Effects of end conditions on compressive strength and static elastic modulus of very high strength concrete

The use of bonded and unbonded caps in testing very high strength concrete cylinders has been investigated experimentally. A hundred and ninety-two concrete cylinder specimens of 150-mm diameter and 300-mm height were cast and tested using packing with softboard, neat cement paste, neoprene pad and sulfur mortar. The design strength level of 75-100 MPa was achieved using water-cementitious material ratios of 0.22, 0.26 and 0.31. The results of the study were compared considering compressive strength and static elastic moduli values. A two-way analysis of variance was performed at a .01 level of significance in order to compare the effect of end conditions. It was found that the overall mean compressive strengths of specimens capped with neat cement paste, neoprene pad sulfur mortar were not significantly different. The packed specimens exhibited a significant difference from the others. On the other hand, there was no statistical difference in the static elastic moduli values when different capping types were used. Several modulus prediction equations were also examined. Experimental values were consistently higher than the predicted values.     

Effect of temperature and aging on the mechanical properties of concrete: Part I. Experimental results

This paper reports the results of curing temperature and aging on the strength and elastic modulus and the Part II paper suggests a prediction model based on these experimental results. Tests of 480 cylinders made of Types I, V, and V cement+fly ash concretes, cured in isothermal conditions of 10, 23, 35, and 50 °C and tested at the ages of 1, 3, 7, and 28 days are reported. According to the experimental results, concretes subjected to high temperatures at early ages attain higher early-age compressive and splitting tensile strengths but lower later-age compressive and splitting tensile strengths than concretes subjected to normal temperature. Even though the elastic modulus has the same tendency, the variation of elastic modulus with curing temperature is not so obvious as compressive strength. Based on the experimental result, the relationships among compressive strength, elastic modulus, and splitting tensile strength are analyzed, considering the effects of curing temperature, aging, and cement type.     

Effect of temperature and aging on the mechanical properties of concrete: Part II. Prediction model

In Part I, empirical relationships between compressive strength and splitting tensile strength or elastic modulus with temperature and aging were proposed. This paper investigates new prediction models estimating splitting tensile strength and elastic modulus without knowing compressive strength. The prediction model is suggested on the basis of the equation that was suggested to predict compressive strength. The mechanical properties calculated by the model are compared with empirical results presented in Part I. To evaluate in-place applicability of the model, the empirical data on strength and elastic modulus of concrete cured at variable temperature are compared with the values estimated using the prediction model. The prediction model properly estimates the strength and elastic modulus of Types I and V cement concretes cured at constant and variable temperature conditions.     

Experimental study of the mechanical behavior of plastic concrete in triaxial compression

This paper is going to present the results of an extensive experimental parametric study of the mechanical responses of various types of plastic concrete in unconfined and triaxial compression tests. Plastic concrete consists of aggregates, cement, water and bentonite, mixed at a high water cement ratio, to produce a ductile material. It is used for creating an impermeable barrier (cut-off wall) for containment of contaminated sites or seepage control in highly permeable dam foundations. A plastic concrete cut-off wall acts essentially as a barrier to stop or reduce the groundwater flow. In this study the effect of specimen age, cement factor, bentonite content and confining pressure on shear strength and permeability of plastic concrete were investigated. The observed behavior is more and more ductile for increasing confining pressure. It is shown, also, that any increase in confining pressure increases the compressive strength as well as the elastic modulus and the deformability of the specimen. It is shown that an increase in cement factor increases the shear strength as well as the elastic modulus. It is obtained that increase of bentonite content, decreases the compressive strength as well as the elastic modulus. Increasing the age of the specimens causes an increase of the compressive strength as well as the elastic modulus and also the shear strength parameters are affected. Also, it is obtained that increase in confining pressure and cement factor reduces the permeability.     

沥青混凝土抗压强度与回弹模量的试验研究

通过对5种不同沥青含量的沥青混凝土进行单轴压缩试验,得到沥青混凝土抗压强度和沥青用量之间的关系;在测得抗压强度的基础上进行抗压回弹模量试验,得到抗压回弹模量和沥青用量之间的关系;同时根据动、静回弹模量的关系,将静态回弹模量换算为动态回弹模量,以促进路面设计理论由静态向动态的过渡.     

