养护条件对混凝土表面层性能的影响

研究了自然环境、水中养护、薄膜覆盖3种养护条件对不同粉煤灰掺量混凝土表面吸水率以及表面层(1～5 cm)混凝土的电阻率影响.结果表明,混凝土表面层的吸水率以及电阻率对养护条件特别敏感,掺粉煤灰可降低混凝土的表面吸水率;养护条件主要影响距离表面1～3cm处混凝土的电阻率,27 d水中养护后,掺粉煤灰试件电阻率与基准试件相差不大,而6d水中养护和自然养护后,掺粉煤灰试件电阻率明显比基准试件大,且随粉煤灰掺量的增加而增大,从电阻率变化范围来看,27 d水中养护试件的电阻率最小,其次是6 d水中养护的试件,而自然养护的试件最大.混凝土表面电阻率变化能较好反映养护质量的好坏,基于这种变化,对混凝土的养护效果进行定量评价是可行的.     

一种评定高性能混凝土养护效果的新方法

本文提出了一种改进的表面吸水率试验用于评估混凝土的养护效果。养护的充分与否通过混凝土试件不同深度处的孔结构的差异来判定的方法。研究中利用高性能混凝土的自养护来对此方法的可行性进行了试验验证。试验结果表明：利用此方法测试的孔结构参数受养护条件的显著影响；养护措施不好的情况下，不同深度处混凝土试件的吸水率与孔结构参数差异显著；本方法操作简单、快捷，可以用来评价混凝土的养护效果。尤其适用于高性能混凝土。     

导电混凝土的研究

一、导电混凝土的用途干燥状态的混凝土的电阻率为6.54～11.4kΩm,是良好的绝缘体。但混凝土是一种多孔材料,在混凝土的内部存在着各种尺寸的孔隙,当这些孔缝中充满水时,混凝土的导电性能就会大大地增加。处于潮湿状态的混凝土,其电阻率为25～45Ω·m,这时的混凝土就     

冻融循环过程中混凝土内部温度-相对湿度关系

研究了不同配合比混凝土试件在经历不同冻融循环次数时的内部温度-相对湿度关系,并将这一关系划分为4个阶段,即降温增湿段、降温降湿段、增温增湿段和增温降湿段.在降温增湿段,混凝土内部相对湿度随温度降低而增大,当相对湿度出现转折表现为降低趋势时,表明混凝土内部开始结冰,温度-相对湿度关系进入降温降湿段;在增温增湿段,混凝土内部相对湿度随温度升高而增大,当相对湿度出现转折表现为降低趋势时,表明混凝土内部的冰晶开始融化,温度-相对湿度关系进入增温降湿段.当混凝土水灰比相同时,试件内部相对湿度随着深度增大而增大;当深度相同时,试件内部相对湿度随着水灰比的减小而降低,且结冰温度和融化温度呈现减小的趋势.低水灰比混凝土内部较低的相对湿度可能会导致更大的自收缩和较小的冻胀作用力.     

高性能轻骨料混凝土的电阻及氯离子电渗特性之研究

由于传统轻骨料吸水率高及连通孔洞多,而传统轻骨料混凝土又使用大量的水泥及水,因此在台湾的海洋性气候所形成的盐雾侵蚀环境下,传统轻骨料混凝土在达到隔热效果的同时却降低了其耐久性能,并对混凝土结构物的服务寿命也造成了影响.采用低吸水率的淤泥轻骨料,并通过致密配比设计,制成了高流动度、高性能的轻骨料混凝土(HPLWC);以3种水胶比(0.28,0.32,0.40)及3种用水量(140,150,160kg/m3),探讨了水泥浆的质与量对高性能轻骨料混凝土耐久性能的影响.对混凝土试件730d龄期的测量结果显示,各组轻骨料混凝土可同时达到具有足够强度、高流动度、高电阻率(在91d及730d可分别达到33～82kΩ·cm及137～175kΩ·cm)、低电渗(在91d可达到650～980C)的工程性质.     

养护方式对混凝土碳化作用的影响

为研究使用不同养护方式对混凝土碳化速度的影响进行了快速试验,本文对其结果予以论述.制作了混凝土棱柱体试件,并将其置于水中和/或空气中养护不同龄期后,放入一个不断充满二氧化碳气体的箱内.切下棱柱体薄片,并用酚酞溶液喷洒,以测定碳化深度.结果表明,当水养护时间增长时,混凝土碳化速度降低.在水中养护28天的棱柱体的碳化速度,仅为在空气中养护相同龄期的17%.     

