMBV） and concrete properties were investigated. The results show that relationships between MMBV and fresh and hardened properties of concrete can be fully established, and the correlation between MMBV and C60 concrete property is higher than the correlation between MMBV and C30 concrete. With the increase of MMBV, concrete workability and frost resistance decrease while drying shrinkage decreases; however, compressive strength and chloride-ion penetration resistance of C30 concrete have not been negatively affected whereas those of C60 concrete are significantly deteriorated when MMBV exceeds 100. To make use of microfines without remarkably damaging concrete quality, it is suggested that MMBV of microfines in MS used in C30 and C60 concrete be no more than 100.

Effects of the Lithologic Character of Manufactured Sand on Properties of Concrete

Six representative parent rocks of sand, including limestone, quartzite, gneisses, granite, Basalt and Marble were selected to conduct a systematical research on the effects of various lithologies of manufactured sand on the workability, mechanism properties, volume stability and durability of manufacturedsand concrete. The experimental results show that the strength of manufactured-sand concrete is slightly higher than that of natural-sand concrete. Furthermore, substituting 15% cement of the concrete mixture with equal quantity of the six different lithology stone powder respectively, the data indicated that they can improve the concrete’s workability, postpone the plastic cracking time, enhance the anti-cracking grade, and have no obvious effect on the properties of antifreeze and sulfate attack resistance but reduce the capability to resist chloride ion penetration. Moreover, the differences in concrete’s workability, mechanism properties, volume stability and durability caused by various lithologies of manufactured sand and stone powder were not significant and the influence of lithology variety on the macro properties of concrete could be neglected eventually.

Self-compacting Concrete-filled Steel Tubes Prepared from Manufactured Sand with a High Content of Limestone Fines

To meet the requirements of construction of concretes filled in the steel tube arches,a C60 grade micro-expansive self-compacting concrete （SCC） was prepared from manufactured sand （MS）.The utilization of MS with a high content of quarry limestone fines was dealed for SCC applications.The workability,compressive and splitting strength,modulus of elasticity,restrained expansion and chloride ion permeability as well as freeze-thaw resistance of three MS-SCC mixes with fines content of 3%,7% and 10% were tested and compared with those of the natural sand （NS）-SCC mix.The experimental results indicate that the performances of the C60 MS-SCC with fines content of 7% are excellent and compared favorably with those of C60 NS-SCC.

Effects of Manufactured-sand on Dry Shrinkage and Crccp of High-strength Concrete

The influences of natural sand, manufactured-sand （MS） and stone-dust （SD） in the manufactured-sand on workability, compressive strength, elastic modulus, drying shrinkage and creep properties of high-strength concrete （HSC） were tested and compared. The results show that the reasonable content （7%-10.5%） of SD in MS will not deteriorate the workability of MS-HSC. It could even improve the workability. Moreover, the compressive strength increases gradually with the increasing SD content,and the MS- HSC with low SD content （smaller than 7%） has the elastic modulus which approaches that of the natural sand HSC, but the elastic modulus reduces when the SD content is high. The influence of the SD content on drying shrinkage performance of MS-HSC is closely related to the hydration age. The shrinkage rate of MS-HSC in the former 7 d age is higher than that of the natural sand HSC, but the difference of the shrinkage rate in the late age is not marked. Meanwhile the shrinkage rate reduces as the fly ash is added; the specific creep and creep coefficient of MS-HSC with 7% SD are close to those of the natural sand HSC.

An Investigation into the Compressive Strength,Permeability and Microstructure of Quartzite-Rock-Sand Mortar

River sand is an essential component used as a fine aggregate in mortar and concrete.Due to unrestrained exploitation,river sand resources are gradually being exhausted.This requires alternative solutions.This study deals with the properties of cement mortar containing different levels of manufactured sand(MS)based on quartzite,used to replace river sand.The river sand was replaced at 20%,40%,60%and 80%with MS(by weight or volume).The mechanical properties,transfer properties,and microstructure were examined and compared to a control group to study the impact of the replacement level.The results indicate that the compressive strength can be improved by increasing such a level.The strength was improved by 35.1%and 45.5%over that of the control mortar at replacement levels of 60%and 80%,respectively.Although there was a weak link between porosity and gas permeability in the mortars with manufactured sand,the gas permeability decreased with growing the replacement level.The microstructure of the MS mortar was denser,and the cement paste had fewer microcracks with increasing the replacement level.

