高抛免振自密实混凝土在全钢结构工程中的施工技术

自密实混凝土是混凝土工艺的一次革命,已成为当前混凝土研究的一个热点,并将被推广和广泛应用。文章结合工程实例介绍了自密实混凝土的性能,重点讲述了高抛免振自密实混凝土的施工要点和质量控制。     

Workability of hybrid fiber reinforced self-compacting concrete

Compared to fiber reinforced concrete (FRC), self-compacting concrete (SCC) is a relatively new type of concrete with high flowability and good cohesiveness. It offers very attractive economical and technical benefits, which can be further extended when combined with FRC. In this article two different types of steel fibers were used, in combination, and the effects of fiber inclusion on the workability of hybrid fiber reinforced self-compacting concrete (HFR-SCC) is studied. The effects of fibers are quantified based on the fiber volume, length, and aspect ratios of the fibers. It was concluded that in addition to the above-mentioned quantifiable three properties, other properties of fibers such as shape and surface roughness are also found to be important but they cannot be quantified at this stage.     

The effect of polypropylene fibers on the properties of fresh and hardened lightweight self-compacting concrete

This paper evaluates the LECA Lightweight Self-Compacting Concrete (LLSCC) manufactured by Nan-Su, of which the Packing Factor (PF) of its design mixing method has been modified and improved.The study analyzes the impact of polypropylene fibers on LLSCC performance at its fresh condition as well as its mechanical properties at the hardened condition.The evaluation of Fiber Reinforced LLSCC (FR-LLSCC) fluidity has been conducted per the standard of second class rating of JSCE, by three categories of flowability, segregation resistance ability and filling ability of fresh concrete.For the mechanical properties of LLSCC, the study has been conducted as follows: compressive strength with elapsed age, splitting tensile strength, elastic modulus and flexural strength, all of which were measured after the sample being cured for 28 days.When self-compacting concretes were lightened to 75% of their normal weight, their fresh properties are affected immensely.Applying 0.3% volume fractions of polypropylene fiber to the LLSCC resulted in 40% reduction in the slump flow (from 720 mm to 430 mm). In general, the rate of slump flow over Super Plasticizer (SP) volume percentage reduced with the use of polypropylene fibers in the FR-LLSC.Polypropylene fibers did not influence the compressive strength and elastic modulus of LLSCC, however applying these fibers at their maximum percentage volume determined through this study, increased the tensile strength by 14.4% in the splitting tensile strength test, and 10.7% in the flexural strength.     

大掺量粉煤灰自密实高性能混凝土的实践

自密实混凝土作为一种高性能混凝土应用的工程已经越来越普遍,本文就威海市环翠楼公园钢结构改造工程实践了通过大掺量Ⅱ级粉煤灰的方式提高自密实混凝土的保水性、流动性及强度。同时节约了大量的水泥,中低强度等级自密实混凝土的实现也为绿色无噪音施工带来了可行性,也为钢管柱的施工带来了方便。     

基于BP神经网络的硬化混凝土水胶比预测

混凝土一旦凝结硬化,再次评价其原始组分非常困难,目前针对水胶比的测试方法,试验复杂,精度不高。而水胶比是混凝土的重要参数,对混凝土的性能发挥至关重要的作用。试验制备了不同水胶比的混凝土,测试了不同龄期时混凝土强度、水泥浆钙元素含量、粗细集料用量。由于水胶比与上述因素间存在相关性,采用BP神经网络分析了硬化混凝土强度、龄期、粗细集料用量、水泥浆钙元素含量与硬化混凝土水胶比的非线性映射关系。研究结果表明:BP神经网络模型预测结果与实际水胶比吻合度较高,具有良好的预测效果。     

混凝土水灰比的影响因素

针对水灰比对混凝土性能的重要性,从拌合条件、材料、外加剂和气温几方面研究了影响混凝土水灰比的因素,以实现在混凝土的各项性能满足要求的情况下,合理减小水灰比的目的,从而提高经济效益。     

Influence of limestone filler and viscosity-modifying admixture on the porous structure of self-compacting concrete

The objective of this experimental work is to study porosity in self-compacting concrete (SCC) made without adding limestone filler, comparing the results with other SCC and with normally-vibrated concrete (NVC). Several types of concrete were made, keeping the w/c ratio constant. The results show that the air content in SCC depends on the flowability and viscosity of the material, putting a forward an expression to estimate the air content in accordance with these two parameters. SCC shows a finer and more tortuous porous structure than NVC, leading to lower permeability to water under pressure. Nevertheless, in the absence of pressure, when water penetrates by capillary action, the results obtained from the different types of concrete were very similar, with differences below 3.5%. This is due to the fact that the content of pores over 0.5 μm is practically the same in SCC and NVC, but for smaller pore sizes, which are therefore only accessed when water under pressure is applied, the differences in porosity between the different samples is more pronounced. On the other hand, it was observed that the use of more fluid mixtures permitted more impermeable concrete to be obtained. The use of viscosity-modifying admixture on SCC as a replacement for limestone filler does not affect the total volume of pores, but generates a slightly more coarse porous microstructure, thereby leading to concretes in which water penetration depth under pressure is a little higher (around 4 mm).     

