Effect of Freeze-thaw Cycles on Bond Strength between Steel Bars and Concrete

The effect of freezing and thawing cycles on mechanical properties of concrete （compressive, splitting tensile strength） was experimentally investigated. According to the pullout test data of three kinds of deformed steel bars, the bond stress-slip curves after freezing and thawing were obtained. The empirical equations of peak bond strength were proposed that the damage accounted for effects of freezing and thawing cycle. Meanwhile, the mechanism of bond deterioration between steel bars and concrete after freezing and thawing cycles was discussed. All these conclusions will be useful to the durability design and reliability calculation of RC structures in cold region.

The Influence of Freeze-Thaw Cycles on Unconfined Compressive Strength of Lignin Fiber-Reinforced Loess

In the seasonal permafrost region with loess distribution,the influence of freeze-thaw cycles on the engineering performance of reinforced loess must be paid attention to.Many studies have shown that the use of fiber materials can improve the engineering performance of soil and its ability to resist freeze-thaw cycles.At the same time,as eco-environmental protection has become the focus,which has been paid more and more attention to,it has become a trend to find new environmentally friendly improved materials that can replace traditional chemical additives.The purpose of this paper uses new environmental-friendly improved materials to reinforce the engineering performance of loess,improve the ability of loess to resist freeze-thaw cycles,and reduce the negative impact on the ecological environment.To reinforce the engineering performance of loess and improve its ability to resist freeze-thaw cycles,lignin fiber is used as a reinforcing material.Through a series of laboratory tests,the unconfined compressive strength(UCS)of lignin fiber-reinforced loess under different freeze-thaw cycles was studied.The effects of lignin fiber content and freeze-thaw cycles on the strength and deformation modulus of loess were analyzed.Combined with the microstructure features,the change mechanism of lignin fiber-reinforced loess strength under freeze-thaw cycles was discussed.The results show that lignin fiber can improve the UCS of loess under freeze-thaw cycles,but the strengthening effect no longer increases with the increase of fiber content.When the fiber content is less than 1%,the UCS growth rate of loess is the fastest under freeze-thaw cycles.And the UCS of loess with 1%fiber content is the most stable under freeze-thaw cycles.The freeze-thaw cycles increase the deformation modulus of loess with 1%fiber content,and its ability to resist deformation is obviously better than loess with 1.5%,2%and 3%fiber content.The fiber content over 1%will weaken the strengthening effect of lignin fiber-reinforced loess,and the optimum fiber content of lignin fiber-reinforced loess under freeze-thaw cycles is 1%.

Influence of particle size and ionic strength on the freeze-thaw stability of emulsions stabilized by whey protein isolate

The influence of particle size and ionic strength on the freeze-thaw(FT) stability of emulsions stabilized by whey protein isolate(WPI) was investigated in this study. The destabilization of emulsions during the FT process could be suppressed in a way by decreasing the particle size of the initial emulsions, which was the result of retarding the coalescence between oil droplets. To further improve the FT stability of emulsions, different amounts of Na Cl were added before emulsification. The emulsions with the ionic strength at 30–50 mmol/L exhibited good FT stability. Notably, the ionic strength in this range would not lower the freezing point of emulsions below the freezing temperature used in this study. Salt addition could improve the structural properties of proteins, which was available to strengthen the rigidity and thickness of interfacial layers, sequentially building up the resistance that the destruction of ice crystals to emulsions. Moreover, stronger flocculation between emulsion droplets could promote the formation of a gel-like network structure dominated by elasticity in the emulsion system, which might effectively inhibit the movement of droplets, and improve the FT stability of emulsions eventually. The result was of great significance for the preparation of emulsion-based foods with improved FT stability.

