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.     

Use of porous ceramic waste aggregates for internal curing of high-performance concrete

Internal curing has become extensively used to reduce autogenous shrinkage and consequently mitigate the high risk of early age cracking of high-performance concrete (HPC). This paper investigates the efficiency of internal wet curing provided by a new type of aggregate, "recycled waste porous ceramic coarse aggregates" (PCCA). Six different silica fume HPCs with and without the PCCA are examined with respect to measured physical and mechanical property development. Four different replacement proportions of normal weight coarse aggregate (NCA) by the PCCA have been evaluated. The results have shown a high effectiveness of the PCCA for internal curing purposes, to drastically reduce and even to completely eliminate autogenous shrinkage of HPC prepared with a very low water/binder ratio (w/b) of 0.15. It has been found that the incorporation of 40% of the PCCA leads to a non-shrinking HPC that results in an insignificant internal stress accompanied by a significant increase of the compressive strength. It should be noted that for the different proportions of the PCCA incorporated no decrease of the compressive strength has been observed at either early or later ages, as is the case with some conventional lightweight aggregates.     

High-strength lightweight concrete made with scoria aggregate containing mineral admixtures

This paper presents a part of the results of an ongoing laboratory work carried out to design a structural lightweight high strength concrete (SLWHSC) made with and without mineral admixtures. In the mixtures, basaltic-pumice (scoria) was used as lightweight aggregate.A control lightweight concrete mixture made with lightweight basaltic-pumice (scoria) containing normal Portland cement as the binder was prepared. The control lightweight concrete mixture was modified by replacing 20% of the cement with fly ash. The control lightweight concrete mixture was also modified by replacing 10% of the cement with silica fume. A ternary lightweight concrete mixture was also prepared modifying the control lightweight concrete by replacing 20% of cement with fly ash and 10% of cement with silica fume. Two normal weight concrete (NWC) were also prepared for comparison purpose.Fly ash and silica fume are used for economical and environmental concerns. Cylinder specimens with 150 mm diameter and 300 mm height and prismatic specimens with dimension 100×100×500 mm were cast from the fresh mixtures to measure compressive and flexural tensile strength. The concrete samples were cured at 65% relative humidity with 20 °C temperature. The density and slump workability of fresh concrete mixtures were also measured.Laboratory test results showed that structural lightweight concrete (SLWC) can be produced by the use of scoria. However, the use of mineral additives seems to be mandatory for production of SLWHSC. The use of ternary mixture was recommended due to its satisfactory strength development and environmental friendliness.     

Properties of lightweight expanded polystyrene concrete reinforced with steel fiber

Expanded polystyrene (EPS) concrete is a lightweight, low strength material with good energy-absorbing characteristics. However, due to the light weight of EPS beads and their hydrophobic surface, EPS concrete is prone to segregation during casting, which results in poor workability and lower strength. In this study, a premix method similar to the ‘sand-wrapping’ technique was utilized to make EPS concrete. Its mechanical properties were investigated as well. The research showed that EPS concrete with a density of 800-1800 kg/m³ and a compressive strength of 10-25 MPa can be made by partially replacing coarse and fine aggregate by EPS beads. Fine silica fume greatly improved the bond between the EPS beads and cement paste and increased the compressive strength of EPS concrete. In addition, adding steel fiber significantly improved the drying shrinkage.     

Characterization of elastic and time-dependent deformations in high performance lightweight concrete by image analysis

Image analysis and strain mapping were used to examine the nature of elastic, creep and shrinkage strains in high performance lightweight concrete (HPLC). The strain maps showed non-uniform deformations related to microstructural features. Both average strain and non-uniformity increased with time under testing. Paste-rich regions exhibited higher creep plus shrinkage than the lightweight aggregate (LWA) particles examined herein; it is suggested that LWA could have a role in reducing deformations of the paste. Compared to normal weight high performance concrete (HPC), the paste and LWA in the HPLC exhibited more gradual spatial differences in elastic deformations, creep and shrinkage. It is proposed that this difference results from the lower stiffness of the LWA compared to granite used in the HPC. The results indicate that improvement in elastic property matching between the lightweight aggregate and high performance paste reduces stress concentrations at the aggregate/paste interface and contributes to reductions in deformations of HPLC compared to HPC.     

