养护条件对水泥砂浆干缩性能的影响

实验研究了养护条件(包括干燥前养护时间、干燥前养护温度和干燥时相对湿度)对砂浆干缩性能的影响。比较了养护时间(1 d,3 d,14 d),养护温度(20℃,60℃),相对湿度(30%,43%,75%)等对水泥砂浆干缩性能的影响。研究结果表明:延长干燥前养护时间,各砂浆干缩率增大;提高干缩前养护温度,不掺矿物外加剂的水泥砂浆干缩率减小,而掺矿物外加剂的水泥砂浆干缩率增大;干燥时相对湿度增加,水泥砂浆干缩率降低。     

Autogenous relative humidity change and autogenous shrinkage of high-performance cement pastes

In this paper, the effects of water to cementitious material ratio (w/cm), silica fume (SF) and ground blast-furnace slag (GBFS) on autogenous relative humidity (RH) change and autogenous shrinkage (AS) of high-performance cement pastes were studied. The mechanism of self-desiccation caused by mineral admixture and reduction of w/cm were studied by the parameters of mineral admixture self-desiccation-effect coefficient k and efficient w/cm r_e proposed. Furthermore, the relationship between autogenous RH and AS of high-performance paste was established. The results indicate that w/cm is a chief factor that affects autogenous RH change and AS of cement pastes. The lower the w/cm of paste is, the more reduction the autogenous RH and the increment of AS are. SF increases autogenous RH reduction and AS increment of cement paste at early ages, and GBFS increases autogenous RH reduction and AS increment at later ages. The effect of mineral admixtures on autogenous RH change of paste resulting from self-desiccation can be reflected effectively by the nonlinear equation with the parameters of k and r_e. There exists a good linear correlation between autogenous RH change and AS of cement pastes.     

Drying characteristics and modeling of yam slices under different relative humidity conditions

The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification from an initial RH of 40%. Using the developed Bi-Di correlation, the estimated Biot number, effective moisture diffusivity, and mass transfer coefficient ranged from 0.1024 to 0.1182, 1.1133 × 10^?10 to 8.8144 × 10^?9 m²/s, and 1.8992 × 10^?9 to 1.7364 × 10^?7 m/s, respectively. A rather high correlation coefficient of determination (R² between 0.9871 and 0.9971) was determined between the experimental and predicted moisture contents. The present findings contribute to a better understanding of the effect of relative humidity on drying characteristics. The developed Bi-Di correlation provided a new method to determine the effective diffusivity of moisture in drying.     

温度和相对湿度对褐煤干燥动力学特性的影响

为确定褐煤干燥最佳工艺参数,在恒温恒湿热风干燥实验台上进行了温度为50～90℃,相对湿度为10％～30％条件下的褐煤颗粒热风干燥实验.将实验数据与9种经验、半经验模型进行拟合,利用相关系数、方差、均方根误差3种统计学参数对不同模型进行评价.结果表明:Page模型相比其他模型更适合用于描述干燥过程中褐煤颗粒水分随时间的变...     

Effect of relative humidity on the oxidative and physical stability of encapsulated milk fat

Milk fat was used in this work as a model to study the effects of humidity and physical properties on lipid oxidation. Although milk fat is considered a relatively stable fat because of its low content of unsaturated FA, it can oxidize significantly under certain conditions, as observed, for example, in the case, of dairy-based powders. Humidity and physical properties have a profound influence on the oxidative stability of powders, containing fat, and these factors affect the surface and encapsulated fractions of the fat differently. To examine these effects, encapsulated milk fat powders were stored under conditions of controlled relative humidity. Oxidation of the encapsulated fat as assessed by measurements of PV, losses of FA, and hexanal production increased, with increasing relative humidity (RH). At higher RH, moisture penetrates into the hydrophilic wall, interacting with and plasticizing the components, thereby making them less effective as moisture and oxygen barriers. Total oxidation of the powders was strongly influenced by the extent of oxidation in the encapsulated fraction (>98% of total lipids) although the surface fat fraction was oxidized more rapidly. Better protection against oxidation was obtained when fats were encapsulated and stored at 14 and 44% RH than at 52% RH.     