增黏剂对新拌自密实混凝土性能及模板侧压力的影响

研究了增黏剂PEG400和PEG800对新拌水泥净浆流变特性、新拌自密实混凝土（SCC）扩展度及经时损失、对模板的初始侧压力及其经时衰减速度的影响。结果表明,未掺增黏剂的水泥净浆呈典型的剪切变稀行为;掺增黏剂后,随剪切时间延长,水泥净浆均呈先剪切变稀后剪切变稠行为,达黏度峰值后,再呈剪切变稀行为,最后又呈剪切变稠行为。掺增黏剂对SCC的初始扩展度影响较小,但2种增黏剂均导致其20 min扩展度损失增大,掺PEG800时20 min扩展度损失更明显。掺增黏剂对减小SCC初始模板侧压力不利,但两者均可有效加快模板侧压力的衰减速度。     

Genetic programming based formulation for fresh and hardened properties of self-compacting concrete containing pulverised fuel ash

Self-compacting concrete (SCC) flows into place and around obstructions under its own weight to fill the formwork completely and self-compact without any segregation and blocking. Elimination of the need for compaction leads to better quality concrete and substantial improvement of working conditions. This investigation aimed to show possible applicability of genetic programming (GP) to model and formulate the fresh and hardened properties of self-compacting concrete (SCC) containing pulverised fuel ash (PFA) based on experimental data. Twenty-six mixes were made with 0.38 to 0.72 water-to-binder ratio (W/B), 183–317 kg/m³ of cement content, 29–261 kg/m³ of PFA, and 0 to 1% of superplasticizer, by mass of powder. Parameters of SCC mixes modelled by genetic programming were the slump flow, JRing combined to the Orimet, JRing combined to cone, and the compressive strength at 7, 28 and 90 days. GP is constructed of training and testing data using the experimental results obtained in this study. The results of genetic programming models are compared with experimental results and are found to be quite accurate. GP has showed a strong potential as a feasible tool for modelling the fresh properties and the compressive strength of SCC containing PFA and produced analytical prediction of these properties as a function as the mix ingredients. Results showed that the GP model thus developed is not only capable of accurately predicting the slump flow, JRing combined to the Orimet, JRing combined to cone, and the compressive strength used in the training process, but it can also effectively predict the above properties for new mixes designed within the practical range with the variation of mix ingredients.     

水灰比对混凝土气泡特征参数影响的试验研究

为指导抗冻混凝土的设计与评估,采用先进的硬化混凝土气泡结构测试设备,通过试验研究了水灰比对混凝土气泡特征参数的影响规律.结果表明:混凝土拌合物含气量相同时,水灰比对硬化混凝土的气泡间隔系数、平均气泡直径及比表面积、气泡的孔径分布等特征会产生显著的影响.随水灰比的增加,气泡间隔系数变大,气泡结构变差,这将对混凝土抗冻性产生重要影响.建议相关规范在对引气混凝土的含气量及气泡间隔系数进行规定时,应充分考虑水灰比的影响.     

高抛免振捣钢管自密实混凝土施工质量控制技术探索

本文通过钢管自密实混凝土模拟试验及钻芯法、超声波法等测试手段,研究钢管自密实混凝土施工质量控制技术。结果表明,在钢管壁合理设置排气孔及控制浇筑速度有利于混凝土密实性;控制原材料入机温度、增加矿物掺合料用量及避开高温时段浇筑等措施可降低混凝土入模温度,减小混凝土水化温升;采用蓄水覆盖养护可有效防止钢管自密实混凝土早期收缩...     

