基于孔结构的蒸压加气混凝土的冻融循环耐久性试验研究

为证明混凝土的毛细吸水系数可以作为其抗冻性能的表现形式,本工作通过蒸压加气混凝土和普通混凝土的冻融循环试验,共三组试件:B07、B08、C40普通混凝土,分别对其进行吸水率和毛细吸水系数的测定以及5次、15次、30次冻融循环试验,研究孔结构对混凝土抗冻性能的影响,通过数据分析和微观分析得出的结果:孔的数量越多,毛细吸水系数越大,在相同冻融循环情况下质量损失率、强度损失率越大,抗冻性能越差,证明了这一说法的可行性,为混凝土的应用提供了可靠的性能研究理论.     

蒸压加气混凝土孔结构表征的图像分析方法

采用数码相机采集图像.利用Matlab将加气混凝土砌块断面照片转化为灰度照片,并对照片进行去噪、直方图均衡化、二值化处理,采用图像分析法实现了对蒸压加气混凝土的宏观孔孔隙率、孔面积及其分布等孔结构特征的计量.分析结果表明,基于Matlab图像分析法可以对蒸压加气混凝土的孔结构进行初步的量化表征,但限于所采用数码相机的分辨率,该方法只适合表征蒸压加气混凝土中的宏观孔.通过改进程序分辨能力、相片拍摄技巧以及样品准备与处理方法可以大幅减小图像分析法的表征误差,提高方法的准确性.     

蒸压加气混凝土的孔结构及表征方法研究进展

蒸压加气混凝土因其质轻、保温性好、环保等优点而受到人们的重视。作为一种典型的宏观多孔建筑材料,蒸压加气混凝土的孔结构特征与其微观结构和性能有密切的关系。钙硅比、水料比、铝粉及工艺参数是影响蒸压加气混凝土孔结构的重要因素,孔隙率、孔径分布等孔结构特征与蒸压加气混凝土的强度、吸水性、干燥收缩、导热性能及耐久性等性能有紧密的联系。本文重点阐述了蒸压加气混凝土的孔结构特征及影响孔结构的主要因素,孔结构对蒸压加气混凝土性能的影响,并简述了蒸压加气混凝土孔结构现阶段主要采用的表征方法。     

蒸压加气混凝土孔结构及其对性能的影响研究进展

蒸压加气混凝土的主要特征是多孔性,其孔结构是影响性能的主要因素之一。简述了蒸压加气混凝土中孔的分类、特征以及表征方法,综述了国内外蒸压加气混凝土强度、吸水导湿、干燥收缩、热工性能、耐久性能等方面的最新研究进展,探讨了蒸压加气混凝土孔结构与性能之间的关系。最后阐述了蒸压加气混凝土孔结构与性能今后的研究方向和研究重点。     

掺合料粉体种类对泡沫混凝土性能的影响

以普通硅酸盐水泥为结合剂,用粉煤灰和硅灰取代砂和部分水泥,研究掺和料种类对泡沫混凝土抗压强度、吸水率以及抗冻性的影响。结果表明:泡沫混凝土的性能不仅与孔隙率有关,还与基体材料中掺合料的种类有关。加入硅灰可引起泡沫混凝土的成型水胶比增加,显著提高了泡沫混凝土的早期强度,但同时引起吸水率的增加,也不利于抗冻;当掺合料为粉煤灰时,提高了泡沫混凝土的抗冻耐久性,当将原状粉煤灰磨细,使泡沫混凝土的后期强度增长较快,并大幅度的降低了吸水率,但对抗冻性影响不大。     

养护方式对磷渣加气混凝土性能的影响

利用磷渣的潜在活性,研制了以磷渣为原材料的加气混凝土。研究了标准养护和蒸压养护两种养护方式对磷渣加气混凝土体积密度、抗压强度、干燥收缩、抗冻性和导热系数等性能的影响。结果表明,与标准养护相比,蒸压养护可以降低磷渣加气混凝土的体积密度、干燥收缩值和导热系数,提高加气混凝土的抗冻性能。     

养护方式对磷渣加气混凝土性能的影响

利用磷渣的潜在活性，研制了以磷渣为原材料的加气混凝土。研究了标准养护和蒸压养护两种养护方式对磷渣加气混凝土体积密度、抗压强度、干燥收缩、抗冻性和导热系数等性能的影响。结果表明，与标准养护相比，蒸压养护可以降低磷渣加气混凝土的体积密度、干燥收缩值和导热系数，提高加气混凝土的抗冻性能。     

风积沙混凝土盐冻多尺度劣化机制

研究风积沙混凝土盐冻劣化规律，揭示劣化机制对其推广应用有重要指导意义。基于室内快速冻融试验及力学特性试验研究了风积沙混凝土盐冻劣化规律，结合SEM、NMR、XRD等表征技术及损伤力学理论从多尺度揭示了盐冻劣化机制。结果表明：风积沙影响混凝土的抗冻性，100%掺量风积沙混凝土强度低，但抗冻性最好。混凝土质量损失率及抗压强度损失率均随盐冻循环次数的增加而增大，相对动弹性模量随盐冻循环次数的增大而减小。风积沙混凝土的盐冻损伤是一个物理-化学过程，界面过渡区(ITZ)骨-浆剥离及附近砂浆基质开裂是导致其宏观物理、力学性能退化的主要原因。风积沙可以改变混凝土内部的孔隙结构及水分传输路径，进而影响孔隙饱和度及混凝土的抗盐冻性能。     

