Vinh Dao

Tensile Properties of Early-Age Concrete

In this paper, the tensile properties of concrete at very early ages and their measurements are reviewed, and the need for further study is clearly highlighted, A newly developed apparatus and procedures for uniaxial direct tensile testing of concrete specimens at ages of 1.5 hours or more after mixing, together with the experimental results obtained, are reported. An improved knowledge of various important early-age properties is presented. The tensile strength, Youngs modulus, and fracture energy of early-age concrete are all found to increase very slowly during the first 3 hours or so, but significantly increase thereafter. Strong correlations are shown to exist between these three parameters, especially between tensile strength and fracture energy. The high values of fracture energy obtained strongly suggest that early-age cracking of concrete involves a significant zone of plastic straining or microcracking in the vicinity of the crack tip. Early-age concrete is also shown to be more ductile than mature concrete.

Performance of Permeability-Reducing Admixtures in Marine Concrete Structures

The use of permeability-reducing admixtures is a potential preventative of the chloride-induced corrosion of steel reinforcement, which is the main cause of the deterioration of concrete structures exposed to coastal environments. This paper presents an experimental investigation into the effectiveness of two typical commercially available permeability-reducing admixtures: one characterized by crystallization activity and the other by hydrophobic and pore-blocking effects. Concrete specimens were exposed to simulated coastal environments, and chloride concentration profiles at 28-, 365-, and 730-day exposures were determined by X-ray fluorescence spectrometry. The results suggested that the incorporation of the admixture, characterized by hydrophobic and pore-blocking effects, appeared to considerably enhance the concrete durability with respect to chloride-induced corrosion. The inclusion of the admixture characterized by crystallization activity however, seemed to have almost no detectable effect. This implies the necessity of exercising a degree of caution during specification. Copyright

Plastic Shrinkage Cracking of Concrete

Summary Plastic shrinkage cracking, which occurs during the first several hours after the casting of concrete, is of considerable economic significance in the concrete construction industry. In this paper, the mechanism of plastic shrinkage cracking and its related phenomena of evaporation and bleeding are first reviewed. Then, together with tensile strength of early-age concrete, the concept of matric suction, its effect on plastic shrinkage and an empirical relationship for its estimation are discussed. Practical implications of the discussion are also highlighted.

An investigation into temperature gradient effects on concrete performance at elevated temperatures

To assure adequate fire performance of concrete structures, appropriate knowledge and adequate, practical models of concrete at elevated temperatures are crucial yet current lacking, prompting further research. This paper first highlights the limitations of inconsistent thermal boundary conditions in conventional fire testing; and of using constitutive models developed based on empirical data developed testing concrete under minimised temperature gradients in modelling of concrete with significant temperature gradients. On that basis, the paper outlines key features of a test setup used for the accurate control of the thermal boundary conditions when testing concrete at elevated temperatures, using radiant panels to generate well-defined and reproducible heating regimes. The repeatability, consistency and uniformity of thermal boundary conditions are demonstrated using measurements of heat flux and in-depth temperature of test specimens. Compressive strength is also investigated. The initial data collected clearly suggested that the incident heat fluxes, and thus the associated temperature gradient, has potentially significant effects on concrete mechanical properties at elevated temperatures. Further research is thus ongoing to quantify such effects and also to develop constitutive models accounting for a wide range of heating conditions; from very slow to extremely rapid heating. The proposed models could be included into effective rational knowledge-based fire design and analysis of concrete structures.

Finite element modelling for service life prediction of repaired concrete in marine environment

Summary Service life of concrete structures subjected to a chloride environment can be predicted using chloride transport models based on the partial differential equation (PDE) of Fick’s second law of diffusion. In this paper, a numerical finite element based formulation is proposed for the service life prediction of reinforced concrete structures in general cases, including those that the PDE cannot be solved analytically or by finite difference method. The proposed model effectively accommodates time- and space-dependent chloride transport, chloride binding, as well as the effect of steel reinforcement and the effect of concrete cover replacement/repair. Practical applications are also analysed and discussed, which highlights the model’s validity and capability.

Effects of temperature and temperature gradient on concrete performance at elevated temperatures

To assure adequate fire performance of concrete structures, appropriate knowledge of and models for performance of concrete at elevated temperatures are crucial yet currently lacking, prompting further research. This article first highlights the limitations of inconsistent thermal boundary conditions in conventional fire testing and of using constitutive models developed based on empirical data obtained through testing concrete under minimised temperature gradients in modelling of concrete structures with significant temperature gradients. On that basis, this article outlines key features of a new test setup using radiant panels to ensure well-defined and reproducible thermal and mechanical loadings on concrete specimens. The good repeatability, consistency and uniformity of the thermal boundary conditions are demonstrated using measurements of heat flux and in-depth temperature of test specimens. The initial collected data appear to indicate that the compressive strength and failure mode of test specimens are influenced by both temperature and temperature gradient. More research is thus required to further quantify such effect and also to effectively account for it in rational performance-based fire design and analysis of concrete structures. The new test setup reported in this article, which enables reliable thermal/mechanical loadings and deformation capturing of concrete surface at elevated temperatures using digital image correlation, would be highly beneficial for such further research.

Thermal Cracking Analysis during Pipe Cooling of Mass Concrete Using Particle Flow Code

Pipe cooling systems are among the potentially effective measures to control the temperature of mass concrete. However, if not properly controlled, thermal cracking in concrete, especially near water pipes, might occur, as experienced in many mass concrete structures. In this paper, a new numerical approach to simulate thermal cracking based on particle flow code is used to shed more light onto the process of thermal crack propagation and the effect of thermal cracks on thermal fields. Key details of the simulation, including the procedure of obtaining thermal and mechanical properties of particles, are presented. Importantly, a heat flow boundary based on an analytical solution is proposed and used in particle flow code in two dimensions to simulate the effect of pipe cooling. The simulation results are in good agreement with the monitored temperature data and observations on cored specimens from a real concrete gravity dam, giving confidence to the appropriateness of the adopted simulation. The simulated results also clearly demonstrate why thermal cracks occur and how they propagate, as well as the influence of such cracks on thermal fields.