Compressive behavior of concrete under high strain rates after freeze-thaw cycles

Xudong Chen,Chen Chen,Zhiheng Liu,Jun Lu,Xiangqian Fan 사단법인 한국계산역학회 2018 Computers and Concrete, An International Journal Vol.21 No.2

The dynamic compressive behavior of concrete after freezing and thawing tests are investigated by using the split Hopkinson pressure bar (SHPB) technique. The stress-strain curves of concrete under dynamic loading are measured and analyzed. The setting numbers of freeze-thaw cycles are 0, 25, 50, and 75 cycles. Test results show that the dynamic strength decreases and peak strain increases with the increasing of freeze-thaw cycles. Based on theWeibull distribution model, statistical damage constitutive model for dynamic stress-strain response of concrete after freeze-thaw cycles was proposed. At last, the fragmentation test of concrete subjected to dynamic loading and freeze-thaw cycles is carried out using sieving statistics. The distributions of the fragment sizes are analyzed based on fractal theory. The fractal dimensions of concrete increase with the increasing of both freeze-thaw cycle and strain rate. The relations among the fractal dimension, strain rates and freeze-thawing cycles are developed.

Experimental Study on Damage Evolution Behavior of Self-Compacting Rubberized Concrete under Direct Tensile Fatigue Loading

Xudong Chen,Chen Chen 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.11

Fatigue properties of self-compacting rubberized concrete (SCRC) under axial tension are rare.To explore the fatigue damage evolution of SCRC, the direct tensile fatigue test and residual mechanical test were conducted. According to the experimental results, the linear relationship between the fatigue life and secondary strain rate also exists in tensile fatigue test of SCRC, and the slope value is similar to the compressive and direct tensile fatigue test of ordinary concrete. Further, the downtrend of the residual secant modulus is more obvious than the residual strength. In addition, the direct tensile fatigue damage evolution of SCRC under different stress levels can be predicted by the Isojeh’s model.

Storage Optimization for Energy-Saving Based on Hypergraph in Cloud Data Center

Xudong Chen,Baomin Xu 보안공학연구지원센터 2015 International Journal of Database Theory and Appli Vol.8 No.4

Experimental Study of Evolution of Fracture Process Zone in Dam Concrete under Cyclic Loading Using Digital Image Correlation

Xudong Chen,Shengtao Li,Xiyuan Cheng 대한토목학회 2022 KSCE JOURNAL OF CIVIL ENGINEERING Vol.26 No.2

In this paper, three-point bending tests were carried out o investigate the post-peak cyclic loading behavior of dam concrete under two loading rates. The fracture parameters, residual load capacity, cyclic modulus, dissipated energy, and deformation recovery were presented and analyzed. The analysis was made to understand the evolution of fracture process zone (FPZ) under post-peak cyclic loading for dam concrete by utilizing digital image correlation (DIC) technique. The results showed while the dam concrete under a higher loading rate showed an increase in maximum load, it is not necessarily the same for post-peak bearing capacity. The specimens with higher post-peak bearing capacity had higher residual stiffness, higher dissipation energy, and higher Crack Mouth Opening Displacement (CMOD) reversibility under cyclic loading. The FPZ development was investigated quantitatively, and the extension and retraction of its tip were observed. Moreover, it was found that the FPZ in specimens with higher residual stiffness developed more slowly, and FPZ had a more significant retraction rate during unloading for the specimens with higher residual bearing capacity.

Hydration of Ultrafine and Ordinary Portland Cement at Early Ages

Xudong Chen,Jikai Zhou,Yuanyuan Yan 대한토목학회 2014 KSCE JOURNAL OF CIVIL ENGINEERING Vol.18 No.6

Experimental results of a quantitative of the hydration of ultrafine and ordinary Portland cement pastes were presented in thispaper. The degree of hydration of cement pastes was determined based on the determination of non-evaporable content. The purposeof this study was to explore the mechanism of the effects of particle size distribution on hydration kinetics of cement. The dataobtained from test indicate that at each age before 7 days, the non-evaporable water content of the ultrafine cement paste was higherthan that of the ordinary cement paste. The fineness has a great effect on early 7 days hydration. In the later age, the rate of hydrationslows down and diffusion of silicate ions through layers of existing products becomes the rate-limiting process. The porosity ofultrafine cement paste is lower thod than of ordinary cement paste. The difference in porosity of two cement paste is mostpronounced at early ages before 14 days. A general reduction in porosity with increasing in age is observed from test results. Theultrafine cement consistently produces higher gel/space ratios at all seven testing ages. At an age of 1 day, the ordinary Portlandcement achieves 79% of its fine counterpart.

