The methylene blue value (MBV) is an important index that reflects the quality of manufactured sand (MS). The relation between MBV of MS and its limestone fines content, clay content and clay characteristics was investigated and the effects of MBV ranging from 0.35 to 2.5 on the performance of MS concrete were tested. Results showed that MBV of MS is affected not by the limestone fines content, but by the clay content and the liquid limit index of clay. With an increase in MBV, the workability, flexural strength and 7d compressive strength of the MS concrete decreased, while the 28d compressive strength was not affected. Moreover, the increase of MBV promoted plastic shrinkage and drying shrinkage cracking of the concrete, and remarkably accelerated freeze-thaw damage and abrasion loss. It was concluded that the critical MBV was 1.4, a value below which MS concrete performances are not significantly diminished.
A simulation model to estimate the pore structure of cement hydrates is presented. This paper describes procedures for predicting phase compositions based on the classical hydration model of portland cement, calculating the particle size distribution of constituent phases and evaluating the pore size distribution by stereological and statistical considerations. To evaluate the effectiveness of this model, simulation results were compared with experimental results of the pore size distribution measured by mercury porosimetry. As a result, it was found that the experimental and simulated results were in close agreement, and the simulated results indicated characterization of the pore structure of cement hydrates.
Hot climate concreting requires specific procedures in order to reduce the undesirable effects caused principally by excessive water evaporation from concrete surfaces, which induce plastic shrinkage cracking and thereby reduce durability. This research highlights the effectiveness of some of these measures: plastic sheets, curing agent, cold water and polypropylene fiber. The design of experiments method was used to reduce the number of tests and allow study of the effects of factors (or measures) and the interactions between these factors and the response parameters, which include plastic shrinkage cracks, evaporation rate, and strength. The most efficient way to minimize plastic shrinkage is to use a curing compound followed by the application of a plastic sheet cover; and the most efficient measure to decrease evaporation is the application of a plastic sheet cover.
For the purpose of global sustainability the long life of structures is essential and the durability performance of reinforced concrete structures is one of the key issues to be resolved. This paper reports the results of a series of long term corrosion tests on fiber reinforced cementitious composites containing polyethylene (PE) alone and hybrid steel cord (SC) and PE fibers. The results are also compared with ordinary mortar. The specimens are subjected to accelerated corrosion for one year by applying external potential to the steel bar anode and a cathode made out of a steel wire mesh placed outside the concrete. Durability performances of the specimens are examined through regular monitoring of the time to initiate corrosion, the corrosion area ratio, corrosion depth, and the amount of steel loss. Results show that the hybrid fiber reinforced cementitious composites (HFRCC) containing hybrid SC and PE fibers exhibited excellent performance compared to mortar and fiber reinforced cementitious composites (FRCC) containing PE fiber. The order of the durability performance is HFRCC, FRCC, and Mortar. It is observed that the sacrificial corrosion of some of the SC fibers in the HFRCC specimen played an important role in the significant reduction of steel bar corrosion in the specimen.
Expansion of the rust volume produced from corrosion creates the radial pressure pressing the cylindrical wall around reinforcing bars. To simulate this radial pressure from the expanding rust, a new experiment method is introduced. By this method, the mechanical device has to be embedded in each specimen during casting concrete. This device transfers the applied force and the displacement from vertical axis to the pressure and the displacement in radial direction. Therefore, the radial pressure and the displacement pressing the cylindrical wall around the reinforcing bars can be determined from the applied force and the displacement in the vertical direction. Two mortar specimens with different covering thickness, 50 and 90mm, were tested in order to examine the behaviors of the crack in the thinnest cover direction. The experimental results from this new experiment method are shown and compared with the calculated results of elastic, elastic-partially cracked, and fully plastic theory. In comparison, the experimental results of radial pressure were found to be higher than the elastic and the elastic-partially cracked but lower than the plastic theory. Furthermore, it was found that both normalized forms of the ultimate radial pressure and the ultimate radial displacement have linear relationships with non-dimensional cover thickness.
When designing blast-resistant reinforced concrete (RC) structures, reducing spall damage due to reflected tensile stress waves is a major problem. Furthermore, for rapid construction of the blast-resistant structures against sudden terrorist bomb attacks, it is necessary to build it with precast concrete walls and reduce the weight of the precast elements by reducing their size for ease of transportation and construction. In this study, to propose an idea for rapid construction and better blast resistance of blast-resistant RC structures, double-layered RC slabs composed of precast thin plates, 50 mm thick, were fabricated and utilized for contact detonation tests. The tests were conducted under a condition that the amount of explosives and the dimensions of specimen were constant respectively, and two types of concrete, normal concrete and polyethylene fiber reinforced concrete (PEFRC), were employed as the slab materials. Our results showed that creating an air cavity between the two layers of PEFRC slab was effective in reducing spall damage, while the air space had no advantage in normal RC, under a condition that the thickness of the air space was fixed at 15 mm. Furthermore, the above difference between PEFRC and normal RC slabs was discussed, based on the numerical result on the fracture process of the air-sandwiched normal RC slab.
This paper investigates the failure mode and the damage mechanism of steel-concrete composite slabs under high cycle fatigue loads by using three-dimensional nonlinear finite element analysis. The applicability of the simulation system, which was originally developed for reinforced concrete slabs, is extended to the steel-concrete composites with the proposed interface element, and experimentally verified with fatigue loading tests for bridge decks. The computed midspan deflection of composite slabs shows a fair agreement with data obtained from the experiment, and the horizontally induced cracking observed in reality is properly reproduced by the computational simulation. Finally, the authors predict the ultimate state at which the upper concrete layer separated by horizontal cracks fails in compression fatigue, and the corresponding S-N diagram is computationally predicted for the future discussion.
Despite its importance in the seismic performance of an overall structure, our understanding of the ductility capacity of high-strength concrete (HSC) columns under various loading conditions is relatively limited compared to that of normal-strength concrete (NSC) columns. This study aims to evaluate the seismic performance of HSC columns through a numerical analysis approach. Based on the smeared crack concept, the analysis model consisted of material models for concrete and embedded reinforcement. The brittle behavior and smooth crack surfaces of HSC are some of its main drawbacks in engineering practice. In the proposed models, the shear retention mechanism along the crack surface correctly considered the smooth crack surfaces of HSC, and the confining effect of the column core was taken into account by modifying the concrete compressive model according to an appropriate confinement model for HSC. As part of the investigation, five large-scale HSC columns were constructed and tested under simulated seismic loading. The proposed numerical method was applied in predicting the seismic performance of various HSC columns tested in this research program and obtained from other research in the literature. The analytically predicted hysteretic behavior, ductility level, and failure mode of the columns showed reasonable agreement with experimental data.