回字形框架结构在温度变化及混凝土收缩下的应变分析

采用继效流动理论分析了洪泽文化中心框架结构在温度变化及混凝土收缩情况下的徐变。通过实验结果及理论结果对比,发现以继效流动理论为基础计算回字形框架结构的应变是一种可行的方法,计算结果与实验结果相近。

钢筋混凝土超长结构裂缝控制技术

近年来,随着城市建设不断加快,建筑工程不断朝着大规模、多功能化方向发展,超长、超大体量的建筑愈来愈多。因结构越长,其温度及收缩变形就越大,约束内力往往引起结构开裂,从而会影响结构安全和耐久性。因此,超长结构裂缝控制成为施工质量控制的重点。本文笔者对钢筋混凝土超长结构裂缝控制问题进行了深入地研究、分析、实践和总结,通过优化结构配筋、试验优化混凝土配合比、浇筑过程严格管控、温度监测及加强养护等系统管理控制措施,取得了较好的裂缝控制效果。

Numerical simulation of concrete shrinkage based on heat and moisture transfer in porous medium

To simulate the concrete shrinkage in varying temperature and moisture environments, a simulate procedure comprising an analytical process and a finite element analysis is proposed based on the coupled partial differential equations describing heat and moisture transfer in a porous medium. Using the Laplace transformation method and transfer function to simplify and solve the coupled equations in Laplace domain, the moisture and temperature distribution in time domain are obtained by inverse Laplace transformation. The shrinkage deformations of concrete are numerically simulated by the finite element method （FEM） based on the obtained temperature and moisture distribution. This approach avoids the complex eigenvalues, coupling difficulty and low accuracy found in other solving methods, and also effectively calculates the moisture induced shrinkage which is almost impossible using familiar FEM software. The validity of the simulation procedure is verified by Hundt＇s test data. The results reveal that the proposed approach can be considered a reliable and efficient method to simulate the coupling moisture and temperature shrinkage of concrete.

Nonlinear finite element analysis of steel-concrete composite beams

Proposes a simplified finite element model for steel-concrete composite beams. The effects of slip can be taken into account by creating a special matrix of shear connector stiffness and using the iteration method. Meanwhile, the effect of material non-linearity of steel and concrete on rigidity and strength of composite beams is considered. With the age-adjusted effective modulus method, the analysis for the whole process of shrinkage and creep under long-term load can be performed. The ultimate load, deflection, stress and slip of continuous composite beams under short-term and long-term load are computed using the proposed finite element model. The numerical results are compared with the experimental results and existing values based on other numerical methods, and are found to be in good agreement.

Health monitoring and comparative analysis of time-dependent effect using different prediction models for self-anchored suspension bridge with extra-wide concrete girder

The structural health status of Hunan Road Bridge during its two-year service period from April 2015 to April 2017 was studied based on monitored data.The Hunan Road Bridge is the widest concrete self-anchored suspension bridge in China at present.Its structural changes and safety were evaluated using the health monitoring data,which included deformations,detailed stresses,and vibration characteristics.The influences of the single and dual effects comprising the ambient temperature changes and concrete shrinkage and creep(S&C)were analyzed based on the measured data.The ANSYS beam finite element model was established and validated by the measured bridge completion state.The comparative analyses of the prediction results of long-term concrete S&C effects were conducted using CEB-FIP 90 and B3 prediction models.The age-adjusted effective modulus method was adopted to simulate the aging behavior of concrete.Prestress relaxation was considered in the stepwise calculation.The results show that the transverse deviations of the towers are noteworthy.The spatial effect of the extra-wide girder is significant,as the compressive stress variations at the girder were uneven along the transverse direction.General increase and decrease in the girder compressive stresses were caused by seasonal ambient warming and cooling,respectively.The temperature gradient effects in the main girder were significant.Comparisons with the measured data showed that more accurate prediction results were obtained with the B3 prediction model,which can consider the concrete material parameters,than with the CEB-FIP 90 model.Significant deflection of the midspan girder in the middle region will be caused by the deviations of the cable anchoring positions at the girder ends and tower tops toward the midspan due to concrete S&C.The increase in the compressive stresses at the top plate and decrease in the stresses at the bottom plate at the middle midspan will be significant.The pre-deviations of the towers toward the sidespan and pre-lift of the midspan girder can reduce the adverse influences of concrete S&C on the structural health of the self-anchored suspension bridge with extra-wide concrete girder.

Self-desiccation mechanism of high-performance concrete

Investigations on the effects of W/C ratio and silica fume on the autogenous shrinkage and internal relative humidity of high performance concrete (HPC), and analysis of the self-desiccation mechanisms of HPC showed that the autogenous shrinkage and internal relative humidity of HPC increases and decreases with the reduction of W/C respectively; and that these phenomena were amplified by the addition of silica fume. Theoretical analyses indicated that the reduction of RH in HPC was not due to shortage of water, but due to the fact that the evaporable water in HPC was not evaporated freely. The reduction of internal relative humidity or the so-called self-desiccation of HPC was chiefly caused by the increase in mole concentration of soluble ions in HPC and the reduction of pore size or the increase in the fraction of micro-pore water in the total evaporable water (Tr/Tte ratio).