高性能再生骨料混凝土性质之探讨

对营建拆除废弃物所制成的再生粗细骨料应用于高性能混凝土进行了初步探讨.结果显示:高性能再生骨料混凝土其91 d的抗压强度最高可达44(DR系列),46(DN系列)MPa,电阻率平均在40 kΩ.cm以上,Cl-渗透值均低于2 000 C.     

直接电养护对混凝土性能的影响

以石墨和碳纤维为导电相材料,研究了石墨掺量（2%、4%）和养护方式（标准养护、直接电养护）对混凝土抗压强度和微观结构的影响,分析了直接电养护时间与试件内部温度和电阻率之间的关系。结果表明：掺入适量导电相材料可以改善混凝土的力学性能,提高混凝土的导电性能;直接电养护可以提高混凝土的内部温度,促进水化反应进程,提高混凝土的抗压强度,且对早期抗压强度的提高效果更大;随着直接电养护时间的增加,混凝土的电阻率不断增大,导电性能降低;直接电养护可以有效减小混凝土中凝胶孔的孔径,改善混凝土的孔结构。     

预养护对砌块混凝土二氧化碳养护的影响

研究了预养护对二氧化碳养护砌块混凝土制品的影响,结果表明:在利用二氧化碳养护砌块混凝土时,影响二氧化碳养护过程的重要因素是预养护期间试件含湿量而不是预养护时间;当试件在预养护期间失去一定量的水时,二氧化碳养护的程度达到最大值.试件的抗压强度随二氧化碳养护程度的增加而增加.利用二氧化碳养护的砌块混凝土其抗压强度与蒸养砌块混凝土相差不大,因此,二氧化碳养护技术是一种非常有前景的可持续发展的新技术.     

水灰比和矿物掺合料对混凝土结构中钢筋极化电阻的影响

采用加速渗透试验方法和埋入式自制CB型氯离子传感器和MB型参比电极研究了水灰比和矿物掺合料组合对6组混凝土结构中钢筋极化电阻Rp的影响。研究表明:当水灰比为0.35时,混凝土在25 h内可保持较高的极化电阻钢筋,钢筋严重锈蚀要60 h,而水灰比为0.40和0.45混凝土中的钢筋极化电阻则持续下降,特别是对于水灰比较高的混凝土试件,氯离子渗透时间超过18 h后,钢筋已严重锈蚀。矿物掺合料有助于减缓钢筋的锈蚀,且复掺粉煤灰和硅灰效果最佳,复掺粉煤灰和矿渣次之;且掺有矿物掺合料混凝土试件中钢筋极化电阻都大于未掺矿物掺合料混凝土试件。当钢筋极化电阻下降至2.7×105Ω.cm2时,继续加速,钢筋极化电阻Rp值会突然下降,钢筋开始锈蚀,钢筋钝化膜发生了破坏,结论认为钢筋极化电阻Rp值为2.7×105Ω.cm2是钢筋在混凝土中钝化膜破坏的临界点。     

80℃蒸汽养护条件下外掺MgO膨胀剂三级配混凝土的膨胀

在80℃蒸汽养护条件下,粗骨料粒径为5～20 mm、20～40 mm、40～80 mm三级配MgO混凝土试件的膨胀。结果表明,80℃蒸汽养护条件下,三级配MgO混凝土试件,28 d前膨胀快之后膨胀放缓,90 d时膨胀趋于稳定。MgO膨胀剂的活性越高,MgO混凝土试件最终产生的膨胀越小。粉煤灰降低掺低活性MgO膨胀剂混凝土试件膨胀,而对掺高活性MgO膨胀剂的膨胀影响较小。     

The effects of low temperature curing on the compressive strength of ordinary and high performance concrete

In this study, the changing of the compressive strength of ordinary and high performance concrete after having cured at low temperature was investigated experimentally. To accomplish this purpose, concrete specimens of 150 mm diameter and 300 mm high were prepared. After their production the specimens were cured at different conditions for 7 days. Some of them were at 23 ± 2 °C (standard curing); the others were at 10, 5, 0 and −5 °C, respectively. In the 7th day, some of the specimens cured at different temperature (10, 5, 0 and −5 °C) were broken under uniaxial compression. On the other hand, some of the specimens were applied to standard curing during 28 days. In the end of 28 days, compressive strength of all specimens was obtained. According to the results, compressive strength of the specimens at 10 °C and less than 10 °C during 7 days was lower than that of the specimens at standard curing. In the end of 28th day loss of compressive strength of concrete specimens cured at different temperatures were more than that of specimens cured at standard cure.     

