Mix design of concrete for prestressed concrete sleepers using blast furnace slag and steel fibers

The application of ground granulated blast furnace slag (GGBFS) and steel fibers in prestressed concrete railway sleepers was investigated in this study. The use of GGBFS was considered as an eco-friendly material aimed at reducing CO₂ emissions and energy consumption as well as to enhance the durability performance of railway sleepers. Steel fibers improves the durability and structural performance in terms of crack control and reduction of spalling and can replace shear reinforcement. The mix proportions of the concrete incorporating GGBFS (56% GGBFS) and GGBFS with steel fibers (56% GGBFS and 0.75% steel fibers) were determined through a series laboratory tests and a life cycle assessment. These mixes satisfied the requirements of the Korean Railway Standard and resulted in improved flexural capacity as well as less CO₂ emissions compared with current railway sleepers. Using these mixes, a total of ninety prestressed concrete sleepers were produced in a factory under the same manufacturing process as current railway sleepers, and their mechanical properties as well as durability performance were evaluated. The mix with partial replacement of Type III Portland cement by GGBFS showed an improved resistance to chloride ion penetration and freeze-thaw cycles compared with the concrete used for current railway sleepers. However, these mixes were more vulnerable to carbonation. The mix with GGBFS and steel fibers (mix BSF) showed a slightly better durability performance than the mix with GGBFS only (mix BS), including better carbonation and freeze-thaw resistances. The mix BSF showed decreased chloride ion penetration depth than mix BS but showed a slightly higher chloride ion diffusion coefficient.     

水泥-粉煤灰-硅灰基超高性能混凝土水化过程微观结构的演变规律

超高性能混凝土(UHPC)具有卓越的力学性能和耐久性能,应用前景广阔。采用扫描电镜背散射电子图像、热重法和氮气吸附法系统研究了水泥-粉煤灰-硅灰基UHPC浆体水化过程中微观结构的演变过程。结果表明:UHPC浆体在早期水泥水化较快,但7d后水化变得较为缓慢,粉煤灰在UHPC浆体中反应较为缓慢,28d时反应程度仅为7%;UHPC浆体中Ca(OH)2含量早期上升快速,由于硅灰和粉煤灰的火山灰反应逐渐消耗,3d后含量开始下降,但28d时浆体中仍存在部分Ca(OH)2;此外,在水化过程中,UHPC浆体的比表面积不断降低,孔隙率逐渐下降,水化产物变得更为致密。     

极地低温下冻融作用对混凝土断裂性能的影响

为研究混凝土在极地及严寒地区经历冻融循环作用后的耐久性能变化情况,该文通过慢冻法开展了84个三点弯曲梁的冻融循环及加载试验。试验以冻融循环下限温度(低至-80℃)、冻融循环次数、混凝土强度、混凝土类型为研究变量,对比分析了冻融循环前后混凝土基本力学性能和关键断裂参数的变化规律。研究结果表明：随着循环下限温度的降低以及循环次数的增加,混凝土的基本力学性能以及起裂韧度和断裂能均呈下降趋势,但失稳韧度以及特征长度则呈相反趋势,表明混凝土在经历冻融循环后阻裂的能力下降,但混凝土变形性能有明显改善;随着混凝土强度的提高,抗冻耐久性能有一定程度的提升;海水海砂混凝土经受冻融循环后断裂性能不低于普通混凝土;提出冻融损伤累积量的概念,可利用其反映断裂参数的变化情况和不同冻融工况的定量化比较。     

废弃瓷砖粉对超高性能混凝土的抗压强度影响规律与机制

超高性能混凝土极低的水胶比和较高的水泥用量,使其在广泛应用中面临着水泥基体高自收缩和高成本等问题,而使用工业副产品或废弃物取代部分水泥是有效的解决方法之一。废品瓷砖已成为一种大宗工业废弃物,应用瓷砖粉在超高性能混凝土中可有效地解决水泥的高消耗和废弃瓷砖的堆积问题。因此,使用瓷砖粉取代水泥质量的10wt%、15wt%、20wt%和25wt%来制备新型绿色低碳超高性能混凝土,主要研究了瓷砖粉对超高性能混凝土抗压强度的影响规律,并采用修正Andreasen堆积模型、XRD分析、TG/DTG、SEM观察探讨了瓷砖粉对超高性能混凝土的改性机制,同时对瓷砖粉超高性能混凝土的环境足迹和成本进行了分析。研究结果表明,瓷砖粉的掺入对超高性能混凝土各龄期抗压影响在±10%以内,但对7~28天和28~60天的抗压强度发展影响显著,在25wt%掺量时抗压强度增长率分别达到了104.6%和51.8%。这主要是由于瓷砖粉的掺入提高了超高性能混凝土的堆积密实度,发生二次水化反应并生成了低钙硅比的水化硅酸钙凝胶,提高了水泥的水化程度,降低了界面过渡区的宽度。并且由环境影响和成本计算可知,瓷砖粉可有效降低超高性能混凝土的能耗、CO₂排放量和成本。      

