Effect of silica fume on the fresh and hardened properties of fly ash-based self-compacting geopolymer concrete

The effect of silica fume on the fresh and hardened properties of fly ash-based self-compacting geopolymer concrete （SCGC） was investigated in this paper. The work focused on the concrete mixes with a fixed water-to-geopolymer solid （W/Gs） ratio of 0.33 by mass and a constant total binder content of 400 kg/m3. The mass fractions of silica fume that replaced fly ash in this research were 0wt%, 5wt%, 10wt%, and 15wt%. The workability-related fresh properties of SCGC were assessed through slump flow, V-funnel, and L-box test methods. Hardened concrete tests were limited to compressive, splitting tensile and flexural strengths, all of which were measured at the age of 1, 7, and 28 d after 48-h oven curing. The results indicate that the addition of silica fume as a partial replacement of fly ash results in the loss of workability; nevertheless, the mechanical properties of hardened SCGC are significantly improved by incorporating silica fume, especially up to 10wt%. Applying this percentage of silica fume results in 4.3% reduction in the slump flow; however, it increases the compressive strength by 6.9%, tensile strength by 12.8% and flexural strength by 11.5%.

Air-void parameters measurement of fresh concrete and hardened concrete

Air content, spacing factor and specific surface of fresh concrete and hardened concrete with different air contents, slumps and mineral admixtures (fly ash, slag, fly ash + slag, fly ash + slag + silica fume composite) were studied by the air-void analyzer (AVA) method and the microscopical method. The correlations between the test results obtained from different methods were analyzed. The results show that, there is a close correlation of air content and spacing factor between the fresh concrete and the hardened concrete, but the specific surface correlation is weak. The air content of concrete measured by the AVA method is smaller than that of the pressure method and the microscopical method, because AVA device captures only the air voids with the size smaller than 3 mm. Spacing factor of the fresh concrete measured by the AVA method is greater than that of the hardened concrete measured by the microscopical method, while the specific surface is smaller. When the criterion of 4%-7% air content measured by the pressure method and microscopical method is acceptable for concrete freezing-thawing (F-T) durability in cold weather, the air content measured by the AVA method should be 2.4%-4.6%. For the concrete F-T durability, when the criterion of the spacing factor measured by the microscopical method is 300 μm, the spacing factor measured by the AVA method should be 360 μm.

Air-void Parameters Measurement of Fresh Concrete

To explore the method to evaluate air-void parameters of fresh concrete rapidly, the spacing factor and specific surface of freshly mixed concrete of different air contents, different slumps and different mineral admixtures （fly ash, fly ash-slag, fly ash-slag-silica fume composite） are studied by air-void analyzer （AVA）, and the correlation between the air content measured by AVA and air content determinator is compared. The results show that the spacing factor of the fresh concrete decreases firstly and then increases with the increase of air content, however the specific surface has the opposite rule. When the air content is more than 8%, the spacing factor of fresh concrete increases and the specific surface decreases. For the fresh concrete samples with similar air content, the specific surface increases firstly and then decreases with the increase of slump, but the spacing factor decreases firstly and then increases. Mineral admixalre can reduce the spacing factor of fresh concrete and increase the specific surface to some degree. There is a good correlation between the air content measured by two methods, and the air content measured by AVA is about 70% of the air content measured by the air content determinator.

Damage model of fresh concrete in sulphate environment

A model of damage to fresh concrete in a corrosive sulphate environment was formulated to investigate how and why the strength of corroded concrete changes over time. First, a corroded concrete block was divided into three regions: an expanded and dense region; a crack-development region; and a noncorroded region. Second, based on the thickness of the surface corrosion layer and the rate of loss of compressive strength of the corroding region, a computational model of the concrete blocks' corrosion-resistance coefficient of compressive strength in a sulphate environment was generated. Third, experimental tests of the corrosion of concrete were conducted by immersing specimens in a corrosive medium for 270 d. A comparison of the experimental results with the computational formulae shows that the calculation results and test results are in good agreement. A parameter analysis reveals that the corrosion reaction plays a major role in the corrosion of fresh concrete containing ordinary Portland cement,but the diffusion of the corrosion medium plays a major role in the corrosion of concrete mixtures containing fly ash and sulphate-resistant cement. Fresh concrete with a high water-to-cement ratio shows high performance during the whole experiment process whereas fresh concrete with a low water-to-cement ratio shows poor performance during the late experiment period.

Bond interface crack propagation of fresh foundation concrete and rock under blasting load

According to concrete age, the dynamic stress intensity factors of bond interface crack of concrete-rock was calculated. Result shows that the propagation of concreteinterface crack is mainly caused by tensile stress and shear stress for stress wave reflection. With the growth of concrete age, interface crack fracture toughness increases, and itscapacity of resisting blasting load strengthens. Therefore, blasting vibration should bestrictly controlled for fresh concrete.