Determination of long-term strength of frozen loess after numerous freeze-thaw cycles

In order to determine the changing rule of long-term frozen soil strength and elucidate the connection between long-term strength and soil physical properties,frozen loess was subjected to 4,6,8,10,and 50 freeze-thaw cycles,under closed-state conditions in a constant-temperature box.The frozen samples were tested on a spherical template indenter,and the results show that under the effect of repeated freeze-thaw cycles,the long-term strength of frozen loess decreased; changes in the mechanical property indices were highly unstable during the first 10 cycles; the soil strength and density were the greatest at the eighth cycle while the void ratio was the smallest; and after eight cycles all of the indices had less fluctuation and certain rising or falling tendencies.By converting the number of freeze-thaw cycles into elapsed time in the tests,three different forecasting methods of long-term soil strength could be assessed and the soil equivalent cohesive force after 10 years,20 years,or 30 years could be estimated.

Pore evolution and shear characteristics of a soil-rock mixture upon freeze-thaw cycling

The changes in pore structure within soil-rock mixtures under freeze-thaw cycles in cold regions result in strength deterioration,leading to instability and slope failure.However,the existing studies mainly provided qualitative analysis of the changes in pore or strength of soil-rock mixture under freeze-thaw cycles.In contrast,few studies focused on the quantitative evaluation of pore change and the relationship between the freeze-thaw strength deterioration and pore change of soil-rock mixture.This study aims to explore the correlation between the micro-pore evolution characteristics and macro-mechanics of a soil-rock mixture after frequent freeze-thaw cycles during the construction and subsequent operation in a permafrost region.The pore characteristics of remolded soil samples with different rock contents(i.e.,25%,35%,45%,and 55%)subjected to various freeze-thaw cycles(i.e.,0,1,3,6,and 10)were quantitatively analyzed using nuclear magnetic resonance(NMR).Shear tests of soil-rock samples under different normal pressures were carried out simultaneously to explore the correlation between the soil strength changes and pore characteristics.The results indicate that with an increase in the number of freeze-thaw cycles,the cohesion of the soil-rock mixture generally decreases first,then increases,and finally decreases;however,the internal friction angle shows no apparent change.With the increase in rock content,the peak shear strength of the soil-rock mixture rises first and then decreases and peaks when the rock content is at 45%.When the rock content remains constant,as the number of freeze-thaw cycles rises,the shear strength of the sample reaches its peak after three freeze-thaw cycles.Studies have shown that with an increase in freeze-thaw cycles,the medium and large pores develop rapidly,especially for pores with a size of 0.2–20μm.Freeze-thaw cycling affects the internal pores of the soil-rock mixture by altering its skeleton and,therefore,impacts its macro-mechanical characteristics.

激光辐照对竹材抗劈力的影响

竹材内应力大、横向连接弱是造成竹材容易劈裂、影响其使用的重要原因。激光处理热效率高,可调整基体结构,细化晶粒,消除内应力。为研究激光对竹材抗劈力的影响,通过激光辐照毛竹材,分析激光功率、辐照时间、试件厚度及竹材表皮保留情况对毛竹抗劈力的影响,结果表明:毛竹抗劈力较低,小于2.5 N/mm;毛竹抗劈力随激光功率的增大而增大,激光功率为2 100 mW时较未处理毛竹的抗劈力提高13.5%;激光辐照处理30 s时较未处理毛竹抗劈力提高13.5%;随辐照时间延长、试件厚度增加,毛竹抗劈力减小;保留竹青有利于提高竹材的抗劈力;激光辐照对竹黄侧抗劈力的提高效果最好,可使其提高20%。

Effect of Microstructure Refinement on the Strength and Toughness of Low Alloy Martensitic Steel

Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a HalI-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet.