Effect of cement type and limestone particle size on the durability of steam cured self-consolidating concrete

This paper describes a laboratory program to investigate the influence of cement and limestone filler (LF) particle size on the hardened properties and durability performance of steam cured self-consolidating concrete. In addition, the interplay between cement type and LF particle size was investigated. CSA (Canadian Standards Association) Type GU (General Use) and HE (High Early-strength) cements were used with 5% silica fume (SF) [1]. The water-to-cement ratio was 0.34. LF with two nominal particle sizes of 17 μm and 3 μm, which correspond to Blaine fineness of 475 and 1125 m²/kg, respectively, were used. In addition to fresh concrete properties, hardened properties including compressive strength, elastic modulus, ultrasonic pulse velocity and density were measured at 12 h and 16 h, and at 3, 7 and 28 days. Indicators of durability performance including rapid chloride permeability testing (RCPT), sulfate resistance, linear shrinkage, salt scaling resistance and freeze-thaw resistance were evaluated. The results showed that LF improved the 12 and 16-h strength with no influence on later age strength (i.e., 3–28 days). The linear shrinkage and RCPT decreased with the addition of LF. This reduction was linked to the production of calcium mono-carboaluminate. LF did not impact the sulfate resistance, salt scaling resistance or freeze-thaw resistance of concrete.     

Influence of a fine glass powder on the durability characteristics of concrete and its comparison to fly ash

A detailed investigation carried out to ascertain the durability characteristics of fine glass powder modified concretes is reported in this paper. Tests were designed to facilitate comparisons between concretes modified with either glass powder or fly ash at the same cement replacement level. The optimal replacement level of cement by glass powder is determined from strength and hydration tests as 10%. The later age compressive strengths of glass powder and fly ash modified concretes are seen to differ by only 5%. The durability characteristics are ascertained using tests for rapid chloride permeability, alkali–silica reactivity, and moisture transport parameters. The chloride penetrability values indicate some amount of pore refinement. The potential of glass powder to reduce the expansion due to alkali–silica reaction is established from tests conducted in accordance with ASTM C 1260, but fly ash is found to perform better at similar replacement levels. Glass powder–fly ash blends that make up a 20% cement replacement level are found to be as efficient as 20% fly ash in reducing expansion. The control concrete is seen to exhibit the lowest overall moisture intake after 14 days of curing, and fly ash concrete the highest, with the glass powder concrete in between. The trend is reversed at later ages, demonstrating that both the replacement materials contribute to improved durability characteristics. The sorptivity and moisture diffusion coefficient values calculated from the moisture intake-time data also demonstrate a similar trend. These studies show that fine glass powder has the potential to improve the durability of concretes.     

Compressive strength and durability properties of ceramic wastes based concrete

This paper presents an experimental study on the properties and on the durability of concrete containing ceramic wastes. Several concrete mixes possessing a target mean compressive strength of 30 MPa were prepared with 20% cement replacement by ceramic powder (W/B = 0.6). A concrete mix with ceramic sand and granite aggregates were also prepared as well as a concrete mix with natural sand and coarse ceramic aggregates (W/B = 0.5). The mechanical and durability performance of ceramic waste based concrete are assessed by means of mechanical tests, water performance, permeability, chloride diffusion and also accelerated aging tests. Results show that concrete with partial cement replacement by ceramic powder although it has minor strength loss possess increase durability performance. Results also shows that concrete mixtures with ceramic aggregates perform better than the control concrete mixtures concerning compressive strength, capillarity water absorption, oxygen permeability and chloride diffusion. The replacement of cement and aggregates in concrete by ceramic wastes will have major environmental benefits.     