加气混凝土宏观孔分析简化方法及验证

利用MATLAB图像处理技术对加气混凝土的宏观孔特性进行分析,初步量化加气混凝土孔分布、平均孔径和孔隙率等特征。通过对已知孔特性的标准图形分析,验证该方法的准确性;通过与图像分析系统测试的加气混凝土孔特性结果对比,验证该方法的可行性。实验表明加气混凝土宏观孔分析简化方法能够较精确分析加气混凝土的宏观孔特性。     

孔径分布对多孔镍孔体积分形维数的影响

用压汞法对用粉末冶金法制备的多孔镍试件进行了测试,得到了多孔镍试件的孔隙率及孔径分布曲线,并以Menger海绵体模型为基础,根据压汞实验数据确定了多孔镍试件的孔体积分形维数,发现多孔镍的孔隙结构具有明显的分形特征,分形维数在2.8～3.0之间.孔径分布范围越宽、平均孔径越大,孔体积分形维数越大.多孔镍的孔体积分形维数在一定程度上反映了多孔镍孔隙结构的均匀性及填充能力.     

矿物掺合料对地聚合物抗冻性能的影响

以碱激发偏高岭土制备地聚合物混凝土,分别研究了掺入15%的钢渣、矿渣或粉煤灰的地聚合物混凝土的力学抗压强度和抗冻性能,测试了地聚合物混凝土的真空饱水体积吸液率,运用XRD、SEM和DSC-TG等测试方法分析了矿物掺合料对地聚合物微观结构和水化产物的影响。结果表明:钢渣或矿渣能有效提高地聚合物混凝土的抗压强度,而粉煤灰的掺入使其强度稍有降低;地聚合物表观形貌中存在较多的孔洞和微裂缝导致其抗冻性能较差,掺入钢渣或者矿渣后地聚合物形成了新的产物C-S-H凝胶、C-A-S-H凝胶等并填充在结构中形成更加密实的板状结构,降低了地聚合物混凝土冻融破坏速率,五次冻融循环后地聚合物的相对强度均在90%以上,抗冻性能得到提高;粉煤灰降低了制备地聚合物混凝土的用水量且未水化的粉煤灰颗粒镶嵌在结构中增加了其密实性和抗冻性能,五次冻融循环后相对强度为86.9%,基准组的相对强度仅为79.7%。     

Air void structure and frost resistance: a challenge to Powers’ spacing factor

This article compiles results from 4 independent laboratory studies. In each study, the same type of concrete is tested at least 10 times, the air void structure being the only variable. For each concrete mix both air void analysis of the hardened concrete and a salt frost scaling test are conducted. Results were not originally presented in a way, which made comparison possible. Here the amount of scaled material is depicted as function of air voids parameters: total air content, specific surface, spacing factor, and total surface area of air voids. The total surface area of air voids is proportional to the product of total air content and specific surface. In all 4 cases, the conclusion is concurrent that the parameter of total surface area of air voids performs equally well or better than the spacing factor when linking air void characteristics to frost resistance (salt frost scaling). This observation is interesting as the parameter of total surface area of air voids normally is not included in air void analysis. The following reason for the finding is suggested: In the air voids conditions are favourable for ice nucleation. When a capillary pore is connected to an air void, ice formation will take place in the air void, being feed from the capillary, but without pressure build-up in the capillary. If the capillary is not connected to an air void, ice formation will take place in the capillary pore, where it can generate substantial pressure. Like this, frost resistance depends on that capillary pores are connected to air voids. The chance that a capillary pore is connected to an air void depends on the total surface area of air voids in the system, not the spacing factor.     

Transport of organic fluids through concrete

The absorption of fluids in concrete is influenced by various parameters such as the surface tension and dynamic viscosity of the fluid, and also the porosity, pore size distribution and the interconnection of the pores in concrete. In addition, chemical reactions between the penetrating fluid and concrete may occur, which affect the absorption behaviour. Chemical reactions can cause deviations from the square-root-of-time relation and a reduction of sorptivity. Both are observed if using water or ethylene glycol as testing fluids. The deviation from the square-root-of-time relation is caused by the dissolving of Ca(OH)₂ from the cement matrix by the absorbed fluid. In the case of water, the reduction of the sorptivity is due to rehydration and wetting expansion. If a fluid is being absorbed in concrete, the effective porosity is smaller than the available porosity. The greater the surface tension of the fluid, the greater the measured effective porosity. This can be explained by trapped air inside the pores, which cannot get out and is being compressed by the capillary pressure, either in dead-end pores or due to fingering of the penetrating fluid. Experimental results on sequential absorption of two different fluids in concrete can only be explained if a trapped pore volume is assumed.     