高火山灰活性煅烧煤矸石添加量对水泥抗压强度的影响

为研究高火山灰活性下煤矸石添加量对水泥抗压强度影响,以龙岩翠屏山煤矿煤矸石为研究对象,分析了温度对煤矸石活性的影响以及煤矸石添加量对水泥强度的影响。结果表明:随着煅烧温度的增大,煤矸石烧失量在逐渐增大,烧失量与煅烧温度呈幂函数关系;随着煅烧温度的增大,煤矸石活性呈现先增大后减小的规律,煅烧煤矸石吸钙量与温度成二次多项式关系,推断实验煤矸石的煅烧最佳温度为750 ℃;随着煤矸石添加量的增加,水泥单轴抗压强度呈下降趋势,试件的抗压强度与煤矸石添加量成指数关系;随着龄期的增大,添加煤矸石的试件强度具有增长的趋势。研究结果对确定煤矸石添加量提供了理论依据,对指导煤矸石在凝胶材料中应用具有重要意义。     

水泥细度对早龄期碾压混凝土综合抗裂性的影响(英文)

水泥水化热与比表面积和化学组成有关,但是相对于调整水泥的化学组成来说,通过减小水泥的比表面积来降低水泥水化热要容易得多。为了探索水泥比表面积与碾压混凝土抗裂性能的关系,采用相同熟料磨制了3种细度的水泥,研究了水泥细度对水化热、胶砂强度的影响,以及对混凝土的工作性、力学性能(抗压强度、抗拉强度和抗拉弹性模量)、极限拉伸值、绝热温升等性能的影响;同时,采用温度–应力试验机,评估了在100%约束和近似绝热条件下水泥细度对早龄期碾压混凝土综合抗裂性能的影响。结果表明:水化热与比表面积成线性关系,降低水泥比表面积是降低混凝土温升的有效、便捷的措施;粗磨水泥提高了碾压混凝土的工作性,降低了混凝土的抗压强度和弹性模量,但混凝土极限拉伸值没有明显变化;温度–应力试验表明,随着水泥比表面积的降低,混凝土第二零应力温度更低,粗磨水泥碾压混凝土综合抗裂风险更低。     

Strength of corundum concretes

Conclusions Studies were made of the strength properties of corundum concretes containing a hydraulic bond, and an assessment was made of bending strength in relation to the heat-treatment temperature.It is shown that the concretes investigated belong to nonlinearly deformable materials. We determined the true strength of the concretes, which with rise in temperature increases; the strongest are concretes with a bond from hydraulically setting cement — Talum cement.We determined the maximum bending deformation of the concretes. It is shown that the lower the elastic deformation shown by the refractory material before failure then the less catastrophically will the destruction occur.We determined the densities of the full energy of deformation and the elastic energy at temperatures of 120, 1400, and 1750°C. It was established that concretes characterized by maximum strength possess the greatest stock of densities of full and elastic energy.On the basis of the deformation diagrams obtained for nonlinear-elastic concretes we determined the magnitudes of static and dynamic elasticity moduli. The strongest concretes are characterized by the maximum modulus values.Translated from Ogneupory, No. 6, pp. 9–14, June, 1986.     

低温干湿循环条件下砂浆抗硫酸盐侵蚀性能试验研究

研究了不同水胶比水泥砂浆试件在低温干湿循环条件作用下的抗硫酸盐侵蚀性能.试验制作了0.36与0.5两种水胶比的普通硅酸盐水泥、中抗硫水泥以及矿粉-硅灰复掺的水泥砂浆试件,检测了试件标养28 d后的孔结构及各试件在(5±1)℃的3％Na2 SO4溶液中干湿循环后的强度、动弹性模量变化情况,对砂浆在低温干湿循环条件下的抗硫酸盐侵蚀性能进行了评价,并分析了检测指标间的相关性.结果表明:低温干湿循环条件下,0.5水胶比砂浆抗硫酸盐侵蚀性能低于0.36水胶比砂浆,抗硫酸盐侵蚀性能随着水胶比的降低而提高;低水胶比砂浆复掺矿粉-硅灰后抗低温干湿循环条件下的硫酸盐侵蚀性能提升明显;两个水胶比砂浆的相对动弹模均与抗压强度高度相关.     

Temperature Dependence of Elastic Constants of Vitreous Silica

The relation between the elastic moduli and temperature from room temperature to about 1300°C was determined for a group of vitreous silica specimens by a dynamic resonance method. All the curves were approximately parabolic in shape, reaching a maximum near 1050° to 1200°C. At the maximum value, Young's modulus was more than 11% higher and the shear modulus was about 9% higher than their room-temperature values. Poisson's ratio was then computed to rise from about 1/6 at room temperature to about 1/5 at the temperature of maximum elastic modulus. Small but significant differences were observed in the temperature-modulus curves for specimens from different sources. These differences were found to be related to differences in the infrared transmission curves.     

