Yunping Xi - Academia.edu (original) (raw)

Papers by Yunping Xi

Cement and Concrete Research, 2019

The purpose of this paper is to develop a model for predicting water vapour sorption hysteresis o... more The purpose of this paper is to develop a model for predicting water vapour sorption hysteresis of cement-based materials. The sorption hysteresis was modelled based on the pore size distribution (PSD) and the pore network. The PSD of gel and capillary pores was predicted from the adsorption isotherm. The results indicated that the distribution had two peaks at 3.8 nm and 11 nm of the pore diameters, referring to gel and small capillary pores, respectively. The predicted ratio of gel water to evaporable interlayer water was not a constant, but decreased with an increasing hydration degree. When RH was below 0.25, the predicted sorption hysteresis was solely influenced by the hysteresis of water in interlayer spaces and minor hydrates. The hysteresis in gel pores, small and large capillary pores influenced the shape of desorption isotherms within the RH ranges of 0.25-0.85, 0.35-0.98 and 0.4-1, respectively.

Procedia Manufacturing, 2020

The primary objective of this paper is to assess model-based simulation for the coupled transport... more The primary objective of this paper is to assess model-based simulation for the coupled transport-degradation behavior of concrete using smart sensing technologies and experimental investigations. Twenty humidity and temperature sensors were installed inside five cylindrical cement mortar specimens (120 mm diameter and 160 mm height) prepared with different watercement ratios and aggregate volume fractions. The mortar specimens were then exposed to 8% sodium chloride solution on the specimen top surface and placed in an environmental chamber with programmable freezing and thawing cycle. Model-based simulations were obtained for the temperature and humidity variations with time, and the chloride concentration profiles and compared with the sensor and experimental measurements. The comparison shows that the temperature, relative humidity and chloride concentration predicted by the model had the same general trend of the sensor and experimental measurements, but the model underestimates their values. Validating model-based simulations is an essential step towards the integration of modelbased simulations and sensors technologies for the structural health management of civil structures.

Cement and Concrete Composites, 2022

This project focused on the chloride-induced corrosion of reinforcing steel in structural concret... more This project focused on the chloride-induced corrosion of reinforcing steel in structural concrete. The primary goal of this project is to analyze the surface chloride concentration level of the concrete bridge decks throughout Colorado. The study indicates three factors that can affect chloride concentration levels in bridge decks: age of the concrete, traffic, and weather. Samples were collected from decks and curbs of bridges in different climate regions with various concrete ages and traffic levels. Water-soluble chloride concentrations were tested for all samples. Chloride concentration profiles for all the locations were listed and plotted. The deepest concrete powder was collected at a depth of 2 inches. The rebar level of the bridge was usually at or below this depth. The chloride concentrations of most bridge decks were below the critical values at the rebar level. The chloride concentrations of bridge decks are usually greater than that of bridge curbs. However, these bridge curbs showed deeper chloride penetration than the bridge decks. Younger bridges had much lower chloride concentrations, which is expected. Heavier traffic resulted in higher chloride concentrations. The bridges built in colder regions had a higher chloride concentration up to 2" depth (the rebar level). Climate may be the most significant influential factor among age, traffic, and climate when considering chloride concentration of bridge decks in Colorado. Corrosion protection should focus on the bridges decks who locate in the cold climate zone and with high traffic volume.

Engineering Journal, 2021

This article presents an experimental study on the effect of different types of chloride-based de... more This article presents an experimental study on the effect of different types of chloride-based deicing salts on the rate of chloride penetration into concrete structures. Three different types of deicers commonly used for deicing and anti-icing operations, NaCl, CaCl2, and MgCl2, were selected to investigate the influence of associated cations on the rate of chloride ingress into concrete. The concentration profiles of various ions were measured including Cl-, Na + , K + , Mg 2+ and the effect of different cations on the diffusion rate of chloride ion into concrete was systematically studied. The test data were compared to numerical results obtained from a numerical model developed based on the Nernst-Planck equation which considered the ionic coupling effects among ions in chloride solutions and ions in concrete pore solution, Na + , K + , OH-. The test results illustrated that cations in different chloride solutions have significant effects on the penetration rate of chloride ions. This can be summarized in the order of CaCl2 > NaCl > MgCl2 from the fastest to slowest chloride ion penetration. In the case of various salt combinations tested, the diffusion rate of chloride ions can be ranked as NaCl + CaCl2 > CaCl2 + MgCl2 > NaCl + MgCl2. The total chloride concentration obtained from experimental results were compared to the numerical model and a good agreement was observed.