Low-velocity flexural impact response of fiber-reinforced aerated concrete

Impact response of fiber-reinforced aerated concrete was investigated under a three-point bending configuration based on free-fall of an instrumented impact device. Two types of aerated concrete: plain autoclaved aerated concrete (AAC) and polymeric fiber-reinforced aerated concrete (FRAC) were tested. Comparisons were made in terms of stiffness, flexural strength, deformation capacity and energy absorption capacity. The effect of impact energy on the mechanical properties was investigated for various drop heights and different specimen sizes. It was observed that dynamic flexural strength under impact was more than 1.5 times higher than the static flexural strength. Both materials showed similar flexural load carrying capacity under impact, however, use of 0.5% volume fraction of polypropylene fibers resulted in more than three times higher flexural toughness. The performed instrumented impact test was found to be a good method for quantifying the impact resistance of cement-based materials such as aerated concrete masonry products.     

Frost resistance and pore size distribution in bricks

Correlation between frost resistance, porosity and pore size distribution was examined. Different test methods were used to evaluate the frost resistance. Porosity and pore size distribution were examined in mercury intrusion porosimeter (MIP). Scanning electron microscopy gave a visual view of the pore geometry, pore size and porosity. A linear correlation was found between frost resistance and the inverse value of the intruded pore volume. A linear correlation was also found between frost resistance and percentage of pores with diameter bigger than 3 μm. The test results were analysed using statistical methods. From the MIP-test a frost resistance number can be calculated which indicates the frost resistance of the brick.     

Experimental Determination of Frost Resistance of Autoclaved Aerated Concrete at Different Levels of Moisture Saturation

The ability of porous building materials to stand up to moisture phase changes induced by alternating environment is described mostly by means of their frost resistance. However, the test conditions defined by relevant standards might not capture the real situation on building site in various locations. In particular, the prescribed full water saturation of analyzed specimens during the whole time of a freeze/thaw experiment presents an ultimate case only but certainly not an everyday reality. Even the materials of surface layers are mostly exposed to such severe conditions just for a limited period of time. In this paper, the experimental analysis of frost resistance of three different types of autoclaved aerated concrete (AAC) is performed in an extended way, including not only the standard testing but also the investigation of dry- and partially saturated samples. A complementary computational analysis of an AAC building envelope in Central European climate is presented as well, in order to illustrate the likely hygric conditions in the wall. Experimental results show that according to the standard test the loss of compressive strength, as well as the loss of mass after 25 cycles, is acceptable for all studied samples but after 50 cycles only the material with the compressive strength of 4 MPa performs satisfactorily. On the other hand, the tests with initially dried or partially saturated samples indicate a good frost resistance of all studied materials for both 25 and 50 cycles.     

Resistance to freezing and thawing of mortar specimens made from sulphoaluminate—belite cement

Resistance to freezing and thawing of mortar specimens made from sulphoaluminate-belite cement (M-SAB) is compared with that of mortars made from portland cement (M-PC). The results suggest that larger median radius of the pores and total porosity of M-SAB compared to those of M-PC are primarily caused by the rapid setting of the SAB cement. The ‘coarsening’ of pore structure of mortar specimens under action of freezing and thawing is proved by the increase in the macropores portion, median pore radius, and total porosity values. This process is more intense in M-SAB. The effect of the frost attack is confirmed by lower compressive strength and dynamic modulus of elasticity on the one hand and higher absorption capacity, expansion, and crack propagation of M-SAB compared with those of M-PC on the other hand. Besides the decreased frost resistance of M-SAB as compared with that of M-PC, unsatisfactory passivation of steel in M-SAB was found. The reason of this fact is the pH value decrease to less than 11.5 of the M-SAB extract     

消泡剂和增稠剂复掺对模袋混凝土抗冻性的影响

为使模袋混凝土在大流动性下达到高抗冻性,应用正交试验法研究了含气量、消泡剂和增稠剂对混凝土工作性和抗冻性的影响。结果表明,含气量和消泡剂掺量是影响模袋混凝土相对动弹性模量的显著因素,且含气量影响效果显著大于消泡剂掺量,增稠剂掺量对相对动弹性模量的提高幅度较小。消泡剂和增稠剂复掺能够达到协同改善模袋混凝土工作性、孔结构和抗冻性的效果,当含气量为5vol%~6vol%时,0.15wt%消泡剂和0.03wt%增稠剂复掺使模袋混凝土含气量损失率减小了64.28%、扩展度损失率减小了55.04%;主要消除有害大气泡数量达81.38%,增加小气泡数量达14.89%,使气泡间距系数减小了11.54%,气泡比表面积减小了20.49%,相对动弹性模量增大了11.97%。气泡间距系数和气泡比表面积均与相对动弹性模量具有良好的相关性;当气泡间距系数不大于361 μm、气泡比表面积不小于16.13 mm?1时,模袋混凝土抗冻等级可达F300。通过对试验结果的回归分析,建立了模袋混凝土抗冻性预测模型。