Plastic shrinkage of mortars with shrinkage reducing admixture and lightweight aggregates studied by neutron tomography

Water transport in fresh, highly permeable concrete and rapid water evaporation from the concrete surface during the first few hours after placement are the key parameters influencing plastic shrinkage cracking. In this work, neutron tomography was used to determine both the water loss from the concrete surface due to evaporation and the redistribution of fluid that occurs in fresh mortars exposed to external drying. In addition to the reference mortar with a water to cement ratio (w/c) of 0.30, a mortar with the addition of pre-wetted lightweight aggregates (LWA) and a mortar with a shrinkage reducing admixture (SRA) were tested. The addition of SRA reduced the evaporation rate from the mortar at the initial stages of drying and reduced the total water loss. The pre-wetted LWA released a large part of the absorbed water as a consequence of capillary pressure developing in the fresh mortar due to evaporation.     

矿物外加剂及测试方法对硬化水泥浆体自收缩值的影响

分别采用两种不同试验方法对比研究了单掺粉煤灰、硅灰和矿渣微粉这三种常用的矿物外加剂(即水泥混合材)对水泥浆体早期自收缩的影响。研究结果表明,单掺粉煤灰、硅灰和矿渣微粉后,硬化水泥浆体的自收缩值分别随其掺量的增加而减小、增大和增大。本文分析研究了这三种常用的矿物外加剂对硬化水泥浆体自收缩产生影响的原因,并对比分析了本试验采用的两种试验方法,发现波纹管法更能准确地测量水泥浆体的早期自收缩,参照现行行业标准JC/T313-1982测试无法正确地反映硬化水泥浆体更早期的收缩现象。     

Transport properties of water and glycol in an ultra high performance fiber reinforced concrete (UHPFRC) under high tensile deformation

Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability. Those characteristics make them very attractive for the rehabilitation of existing structures and the conception of new structures. To define the range of admissible tensile deformation in those materials, the influence of imposed tensile deformation and subsequent cracking on permeability and absorption was studied. The transport properties of water and glycol were assessed in order to estimate the effect of the interaction of water with a specific UHPFRC. The experimental results demonstrate that permeability and absorption increase steadily until a residual tensile deformation of 0.13% is reached in the material, then water seeping rises distinctly. During experiments, the interaction of water with the UHPFRC decreases by 1 to 3 orders of magnitude the permeability and reduces absorption by approximately 50 to 85%. Test results reveal the high capability of the material to seal cracks and improve its water-tightness with time.     

Effect of bagasse fiber on the flexural properties of biodegradable composites

Flexural modulus of the press‐molding composites made from bagasse fiber and biodegradable resin was investigated by experiment and numerical prediction with Cox's model that incorporates the compression ratio of the bagasse fiber in the cross section. The effect of the volume fraction of bagasse fiber and its length on the flexural modulus was examined. Up to 65% volume fraction in the experiment, the flexural modulus increased with increase of the volume fraction of the bagasse fiber. The numerical prediction was in good agreement with the experimental result. Above 65% volume fraction, however, the flexural modulus decreased in the experiment, while the prediction increased. It seemed that the biodegradable resin was insufficient to cover all the surface of bagasse fiber in the composite. Moreover, the decrease of the flexural modulus was found below 3 mm at the fiber length in the experimental and the same trend was shown in the numerical prediction. POLYM. COMPOS. 26:689–694, 2005. © 2005 Society of Plastics Engineers     

Early age creep and relaxation of UHPFRC under low to high tensile stresses

This paper presents experimental results on creep and relaxation of UHPFRC at early age (3–8 days), under low to high tensile stresses (1.2, 2.7, 5.4, and 8.1 MPa—estimated as 13, 30, 60, and 90% of the tensile strength, respectively) up to the strain hardening domain. Various loading programs (single and incremental loading steps) were used to reveal non-linear viscoelasticity. For a very low load level of 1.2 MPa, an unexpected important decrease of the creep response was observed. For the 2.7 MPa load level, the viscoelastic responses obtained for creep and relaxation were in the same range. Non-ageing linear viscoelastic models were used to analyze and discuss the obtained results. Non-linear viscoelasticity was observed in several test series and possible underlying mechanisms were discussed. The hypothesis of shrinkage being the same for free and loaded specimen remains an open question for the analysis of test results.     

Axial compressive strength of carbon fiber with tensile strength distribution

Measuring the fiber lengths of the broken pieces and estimating the mean tensile strength from the length just before the final fragment length in tension, efforts were made to estimate the axial compressive strengths of carbon fibers when the tensile strength varies with the length. The estimated compressive strength of carbon fibers decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compressive force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compressing force in the temperature range from room temperature to 80°C. The real compressive strength of the fibers, determined by extrapolating this straight line until the radial compressing force is zero, is about 20% higher than the compressive strength estimated by assuming that the tensile strength is uniform. It is approximately 10–50% of tensile strength. A linear relationship between the fiber axial compressive strength and compressive strength of the unidirectional composites is found. The experimental values agree with the values calculated by the rule of mixtures.     

