混杂纤维增强高强混凝土性能研究

为研究钢纤维、聚乙烯醇纤维混杂比例对高强混凝土性能的影响,通过合理设计坍落度试验、力学强度试验、收缩试验、抗裂试验、抗氯离子侵蚀试验,对比评价了纤维混杂比例对高强混凝土工作性、抗折强度、收缩性、抗裂性能以及氯离子渗透系数的影响。结果表明,钢纤维和聚乙烯醇纤维降低了新拌混合物的工作性。与单掺纤维相比,混杂纤维对高强混凝土力学性能改善效果不明显,但可明显改善混凝土抗裂性能,开裂面积抑制率最大为95.8%,同时能使高强混凝土收缩率和氯离子分别降低27.7%和66.5%,明显提高高强混凝土的耐久性能。通过扫描电镜试验分析探讨了纤维增强混凝土的作用机理,结果表明混杂纤维对基体内部结构的改善实现了对混凝土宏观性能的提升,最终推荐采用0.75%(体积分数)钢纤维和0.25%(体积分数)聚乙烯醇纤维。     

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.     

The effects of gradation and admixture on the pumice lightweight aggregate concrete

The usage of lightweight concrete, which has some advantage over ordinary concrete, has increased to a remarkable level in recent years. Many researchers have investigated the possible uses of lightweight concrete in terms of its strength, density and other mechanical and physical properties. The desired quality for lightweight concrete can be obtained through the proper selection of admixtures and proper grading of the lightweight aggregate.In this article, an experimental investigation on the production of moderate-strength lightweight concrete with pumice, according to the ACI standard, is presented. The gradation curves' (which fall within A16-C16 gradation curves, Turkish Standard Code, TS706) performances were investigated in terms of strength and density. The addition of superplasticizer and air-entraining admixtures improved the strength-to-density ratio of the hardened concrete and the workability of fresh concrete. As a result of this study, lightweight concrete blocks having a minimum compressive strength of 6.56 N/mm² and a density of 1300 kg/m³ were obtained.     

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.     

Investigation of brucite-fiber-reinforced concrete

Laboratory experiments were made on the brucite-fiber-reinforced concrete composites. Effects of brucite fiber grades and the dosage on flexural strength, compressive strength, impact strength, sulfate corrosion resistance and the slump, cohesiveness, as well as the water retentiveness were also investigated. Different water reducers were tested. The particle-size characteristics of brucite fibers, the densities of the concrete, and the viscosities of the fiber/water-reducer suspensions were also measured. Results show that proper addition of brucite fibers in concrete can improve the mechanical properties, especially the flexural strength. In the test, the optimum quantity was about 0.5 wt.% of concrete. With the dosage increase of brucite fibers in concrete, the fluidity and the density of the concrete decrease. The performance of the concrete strengths is the collective interactions of the fiber reinforcement and the density reduction. The aspect ratio and the surface area of brucite fibers are the important affecting factors to the workability and the mechanical properties of the fiber concrete. Larger aspect ratios and smaller surface areas benefit the reinforcement. Water reducers with lower fiber suspension viscosities are favorable in improving the workability and strengths of the brucite fiber concrete.     

掺入塑钢纤维的轻骨料混凝土与钢筋间的力学性能研究

为了分析塑钢轻质骨料加入到混凝土中的力学以及粘结性能,基于试验研究并分析了影响混凝土试件力学以及粘结性能的影响因素,实验结果表明：加入不同比例的塑钢纤维混凝土试件的力学以及粘结性能均表现出了不同的变化特性,混凝土及砂浆的用水量、砂率的不同,会导致塑钢纤维对混凝土的增强效果差异较大;素混凝土的强度较高时,混凝土试件的粘结性能越高,并且其钢筋产生的位移大大降低;加入塑钢纤维的混凝土试件的力学以及粘结性能均会随着轻质骨料筒压的降低而降低。     