Mathematical Modelling in Placing of Fresh Concrete

The main problem in working with fresh concrete is the aggregate segregation during filling of formwork. The segregation is strongly related to W/C (Water/Cement) ratio. The fresh concrete is usually considered as a Non-Newtonian fluid since it is a mixture of aggregate, cement and water. The flow behavior of the fresh concrete to W/C ratio plays a crucial role in the quality of the high performance concretes by affecting the flow behavior of the fresh concrete. The aggregates in the fresh concrete usually cause segregation in the final product depending on the flow condition. In this study, the mechanism of segregation in such systems was theoretically investigated. The mould filling of fresh concrete was numerically investigated and aggregates were considered as Lagrangian particles and segregation was identified from trajectories of such particles. It was found that the aggregates were trapped at the dead zones leading to segregation in the system. The particle size and geometry of the mould was found to have significantly affect the segregation in the system.

FC纤维混凝土在机场道面施工中的应用

长期以来,机场道面经常出现断角、断板、掉边等现象,产生大量砂粒,容易造成发动机、轮胎损坏,甚至导致飞机发生事故。而采用FC纤维混凝土能有效解决道面龟裂破坏、道面耐久性病害等问题,显著减少飞机运营过程中FOD的产生,降低机场道面的维护成本和难度。基于此,文章深入研究了FC纤维混凝土在机场道面施工中的应用。

In-situ Monitoring the Setting Behavior of Foamed Concrete Using Ultrasonic Pulse Velocity Method

The applicability of ultrasonic pulse velocity （UPV） method to in-situ monitor setting and hardening process of foamed concrete （FC） was systematically investigated. The UPVs of various FC pastes were automatically and continuously measured by a specially designed ultrasonic monitoring apparatus （UMA）. Ultrasonic tests were performed on FC mixtures with different density （300, 500, 800 and 1 000 kg/m3）, and different fly ash contents （0%, 20%, 40% and 60%）. The influence of curing temperatures （20, 40, 60 and 80~C） was also studied. The experimental results show that three characteristic stages can be clearly identified during the setting process of an arbitrary FC paste： dormant stage, acceleration stage, and deceleration stage. Wet density, fly ash content, and curing temperature have great impact on setting behavior. A stepwise increase of the wet density results in shorter dormant stage and larger final UPV. Hydration reaction rate is obviously promoted with an increase in curing temperature. However, the addition fly ash retards the microstn,lcture formation. To aid in comparing with the ultrasonic results, the consistence spread test and Vicat needle test （VNT） were also conducted. A correlation between ultrasonic and VNT results was also established to evaluate the initial and final setting time of the FC mixtures. Finally, certain ranges of UPV with reasonable widths were suggested for the initial and final setting time, respectively.

Utilization of Cement Kiln Dust (CKD) with Silica Fume (SF) as a Partial Replacement of Cement in Concrete Production

This research aimed to clarify the role of by-product materials, such as CKD with SF as partial replacement by weight of cement in concrete manufacturing and inclusion on different characteristics of concrete. Concrete test specimens were mixed with 0%, 5%, 10%, 15%, 20% and 25% (CKD) with 15% (SF) as partial replacement by weight of Cement (CEM I-52.5N). Fresh concrete properties have been evaluated by workability measurement slump test. While hardened concrete properties have been evaluated by compressive, split tensile and flexural strengths tests at ages 7, 28 and 56 days, but evaluated for bond strength, modulus of elasticity and chemical composition measurement with X-Ray Fluorescence at age of 28 days. The test results have revealed that the increase of CKD amount with fixed amount of SF in concrete mixtures as partial replacement by weight of cement leads to gradual decrease of fresh concrete workability. In concrete mixtures, 20% CKD in the presence of 15% SF as partial replacement by the weight of cement are the optimum ratios which can be used without any negative effect on mechanical properties compressive, indirect tensile, flexural and bond strength at all the ages of concrete. Also modulus of elasticity and bond strength increased by 8.81% and 0.69% respectively at the age 28 days compared with control mixture.