Behavior of High Water-cement Ratio Concrete under Biaxial Compression after Freeze-thaw Cycles

The high water-cement ratio concrete specimens under biaxial compression that completed in a triaxial testing machine were experimentally studied. Strength and deformations of plain concrete specimens in two loading direction under biaxial compression with stress ratio of a=0, 0.25, 0.5, 0.75, 1.0 were obtained after 0, 25, 50 cycles of freeze-thaw. Influences of freeze-thaw cycles and stress ratio on the peak stress and deformation of this point were analyzed according to the experimental results, Based on the test data, the failure criterion expressed in terms of principal stress after different cycles of freeze-thaw, and the failure criterion with consideration of the influence of freeze-thaw cycle and stress ratio were proposed respectively.

Experimental investigation and prediction model for UCS loss of unsaturated sandstones under freeze-thaw action

Sandstone is widely distributed in cold regions and the freeze-thaw deterioration of them has caused many geological engineering disasters.As an important and direct index of frost resistance,the strength loss of sandstones under freeze-thaw actions should be investigated to provide a guidance for the stability assessment of geological engineering.In this research,the UCS(Uniaxial compressive strength)loss of six typical sandstones with different water contents after 0,20,40 and 60 freeze-thaw cycles was measured in the laboratory.The experimental results indicated that the freeze-thaw damage was more serious in sandstones containing high water contents,and the critical saturations for causing a significant loss of UCS under freeze-thaw were 60%-80%for these sandstones.Below this critical saturation,the UCS loss of the sandstones was mainly caused by water weakening rather than freeze-thaw damage.Besides,a developed strength prediction model was proposed by combining the exponential decay function and multiple linear regression method.The initial porosity,elastic modulus and tensile strength of fresh sandstones were a good parameter combination to accurately determine the decay constant in this developed model.The main novelty of this model is that it can accurately and easily estimate the UCS loss of sandstones after any freeze-thaw cycle only using the initial parameters of fresh sandstones,but it does not need to perform freeze-thaw and mechanical strength experiments.This study not only provides an accurate prediction model of UCS under freeze-thaw,but also makes a contribution to better understanding the frost resistance mechanism of sandstones.

Impact of Freeze-Thaw Cycles on Compressive Characteristics of Asphalt Mixture in Cold Regions

Low average temperature, large temperature difference and continual freeze-thaw （F-T） cycles have significant impacts on mechanical property of asphalt pavement. F-T cycles test was applied to illustrate the mixtures＇ compressive characteristics. Exponential model was applied to analyze the variation of compressive characteristics with F-T cycles; Loss ratio model and Logistic model were used to present the deterioration trend with the increase of F-T cycles. ANOVA was applied to show the significant impact of F-T cycles and asphalt- aggregate ratio. The experiment results show that the compressive strength and resilient modulus decline with increasing F-T cycles; the degradation is sharp during the initial F-T cycles, after 8 F-T cycles it turns to gentle. ANOVA results show that F-T cycles, and asphalt-aggregate ratio have significant influence on the compressive characteristics. Exponential model, Loss ratio model and Logistic model are significantly fitting the test data from statistics view. These models well reflect the compressive characteristics of asphalt mixture degradation trend with increasing F-T cycles.

Investigation on Thermal Insulation and Mechanical Strength of Lightweight Aggregate Concrete and Porous Mortar in Cold Regions

Thermal insulation is an important indicator to evaluate the construction material in cold region engineering.As we know,adding the industrial waste as lightweight aggregate or creating the pore inside the cement-based composite could make the texture loose,and the thermal insulating capacity of the material would be improved with this texture.Using these methods,the industrial by-product and engineering waste could be cycled in an efficient way.Moreover,after service the fragmented cement composites paste could be used as aggregate in the thermal insulating concrete again.While the porous texture is not favorable for the mechanical strength and long-term durability in a cold environment.To balance the above three requirements from two opposite directions,different processing methods were applied to create the thermal insulation concrete/mortar.Firstly,the organic/inorganic lightweight aggregate,including the Expanded Polystyrene(EPS),Expanded Perlite(EP),and Ceramsite(CRMST)particles,were applied to create the Lightweight Aggregate Concrete(LWAC).As the comparative tests,the expanded Superabsorbent Polymer(SAP)hydrogel and Air-Entraining Agent(AEA)were also introduced to create the porous mortar.The above concrete/mortar was tested in the normal state and under the Freeze-Thaw cycle to explore the engineering performance in cold regions.During the experimenting process,the thermal insulation,mechanical strength,and frost resistance of these cement-based composites were investigated,and an optimal thermal insulation concrete/mortar was determined.