Long term behavior of self-compacting reinforced concrete beams

Tests were carried out on 8 self-compacting reinforced concrete(SCC) beams and 4 normal reinforced concrete beams. The effects of mode of consolidation,load level,reinforcing ratio and structural type on long term behavior of SCC were investigated. Under the same environmental conditions,the shrinkage-time curve of self-compacting concrete beam is very similar to that of normal concrete beam. For both self-compacting reinforced concrete beams and normal reinforced concrete beams,the rate of shrinkage at early stages is higher,the shrinkage strain at 2 months is about 60% of the maximum value at one year. The shrinkage strain of self-compacting reinforced concrete beam after one year is about 450×10-6. Creep deflection of self-compacting reinforced concrete beam decreases as the tensile reinforcing ratio increases. The deflection creep coefficient of self-compacting reinforced concrete beam after one and a half year is about 1.6,which is very close to that of normal reinforced concrete beams cast with vibration. Extra cautions considering shrinkage and creep behavior are not needed for the use of SCC in engineering practices.

Creep and Shrinkage of Concrete in Eurocode 2 and Polish Bridge Standards--Necessity for Implementation

Estimation of creep and shrinkage in concrete bridges is still approximate and uncertain. Over the years, Polish Codes for Concrete Structures partially adapted the CEB （Euro-International Concrete Committee）-FIP （International Federation for Prestressing） models used to predict creep and shrinkage of concrete. In the currently used Polish concrete bridge code, modified CEB-FIP 1970 recommendations are used. At the time the standard was implemented, it introduced simple methods for the evaluation of final creep coefficients and shrinkage strains. It was sufficient for simple bridge structures and concrete technology used at that times. As modern bridge structures have become increasingly complex with variable construction techniques and developing concrete technology, the implementation of Eurocode 2 is necessary as it gives more practical and accurate methods for the prediction of creep and shrinkage effects. A comparative analysis of the time-dependent deformation of concrete included in Eurocode 2 and in Polish Bridge Codes is pointing out that there is a necessity for more adequate criteria for the rapidly growing concrete bridge stock in Poland.

贮水和水处理构筑物的裂缝控制浅析

本文通过分析贮水及水处理构筑物裂缝产生的主要原因,提出设计、施工等方面进行裂缝控制的综合技术措施,以引起各方面对各储水及水处理构筑物裂缝控制的重视,使结构设计更完善,施工及使用更合理,提高工程整体质量水平。

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.

Design of Monolithic Concrete Ground Floors

It is known that the slabs on soil constitute one of the most difficult types of structures despite their apparent simplicity. The objective of this paper is to give a general survey of the design of ground supported slabs with the interposition of a suitable subbase. A solution is proposed with the following characteristics： （1） complete suppression of joints; （2） conventional reinforcement with meshes in the upper and lower fiber of the slab in order to confront and distribute cracking that is caused by hindrance of free contractions and expansions; （3） effective confrontation of problems of bulging. The proposal is in effect on one hand for industrial floorings and on the other hand for concrete pavings with large durability requirement.

Creep and Shrinkage Self-Compacting Concrete （SCC） Analytical Models

In the structures whose long-term behavior should be monitored and controlled, creep and shrinkage effects have to be included precisely in the analysis and design procedures. Creep and shrinkage, vary with the constituent and mixtures proportions, and depend on the curing conditions and work environment as well. Self-compacting concrete （SCC） contains combinations of various components, such as aggregate, cement, superplasticizer, water-reducing agent and other ingredients which affect the properties of the SCC including creep and shrinkage of the SCC. Hence, the realistic prediction creep and shrinkage strains of SCC are an important requirement of the design process of this type of concrete structures. In this study, three proposed creep models and four shrinkage models available in the literature are compared with the measured results of 52 mixtures for creep and 165 mixtures for shrinkage of SCC. The influence of various parameters, such as mixture design, cement content, filler content, aggregate content, and water cement ratio （w/c） on the creep and shrinkage of SCC are also compared and discussed.

Coupling model for calculating prestress loss caused by relaxation loss,shrinkage,and creep of concrete

The calculation model for the relaxation loss of concrete mentioned in the Code for Design of Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts(JTG D62—2004) was modified according to experimental data. Time-varying relaxation loss was considered in the new model. Moreover, prestressed reinforcement with varying lengths(caused by the shrinkage and creep of concrete) might influence the final values and the time-varying function of the forecast relaxation loss. Hence, the effects of concrete shrinkage and creep were considered when calculating prestress loss, which reflected the coupling relation between these effects and relaxation loss in concrete. Hence, the forecast relaxation loss of prestressed reinforcement under the effects of different initial stress levels at any time point can be calculated using the modified model. To simplify the calculation, the integral expression of the model can be changed into an algebraic equation. The accuracy of the result is related to the division of the periods within the ending time of deriving the final value of the relaxation loss of prestressed reinforcement. When the time division is reasonable, result accuracy is high. The modified model works excellently according to the comparison of the test results. The calculation result of the modified model mainly reflects the prestress loss values of prestressed reinforcement at each time point, which confirms that adopting the finding in practical applications is reasonable.