Properties of volcanic pumice based cement and lightweight concrete

The results of investigations on the suitability of using volcanic pumice (VP) as cement replacement material and as coarse aggregate in lightweight concrete production are reported. Tests were conducted on cement by replacing 0% to 25% of cement by weight and on concrete by replacing 0% to 100% of coarse aggregate by volume. The physical and chemical properties of VP are critically reviewed to evaluate the possible influence on both fresh and hardened state of cement and concrete. The standard tests on different Portland cement-volcanic pumice powder (VPP) mixes provided encouraging results and showed good potential of manufacturing Portland volcanic pumice cement (PVPC) with higher setting time using up to 15% of VPP. The properties of volcanic pumice concrete (VPC) using different percentages of volcanic pumice aggregate (VPA) were evaluated by conducting comprehensive series of tests on workability, strength, drying shrinkage, surface absorption and water permeability. It is concluded that the VPC has sufficient strength and adequate density to be accepted as structural lightweight concrete. However, compared to control concrete, the VPC has lower modulus of elasticity and has more permeability and initial surface absorption.     

Parameter-study on the influence of steel fibers and coarse aggregate content on the fresh properties of self-compacting concrete

Self-compacting concrete (SCC) offers several economic and technical benefits; the use of steel fibers extends its possibilities. Steel fibers bridge cracks, retard their propagation, and improve several characteristics and properties of the concrete. Fibers are known to significantly affect the workability of concrete. Therefore, an investigation was performed to compare the properties of plain SCC and SCC reinforced with steel fibers. Two mixtures of SCC with different aggregate contents were used as reference. Each of the concretes was tested with four types of steel fibers at different contents in order to answer the question to what extent the workability of SCC is influenced. The slump flow, a fiber funnel and the J-ring test were used to evaluate the material characteristics of the fresh concrete. This paper discusses the suitability of the applied test methods and the effect of the coarse aggregate content, the content and type of steel fibers on the workability of SCC.     

Optimum mix parameters of high-strength self-compacting concrete with ultrapulverized fly ash

This paper presents the preparation technology of high-strength self-compacting concrete (SCC) containing ultrapulverized fly ash (UPFA) and superplasticizer (SP). After selecting the parameters of mix proportions, a SCC with good workability, high mechanical properties, and high durability is developed. The experimental results indicate that the fresh mixture has low slump loss. The compressive strength of concrete reaches 80 MPa, and the concrete presents low permeability, good freeze-thaw resistance, and low drying shrinkage.     

碳化作用下轻骨料混凝土干缩变形及影响规律

采用碳化试验和干燥收缩试验相结合,研究了轻骨料混凝土在碳化作用下的干缩变形发展规律及其影响因素。结果表明,加速碳化作用下,轻骨料混凝土的干缩变形显著增长,而随着矿物掺合料的掺入,干缩率得到一定程度的抑制,其中掺超细粉煤灰轻骨料混凝土碳化干缩率最小;骨料预湿完全条件下轻骨料混凝土水胶比越大,碳化干缩变形越大;轻骨料混凝土在高浓度CO2加速碳化作用下干缩变形显著高于自然碳化混凝土;而在相对湿度50%条件下碳化时,其干缩率要大于其它湿度条件下的试样;另外,轻骨料预湿后混凝土碳化干缩率要低于未预湿轻骨料混凝土。因此,在实际工程中,选择合适的矿物掺合料、水胶比、养护条件以及骨料的预湿工艺对轻骨料混凝土的减缩防裂具有较好的效果。     

Properties and possible recycling of solid waste from ready-mix concrete

Due to more restrictive environment legislation, many ready-mix concrete suppliers are often obligated to find adequate means of handling, recycling and marketing their waste solids. This paper discusses the nature, properties and possible uses of the fine fraction of the waste slurry. Chemical, physical and mechanical properties of the slurry solids are presented. The influence of addition of waste solids to mortar and concrete made with three brands of cements are examined. The effects of waste solids on compressive and tensile strength, setting time, shrinkage and sulfate expansion and workability were studied.     

