国内聚羧酸系高性能减水剂的合成及研究状况

简述了聚羧酸类减水剂合成方法、分子结构与性能的关系,讨论了大单体聚乙二醇丙烯酸酯等的制备工艺技术。     

Thermal field in water pipe cooling concrete hydrostructures simulated with singular boundary method

The embedded water pipe system is often used as a standard cooling technique during the construction of large-scale mass concrete hydrostructures. The prediction of the temperature distribution considering the cooling effects of embedded pipes plays an essential role in the design of the structure and its cooling system. In this study, the singular boundary method, a semi-analytical meshless technique, was employed to analyze the temperature distribution. A numerical algorithm solved the transient temperature field with consideration of the effects of cooling pipe specification, isolation of heat of hydration, and ambient temperature. Numerical results are verified through comparison with those of the finite element method, demonstrating that the proposed approach is accurate in the simulation of the thermal field in concrete structures with a water cooling pipe.     

Fracture of model concrete: 2. Fracture energy and characteristic length

The specific fracture energy G_F was measured in six types of simple concrete: all from the same matrix. The aggregates were spheres of the same diameter (strong aggregates, that debonded during concrete fracture, and weak aggregates, able to break); three kinds of matrix-aggregate interface (weak, intermediate and strong) were used. All in all, 55 test results are reported. These results are intended to be used as an experimental benchmark for checking numerical models of concrete fracture.A meso-level analysis of these results showed a correlation between the measured G_F values and the properties of the matrix, aggregates and interfaces, particularly with the actual area of the fracture surface. The strength of the matrix-aggregate interface correlates quite well with G_F, and concrete ductility, measured by means of the characteristic length, correlates also with the strength of the matrix-aggregate interface.     

Permeability characteristics of carbonated concrete considering capillary pore structure

During carbonation process, the calcium phases present in cement are attacked by CO₂ and converted into CaCO₃ and the permeability of concrete is changing due to the change in porosity. The rate of carbonation depends upon porosity and moisture content of the concrete. Especially in underground reinforced concrete structures, the interior portion of concrete surface may be exposed to carbonation and the exterior portion of concrete surface exposed to wet soil or underground water. As carbonation proceeds from outer surface into internal portion of concrete, microstructure is also changed continuously from outer surface into internal portion of concrete. Even the deteriorations in the structures due to the carbonation have been reported more, research on permeability characteristics of concrete considering carbonation and micro-structural information is very scarce.In this study, the permeability coefficient in carbonated concrete is derived by applying a capillary pore structure formation model in carbonated cement mortar and assuming that aggregates do not affect carbonation process in early-aged concrete as a function of porosity. The permeability obtained from the micro-level modeling for carbonated concrete is verified with the results of accelerated carbonation test and water penetration test in cement mortar.     

某面板堆石坝渗流控制措施有效性评估

本文基于ANSYS运用三维稳定渗流有限元法对某面板堆石坝进行渗流稳定性分析,研究了渗控措施设计方案下工程的渗流场分布及渗透稳定性,评价渗控措施的有效性,并对混凝土面板和防渗帷幕的渗透系数的变化对大坝渗流场的影响进行模拟.研究结果表明:大坝防渗系统防渗效果较好,有效的控制了坝内渗流场的分布;混凝土面板及防渗帷幕渗透系数的改变对渗流场影响较大,在渗控系统的施工过程中须严格控制施工质量.     

A multiscale model for effective moduli of concrete incorporating ITZ water-cement ratio gradients, aggregate size distributions, and entrapped voids

This paper develops a model for the effective elastic properties of concrete, which is a function of the volume fractions, size distributions, and elastic properties of fine aggregate (FA) and coarse aggregate (FA) and entrapped voids. Furthermore, the model is a function of the overall water-cement ratio and specific gravity of cement. Explicitly modeled are the water-cement ratio gradients through the interfacial transition zone (ITZ), which, in turn, affect the variation of the cement paste elastic properties through the ITZ, while maintaining the total fractions of cement and water consistent with the overall water-cement ratio. The ITZ volume is also conserved.     

Redefining cement characteristics for sulfate-resistant Portland cement

Experimental research was performed to relate specific cement characteristics to expansion due to sulfate attack. Twenty-one North American cement of statistically diverse chemical composition were used in the study. ASTM 1012 “Standard Test Method for Length Change of Hydraulic Cement Mortars Exposed to a Sulfate Solution” was performed using mortars prepared with each of the cement. First-order and multivariate relationships between cement characteristics and sulfate expansion were correlated at different ages. Analysis revealed that while tricalcium aluminate (C₃A) has typically been targeted as the chief contributor to sulfate attack, iron oxide (Fe₂O₃) or tetracalcium aluminoferrite (C₄AF) content, combined with total equivalent alkalis, showed a much stronger negative correlation with expansions at all ages. These results are in agreement with a broad spectrum of sulfate expansion theories and can provide a better means of specifying sulfate-resistant cement.     

Stress and strain state of concrete during freezing and thawing cycles

The objective of this work is to calculate the pressures, stresses, and strains induced into moist concrete during freezing and thawing. The applied theory is based on thermodynamics and the linear theory of elasticity. If no additional salts are dissolved in the pore water the inputs needed in the theory are relative humidity and temperature measured in the sample chamber and inside concrete and evaporable water amount in the pore structure. Theoretical results were compared with the test results made with two concretes cured under water or at 96% relative humidity. One of the concretes was air entrained and in the comparison concrete no air-entraining agents were used. In the test cylinders cured under water the largest tensional stresses in freezing occurred on the surface of the test cylinders both in the axial and tangential direction. The largest tensional stress was 2.2 MPa, both in air-entrained and in non air-entrained concretes. The largest tensional stresses in the warming phase took place at the end of the thawing period when the chamber temperature was around +5 °C. Then the maximum tension occurred in the middle of the concrete cylinder in the axial direction of the cylinder. This maximum tensional stress was over 2.5 MPa in the air-entrained concrete cured in the relative humidity of 96%. The thermodynamic pumping effect at the end of the thawing phase in every cycle can increase the pore water amount remarkably if free water or moisture is available on the surface of the structure or in the environment vapor. The thermodynamic pumping effect seems to be remarkably greater and more dangerous in air-entrained concretes.     

Behavior of high strength concrete with and without steel fiber reinforcement in triaxial compression

Based on an extensive experimental program, this paper studies the behavior of high strength concrete and steel fiber reinforced high strength concrete under uniaxial and triaxial compression. Triaxial stress-strain relations and failure criteria are used to evaluate the effect of steel fiber reinforcement on the mechanical properties of high strength concrete in triaxial compression, which is found to be insignificant.