Constraints in Using Manufactured Sands in Concrete Pavements in Australia

Due to the shortages of natural sands along the east coast of Australia in particular and the need to fully utilise fines produced in quarry operations, progress has been made in utilising blends of manufactured sands and natural sands in concrete pavements. This paper documents some of the constraints in utilising larger proportions of manufactured sands in concrete pavements. These constraints are mainly caused by the current level of knowledge regarding the impact of manufactured sands on skid and abrasion resistance of concrete pavements. This paper presents a brief review of literature on this subject in the USA, France and UK. It also briefly documents work recently carried out in Australia by CCAA （Cement Concrete and Aggregates Australia）, referring to the skid and abrasion resistance of concrete pavements using manufactured sands. The paper concludes that there is no relationship between the free silica content and the skid resistance. With regard to the abrasion resistance, it is rather the curing conditions and the compressive strength that are more important in achieving good results.

煤矸石机制砂C20混凝土力学性能研究

为探究煤矸石机制砂对低强度等级C20混凝土力学性能的影响,调控煤矸石机制砂掺量和水灰比浇筑混凝土试块,开展抗压和抗折强度试验。结果表明:相同水灰比下,随煤矸石机制砂掺量增加,混凝土的抗压强度先增强后降低;相同取代率下,混凝土的抗压和抗折强度随水灰比增大而降低。相较于煤矸石机制砂取代率,水灰比对混凝土抗折强度的影响更大。取代率不超过30%时,对混凝土的抗压和抗折强度均有利,水灰比为0.55、取代率为25%时,混凝土抗压和抗折强度提高最多。

机制砂再生粗骨料混凝土抗压强度试验研究

为探究机制砂原料种类和机制砂级配对机制砂再生混凝土抗压强度的影响,以细骨料类型(天然砂、石灰石机制砂、卵石机制砂)细骨料级配、水灰比为变量制作了30组机制砂再生混凝土立方体试块和15组天然砂再生混凝土立方体试块。结果表明:在石粉掺量10%时,机制砂再生混凝土抗压强度高于天然砂再生混凝土,且机制砂原料为石灰石的抗压强度高于卵石;机制砂各筛孔量占比分别为5%、7%、18%、25%、26%和14%时,机制砂再生粗骨料混凝土的抗压强度最高。研究表明:严格控制水灰比、颗粒级配可使机制砂再生混凝土达到设计强度、满足生产应用要求,采用机制砂替代天然砂制备再生骨料混凝土更有优势,是缓解天然砂资源短缺的可行途径之一。

振动台试验饱和机制砂模型土动力特性研究

根据饱和砂土-结构相互作用振动台试验的相似性要求,对饱和砂土的相似性要求进行探讨,提出以机制砂作为通用的饱和砂土模型材料。通过动力三轴分级循环加载试验,研究饱和机制砂模型土的最大动剪切模量Gmax、动剪切模量比G/Gmax-γ曲线、阻尼比λ-γ曲线、孔压比ru-ΔW曲线的变化范围,得到其动力本构模型参数随有效粒径d10、不均匀系数Cu、曲率系数Cc和相对密度Dr的变化规律和拟合公式,并根据相似要求和拟合公式分不同情况给出饱和机制砂模型土的配制方法。与文献中多种原型砂土的相似性分析表明,饱和机制砂模型土可以对大部分原型砂土有效实现动力特性相似和液化特性相似,证明其广泛适用性。

