Tensile creep due to restraining stresses in high-strength concrete at early ages

This paper reports an experimental study on the early-age tensile creep behavior of high strength concrete (HSC) comprising of silica fume concrete, fly ash concrete and plain concrete under uniaxial restraining stresses. A series of restraint shrinkage tests were carried out adopting semi-adiabatic and isothermal conditions to determine the effects of temperature history on the tensile creep properties for young concretes. Furthermore, the effects of restraining stress history on creep were also discussed under three different degrees of restraint conditions. It was found that the initial thermal dilation deformation delayed the development of tensile creep and weakened the creep potential of early age concretes. It was also observed that the young concrete subjected to a lower restraining tensile stress history had a higher potential of visco-elastic response in tension at early ages.     

Dissolution rate of silica fume in very high strength concrete

A very high strength concrete, having a 91 day compressive strength of 113 MPa, was developed using Type III cement, limestone aggregates, sodium naphthalene superplasticizer and silica fume, with W/C ratio of 0.24. SEM-EDXA and AEM were used to study the rate of dissolution of silica fume in this concrete, with progressive hydration. The ultrafine particle size of silica fume (< 1 μm) makes it difficult to view the state of these particles in concrete under the SEM. With AEM, however, it was possible to observe the dissolution process of silica fume particles, which begin at an early stage. Within 28 days, most of the silica fume is consumed in the pozzolanic reaction. The initial reaction product is a silica rich gel which later transforms into different morphological types of C-S-H which are compacted together. This is a major contributory factor for the very high strength of this concrete. Some partly reacted silica fume particles, however, remain in the hardened paste; lack of water most probably inhibits their complete transformation.     

Characteristics of a thermally activated alumino-silicate pozzolanic material and its use in concrete

This paper presents the results of the physical and chemical properties of a thermally activated alumino-silicate material (MK), and deals with the properties of fresh and hardened concrete incorporating this material. The properties of fresh concrete investigated included workability, bleeding, setting time, and autogenous temperature rise. The properties of the hardened concrete investigated included compressive, splitting-tensile and flexural strengths, Young's modulus of elasticity, drying shrinkage, resistance to chloride-ion penetration, freezing and thawing, and saltscaling resistance. The properties of the MK concrete were also compared with those of the control portland cement concrete and the silica fume concrete.

The test results indicate that the MK material is highly pozzolanic and can be used as a supplementary cementing material to produce high-performance concrete. Although it requires a higher dosage of the superplasticizer and air-entraining admixture compared with that of the control concrete, the MK concrete can be produced with satisfactory slump, air content, and setting time. The concrete incorporating 10% MK had higher strength at all ages up to 180 days compared with the control concrete; in comparison with the silica fume concrete the MK concrete showed a faster strength development at early ages, but had lower strength after 28 days. At 28 days, the MK concrete had somewhat higher splitting-tensile and flexural strengths, Young's modulus of elasticity, and lower drying shrinkage compared with that of the control and the silica fume concretes. The resistance of the MK concrete to the chloride-ion penetration was significantly higher than that of the control concrete, but similar to that of the silica fume concrete. The MK concrete showed excellent performance in the freezing and thawing test. The performance of the MK concrete subjected to the de-icing salt scaling test was similar to that of the silica fume concrete, but marginally inferior to the control concrete.     

Properties of rubberized concretes containing silica fume

A test program was carried out to develop information about the mechanical properties of rubberized concretes with and without silica fume. Two types of tire rubber, crumb rubber and tire chips, were used as fine and coarse aggregate, respectively, in the production of rubberized concrete mixtures which were obtained by partially replacing the aggregate with rubber. Six designated rubber contents varying from 2.5% to 50% by total aggregate volume were used. The concretes with silica fume were produced by partial substitution of cement with silica fume at varying amounts of 5–20%. Totally, 70 concrete mixtures were cast and tested for compressive and splitting tensile strengths, and static modulus of elasticity in accordance to ASTM standards. The design strength level ranging from 54 to 86 MPa was achieved using water–cementitious material (w/cm) ratios of 0.60 and 0.40. Test results indicated that there was a large reduction in the strength and modulus values with the increase in rubber content. However, the addition of silica fume into the matrix improved the mechanical properties of the rubberized concretes and diminished the rate of strength loss. Results also revealed that a rubber content of as high as 25% by total aggregate volume might be practically used to produce rubberized concretes with compressive strength of 16–32 MPa.     