轻质超高性能混凝土的制备及性能形成机理

以预湿轻集料、硅酸盐水泥、硅灰、粉煤灰微珠、钢纤维、膨胀剂等为原材料,采用轻质、超高强度与低收缩协同设计方法制备了一种轻质超高性能混凝土材料,并研究其性能的主要影响因素和形成机理。结果表明:制备的轻质超高性能混凝土可实现自流平、干表观密度低至2040 kg/m3、28 d抗压/抗折强度可达120/18 MPa、弯曲韧性指数I₂₀可至21.8、56 d收缩率低至2.62×10^–4;其性能的形成主要源于粉煤灰微珠的滚珠效应、轻集料的轻质多孔效应、预湿轻集料的内养护与拱壳效应,以及预湿轻集料内养护与膨胀剂的叠加效应。     

再生轻骨料混凝土抗压强度指标换算关系研究

为了研究再生轻骨料混凝土立方体抗压强度和轴心抗压强度,以及两者之间的换算关系,以再生轻粗骨料取代率为变化参数,制作了15个标准立方体试块和15个标准棱柱体试块,进行抗压强度试验.实测了试块的抗压强度,探讨了不同再生轻粗骨料取代率对于再生轻骨料混凝土抗压强度的影响规律,采用数据统计回归的方法,得到了再生轻骨料混凝土轴心抗压强度和立方体抗压强度之间的换算关系.研究结果表明:再生轻骨料混凝土的抗压强度均低于普通混凝土的抗压强度;随着再生轻粗骨料取代率的增加,再生轻骨料混凝土的抗压强度逐渐降低;再生轻骨料混凝土轴心和立方体抗压强度之间的换算关系和普通混凝土并不相同;基于实测数据,建立了再生轻骨料混凝土抗压强度指标之间的换算关系式.     

Internal curing of high-performance concrete with pre-soaked fine lightweight aggregate for prevention of autogenous shrinkage cracking

The effectiveness of internal curing (IC) to reduce autogenous shrinkage cracking in high-performance concrete (HPC) was investigated using different levels of internal curing on four pairs of large-size prismatic HPC specimens tested simultaneously under free and restrained shrinkage. Internal curing was supplied by pre-soaked fine lightweight aggregate (LWA) as a partial replacement to regular sand. It was found that the use of 178 kg/m³ of saturated LWA in HPC, providing 27 kg/m³ of IC water, eliminated the tensile stress due to restrained autogenous shrinkage without compromising the early-age strength and elastic modulus of HPC. It was shown that the risk of concrete cracking could be conservatively estimated from the extent of free shrinkage strain occurring after the peak expansion strain that may develop at very early ages. Autogenous expansion, observed during the first day for high levels of internal curing, can significantly reduce the risk of cracking in concrete structures, as both the elastic and creep strains develop initially in compression, enabling the tensile strength to increase further before tensile stresses start to initiate later.     

冻融作用后塑钢纤维轻骨料混凝土力学性能试验研究

以圆球形粉煤灰陶粒为粗骨料,对塑钢纤维(0kg/m3、3kg/m3、6kg/m3、9kg/m3)轻骨料混凝土试件进行快速冻融(0次、50次、100次、150次)试验,研究冻融后试件的抗压性能、劈裂抗拉性能、抗折性能、抗冲击性能.结果表明:冻融循环作用下,适量的塑钢纤维掺入可以明显增强轻骨料混凝土的劈裂抗拉强度和抗折强度;提高轻骨料混凝土折压比,增强轻骨料混凝土抗裂性能;并能显著改善轻骨料混凝土的抗冲击性能.综合各项力学性能指标,冻融后轻骨料混凝土塑钢纤维最优掺量为6kg/m3.     

钢管自密实轻骨料混凝土变形性能试验研究

采用堆积密度分别为500 kg/m³、800 kg/m³的页岩陶粒和堆积密度为1 600 kg/m³的普通碎石作为粗骨料配制自密实轻骨料混凝土和普通混凝土,并成型了钢管轻骨料混凝土与钢管普通混凝土,对比研究了二者的收缩应变、轴压应力-应变变化规律和温度-应变变化规律。结果表明:随着粗骨料堆积密度的降低,同配比轻骨料混凝土的密度、强度和弹性模量均逐渐降低;相同龄期时,钢管轻骨料混凝土及其核心轻骨料混凝土的收缩应变均小于钢管普通混凝土及其核心混凝土的收缩应变,钢管与核心轻骨料混凝土的密贴性更好;钢管轻骨料混凝土的轴压应力-应变变化规律与钢管普通混凝土的基本相同,与钢管普通混凝土相比,钢管轻骨料混凝土的弹性模量有所降低,但比非钢管约束状态下轻骨料混凝土弹性模量的降低幅值有所减小;钢管轻骨料混凝土与钢管普通混凝土的温度-应变相当,均为4.0 με/℃左右。     

Investigations on the compressive strength of silica fume concrete using statistical methods