Construction and Building Materials, 2020

The chemo-mechanical model has been solved for predicting period of rust production. The Timoshen... more The chemo-mechanical model has been solved for predicting period of rust production. The Timoshenko solution has been used to solve the pressure build-up in the system. The fictitious crack model has been used for solving the crack propagation period. The simulation results have been compared with the previous experimental results. The lifetime of the well cement on small-scale borehole system has been predicted.

Cement and Concrete Composites, 2019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

The expansion of concrete subjected to extreme elevated temperature is linked with intricate micr... more The expansion of concrete subjected to extreme elevated temperature is linked with intricate micro-structural variations, such as the transformation of the constituent phases. This study proposes a model to predict the thermal expansion of cement paste and concrete considering micro-structural changes under elevated temperatures ranging from 20°C to 800°C. The model presented can consider characteristics of various aggregates in the calculation of thermal expansion for concrete. The model is a combination of a multi-scale stoichiometric model and a multi-scale composite model. At the cement paste level, the model satisfactorily predicted a test result. At concrete level, upper bounds from the model were matched relatively well with test results by previous researcher. If the mechanical properties, such as elastic modulus (E), Poisson’s ratio (ν), and thermal deformation, of the aggregates used in concrete are given, it is likely that the model will reasonably predict experimental re...

Computers and Concrete, 2015

Moisture transport in concrete in atmospheric environment was studied in this paper. Based on the... more Moisture transport in concrete in atmospheric environment was studied in this paper. Based on the simplified formula of the thickness of the adsorbed layer, the pore-size distribution function of cement paste was calculated utilizing the water adsorption isotherms. Taking into consideration of the hysteresis effect in cement paste, the moisture diffusivity of cement paste was obtained by the integration of the pore-size distribution. Concrete is regarded as a two-phase composite with cement paste and aggregate, neglecting the moisture diffusivity of aggregate, then moisture diffusivity of concrete was evaluated using the composite theory. Finally, numerical simulation of humidity response during both wetting and drying process was carried out by the finite difference method of partialdifferential equation for moisture transport, and the numerical results well capture the trend of the measured data.

Steel and Composite Structures, 2013

ABSTRACT Fire performance and fire safety of high-rise buildings have become major concerns after... more ABSTRACT Fire performance and fire safety of high-rise buildings have become major concerns after the disasters of World Trade Center in the U.S. in 2001 and Windsor tower in Spain in 2005. Performance based design (PBD) approaches have been considered as a better method for fire resistance design of structures because it is capable of incorporating test results of most recent fire resistance technologies. However, there is a difficulty to evaluate fireproof performance of large structures, which have multiple structural members such as columns, slabs, and walls. The difficulty is mainly due to the limitation in the testing equipment, such as size of furnace that can be used to carry out fire tests with existing criteria like ISO 834, BS 476, and KS F 2257. In the present research, a large scale calorie meter (10 MW) was used to conduct three full scale fire tests on medical modular blocks. Average fire load of 13.99 was used in the first test. In the second test, the weighting coefficient of 3.5 (the fire load of 50 ) was used to simulate the worst fire scenario. The flashover of the medical modular block occurred at 62 minutes in the first test and 12 minutes in the second test. The heat resistance capacity of the external wall, the temperatures and deformations of the structural members satisfied the requirements of fire resistance performance of 90 minutes burning period. The total heat loads and the heat values for each test are calculated by theoretical equations. The duration of burning was predicted. The predicted results were compared with the test results, and they agree quite well.

Journal of Materials in Civil Engineering, 2016

AbstractThe purpose of this paper is to investigate the diffusivity of the interfacial transition... more AbstractThe purpose of this paper is to investigate the diffusivity of the interfacial transition zone (ITZ) experimentally using cementitious composite materials made of rod-shaped aggregates and cement mortar. Two types of tests were conducted: the porosity distribution of the ITZ at different distances from aggregate surface was measured by backscattered electron images, and the water vapor diffusivity of the composites was measured by the penetration rate of water vapor through thin slides of specimens. The surface roughness of rod-shaped aggregates was considered as a testing parameter. The diffusivity of ITZ was calculated based on the measured diffusivities of the composite and it was also modeled according to the measured porosity distribution within the ITZ. The results showed that the porosity of ITZ decreases with increasing distance from the aggregate surface, a rougher aggregate surface decreases the porosity of ITZ in the first 10 μm from the aggregate surface, and the ratio of averaged wate...