盐冻作用下玄武岩纤维混凝土力学性能损伤研究

针对西北寒旱地区混凝土结构易开裂耐久性降低的问题,选取力学性能优异的玄武岩纤维作为混凝土增强材料,采用室内快速冻融试验,以纤维体积掺量为变量,研究了不同纤维体积掺量(0.05%、0.1%、0.15%、0.2%)混凝土试件分别在清水、质量分数为3%的NaCl溶液、质量分数为5%的Na₂SO₄溶液冻融作用下动弹性模量、抗压强度、抗折强度三个力学性指标的变化。研究发现,玄武岩纤维的掺入能有效提升混凝土的初始抗折强度和抗盐冻能力,纤维体积掺量在0.15%~0.2%时混凝土试件动弹性模量、抗压强度与抗折强度在盐冻作用下的衰减速率减缓明显,玄武岩纤维混凝土在三种冻融介质中力学性能下降速率排序为清水<5%Na₂SO₄溶液<3%NaCl溶液。以动弹性模量为损伤变量,拟合混凝土相对抗压强度、相对抗折强度与损伤度的相关模型,模型相关性良好。研究结果可为玄武岩纤维混凝土的实际运用与后期维护提供理论依据与参考。     

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.     

高温后混凝土气体渗透性及力学性能研究

火灾高温不仅会降低混凝土结构的力学性能,还会对混凝土的耐久性产生重大的影响.通过高温电阻炉模拟火灾试验,研究高温下混凝土的气体渗透性、弹性模量和轴心抗压强度的变化特征,以及气体渗透性和力学性能的相互关系.试验结果表明:混凝土气体渗透性随火灾温度的升高而逐渐增大,且在500℃时气体渗透性增长72.6倍;在200 ℃高温下,混凝土的弹性模量和轴心抗压强度略有提高,而在350 ℃及以上高温下,其力学性能大幅降低.     

Experimental and numerical investigation of the dynamic response of a ZrB2-SiC ceramic under uniaxial compression

The dynamic experimental tests were performed on cylindrical zirconium diboride-silicon carbide ceramic specimens under the uniaxial compression from 519 to 2861 s⁻¹. The effect of the strain rate on the dynamic response of a ZrB₂-SiC ceramic was investigated using experimental and numerical methods. A significant increase on the dynamic compressive strength, elastic modulus, and the dynamic tensile strength was found with the increase of the strain rate. The damage process and fracture pattern of the ZrB₂-SiC ceramic exhibited a significant strain-rate dependence under the dynamic compression. The strain rate-dependent elastic modulus and tensile strength were introduced into Johnson–Holmquist (JH-2) model to predict the dynamic compression behavior of the ZrB₂-SiC ceramic. The simulation results of the dynamic compressive strength, stress–strain relation, and fracture patterns were in good accordance with the dynamic experimental results.     

The effect of ultra-high molecular weight polyethylene fiber on the mechanical properties of acrylic bone cement

The effects of the addition of ultra-high molecular weight polyethylene fiber (UHMWPE) on the mechanical properties of standard surgical Simplex-P radiopaque bone cement have been investigated. It was found that the tensile strength and tensile modulus were apparently not improved by the incorporation of UHMWPE in the acrylic bone cement. The results of bending strength and bending modulus indicated that a reinforcing effect is obtained at UHMWPE contents as low as 1 wt%, and then levelled off with increasing UHMWPE contents. When the UHMWPE contents as low as 2 wt%, the values of compressive strength and modulus seemed approximate the same; whereas the values of compressive strength and modulus decreased with increasing UHMWPE contents. From the results of dynamic mechanical analysis (DMA), the values of dynamic storage modulus of bone cement increased at UHMWPE fiber as low as 2 wt%, but beyond that UHMWPE content the value of the dynamic storage modulus decreased with increasing UHMWPE contents. The same results were also found for the dynamic loss modulus. When methyl methacrylate was grafted onto UHMWPE by plasma and UV irradiation treatment, it was found that by adding the treated UHMWPE fiber in acrylic bone cement had a significant reinforcing effect on the mechanical properties of bone cement.     