Production and Properties of Superplasticized Concrete

The objective of this research is to study the effect of grinding powdered superplasticizer, Portland cement, sand, and silica fume on the properties of fresh and hardened concrete. Lose Angeles Machine was used to grind these constituents. The program was arranged to determine the effect of cycles＇ number, superplasticizer type and dosage, silica fume dosage and condition, and gravel to sand ratio on properties of concrete. Naphthalene sulphonated formaldehyde （NSF） superplasticizers in the forms of liquid and powdered were used. Silica fume may be grinded with the other constituents （grinded）, or added to concrete mixer （normal）. The water/cement （w/c） ratio varied from 0.35 to 0.55 to achieve a constant slump （50-90 mm）. Slumps, bulk density and mechanical properties of concrete were measured. Scanning electron microscope （SEM） was also used to show the differences between traditional and superplasticized concrete. The results showed that grinding the mixture enhances fresh and hardened concrete properties. It is also observed that grinding the mixture for 500 cycles is more effective than other numbers of grinding. In addition, superplasticized concrete exhibits compressive strength higher than traditional one at varied ages. Moreover, using powdered superplasticizer has a remarkable effect on enhancing concrete properties rather than using it in a liquid form. A dosage of 1% by weight of cement gave the highest results of compressive strength. Silica fume has an essential role in improving concrete strength and durability since it acts as very efficient void filler and as a super pozzolana. SEM observations illustrate that grinding the mixture enhances transition zone （TZ） properties and makes it denser. On the other hand, grinded mixture can be packaged in bags and transported for use in crowded cities, and so, enhances quality control, since the only requirement to obtain superplasticized concrete is to add water and gravel. This technique has many benefits such as; saving cement, labor and noise, high quality control, and enhancing concrete permeability and durability. There are many fields of application of superplasticized concrete such as; in locations which are not easily accessible by ordinary concreting techniques, in repairing and strengthen, thin coating, and for small projects when ready mix supply is not feasible.

新拌混凝土均匀度的超声检测

混凝土均匀度检测是混凝土施工质量控制中的关键技术之一。基于超声技术开展了不同搅拌时间下的新拌混凝土均匀度检测试验,得出不同点位的混凝土密度,从而计算出不同搅拌时间下混凝土的均匀度指数。试验结果表明,新拌混凝土使用单卧轴搅拌机的合理搅拌时间为100~150 s,通过分析3种拟合函数的回归曲线与实测关系曲线的符合程度,确定了符合程度较高的专用均匀度检测曲线拟合函数。由此可见,超声技术可有效检测混凝土的均匀度,在一定程度上可指导新拌混凝土的现场施工。

新拌混凝土离散元参数的测量与标定

随着计算机技术的发展,可以采用仿真的方法来研究新拌混凝土的流变性能.离散元方法适合于新拌混凝土的大变形流动.颗粒的物性参数和接触参数的设定是模拟结果真实可靠的关键.在本研究中,将混凝土分为机制石和砂浆两相.首先测量了物性参数,包含密度、恢复系数、静摩擦系数和滚动摩擦系数.使用Hertz-Mindlin(no slip)接触模型表示粗骨料-边界和粗骨料-粗骨料之间的相互作用,并通过休止角的实验进行了验证;采用Hertz-Mindlin with JKR接触模型来描述粗骨料-砂浆、砂浆-砂浆、砂浆-边界之间的相互作用,用坍落度实验对JKR参数进行了标定,并采用响应曲面法确定了最佳的参数组合值.最后通过L型箱试验对新拌混凝土的离散元仿真方法进行了验证.

Optimization of Strength Properties of Reactive Powder Concrete by Response Surface Methodology

The main objective of this study is to optimize the fresh and strength properties of reactive powder concrete incorporated with industrial by-products like ultra-fine ground granulated blast furnace slag as cement substitute and added with coal bottom ash and recycled concrete fines as partial replacement of quartz sand by response surface methodology through design of experiment approach.Totally four responses namely slump,compressive strength(C-28),flexural strength(F-28),and split-tensile strength(S-28)after 28 days of curing period were considered.The statistical study on the reactive powder concrete includes the modeling of regression,normal probability plots,surface plot analysis,and optimization of process variables.The regression models of the considered responses(slump,C-28,F-28,and S-28)were tested.The results obtained from the analysis of variance(ANOVA)and Pareto chart were used to determine the statistical significance of the process variables.The influence of the variables on the responses was studied by means of the surface plot analysis.The optimal proportion of the variables against the responses was obtained through optimization response.The resulted regression equations were in the form of second-order polynomial equation and the prediction of strength properties was found to be in line with the experimental results.The difference of proportion of variance indicated that only 0.43%,6.42%,5.15%,and 9.7%of deviations cannot be expressed by the analysis.The ANOVA and Pareto charts represented the high significance and appropriateness of the linear term of slump response and the two-way interaction term of strength responses.The results of the optimization response revealed the optimal proportions of recycled concrete fines and coal bottom ash as 19.15%and 7.02%,respectively.