Construction and Building Materials, 2015

h i g h l i g h t s Influences of different factors on thermal conductivity of concrete were pres... more h i g h l i g h t s Influences of different factors on thermal conductivity of concrete were presented. The theory of composite materials was applied to calculate thermal conductivity. Porosity was chosen as a key parameter in a new thermal conductivity model. Interfacial thermal resistance was considered to predict thermal conductivity. Thermal conductivity models for unsaturated and damaged concrete were proposed.

Structure and Infrastructure Engineering, 2008

... 199821. Liu, Y. and Weyers, RE 1998. Modeling of time-to-corrosion cracking in chloride conta... more ... 199821. Liu, Y. and Weyers, RE 1998. Modeling of time-to-corrosion cracking in chloride contaminated reinforced concrete structures. ACI Mater. J , 95: 675681. [Web of Science ®], [CSA] View all references). In the ...

Journal of Materials Research, 1997

A model based on mosaic pattern analysis is shown to have the potential to describe the complex s... more A model based on mosaic pattern analysis is shown to have the potential to describe the complex shapes and spatial distribution of phases in the microstructures of multiphase materials. Several characteristics of both micrographs of portland cement pastes and images generated using the few parameters of the model are determined and, for the most part, agreement is good. The advantage is that spatial features of the microstructures can be captured by a few parameters.

Journal of Materials in Civil Engineering, 2015

Because of the increasing difficulty in obtaining natural aggregate, alternative aggregates have ... more Because of the increasing difficulty in obtaining natural aggregate, alternative aggregates have been considered in the construction industry. One of the feasible alternatives is to reuse construction and demolition waste as aggregates, called recycled aggregate, to make new concrete called recycled aggregate concrete (RAC). However, the compressive strength of RAC is usually poor due primarily to the high water absorption capacity, high porosity, and weaker bond of interfacial transition zone between recycled aggregate and new cement mortar. This paper presents the results of a comprehensive experimental study on RACs made of 100% recycled coarse aggregate. Three different mixing approaches, several surface pretreatment methods with two different pretreatment materials were used to improve the compressive strength of RAC. The experimental results showed that proper surface pretreatments and mixing approaches can improve the quality of RAC significantly. The water-cement ratio of pretreatment materials is very important. The compressive strength of an optimized RAC with 100% recycled coarse aggregate can reach as high as 43.3 MPa (6,277 psi) on 28 days.

Journal of Construction Engineering and Management, 2010

Constant exposure to the environment makes highway construction highly dependent on weather. Howe... more Constant exposure to the environment makes highway construction highly dependent on weather. However, highway construction contracts are often unclear about the potential influence of weather-related delays on highway construction project schedules. There is a need to ...

Journal of Materials in Civil Engineering, 2003

Due to the random nature of material properties, crack growth is a random process. A probabilisti... more Due to the random nature of material properties, crack growth is a random process. A probabilistic model for randomness of the progressive crack growth in a quasibrittle material such as concrete is presented. The model consists of a Markov chain adapted to R-curve fracture behavior. The method is useful particularly for concrete structures with existing major cracks. The R-curve serves as the mean curve for the fracture process. The R-curve under mon<;>tonic loading is obtained from the given test data on the effect of structural sizes. The standard deviation of the mean curve is predicted from the standard deviation of the load carrying capacity of specimens. The parameter estimation method is fonnulated and applications to existing structures are illustrated.

Abstract-A probabilistic model for the randomness of the progressive crack growth in a quasi-brit... more Abstract-A probabilistic model for the randomness of the progressive crack growth in a quasi-brittle material such as concrete is presented. The model consists of a Markov chain adapted to R-curve behavior. It yields the crack propagation probability in any loading step as well as the probability of failure at any stage of the fracture process. The R-curve is obtained from the given test data on the effect of structure size on the maximum loads. The standard deviation of the peak load is the minimum statistical information required. According to the available test results, this standard deviation is approximately a linear function of the crack propagation distance. The parameter estimation method is formulated and some applications are illustrated. 0 1997 Elsevier Science Ltd