Modified water-cement ratio law for silica fume concretes

The application of condensed silica fume as a mineral admixture in concrete is almost a routine one nowadays for the production of tailor-made high-performance concretes. Abrams' Law, which was originally formulated for conventional concrete containing cement as the only cementitious material, is not directly applicable to these new-generation concretes. In the present paper, modified relationships have been proposed to evaluate the strength of silica fume concrete. An extensive experimentation was carried out to determine the isolated effect of silica fume on concrete, and, analyzing the 28-day strength results of 32 concrete mixes performed over a wide range of water-binder ratios and silica fume replacement percentages, simplified relationships have been proposed. These simplified models might serve as useful guides for proportioning concrete mixes incorporating silica fume.     

Evaluation of capillary pore size characteristics in high-strength concrete at early ages

The quantitative scanning electron microscope-backscattered electron (SEM-BSE) image analysis was used to evaluate capillary porosity and pore size distributions in high-strength concretes at early ages. The Powers model for the hydration of cement was applied to the interpretation of the results of image analysis. The image analysis revealed that pore size distributions in concretes with an extremely low water/binder ratio of 0.25 at early ages were discontinuous in the range of finer capillary pores. However, silica-fume-containing concretes with a water/binder ratio of 0.25 had larger amounts of fine pores than did concretes without silica fume. The presence of larger amounts of fine capillary pores in the concretes with silica fume may be responsible for greater autogenous shrinkage in the silica-fume-containing concretes at early ages.     

辅助胶凝材料对预应力高强混凝土管桩强度及工艺的影响

为优化混凝土管桩生产工艺,以硅灰(SF)和偏高岭土(MK)作为辅助胶凝材料,研究硅灰和偏高岭土对不同蒸养时间下混凝土抗压强度的影响,并使用X射线衍射(XRD)和扫描式电子显微镜结合能量色散谱(SEM-EDS)分析其水化产物及微观结构。通过Design-Expert8.0软件设计Box-Behnken试验,以硅灰掺量、偏高岭土掺量和蒸养时间三个因素为自变量,蒸养混凝土抗压强度为响应值,构建多因素回归方程模型。结果表明:硅灰掺量为胶凝材料质量分数8%时,对抗压强度略有提高,提高幅度为6.2%,达到83.6 MPa;5%、8%和10%(质量分数)掺量的偏高岭土均可提高蒸养混凝土的抗压强度,蒸养4 h、8 h、12 h时,10%掺量的偏高岭土对混凝土抗压强度的提升幅度依次为15.6%、13.2%和13.6%,蒸养4 h、8 h和12 h对混凝土抗压强度影响不大。XRD和SEM-EDS结果表明,硅灰和偏高岭土均消耗了Ca(OH)₂,提升了水泥早期水化程度,可以改善内部孔结构。通过响应面法建立模型可以预测,当硅灰质量分数为6.6%、偏高岭土质量分数为10%、蒸养时间为8.6 h时,混凝土抗压强度最高,达到104.8 MPa,且具有较高置信度。     

Behaviour of lightweight expanded polystyrene concrete containing silica fume

Lightweight concrete can be produced by replacing the normal aggregate with lightweight aggregate, either partially or fully, depending upon the requirements of density and strength. The present study covers the use of expanded polystyrene (EPS) beads as lightweight aggregate both in concretes and mortars containing silica fume as a supplementary cementitious material. The main aim of this project is to study the strength and the durability performance of EPS concretes. These mixes were designed by using the efficiency of silica fume at the different percentages. The resulting concretes were seen to have densities varying from 1500 to 2000 kg/m³, with the corresponding strengths varying from 10 to 21 MPa. The rate of strength gain for these concretes shows that an increase in the percentage of silica fume increases the 7-day strength. This was observed to be about 75%, 85%, and 95% of the corresponding 28-day strength at the silica fume replacement levels of 3%, 5%, and 9%, respectively. The results of absorption, at 30 min and the final absorption, show that the EPS mixes made with sand have lower levels of absorption compared to the mixes containing normal aggregates. Further, the absorption values were seen to be decreasing with increasing cementitious content. The performance of these concretes, in terms of their chloride permeability and corrosion resistance, even at the minimal silica fume content level was observed to be very good.     

电伴热预养护条件对混凝土受冻临界强度的影响

电伴热预养护是一种保证预拌混凝土冬期施工养护温度和强度增长的简单高效的方法。本文采用7 d恒负温(-5 ℃、-10 ℃、-15 ℃)一次冻结转标准养护的试验,研究电伴热预养护不同温度和时间对一种高坍落度C30普通混凝土抗压强度的影响。依据混凝土受冻临界强度的定义,确定基于电伴热预养护条件下的混凝土受冻临界强度值及其合理的预养护时间。结果表明:与标准养护相比,经电伴热高温预养护的混凝土抗压强度均得到了提高,但电伴热预养护温度宜控制在30 ℃,较高的预养护温度下强度发展速率和R_-7+28(负温养护7 d再转标养28 d的抗压强度)值反而降低;当预养护温度为30 ℃,硬化温度不低于-15 ℃时,合理的预养护时间在36～48 h之间;恒负温(-5 ℃、-10 ℃、-15 ℃)硬化条件下,采用电伴热预养护的混凝土受冻临界强度的范围是6.6～17.8 MPa,为混凝土立方体抗压强度标准值的22.0%～59.3%。研究旨在比较电伴热预养护制度对普通混凝土力学性能的影响,进而指导相关工程应用。     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