加强筋结构对离子膜内场分布特性的影响

为研究加强筋结构对离子膜内场分布特性的影响,采用多孔介质渗流模型、稀物质传递及二次电流分布模型,对电场条件下含加强筋的离子膜内物质迁移过程进行了数值模拟,得到了物质在膜内的对流速度、浓度及电流密度分布,考察了加强筋形状、间距及成网方式对场分布特性的影响。利用拍照法和图像处理技术对电解后膜表面离子浓度进行了测量,实验结果与模拟结果吻合良好。结果表明,在加强筋所围成的空隙中心处对流速度最大;越靠近加强筋,速度越低,浓度和电流密度越大;采用正交型加强筋,膜内具有较高的对流速度和电流密度;加强筋间距越大,电流密度越低,分布越均匀;采用椭圆形长轴迎流的加强筋时,膜内具有最高的电流密度,但其电流密度分布最不均匀。     

Effect of fiber misorientation on the tensile strength of compression molded continuous fiber composites

Resin cross-flow during compression molding of unidirectional sheet molding compound composites, such as CSMC and XMC, may cause severe misorientation of the continuous fibers in the outer layers. The extent of fiber misorientation depends on the type of molding compound, the length of cross-flow, and the location of the charge in the mold. The tensile strength is reduced in the direction of cross-flow with decreasing mold surface coverage. However, since severe fiber misorientation is generally restricted to the outer layers, increasing the number of plies improves the tensile strength to the level observed with little or no misorientation.     

Influence of limestone filler and viscosity-modifying admixture on the shrinkage of self-compacting concrete

The aim of this experimental work is to study shrinkage evolution with age in self-compacting concretes (SCC) made with w/c = 0.6 and different limestone filler and viscosity-modifying admixture (VMA) contents. The results show that limestone fillers speed up hydration reactions and provide a finer porous structure. As a result, when specimens are hardened under water, SCC made with limestone fillers tends to shrink, since the water only penetrates the outer layers of the specimen, while the interior is subject to self-desiccation. If the concrete contains substantial air content (3.2%) the water finds it easier to penetrate and the concrete swells. When hardening takes place in the open air, autogenous shrinkage in SCC is lower than in normally-vibrated concretes (NVC) and the higher the fines content, the lower the shrinkage. This is more than likely due to the use of limestone filler as addition, finer porous structure and higher amount of absorbed water from the aggregate which compensates for the auto-desiccation of the concrete. On the whole, drying shrinkage in SCC is greater than in NVC. However, when filler is replaced by VMA the porous structure is coarser and shrinkage is reduced by 33%, thanks to the lower capillary pressure. This means that due to the reduction in autogenous and drying shrinkage, SCC made solely with VMA presents 7.7% lower total shrinkage than NVC. In SCC made with limestone filler, total shrinkage is only slightly higher than in NVC, the differences being under 9.2%.     

聚乙二醇对聚甲醛结晶行为及其纤维性能的影响

在聚甲醛(POM)中添加少量的聚乙二醇(PEG),熔融纺丝制得POM/PEG共混纤维。通过光学解偏振法、偏光显微镜观察(PLM)研究了POM与不同相对分子质量的聚乙二醇(PEG)的不同比例共混物的等温结晶行为,通过单纤强力测试仪、差示扫描量热(DSC)测试分析了共混纤维的力学性能。结果表明:PEG的加入能够加快POM的结晶,结晶温度升高,晶粒生长尺寸减小;随着PEG加入比例的增加,纤维断裂强度先降低后升高;相对分子质量为2 000的PEG与POM质量比为99/1时,纤维断裂强度比纯POM纤维提高约7%。     

M_η对PS静电纺丝及超细纤维膜拉伸性能的影响

采用乳液聚合制备了不同相对分子质量的聚苯乙烯(PS),研究了相对分子质量对静电纺丝过程和超细纤维膜形态和性能的影响。使用乌氏粘度计测定PS的粘均相对分子质量(Mη),使用高速摄相机观察射流分裂行为,通过扫描电镜,万能拉伸机对制得的PS超细纤维膜形态和力学性能进行分析测试。结果表明:聚合所得不同Mη的PS都具有可纺性,超细纤维形态优良,Mη增大,稳定射流越长,平均直径从0.3μm增至3.0μm。纤维膜断裂强度也随Mη增大而增大。     

Effect of fiber size on structural and tensile properties of electrospun polyvinylidene fluoride fibers