Foreign Banks’ Entry and Local Banks’ Strategies in China

Xudong Chen,Liming Yao 한국증권학회 2017 Asia-Pacific Journal of Financial Studies Vol.46 No.4

The opening of the Chinese banking sector encourages the entry of foreign banks through greenfield investment or cooperation with local banks. The entry strategies of foreign banks are therefore influenced by the strategies of local banks. This paper examines the interaction between local and foreign banks. The results demonstrate that the superior capabilities of foreign banks to liquidate collateral facilitate their greenfield investment. Otherwise, foreign banks will cooperate with local banks, whose cooperation depends on local banks’ self-development capability. Our results suggest that foreign banks are more likely to cooperate with large local banks in China unless small local banks have greater self-development capabilities.

Fractal Characterization of Pore System Evolution in Cementitious Materials

Xudong Chen,Jikai Zhou,Ning Ding 대한토목학회 2015 KSCE JOURNAL OF CIVIL ENGINEERING Vol.19 No.3

The porous microstructure of cementitious materials can be examined using the fractal approach. The aim of our study is to makean evaluation of the fractal characteristics of pore size distributions of cementitious materials measured by the mercury intrusionporosimetry technique. The cement paste, mortar, and concrete specimens were mixed with various water-cement ratios and curingages. The fractal characteristics of these materials were examined through fractal models for the pore size distribution, and wererepresented by only one physical parameter, namely-the fractal dimension. The results illustrate that cementitious materials areporous fractal structure. Five different calculation models of the fractal dimensions were developed. Overall, the modeled fractaldimension values were consistent. A final discussion on the relationship between the modeled values and the evolution of the porestructure was presented.

Experimental Study and Analytical Modeling on Fatigue Properties of Pervious Concrete Made with Natural and Recycled Aggregates

Xudong Chen,Dandan Shi,Nan Shen,Shengtao Li,Saisai Liu 한국콘크리트학회 2019 International Journal of Concrete Structures and M Vol.13 No.1

As a solid pollutant, the recycled aggregate can be reused to replace the natural aggregate to cast pervious concrete, promoting resource recycling and reducing environmental pollution. Pervious concrete is usually applied to transportation engineering as pavements and decks, which are often subjected to fatigue loads in service. Therefore, it is essential to investigate the fatigue mechanical properties of pervious concrete. In this study, four-point cyclic bending loading test of natural aggregate pervious concrete and recycled aggregate pervious concrete were conducted under four different stress levels. By analyzing the experimental results, the mechanical performances, including hysteresis curve characteristics, damping ratio, dynamic elastic modulus and cyclic strain, of two kinds of pervious concrete under cyclic loading were revealed. Based on the improved EPF model, the relationship between fracture parameters, plastic strain and unloading strain were obtained. Besides, the relationship between the loading cycles and the ratio of plastic strain to unloading strain was received according to fatigue testing data under different stress levels. Further, the simplified fatigue model of pervious concrete was proposed and the experimental data was fitted with the model results. The fitting result reached a good agreement.

Investigation of the Deformation and Failure Characteristics of High-Strength Concrete in Dynamic Splitting Tests

Xudong Chen,Jin Wu,Kai Shang,Yingjie Ning,Lihui Bai 한국콘크리트학회 2022 International Journal of Concrete Structures and M Vol.16 No.6

The dynamic response properties of concrete have been of interest during the use of buildings due to seismic, impact, and explosion events. The splitting Hopkinson lever is a classical device for testing the dynamic mechanical properties of materials. In this paper, dynamic splitting tests on concrete were conducted using it, and a time series predictive computational model for the incident, reflected and transmitted pulses of high-strength concrete specimens at high strain rates was developed, and the extension mechanism of splitting tensile cracks in high-strength concrete was detected and analyzed based on the DIC technique. The results show that: the peak strength of C60 specimens and C80 specimens increased by about 60% and 90%, respectively, from 0.05 MPa to 0.09 MPa in impact strength; the triangular damaged area at the end of the contact surface of the specimen and the rod subjected to high impact pressure increased significantly, the dynamic energy dissipation increased, and the damage degree of the specimens increased; under the action of high strain rate, the brittleness of the concrete specimens with higher strength increased, the damage rate The higher strength concrete specimens have increased brittleness, faster damage rate and higher crack extension under high strain rate. The results of the paper can provide important references for the design of buildings under impact loading.

A Note on the Flexural Behavior of Concrete under Different Loading Rates

Xudong Chen,Jikai Zhou,Yuanyuan Yan 대한토목학회 2015 KSCE JOURNAL OF CIVIL ENGINEERING Vol.19 No.3

This paper examined the flexural behavior of large concrete specimens (300 × 300 × 1100 mm) under different strain rate. Fourstrain rates were examined: 10−6, 10−5, 10−4, and 10−3 s−1. The changes of flexural strength, dynamically increased factor, as well aselastic modulus in response to different strain rates were well addressed. The results show that increasing strain rate led to moreflattened surfaces of the specimens, In addition, more aggregates found to be broken on the fractured surface. The elastic modulusand flexural strength of the concrete were positively correlated to the strain rate. Nevertheless, the increase in elastic modulus wasapproximately half of the percentage of increase in flexural strength. There was no clear change for the Poisson’s ratio in response tothe change in strain rates.