蒸养条件下机制砂混凝土的力学性能及干缩性能

为推动机制砂在预制混凝土构件中的应用,实现机制砂在建筑行业的更广阔应用,进而有效减缓天然砂资源短缺的问题,通过混凝土抗压强度、动弹性模量和干燥收缩试验等宏观性能测试和扫描电镜测试、压汞测试及显微硬度测试等微观测试方法,探讨养护条件和砂型对混凝土力学性能及干缩性能的影响规律及相应机理。试验结果表明:无论采用河砂还是机制砂,蒸养混凝土相较标养混凝土具有更高的早期力学性能和更低的干燥收缩,但高温导致水化产物分布不均,阻碍了后续的水化反应,使其后期力学性能出现不同程度的劣化。结合微观测试结果可知,孔径粗化、界面过渡区间隙增大和水化产物结构疏松是造成蒸养混凝土后期性能劣化的主要原因,但采用机制砂替代河砂能有效减弱蒸养热伤损效应产生的劣化影响。此外,蒸养机制砂混凝土相较蒸养河砂混凝土具有更高的力学性能和更优的微观结构,具体表现为:蒸养机制砂混凝土孔结构更加细化,界面过渡区间隙更小,同时基体显微硬度均值更高,整体密实度更高。但小于50 nm毛细孔增加将导致蒸养机制砂混凝土干缩变形增大。基于GL 2000收缩模型,通过引入考虑机制砂的影响因子,建立了蒸养机制砂混凝土干燥收缩预测模型,可较准确且有效地进行蒸养机制砂混凝土的干燥收缩预测。

石灰石粉作为矿物掺合料对机制砂混凝土力学性能及耐久性的影响

石灰石粉可作为机制砂细粉料或矿物掺合料两种途径掺入混凝土中,然而两种不同掺入途径对混凝土的配合比设计及其性能影响截然不同。基于我国某桥梁工程机制砂混凝土的实际应用需求与服役环境特征,通过测试不同石灰石粉掺量下的机制砂混凝土工作性、抗压强度与氯离子扩散系数,对石灰石粉作为单一矿物掺合料制备承台、挡土墙和索塔用机制砂混凝土的可行性进行了分析。研究结果表明:随着石粉掺量增加,机制砂混凝土的抗压强度性能呈现下降趋势,可降低水胶比以保障混凝土强度;石粉会提高C30机制砂混凝土的氯离子迁移系数,但对C50机制砂混凝土的氯离子迁移系数影响较小。初步认为石粉作为矿物掺合料用于不同类型机制砂混凝土具有可行性,更高掺量的石粉对机制砂混凝土耐久性的影响有待进一步探究。

石粉对轨道交通工程用机制砂混凝土性能的影响研究

研究了石粉含量对轨道工程用C20、C35、C50机制砂混凝土性能的影响及石粉含量的合理范围。结果表明:对于工作性和抗压强度,机制砂混凝土的强度等级越高,石粉的最佳含量越小,C20机制砂混凝土的石粉含量宜为10%~20%,C35宜为5%~10%,C50宜为5%以内;对于早期抗裂性能,C35机制砂混凝土的石粉含量宜控制在3%~7%,C50宜控制在5%以内;对于抗氯离子渗透性能,C35机制砂混凝土的石粉含量宜控制在5%~15%,C50机制砂混凝土的性能受石粉含量的影响较小;对于抗冻性能,当石粉含量在5%以内时,C35机制砂混凝土的抗冻性能变化不明显,超过5%后,抗冻性能下降,石粉含量对C50机制砂混凝土抗冻性能的影响不明显。

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。

橡胶-机制砂UHPC压缩破坏特性及能量演化特征

为减少超高性能混凝土(UHPC)的河砂用量,改善其压缩破坏特性,本研究通过设计四因素四水平正交试验,分析了机制砂、橡胶颗粒、水胶比和钢纤维在不同水平状态下对UHPC流动性和抗压强度的影响,获得了综合性能最佳的优选组,并探究了橡胶-机制砂UHPC的压缩破坏能量演化特征。结果表明:1)在各因素中,对UHPC拌合物的流动性和抗压强度影响最大的因素分别为水胶比和钢纤维,随着水胶比增加,UHPC的流动性显著增加,而抗压强度与钢纤维掺量之间为正效应;2)当橡胶和机制砂替代河砂掺量分别10%和40%(橡胶为等体积替代河砂,机制砂为等质量替代河砂,相当于节约了50%的河砂)时,试件抗压强度仍达到118.5 MPa;3)从破坏形态和能量演化看,钢纤维和橡胶颗粒混掺后UHPC表现出脆-延转换力学特征,优选组UHPC在达到应力峰值时较对照组保持了较好的完整度。