We use electrospinning to obtain polyvinylidene fluoride (PVDF) fibers and demonstrate simultaneous improvements in β‐crystal microstructure and in tensile properties of fibers with reduction of their diameter. PVDF fibers with average diameters ranging from 70 to 400 nm are obtained by controlling the concentration of the polymer in the electrospinning solution. The amount of β‐crystals present is found to be greater for finer diameter fibers, yielding a maximum β‐phase fraction of 0.86 in the 70‐nm fibers. Moreover, the deformation behavior of the fibers reveals that the tensile modulus and strength improve with reductions in fiber size. Sharp increases in tensile properties are demonstrated when the size of the fibers is reduced below 175 nm. We attribute the enhanced concentration of β‐crystals and the tensile behavior of finer diameter fibers to the extensional forces experienced by the material during electrospinning. POLYM. ENG. SCI., 55:1812–1817, 2015. © 2014 Society of Plastics Engineers     

Transverse tensile characteristics of fiber composites with flexible resins: Theory and test results

A theoretical and experimental investigation has been conducted of the transverse tensile properties of flexible-resin/ glass-fiber composite lamina. The objective is to increase the elongation at failure of a lamina in a state of transverse tension so that when a lamina is used in a laminate configuration, it does not suffer premature failure (e.g., failure at a lower strain level than that for a lamina in fiber direction tension). We show that the mode of failure in the transverse tensile lamina differs greatly from that in the neat resin. The failure mode in the lamina form is found to be brittle fracture. From this, it follows that the controlling neat resin property is the low elongation modulus rather than the elongation at failure. Significant improvements in the transverse tensile strain at failure were obtained for the flexible-resin/glass-fiber systems studied.     

Effect of fiber reinforcement on the tensile, fracture and thermal properties of syntactic foam

This paper examines the effect of the fiber content and fiber length on tensile, fracture and thermal properties of syntactic foam. Results showed that a hybrid structure demonstrates a significant increase in the ultimate tensile strength, σ_uts, and Young's modulus, E, with increasing fiber loading. Interestingly, the fracture toughness, K_Ic, and energy release rate, G_Ic, increased by 95% and 90%, respectively, upon introduction of 3 wt% short carbon fibers in syntactic foam, indicating the potent toughening potential for short carbon fibers in syntactic foam systems. SEM and OM studies identified the presence of several toughening mechanisms. An estimate of the contribution from each toughening mechanism by composite theory and fractography revealed that the specific energy required to create new surfaces was enhanced by the presence of fibers and was the main contributor to the toughness of the short fiber reinforced syntactic foam.     

高收缩聚酯合成及其纤维的收缩性能

在聚酯中添加间苯二甲酸(IPA)和具有侧链结构的新戊二醇(DTG),制成了高收缩聚酯切片及纤 维。研究了IPA,DTG的添加量对切片的热性能和纤维的热收缩率及经时稳定性的影响。研究表明,随着 IPA、DTG加入量增加,共聚酯的结晶性能降低,而纤维的收缩率则呈规律提高,可达到50.0％以上,并具有 较好的经时稳定性。     

Binder fiber distribution and tensile properties of thermally point bonded cotton‐based nonwovens

Cotton‐based nonwovens are generally produced by carding and then bonding. One of the most important characteristics of nonwoven materials is the uniformity of their structure and properties. However, the carded webs always have irregularities caused by processing and material variables. The binder fiber distribution in carded cotton‐based nonwoven fabrics was analyzed based on the crystallization behavior of one of the components of the binder fibers by DSC. The effects of process parameters, such as bonding temperature and binder fiber component, on the uniformity were discussed in detail in this article. Also, the relationship of binder fiber distribution and the strip tensile property and single‐bond tensile strength were investigated. The results showed that if the binder fibers were not well distributed in the fabric, it would be hard to get the same trend of temperature effect on tensile property for the strip and single‐bond tests. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3148–3155, 2004     

Size effect on flexural, splitting tensile, and torsional strengths of high-strength concrete

This paper presents the results of an investigation into the size effect on flexural, splitting tensile, and torsional strengths of high-strength concrete (HSC) with normal aggregate (crushed limestone) and lightweight aggregate (sintered fly ash). The Bazant’s size effect law gives a very good fit to the flexural strengths of both normal and lightweight aggregate HSC measured from beams of different sizes. As observed in the size effect curve, the fracture behavior of the lightweight HSC seems more brittle than that of the normal HSC. Linear elastic fracture mechanics may still be less applicable to HSC in the normal size range than nonlinear fracture mechanics. A reverse size effect is observed in the prism splitting tensile strengths of both normal and lightweight HSC and possible mechanisms of the reverse size effect are discussed. The torsional strength of the lightweight HSC appears to have a stronger size dependency than that of the normal HSC.