Fracture process zone of notched three-point-bending concrete beams

By measuring and analyzing the load-deflection curve and acoustic emission characteristics of notched three-point-bending concrete beams with different notch depths, the length of the fracture process zone (FPZ) was calculated from the difference between the equivalent crack lengths obtained by different kinds of equivalence. Then, the evolution of the FPZ was quantitatively described. It was found that length of the FPZ is not a material parameter; it is greatly influenced by the specimen size. But in a relative sense, the influence of the specimen size can be eliminated. Also, the evolution of the FPZ in specimens of different sizes is almost the same. When the crack length is small, the FPZ increases linearly with the crack extension. As the crack extends to half of the ligament, the FPZ reaches its maximum size. Thereafter, the FPZ moves ahead and shrinks, but the ratio of its length to the length of the residual ligament remains constant, approximately equal to 0.77.     

混凝土泵车臂架油缸抗偏载研究与应用

通过试验分析找到了混凝土泵车臂架油缸偏载的原因,提出了一种提高混凝土泵车臂架油缸抗偏载能力的方案,设计出了一种新型的导向结构,从而提高了油缸的抗偏载力,为进一步优化超长臂架油缸提供了参考。     

Corrosion of reinforcing steel in neutral and acid solutions simulating the electrolytic environments in the micropores of concrete in the propagation period

Steel embedded in concrete passivates due to the alkaline nature of the concrete pore solution. Reinforcement corrosion develops when the alkalinity is neutralized. Corrosion, in turn, induces acidification of the surrounding pore solution. In the present paper, corrosion rate of corrugated steel bars is studied and measured at different pH values in solutions simulating chloride environments. The media considered consists of saturated calcium hydroxide solutions containing Na and K⁺, neutralized with ferrous chloride. This latter substance is the soluble compound produced during the corrosion of steel after chloride attack. Hydrochloric acid solutions of different pHs were prepared in order to compare the steel corrosion rates in these solutions with those observed in ferrous chloride solutions of the same pH. A comparison of polarization resistance measurements (R_p) with gravimetrically weight loss determined is presented. Tafel slopes results are also included. Additionally, a comparison is made between measurements of AC impedance with those of the R_p method. The results indicate that the corrosion rate in the studied media follows the general trend found in other media of similar pH values: corrosion increases in acidic solutions, remains rather stable for pH range 3-11 and decreases significantly in highly alkaline solutions.     

High-frequency ultrasonic testing of young cement-based materials using the ‘prism technique’

A new technique for testing young cement paste and mortar is presented. It is based on a method, which uses unconventional prism-shaped specimens. It involves the precise measurement at fixed intervals of 24 h of the time-of-flight of both compressional and shear waves using high-frequency ultrasound. A water tank was designed that allows for easy coupling and handling of the specimens. Both waves are generated by mode conversion using the same transducer. The echoes from all the specimens were strong, and no signal processing was required. Thus, reliable and repeatable results were achieved using a very basic apparatus. Second-order elastic constants were evaluated, and correlation was made with the porosity and age of mortar. A new standard based on this method is proposed which would help in the making of prediction models for evaluating the strength of cement-based materials.     

水泥混凝土水化热的研究与进展

水泥的水化反应是一个放热反应。水泥水化放热的周期很长,但大部分热量是在3天内放出的,尤其是在水泥浆发生凝结、硬化的初期放出。大多数情况下,硬化水泥浆体和混凝土的早期体积变形,主要源于水泥的水化热温升,因此,降低水泥混凝土的水化热是防止其早期开裂的有效途径。本文综合分析了水泥混凝土水化热对其性能的影响,总结了前人在水泥混凝土水化热研究方面提出的一些理论计算公式,介绍了国内外关于水泥混凝土水化热的最新研究进展和水泥生产中降低水化热的技术措施。     

Influence of the depth and morphology of real cracks on diffuse ultrasound in concrete: A simulation study

The aim of the present paper is to simulate the propagation of diffuse ultrasonic energy in concrete in the presence of a real crack. The numerical model is presented and validated by the comparison with experimental data from the literature. Unlike most of the studies which consider a crack as a notch, a realistic crack morphology exhibits partial contacts along its lips. These contacts are modeled in order to study their influence on the diffusion parameters. The feasibility of determining the contact density of the crack is shown, revealing practice implications for non-destructive crack sizing and imaging in concrete.     

ASR pessimum behaviour of siliceous limestone aggregates

Siliceous limestone aggregates have “pessimum” behaviours similar to those observed for pure siliceous aggregates such as flint or opal. For high alkali contents concretes based on fine and coarse reactive siliceous limestone aggregates swell less than concretes based on fine reactive siliceous limestone aggregates and non-reactive coarse aggregates. The reduction of the swelling is more significant for a “micritic” limestone containing highly reactive free silica than for a “sparitic” limestone containing less reactive free silica. The consumption of a part of the alkalis by non-expansive processes such as chemical sorption on silanol sites of reactive silica and pozzolanic C-S-H is expected to explain the reduction of the swelling.     

Effects of seawater on AAR expansion of concrete

Recently, AAR was identified in submerged piles of some bridges in tidal waters. Microstructural examination detected chloroaluminate salts in some cracks. To clarify whether seawater had influenced the deterioration an experimental program was planned to examine the effects of sodium chloride on AAR under various curing conditions.Concrete prisms containing either of highly-reactive, slowly-reactive or nonreactive aggregate, and either low or high alkali contents, were stored in saltwater (representing seawater) or at 100% RH, at temperatures of 38, 60 and 80 °C, for expansion measurement over 600 days, after which the temperature for those stored in saltwater was lowered to 23 °C, to check its effect on further expansion, which could be attributed to precipitation of ettringite and/or Ca-chloroaluminate.The results indicate that the type of aggregate and concrete alkali content had the greatest effect on AAR expansion. Exposure to saltwater did not have any significant effect on the AAR expansion.     

Nonlinear ultrasonic evaluation of load effects on discontinuities in concrete

The presence of discontinuity surfaces in concrete structures, i.e. two or more layers in contact, may be an existing situation with evident relapses on damage formation and progression. Differences occur depending on the type of discontinuity, which could be a thin weaker layer or a pre-existing crack. The behavior of pre-existing interfaces is here studied by means of the Scaling Subtraction Method, a Nonlinear Ultrasonic Non-Destructive Technique, that revealed to be effective in describing the mechanical evolution of concrete samples with discontinuity surfaces under the effects of compressive loads.     

Effect of elevated temperatures on geopolymer paste, mortar and concrete

Geopolymers are generally believed to provide good fire resistance due to their ceramic-like properties. Previous experimental studies on geopolymer under elevated temperatures have mainly focused on metakaolin-based geopolymers. This paper presents the results of a study on the effect of elevated temperature on geopolymer paste, mortar and concrete made using fly ash as a precursor. The geopolymer was synthesized with sodium silicate and potassium hydroxide solutions. Various experimental parameters have been examined such as specimen sizing, aggregate sizing, aggregate type and superplasticizer type. The study identifies specimen size and aggregate size as the two main factors that govern geopolymer behavior at elevated temperatures (800 °C). Aggregate sizes larger than 10 mm resulted in good strength performances in both ambient and elevated temperatures. Strength loss in geopolymer concrete at elevated temperatures is attributed to the thermal mismatch between the geopolymer matrix and the aggregates.