424 publications from this institution
The fracture processes in cement paste at microscale are simulated by the 3D lattice fracture model based on the microstructure of hydrating cement paste. The uniaxial tensile test simulation is carried out to obtain the load-displacement diagram and microcracks propagation for a Portland cement paste specimen in the size of 100×100×100 µm3 at the degree of hydration 69%. The Young's modulus, tensile strength, strain at peak load and fracture energy are computed on the basis of the load-displacement diagram.
Abstract This paper presents an experimental study on the development of material properties over time (up to around 2 years) and the structural behavior of reinforced beams, for two types of alkali‐activated concrete (AAC). Compressive strength, flexural strength, tensile splitting strength, elastic modulus, and flexural behavior of reinforced beams are investigated. Tested material properties of AAC are compared with the properties of conventional concrete, as predicted by Eurocode. For the mixes of AAC and the conventional concretes with the same 28 days compressive strength, flexural and tensile splitting strength at 28 days are found to be similar, whereas the elastic moduli of AAC mixtures is up to 30% lower than those of conventional concrete. Related to the long term behavior, after 28 days moist‐curing and subsequently exposing AAC specimens to laboratory conditions (50% RH/20°C), a reduction of flexural strength, tensile splitting strength and elastic modulus, is observed. Structural behavior of the reinforced AAC beams in four‐point bending test seems not to be affected significantly by the observed decrease in material properties, and is found to be similar to that of conventional concrete beams. The acquired results indicate that the observed decrease of material properties over time might be related to drying (moisture loss). However, more research is needed to understand the phenomenon, especially related to the aimed structural application and safe upscaling of AAC.
A key parameter of the performance of concrete repairs is the bond strength between the repair material and the concrete substrate. Sufficient bond strength is required to withstand the stresses at the interface induced by the environmental and/or mechanical loads. The bond strength is determined by the microstructure of the repair-substrate interface. The microstructure development of the repair-substrate interface is influenced by the moisture exchange between the repair material and the concrete substrate and the “wall effect”. To understand the fundamental bond mechanisms, the microstructure development in concrete repairs are experimentally investigated using the scanning electron microscopy. The experimental results reveal that when the concrete substrate has lower water content, it absorbs more water from the repair material and results in a lower porosity both in the interface and in the repair material. Due to the “wall effect”, the cement particles have a poor packing at the repair-substrate interface and the w/c ratio locally increases. The increased w/c ratio results in a porous interfacial zone and poor mechanical properties.
The cement industry is aware of the environmental problem due to large CO2 emission. Using less Portland cement can reduce the carbon footprint of the cement industry. This can be achieved by replacing part of Portland cement with inert additives such as micronized sand and limestone powder. With these materials economic and environmental friendly solutions are conceivable. However, because of its potential negative impact on the mechanical properties of cement paste, the influence of inert filler on the durability of the development of cracks in cement paste should be considered. One of the reasons is that additives reduce the concentration of Portland cement clinker and their strength-providing reaction products. The purpose of this study is to investigate the influence of inert fillers on the development of cracks in cement paste under compressive loading. A combined experimental and numerical investigation was performed in this study. The crack path in the loaded cement pastes was investigated by scanning electron microscopy (SEM) observations. Fracture processes of cement-based materials were simulated with a numerical model. The simulation results are validated by experiments. The results indicate that the mechanical properties of interface between hydration products and inert filler in cement paste systems have an important influence on the development of fracture in blended cement paste under compressive loading. The micronized with poor adhesion properties leads to a high probability of cracking in blended cement paste. On the contrary, the presence of LP particles has no significant impact on the propagation of cracks.
A large number of studies have been conducted to investigate the pore structure of cement-based materials with the mercury intrusion porosimetry (MIP) method. However, the conventional MIP method has some drawbacks in the measurement of pore size distributions of most porous materials, including cement-based materials. This is mainly because of the lack of direct accessibility of most pore volumes. This paper reports an alternative method to investigate the pore structure with conventional MIP set-up but different testing sequence and different data analysis method. By repeating the intrusion and extrusion cycles in every pressure step, the volume of throat pores and the volume of corresponding ink-bottle pores can be determined at each pore diameter. With the assumption that the size distribution of these ink-bottle pores at each pore diameter is equal to that of the pores larger than the corresponding throat pores, the pore size distribution can be recalculated. The new MIP method is used to investigate the pore structure of Portland cement pastes. The results measured with the new MIP method are compared with those determined with conventional MIP method and backscanning (BSE) image analysis. It is found that the new and conventional MIP measurements give the same total porosity and BSE image analysis gives a relatively lower total porosity. Regardless of the curing age, the result of the new MIP method shows a critial pore diameter smaller than that of the old method and larger than that of the BSE image analysis.