Carbon Nanotubes and Resistance to Freeze-Thaw Cycles

The research of materials with good properties is one of the important concerns of scientists groups, and more again in region where materials are subjected to freeze and thaw cycles. In the case of this paper, it has been a matter of evaluating of the effect of carbon nanotubes on concrete resistance to freeze and thaw cycles. Thus, it has been manufactured concretes with different rates of addition (0%, 0.1%, 0.5%, 1% bwc) of cement by carbon nanotubes. The durability factor, determined for C30 specimens at 28 days, shows that C005 provides a better resistance to freezing-thawing cycles with a 54.96 as index.

水泥-纤维固化风积砂劈裂特性跨尺度研究

目的 研究水泥掺量、纤维掺量和纤维长度对固化风积砂劈裂强度的影响规律,揭示添加剂掺量,纤维赋存形态及长度对水泥-纤维固化风积砂劈裂强度的影响机理。方法 将水泥、玻璃纤维作为添加剂掺入风积砂中,采用劈裂试验测定试样的劈裂强度,采用CT断层扫描技术,获取水泥-纤维固化风积砂不同层位平扫切片,并借助Avizo图像处理软件对CT平扫切片进行内部形态三维重构。结果 在掺量为0.3%的6 mm纤维及水泥掺量10%的组合条件下,试样劈裂强度达到最大值;当纤维掺量控制在0.3%时,纤维可均匀地赋存在试件内部,形成交错网状的三维结构;采用CT三维重构方法从细观证实了当纤维在试样内部达到理想赋存形态时,纤维对水泥-纤维固化风积砂的劈裂强度起到增益效果。结论 掺入适量的水泥和纤维可改善风积砂的工程力学特性,研究成果可为水泥-纤维固化风积砂路基病害防治提供科学依据。

The geomechanical properties of soils treated with nanosilica particles

This study examines the effect of nanosilica(NS)additive to improve the mechanical properties of clay,clayey sand,and sand.The engineering properties of the soils were investigated through Atterberg limits,compaction,unconfined compression,ultrasonic pulse velocity(UPV),freeze-thaw,and direct shear tests.The NS content varied from 0%to 0.7%and cement content was 5%and 10%by the dry weight of the soil.The curing period varied from 7 d to 150 d.The consistency,compaction,and strength properties of the soils were affected by the presence of NS and cement.The optimum NS contents in clay specimens with 5%and 10%cement were 0.5%and 0.7%,respectively.It was 0.7%in sand specimens with both cement ratios,as well as 0.3%and 0.7%in clayey sand specimens with 5%and 10%cement,respectively.In terms of freeze-thaw resistance,clayey sand specimens containing 0.5%NS and 10%cement had the minimum strength loss.Exponential relationships existed between the ultrasonic pulse velocity(UPV)and the unconfined compressive strength(UCS)of soil specimens having the same curing period.The shear strength parameters of the soils also improved with the addition of NS.Scanning electron microscope(SEM)images demonstrated that cement and NS contributed to the improvement of the soils by producing a denser and more uniform structure.It was concluded that the minor addition of NS could potentially improve the geomechanical properties of the soils.