冻融作用下活化煤矸石粉混凝土损伤劣化规律

为促进寒冷地区煤矸石在混凝土中的应用,通过机械-微波方式对煤矸石进行复合活化,从表观形貌、质量损失率、相对动弹性模量、抗压强度和劈裂抗拉强度等方面对煤矸石粉混凝土的损伤劣化规律进行了研究,并使用扫描电子显微镜、核磁共振波谱仪和X射线衍射仪等研究了活化煤矸石粉对混凝土抗冻性能的改性机理。结果表明:20%(质量分数)掺量下活化煤矸石粉对混凝土抗冻性能改善效果最好,且经300次冻融循环后,质量损失率和相对动弹性模量分别为2.32%和91.32%,抗压强度和劈裂抗拉强度分别下降了16.40%和26.12%;活化煤矸石粉能填充、细化孔隙,且其二次水化能消耗水泥水化产物Ca(OH)₂,产生C-S-H和C-A-S-H,提升水泥石密实程度,改善孔结构,使大孔占比减少。     

Compressive strength studies on portland cement mortars containing fly ash and superplasticizer

The effects of two super water-reducing admixtures on the compressive strengths and elastic moduli of portland cement mortars containing fly ash were determined. The results show that the effectiveness of the chemicals for improving the strengths of mixtures in which portland cement is partially replaced by fly ash decreases with an increase in the percentage of ash. Increased strength can be nil or too low to justify the additional expense associated with the chemicals. The mixtures did exhibit normal stress-strain relationships: the elastic moduli were related to the ultimate compressive strengths in the usual manner. The results are based on mixes with equal workabilities.     

聚丙烯酸酯乳液在水泥砂浆中的应用

为了提高水泥砂浆的韧性,选用了能形成柔性薄膜结构的聚合物--聚丙烯酸酯乳液对水泥砂浆进行改性.研究了聚丙烯酸酯乳液对水泥砂浆体积密度、抗压强度、抗折强度、韧性、动弹模量和黏结抗拉强度的影响.结果表明:聚丙烯酸酯乳液在一定程度上降低了水泥砂浆体积密度和抗压强度,对抗折强度影响较小,改善了水泥砂浆的韧性,并且可提高黏结抗拉强度.当乳液掺量(质量分数)大于5%时,28 d混合养护聚丙烯酸酯乳液水泥砂浆的压折比降低到3以下,只有当乳液掺量大于10%时,水泥砂浆的黏结抗拉强度才明显提高.     

含粉煤灰微珠混凝土的强度和徐变特性研究

利用粉煤灰微珠按10%、20%和30%（质量分数）部分替代水泥制备了混凝土试样,并对混凝土试样的抗压强度、弹性模量和徐变进行了测试,同时通过压汞孔隙率实验对试样的微观结构进行了研究。结果表明,粉煤灰微珠会使混凝土的早期抗压强度和弹性模量下降,但是对混凝土长期强度增长和弹性模量增长有明显的促进作用,在90 d时粉煤灰微珠掺量为20%的试样强度和弹性模量最高。掺入20%的粉煤灰微珠可以降低混凝土的比徐变,但是过多的粉煤灰微珠反而会增加混凝土的徐变变形。孔隙分析结果表明,总孔隙体积较高时,混凝土的强度较低,徐变变形较大;而加入20%粉煤灰微珠会降低混凝土的中孔隙、大孔隙和总孔隙体积,从而改善混凝土的强度和徐变特性。     

铁尾矿粉改良水泥土的强度与动力特性试验研究

为提高水泥改良土强度,验证尾矿粉能够起到对水泥土的增强效果,对已用水泥改良过的路基土,在尾矿粉掺入量不同的条件下,经改良后水泥土的强度特性,以及对被最佳掺量的尾矿粉改良后的水泥土,在围压和频率不同的条件下进行动力特性的试验研究,制备了不同尾矿粉掺量(0%、2%、4%、6%、8%、10%)的试验土样进行无侧限抗压强度试验和动三轴试验;试验结果表明:随着尾矿粉掺入量的增加,无侧限抗压强度先增大后减小,土的应力应变曲线先急剧增加达到峰值后逐渐趋于平缓.当尾矿粉掺量达到最佳时,分析不同围压和不同频率的条件下G/Gmax~γ、λ~γ变化规律.对于同一动剪应变幅,动剪模量比随围压的增加而增大.阻尼比随动剪应变幅的增大而增大,随围压的增大而减小.G/Gmax~γ、λ~γ关系受频率影响不太敏感,都集中于一狭小范围.该试验能够提高水泥土强度的尾矿粉最佳掺量为6%.