Toughness enhancement in steel fiber reinforced concrete through fiber hybridization

Crimped steel fibers with large diameters are often used in concrete as reinforcement. Such large diameter fibers are inexpensive, disperse easily and do not unduly reduce the workability of concrete. However, due to their large diameters, such fibers also tend to be inefficient and the toughness of the resulting fiber reinforced concrete (FRC) tends to be low. An experimental program was carried out to investigate if the toughness of FRC with large diameter crimped fibers can be enhanced by hybridization with smaller diameter crimped fibers while maintaining workability, fiber dispersability and low cost. The results show that such hybridization indeed is a promising concept and replacing a portion of the large diameters crimped fibers with smaller diameter crimped fibers can significantly enhance toughness. The results also suggest, however, that such hybrid FRCs fail to reach the toughness levels demonstrated by the smaller diameter fibers alone.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Mechanical and acoustic absorption properties of lightweight fly ash/slag-based geopolymer concrete with various aggregates

Acoustic absorption and thermal insulation play a key role in modern buildings to make living comfortable and energy-saving. This paper aims to study the workability, physical and mechanical properties, thermal conductivity, and acoustic absorption of modified geopolymer concrete (GPC) with various types of lightweight aggregates (LWA) such as extruded polystyrene foam beads waste (EPS), vermiculite, or lightweight expanded clay aggregate (LECA). The mixtures of geopolymer concrete have been modified by substituting for the ordinary aggregates (dolomite) by volume with various ratios of 0, 25, 50, 75, and 100% for each type of LWA. Besides, the mechanisms of specimens were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and mapping. The results illustrated that the compressive strength values range between 8.5 and 47.50 MPa. The hardened density of concrete was between 1500 and 2450 kg/m³, and thermal conductivity was between 0.45 and 1.16 W/m.K. Geopolymer concrete was considered an acoustic absorption and thermally insulating material. Geopolymer concrete was considered an acoustic absorption and thermally insulating material. EPS, vermiculite, and LECA will be beneficial for applications in lightweight geopolymer concrete due to their capability to reduce weight and excellent thermal conductivity, and the property of improving acoustic absorption. The mechanical results indicated that 25% LECA was the best compared with the ratios of other LWA and gained 35.0, 2.7, and 4.3 MPa of compressive, splitting tensile and flexural strength, respectively. It had positive workability; the thermal conductivity was 1.1 W/m.K, and hardened density was decreased to 10% compared to the control. In addition, LECA is considered the superior and suitable material for acoustic absorption compared with other aggregates.     

改性聚丙烯纤维砂浆和混凝土的性能试验

试验采用P.P.Kraai提出的砂浆及混凝土干燥收缩裂缝测试方法、混凝土力学性能试验、抗冻等耐久性能试验方法,研究了改性聚丙烯纤维对砂浆和混凝土性能的影响。结果表明,在混凝土中掺入一定量的改性聚丙烯纤维,混凝土的抗压强度略有下降;纤维在混凝土中形成的乱向支撑体系,产生了有效的增强效果,减少了裂缝的产生,提高了混凝土的抗折、抗拉强度,从而改善了混凝土抗裂、抗渗、抗冲击和抗冻等性能。     

轻质骨料对泡沫混凝土性能的影响及其内养护机制

为降低泡沫混凝土干燥收缩,采用轻质骨料(页岩陶粒及粉煤灰陶粒)制备泡沫混凝土,研究了轻质骨料对泡沫混凝土抗压强度、干燥收缩、内部湿度及孔结构等的影响,并分析了轻质骨料的内养护机制。结果表明,轻质骨料能够调节泡沫混凝土的内部湿度,约束基体的变形,降低泡沫混凝土的干燥收缩。而轻质骨料也会在泡沫混凝土中引入缺陷,导致其强度降低。轻质骨料中大于100 nm的墨水瓶孔是内养护效果的控制因素,大于100 nm的墨水瓶孔数量越多,轻质骨料释水能力越强,内养护效果越好,制备的泡沫混凝土干燥收缩越小。     