配筋超高性能混凝土用作免拆模板对短柱力学性能影响的实验研究

为进一步提高在海洋浪溅区等严重氯盐腐蚀环境下混凝土结构的耐久性,采用以配筋超高性能混凝土(UHPC)用作免拆模板加素填芯混凝土的技术思路,通过轴压实验研究了配筋UHPC用作免拆模板对短柱轴压力学性能的影响。结果表明,与破坏时海工混凝土柱出现大量的混凝土剥落和钢筋裸露不同,UHPC免拆模板柱的整体性良好,表面的UHPC仅出现少量剥落且钢筋不外露。同时,在等截面和相同配筋的情况下,UHPC免拆模板方柱和圆柱的开裂荷载明显高于高强海工混凝土方柱和圆柱的极限荷载,且分别达到其自身极限荷载的93%和88%。考虑到混凝土对钢筋的有效保护是影响海工混凝土结构耐久性的重要因素,配筋UHPC用作免拆模板这一思路在提高柱体的极限承载力、延性、刚度的同时还可以提高结构的开裂荷载、抗裂性能,有效地提高结构耐久性,适用于沿海工程。其中圆柱的力学性能提高幅度要远高于方柱,从配筋UHPC免拆模板的使用效果和施工便利性考虑,优选UHPC免拆模板圆柱。     

碱矿渣高性能混凝土冻融耐久性与损伤模型研究

采用矿渣、Na2SiO3和NaOH复合激发剂制备了高性能碱矿渣混凝土(Alakali-slag Concrete,ASC),通过冻融循环试验、扫描电镜和能谱分析测试,研究了ASC的冻融耐久性、微观结构、性能机理、冻融后内部损伤变量的变化规律及损伤模型。结果表明:ASC抗冻等级在F300以上,具有优异的抗冻融耐久性;ASC的水化产物主要为低Ca/Si比的C-S-H(Ⅰ)、碱性铝硅酸盐和沸石型矿物等,没有Ca(OH)2和过渡带,结构的密实性和均匀性好,抗压强度高达90MPa,因此ASC具有优异的抗冻性;以相对动弹模量为损伤变量建立了ASC冻融损伤模型,发现动弹模量衰减模型的拟合精度优于冻融累积损伤模型,幂函数模型比指数函数模型有更好的拟合精度和相关性,模型能够较好地反映ASC的冻融损伤规律和损伤程度。     

玄武岩纤维混凝土腐蚀条件下抗冻融性能试验研究

为了研究玄武岩纤维增强混凝土(BFRC)工程应用的耐久性能,通过试验的方法开展了这种新型复合建筑材料在腐蚀条件下抗冻融性能研究。试验分别测试了玄武岩纤维增强混凝土在无腐蚀环境下、在溶液、溶液和复合溶液(+)中的抗冻融寿命。试验结果表明:在上述四种情况下,复合材料经过冻融循环时相对弹性模量下降速度依次减缓。结论玄武岩纤维对混凝土在腐蚀条件下抗冻融破坏能力增强效果显著,有效地延长了混凝土在腐蚀条件下的使用寿命。     

Eco-efficient ultra-high performance concrete development by means of response surface methodology

The research described in this paper represents a statistically based model with the help of response surface methodology (RSM) aiming to study the applicability of this method to ultra-high performance concrete (UHPC) mixture design and its optimization. Besides, the effects of silica fume, ultra-fine fly ash (UFFA) and sand as three main variable constituents of UHPC on workability and compressive strength as the main performance criteria and responses of this high-tech material were investigated. The models proposed here demonstrate a perfect correlation among variables and responses. Furthermore, through performing a multi-objective optimization, cement and silica fume, as two main constituents of UHPC affecting its eco-efficiency and cost, were substituted by UFFA and sand as much as possible. Finally, an eco-efficient UHPC with cement and silica fume content of 640 kg/m³ and 56.3 kg/m³ respectively and compressive strength and flow diameter of 160.3 MPa and 19 cm was developed.     

The Macro and Micro Effects of Graphene Oxide and Silica Fume on the Performance of Concrete under Standard Curing Conditions

Herein, the properties of concrete containing graphene oxide (GO) and silica fume (SF) are systematically investigated. The results show that the excellent properties of GO and SF have a significant improvement effect on the performance of concrete. Among them, the compressive, splitting tensile, and flexural strength of B-1 group (GO, 0.01 wt% and SF, 2.5 wt%) are increased by 11.04%, 12.23%, 20.11% compared with the reference group after 3 days of curing. Meanwhile, the water penetration height of B-1 group is decreased by 59.67% compared with reference group. The mass loss and relative dynamic modulus loss are 5.32% and 8.89% after 75 freeze-thaw cycles, which are much lower than the reference group. Moreover, the micromodification mechanism of GO and SF on the concrete is investigated by using X-ray diffraction analysis, scanning electron microscopy, and mercury intrusion porosimetry. The results show that the incorporation of GO and SF promotes the early hydration reaction of cement, improves the internal structure of the matrix, and enhances the compactness of the matrix.     