Effect of used engine oil on properties of fresh and hardened concrete

There is a current trend all over the world to investigate the utilization of processed and unprocessed industrial by-products and domestic wastes as raw materials in cement and concrete. This has a positive environmental impact due to the ever-increasing cost of waste disposal and stricter environmental regulations. Historically, reference books on concrete technology and cement chemistry indicate that the leakage of oil into the cement in older grinding units resulted in concrete with greater resistance to freezing and thawing. This effect is similar to adding an air-entraining chemical admixture to the concrete. Such information is not backed by any research study reported in the literature. The objective of the research reported in this paper was to investigate the effects of used engine oil on properties of fresh and hardened concrete. The main variables included the type and dosage of an air-entraining agent (commercial type, used engine oil, or new engine oil), mixing time, and the water/cement ratio of the concrete. Results showed that used engine oil increased the slump and percentage of entrained air of the fresh concrete mix, and did not adversely affect the strength properties of hardened concrete.     

自密实混凝土的力学性能研究

对自密实混凝土的力学性能进行了研究,探讨了自密实混凝土的抗压强度、抗压尺寸效应、劈拉强度、养护龄期、抗折强度以及各强度指标之间的关系.     

自密实超高强混凝土力学性能试验研究

首先对自密实混凝土的定义及优点作了简介,并列举了几篇国内外关于自密实混凝土的基本力学性能、自密实混凝土与振捣混凝土力学性能的比较以及掺入超细粉煤灰和超塑化剂对自密实混凝土性能的改善。随后,作者通过试验的方法对自密实混凝土的基本力学性能做了研究,并比较了圆柱体抗压强度与劈拉强度、圆柱体抗压强度与弹性模量以及三者与混凝土密度之间的关系。     

谈水灰比对混凝土强度的影响

论述了混凝土水灰比中水和水泥的称量发生误差对混凝土强度的影响并加以比较,介绍了在不影响混凝土强度的条件下如何调整混凝土的流动性,为混凝土的推广奠定了基础。     

Hydric, thermal and mechanical properties of self-compacting concrete containing different fillers

Water vapor diffusion coefficient, water absorption coefficient, water permeability, thermal conductivity, compressive strength and freeze resistance properties of two self-compacting concretes (SCC) containing limestone filler and fly ash are determined in the paper. The results indicate that strength development is faster in the material containing limestone filler but after 90 days the strength values in both materials are almost the same. Liquid water transport and heat transport are faster in SCC with limestone filler while the water vapor transport is faster in the material containing fly ash. This corresponds well with the much better freeze resistance of the material with fly ash compared to that with limestone filler and with the results of mercury intrusion porosimetry.     

A review of the fresh/hardened properties and applications for plain- (PRC) and self-compacting rubberised concrete (SCRC)

Significant research has been undertaken to better understand the performance characteristics and commercial potential for scrap vehicles tyres to be utilised as an alternative to conventional aggregates in concrete. Substituting recycled rubber for natural aggregates gives plain rubberised concrete (PRC) and is typically for non-structural applications. Several studies have also focussed on self-compacting rubberised concrete (SCRC). This paper summarises, compares and draws general conclusions from the findings of more than 70 independent studies, in terms of the fresh and hardened state properties of rubberised concrete to highlight and confirm the key characteristics of the material. Moreover, recommendations for future work and directions for this field of research have been detailed.     

Influence of aggregates grading and water/cement ratio in workability and hardened properties of mortars

Mortar is the material responsible for the distribution of stresses in masonry structures. The knowledge about the fresh and hardened properties of mortar is fundamental to ensure a good performance of masonry walls. Water/cement ratio and aggregates grading are among several variables that influence physical and mechanical behaviour of mortars. An experimental program is presented in order to evaluate the influence of aggregates grading and water/cement ratio in workability and hardened properties of mortars. Eighteen compositions of mortar are prepared using three relations cement:lime:sand, two types of sand and three water/cement ratios. Specimens are analyzed through flow table test, compressive and flexural strength tests. Results indicate that the increase of water/cement ratio reduces the values of hardened properties and increases the workability. Besides, sands grading has no influence in compressive strength. On the other hand, significant differences in deformation capacity of mortars were verified with the variation of the type of sand. Finally, some correlations are presented among hardened properties and the compressive strength.     

硬化混凝土中水泥用量的测定方法概述

水泥用量是混凝土配合比的重要参数,测定混凝土中的水泥用量对耐久性判断及配合比检查具有重要意义。硬化混凝土中的水泥用量可用化学分析方法及光学显微镜法测得。化学分析法又分为水泥溶解法与可溶性SiO2和CaO分析法,光学显微镜法包括荧光显微镜和扫描电镜。重点对化学分析方法的测定原理、试验方法及结果计算做了介绍,评价了各种方法的精密度与准确性,并总结了可能对测定造成影响的因素。