Yunping Xi

Moisture diffusion in cementitious materials Adsorption isotherms

Abstract This article describes an improvement on a previous model proposed by Bažant and Najjar, in which moisture diffusivity and moisture capacity are treated as separate parameters. These parameters are evaluated from independent test results, and are shown to depend on the water:cement ratio, curing time, temperature, and cement type. The moisture capacity is obtained as the slope of the adsorption isotherm. A mathematical model is developed and is shown to predict experimental adsorption isotherms of Portland cement paste very well. In the present form, the model is not applicable to high temperatures.

Moisture diffusion in cementitious materials Moisture capacity and diffusivity

Abstract Based on a model by Bažant and Najjar, and using a new model for adsorption isotherms, moisture capacity and diffusivity of concrete are analyzed. The moisture capacity, obtained as a derivative of the ] adsorption isotherm, first drops as the humidity increases from zero, then levels off as a consuant, and finally again increases when the humidity approaches saturation, regardless of the age, cement type, temperature, and water:cement ratio. The well-known diffusion mechanisms,including the ordinary diffusion, Knudsen diffusion, and surface diffusion, are analyzed and the diffusion in concrete is treated as a combination of these mechanisms. An improved formula for the dependence of diffusivity on pore humidity is proposed. The improved model for moisture diffusion is found to give satisfactory diffusion profiles and long-term drying predictions. The model is suited for incorporation into finite element programs for shrinkage and creep effects in concrete structures.

Hardening mechanisms of an alkaline-activated class F fly ash

The hardening mechanism of a paste composed of a low calcium fly ash and alkali was investigated. It was found that a fraction of fly ash reacted with water-glass and formed amorphous or low-ordered crystalline compounds of the type of Na2O–Al2O3–SiO2, after the paste was cured at 60°C for 24 h. For the water-glass with a modulus of 1.64, the strength of the paste is mainly attributed to the gel-like reaction products that bind the particles of fly ash together. When the modulus is decreased to 1.0, crystalline sodium silicate is formed in the matrix, which helps to achieve high strengths.

Reliability analysis of chloride penetration in saturated concrete

Abstract Corrosion of reinforcement in concrete is a major durability problem of reinforced concrete structures. The corrosion is initiated by chloride penetration into the concrete, which is a diffusion-controlled process involving many complex physical and chemical mechanisms. Large random variation has shown in the corrosion damage of reinforced concrete structures, and there is a pressing need to develop a reliability analysis method for chloride penetration and for the onset of steel corrosion in concrete. In order to conduct a reliability analysis, a comprehensive material model for chloride concentration is developed and described, and some of the model parameters (water–cement ratio and curing time) are selected as random variables. By including uncertainties in the selected variables, the chloride penetration front at a point in time can be represented by a time-dependent probabilistic distribution. Similar to the concepts of supply and demand used in structural reliability analysis, the chloride penetration front at a target level (depth) with respect to time can be represented by the probability distribution of crossing the target level. A detailed description of the basic concepts and numerical examples are given. To include the effect of uncertainties of the material parameters, a recently developed Monte Carlo simulation program was used in numerical examples.

Experimental study on the effect of aggregate content on fracture behavior of concrete

Abstract Effect of aggregate content on fracture behavior of concrete is studied by testing on 48 geometrically similar three-point bend concrete beams. The results are analyzed by using a size effect method, in which the fracture behavior of concrete is characterized by two parameters: fracture energy Gf and effective fracture process zone cf. Test results showed that with increasing volume fraction of aggregate in the range 45–75%: (1) the compressive strength of concrete decreases slightly (15%), and can be practically considered to be a constant; (2) fracture energy Gf varies within 25%, and there is not critical volume fraction which gives the maximum Gf; and (3) the size of the fracture process zone decreases, which may be explained by the change in coarseness of grain structures defined in terms of mosaic patterns.

Effect of Phase-Change Materials on Properties of Concrete

The results of an experimental investigation of using phase-change materials (PCMs) in portland cement concrete are presented in this paper. The objective of this paper is to improve the thermal properties of concrete as a structural material. A compression test, flexural test, drying shrinkage test and thermal conductivity test were conducted. The paper shows how PCMs are used in concrete mixtures as both sand replacement and additives. The results revealed that in the replacement method, the loss of compressive strength due to the addition of PCM is not as high as the additive method. For up to 20% sand replacement by PCM, the strength loss is not significant for structural applications. The specific heat of the concrete increased considerably. Therefore, the thermal conductivity of the concrete is reduced and the insulation capacity of the concrete is also improved. A flexural test, drying shrinkage test, and microstructure analysis provide a good understanding of the PCM-modified concrete. Overall, the results presented in the paper show that it is quite promising to use PCM in concrete to improve its insulation capacity with decreased thermal conductivity and, at the same time, it is possible to keep the strength loss of the concrete in an acceptable range.

A model for moisture capacities of composite materials Part I: formulation

Abstract Moisture capacity is defined as the change of moisture content in a porous material due to a change of internal pressure at certain temperature. It is one of the material parameters characterizing transport behaviors. A composite model for effective moisture capacity is developed based on extreme energy principles, which have been used to evaluate effective heat capacities of two phase composite materials. In the method, potential energy is expressed in terms of Helmholtz free energy; complementary energy in terms of Gibbs free energy. These two free energies are derived in terms of material parameters related to moisture transfer, and based on the relevant thermodynamic principals. The effective moisture capacities obtained in the present study are composed of two terms. One is the volumetric average of the moisture capacities of the constituent phases, and the other is the coupling effect which depends on bulk moduli, shrinkage coefficients of the constituents, as well as on volume fraction of the inclusion. Among these influential parameters, the coupling effect is controlled mainly by the difference of the shrinkage coefficients between the two constituent phases. When the difference increases, so does the contribution of the coupling effect.

Integrated Framework for Quantifying and Predicting Weather-Related Highway Construction Delays

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 discourage litigation arising from weather-related delays by including in contracts a reasonable number of nonwork days as a consequence of adverse weather and providing an equitable criteria for the course of action when the predictions in the contracts turn out to be inaccurate. To address this need, an integrated framework consisting of the following two key components is proposed: (1) identification of attributes of weather that cause construction delays and (2) generation of synthetic weather sequences using a stochastic weather generator to quantify and provide probabilistic forecasts of weather threshold values. The utility of this framework is demonstrated through its application to construction work on a project in Texas. The use of probabilistic forecast of construction delay attributes provided by a semiparametric weather generator in this research is an example of interdisciplinary study to help address this problem. The result of the research is better decision support for agencies who wish to author contracts that more equitably allow for the influence of weather during construction.

Measurement of time-dependent strains of concrete: Prepared by subcommittee 4: Standardized test methods for creep and shrinkage

1359-5997/98

Drying shrinkage of cement paste as measured in an environmental scanning electron microscope and comparison with microstructural models

A recently developed image-intensity-matching technique has been used to analyse images of cement paste which were dried in an environmental scanning electron microscope. Shrinkage that occurs during changes in relative humidity is reported, together with some of the influences of water-to-cement ratio, temperature and age. Results from microstructurally based models are compared with experimental results. The best fit of models to experiment is achieved if calcium silicate hydrate (C–S–H) is divided into two types: high density C–S–H, which does not shrink, and low density C–S–H, which does shrink. Approximate values of unrestrained shrinkage of the low density C–S–H are attained as a function of relative humidity.