The effect of nanosilica addition on flowability,strength and transport properties of ultra high performance concrete

The experimental study herein presented was conducted aiming to evaluate the influence of nanosilica (nS) addition on properties of ultra-high performance concrete (UHPC). Thermo gravimetric analysis results indicated that nS consumes much more Ca(OH)₂ as compared to silica fume, specifically at the early ages. Mercury intrusion porosimetry measurements proved that the addition of nS particles leads to reduction of capillary pores. Scanning electron microscope observation revealed that the inclusion of nS can also efficiently improve the interfacial transition zone between the aggregates and the binding paste. The addition of nS also resulted in an enhancement in compressive strength as well as in transport properties of UHPC. The optimum amount of cement replacement by nS in cement paste to achieve the best performance was 3 wt.%. However, the improper dispersion of nS was found as a deterrent factor to introduce higher percentage of nS into the cement paste.     

Creep of UHPC in tension and compression: Effect of thermal treatment

Steel fiber-reinforced ultra-high performance concrete (UHPC) is of increasing interest for use in precast prestressed concrete highway bridge girders due to its superior durability and the potential for reducing or eliminating shear reinforcement, due to the presence of steel fibers. However, the contributions of creep, and especially tensile creep, to long-term performance must be better understood to develop appropriate design specifications. Due to practical considerations, it is also of interest to investigate the influence of varying thermal treatment, including temperatures lower than those recommended by the manufacturer (i.e. 90 °C), on the creep of UHPC. In this 1-year study, the effects of three different thermal treatment regimes on tensile and compressive creep performance of UHPC are examined, with complementary characterization by nanoindentation and scanning electron microscopy. Results show that UHPC creeps phenomenologically differently in tension and compression. Both thermal treatments examined resulted in similar tensile creep behavior, suggesting that a lower temperature applied over a longer period could effectively cure UHPC. For the non-thermally cured UHPC, a 10 μm wide region observed at the fiber/matrix interface was characterized by reductions in elastic modulus as well as greater porosity and microcracking than the bulk paste. It is suggested that the quality of the fiber/matrix interface is a major contributor to the measured increased creep of non-thermally treated UHPC as compared to UHPC treated at 60 °C or 90 °C.     

Primary particle size and agglomerate size effects of amorphous silica in ultra-high performance concrete

Silica fume is widely used in ultra-high performance concrete (UHPC). However, it is a by-product in the industrial silicon production and therefore far from an optimized additive. Silica fume improves the compressive strength, but its detailed reaction mechanisms in concretes with low water/cement ratios are not yet fully understood. This study focuses on the influence of primary particle sizes and sizes of agglomerates of different amorphous silicas in UHPC. As a reference system, wet-chemically synthesized silica was used with very high purity, defined particle sizes, narrow primary particle size distributions and controllable agglomerate sizes. The obtained data were compared to silica fume. The results indicate that non-agglomerated silica particles produce the highest strength after 7 d, but a clear dependence on primary particle sizes, as suggested by calculations of packing density, was not confirmed. UHPC may be improved by incorporating an ameliorated dispersion of silica e.g. through commercial silica sols.     

A model predicting carbonation depth of concrete containing silica fume

Silica fume (SF) has been used since long as a mineral admixture to improve durability and produce high strength and high performance concrete. Due to the pozzolanic reaction between calcium hydroxide and silica fume, compared with ordinary Portland cement, the carbonation of concrete containing silica fume is much more complex. In this paper, based on a multi-component concept, a numerical model is built which can predict the carbonation of concrete containing silica fume. The proposed model starts with the mix proportions of concrete and considers both Portland cement hydration reaction and pozzolanic reaction. The amount of hydration products which are susceptible to carbonate, such as calcium hydroxide (CH) and calcium silicate hydrate (CSH), as well as porosity can be obtained as associated results of the proposed model during the hydration period. The influence of water-binder ratio and silica fume content on carbonation is considered. The predicted results agree well with experimental results.     