Effect of W/C ratio on covering depth of fly ash concrete in marine environment

This investigation studied the effect of W/C ratio on covering depth required against the corrosion of embedded steel of fly ash concrete in marine environment up to 4-year exposure. Fly ash was used to partially replace Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of cementitious material. Water to cementitious material ratios (W/C) of fly ash concretes were varied at 0.45, 0.55, and 0.65. The 200-mm concrete cube specimens were cast and steel bars with 12-mm diameter and 50 mm in length were inserted in the concrete with the covering depth of 10, 20, 50, and 75 mm. The specimens were cured in water for 28 days, and then placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the concrete specimens were tested for the compressive strength, chloride penetration profile and corrosion of embedded steel bar after being exposed to tidal zone for 2, 3, and 4 years. The results showed that the concrete mixed with Portland cement type I exhibited higher rate of the chloride penetration than the fly ash concrete. The chloride penetration of fly ash concrete was comparatively low and decreased with the increasing of fly ash content. The increase of fly ash replacement and the decrease of W/C ratio could reduce the covering depth required for the initial corrosion of the steel bar. Interestingly, fly ash concretes with 35% and 50% cement replacement and having W/C ratio of 0.65 provided better corrosion resistance at 4-year exposure than the control concrete with W/C ratio of 0.45. In addition, the covering depth of concrete with compressive strength of 30 MPa (W/C ratio of 0.65) could be reduced from 50 to 30 mm by the addition of fly ash up to 50%.     

海水海砂再生混凝土的基本力学性能

利用海水、原状海砂及再生粗骨料,制备了设计预期强度为C20~C50的海水海砂再生混凝土。通过240个标准立方体(150 mm×150 mm×150 mm)和96个棱柱体(150 mm×150 mm×300 mm)试件,完成了工作性能、立方体抗压强度、轴心抗压强度、劈裂抗拉强度以及弹性模量试验,研究了海水海砂再生混凝土的基本力学性能;最后基于试验数据,得到了海水海砂再生混凝土立方体抗压强度与轴心抗压强度关系公式以及弹性模量与轴心抗压强度关系公式。结果表明:海水海砂再生混凝土工作性能良好,C40和C50强度等级的坍落度比一般再生混凝土分别提高5%和33%;立方体抗压强度、轴心抗压强度和劈裂抗拉强度随着龄期变长而增加,且长期强度趋于稳定;与普通混凝土相比,海水海砂再生混凝土7 d立方体抗压强度提高13%~52%,28 d抗压强度降低约5%,90 d抗压强度降低约15%,180 d抗压强度降低18%~29%;海水海砂再生混凝土28 d弹性模量比普通混凝土略有降低,降低幅度在14%以内;再生粗骨料对混凝土力学性能、工作性能的影响大于海水海砂。     

Effect of curing methods on the properties of plain and blended cement concretes

This paper reports result of a study conducted to investigate the effect of curing methods on the properties of plain and blended cement concretes. The concrete specimens were prepared with Type I, silica fume, and fly ash cement concretes. They were cured either by covering with wet burlap or by applying two types of curing compounds, namely water-based and acrylic-based. The effect of curing methods on the properties of plain and blended cement concretes was assessed by measuring plastic and drying shrinkage, compressive strength, and pulse velocity. Results indicated that the strength development in the concrete specimens cured by covering with wet burlap was more than that in the specimens cured by applying water – and acrylic-based curing compounds. Concrete specimens cured by applying curing compounds exhibited higher efficiency in decreasing plastic and drying shrinkage strain than specimens cured by covering with wet burlap. The performance of acrylic-based curing compound was better than that of water-based curing compound. The data developed in this study indicate that curing compounds could be utilized in situations where curing with water is difficult. Among the two curing compounds investigated, acrylic-based curing compound performed better than the water-based curing compound.     

Behaviour of eccentric loading of FRP confined fibre steel reinforced concrete columns

This paper presents results of testing 16 specimens, 12 of which as columns under different eccentricities and four as beams under four point loading regime. All 16 specimens were circular in cross section and were made of reinforced concrete. Four specimens served as reference specimens and were just made of reinforced concrete. The next four specimens were wrapped with carbon fibre reinforced polymers (CFRP). The next four specimens had steel fibres added to the concrete. The final four specimens were reinforced with steel fibres and wrapped with CFRP. From each group of specimens, one specimen was tested as a column under a concentric load, the second specimen was tested as a column under 25 mm eccentricity, the third specimen was tested as a column under 50 mm eccentricity, and the final specimen was tested as a beam under four point loading regime. The experimental programme proved that the introduction of fibres as well as wrapping the specimens with FRP improve the properties of concrete, especially its ductility.     