A differential shrinkage approach for evaluating particle formation behavior during drying of sucrose,lactose, mannitol,skim milk,and other solid-containing droplets

Particle formation process during drying of solid-containing droplets exerts profound influence on the property and quality of dried particles. A differential shrinkage approach is proposed to describe the particle formation behavior of different materials, by evaluating how the droplet shrinkage kinetics of the given material(s) deviates from the ideal shrinkage. Sucrose was selected as the reference material to establish ideal shrinkage kinetics, as its shrinkage at 10, 30, and 50?wt.% initial concentrations well followed the ideal shrinkage at all temperatures tested (70, 90, and 110?°C). Comparing the differential shrinkage kinetics of sucrose, lactose, and mannitol showed that mannitol has a strong crust-forming tendency, which could not be explained by the difference in the solubility of the three materials. By establishing a differential shrinkage curve for each material, this approach offers a straightforward and powerful method to evaluate the particle formation property of the material at various initial concentrations.     

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.     

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。     

SAP内养生路面混凝土收缩性能及力学性能研究

超吸水性树脂(SAP)作为一种内养生材料,对混凝土力学性能、收缩性能等均具有显著改善作用,在公路混凝土路面中具有十分重要的应用价值.本文通过对混凝土力学性能、收缩变形及内部相对湿度进行研究,并通过SEM对SAP内养生混凝土内部微观形貌、裂缝特征进行分析.结果表明:SAP的加入能显著降低混凝土毛细孔隙间的范德华力,抑制混...     

不同相对湿度下亲疏水纳米纤维膜空气过滤性能实验研究

利用静电纺丝法制备了表面静态接触角为23.6°的具有亲水功能的PAN/PVP复合纳米纤维膜、接触角为81.2°的PAN纳米纤维膜、接触角为131.9°的具有疏水功能的PAN/PVDF复合纳米纤维膜。利用自行搭建的空气过滤实验台,在40%、55%、70%三种相对湿度下对三种纳米纤维膜进行空气过滤实验,对纳米纤维膜的过滤效率、阻力损失及品质因子进行分析。结果表明：三种纳米纤维膜的过滤效率随着相对湿度的增大而升高,PAN/PVP膜和PAN膜的阻力损失随着相对湿度的增大而增加,PAN/PVDF的阻力损失随着相对湿度的增大而减小;PAN/PVP膜和PAN膜的品质因子随着相对湿度的增大而减小,PAN/PVDF膜的品质因子随着相对湿度的增大而增大,湿度越大,PAN/PVDF纳米纤维膜的过滤性能越显著。     

Effects of poly(vinyl acetate) and poly(vinyl chloride‐ co‐vinyl acetate) low‐profile additives on properties of cured unsaturated polyester resins. I. Volume shrinkage characteristics and internal pigmentability

Three series of self‐synthesized poly(vinyl acetate)‐based low‐profile additives (LPAs) with different chemical structures and molecular weights, including poly(vinyl acetate), poly(vinyl chloride‐co‐vinyl acetate), and poly(vinyl chloride‐co‐vinyl acetate‐co‐maleic anhydride), were studied. Their effects on the volume shrinkage characteristics and internal pigmentability for low‐shrink unsaturated polyester (UP) resins during cure were investigated. The experimental results were examined with an integrated approach involving measurements of the static phase characteristics of the ternary styrene/UP/LPA system, the reaction kinetics, the cured sample morphology, and microvoid formation by using differential scanning calorimetry, scanning electron microscopy, optical microscopy, and image analysis. Based on the Takayanagi mechanical model, factors leading to both good volume shrinkage control and acceptable internal pigmentability for the molded parts were explored. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3336–3346, 2003     

Effect of relative humidity conditions on responsiveness of European corn borer (Ostrinia nubilalis) males to female sex pheromone in a wind tunnel

The responsiveness pf 3-day-old European corn borer males to three concentrations of 973 ZE 11-tetradecenyl acetate, the female sex pheromone, was studied over a range of relative humidities (43–100%) in a wind tunnel. The proportion of males taking flight and reaching the source decreased, while the proportion exhibiting in-flight arrestment of upwind progress increased under high humidity conditions at all three concentrations of pheromone tested. The relationships between relative humidity and these behaviors were best described by polynomial equations.     

Prediction of cracking within early-age concrete due to thermal, drying and creep behavior

To predict potential early-age cracking after concrete placing, a numerical simulation procedure has been completed based on a micromechanical model and empirical formulas on the property development of young concrete. The numerical model could account for the effects of hydration, moisture transport and creep. Environmental influences, such as removal of formworks, curing conditions and variations of surrounding temperature and relative humidity, have been investigated. In calculating stress field with age caused by these synthetic physical-mechanical processes, three-dimensional finite element and finite difference (3D-FE-FD) methods are combined together.