Frost resistance and microstructure of Engineered Cementitious Composites: Influence of fly ash and micro poly-vinyl-alcohol fiber

One of the most damaging environmental conditions to concrete structure is cyclic freezing and thawing. This paper discusses the influence of the high volumes of fly ash (FA) and micro poly-vinyl-alcohol (PVA) fibers on the cyclic freeze-thaw resistance and microstructure of the Engineered Cementitious Composites (ECC). ECC mixtures with two different FA-cement (FA/C) ratios (1.2 and 2.2 by weight), and at constant water-cementitious materials (fly ash and cement) ratio of 0.27 are prepared. To compare the behavior of ECC with ECC matrix, all of the preceding properties are also investigated for ECC matrix mixtures of same composition without PVA fiber. For frost resistance, mixtures are exposed to the freeze and thaw cycles up to 300 cycles in accordance with ASTM C666, Procedure A. Experimental tests consist of measuring the residual mechanical properties (flexural strength, mid-span beam deflection and flexural stress - deflection curve), ultrasonic pulse velocity and mass loss. The roles of PVA fibers and FA are discussed through the analysis of microstructure and fiber-matrix interactions as function of frost exposure. The microstructural characterization by measuring pore size distributions is examined before and after exposure to frost deterioration by using mercury intrusion porosimetry (MIP). The air-void characteristics of mixtures are also studied using linear transverse method. Test results confirm that both ECC mixtures with high volumes of FA remain durable, and show a tensile strain capacity of more than 2% even after 300 freezing and thawing cycles. On the other hand, before completing 300 freezing and thawing cycles, matrix (ECC without fiber) specimens have severely deteriorated, requiring removal from the freeze-thaw machine. Therefore, results indicate that the addition of micro PVA fiber to the ECC matrix substantially improved the frost resistance. The results of freeze-thaw tests also indicated that the reduction of residual physical and mechanical properties with increasing number of freeze-thaw cycles is relatively more for ECC mixture with FA/C ratio of 2.2 than for ECC mixture with FA/C ratio of 1.2.     

基于静水压理论的混凝土冻融破坏演化模型EI北大核心CSCD

定量分析混凝土材料的抗冻性能是数字混凝土时代的基本要求之一,在全面分析混凝土抗冻性能影响因素和现有混凝土冻融破坏假说的基础上,推导了混凝土冻融破坏理论模型,该模型以内因和外因为自变量,以材料的微、细观结构演变为核心,实现了混凝土抗冻性能的定量分析。模型计算分析表明:水泥水化产物物理力学性能、引气泡含量及孔径分布、最低温度、降温速率、应力水平、材料溢出距离等内、外因素,从不同角度、不同程度影响水泥基材料的冻融损伤特性,合理设计引气泡含量及引气泡孔径,方能有效提高混凝土的抗冻耐久性。且上述分析结果同已有研究成果有较好契合。研究成果能为混凝土的抗冻设计提供参考。     

Development of vegetable fibre–mortar composites of improved durability

The primary concern for vegetable fibre reinforced mortar composites (VFRMC) is the durability of the fibres in the alkaline environment of cement. The composites may undergo a reduction in strength and toughness as a result of weakening of the fibres by a combination of alkali attack and mineralisation through the migration of hydration products to lumens and spaces. This paper presents several approaches used to improve the durability performance of VFRMCs incorporating sisal and coconut fibres. These include carbonation of the matrix in a CO₂-rich environment; the immersion of fibres in slurried silica fume prior to incorporation in the ordinary Portland cement (OPC) matrix; partial replacement of OPC matrix by undensified silica fume or blast-furnace slag and a combination of fibre immersion in slurried silica fume and cement replacement. The durability of the modified VFRMC was studied by determining the effects of ageing in water, exposure to cycles of wetting and drying and open air weathering on the microstructures and flexural behaviour of the composites. Immersion of natural fibres in a silica fume slurry before their addition to cement-based composites was found to be an effective means of reducing embrittlement of the composite in the environments studied. Early cure of composites in a CO2-rich environment and the partial replacement of OPC by undensified silica fume were also efficient approaches in obtaining a composite of improved durability. The use of slag as a partial cement replacement had no effect on reducing the embrittlement of the composite.     

超高性能混凝土在国内外桥梁工程中的研究与应用进展

超高性能混凝土(UHPC)是一种新型的水泥基复合材料,因具有优异的力学性能和耐久性能而受到了广泛关注,并开始应用于桥梁工程领域。着重介绍了UHPC在国内外桥梁工程中的应用进展,并分析了其在国内桥梁工程中的应用前景。据不完全统计,截至2016年底,国内外已有超过400座桥梁采用UHPC作为主要或部分建筑材料。国内学者针对UHPC的优异性能,开展了一系列的UHPC桥梁结构研发工作,结果表明UHPC结构可有效减轻结构自重,提高结构的跨越能力,适合装配化施工,有望解决现有常规桥梁结构存在的诸多技术难题,具有广阔的应用前景。目前,UHPC在国内桥梁工程中已逐渐开始获得应用,今后应进一步结合实际工程需要开展应用技术研究,形成应用指南或技术标准。