基于机制砂棱角性表征的级配测量修正方法及实验研究

机制砂的形状特征和级配对混凝土性能有着重要影响,图像法可以同时测量颗粒的形态和级配。在机制砂的级配测量中,需要考虑颗粒的形态特征对级配的影响。因此,提出利用图像分析技术实现机制砂棱角性的表征和级配测量修正,分别进行了机制砂的棱角性对比实验,级配重复性实验,图像法与筛分法级配测量对比实验以及修正前后的对比实验。实验结果表明,拟合椭圆棱角性算法可以独立地表征棱角性,不受粒径大小的影响。等效Feret椭圆短径作为等效粒径,粒度区间最大重复性误差出现在1.18~2.36 mm的尺寸区间,为1.26%。图像法测量的最大级配误差从修正前的6.29%减少到修正后的1.01%,细度模数的最大误差从0.17降低到0.04。相比传统的筛分方法,该测量系统具有良好的稳定性和功能性,满足测量的标准和要求。

机制砂对隧道衬砌混凝土收缩性能的影响

使用机制砂制备铁路隧道衬砌混凝土是未来发展趋势,衬砌混凝土是隧道工程的重要组成部分,而由于混凝土收缩导致的环向裂缝是其常见缺陷,因此,研究采用机制砂后的衬砌混凝土收缩性能对保障隧道工程质量有重要意义。本研究对强度等级为C40的花岗岩机制砂混凝土(GSC)、凝灰岩机制砂混凝土(TSC)和河砂混凝土(RSC)的工作性能、抗压强度、收缩性能、早期抗裂性能进行了对比试验研究,并结合水分蒸发量和孔结构进行了内在机理分析。研究结果表明:在相同外加剂掺量下,两种机制砂混凝土的工作性能与河砂混凝土相近,较河砂混凝土具有更好的保坍性;机制砂混凝土的抗压强度可以满足设计要求,且凝灰岩机制砂混凝土在各龄期均较河砂混凝土表现出更高的抗压强度;若早期良好养护,机制砂不会增加混凝土的收缩,花岗岩机制砂还能显著降低收缩;若早期缺乏良好养护,两种机制砂均会增加混凝土早期收缩,早期抗裂性能也下降,增加衬砌混凝土环向开裂风险。

机制砂与特细砂抗氯盐侵蚀混凝土的制备及性能研究

利用机制砂、特细砂等制备抗氯盐侵蚀混凝土对降低生产成本、提升桥梁服役寿命具有重要意义。本研究通过优化骨料设计与水泥浆体组成提升混凝土的抗氯离子侵蚀性能,研究了骨料级配、特细砂用量、砂率、水胶比、胶凝材料种类对混凝土性能的影响。结果表明:8~16 mm碎石质量分数由50%增加至80%时,氯离子扩散系数由4.0×10^(-12)m^(2)/s降低至2.9×10^(-12)m^(2)/s;特细砂替代50%(质量分数)机制砂时,氯离子扩散系数为3.6×10^(-12)m^(2)/s,仍可以满足设计要求;砂率增大,氯离子扩散系数先减小后增大。水胶比在0.36~0.38时,氯离子扩散系数低于3.5×10^(-12)m^(2)/s;矿渣组分水化改善了混凝土的孔结构,并形成了更多的AFm、C-S-H产物,提升了混凝土对氯离子的固化能力,矿渣水泥混凝土氯离子扩散系数较普通硅酸盐水泥混凝土降低了40%。