高强FC合成纤维道面混凝土力学性能试验研究

为了提高道面混凝土的基本力学性能,对FC纤维道面混凝土(简称FCRPC)的工作性及基本力学性能进行了系统的试验研究,对比分析了FC纤维以0.8%,1.0%,1.2%,1.4%,1.6%五种体积掺率对道面混凝土抗折、抗压强度性能的影响规律,试验结果表明:高强FC纤维对道面混凝土有较好的力学增强性能,在高强FC纤维掺量为1.2%时达到最佳。

FRP-混凝土-钢双壁空心柱的抗爆性能分析

纤维增强复合材料(fiber reinforced polymer,FRP)-混凝土-钢双壁空心柱是一种新型混凝土组合柱,由外层FRP管、内层钢管及两管间填充的混凝土组成。针对FRP-混凝土-钢双壁空心柱在近场爆炸荷载下的抗爆性能进行了数值模拟。采用任意拉格朗日欧拉法(arbitrary Lagrangian Eulerian,ALE)进行建模,并基于已有的试验结果验证所建模型的准确性。利用验证后的数值模型,对爆炸冲击波传递过程、混凝土损伤发展、FRP管环向应变、爆炸后剩余力学性能以及参数进行了分析。结果表明,FRP管能有效约束爆炸区域内的夹层混凝土;爆炸后双壁空心柱的轴向承载力明显减小、轴向刚度明显下降;通过增加FRP管、钢管厚度,以及减小空心率能提高双壁空心柱的抗爆性能,其中前两项的影响更为显著。

新拌混凝土氯离子含量现场检测的现状研究

新拌混凝土氯离子含量超标对钢筋混凝土的耐久性危害很大。对新拌混凝土中的氯离子含量进行现场快速检测并完善其检测方法是目前工程界的当务之急。本文阐明了氯离子选择性电极法是新拌混凝土氯离子含量现场检测的重要方法,并提出了仍须解决的几个技术问题。

FC纤维机场道面混凝土试验研究及施工技术

根据清华大学FC研究中心提供的FC纤维性能,按照某机场道面混凝土中掺加FC纤维的要求,经大量的试验研究并结合现场施工条件,在保证道面混凝土基本性能的前提下,探讨了FC纤维混凝土的配合比、施工工艺及关键技术的质量控制措施,为机场道面混凝土掺加FC纤维的广泛应用奠定了技术基础。

新拌混凝土配合比快速分析方法综述

在一定的原材料和生产工艺下,配合比决定了产品的性能。新拌混凝土配合比的快速分析对提前控制混凝土的最终质量有重大意义。本文归纳了国内外对新拌混凝土中粗细集料、胶凝材料、水等关键材料用量的测定方法的原理及方法概要,分析了国标中配合比分析试验存在的问题,提出应采用微波法测定单位用水量,再用4.75和0.08 mm筛筛分出粗细集料后参考Dunagan法测定集料单位用量,然后用减量法计算出胶凝材料总量,并将选择性溶解法优化为溶解减量法以测定掺合料用量。此方法符合准确和快速的原则,可适用于搅拌站和施工现场的新拌混凝土配合比快速分析。

振捣作用下新拌混凝土流变性和气泡结构研究

振捣是混凝土密实成型的重要手段,振捣效果直接决定了混凝土成型后的力学性能和耐久性能。本文提出了基于斯托克斯定律、适用于振捣作用下新拌混凝土流变性的测试和评价方法——拉球法,分析了静态下该方法与流变仪所测流变参数的相关性,论证了其评价新拌混凝土流变性的可靠性。结果表明,拉球法与流变仪所测塑性黏度和屈服应力具有良好的线性相关性。振捣可使近振源处浆体的黏性阻力降低90%以上,并且可以有效排出新拌砂浆中的较大气泡(孔径大于500μm)而保留微小气泡(孔径小于200μm)。高引气剂掺量下振捣排出较大气泡的作用效果更强,当引气剂掺量由0.015%增大至0.030%(质量分数)时,孔径在[500,1000)μm的气泡含量降幅由31.0%增大至84.8%,孔径在[1000,2000)μm的气泡含量降幅由10.3%增大至36.4%。

粉煤灰的颗粒特征对混凝土流变性能的影响

研究了粉煤灰的颗粒特征——细度、最可几粒径和比表面积对混凝土新拌浆体流变性能的影响。结果表明,当粉煤灰的细度和最可几粒径较小时,比表面积对静态屈服应力值的影响较小;当灰样粒径较大时,比表面积对静态屈服应力值的影响较大;当粉煤灰的细度和粒径相近时,比表面积与动态屈服应力值相关性较高;掺入粉煤灰提高混凝土浆体的塑性黏度,塑性黏度与粉煤灰颗粒的细度、最可几粒径及比表面积没有明显的相关性。