The present paper deals with a mathematical model developed using statistical methods to predict the 28-day compressive strength of silica fume concrete with water-to-cementitious material (w/cm) ratios ranging from 0.3 to 0.42 and silica fume replacement percentages from 5 to 30. Strength results of 26 concrete mixes, on more than 300 test specimens, have been analyzed for statistical modeling. The ratios of compressive strengths between silica fume and control concrete have been related to silica fume replacement percentage. The expression, being derived with strength ratios and not with absolute values of strength, is independent of the specimen parameters and is applicable to all types of specimens. On examining the validity of the model with the results of previous researchers, it was observed that for results on both cubes and cylinders, predictions were obtained within 7.5% of the experimentally obtained values.     

外掺粉煤灰天然轻骨料混凝土早期力学性能研究

以天然浮石作为粗骨料,通过外掺法分别掺加0％、20％、30％和40％体积的粉煤灰替代相同体积的砂,配置轻骨料混凝土.通过对混凝土在3d、14 d、28 d的抗压强度,以及28 d的弹性模量与轴心抗压强度的研究,得出天然浮石轻骨料混凝土外掺粉煤灰最优掺量为30％.浮石轻骨料混凝土在此掺量下,不仅改善了混凝土的和易性,而且提高了基本力学性能.     

The effects of different fine and coarse pumice aggregate/cement ratios on the structural concrete properties without using any admixtures

Structural lightweight concrete solves weight and durability problems in buildings and structures. In order to produce the high strength concrete in the civil engineering applications, lightweight concrete mixtures containing the fine pumice aggregate (FPA) from Nev°ehir region in Turkey and coarse pumice aggregate (CPA) from Yali Island in the Eastern Mediterranean were tested and the research findings were discussed in this paper. To analyse the effects of FPA and CPA/cement ratios on the structural concrete engineering properties, the range of different pumice aggregate/cement (A/C) ratios of 2:1, 2 1/2:1, 3:1, 3 1/2:1 and 4:1 by weight and cement contents of 440, 375, 320, 280 and 245 kg/m³, respectively, were used to make pumice aggregate lightweight concrete (PALC) mixture testing samples with a slump of from 35 to 45 mm.The experimental research findings showed that PALC has strengths comparable to normal weight concrete, yet is typically 30-40% lighter. PALC showed the design flexibility and substantial cost savings by providing less dead load due to its lower density values. The properties, which increase in value and indicate the increasing quality with lower A/C ratios (high cement contents), are compressive strength, modulus of elasticity and density. Properties, which decrease in value and indicate the increasing quality, with lower A/C ratios are water absorption and carbonation depth. In all cases, lowering the A/C ratio (higher cement content) increases quality. The research showed that structural lightweight concrete can be produced by the use of fine and coarse pumice aggregates mixes without using any additions or admixtures.     

Study on effects of solar radiation and rain on shrinkage, shrinkage cracking and creep of concrete

In this paper, the effects of actual environmental actions on shrinkage, creep and shrinkage cracking of concrete are studied comprehensively. Prismatic specimens of plain concrete were exposed to three sets of artificial outdoor conditions with or without solar radiation and rain to examine the shrinkage. For the purpose of studying shrinkage cracking behavior, prismatic concrete specimens with reinforcing steel were also subjected to the above conditions at the same time. The shrinkage behavior is described focusing on the effects of solar radiation and rain based on the moisture loss. The significant environment actions to induce shrinkage cracks are investigated from viewpoints of the amount of the shrinkage and the tensile strength. Finally, specific compressive creep behavior according to solar radiation and rainfall is discussed. It is found that rain can greatly inhibit the progresses of concrete shrinkage and creep while solar radiation is likely to promote shrinkage cracking and creep.     

PVC粗骨料混凝土的力学和吸能性能分析

将聚氯乙烯(PVC)软板进行粉碎得到级配良好的片状PVC颗粒以不同掺量等体积替代天然粗骨料后加入混凝土中制成试件,做单轴压缩试验和钢球自由落体冲击试验,得到立方体抗压强度、劈裂抗拉强度、卸载弹性模量和能量吸收率,用来探究不同掺量的PVC粗骨料混凝土力学和吸能性能。结果表明,随PVC粗骨料掺量增加,混凝土的抗压强度和劈裂抗拉强度均呈显著降低趋势,在实际工程中,PVC粗骨料掺量应该控制在20%左右;随PVC粗骨料掺量增加,混凝土脆性得到改善以及延性增强;混凝土的能量吸收能力随PVC粗骨料掺量增加呈显著增加趋势。     