Effect of high temperatures on high performance steel fibre reinforced concrete

After being subjected to different elevated heating temperatures, ranging between 105 °C and 1200 °C, the compressive strength, flexural strength, elastic modulus and porosity of concrete reinforced with 1% steel fibre (SFRC) and changes of colour to the heated concrete have been investigated.The results show a loss of concrete strength with increased maximum heating temperature and with increased initial saturation percentage before firing. For maximum exposure temperatures below 400 °C, the loss in compressive strength was relatively small. Significant further reductions in compressive strength are observed, as maximum temperature increases, for all concretes heated to temperatures exceeding 400 °C. High performance concretes (HPC) start to suffer a greater compressive strength loss than normal strength concrete (NSC) at maximum exposure temperatures of 600 °C. It is suggested that HPC suffers both chemical decomposition and pore-structure coarsening of the hardened cement paste when C-S-H starts to decompose at this high temperature. Strengths for all mixes reached minimum values at 1000 or 1100 °C. No evidence of spalling was encountered. When steel fibres are incorporated, at 1%, an improvement of fire resistance and crack [F.M. Lea, Cement research: retrospect and prospect. Proc. 4th Int. Symp. On the Chemistry of Cement, pp. 5-8 (Washington, DC, 1960).] resistance as characterized by the residual strengths were observed. Mechanical strength results indicated that SFRC performs better than non-SFRC for maximum exposure temperatures below 1000 °C, even though the residual strength was very low for all mixes at this high temperature. The variations with colour, which occured, are associated with maximum temperatures of exposure.     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Steam-cured concrete incorporating mineral admixtures

This paper explores the potential benefits of steam-cured concrete, particularly on mixes incorporating mineral admixtures. Twenty mixes with various combinations of Portland cement, fly ash (FA), slag and silica fume (SF) were investigated. For each mix, specimens were either standard-cured in a water bath of 27 °C or steam-cured at 55 °C maximum temperature over 8 h. For the materials and test conditions reported in this study, it was found that steam-cured concretes were more porous as indicated by the much higher sorptivity values compared with standard-cured specimens. Mixes with SF have the best performance and hold promise in precast manufacturing due to their high early strength development and low sorptivity values.     

Studies on chemical resistance of low water/cement ratio concretes

Solutions containing mineral acids, and certain organic acids and salts are highly corrosive to portland cement concrete. Since permeability is the key factor governing the rate of deterioration, it is customary to use a low water-cement ratio in making concretes or concrete overlays required to resist corrosive action of aggressive chemical solutions. Pozzolanic admixtures are often used to provide additional protection against acidic attack. Highly reactive pozzolanic admixtures, such as condensed silica fume, which rapidly react with calcium hydroxide and reduce both the alkalinity and permeability of concrete are now being used for improving durability. During the last two decades, latex admixtures have also found widespread application. The polymeric constituents of a latex seem to coat the alkaline hydration products of portland cement, thus protecting them from attack by aggressive solutions.

An experimental study was undertaken to evaluate the relative chemical resistance of low water-cement ratio concretes, containing either a styrene-butadiene latex or a silica fume admixture, to the following solutions; 1% HCl, 1% H₂SO₄, 1% lactic acid, 5% acetic acid, 5% ammonium sulfate, and 5% sodium sulfate. Time taken to register 25 percent weight loss by fully submerged concrete specimens was used as a criterion for failure. From the data it appears that, except for the ammonium sulfate solution, the concrete containing the silica fume generally showed better resistance to chemical attack than other concrete types.     

Influence of amorphous silica on the hydration in ultra-high performance concrete

Amorphous silica particles (silica) are used in ultra-high performance concretes to densify the microstructure and accelerate the clinker hydration. It is still unclear whether silica predominantly increases the surface for the nucleation of C–S–H phases or dissolves and reacts pozzolanically. Furthermore, varying types of silica may have different and time dependent effects on the clinker hydration. The effects of different silica types were compared in this study by calorimetric analysis, scanning and transmission electron microscopy, in situ X-ray diffraction and compressive strength measurements. The silica component was silica fume, pyrogenic silica or silica synthesized by a wet-chemical route (Stoeber particles). Water-to-cement ratios were 0.23. Differences are observed between the silica for short reaction times (up to 3 days). Results indicate that silica fume and pyrogenic silica accelerate alite hydration by increasing the surface for nucleation of C–S–H phases whereas Stoeber particles show no accelerating effect.     

