粉煤灰掺量对非连续级配混凝土新拌流变性影响

利用自研流变仪设备,基于非连续级配粗骨料条件,试验研究了粉煤灰掺量对新拌混凝土的浆体包裹层厚度、流变屈服应力与塑性粘度的影响;同时分析了掺粉煤灰非连续级配混凝土的坍损、流变参数经时变化。结果表明,与连续级配混凝土相比,缺级配粗骨料新拌混凝土掺入粉煤灰后浆体包裹层厚度有所增加;屈服应力先增后减,而塑性粘度则先减后增;粉煤灰掺入同时可降低级配不良新拌混凝土塌损和减缓流变参数的经时变化,改善非连续级配拌合物的流变性能效果显著。

Air-void parameters measurement of fresh concrete and hardened concrete

Air content, spacing factor and specific surface of fresh concrete and hardened concrete with different air contents, slumps and mineral admixtures (fly ash, slag, fly ash + slag, fly ash + slag + silica fume composite) were studied by the air-void analyzer (AVA) method and the microscopical method. The correlations between the test results obtained from different methods were analyzed. The results show that, there is a close correlation of air content and spacing factor between the fresh concrete and the hardened concrete, but the specific surface correlation is weak. The air content of concrete measured by the AVA method is smaller than that of the pressure method and the microscopical method, because AVA device captures only the air voids with the size smaller than 3 mm. Spacing factor of the fresh concrete measured by the AVA method is greater than that of the hardened concrete measured by the microscopical method, while the specific surface is smaller. When the criterion of 4%-7% air content measured by the pressure method and microscopical method is acceptable for concrete freezing-thawing (F-T) durability in cold weather, the air content measured by the AVA method should be 2.4%-4.6%. For the concrete F-T durability, when the criterion of the spacing factor measured by the microscopical method is 300 μm, the spacing factor measured by the AVA method should be 360 μm.

Damage model of fresh concrete in sulphate environment

A model of damage to fresh concrete in a corrosive sulphate environment was formulated to investigate how and why the strength of corroded concrete changes over time. First, a corroded concrete block was divided into three regions: an expanded and dense region; a crack-development region; and a noncorroded region. Second, based on the thickness of the surface corrosion layer and the rate of loss of compressive strength of the corroding region, a computational model of the concrete blocks' corrosion-resistance coefficient of compressive strength in a sulphate environment was generated. Third, experimental tests of the corrosion of concrete were conducted by immersing specimens in a corrosive medium for 270 d. A comparison of the experimental results with the computational formulae shows that the calculation results and test results are in good agreement. A parameter analysis reveals that the corrosion reaction plays a major role in the corrosion of fresh concrete containing ordinary Portland cement,but the diffusion of the corrosion medium plays a major role in the corrosion of concrete mixtures containing fly ash and sulphate-resistant cement. Fresh concrete with a high water-to-cement ratio shows high performance during the whole experiment process whereas fresh concrete with a low water-to-cement ratio shows poor performance during the late experiment period.

Bond interface crack propagation of fresh foundation concrete and rock under blasting load

According to concrete age, the dynamic stress intensity factors of bond interface crack of concrete-rock was calculated. Result shows that the propagation of concreteinterface crack is mainly caused by tensile stress and shear stress for stress wave reflection. With the growth of concrete age, interface crack fracture toughness increases, and itscapacity of resisting blasting load strengthens. Therefore, blasting vibration should bestrictly controlled for fresh concrete.

The Application of Maturity Method on Whitetopping Construction

The primary objective of this study was to assess the use of the maturity method to determine the joint sawing window and the traffic opening time on whitetopping construction in Korea. To determine joint sawing time, it was necessary to find the minimum strength not to cause raveling and to identify the time to the occurrence of drying shrinkage. This study found that the minimum compressive strength for joint sawing was 4.41MPa （45kg/cm2） and drying shrinkage occurred just after the concrete temperature reached at the peak. To develop the relationship between compressive strength and maturity values, thermachron i-buttons were inserted into the top and mid-depth of the fresh concrete in the test slabs. The results of the laboratory tests indicated that the Arrhenius equation better fitted the relationship between the compressive strength and maturity values than did the Nurse-Saul equation. However, the Nurse-Saul function estimated in-place strength quite well in this study. Therefore, the Nurse-Saul equation was used to determine the joint sawing window and the traffic opening time for whitetopping construction.

Pore Water Pressure Arising during Pile Drilling in Sand

The pile working load depends on the imperfections which may be taken place in pile-soil system, during pile construction, among many other factors. This subject attracted the researcher＇s attention world wide in the last decades. Types of imperfections either geotechnical or structural are documented in literature and well explained. Nevertheless, the influence of these imperfections in pile load calculations is still ambiguous. The work presented herein is devoted to study soil disturbance during construction of piles using continuous flight auger, CFA. The study of soil disturbance due to drilling needs some evidence. The source of this evidence is field observations collected from four different construction sites, which are documented in this paper. The study concluded that the disturbed zone of soil by CFA has a conical shape and extending laterally to a distance equivalent to ten times of the pile diameter around the auger at the cutting bits and has an inclined surface of4：1 （vertical ： horizontal）. Furthermore excess pore water pressure was induced in soil in the vicinity of pile drilling. Due to this excess pore water pressure, 3.5% to 6.5% of piles constructed by CFA showed percolation of water from the top of the piles through fresh concrete. Also, subsidence of fresh concrete in pile hole was recorded in few of the constructed piles. Pile loading tests showed that the percolation of water and/or subsidence of fresh concrete have not appreciable influence on the load-displacement characteristics of the piles. Moreover, percolation of water at pile heads.