Properties of autoclaved lightweight aggregate concrete

Many researches have been carried out on production and properties of pre-cast concretes. Currently, most of them have focused on normal concrete, and are unable to completely represent the behavior of lightweight concrete (LWC). In this study, physical and mechanical properties of LWC produced with diatomite and pumice lightweight aggregates after autoclave curing were investigated. In the production of LWC, 0–4 mm maximum sizes of aggregates were used. Cement content and water/cement ratio were kept at 300 kg/m³ and 0.20, respectively. The specimens were prepared in 50×100 mm cylindrical shape, and after 24 h of demoulding exposed to autoclave curing for 2, 4, 6, 8 and 10 h. Besides, two different cures were applied on the specimens as in water and in air at 20 °C±2, respectively. At the end of autoclaving and environmental cure, compressive strength in 7, 28 and 590 d, unit weight, specific porosity, thermal conductivity and water absorption were tested. Also, microstructures of LWC produced with diatomite and pumice aggregate were investigated. As a result, it is concluded that by autoclaving of specimens in 8–10 h, especially, compressive strength of specimens have increased 75% of strength of 28 aged specimens cured in water.     

氯离子在带接缝混凝土内的传输规律研究

为得出不同类型接缝及石灰石粉含量对混凝土构件在氯盐环境中的耐久性影响,设计了一侧为普通混凝土、另一侧为石灰石粉混凝土、中间为不同类型接缝的组合试件。将养护后的试件在10%浓度的氯盐溶液中浸泡270 d后取样,检测接缝处及其两侧混凝土内的自由氯离子浓度。结果表明:在同一试件内,接缝处的氯离子浓度最大,距接缝0～20 mm处混凝土内的氯离子浓度向远离接缝方向逐渐减小,距接缝20 mm之外混凝土内的氯离子浓度基本相等; 相同侵蚀时间下,同一深度处直接湿接缝内的氯离子浓度最大,凿毛接缝处次之,界面剂接缝处最小,拟合得出的3种接缝处的表观氯离子扩散系数分别为基体混凝土的1.95倍、1.87倍、1.83倍; 接缝两侧相同距离处,掺石灰石粉混凝土内的氯离子浓度均大于普通混凝土,石灰石粉掺量(质量分数)为10%、20%、30%时混凝土表观氯离子扩散系数分别增大9.8%、11.8%、65.8%; 数值模拟得到的氯离子在带接缝混凝土内的分布规律与试验结果吻合较好。     

Physical, mechanical and thermal properties of concrete in different curing media containing ZnO2 nanoparticles

In the present work, the effect of curing medium on microstructure together with physical, mechanical and thermal properties of concrete containing ZnO₂ nanoparticles have been investigated. Portland cement was partially replaced by ZnO₂ nanoparticles with the average particle size of 15 nm and the specimens were cured in water and saturated limewater for specific ages. The results indicate that ZnO₂ nanoparticles up to maximum of 2.0% produces concrete with improved compressive strength and setting time when the specimens cured in saturated limewater. The optimum level of replacement for cured specimens in water is 1.0 wt%. Although the limewater reduces the strength of concrete without nanoparticles when it is compared with the specimens cured in water, curing the specimens bearing nanoparticles in saturated limewater results in more strengthening gel formation around ZnO₂ nanoparticles causes more rapid setting time together with high strength. Accelerated peak appearance in conduction calorimetry tests, more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results, all indicate that ZnO₂ nanoparticles could improve mechanical and physical properties of the specimens.     

The influence of sulfate attack on the dynamic properties of concrete column

Concrete structures in service may be suffer from sulfate attack, which may lead to the deterioration of both static and dynamic properties of concrete, and therefore jeopardize their performance in earthquake events. To predict the seismic performance of sulfate-damaged concrete structures, the rate-dependent strength, deformation behavior and energy absorption capacity of concrete specimens under sulfate attack were investigated. First, half of the specimens were initially cured under sulfate attack for 4 months and the other half of the specimens were cured in a room environment for comparison. Then axial compressive experiments for concrete specimens with different strain rates were carried out. Experimental results show that sulfate attack had a significant impact on the dynamic properties of concrete. Sulfate attack makes concrete strength more sensitive to strain rate. With increasing strain rates, the peak strain and energy absorption capacity of control and sulfate attacked concrete tend to increase, but they are greatly decreased by disruptive expansion and chemical deterioration induced by sulfate attack. Finally, the failure phenomenon was shown and the damage mechanism of specimens was discussed.