稻壳灰抑制超高性能混凝土的自收缩机理分析

在过去的十几年中,超高性能混凝土（UHPC）由于其优异的性能（如：超高强度,低渗透性和优良的耐久性）已经成为一种非常有前景的建筑材料。然而,UHPC也像普通高性能混凝土一样,由于水泥和硅灰的掺量比较高,使其具有很大的自收缩性能。寻求一种有效减少自收缩的方法是近年来水泥及混凝土研究领域的一个非常重要的任务。本研究中采用稻...     

超细高活性矿物掺合料对UHPC水化和收缩性能的影响

本试验研究了超细高活性矿物掺合料(超细掺合料)与硅灰以单掺、复掺的方式制备超高性能混凝土(UHPC),分析了复掺不同掺量超细掺合料对UHPC的工作性、力学性能、水化热和收缩性能的影响。结果表明:UHPC流动性随超细掺合料掺量的增加而增加,跳桌流动度最高为275 mm;将超细掺合料与质量分数为10%的硅灰以复掺的方式制备UHPC时,随超细掺合料掺量的增加,UHPC抗折强度先增加后降低,抗压强度先增加后趋于平稳,最大抗折强度和抗压强度分别为25.9 MPa和150.0 MPa;超细掺合料与质量分数为10%的硅灰复掺制备的UHPC水化热随超细掺合料掺量增加,先增大后减小;复掺质量分数为10%的超细掺合料与质量分数为10%的硅灰制备的UHPC早期收缩量最小,比单掺质量分数为20%的硅灰制备的UHPC低50.92%。     

低温养护下矿物掺合料湿喷混凝土力学性能及配比优化研究

为系统研究粉煤灰掺量、硅灰掺量及养护温度对湿喷混凝土力学性能的影响规律,通过设计正交试验对湿喷混凝土抗压强度进行极差和方差分析。结果表明:湿喷混凝土抗压强度随养护龄期增加而增大,但抗压强度增幅随养护龄期延长而减弱;增加硅灰和粉煤掺量均能有效提高湿喷混凝土抗压强度,但粉煤灰掺量超过10%(质量分数,下同)后,粉煤灰掺量的增加对混凝土后期抗压强度没有显著的影响;三因素对湿喷混凝土抗压强度影响程度顺序为硅灰掺量>养护温度>粉煤灰掺量;湿喷混凝土抗压强度对矿物掺合料的敏感性与养护温度呈正相关,增大养护温度能够提高矿物掺合料对湿喷混凝土抗压强度的改善效果;随着养护温度的提高,团絮状胶凝物质大量生成,水化产物黏结得更为密实,混凝土的抗压强度和承载性能得到进一步增强;构建多元非线性回归模型能够对混凝土抗压强度进行预测,并且湿喷混凝土在硅灰掺量、粉煤灰掺量及养护温度分别为15%、15%和10 ℃时具有最佳的抗压强度。     

Effect of magnesium and sulfate ions on durability of silica fume blended mixes exposed to the seawater tidal zone

The effect of silica fume on deterioration resistance to sulfate attack in seawater within tidal zone and simulated wetting-drying condition has been studied in Portland cement concretes and pastes containing silica fume (SF) with/without ground granulated blast furnace slag (GGBS). Changes in the compressive strength and capillary water absorption of specimens as a function of SF content have been investigated combined with phases determination by means of scanning electron microscopy and X-ray energy dispersion analysis. The strength change factors (SCFs) of specimens with SF (the more SF content, the higher strength loss) were greater than that of the mixes without SF or cured under tap water. Mg²⁺ ion originated attack found to be the dominating deterioration mechanism as confirmed by X-ray and chemical analyses.Further, the incorporation of GGBS with SF mixes in different exposure conditions led to the worst performance in all of the test environments. Lower cement content and hydration rate accompanied with particular chemical composition of GGBS made concrete and paste specimens to be more susceptible to deleterious seawater environment.     

硅灰对钢纤维混凝土耐久性能的影响研究

为提高钢纤维混凝土耐久性能,采用复掺的方式,选择高品质的硅灰掺入到钢纤维的混凝土结构中。在保证基准配合比相同的情况下,通过不同的硅灰与钢纤维配合比,探讨硅灰对钢纤维混凝土耐久性能的影响。通过实验结果表明,随着硅灰掺入钢纤维混凝土量的增加,混凝土的抗折强度、抗压强度和劈裂强度、抗冻性能都明显提高,并在12%硅灰+1.2%钢纤维时达到最大。