矿物掺合料对UHPC性能的影响

传统超高性能混凝土(UHPC)的硅灰用量一般都比较高,导致其制作成本较高,而且自收缩比较大,对实际工程应用造成了一定的影响。本文用粉煤灰和矿粉部分或全部替代硅灰制备UHPC,并对其工作性能、力学性能、自收缩及孔结构特征进行了试验研究。结果表明:采用粉煤灰或矿粉替代硅灰可以改善UHPC拌合物的流动性,替代率越高,拌合物的流动度越大;当采用粉煤灰或矿粉替代50%(质量分数)硅灰时,在标准养护下,对28 d抗压强度的影响较小,而在高温蒸养下,则会导致28 d抗压强度下降,当替代率达到100%(质量分数)时,无论是标准养护还是高温蒸养,都会显著降低28 d抗压强度;采用粉煤灰或矿粉替代硅灰能降低细孔的占比,增大孔径,减少自收缩,且粉煤灰对于自收缩的抑制效果优于矿粉。     

硅灰对碳纤维混凝土强度及电阻率影响研究

通过在碳纤维混凝土中加入硅灰,研究了硅灰对碳纤维混凝土抗压强度、抗折强度和劈裂强度的影响规律,同时研究了硅灰对碳纤维混凝土电阻率的影响作用。结果表明:碳纤维混凝土相对素混凝土,抗压强度降低了5.9%,抗折强度提高了25.8%,劈裂强度增加了了21.1%;加入硅灰后,碳纤维混凝土抗压强度、抗折强度和劈裂强度都有所增加;10%硅灰掺量时,抗压强度增加了39%,抗折强度提高了40.6%,劈裂强度提高了34.3%。加入硅灰后碳纤维混凝土电阻率也明显降低,3 d时两者电阻率降低了8.6Ω,随着时间的推移,差值越来越小。说明硅灰对碳纤维混凝土的电阻率也有明显作用,进一步证明了加入硅灰可以有效提高碳纤维在混凝土中的分散性。     

Comparative study of epoxy and polyester resin-based polymer concretes

This paper presents a comparative evaluation of two types of polymer concretes, prepared with two different polymer binders (epoxy and polyester resins) with identical aggregate in both cases (crushed quartzite and silica sand). Comparison is also made with a conventional cement concrete. Properties studied include strength (compressive, split-tensile, flexural and impact), setting shrinkage, abrasion and resistance to various chemicals. Both polyester concrete and epoxy concrete show far superior properties than cement concrete. It is further shown that the polyester concrete properties can be improved upon to approach those of the epoxy concrete by modifications such as incorporation of calcium carbonate microfiller and/or addition of a suitable silane coupling agent.     

外加剂对混凝土收缩与开裂性能影响的试验研究

针对传统混凝土中加入矿物外加剂会导致混凝土开裂的问题,提出在传统方法上加入粉煤灰和硅灰。为验证方法的可行性,采用不同实验配方比,并结合单掺和复掺方式,对上述方法进行验证。实验表明:复掺30%~40%的粉煤灰和矿粉可有效降低混凝土的开裂面积;硅灰与粉煤灰和矿粉在二元或三元复合下可降低砂浆自收缩和干燥收缩,且混凝土强度不会降低。由此得出混凝土收缩与开裂和砂浆收缩存在显著相关性,砂浆的自收缩和干燥收缩是影响开裂的主要因素。     

粗骨料对混凝土单轴抗压强度及破坏特征影响的数值分析

采用PFC2D离散元首先生成混凝土基质,然后利用Clump技术形成不同形态的粗骨料混凝土数值模型,分析研究粗骨料含量和形态对混凝土强度和损伤演化的影响。获得以下主要结论：随着粗骨料含量的增加混凝土单轴抗压强度增大,相同含量下三角形粗骨料混凝土的强度最高,其次是五边形粗骨料,圆形粗骨料混凝土的强度最低;粗骨料含量小于50％时,随粗骨料含量的增大弹性模量增速较小,而大于50％时增速较大;混凝土破坏的初始微损伤主要集中在结合面处,圆形粗骨料微裂隙沿着结合面切线方向发展,主控破裂面沿着相邻粗骨料的公切线方向发展,多边形粗骨料微裂隙沿边延伸进入基质,主控破裂面为相邻粗骨料角度基本一致边的微裂隙沿边发展贯通而成。混凝土的损伤演化分为：微损伤的随机分布、微裂隙形成和微裂隙贯通形成破裂面三个阶段,粗骨料含量低时混凝土损伤比较集中,形成明显的主控破裂面,粗骨料含量高时内部损伤严重形成网状损伤裂隙。