二灰钢渣混合料力学性能试验研究

为了研究二灰钢渣混合料的力学性能,进行了抗压强度、抗压回弹模量、劈裂及弯拉强度试验。试验表明,二灰钢渣的抗压强度随粉煤灰与石灰比值的变化而变化,当粉煤灰与石灰的比值为3—5,钢渣的含量在75%—80%时7 d抗压强度较高,最高可达2.62 MPa;二灰钢渣混合料前期强度增加较慢,后期增加较前期快。二灰钢渣混合料的抗压回弹模量随龄期的增加而增长,在28—90 d时模量增加较慢,而90—180 d增大明显,增幅最高可达78.8%。二灰钢渣劈裂强度的形成主要是受钢渣含量的影响;劈裂强度随龄期的增加而增长,后期比前期增长快,28 d劈裂强度在0.14—0.20 MPa之间。

冷再生混合料力学性能试验研究

在对冷再生混合料的配比和力学性能(抗压强度、抗压回弹模量和劈裂强度)室内试验的基础上,探讨了冷再生混合料力学性能的变化规律以及其影响因素。结果表明:旧的沥青混合料中掺入一定量的添加剂和适量的水之后,其物理化学性质得到了改善,抗压强度和劈裂强度有明显提高;再生混合料的力学性能主要取决于添加剂的种类、剂量和集料的级配,研究结果为有效控制和检验冷再生技术的实际应用提供了依据。

钢纤维体积掺量对超高性能混凝土力学性能的影响

摘要：通过立方体抗压试验、劈裂抗拉试验、梁抗折试验，分析了钢纤维体积掺量分别为0．5％、1．0％、1．5％、2．0％、2．5％时对超高性能混凝土（UHPC）力学性能的影响。结果表明，UHPC的抗压强度、抗折强度、劈裂抗拉强度随着钢纤维体积掺量的增加都有不同程度的提高，劈裂抗拉强度在钢纤维体积掺量为1．0％～1．5％时增长最快，抗折强度在钢纤维体积掺量为1．0％-2．5％时增长最快。

冷再生材料路用性能的试验分析

目的研究废旧沥青路面材料的冷再生利用.方法通过系统的室内试验对水泥稳定废旧沥青混合料的物理力学性质进行了分析与评定,对无侧限抗压强度、抗压回弹模量、劈裂强度等主要力学性能进行了研究.结果实验表明,当水泥含量达到5%时,冷再生材料的无侧限抗压强度达到现行规范中规定的要求,抗压回弹模量、劈裂强度达到常用的无机结合料稳定基层材料相应指标的数值,同时冷再生材料也具有较强地抗冲刷性能.结论对于试验所用冷再生材料,只要科学合理地进行配合比设计,同时再生剂含量达到5%以上时,冷再生材料可以将之运用于沥青路面基层之中.

硫酸盐环境下再生混凝土性能试验研究

用强度等级为C25废弃普通混凝土制备再生骨料,然后分别用20%(A类)和40%(B类)的再生骨料取代率来取代天然骨料,配制成再生混凝土。对再生混凝土进行了6%(质量分数)浓度的硫酸纳溶液作用下的室内侵蚀模拟试验,研究了在干-湿循环作用下硫酸纳对再生混凝土的腐蚀特性。试验结果表明:在干湿循环作用的初期阶段,对于20%和40%再生骨料取代率的再生混凝土来说,它们的动弹性模量、劈裂强度都有一定的提高,提高幅度前者略低于后者。随着干湿循环周期数的继续增加,无论是20%再生骨料取代率再混凝土,还是40%再生骨料取代率再混凝土,它们的动弹性模量和劈裂强度都开始出现缓慢下降,继而又快速下降,且后者的下降速度大大高于前者。此研究可为再生混凝土在硫酸纳环境中的工程应用提供一定参考。

外掺氧化镁混凝土性能试验研究

通过采用锦屏一级水电站的原材料,对外掺氧化镁的水工混凝土性能试验研究表明,外掺氧化镁对混凝土的抗压强度略有降低,对劈拉强度、轴拉强度、弹性模量、极限拉伸值略有提高;未掺氧化镁的砂岩混凝土180d自身体积变形为-57.8个微应变,外掺4%的氧化镁可以使砂岩混凝土的自身体积变形接近0。