Engineered Cementitious Composite (ECC) has been proposed to be one of the most promising repair materials due to its unique high ductility and tight crack width control. In concrete repairs, the shrinkage of repair materials is restrained by concrete substrate, and the repair material therefore often cracks. When ECC is used as repair material, the crack width is much smaller compared to normal concrete. The tight crack width of ECC retards the penetration of water and harmful substances and thus enhances the durability of concrete repairs. This paper is aimed to explore the chloride penetration in cracked ECC repairs and to assess the service life of the repair systems. Rapid chloride migration tests was conducted to investigate the chloride penetration profile. Based on the experimental results, the service life of repair systems was evaluated.
Concrete is seldom saturated due to its self-desiccation. Even the submerged concrete structures may remain unsaturated for quite a long time. It has been reported the saturation level of pore solution has significant effect on species penetration. However, very little work was proposed regarding the transport properties and serviceability of concrete structures made of blended cementitious materials. This paper initiated the study of chloride ion diffusion in various blended cement-based system under non-saturated condition by resistivity measurements. Experiments have been performed on mortars made of different cement-based materials (Portland cement, fly ash, blast furnace slag, limestone powder) with different water to binding ratio (w/b=0.4, 0.5, 0.6). The mortar specimens have been curing for 200 days conditioning with 98% RH and 20°C, followed by oven drying at 50°C until the specimens reach different saturation levels from 95% down to 18%. The resistivity measurements for different cement-based systems are performed. The results showed that saturation level has significant effect on the chloride diffusion coefficient. As to the relation between relative diffusivity and water saturation, the effect of w/b is less obvious in system with higher w/b. Compared with FA and LP system, the deepest decrease in relative diffusivity was found in BFS-blended system with the decrease of water saturation level.
Due to the high CO2 emissions and scarcity involved in producing cement, ways are sought to reduce the amount of cement needed. In this paper the possibility of using micronized sand as cement replacement is investigated. Material properties are determined and the influence on the cement paste is assessed via workability and strength measurements. It is found that the workability decreases with increasing fineness and higher replacement levels. Strength reduction seems to be slightly more than proportional to the replacement level. Theoretical analysis has been done on the subjects of particle size distribution, specific surface and particle packing. A good correlation is found between the specific surface and the workability indicators. Particle packing models give promising results for optimizing the mix composition. The numerical simulation model HUMPSTRUC3D is used to evaluate the correlation between the simulated contact area of particles in the paste, and strength development measured. Finally discussion is given on economical and environmental aspects.
Fresh model cement mixtures, with the same w/c ratio and particle size distribution, were simulated by the SPACE system that is based on a dynamic mixing algorithm. Thereupon, they were hydrated by the HYMOSTRUC 3D system. Boundary conditions were varied, rendering possible assessment of their influence on percolation of capillary porosity by serial sectioning and using the overlap of slices. Simulation results revealed increases in total porosity and in connected fraction of capillary pores due to the existence of aggregate. The de-percolation threshold of capillary porosity was found not only related to total porosity and image resolution, but also governed by the spatial distribution of capillary pores.
In this work, a numerical model is proposed to study chloride transport in alkali-activated materials. This model is based on multiple-relaxation-time lattice Boltzmann method, where particle distribution function is introduced to simulate the chloride binding and diffusion. This model takes into account diffusion, homogenous reaction between chloride ions and diffusive solid, and heterogeneous reaction between chloride ions and non-diffusive solid. The accuracy of the model is confirmed by a benchmark simulation of transient reactive transport problem. As a demonstration, this model is then applied to simulate chloride transport in alkali-activated slag paste with varying alkaline activators. The influence of alkali content, silica content, curing age and chloride binding on the chloride transport property is briefly discussed.
In concrete repair systems, material properties in the repair material and interface are greatly influenced by the initial moisture content of the concrete (or mortar) substrate. In order to quantify moisture profiles inside the repair system, X-ray absorption was used. Preliminary studies are performed first to determine the absorption rate of a two-year-old mortar substrate. Cumulative water absorption and the penetration front of water are monitored as a function of time. Furthermore, absorption by the mortar substrate is investigated when a repair material is cast on top. Influence of water-to-cement ratio (w/c) of the repair material on water exchange, and moisture content change during drying in the repair system are studied. Ordinary Portland cement (OPC) paste was used as a repair material. It was shown that, initially, water from the repair material is absorbed by the substrate with the same speed as “free” water. Furthermore, not only can water be absorbed by the substrate, but it can also migrate back from the substrate to the repair material due to hydration of the repair material. Quantifying the dynamics of water exchange at an early stage between the repair material and the concrete substrate is a first step towards understanding the development of material and interface properties in repair systems. This can provide engineers with recommendations about substrate preconditioning in experiments and field practice.
In this study a novel fire test system including in situ monitoring of internal pore vapor pressure and temperature history inside the concrete is first introduced. The fire test system was used to measure high performance concrete slabs and self-compacting concrete slabs at different layer depths from the fire front, i.e., 10 mm, 20 mm and 30 mm when the slabs were subjected to the standard ISO 834 fire conditions. Microstructural investigation was performed on parallel samples exposed to small furnace. The total porosity and pore size distribution were measured by mercury intrusion porosimetry. The vapor pressure build up is investigated, as wall as the correlation between the vapor pressure distribution, the temperature history, the initial moisture content, the porosity/pore size distribution. Some possibilities of preventing concrete spalling are discussed.