膨胀剂和减缩剂对预制箱梁混凝土的防裂效果研究

测试C50箱梁混凝土的力学、抗裂、变形、热学性能,采用B4Cast软件仿真分析构件混凝土的温度、应力发展规律和开裂趋势,并研究膨胀剂、减缩剂对箱梁混凝土的防裂效果。结果表明:C50箱梁混凝土抗裂性较差,早期平板总开裂面积达310 mm²/m²,圆环开裂时间约为52 h;箱梁混凝土与外部环境最大温差为16~18 ℃,在翼板两侧和底板两侧拐角处混凝土表层拉应力达到最大,分别约为0.8 MPa、0.9 MPa。膨胀剂、减缩剂对C50混凝土拌合物工作性、力学性能、热学性能影响不大,仅单掺减缩剂会使混凝土力学性能略有降低。膨胀剂、减缩剂的掺入,可降低C50混凝土平板总开裂面积68%~95%,延后圆环开裂时间105.5~227.5 h;可使自生体积收缩变形减小63%~78%,干缩减小38%。膨胀剂与减缩剂复掺,可削减箱梁混凝土温峰达4 ℃,降低最大拉应力52%,混凝土抗裂安全系数可达3.05,开裂风险明显下降。     

Effect of fiber factor on the workability and mechanical properties of polyethylene fiber-reinforced high toughness geopolymers

In this work, we studied the influence of fiber factor (F) on the workability, mechanical properties, and internal defects of polyethylene (PE) fiber-reinforced high toughness geopolymers, and two specific fiber parameters, the critical fiber factor (F_c = 150) and dense fiber factor (F_d = 600), were proposed. Subsequently, the workability and hardening properties of the 26 PE fiber-reinforced geopolymer (FRG) groups were divided into three grades. When F was lower than F_c, the fibers had an insignificant effect on the workability and hardening properties of the material. However, when F was higher than F_d, many fibers agglomerated and deteriorated the workability and mechanical properties. Thus, when F was between F_c and F_d, satisfactory workability and excellent mechanical properties were simultaneously obtained. Furthermore, with similar fiber stiffness coefficients, the relative yield stress of FRG was similar to the fiber-reinforced Portland cement-based composites. Moreover, the flexural and compressive strength values of the PE FRG were both higher than the PE fiber-reinforced Portland cement-based composites. As a result, this work provides relevant conclusions for the theoretical reference and design of FRG with satisfactory workability and mechanical behaviors for concrete engineering.     

Effect of the length and the volume fraction of wavy steel fibers on the behavior of self-compacting concrete

Self-compacting concrete (SCC) is a highly workable concrete that fills formwork under its own weight (without any vibration or impact). It also passes easily through small spaces between reinforcement bars. The inclusion of fibers in such concrete limits the concrete shrinkage cracks at early age and enhances some of its properties. However, fibers may affect the flow characteristics of SCC. In this paper, three wavy steel fibers (SF) of different lengths, 35, 40, and 50 ± 2 mm with six different volume fractions (V_f) of 0.3, 0.5, 0.8, 1, 1.2, and 1.4% were used in SCC. The experimental results showed that the addition of SF with higher V_f content and longer length decreases the workability of SCC, reduces its passing ability and increases the possibility of blockage. Mechanical performances of concrete in terms of flexural strength and elasticity modulus were improved, where the slightly compressive strength decreased with an increase in V_f content of SF.     

石粉含量对机制砂混凝土性能的影响

试验研究了石粉含量对机制砂混凝土和易性、力学性能、干缩性能和抗渗性能的影响.结果表明,机制砂中石粉含量10％～15％能改善混凝土的和易性和力学性能,减小混凝土的干缩,提高混凝土的抗渗性.